Tài liệu Báo cáo Y học: Receptor crosstalk Implications for cardiovascular function, disease and therapy ppt

REVIEW ARTICLE
Receptor crosstalk
Implications for cardiovascular function, disease and therapy
Nduna Dzimiri
Cardiovascular Pharmacology Laboratory, Biological and Medical Research Department, King Faisal Specialist Hospital & Research
Centre, Riyadh, Saudi Arabia
There are at least three well-defined signalling cascades
engaged directly in the physiological regulation of cardiac
circulatory function: the b
1
-adrenoceptors that control the
cardiac contractile apparatus, the renin-angiotensin-aldo-
sterone system involved in regulating blood pressure and the
natriuretic peptides contributing at least to the factors
determining circulating volume. Apart from these pathways,
other cardiac receptor systems, particularly the a
1
-adreno-
ceptors, adenosine, endothelin and opioid receptors, whose
physiological role may not be immediately evident, are also
important with respect to regulating cardiovascular function
especially in disease. These and the majority of other car-
diovascular receptors identified to date belong to the guanine
nucleotide binding (G) protein-coupled receptor families
that mediate signalling by coupling primarily to three G
proteins, the stimulatory (G
s
), inhibitory (G
i
)andG
q/11

proteins to stimulate the adenylate cyclases and phospho-
lipases, activating a small but diverse subset of effectors and
ion channels. These receptor pathways are engaged in
crosstalk utilizing second messengers and protein kinases as
checkpoints and hubs for diverting, converging, sieving and
directing the G protein-mediated messages resulting in dif-
ferent signalling products. Besides, the heart itself is endowed
with the means to harmonize these signalling mechanisms
and to fend off potentially fatal consequences of functional
loss of the essential signalling pathways via compensatory
reserve pathways, or by inducing some adaptive mechanisms
to be turned on, if and when required. This receptor crosstalk
constitutes the underlying basis for sustaining a coherently
functional circulatory entity comprising mechanisms con-
trolling the contractile apparatus, blood pressure and cir-
culating volume, both in normal physiology and in disease.
Keywords: receptor crosstalk; heart; vasculature; regulatory
systems; subcellular; contractile function; G-proteins; heart
failure; hypertension; hypertrophy; signal transduction.
INTRODUCTION
The cardiovascular circulatory function constitutes a very
sophisticated network of several highly synchronized cir-
cuits to ensure the sustention of human life by maintaining
or increasing blood supply providing oxygen and nutrients
to active tissue, and by redistributing the blood to prevent
heat loss from the body. In humans, multiple cardiovascular
regulatory mechanisms have evolved to uphold this function
at three major levels: contractile apparatus, blood pressure
and circulating volume. Apart from its ability to ensure a
smooth supply of nutrients to various organs, this network

also has the capacity to adapt to minor changes in vascular
resistance that may influence the caliber of arterioles and
other resistance vessels, and thus alter capillary hydrostatic
pressure. Such circulatory adjustments are effected syner-
gistically by local (autoregulatory) as well as systemic
mechanisms in both the heart and peripheral circulatory
organs. The autoregulatory mechanisms are a result of the
intrinsic contractile response of smooth muscle to stretch, in
combination with vasodilatation produced by metabolic
changes leading to a decrease in oxygen tension, pH, and
local vasoconstrictors, such as serotonin. Systemic regula-
tory mechanisms involve vasodilators such as the kinins,
Correspondence to N. Dzimiri, Biological & Medical Research Department (MBC-03), PO Box 3354, Riyadh 11211, Saudi Arabia.
Fax: + 966 1442 7858, Tel.: + 966 1442 7870, E-mail:
Abbreviations: A, adenosine receptor subtype; AC, adenylate cyclase; Ach, acetylcholine; ACE, angiotensin converting enzyme; ADO, adenosine
receptors; ANG II, angiotensin II; ANP, atrial natriuretic peptide; ANPR, ANP receptor; AP-1, activating protein; AR, adrenoceptors; ATR,
angiotensin receptor; BNP, brain natriuretic peptide; BNPR, BNP receptor; BP, blood pressure; [Ca
2+
], calcium channel; [Ca
2+
]
i
, intracellular
calcium; [Ca]
v
, voltage-gated Ca
2+
channel; cAMP, 3¢,5¢-cyclic adenosine monophosphate; cGMP, 3¢,5¢-cyclic guanosine monophosphate;
CNS, central nervous system; CV, cardiovascular; diacylglycerol, diacylglycerol; ET-1, endothelin-1; ETR, endothelin receptor; ETC, endothelial
cells; 5-HT

4
, 5-hydroxytryptamine; I
Ca
, calcium current; I
K
, inward rectifying K
+
current; GC, guanylate cyclase; iNOS, inducible nitric oxide
synthase; InsP
3
, inositol triphosphate; IPN, isoproterenol; [K
+
], potassium channel; [K]
ATP
, ATP-dependent K
+
channel; [K
+
]
D
, delayed rectified
K
+
channel; MAPK, mitogen-activated protein kinase; MR, muscarinic cholinergic receptors; Na, sodium; NE, norepinephrine; NO, nitric oxide;
eNOS, cardiac nitric oxide synthase; OP, opioid receptors; PE, phenylephrine; PIE, positive inotropic effect; PKC, protein kinase C; PLA
2
,
phospholipase A
2
; PLC, phospholipase C; PLD, phospholipase D; PKG, cGMP-dependent protein kinase; PPase, phosphoprotein phosphatase;

PP2A, phosphoprotein phosphatase 2A; PTK, protein tyrosine kinase; PTPase, protein tyrosine phosphatase; PTX, Pertussis toxin; RAS, renin-
angiotensin aldosterone system; RPIA, N
6
-phenylisopropyladenosine; VECs, vascular endothelial cells; VSM, vascular smooth muscle.
(Received 12 March 2002, revised 29 June 2002, accepted 14 August 2002)
Eur. J. Biochem. 269, 4713–4730 (2002) Ó FEBS 2002 doi:10.1046/j.1432-1033.2002.03181.x
circulating vasoconstrictors, such as catecholamines and
angiotensin II (Ang II), neural regulatory mechanisms,
sympathetic vasodilator systems and the vagal tone.
Locally, the ability of the heart to maintain its regular
contractility and pumping rate is facilitated primarily by
postganglionic sympathetic-adrenergic nerve endings that
terminate within the myocardium and the autonomic
electrical stimulus originating in the heart. Thereby, the
noradrenergic sympathetic impulses increase the cardiac
rate, contractile force and accelerate relaxation via the
b-adrenoceptors (b-ARs), while impulses in the vagal
cardiac fibres decrease heart rate via the cholinergic
pathway [1–3]. Stimulation of the cardiac b-ARs activates
the stimulatory guanine nucleotide binding (G
s
)protein–
adenylate cyclase (AC))3¢,5¢-cyclic adenosine monophos-
phate (cAMP) cascade, initiating the protein kinase A
(PKA)-dependent phosphorylation of several ion channels
and regulatory proteins, such as the L-type Ca
2+
channels,
phospholamban and myofibrillar proteins involved in the
cardiac excitation-contraction coupling and energy meta-

bolism [1,2]. Regulation of the blood pressure involves
both systemic mechanisms and local angiotensin receptor-
mediated actions of Ang II on the renin-angiotensin-
aldesterone system (RAS) [4]. The angiotensin receptors
(ATRs) couple to G
q/11
or G
i/o
proteins to stimulate
several intracellular signalling pathways, transduced via
activation of at least five different effector systems: (a)
phospholipase C (PLC) leading to the formation of
inositol-1,4,5-triphosphate (InsP
3
) and diacylglycerol
(DAG); (b) voltage-dependent Ca
2+
channels stimulating
several downstream effectors; (c) phospholipase D clea-
ving phosphatidylcholine; (d) phospholipase A
2
synthesi-
zing prostaglandins and procanoids; and (e) AC inhibition
leading to a decrease in cAMP production [4].
By virtue of its nature, circulating blood volume is an
indirect product of mechanisms controlling the contractile
apparatus, blood pressure and valvular function. This
function is regulated primarily by the release of natriuretic
peptides (NPs), particularly the atrial natriuretic peptide
(ANP), in response to volume expansion through activa-

tion of the hypothalamic muscarinic cholinergic neurons
by a-AR synapses [5,6]. Unlike the other essential cardiac
G-protein coupled receptors, such as ARs or ATRs, the
NP receptors (NPRs) belong to the so-called type I
transmembrane single-chain receptors containing three
domains: intracellular catalytic GC, adjacent kinase-like
and extracellular ligand-binding domains. They transmit
NP signalling by activating the guanylate cyclases to
produce the second messenger cGMP, leading to cGMP-
dependent protein kinase-mediated cellular actions [5].
Apart from the aforementioned pathways, the cardiovas-
cular system maintains several other receptor systems,
particularly the a
1
-ARs [2], adenosine (ADO) [7], endo-
thelin (ETR) [8], and opioid (OPR) [9] receptors, with yet
no fully defined cardiac physiological function, but may
contribute primarily to events associated with its adaptive
functional performance in disease. The existence of this
particular group of receptors in the heart not only raises
important questions regarding the complexity of the
circulatory function, but also unequivocally points to the
fact that no cardiovascular functional entity is attributable
to a single signalling mechanism. Growing evidence points
to crosstalk as a primary means by which these mecha-
nisms regulate cardiovascular circulatory function, and
which is the subject of this review.
CARDIOVASCULAR RECEPTOR
CROSSTALK SIGNALLING
Crosstalk among adrenoceptor subtypes

The concept of receptor cross talk has its origin in the early
1980s, when efforts were directed at explaining some of the
apparently inconsistent behaviour of certain pharmacolo-
gical agents, such as the a-AR and b-AR agonists. Hence,
among the most elaborately described crosstalk to date is
that involving the cardiac AR subtypes, particularly
between the b
1
-AR and a
1
-AR, in the regulation of the
cardiac contractility and rhythm [10–14]. This is partly
attributable to the fact that, initially, differentiation among
the AR subtypes has been hypothetically based on
differences in the potencies of the three agonists epineph-
rine, norepinephrine (NE) and isoproterenol (IPN) to the
a-andb-AR subfamilies. Early studies had already
demonstrated that such a receptor classification could
not be sustained, because of compelling evidence revealing
that catecholamines not only transduce their signalling via
both a-andb-AR subtypes [11], but also influence the
downstream signalling components of the individual
pathways in virtually the same fashion under different
conditions [12]. In rat neonatal cardiomyocytes for exam-
ple, stimulation of a
1
-AR inhibits b-AR-mediated cAMP
accumulation, presumably by coupling to the G
i
protein,

indicating that the former pathway may regulate the b-AR
signalling downstream of agonist–receptor interactions
[10,14]. These studies led to the appreciation of the
probability that both the convergence of these two
pathways at the receptor–G protein–AC circuit, and their
cross regulation via G
s
and G
i
serve to regulate mecha-
nisms controlling cardiac contractile function under phy-
siological conditions [10,13]. However, the mode(s) by
which this crosstalk is transmitted downstream of the AC
appears to differ depending on the experimental setup. In
one study using transgenic mouse lines for example,
cardiac-specific overexpression of a
1B
-AR was found not
to affect b-AR density or affinity to antagonists, yet the
basal AC activity was increased without influencing basal
cAMP levels, while IPN-stimulated AC was attenuated in
association with increased Ca
2+
-dependent PKC-d and
PKC-e as well as Ca
2+
-independent PKC-b2 levels in
particulate cellular fractions [13]. These findings led to the
suggestion that overexpression of a
1B

-AR triggers uncoup-
ling or desensitization of the b-AR by molecular crosstalk,
via the PKC pathway. In contrast, in another transgenic
mouse model, similar overexpression of the a
1B
-AR was
associated with significant depression of left ventricular
contractility, accompanied by both attenuated basal and
catecholamine-stimulated AC activity [14]. Treatment of
the mice with pertussis toxin (PTX) led to a reversal
of these changes, presumably pointing to an induction of
coupling to PTX-sensitive G
i
proteins resulting from
elevated levels of a
1B
-AR. This, together with the obser-
vation of elevated GRK2 activity in these animals stimu-
lated the notion that down-regulation of b-ARs may cause
an elevation in a
1
-AR levels. It is evident that under
experimental conditions, the a
1
-andb
1
-AR pathways
4714 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002
influence each other in a variety of ways, utilizing different
signalling conduits. The cardiac a

1
-AR may influence the
b
1
-AR signalling via two distinct receptor-mediated mech-
anisms: via pertussis toxin (PTX)-sensitive G proteins
possibly to regulate positive inotropic function and by
enhanced a
1
-AR under disease conditions.
Adrenoceptor crosstalk with other cardiovascular
receptors
Rapidly accumulating literature has demonstrated that,
apart from crossregulation of each other, stimulation of the
AR pathways triggers alterations in the signal transduction
of other cardiovascular systems, particularly the ATR,
ETR, muscarinic acetylcholine receptor (MR), NPR and
nitric oxide synthase (NOS) pathways [15–31] (Table 2). In
neonatal rat cardiomyocytes, the a
1
-andb
1
-AR agonists
induce ANP transcription [15,16], while Ang II stimulation
of AT1R leads to a decrease in a
1A
-AR mRNA levels and
stability as well as its induction of immediate early gene
c-fos expression, demonstrating that crosstalk among these
receptors occurs at the level of gene transcriptional regula-

tion [17]. Furthermore, in rat-1 fibroblasts, activation of
ET-1 receptors was found to induce a
1A
-AR phosphoryla-
tion, possibly involving the PKC and MAPK signalling [18],
while in human pericardial smooth muscle, it was found to
counter-regulate cAMP and MAPK signalling [20]. Grow-
ing evidence demonstrates that stimulation of these path-
ways can enhance or inhibit the release of endogenous
catecholamines often associated with the down-regulation
or desensitization of the ARs. Thus, it appears that some of
this crosstalk may be a direct product of altered receptor
turnover. In the vascular system and peripheral circulatory
organs, complex crosstalk regulating AR signalling often
involves synergistic actions of several pathways, or may be
an indirect product of interactions between some nonadre-
nergic pathways acting on local catecholamine release. A
typical example is the interaction between the MR and
a-AR pathways in the hypothalamus, which is thought to
be responsible for the ANP release in the regulation of
circulating blood volume [6]. Also, in rabbit cerebral
arteries, activation of a
2
-AR triggers endothelium-depend-
ent ET-1-mediated contractile response to acetylcholine
(Ach), leading to a reversal of MR effects [18,19]. Crosstalk
in which nonadrenergic pathways influence b-AR include
the attenuation of IPN-stimulated cAMP accumulation by
ET-1 in human pericardial smooth muscle cells [20],
inhibition of epinephrine release and b-AR responsiveness

by adenosine [21,22] and inhibition of b-AR contractility by
either Ang II via AT1R [23–25], nitric oxide (NO) produc-
tion [26–28] or OP
2
receptors [29–31]. Of these interactions,
probably the most exhaustively studied crosstalk is that
between b
1
-AR and the AT pathways, suggesting that
Ang II-mediated AT1R stimulation decreases b
1
-AR
responsiveness via PKC activation [24] and inhibits b-AR-
stimulated AC activity via the G
i
protein in cell cultures [23].
Some studies have indicated that such AT1R activity
induces local catecholamine release from the cardiac
sympathetic neurons causing myocardial damage probably
resulting from down-regulation of the b-AR [24,25]. The
regulation of NE via neuronal AT1R pathway appears to
follow two courses defined as evoked or enhanced neuro-
modulation [23]. Accordingly, evoked neuromodulation
involves AT1R-mediated, antagonist-dependent rapid NE
release and inhibition of K
+
channels, while enhanced NE
neuromodulation involves the MAPK cascade ultimately
leading to an increase in NE transporter, tyrosine hydroxy-
lase and dopamine b-hydroxylase mRNA transcription [23].

In contrast to Ang II or ET-1, adenosine has been shown to
inhibit NE release from sympathetic nerve endings in rat
adrenal medulla partially through its inhibitory effects on
RAS pathway [21], and to exert antiadrenergic effect in rat
hearts through crosstalk between its two receptor subtypes
A
1
-andA
2a
-ADO [22]. The cardiac nitric oxide synthase
(eNOS) pathway may contribute to a number of such
mechanisms involving the crosstalk among ET, MR and
AR pathways [8,26–28]. Stimulation of the eNOS may
attenuate both inotropic and lustropic responses to b-AR
stimulation, and appears to regulate baseline ventricular
relaxation in conjunction with ANP [26,27]. It was shown
for example, that the inhibition of b-AR-stimulated increase
in the slow-inward Ca
2+
current (I
Ca
) and reduction in
Ca
2+
affinity of the contractile apparatus may be a result of
MR stimulation of the heart activating NO production
of cGMP [27]. In neonatal ventricular myocytes and
fibroblasts, ANP and NO were shown to synergistically
attenuate the growth-promoting effects of NE by a cGMP-
mediated inhibition of NE-stimulated Ca

2+
-influx [28].
Some of the crosstalk leading to the inhibition of the b
1
-AR
activity can be explained as resulting from a corelease of the
endogenous hormones with the catecholamines in cardio-
myocytes, like in the inhibition of b
1
-AR-stimulated AC in
myocyte sarcolemma by OP
2
agonists via G
i/o
pathways
[29,30], or the OP
3
agonists in rat ventricular myocytes,
devoid of the phosphoinositol pathway [31].
Crosstalk among nonadrenoceptor pathways
Apart from their interaction with the ARs, activation of
several G protein-linked cardiovascular pathways, notably
the ATR, ETR and MR systems, can also trigger the release
of, and enhance cardiovascular responses to, other vaso-
active peptides such as ANP, vasopressin or aldosterone.
Perhaps the most comprehensively studied crosstalk to date
is that involving the ANP and RAS pathways, whereby the
former is thought to exert its actions by inducing an increase
in angiotensin converting enzyme (ACE) to counteract RAS
effects in regulating circulating volume. However, studies so

far have not delineated exactly how this may occur. In sliced
rat atrial tissue, Ang II induces inositol phosphate accumu-
lation and ANP release [32], but seems to impair ANP-
mediated inhibition of AC in the vascular smooth muscle
[33]. Other interesting crosstalk involving the RAS pathway
includes the observation of a transcriptional regulation of
ATR through inhibition of NO synthesis in rats [34] and the
influence of Ang II on ET-1 synthesis and/or release
apparently without influencing its circulating levels in
human endothelial cells [35]. These Ang II actions are
probably mediated through AT1R stimulation of [Ca
2+
]
i
activity. Complex interactions have also been reported
involving the ET and ANP pathways. In this crosstalk,
ANP was reported to inhibit ET-1 in dogs with congestive
heart failure [36], while ET
A
is thought to regulate ANP
gene expression via multiple pathways involving G
i
and G
q
in addition to MAPK activation [37]. The crosstalk among
the vasoactive pathways occurs at two distinct levels: the
Ó FEBS 2002 Cardiovascular receptor crosstalk (Eur. J. Biochem. 269) 4715
central nervous system and cardiac humoral regulatory
mechanisms. One classical example is the release of the NP
through hypothalamic MR and a

1
-AR crosstalk, which is
probably humorally regulated by the heart through several,
diverse feedback mechanisms [6]. However, the contributory
mechanisms remain highly speculative.
The role of crosstalk in cardiovascular function would be
incomplete without a brief consideration of its involvement
in the regulation of the cardiac chloride (Cl
–
), sodium
(Na
+
), K
+
and Ca
2+
channels, as they regulate the mem-
brane potential and transportation of ions and substrates,
controlling excitation and excitation-contraction coupling
of the contractile apparatus. Regulation of these channels is
mediated often through interplay between G
s
-and
G
i
-coupled pathways. For example, it has been suggested
that in cardiac myocytes, b-AR-mediated activation of the
Cl
–
requires the stimulation of both cAMP-dependent

PKA-mediated phosphorylation and cAMP-independent
pathways [38], and may be inhibited by a
1
-AR stimulation
via a PTX-insensitive G-protein [39]. This crosstalk may
lead to the inhibition of b-AR-stimulated increase in
intracellular Ca
2+
([Ca
2+
]
i
), and/or reduction in the affinity
of the contractile apparatus to Ca
2+
. In the heart,
membrane-delimited activation of muscarinic K
+
channels
by G
bc
plays an important role in the inhibitory synaptic
transmission [3]. The activation of the Na
+
pump and
voltage-dependent [K
+
] mediates smooth muscle hyperpo-
larization in the relaxation elicited by Ach, possibly through
enhancement of cGMP activity [40]. Furthermore, it has

been speculated that AT1R-evoked NE neuromodulation
involves the inhibition of K
+
channels and stimulation of
Ca
2+
channels [23]. In the heart, the Na
+
/H
+
exchanger
and mitochondrial K
+
channels may be important in
apoptosis and ischemia/ischemic preconditioning signalling
discussed later. This summary is far from being exhaustive,
and represents only a taste of the rapidly growing know-
ledge about receptor crosstalk with potential relevance for
the physiological regulation of circulatory function. Inter-
estingly, although this pool of interactions among cardio-
vascular systems appears to be congested and not very
transparent, it is regulated by just a couple of G proteins,
protein kinases and signalling junctions.
LONG-TERM AND SHORT-TERM
EFFECTS OF CARDIOVASCULAR
RECEPTOR CROSSTALK
In general, cardiovascular signalling may be regulated at the
level of a single functional entity such as contractile
apparatus, but more importantly so, in coordinating the
different functions into a synchronized unit. In the execution

of these functions, two types of cellular responses, the short-
term and long-term responses may ensue. Short-term events
include, for example, activation of Ca
2+
turnover to
stimulate the contractile apparatus or vasoconstriction,
while long-term actions are essentially involved in gene
transcriptional regulation or altered expression, often as an
adaptive mechanism in disorders such as left ventricular
hypertrophy (Fig. 1). Cardiovascular signalling crosstalk
mediates both short- and long-term events, and coordina-
tion of the individual contributory pathways is regulated at
various signalling junctions, particularly the G protein, AC,
PK and MAPK levels. The existence in the human
cardiovascular system of at least 16 a, 11 b and 5 c subunits
of the heterotrimeric G proteins [41], 10 mammalian AC
isoforms (ACI–ACX) [42] and multiple PKC isoenzymes
exhibiting specificity and diversity in their activation of
G-protein coupled receptor downstream signalling compo-
nents clearly endows the heart with an enormous potential
to assemble numerous signalling products to regulate
intracellular Ca
2+
turnover, and therefore positive inotro-
pism, as well as vasoconstriction and vasodilatation.
However, despite the diversity in the multiple signalling
systems engaged in the regulation of cardiovascular func-
tion, these pathways transduce their messages by coupling
primarily to three G proteins, the G
s

,G
i
and G
q/11
to
stimulate cardiac-specific ACII and ACV or PLC isoforms,
utilizing cAMP-dependent PKA, PKC-a and PKC-f to
activate a small subset of downstream effectors and ion
channels. In particular, the G
i
-coupled, PLC-mediated
signalling cascades appear to occupy a central role in this
crosstalk (Fig. 1). This pathway mediates among other
factors, the inhibition of b-AR-stimulated cAMP accumu-
lation resulting from the crosstalk between the a
1
-AR and
b-AR [10], b
1
-AR and OPRs [29] as well as the AT1R and
ANP systems [32], although a G
i
-mediated crosstalk
bypassing PLC stimulation has also been proposed between
b-AR and OPRs [30]. The same pathway has been
postulated for the ET
A
-induced regulation of ANP signal-
ling and gene expression by coupling to both G
i

and G
q
proteins [36,37]. In these interactions, the G
i
appears to
couple negatively, while the G
q
may do so in a supportive
fashion, especially in the regulation of the contractile
apparatus (Fig. 2). Another important regulatory pathway
is the cGMP-mediated signalling, which is thought to be
involved in the crosstalk between ANPR and AT1R [37],
the inhibition of ET-1 secretion by ANP [36], and the
synergistic actions of NO and ANP in attenuating NE-
stimulated Ca
2+
influx [27]. Although hardly any specific
physiological role in cardiovascular signalling has been
clearly defined for the majority of the crosstalk among these
pathways, its existence in the cardiovascular system strongly
points to an orchestrated ancillary functional role in support
of the classically defined pathways.
Perhaps the most challenging question at present is how
crosstalk is regulated beyond the primary receptor–
G protein–second messenger circuit. Although this question
is far from being answered, it can be plausibly assumed that
the majority of the players have already been identified.
While the short-term crosstalk events appear to be mediated
primarily via second messenger-dependent PKA and PKC,
regulation of the long-term events probably underlies

crosstalk involving both PKs and the MAPK pathways.
Thus, downstream of the second messengers, thePKs and the
MAPKs serve as hubs for diverting, converging, sieving as
well as directing signalling messages mediated by the various
G proteins coupling via the PLC pathway, for example.
Established crosstalk regulation by PKs includes, among
others, the cross-regulation of the a
1
-andb-ARs [11–13,43]
and attenuation of ANP-mediated inhibition of AC activity
by Ang II [33], while crosstalk at the MAPK level has been
ascribed to stimulation of gene expression via different
pathways [11,37]. Apparently, the GPCRs stimulate the
MAPK pathways mainly by coupling via G
i
protein [44], and
to some extent by the bc complex of the G
q
protein [45–47].
Some phosphoinositide 3-kinase (PI3K) isoforms may also
4716 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002
function as crosstalk mediators between nonreceptor protein
tyrosine kinase (PTK) and G-protein coupled receptor
signalling to stimulate the Ras-Raf-MAPK cascades [48,49].
The fact that cardiac function is regulated by diverse
signalling cascades linked via autoregulatory and systemic
regulatory mechanisms renders it mandatory for the heart
to possess an inherent machinery to integrate the commu-
nication among these individual pathways into a single
functional entity. To achieve this, the heart probably

functions as an endocrine and paracrine organ [50,51] that
determines its own fate by regulating the various signalling
mechanisms through receptor crosstalk. Some of these
mechanisms have their origin in the CNS (Fig. 3). These
include: (a) the possible regulation of blood pressure in the
cardiovascular centres of the brain through Ach release via
cholinergic neurons [52]; (b) the negative feedback system
regulating the balance between vasodilatory and vasocon-
strictory effects involving crosstalk between ET-1, ET
B
and
NO [53]; (c) the regulation of both the noradrenergic and
cholinergic systems in cardioinhibitor and vasomotor
centres in the medulla oblongata [3]; and (d) the regulation
of MR by a-AR systems controlling ANP release in the
hypothalamus [6]. Thus, the CNS may be intimately
involved in defining the types, sources and physiological
entities to convey defined messages at the appropriate time,
using sympathetic and parasympathetic routes as links
between the extracardiac and the cardiac signals.
Implication of receptor crosstalk for cardiovascular
physiology
Because circulatory function constitutes an integration of
messages emanating from the contractile apparatus with
those of the various regulators of blood pressure and
circulating volume, a clear demarcation is often not possible
between the mechanisms controlling the different compo-
nents of this complex machinery. Nonetheless, cardiovas-
cular receptor systems may be broadly placed into those
that regulate mainly the contractile apparatus, such as the

AR systems, and those that primarily determine the
circulating blood volume, such as the ATR, ETR and
NPR systems. Interestingly, while the human heart posses-
ses at least three defined b-AR subtypes (b
1
-, b
2
-and
b
3
-AR), it was traditionally believed that catecholamines
preferentially elicit their inotropic, chronotropic, lusitropic
and dromotropic effects via the b
1
-AR–G
s
–AC–cAMP
pathway under physiologic conditions [1,2]. However, the
accumulating evidence that several cardiac pathways can
Fig. 1. Regulation of short and long-term signalling in normal cardiovascular physiology and disease. Short-term signalling is involved in acute and
instantaneous regulation functions such as cardiac contractile function. Stimulation of receptors such as b
1
-AR by the catecholamines (H), for
example, leads to such effects by activating the classical receptor–guanine nucleotide binding protein (G)–second messenger (AC) circuit leading toa
change in the concentration of intracellular messengers such as free cytosolic Ca
2+
, and consequently positive inotropic effects (PIE). Alternatively,
a change in the transcriptional regulation of certain genes or malfunctional signal transduction, such as PKA-mediated b
1
-AR actions, may trigger

long-term cellular effect by stimulating the mitogen-activated protein kinase pathway resulting in altered protein functional expression, as in
apoptosis or mitogenesis. The G
bc
-mediated crosstalk between the MAP kinases and G
i/o
pathways play an important role in mediating long-term
signalling changes especially in cardiac disease. Inhibitory or counteractive functions are indicated as rounded arrow ends. L, L-type Ca
2+
channel;
PLB, phospholamban; Ry, ryadine receptor.
Ó FEBS 2002 Cardiovascular receptor crosstalk (Eur. J. Biochem. 269) 4717
also mediate positive inotropism in vitro at least via the G
s
–
AC–cAMP pathway and that multiple GPCRs can couple
to more than one G protein has cast doubt over the validity
of this paradigm. Moreover, at least three potential
pathways have been delineated so far, that can lead to the
enhancement of cAMP synthesis: directly by coupling to the
G
s
, or to PKC and Ca
2+
via G
q/11
, or signalling via the bc
complex of G
i/o
proteins (Table 1). Accordingly, any
signalling cascade employing members of the G

q/11
and
G
i/o
protein families should also be capable of at least
indirectly regulating or modulating the contractile appar-
atus by their effects on PLC-b, leading to InsP
3
synthesis or
through the PKC-mediated handling of Ca
2+
turnover, or
even bypassing these two routes. While the last alternative
symbolizes physiologically only a theoretical possibility, it
might be relevant in cardiac disease, if and when the other
two are rendered nonfunctional. In contrast, although no
physiological function is currently attributed to the G
q/11
/
PLC–DAG route, compelling evidence points to an essen-
tial function as an inherent supportive resource to be tapped
as required (Fig. 2). Notably, all of the receptors currently
thought to be physiologically dormant in the heart,
particularly b
2
-AR, A
2
-ADO, ET
A
, AT1R and M

1
-MR
subtypes can elevate Ca
2+
levels via the G
q
/PLC route
(Table 1). It is therefore highly unlikely that the expression
of this contingency of Ca
2+
regulators is redundant,
considering the complexity and essence of the cardiovascu-
lar circulatory system. On the other hand, while some of
these pathways have been shown to mediate positive
inotropic effects (PIE) in vitro at least [54], it does not
appear desirable at all that they contribute directly to
cardiac physiological regulation, but rather to avail them-
selves in the event that the primary regulators of the
contractile apparatus falter in disease.
In contrast to enhancing PIE in normal cardiac function,
receptor crosstalk may in fact be desirable, if not absolutely
essential, in rhythmically quenching the b
1
-AR-mediated
stimulation of the excitation/contraction coupling mechan-
ism of the contractile apparatus. This can be accomplished
in several ways or through a combination of factors,
including receptor internalization, dephosphorylation, deg-
radation and negative feedback loops evoking primarily the
inhibition of cAMP-mediated effects. In the heart, the most

obvious mechanism for this action is the direct coupling of
the G
ia
, notably G
ia2
, to inhibit AC signalling. A number of
signalling pathways, including the a
1
-AR, b
2
-AR, M
2
-MR,
A
1
-andA
3
-ADO have actually been found to couple to this
pathway, in fashions that would lead to the inhibition of b
1
-
AR-stimulated AC activity (Table 2). The same route
potentially mediates the negative inotropic effects (NIE) of
some of these pathways, counterbalancing b
1
-AR positive
inotropism by inhibiting Ca
2+
effects. For example, the
Fig. 2. Regulation of the contractile apparatus through receptor crosstalk signalling. Cardiovascular crosstalk engages mainly three G protein

families: the stimulatory (G
s
), inhibitory (G
i/o
)andG
q/11
proteins, employing PLC as a central hub in regulating this crosstalk signalling. In the
regulation of positive inotropism, for example, the G
s
-coupled b
1
-AR pathway constitutes the major stimulator of the contractile apparatus. The
G
q
-coupled PLC/InsP
3
(IP
3
) or PLC/DAG/Ca
2+
pathways provide reserve pathways that can be mobilized, if and when needed. All of these
pathways may be inhibited by G
i/o
-coupled signalling, via the muscarinic cholinergic pathway, for example, either through their inhibition AC
function or via the InsP
3
or DAG pathways to furnish a feedback loop in order to intermittently quench the otherwise immutable b
1
-AR
stimulation of the contractile apparatus. Details of the individual messenger cycles have been omitted for clarity. Inhibitory or counteractive

functions are indicated as rounded arrow ends. H, hormone (ligand); L, L-type Ca
2+
channel; PLB, phospholamban; R, receptor; Ry, ryadine
receptor; X, crosstalk regulatory switch; V, voltage gated Ca
2+
channel.
4718 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002
M
2
-MR-mediated activation of NO-stimulated cGMP
synthesis is thought to contribute to various cardiac
functions including NIE, an abbreviation of contraction,
and enhancement of diastolic relaxation [27]. The NIE may
serve as a physiological role of intermittently quenching
the otherwise immutable b
1
-AR-mediated excitation of
the contractile apparatus. It has been suggested that an
elevation of Ca
2+
entry via L-type Ca
2+
channels in
response to b-AR stimulation, rather than its release from
intracellular stores, may mediate its inhibition of ACV and
ACVI and act as a negative regulator of the receptor-
mediated AC activity [56]. Therefore, some of the cardiac
signalling circuits involving Ca
2+
-dependent inhibition of

these AC isoforms, or inhibition of G-protein coupled
receptor activity via PKA-mediated phosphorylation prob-
ably constitute a feedback loop for controlling cAMP-
dependent increases in [Ca
2+
]
i
. Thus, the dual regulatory
control of the cardiac AC activity by the G
s
and G
i
is
probably designated to provide means for filtering parti-
cularly the b
1
-AR signalling in the regulation of the
contractile apparatus, employing the PKs as feedback
regulatory switches, or for redirecting signalling messages
to meet certain requirements (Fig. 2).
Apart from this function, crosstalk between b
1
-and
b
2
-AR, involving switching of the b
2
-AR from G
s
to G

i
coupling appears also to provide the heart with the ability to
cope with various situations, and to mediate actions that
differentiate these two AR subtypes on cardiac Ca
2+
hand-
ling, contractility, cAMP accumulation, PKA-mediated
protein phosphorylation, and in modulating noncontractile
cellular processes [54,55]. Thus, the dual coupling of the
b
2
-AR to G
s
and G
i
in the heart is thought to result in
compartmentalization of the G
s
-stimulated cAMP signal,
selectively affecting plasma membrane effectors such as
L-type Ca
2+
channels, and bypassing cytoplasmic target
proteins such as phospholamban and myofilament con-
tractile proteins [55]. Accordingly, G
i
-dependent functional
compartmentalization of the b
2
-AR-directed cAMP/PKA

signalling to the sarcolemmal microdomain dissociates the
receptor-induced augmentation of [Ca
2+
]
i
transients and
Fig. 3. Crosstalk in the regulation of cardiovascular circulatory function. The regulation of the blood pressure and circulating blood volume is
maintained through crosstalk of various signalling pathways, some of which are controlled in the cardiovascular control centres in the central
nervous system. Receptor such as AT1R and ET1R stimulate vasoconstriction employing primarily the PLC pathway, whereas the nitric oxide
synthase and NP pathways regulation cell/blood volume by causing smooth cell relaxation. In the CNS crosstalk between the cholinergic and the
a-AR among others, has been implicated in the release of ANP, or regulation of nitric oxide pathways leading to feedback pathways for the
regulation of the blood pressure and volume. Inhibitory or counteractive functions are indicated as rounded arrow ends. Ad, adenosine; Arg,
L
-arginine; L, L-type Ca
2+
channel; V, voltage gated Ca
2+
channel.
Ó FEBS 2002 Cardiovascular receptor crosstalk (Eur. J. Biochem. 269) 4719
contractility from cAMP production and PKA-dependent
cytoplasmic protein phosphorylation, apparently allowing
cAMP to perform selective functions during b-AR subtype
stimulation [55]. Pathways involving the activation of type 1
protein phosphatase (PP1) and structural restriction of
PKA diffusion by its specific anchoring proteins have been
proposed as candidate mechanisms underlying this com-
partmentalization [57]. Besides, the switching from G
s
-to
G

i
-mediated PKA coupling of G-protein coupled receptor
pathways is thought to mediate mitogenesis via the Sos
pathway [43]. Activation of Src (or closely related PTKs),
and subsequently Tyr phosphorylation of adapter or
scaffold proteins, leads to the recruitment of guanine
nucleotide exchange factors, such as the Grb2–mSos
complex to the plasma membrane, followed by sequential
activation of Ras/Raf pathway to regulate gene expression
essential for proliferation [43–47] (Fig. 1). This process,
traditionally conceived as an escape route for G-protein
coupled receptors from unabated stimulation by their
agonists, has recently gained some recognition as a normal
physiological process to regenerate receptors following
desensitization. Other crosstalk involving interaction of G
s
and G
i
signalling may be important in disease manifesta-
tion. These are discussed below.
An interesting feature of cardiac signalling is the fact that
coupling of receptor subtypes within the same family via
different G proteins often generates opposed signalling
products. For example, while coupling of a
2
-AR, M
2
-MR,
A
1

-andA
3
-ADO to PLC via the G
i/o
inhibits AC activity,
the signalling of a
1
-AR, A
2A
-ADO, A
3
-ADO, M
1
-and
M
2
-MR via the G
q
theoretically promotes conditions for
PIE due to their ability to mobilize Ca
2+
(Table 2). These
Table 1. The major human cardiac receptors and their circulatory function-related signalling products. The table summarizes the circulatory function-
related signalling pathways and products of important cardiovascular receptor subtypes in the various organs. The relevant literature has been
quoted in the main text.
Family Type Localization CV function Coupler Signalling mechanism [References]
a-AR a
1A
,
a

1D
Heart, blood
vessels
VSM, myocardial
contraction
G
q/11
PLC-mediated [Ca
2+
]
v
activation [2], inhibition
of cAMP accumulation [10]
a
2B
heart Vasoconstriction G
i/o
Ca
2+
-dependent [K
+
] activation
[Ca
2+
]
v
inhibition [2]
b-AR b
1
Myocardium PIE G

s
AC-mediated cAMP synthesis [1,2]
b
2
Cardiac chambers Heart rate control;
VSM relaxation
G
s
;G
q
G
i/o
AC-mediated cAMP synthesis [2,55]
AC inhibition [2,87]
ADO A
1
Brain, heart Bradycardia G
i/o
AC inhibition; [K]
i
opening [7]
A
2A
Pacemaker Vasodilatation G
s
;G
15
Stimulation of AC and PLC-b [7]
A
2B

VSM, brain VSM relaxation G
q/11
? PLC/AC-mediated Ca
2+
activation [7]
A
3
Heart, kidney A
1
modulation G
i3
G
q/11
AC inhibition [7]
PLC/InsP
3
-mediated Ca
2+
increase [7]
ANG AT1R VSM Heart,
aorta, kidney
VSM contraction G
q/11
PLC/PKC-mediated elevation of [Ca
2+
]
i
level [4]
AT2R Heart, adrenal
medulla

Vasodilatation;
apoptosis promotion
G
ia2
/G
ia3
Ppase-stimulated MAPK activation; [K
+
]
D
opening;
PTPase activation leading to T-type [Ca
2+
] closure [4]
ET ET
A
VSM (blood
vessel, heart)
Vasoconstriction; PIE G
q/11
;
(G
s
?)
PLC/InsP
3
-mediated Ca
2+
influx;
Activation of cAMP synthesis [8]

ET
B
VECs, heart Vasodilatation/vasocon-
striction (ET
B2
)
G
i
G
q/11
Inhibition of cAMP formation [8]
PLC-mediated phosphoinositol-stimulated [Ca
2+
]
i
elevation/Ca
2+
influx [8]
MR M
2
Heart, brain Decrease in heart
rate, force (NIE)
G
i/o
G
bc
[K
+
]
D

opening; AC and [Ca
2+
] inhibition [3,40,45]
M
3
VSM, brain VSM contraction G
q/11
PLC-mediated InsP
3
/Ca
2+
, DAG/PKC activity;
cAMP elevation [3,52]
NP ANP Kidney,
myocardium
Blood volume,
BP regulation
GC cGMP-dependent PKG inhibition of cardiac growth
and function [5,6]
BNP Ventricle BP reduction GC cGMP-dependent PKG inhibition of VEGF synthesis
and function [5,6]
NOS eNOS VSM endothelium; Vasodilator tone
in BP regulation
GC cGMP-mediated NO actions [26], Ca
2+
/
calmodulin activation [26,28]
OP OP
1
Neocortex,

vas differens
Central CV regulation G
ia1
/G
ia3
(G
oa2
)
AC inhibition [9]; inhibition of b-AR function [29,30]
OP
3
Caudate putamen Central CV regulation G
o
I
K
conductance activation; reduction in
neuronal I
Ca
; AC inhibition [9,80]
4720 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002
Table 2. Circulatory function-related effects of cardiovascular receptor crosstalk. The table shows signalling products of crosstalk interaction of
cardiac receptors on other G-protein coupled receptor receptor system. Details of the crosstalk activity are given in the cited references in the main
text.
Receptor Interactive agonist/receptor Cardiovascular signalling product [References]
a
1
-AR G
i
-coupled a
1B

-AR
stimulation
Potentiates PE-mediated protein synthesis; Ca
2+
-dependent PKC-a-mediated early
gene expression in neonatal heart [83]
b-AR agonist (IPN) Synergistic with PE, activates protein synthesis via Raf/MAPK pathway in neonatal
rats myocytes [11];
PKC-mediated depression of b-AR response to IPN in a
1B
-AR overexpressing mice [13,14]
Ang II via AT1R Attenuates a
1
-AR mRNA and stimulated c-fos induction [17]
ET-1 via ET
A
Phosphorylates a
1B
in Rat-1 fibroblasts via PKC and PTKs [18]
b
1
-AR a
1
-AR agonist (PE) Synergistic with IPN, activates protein synthesis via Raf/MAPK pathways in
cultured myocytes in neonatal rats [11];
G
i
-coupled inhibition of b-AR-activated chloride current [39]
G
i

-coupled a
1
-AR (NE) Inhibits b
1
-AR stimulated cAMP accumulation [10]
Ang II on
(neuronal) AT1R
Inhibits cardiac b
1
-AR responsiveness via PKC activation and density in
transgenic mouse [23–25]; Inhibits [K
+
]; stimulates NE release, NE transporter
and [Ca
2+
] [23,24]
AT1R -antagonists
Ang II-stimulated AT2R
Inhibits IPN-stimulated b
1
-AR in transgenic mouse [24],
Stimulates [K
+
] via G
i
coupling to PP2A/PLA
2
; stimulates NE-transporter
and transcription activity [23]
ET-1 Inhibits pericardial cell IPN-stimulated cAMP accumulation [20]

NO via eNOS Inhibits b-AR PIE and chronotropy in transgenic mice and heart failure [26,65]
and NE mitogenic effects in ventricular cells [27]
NO via iNOS Induces b
1
-AR hyporesponsiveness in cardiomyopathy [28]
RPIA-desensitized A
1
Decreased inhibition of IPN-stimulated AC activity [66]
Carbachol-stimulated M
2
Increases IPN-stimulated AC in adult rat cardiomyocytes [66] Inhibits
NE release, b
1
-AR contractility [76]
OP
1
agonists Inhibits NE-mediated b
1
-AR effects in rat ventricular myocytes
via a PTX-sensitive G
i/o
protein [29,30]
ET
A
-stimulated ET-1 Attenuates IPN-induced cAMP accumulation in cardiac smooth muscle [20]
M
2
stimulation Parasympathetic slowing of heart rate; inhibits b
1
-AR contractility (via G

i
?) [3,28,45]
A
1
Isoproterenol-desensitized
b-AR
Decreased AC inhibition by RPIA in adult rat myocytes [66]
Ach-desensitized M
2
Decreased inhibitory action of RPIA on AC [66]
A
2a
Counteracts A
1
-meditaed antiadrenergic actions [22]
AT1R ANP via cGMP Inhibit ET-1 secretion following Ang II-mediated AT stimulation
in porcine aorta [71]; inhibits RAS activation [70]
ET-1 Stimulates RAS system synergistically with Ang II [59]
NO Up-regulation of AT1R by inhibition of NO synthesis [34]
AT2R ANP via ANPR Increases ACE levels via cGMP stimulation, vasorelaxation [98]
ET-1 stimulated ET
A
Synergistically activates Raf-1 kinase, MAPK in myocytes [8]
ET
A
Ang II via AT1R Induces endothelial ET-1 release associated with hypertensive-induced hypertrophy [35].
ANP Counteracts ET-1 activation of AP-1 via cGMP pathway in dogs
with congestive heart failure [36].
Isoproterenol-activated
b-AR

Attenuation of ET-1-induced MAPK activity [21]
NO Inhibits ET synthesis via cGMP synthesis [52]
ANP a
1
-AR agonist (PE) Stimulates ANP synthesis and release [6,16]
b-AR agonists (IPN) Stimulates ANP transcription via Akt [15]
Ang II-activated AT1R Attenuates ANP-mediated AC inhibition via KPC in VCM [33];
stimulates ANP release via inositol phosphate/Ca
2+
activation [32]
eNOS Up-regulates ANP in eNOS deficient mice [26]
ET-1-activated ET
A
Activates G
q
-mediated Ca
2+
-stimulated ANP synthesis in rat myocytes [37]
M
1
/M
2
stimulation ANP release in response to volume expansion (via G
q
) [6]
OP
2
agonists Releases ANP in hypertensive patients [92]
BNP ET-1-mediated activity Increases BNP gene expression in porcine aorta [71]
M

2
Isoproterenol-desen-
sitized b-AR
Increased AC inhibition by carbachol in rat myocytes [66]
Oxymetazoline on a
2
-AR Induces endothelial ET-1-mediated contraction to Ach [19]
ENOS ET-1 stimulated ET
A
, ET
B
Induce NO synthesis in endothelial cells, counterbalances NO vasodilator tone [8]
M
2
agonists Stimulates NO synthesis [28].
Ó FEBS 2002 Cardiovascular receptor crosstalk (Eur. J. Biochem. 269) 4721
overtly antagonistic events imply that the ability to
concomitantly couple to PLC via the G
i
and G
q
pathways
furnishes at least an auxiliary intraregulatory mechanism
that can be mobilized to switch from one pathway to
another to regulate the cardiac contractile apparatus. The
observation that crosstalk between G
i
and G
q
-coupled

receptors is mediated by the G
bc
subunits [58] similarly
underscores the significance of crosstalk at the level of
proteins regulating second messenger function in cardiac
inotropy, and strongly corroborates the probability of the
reserve resources of both PIE and NIE being regulated by
the same components upstream of the second messengers.
Therefore, these manifestations point to PLCs as a nucleus
for sorting and channelling signals that determine auxiliary
cardiac circulatory function, providing a regulatory feed-
back route for harnessing the b
1
-AR stimulation of the
contractile apparatus (Fig. 2).
Apart from serving as a negative feedback in regulating
inotropism, crosstalk is also likely to be required to a greater
extent in the regulation of blood pressure and circulating
volume than in contractile function under physiological
conditions, especially considering the fact that these func-
tions are controlled by vasoactive autocoids with directly
opposing effects on the vascular system. These interactions
are likely to be dominant in the vascular beds, but are
apparently also in the kidney, where ET-1 and Ang-II cause
vasoconstriction, decreasing renal blood flow, and glomer-
ular filtration rate, while bradykinin and ANP cause
vasodilation and increase glomerular capillary permeability
[50,52,59]. Such crosstalk may primarily be designed to finely
tune the balance between vasoconstriction and vasodilata-
tion in regulating circulating volume, cardiovascular hemo-

dynamics and blood pressure. While the involvement of the
crosstalk among these vasoactive receptor systems seems
unequivocal as a physiological regulatory control for circu-
latory function, very little is known to date with respect to the
level at which the different pathways communicate with each
other. Nonetheless, it seems clear that both divergence and
convergence in G-protein coupled receptor pathways at the
various junctions serve to furnish the heart with the means
to respond appropriately to intercellular and intracellular
signals to meet the individual environmental requirements.
Implications of receptor crosstalk for cardiac
adaptation to diseases
Cardiovascular diseases fall broadly into three main cate-
gories. The first group comprises pathologies such as dilated
cardiomyopathy that directly affect the heart muscle,
gravely compromising cardiac contractile function. In these
diseases, the heart may transverse various phases, such as
left ventricular hypertrophy, prior to reaching end-stage
heart failure, a state in which the heart cannot meet its
fundamental functional demands without some form of
assistance. This complex process involves alterations in the
myocyte structure and function, abnormalities in Ca
2+
homeostasis, excitation-contraction coupling and changes
in the cytoskeletal architecture, partly resulting in apoptotic
cell death [60,61]. It appears that adaptive energy metabo-
lism lends itself as the first line of defense to protect cardiac
function from collapse in disorders affecting the contractile
apparatus. The major myocardial energy substrate probably
switches from fatty acids to glucose, associated with

substantial down-regulation of the fatty acid utilization
enzymes [62], under the control of a yet unidentified gene
regulatory program. The transition from compensated to
decompensated heart failure is associated with overexpres-
sion of neurohormones and peptides, such as Ang II, ET,
NE and proinflammatory cytokines. At receptor level, the
paradigm is that the b
1
-ARs are down-regulated, and the
b
2
-ARs uncoupled, accompanied by an up-regulation in
the a-AR [1], b-AR-specific receptor kinase 2/3 (GRK2/3)
and G
i
protein levels [63]. While the mechanisms underlying
increases in a
1
-AR levels through down-regulation of
b
1
-AR are yet to be clarified, the fact that such elevations
are manifest in human disease implies that the origin of such
crosstalk is embedded in the mechanisms leading to cardiac
disease manifestation. Recently, attention has focused on
the role of signalling pathways, such as b
1
-AR in the
manifestation of heart failure. Some studies have suggested
for example, that PKA-mediated b

1
-AR signalling may
induce apoptosis [61] and alterations in this pathway are an
underlying cause of cardiac toxicity. Apparently, in adult
mouse cardiomyocytes, the apoptosis induced by the
G
s
-mediated signalling or other assaulting factors can be
counteracted through a process involving the coupling of
the b
2
-AR to G
i
,G
bc
, PI3K and the serine-threonine kinase
Akt-glycogen synthase kinase 3b pathway, which is thought
to mediate survival mechanisms [61] (Fig. 1). This under-
scores the notion that the cardiomyocyte is equipped with
reserve mechanisms to counter potentially detrimental
effects resulting from malfunctional signal transduction.
Malfunctional crosstalk is certainly an important contribu-
tory component of the progression of cardiac disease to
heart failure. Such crosstalk between b-ARs and, among
others, the ATR, ANPR, ETR, ADO or OPR subtypes has
been implicated in the manifestation of heart failure through
a variety of mechanisms [12,31,36,64,65]. In experimental
right-sided congestive heart failure for example, crosstalk
between myocardial OPRs and b-ARs has been associated
with changes in the regulation of cardiac Ca

2+
metabolism
and contractility in response to stress [64], while in cardio-
myopathy, b-AR hyporesponsiveness has been attributed to
excessive NO production mediated by the eNOS [27,65].
Crosstalk in which ANP inhibits ET-1 secretion [36] and
ET-1 conversely stimulates ANP up-regulation [37] has also
been implicated as a cause of chronic congestive heart
failure. Furthermore, under desensitization conditions,
cardiac G-protein-coupled receptor may be engaged in
complex crosstalk in which the activation of any one of
them may induce desensitization of the other, employing a
pathway involving AC, PKC and PTKs [66]. It remains to
be ascertained however, whether such manifestations are of
any practical significance for human disease conditions.
The second category of cardiac diseases comprises
disorders, such as left ventricular dysfunction from valvular
heart diseases or hypertension that may be triggered as a
result of a defect in the global circulatory function.
Naturally, these diseases often involve both local and global
malfunctions in cardiovascular regulatory pathways as in
valvular heart diseases, in which a concomitant down-
regulation of b-ARs, up-regulation in a
2
-ARs and GRKs 2,
3 and 5 occur both in the myocardium and peripheral blood
circulation [67–69]. Furthermore, in disorders underlying
environmental interactions with genetic factors, such as
hypertension, the role of receptor crosstalk becomes even
4722 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002

more complex and less discernible. Currently, crosstalk
among basically all signalling pathways regulating both
vascular reactivity, such as the ETR, ATR, NOS, and
contractile function such as the b-ARs, has been implicated
in the development of experimental hypertension [53].
However, evidence for the role of these receptor systems
in human hypertension is scanty and often contradictory. In
recent years, attention has also focussed on the potential
role of the deleterious vascular effects of endogeneous ET-1
and its crosstalk with other vasoactive hormones in
hypertension. Some studies have suggested that these effects
can be accentuated by reduced NO generation as a result of
hypertensive endothelial dysfunction [36,53]. A defect in the
ET-derived NO signalling presumably triggers abnormal
response to Ach in hypertensive vessels, and may at least in
part account for the increased vascular resistance observed
in hypertension [33]. On the other hand, despite persuasive
evidence suggesting that ET-1 contributes to the adverse
effects of hypertension-induced cardiac and vascular
remodeling, as well as hypertensive renal damage [53], some
studies show that its generation and responsiveness remains
unaltered in hypertensive subjects. In contrast, animal
models of early stage heart failure suggest that the inhibition
of endogenous ET-1 by ANP may play a critical role in its
inhibition of RAS and maintenance of renal function, partly
by counteracting neurohormone-induced vasoconstriction
[36,70,71]. Another potentially important crosstalk includes
a complex interaction between ET, Ang II, a-AR agonists,
Ca
2+

and growth factors, which has been implicated in the
pathogenesis of hypertension-induced vascular hypertrophy
[53].
The third class of cardiovascular disorders, the coronary
artery diseases, is a sequel of vascular malfunctions leading
to the blockade of coronary circulation resulting in inade-
quate supply of oxygen and nutrients to the heart itself, and
therefore causing ischemia. It appears that these events
involve the interaction of the same vasoactive substances
particularly the a-AR, ATR and ETR agonists that play
important modulatory or initiator roles in the development
of hypertension, arteriosclerosis and apoptosis [53,61,72,73].
An imbalance in the crosstalk between ET-1 and NO has
also been associated with atherosclerosis, hypoxia and
ischemia [53,74]. In contrast to disease manifestation,
crosstalk among various other vascular signalling pathways
has also been associated with ischemic preconditioning
(PC), a protective mechanism against ischemic injury with
immediate and delayed protective effects. These mecha-
nisms involve complex second messenger pathways com-
prising components such as adenosine, ADO subtypes,
PKC-e, mitochondrial ATP-sensitive [K
+
], eNOS and
paradoxical protective role of oxygen radicals [75–78]. The
ADO-mediated PC probably involves the activation and
translocation of PKC-e to mitochondrial membranes lead-
ing to increased ATP-sensitive [K
+
] opening and possibly

stimulation of the MAPK pathway through a yet unknown
mechanism [79]. In contrast, enhanced synthesis of NO by
eNOS is thought to play a dual role, acting initially as the
trigger and subsequently as the mediator of the condition
defined as late PC [77]. Other potential mechanism of PC
include possible neuromodulatory crosstalk of M
2
-MR with
the b-AR pathway through presynaptic inhibition of NE
release in guinea pig hearts, without the involvement of
reflex vagal activity in rat hearts, or ET-1 cross talk with the
NO pathway leading to cholinergic vasodilatation [75–77].
In the rat myocardium, both PKC and a subpopulation of
G
i/o
proteins may be involved in OP
1
and A
1
-ADO-
mediated ischemic preconditioning and protection against
apoptosis [80].
In diseases with an element of pressure overload, such as
hypertension or valvular heart diseases, the heart usually
responds to increased hemodynamic load by hypertrophic
growth, or remodeling of myocytes and the extracellular
matrix [60]. The hypertrophic increase in the left ventricular
mass occurs as a pathologic consequence of the disease, or
as an adaptive compensatory mechanism to reduce systolic
wall stress on the left ventricle as in hypertension. These

changes are characterized by a series of sequential gene
activating events resulting in elevated protein synthesis, a
more regular cell shape, and organization of the contractile
proteins into sarcomeric units [81]. The most important
feature of left ventricular overload-induced hypertrophy is
the Ôswitching onÕ of immediate early genes, particularly
c-fos, c-jun and c-myc, all of which are transcription factors
activated by MAPK-dependent phosphorylation [81–83].
At the onset, homodimerization of Jun/Fos or Jun/Fra
(collectively known as activator protein-1) allows them to
induce RNA polymerase II activity, followed by the
reactivation of genes such as the ANP, skeletal a-actin
and b-myosin heavy chain (b-MHC) that are normally
expressed during the fetal and early neonatal periods, but
are quiescent in the adult myocardium [60,81]. This in turn
triggers the activation of some constitutively expressed
structural genes, e.g. cardiac a-actin and myosin light chain
type 2, encoding the signalling products contributory to the
hypertrophic structural transformations [60]. Although the
initiators of these sequelae of events remain unclear, several
biochemical and mechanical stimuli, including the a
1
-AR
and b
1
-AR agonists, Ang II, ET, tumor-promoting phorbol
esters, growth factors and mechanical stress are capable of
triggering this transcriptional cascade linked in a temporal
sequence by a variety of mechanisms [12,71,82]. Stimulation
of these pathways induces cardiac hypertrophy by promo-

ting individually or synergistically protein synthesis and
immediate early gene expression via the MAPK pathways
[12,81,82]. Besides, complex interactions between ET, ANG
II, a-AR agonists, Ca
2+
and growth factors as well as
transcriptional regulation of G-protein-coupled receptors by
other pathways are also common features of experimental
cellular hypertrophy [83,84]. These interactions are primar-
ily products of their activating PKs to turn on the Ras-GTP
complex and its downstream effectors, raf-1 and MAPKs
[81,82] possibly involving the PLC-b signalling via PKC-e in
crosstalk with phospholipase D [84]. As per current notion,
cardiac hypertrophy may result from stimulation of the
G
q
-coupled receptors, such as AT1R, in cardiac myocytes
triggering rapid Tyr phosphorylation of Shc protein and its
association with the adapter protein Grb2 and mSos-1, a
GEF of p21
ras
[84] to promote the translocation of mSos-1
to the membrane, in order to interact with MAPK or other
pathways. Mechanical stress may also lead to cardiac
hypertrophy by triggering the transcriptional activation of
genes contributing to compensatory responses via the stress-
regulated p38
MAPK
pathway [85].
The need for the heart to maintain a tightly controlled,

reliable signalling machinery becomes even more important
in disease in order to fend off potentially fatal consequences
Ó FEBS 2002 Cardiovascular receptor crosstalk (Eur. J. Biochem. 269) 4723
of a defect or total loss of the contractile function to enable
it sustain its essential task. The heart can fulfill this by either
maintaining some reserve pathway(s) that can always be
tapped in need, or by inducing novel adaptive signalling
schemes. One of the most intriguing manifestations of
contractile function is the fact that the heart muscle tissue
concurrently maintains b
2
-AR and a
1
-AR, both of which
have been regarded to be physiologically dormant, yet they
can be stimulated by the same pharmacological agents
leading to similar or counteractive signalling products as
those of b
1
-AR activation [12,13,53,54]. The observation of
a causal relationship between the attenuation in b
1
-AR and/
or uncoupling of b
2
-AR with an up-regulation in a
1
-AR in
both experimental heart failure and in human disease
strongly argues for the existence of a

1
-AR as a purposeful
and productive resource for adaptive regulation of contrac-
tility in response to cardiac disease. Although there is hardly
any direct evidence for such a mechanism at present, the
logical explanation for this association is the existence of a
signalling switch to turn on the a-AR in response to a
malfunctioning b
1
-AR pathway. Moreover, the overwhelm-
ing notion seems to suggest that stimulation of b
2
-AR is
devoid of the toxic effects associated with stimulation of the
b
1
-AR subtype in the progression of cardiac disease to end-
stage [54,86,87]. Put together, it appears that selective
activation of cardiac b
2
-AR in presence of a defunct b
1
-AR
pathway may provide catecholamine-dependent inotropic
support without cardiotoxic consequences, which might
have beneficial effects in the failing heart [54,58]. Basically,
in order to compensate for a defunct b
1
-AR, the mobiliza-
tion of Ca

2+
required for the PIE is attainable most
probably by a mechanism that bypasses the G
s
-AC
pathway, and signalling via PLC is one important option
for providing such an alternative mechanism. Similar
pathways may be utilized by the b
2
-AR, which has been
shown to be capable of bypassing the L-type Ca
2+
channel
in its modulation of cellular contraction [87]. Alone, the
diversity in the mode by which the b-AR pathways regulate
Ca
2+
metabolism is already evident at the G protein
coupling circuit. At this junction, coupling to at least the G
q
can mediate PIE as an alternative to that attained via the G
s
proteins. As discussed above, the G
q
-coupled activation of
myocardial a
1
-ARs, ETRs and ATRs results in the
accelerated production of InsP
3

and DAG from phospho-
inositol hydrolysis, both of which directly or indirectly lead
to an elevation of [Ca
2+
]
i
and consequently, cardiac
inotropy regulation in vitro at least (Fig. 2). Therefore, the
coexistence of these potent PIE providers in the heart,
without displaying any evident involvement in cardiac
physiologic function, is a clear lead for their potential as
resources that can be availed, if required to support a
defunct or ailing contractile apparatus. A mechanism can be
envisaged in which an inefficient b
1
-AR signalling itself may
utilize the G
q
protein as a switch to turn such reserve
pathways on and off in cardiac disease in case of demand.
The existence of various isoforms and subtypes of major
G-protein coupled receptor downstream signalling compo-
nents furnishes the heart with a sizeable resource of
alternative signalling combinations in cardiac disease. At
least four stimulatory G
sa
gene splice variants and two G
ia
subunits are involved in the regulation of the cardiac AC
signalling [88]. In the heart, the G

sa
is abundantly expressed,
while only the G
ia2
subtype is predominant, and probably
constitutes a primary mediator in the inhibition of the
b
1
-AR-stimulated AC activity through various routes [88].
In cardiac myocytes where AC V and AC VI are most
abundant, inhibition of their catalytic activity results
primarily from the activation of
L
-type Ca
2+
channels
[89,90]. Incidentally, this is the same group of ACs that has
G
sa
as their activator, the G
ia
and Ca
2+
as inhibitors, while
the bc complex appears not to partake in their signalling
[89]. It has been argued that the G
sa
and G
oa
do not inhibit

certain ACs directly, because this effect would be antago-
nistic to those of their analogous G
bc
subunits [89]. In
addition, individual AC isoforms particularly the AC I,
AC III and AC VIII can also be regulated by Ca
2+
directly,
Ca
2+
/CaM mechanism, or through phosphorylation
processes via PKA [90,91]. These Ca
2+
effects present
an opportunity to cross-regulate receptor-stimulated AC
activity without the involvement of G
s
protein, PKC or
calmodulin, and can be utilized by those pathways bypas-
sing or devoid of the G
s
-mediated systems. These are but a
handful of potential interregulatory mechanisms between
AC, Ca
2+
and G proteins, which may be engaged in the
crosstalk among different receptor subtypes in the manage-
ment of PIE in heart disease.
In addition, some receptor proteins, such as ANPR, have
been shown to undergo transcriptional and post-transla-

tional regulation by stimulators of other signalling systems
in the progression of particularly left ventricular hypertro-
phy and hypertension [15–18,92]. Such modulation of one
cardiac gene by the expression of another strongly suggests
that regulators of both transcription and post-translation
mediate various forms of adaptive crosstalk in response to
cardiac disease. A further example is the down-regulation of
medium-chain acyl-CoA dehydrogenase (MCAD) tran-
scription in the hypertrophied but lacking in the nonfailing
ventricle, immediately following the manifestation of pres-
sure overload [62], also indicative of a compensatory control
at the translational or post-translational level. Thus, there
exists a huge contingency of potential regulators of cardiac
receptor signalling at various stages of signal generation to
furnish the heart with the capability to adapt itself to altered
functional conditions in the most appropriate fashion in
disease.
Implications of receptor crosstalk for pharmacological
treatment of cardiac disease
Traditionally, the search for the most appropriate thera-
peutic management of heart failure have focused mainly on
reducing the inotropic load to improve or maintain
sufficient left ventricular function, typically by blocking
the b-AR activity. For quite a long time, the underlying
cardioprotective mechanism of this approach was associ-
ated with the ability of such b-AR blockage to prevent the
deleterious consequences of cardiac sympathetic stimula-
tion. However, despite the fact that b-AR antagonist-
(b-blocker-) induced reduction in heart rate may essentially
contribute to benefits, such as improving myocardial energy

balance, by generating a less negative force-frequency
relationship [93], there is no reason to believe that these
agents are more effective than one exerting NIE in slowing
down the cardiac pumping rate. Moreover, b-blocker
therapy has been associated with several untoward effects,
such as augmenting plasma ANP, BNP and cGMP levels
4724 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002
[94]. It may also provoke other secondary effects, such as
atrial 5-hydroxytryptamine (5-HT
4
) receptor hyperrespon-
siveness or enhanced serotonin-evoked increases in cAMP
levels, as a result of modified crosstalk between the 5-HT
4
,
b-AR and MRs [95]. More importantly, neither heart failure
nor cardiac hypertrophy constitutes a disease, but both
simply indicate the existence of advanced or end-stage
cardiac disease. It is not even certain whether or not either of
them can directly influence the functional expression of
cardiac signalling pathways. For this reason, it has become
increasingly apparent that understanding of the actual
underlying cause of the disease is a prerequisite for both
prevention and effective therapeutic management of car-
diovascular diseases. Until these mechanisms are elucidated,
therapeutic strategies will continue to be directed mainly at
containing these end-stage manifestations rather than
combating the underlying cause of the disease.
It is nonetheless now evident that in the presence of a
primary disorder of myocardial contractility and/or extra-

ordinary pressure on the heart, ventricular performance
depends on several compensatory mechanisms. These
mechanisms include among others, the Frank-Starling
phenomenon, reflex release of cardiac hormones such as
the ANP, vagal innervation of the atria, and sustention of
both the parasympathetic and sympathetic functions in
heart failure [96]. It is also well established that neuro-
humoral activation of the heart in form of increased
sympathetic, RAS, vasopressin or ANP activity may
contribute to the transition from left ventricular dysfunction
to clinical heart failure, and is an independent predictor of
poor prognosis [97]. Thus, while these compensatory
mechanisms provide circulatory support in patients with
acute heart failure, neurohumoral activation over an
extended period of time might be harmful to patients with
chronic congestive heart failure, as several neurohumoral
factors may be inappropriately activated [96]. Moreover, in
providing inotropic support, agonists of different pathways
affect the contractile function in various ways, a fact that
may be important in the treatment of heart failure. For
example, selective partial b
1
-AR agonists and phosphodi-
esterase III inhibitors are thought to cause relatively limited
accumulation of cAMP for a given PIE than does isopren-
aline, while stimulation of a-AR or ETRs can enhance
myocardial contractility by enhancing the Ca
2+
respon-
siveness of the myofilaments [54]. It is also believed that

b-AR stimulation results in a proportionally large increase
in [Ca
2+
]
i
transients compared to other PIE providers, due
to additional cAMP-dependent Ca
2+
-desensitizing effects
on the myofibrils [98]. This differential regulation of Ca
2+
by these receptors might provide a useful therapeutic
strategy, in cases where offloading or reduction in the
inotropic load may be necessary. As a result, new
therapeutic concepts are emerging to inhibit the potentially
deleterious consequences of the compensatory mechanisms
and to indirectly influence signalling pathways using
agonists/antagonists of different receptor systems. A clas-
sical example is the introduction of ACE inhibitors in the
treatment of congestive heart failure, based on the notion
that pharmacological interventions with the myocardial
RAS significantly causes reversal of local sympathetic
neuroeffector defects [96,97,99]. It is thought that the
administration of ACE inhibitors increases cardiac and
peripheral b
2
-AR levels as well as improving prognosis and
cardiac function [100–102]. This prompted the notion that
the ACE inhibitors prevent b-AR down-regulation or
alternatively increase b-AR up-regulation by inhibiting the

Ang II-mediated catecholamine release via AT1R stimula-
tion in heart failure. Unfortunately, the numerous endea-
vors to explain these therapeutic approaches based on the
sympathetic actions of these signalling pathways have
yielded no tangible mechanistic explanation for the rationale
behind these strategies. Some studies have also postulated
that the antihypertensive actions of ACE inhibitors, AT1R
and b
2
-AR antagonists are partly due to their ability to
modulate the expression and function of other vascular
receptor systems, such as the NPs and bradykinin
[70,103,104]. The interchangeability of the inhibition of
the b-AR and the RAS pathways in the treatment of both
heart failure and hypertension intuitively demonstrates the
essence of their crosstalk in controlling circulatory func-
tion. Apart from these interactions, crosstalk amongst AR
subtypes, and with other pathways such as the NP or
ADO, also lends itself attractive as therapeutic target in
heart failure. The crosstalk among AR subtypes may prove
to be the therapy of choice in those disorders exhibiting
b-AR down-regulation in association with a-AR up-
regulation, as these processes rapidly revert to normal
following the correction of the disease [105]. We recently
demonstrated the therapeutic potential of this approach by
successfully administering phentolamine as a last resort in
containing persistent infundibular obstruction unrespon-
sive to classical therapeutic regiments, possibly as a result
of tenaciously elevated a-AR activity, following successful
balloon valvuloplasty of severe pulmonary stenosis [106].

The influence of the AR agonists on the NP pathways may
also be important in the treatment of heart failure, as
b-blockers can modulate crosstalk between these signalling
pathways, even in the absence of other variables such as
hypertension or left ventricular hypertrophy.
Studies in animal disease models have provided import-
ant leads for the relevance of crosstalk in therapeutic
management of heart failure, hypertension and ischemic
heart diseases. One of the most well studied pathways is
crosstalk between A
2A
-ADO and A
1
-ADO [22], which is
thought to be the basis for the proadrenergic actions of the
former. In rats treated with an A
1
-ADO antagonist,
prejunctional a
2
-ARs become supersensitive to their select-
ive agonists accompanied by an increase in plasma renin.
Furthermore, captopril prevents the development of hyper-
tension and morphological changes in the arteries, and
prejunctional b-AR-mediated NE release is enhanced, thus
altering the adrenergic regulation of the cardiac function
[23]. These observations suggest that selectively targeting
A
1
-orA

2A
-ADO may be useful either to protect the failing
heart from chronic, excessive adrenergic stimulation or
to potentiate the inotropic response to such stimulation
when clinically indicated. The antiadrenergic properties of
A
1
-ADO may also offer protection against injury from
myocardial ischemia and reperfusion [22]. In contrast,
crosstalk between A
1
-andA
2A
-ADO suggests a therapeutic
benefit of A
2A
-antagonists in situations where enhanced
antiadrenergic response is desirable, such as acute myocar-
dial ischemia [22]. This lends the possibility to potentiate the
anti-ischemic effects of A
1
-ADO through selective manipu-
lation of the A
2A
-ADO. The clinical implications of the
proadrenergic actions of A
2A
-ADO are not limited to
Ó FEBS 2002 Cardiovascular receptor crosstalk (Eur. J. Biochem. 269) 4725
myocardial ischemia and heart failure, but may also be

important in the treatment of cardiac arrhythmias, whereby
A
2A
-selective antagonists may enhance the actions of
adenosine in terminating supraventricular arrhythmias.
Furthermore, both MR expression and cardiac cellular
responsiveness to muscarinic stimuli have been shown to
increase significantly in hypoxic myocytes exposed to
epinephrine, while in contrast, MR agonists may reduce
NE overflow in animal in situ heart preparations [77]. These
observations also point to potential pharmacological strat-
egies in the prevention of myocardial ischemic injury by
influencing the cholinergic system.
The fact that crosstalk takes place at several junctions of
the signalling pathways bespeaks the prospects of various
potential strategies for the treatment of cardiovascular
diseases in future. Also, the indication that hypertrophic
adaptive mechanisms are regulated at the transcriptional
level renders the components of these processes valuable as
potential therapeutic targets in the prevention of, for
example, cardiac hypertrophy. Future strategies will there-
fore have to focus on preventing or suppressing the initiators
of injury or disease, rather than heart failure or cardiac
hypertrophy emanating from these diseases. Such efforts are
likely to concentrate primarily on developing agents exerting
their influence at four major signalling junctions: (a)
receptor–G protein–effector circuit; (b) activators of tran-
scription factors (TFs); (c) regulators of transcription; and
(d) regulators of protein translational function (Fig. 4).
While the underlying basis for these crosstalk mechanisms

remains largely unknown, it is nonetheless bound to occupy
an increasingly central position in these therapeutic approa-
ches in the foreseeable future.
SUMMARY AND PERSPECTIVES
The above discussion clearly demonstrates the fact that
cardiovascular receptor signalling is a maze comprising
several interlocked receptor pathways that are tightly
coordinated and perfectly synchronized to ensure har-
mony, integrity and continuity of this vital function
throughout life. The signalling pathways involved in the
regulation of this function are extremely diverse and range
from direct programs, such as the AR and AT pathways,
to multistep ones, such as the PKA and Ras/MAPK
pathways. The most important question remains how the
various pathways communicate with one another to
execute this noble function. Currently available data is
concordant with the notion that cardiovascular signal
transduction systems converge at certain checkpoints,
probably under the humoral control of the heart itself. A
malfunction or alteration in the transduction of any one of
these signalling pathways may positively or adversely affect
Fig. 4. Potential therapeutic targets involving
crosstalk signalling in cardiovascular disease.
The changes in progression of cardiac disease
to end-stage heart failure, such as cardiac
hypertrophy, are often a results of long-term
signalling effects. The mechanisms underlie
partly the change in intracellular messengers
triggering the activation of transcription fac-
tors (TFs) and their translocation into the

nucleus, where they bind to their target pro-
moters. A constructive interaction of the TF
with the promoter induces transcription of an
appropriate gene and consequently, synthesis
of the corresponding mRNA, whose message
is translated into the corresponding protein. In
this process, crosstalk can be regulated at
several hotspots, leading to diverse signalling
messages. These hotspots lend themselves
useful therapeutic targets for the future. The
figure shows some examples of such potential
therapeutic targets at some of these hotspots.
H, hormone (ligand); E, enzyme, G, G pro-
tein; RTK, receptor tyrosine kinase.
4726 N. Dzimiri (Eur. J. Biochem. 269) Ó FEBS 2002
the signalling of another in their regulation of this function.
Our present knowledge also strongly testifies that the heart
is indeed furnished with various yet unknown candidates to
protect its circulatory function, if and whenever needed.
The major limitation in our endeavors to comprehend
these concepts is the fact that this knowledge has been
derived from studies mainly in isolated organs or cells. It
goes without saying that any alteration in a signalling
pathway or the removal of any component from its
physiological environment may alter the function of other
components, or even whole systems, and so also signal
transduction mechanisms, leading to new signalling pro-
ducts altogether. Thus, while this information is very
valuable, caution is nonetheless called for in interpreting its
relevance to cardiovascular physiological signalling. De-

spite this and the fact that currently there are no
appropriate noninvasive procedures to verify some of these
important notions, our current understanding of receptor
crosstalk in cardiovascular function presents a milestone in
our endeavors to comprehend the modus operandi of this
complex machinery. It is certainly just a matter of time
before the literature is flooded with valuable pieces of this
maze, which may ultimately enhance our understanding of
the regulation of these signalling pathways, as a whole.
Two important issues are likely to dictate the focus of
research in this field in the near future. The first is with
regard to the mechanisms coordinating the arrays of
signalling messages from various sources to elicit such a
perfectly synchronized physiological function. Given the
fragility and vital nature of the circulatory function and the
potential contribution of receptor crosstalk in its mainten-
ance, it is evident why evolution has had to endow the
heart with the ability to control its own fate. However,
while the place of the heart as an endocrine organ seems
apparent, its ability to mobilize any of the receptor systems
as compensatory measure and/or adaptive mechanism(s) at
its disposal remains to be elucidated. Therefore, future
studies are also to be directed at providing some answers to
the important questions regarding the mode by which the
heart may regulate these mechanisms under normal
physiological and pathophysiological states. It is obviously
impossible to predict how long it will take to overcome
these formidable tasks. Nonetheless, the current pace of the
advancement in our knowledge on the subject justifies the
great expectation and hope to succeed in employing this

knowledge to combat this major killer disease worldwide
sooner rather than later.
ACKNOWLEDGEMENTS
The preparation of this review and the work of the author cited herein
were supported by the Royal Cardiovascular Research Grant of the
King Faisal Specialist Hospital and Research Centre. I would also like
to thank Dr Abdelilah Aboussekhra for his useful suggestions and Paul
Muiya for his assistance in the preparation of the manuscript.
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