New Concepts in Diabetes and Its Treatment - part 2 docx

from the elevated glucose values. In type 2 diabetes there is also an increased
release of proinsulin, which may account for 30% of total insulin compared
to 15% in normal subjects.
Concerning the insulin response to intravenous glucose, as occurs during
IVGTT, in type 2 diabetes there is a marked reduction in the first phase of
insulin release. The second phase may also be reduced in diabetic patients with
fasting glycemia ?250 mg/dl, but may be normal or increased in ‘compensated’
patients with fasting glycemia =200 mg/dl (even if also in these instances the
insulin response should be regarded as diminished, considering the existing
hyperglycemia). Reduced insulin response is also recorded during prolonged
glucose infusion.
The insulin response to nonglucose stimuli, such as intravenous arginine,
secretin, isoproterenol, isoprenaline, tolbutamide, or even a mixed meal, may
be normal in type 2 diabetic patients with fasting glycemia =200 mg/dl. This
is due to the potentiation of the insulin response to nonglucose stimuli exerted
by the hyperglycemia present in the diabetic patients.
Finally, in type 2 diabetic patients the oscillations in insulin secretion,
which are significant for glycemic control, cannot be detected, even in the
patients with mild form of the disease.
Causes of the Insulin Secretory Defect
A major role is certainly played by genetic predisposition, but several
biochemical mechanisms and neurohormonal factors may contribute. Little
is known about susceptibility genes to the common polygenic forms of type
2 diabetes. Studies of genes involved in insulin secretion or insulin action have
been successful to a certain extent by showing the implication of the insulin-
receptor substrate-1 (IRS-1) gene, the ras associated with diabetes (rad) gene,
the glucagon receptor gene, or the sulfonylurea receptor (SUR) gene (among
others) in a low percentage of cases of type 2 diabetes in particular populations.
However, the majority of susceptibility genes are still to be described.
Recently, an inherited or acquired defect of FAD-linked mitochondrial
glycerophosphate dehydrogenase in -cells has been proposed to contribute

to the impairment of insulin release in type 2 diabetes.
Intravenous administration of -endorphins or naloxone to type 2 diabetic
patients enhances both basal and OGTT stimulated insulinemia, which sug-
gests a possible pathogenetic role of these compounds in the dysfunction of
-cells.
Prostaglandins may also be implicated, as suggested by the improvement
of insulin response to intravenous glucose and the increase of the slope of
27Insulin Secretion and Its Pharmacological Stimulation
glucose potentiation after infusion of sodium salicylate (inhibitor of prosta-
glandin synthesis). A similar effect has been observed with the -adrenergic
blocking agent phentolamine, which suggests a role of the -adrenergic system.
It has also been suggested that galanin and pancreostatin, peptides which
inhibit insulin secretion, may be increased in the pancreatic islets of type 2
diabetic patients. Finally, hyperglycemia, once established, may contribute to
aggravate the -cell dysfunction, through several mechanisms most of which
are included in the concept of ‘glucotoxicity’. The glucotoxicity concept may
help to explain the beneficial effect on insulin secretion obtained in type 2
diabetic patients after adequate treatment achieving glycemic control as well
as the transient improvement in the -cell function which may occur in type 1
diabetic patients after therapeutical control of hyperglycemia (‘honeymoon’
phenomenon).
It has been proposed that at least one factor contributing to the pathogen-
esis of type 2 diabetes is desensitization of the GLP-1 receptor on -cells. At
pharmacological doses, infusion of GLP-1, but not of GLP, can improve and
enhance postprandial insulin response in type 2 patients. Agonists of GLP-1
receptor have been proposed as new potential therapeutic agents in type 2
diabetic patients.
It should also be emphasized that complex alterations of glucidic and
lipidic metabolism in the -cells may play a role. In particular, in obese/diabetic
hyperinsulinemic subjects, LC-CoA derived from the enhanced availability of

FFA may affect the -cells’ secretory response according to the following
mechanism: as the glycemic level increases, the -cells utilize more glucose;
this leads to enhanced production of malonyl-CoA, which blocks the intrami-
tochondrial transport of LC-CoA, which therefore accumulates in the cytosol
and (through its complex biological effects) stimulates insulin secretion (see
also chapter III and figure 3 for details).
Altered expression of genes encoding enzymes in the pathway of malonyl-
CoA formation and FFA oxidation contributes to the -cell insensitivity to
glucose in some patients with type 2 diabetes. Clearly, the detrimental impact
of diabetic hyperlipidemia on -cell function has been a relatively neglected
area, but future pharmacological approaches directed at preventing ‘lipotox-
icity’ may prove beneficial in the treatment of diabetes.
Insulin Secretion in Other Types of Diabetes
Various, less common types of diabetes are known to occur, in which the
secretory defect is based upon different mechanisms, as outlined in chapter I
on Etiological Classification.
28Belfiore/Iannello
Pharmacological Stimulation of Insulin Secretion
Insulin Secretion as Modified by Sulfonylureas
The main drugs able to stimulate insulin secretion are the sulfonylureas.
These compounds have been used in the management of type 2 diabetes since
1955 and, when properly utilized, are easy to use and appear to be effective
and safe. It is estimated that 30–40% of diabetic patients are taking oral
sulfonylureas. Indications and contraindications for sulfonylureas are shown
in tables 1 and 2, respectively.
Table 1. Patients candidate for sulfonylurea treatment
Most patients with type 2 diabetes, not well controlled with dietary restriction and exercise
Children and adults with the MODY (maturity-onset diabetes of youth) type of diabetes
Obese-diabetic patients with marked insulin resistance
Lean type 2 diabetic patients with preserved insulin secretory capacity

Table 2. Contraindications to sulfonylurea treatment
Patients with type 1 diabetes
Patients with pancreatic diabetes
Patients with an acute illness or stress or undergoing surgery
Patients with hepatic or liver diseases
Patients predisposed to hypoglycemia:
Underweight or malnourished
Elderly
Diabetic pregnancy:
Potential teratogenicity
Perinatal mortality
Severe neonatal hypoglycemia
Diabetic female patients during lactation
Patients with a history of severe adverse reactions to sulfonylureas
Different Sulfonylureas
The first oral hypoglycemic drug was synthesized in 1926 by altering the
guanidine molecule. The sulfonylureas used today are derived from this native
molecule. The ‘first-generation’ sulfonylureas, which were developed initially,
are effective in large doses, while the ‘second-generation’ drugs, developed more
recently, are effective in smaller doses. Some sulfonylureas, such as tolbutamide,
29Insulin Secretion and Its Pharmacological Stimulation
Table 3. Main characteristics of sulfonylureas
Compound Dose, mg/day Doses q.d. Duration of Metabolism/
hypoglycemic excretion
effect, h
First generation
Acetohexamide 250–1,500 1–2 12–18 Liver/kidney
Tolbutamide 500–3,000 2–3 6–12 Liver
Chlorpropamide 100–150 1 60 Kidney
Tolazamide 100–1,000 1–2 12–14 Liver

Second generation
Glibenclamide (or glyburide) 1.25–20 1–2 16–24 Liver/kidney
Glyburide, micronized 0.75–12 1–2 12–24 Liver/kidney
Glipizide 2.5–40 1–2 12–24 Liver/kidney
Gliclazide 80–320 1–3 10–20 Liver/kidney
Gliquidone 30–120 1–3 6–12 Liver
Glimepiride 1–8 1 #24 Kidney
Repaglinide
1
0.5–16 1–4 4–6 Liver
1
Repaglinide is a nonsulfonylurea hypoglycemic agent of the meglitinide family.
have a short duration of action (6 h), others, such as chlorpropamide, have a
long duration of action (up to 60 h), several others show an action of inter-
mediate duration. Some characteristics of the sulfonylureas which are or have
been in clinical use are summarized in table 3.
‘First-Generation’ Sulfonylureas. Tolbutamide has a ‘short’ duration of
action (see table 3) and is carboxylated by the liver to a totally inactive
derivative. Being metabolized only in the liver, this compound may be useful
in nephropathic diabetic patients.
Tolazamide has a more potent hypoglycemic activity than tolbutamide
and an ‘intermediate’ duration of action (see table 3). It is metabolized only
by the liver with the production of some very little active metabolites excreted
in the urine (85%). It is safer in the elderly and in nephropathic diabetic
patients. Tolazamide also has a diuretic action.
Chlorpropamide has a more potent hypoglycemic activity than tolbuta-
mide and a ‘very long’ duration of action (see table 3), and therefore it can
induce more hypoglycemic episodes than tolbutamide. It is hydroxylated by
the liver with production of some active metabolites excreted in the urine (by
80–90%) and, thus, is contraindicated in the elderly and in nephropathic

diabetic patients. Several side or toxic effects may occur with chlorpropamide,
30Belfiore/Iannello
such as alcohol-induced flushing, occasional hypersensitivity reactions as well
as water retention and hyponatremia (due to sensitization of renal tubules to
antidiuretic hormone).
Acetohexamide has a more potent hypoglycemic activity than tolbutamide
and an intermediate duration of action. It is reduced by the liver to 1-hydroxy-
hexamide which is a potent hypoglycemic drug, excreted by 60% in the urine.
Thus, it is contraindicated in the elderly and in nephropathic diabetic patients.
Acetohexamide also has diuretic and uricosuric actions.
‘Second-Generation’ Sulfonylureas. Glyburide or glibenclamide has been
used since 1969. It has a 50–100 times more potent hypoglycemic activity than
the ‘first-generation’ drugs and has a relatively long duration of action. It is
metabolized by the liver to several both inactive and mildly active metabolites,
excreted partially in the urine (50%) and partially in the bile (50%). It may
induce severe hypoglycemic episodes and is contraindicated in the elderly and
in nephropathic diabetic patients. Glyburide absorption is not affected by food
but it takes 30–60 min to achieve adequate plasma levels, so that this drug
should be taken before the morning meal.
Glipizide has been used since 1973, has a 50–100 times more potent
hypoglycemic activity than the ‘first-generation’ drugs (comparable to that of
glyburide) and has an ‘intermediate’ duration of action (see table 3). It is
metabolized by the liver to several inactive metabolites, excreted in the urine
(by 68%) and in the feces (by 10%). It may induce severe hypoglycemic episodes
(similarly to glyburide) and is contraindicated in the elderly and in nephro-
pathic diabetic patients. The absorption of glipizide is delayed by about 30 min
when it is ingested with a meal, so that it is recommended to take the drug
30 min before meals. Glipizide has a greater effect than glyburide in raising
postprandial plasma insulin level and lowering postprandial plasma glucose
level while glyburide has a better effect than glipizide in raising fasting insuline-

mia and reducing fasting glycemia (probably, reducing fasting hepatic glucose
production). For this metabolic difference, a ‘combined’ administration of the
two sulfonylureas was suggested.
Gliclazide has a potent hypoglycemic activity (comparable to that of
glyburide and glipizide) and has an ‘intermediate’ duration of action. It is
metabolized by the liver to several probably inactive metabolites, excreted in
the urine (by 60–70%). It has been suggested that gliclazide exerts antiplatelet
aggregating activity, with a potential preventing effect on diabetic microangi-
opathy, although this effect has not been confirmed.
Gliquidone has a short duration of action (the mean half-life was approxi-
mately 1.2 h and the mean terminal half-life was 8 h), is metabolized in the
liver to totally inactive or minimally active derivatives, and is excreted in the
intestine (by about 100%). For these reasons, gliquidone is safer in the elderly
31Insulin Secretion and Its Pharmacological Stimulation
and in nephropathic diabetic patients.A newly developed sulfonylurea, glimepi-
ride, has been reported to have a more potent hypoglycemic action than
glibenclamide while its ability to stimulate insulin secretion is much weaker,
possibly due to less stimulation of insulin secretion and more pronounced
extrapancreatic effects. It is effective at lower dosage, has a more rapid onset
of action than glibenclamide and a long duration of action. There is increased
plasma elimination of glimepiride with decreasing kidney function, which is
explainable on the basis of altered protein binding with an increase in unbound
drug.
Efficacy and Interactions. Good response with sulfonylureas will occur
in 70–75% of patients during the first years of treatment, provided that the
patient selection is appropriate. Primary failure occurs in 15–25% of cases
and may depend on a poor selection of the patients (unrecognized type 1
diabetic patients treated with sulfonylureas). Chronic therapy may be associ-
ated with progressively less beneficial effects (secondary failure), sometimes
as result of intercurrent factors which impair insulin action and secretion

(such as stress, infections, dietary disregard, etc.) (see also chapter III on
Insulin Resistance and Its Relevance to Treatment). The response to the
hypoglycemic drugs may be restored with the disappearance of the intercur-
rent event. All sulfonylureas are bound to serum albumin and, since a large
number of drugs may compete for ionic binding sites on albumin, sulfonylu-
reas can influence the effect of many drugs (and these drugs, conversely,
can influence the effect of sulfonylureas). The physician must understand
potential interactions with a number of commonly used drugs, that may
significantly alter the activity of the sulfonylureas both diminishing (diuretics,
-blockers, corticosteroids, estrogens, indomethacin, alcohol, rifampicin, etc.)
or increasing (sulfonamides, salicylates, clofibrate, chloramphenicol, MAO
inhibitors, probenecid, allopurinol, -blockers, alcohol, etc.) their hypogly-
cemic effect. It is noteworthy that some drugs (such as -blockers and
alcohol) can alter sulfonylureas activity in opposite directions. Sulfonylureas
of ‘second generation’ may have less interactions than those of the ‘first
generation’.
Some data of literature demonstrate that serum levels of sulfonylureas
(tolbutamide, chlorpropamide, glyburide and gliquidone) in treated diabetic
patients show extremely interindividual variations, with no correlation between
the dose and the plasma level.
Mechanism of Sulfonylurea Action
Acute Effects on Insulin Secretion. Sulfonylureas act primarily by acutely
stimulating release of insulin from pancreatic -cells (obviously, in presence
of functioning pancreatic islets), and this stimulation of insulin secretion is a
32Belfiore/Iannello
direct effect, as unquestionably proven by studies with perfused pancreases,
isolated perifused islets and cultures of -cells.
Available data suggest that sulfonylureas bind to a specific receptor (closely
associated with the ATP-sensitive K
+

-channels) on the outside of plasma
membrane of the -cells. Recent studies with human pancreatic islets showed
that
3
H-glibenclamide binds to saturable sites in islet membrane preparations
in a linear fashion. This binding was both temperature- and time-dependent.
Scatchard analysis of the equilibrium binding data indicated the presence
of a single class of saturable, high-affinity binding sites. The displacement
experiments showed the following rank order of potency of the oral hypogly-
cemic agents tested: glibenclamide > glimepiride ? tolbutamide ? chlorprop-
amide  metformin. This binding potency order was parallel with the
insulinotropic potency of the evaluated compounds. Glimepiride has been
reported to bind to a 65-kDa subunit of the sulfonylurea receptor. This charac-
teristic may entail a minor effect of the K-channel in other tissues, such as
myocardium (where the closure of K-channels may interfere with the repolar-
ization process).
Upon binding to their receptors, sulfonylureas inhibit the K
+
-channels,
diminish K
+
efflux and cause depolarization of the plasma membrane. This
depolarization induces voltage-dependent Ca
2+
-channels to open and extracel-
lular Ca
2+
to enter the cell. Increased cytoplasmic Ca
2+
stimulates the fusion

of the secretory granule membrane with cell membrane, followed by extrusion
of insulin outside the cell (exocytosis) (see also fig. 1).
Metabolic studies demonstrate that sulfonylureas stimulate the first phase
of insulin release and have little effect on the second phase. They can act in
the absence of glucose but also may potentiate glucose-mediated insulin release.
As consequence of the stimulation of secretion, sulfonylureas can induce mor-
phological alterations of the -cells such as degranulation, loss of zinc and
aspects of emiocytosis.
Chronic Effects on Insulin Secretion. Whether chronic sulfonylurea treat-
ment results in increased insulin secretion is a controversial problem. The
finding that after chronic treatment of type 2 diabetic patients insulinemia
returns to pretreatment level, without deterioration of glucose control, suggests
long-term extrapancreatic effects of sulfonylureas. The lower plasma glucose
achieved with sulfonylurea drugs in type 2 diabetic patients might be expected
to stimulate less insulin secretion (blood glucose is the major stimulus to insulin
release), and this can explain the inability of some studies to demonstrate
the chronic effect of sulfonylurea in stimulating insulin secretion. Available
literature data, however, do not support the concept that the improvement of
glycemia during chronic sulfonylurea treatment can be attributed solely to an
increased insulin secretion.
33Insulin Secretion and Its Pharmacological Stimulation
Table 4. Extrapancreatic effects of sulfonylureas
Hormonal effects
Potentiation of insulin action on skeletal muscle and adipose tissue glucose transport
Potentiation of insulin action on hepatic glucose production (activation of glycogen synthase
and glycogen synthesis)
Decrease of hepatic insulin extraction
Decrease of insulin degradation (inhibition of insulinase activity)
Stimulating effect on gastrointestinal hormone release
Direct metabolic effects

Insulin receptors (partial restoration of their number in plasma membrane in type 2 obese-
diabetic patients)
Liver (increase in fructose 2,6-bisphosphate; increase in glycolysis; decrease in gluconeogen-
esis; decrease in long-chain fatty acid oxidation)
Skeletal muscle (increase of glucose and amino acid transport; increase of fructose 2,6-
bisphosphate)
Myocardial tissue (increase of contractility; increase of oxygen consumption; increase of
glycogenolysis; decrease of Ca
2+
-ATPase; increase of glucose transport and glycolysis;
increase of phosphofructokinase activity and pyruvate oxidation)
Adipose tissue (increase in glycogen synthase; inhibition of lipolysis, increase in glucose
transport)
Platelet arachidonic acid metabolism (inhibition of cycloxygenase and 12-lipoxygenase path-
ways)
Other Effects. Sulfonylurea treatment does not appear to stimulate proin-
sulin biosynthesis. On the other hand, studies performed with in vivo and in
vitro animal perfused pancreases, or with isolated perifused islets and islet-
cell cultures, reported an acute and chronic sulfonylurea-induced inhibition of
the biosynthesis of proinsulin (through unknown mechanisms). Sulfonylureas,
acutely or chronically, do not alter glucagon secretion both in normal subjects
and diabetic patients. Sulfonylureas appear to stimulate pancreatic -cell soma-
tostatin release (with unclear physiological effect).
Extrapancreatic Effects of Sulfonylureas. Diverse in vitro and in vivo
extrapancreatic effects of sulfonylureas have been reported over the last 30
years (most of which, however, were obtained with drug concentrations larger
than those achieved in therapeutic use) (table 4). These effects of sulfonylureas
are due to direct actions on liver and/or muscle and, occurring in the absence
of changes in insulin binding, are probably mediated by postreceptor events.
As a whole, the extrapancreatic effects of sulfonylureas are of minor clinical

significance. A possible exception is glimepiride, which may exert more signifi-
cant extrapancreatic actions, including activation (through dephosphorylation)
of GLUT-4.
34Belfiore/Iannello
Table 5. Sulfonylurea side or toxic effects
Hematologic reactions (agranulocytosis, bone marrow or red cell aplasia, hemolytic anemia)
Skin reactions (rash, pruritus, erythema, purpura, photosensitivity)
Hypersensitivity reaction (rush, fever, arthralgia, angiitis, jaundice, etc.)
Alcohol-induced flushing (most frequently associated with chlorpropamide treatment)
Gastrointestinal complaints (nausea, vomiting, jaundice or hepatitis or cholestasis)
Antithyroid activity
Diuretic effect or antidiuresis with hyponatremia
Cataract formation (reported in some dogs treated with high doses of glimepiride)
Teratogenicity
Side or Adverse Effects of Sulfonylureas. The most important adverse effect
of sulfonylureas is hypoglycemia which, although occurring less often than
with insulin, when it occurs it tends to be more severe, prolonged and sometimes
fatal. The incidence of sulfonylurea-induced hypoglycemia is 0.19–4.2/1,000
treatment years (compared to 100/1,000 patients/year for insulin-induced hy-
poglycemia) and is most frequent in patients taking long-acting drugs (such
as glyburide and chlorpropramide) which, for this reason, should be avoided
in patients with predisposing conditions (the best treatment of hypoglycemia
is prevention). The case fatality rate of hypoglycemia induced by sulfonylureas
is 4.3% (see also chapter VIII on Clinical Emergencies in Diabetes. 2: Hypogly-
cemia). It is noteworthy that sulfonylureas predispose to hypoglycemia during
and after exercise. In this regard, it has been claimed that glimepiride maintains
a more physiological regulation of insulin secretion during physical exercise,
with less risk of hypoglycemia.
Other sulfonylurea side effects or toxic reactions occur at low rate (1.5%
for glyburide) (table 5) and appear within the first 2 months of treatment. The

chlorpropamide alcohol flushing (CPAF), occurring in 30–40% of type 2 and
10% of type 1 diabetic patients, is linked to a genetic predisposition to diabetes
development (autosomic trait) and can be considered a good genetic marker
of type 2 diabetes mellitus.
Other Drugs Modifying Insulin Secretion
Repaglinide is a nonsulfonylurea hypoglycemic agent of the meglitinide
family, a new class of drugs with insulin secretory capacity which exert a
rapid- and also short-acting effect, thus entailing reduced risk of long-lasting,
and hence dangerous, hypoglycemia. Repaglinide appears to bind to receptor
35Insulin Secretion and Its Pharmacological Stimulation
sites different from those of sulfonylureas (two binding sites have been identi-
fied). Repaglinide lowered fasting and postprandial blood glucose levels in
animals, healthy volunteers and patients with type 2 diabetes mellitus. Repagli-
nide is rapidly absorbed and eliminated, which may allow a relatively fast
onset and offset of action. Excretion occurs almost entirely by nonrenal mecha-
nisms. In comparative clinical trials in patients with type 2 diabetes mellitus,
repaglinide 0.5–4 mg twice or 3 times daily before meals provided similar
glycemic control to glibenclamide (glyburide) 2.5–15 mg/day. Addition of
repaglinide to existing metformin therapy resulted in improved glycemic con-
trol. In contrast with glibenclamide, use of repaglinide allowed patients to
miss a meal without apparently increasing the risk of hypoglycemia.
GLP-1 has insulinotropic action, which may explain the increased insulin
response after oral compared to intravenous glucose administration, and exerts
several other functions such as reduction of glucagon concentration, reduction
of gastric emptying, stimulation of proinsulin biosynthesis and reduction of
food intake (upon intracerebroventricular administration in animals). On these
grounds, GLP-1 seems to offer an interesting perspective in treatment of
diabetic patients. The observations that GLP-1 induces both secretion and
production of insulin, and that its activities are mainly glucose-dependent, led
to the suggestion that GLP-1 may present a unique advantage over sulfonylurea

drugs in the treatment of type 2 diabetes. This peptide is able to lower and
perhaps normalize fasting hyperglycemia and to reduce postprandial glycemic
increments (especially in type 2 diabetic patients) but its usefulness is not
completely established. Due to rapid proteolytic cleavage, the half-life of GLP-1
istooshortfortherapeuticalusewithsubcutaneous injections. GLP-1analogues
with different pharmacokinetic properties (or some preparations that could
be orally administered) are in development. Given the large amount of GLP-1
present in L-cells, it appears worthwhile to look for some agents that could
‘mobilize’ this endogenous pool of the ‘antidiabetogenic’ gut hormone GLP-1.
Interference with sucrose digestion using -glucosidase inhibition moves nutri-
ents into distal parts of the gastrointestinal tract and, thereby, prolongs and
augments GLP-1 release.
Antiarrhythmic agents with Vaughan Williams class I
a
action have been
found to induce a sporadic hypoglycemia. Recent investigation has revealed
that these drugs induce insulin secretion from pancreatic -cells by inhibiting
ATP-sensitive K
+
(K-ATP) channels in a manner similar to sulfonylurea drugs.
It is possible that in the future, pharmacological compounds will be found
that may act on GK and improve -cell insulin secretion.
36Belfiore/Iannello
Suggested Reading
Belfiore F, Rabuazzo AM, Iannello S, Campione R, Castorina S, Urzı
´
F: Extrapancreatic action of
glibenclamide: Reduction in vitro of the inhibitory effect of glucagon and epinephrine on the hepatic
key glycolytic enzymes phosphofructokinase and pyruvate kinase. Eur J Clin Invest 1989;19:367–371.
Draeger E: Clinical profile of glimepiride. Diabetes Res Clin Pract 1995;28(suppl):139–146.

Drucker DJ: Glucagon-like peptides. Diabetes 1998;47:159–169.
Goldberg RB, Einhorn D, Lucas CP, Rendell MS, Damsbo P, Huang WC, Strange P, Brodows RG:
A randomized placebo-controlled trial of repaglinide in the treatment of type 2 diabetes. Diabetes
Care 1998;21:1897–1903.
Lebovitz HE: Oral hypoglycemic agents; in Rifkin H, Porte D (eds): Diabetes mellitus. Theory and
Practice, ed 4. New York, Elsevier, 1990, pp 554–574.
Matschinsky FM: Banting Lecture 1995: A lesson in metabolic regulation inspired by glucokinase glucose
sensor paradigm. Diabetes 1996;45:223–241.
Philipson LH, Steiner DF: Pas de deux or more: The sulfonylurea receptor and K
+
channels. Science
1995;268:372–373, 423–429.
F. Belfiore, Institute of Internal Medicine, University of Catania, Ospedale Garibaldi,
I–95123 Catania (Italy)
Tel. +39 095 330981, Fax +39 095 310899, E-Mail francesco.belfi
37Insulin Secretion and Its Pharmacological Stimulation
Chapter III
Belfiore F, Mogensen CE (eds): New Concepts in Diabetes and Its Treatment.
Basel, Karger, 2000, pp 38–55

Insulin Resistance and Its
Relevance to Treatment
F. Belfiore, S. Iannello
Institute of Internal Medicine, University of Catania, Ospedale Garibaldi,
Catania, Italy
Insulin Action
Introduction
Insulin exerts its metabolic effects on the insulin-sensitive tissues, i.e. on
liver, muscle and adipose tissue. In these tissues, insulin action is the result of
complex mechanisms. We can distinguish (1) insulin binding to specific recep-

tors and the following sequence of events along the insulin signalling pathway,
which ultimately lead to (2) the insulin metabolic effects at postreceptor level.
The Insulin Receptor
The insulin receptor is a heterodimer composed of two chains or subunits,
the - and the -chain, linked by disulfide bridges. The -subunit is extracellular
in location and is the site of insulin binding. The -subunit is transmembrane
in location and originates from the signal transduction.
Normally there is a large surplus in the number of receptors (i.e. there is
a large amount of spare receptors). Nevertheless, considering that insulin
binding to its receptors is a random phenomenon, it follows that the higher
the number of insulin molecules or receptor units, the higher the number of
insulin molecules which will bind to the receptor units. An increase in the
insulin level causes a decrease in the receptor number on the plasma membrane
(downregulation of insulin receptors), a phenomenon that may occur in condi-
tions of persistent hyperinsulinemia (insulin-resistant states, including obesity).
38
Fig. 1. Schematic representation of dose-response curves of insulin action in the normal
state and in conditions of impaired insulin action. For explanation, see the text.
When the receptor number is decreased, the number of insulin molecules that
bind to the receptors at a given insulin level will be reduced, and therefore
the insulin effects will be diminished, i.e. there is insulin resistance. However,
by increasing the insulin level, the number of insulin molecules that bind to
the receptors can be increased toward the normal and therefore the insulin
effects can be restored; moreover, by increasing further the insulin level, the
maximum effect can be reached. This condition is called decreased insulin
sensitivity. By plotting the insulin concentrations (on the abscissa) against the
insulin effect (on the ordinate), the insulin dose-response curve is obtained.
This curve, in the case of insulin resistance due to reduced receptor number,
will be shifted to the right, as the maximum effect is reached at very high
insulin levels. On the other hand, when the insulin resistance is due to defects

in postreceptor steps of insulin action (see below), the dose-response curve is
flattened and the maximum insulin effect is not reached even at very high
insulin concentrations. When the two conditions coexist, the insulin dose-
response curve will be shifted to the right and flattened (fig. 1).
Concerning the fate of the insulin-receptor complexes, several data suggest
that they are internalized and delivered to endosomes, the acidic pH of which
induces the dissociation of insulin molecules from insulin receptors and their
sorting in different directions: insulin molecules are targeted to late endosomes
39Insulin Resistance and Its Relevance to Treatment
and lysosomes where they are degraded whereas receptors are recycled back
to the cell surface in order to be reused.
To understand the function of insulin receptors, it should be recalled that
protein kinases that directly phosphorylate proteins are divided into two major
classes: those that phosphorylate tyrosine (tyrosine-specific protein kinases)
and those that phosphorylate serine and threonine (the serine/threonine-spe-
cific protein kinases). The receptor -subunit can be phosphorylated on serine,
threonine and tyrosine residues and possesses intrinsic protein-tyrosine kinase
activity. Insulin stimulates this activity (i.e. the insulin receptor is itself an
insulin-sensitive enzyme) which is responsible for both autophosphorylation
of the receptor itself and phosphorylation of tyrosine residues of various
cellular substrates, including the insulin receptor substrates (IRS-1 and IRS-2).
The latter, through a mechanism not yet fully understood, trigger a sequence
of events which include phosphorylation/dephosphorylation of several cyto-
plasmatic proteins which, in turn, will induce the spectrum of insulin effects
(fig. 2). Two insulin receptor isoforms have been identified, the A and the B
form, which, however, revealed no difference in their tyrosine kinase activity
in vivo.
Protein-tyrosine phosphatases (PTPases) play an essential role in the regu-
lation of reversible tyrosine phosphorylation of cellular proteins that mediate
insulin action. In particular, some data suggest a possible role of the transmem-

brane PTPase in insulin receptor signal transduction.
Recent studies suggest that the insulin receptor tyrosine kinase inhibitor,
the membrane glycoprotein PC-1, may modulate insulin activity (and may
play a role in insulin resistance – see the second part of this chapter).
Metabolic Effects of Insulin (Postreceptor Effects)
The mechanisms of postreceptor insulin effects can be distinguished into:
(a) translocation (and activation) of glucose transporters (the GLUT-4 iso-
form) from the intracellular pool to thecell membrane; (b) activation/inhibition
of several enzymes of intermediary metabolism through either changes in
concentrations of ions or regulatory compounds which bind to the enzyme
at sites distinct from the substrate-binding site (allosteric effectors), or covalent
modifications of the enzyme molecules often consisting of phosphorylation/
dephosphorylation processes; (c) induction/repression mechanisms leading to
changes in enzyme concentration through regulation of the synthesis of the
enzyme proteins. Translocation and activation/inhibition processes are short-
term mechanisms (occurring within seconds or minutes), the induction/repres-
sion processes are long-term mechanisms (hours).
40Belfiore/Iannello
Stimulation of glucose transport across the cell membrane is one of
the main effects of insulin in muscle and adipose tissue, and is the result
of the translocation of glucose transporter (the GLUT-4 isoform)-containing
vesicles from an intracellular storage pool to the surface membrane. This
event is mediated through IRSs, which in turn activate PI-3-kinase isoforms
(fig. 2). Translocation and activation of GLUT-4 is favored by its dephos-
phorylation. In addition to glucose transport, insulin also stimulates the
transport across the cell membrane of amino acids and ions, mainly potassium
and phosphate.
Insulin regulates several key metabolic steps (fig. 1). In doing so, insulin
is opposed by the four counterregulatory hormones (the rapid-acting glucagon
and catecholamines, and the slow-acting growth hormone and cortisol). Insulin

affects the pathways of glucose utilization as well as the synthesis and degrada-
tion of macromolecules (glycogen, triglycerides and proteins) by regulating
the activity of ‘key enzymes’. Indeed, along each metabolic pathway, there is
one or more key step(s) catalyzed by key enzymes. These are enzymes which,
because of their low activity and sensitivity to regulatory factors (including
hormones), regulate the overall rate of the pathway to which they belong.
In particular, insulin (or, better, its prevalence over the counterregulatory
hormones) exerts the following effects (fig. 1):
(a) favors glucose utilization by activating the three key glycolytic kinases,
namely hexokinase (and GK in the liver), phosphofructokinase and pyruvate
kinase; in the liver, this is associated with repression of the opposing key
gluconeogenic enzymes: glucose-6-phosphatase, fructose bisphosphatase and
phosphoenolpyruvate carboxykinase plus pyruvate carboxylase;
(b) stimulates glucose oxidation, by activating the key enzyme pyruvate
dehydrogenase in the mitochondria;
(c) lowers FFA level by inhibiting lipolysis in the adipose tissue and
reduces ketogenesis from FFA in the liver (see chapter VII on ketoacidosis
for further explanation);
(d) favors glycogen synthesis and depresses glycogenolysis by activating
the enzyme glycogen synthase while inhibiting glycogen phosphorylase;
(e) enhances triglyceride synthesis and refrains triglyceride hydrolysis
(lipolysis) by inhibiting the hormone sensitive lipase;
(f) finally, insulin stimulates protein synthesis and opposes protein deg-
radation (or proteolysis). The main insulin actions are summarized in table 1.
Thus, the overall action of insulin is (1) to increase glucose utilization in
muscle, liver and adipose tissue while depressing glucose production in the
liver, which results in blood glucose lowering; (2) to lower FFA level by
refraining lipolysis, and (3) to prevent ketone formation in the liver by opposing
ketogenesis.
41Insulin Resistance and Its Relevance to Treatment

Fig. 2. Simplified representation of the mechanisms of action of insulin, glucagon,
catecholamines (sympathetic activation) and acetylcholine (parasympathetic activation).
(Continuous lines ending with black arrows indicate transformation or translocation of
substrates or ions; dotted lines ending with white arrows indicate stimulation; dotted lines
ending with filled circles indicate inhibition.)
Insulin: Insulin receptors, with their tyrosine kinase activity, phosphorylate several
protein substrates (IRS1, IRS2, Shc, etc.) which in turn phosphorylate (activate) several
protein kinases (PI3-K, MAP-K, PKB, PKCz, PKCl, etc.) and these, through a complex
cascade of phosphorylation/dephosphorylation, produce the various insulin effects (glucose
transport, enzyme activation/inhibition, induction/repression of enzymes and other proteins).
Other hormones: Glucagon and catecholamines (-receptors) activate adenylate cyclase
(with the participation of Gs proteins), thus producing cAMP and stimulating PKA. Cate-
cholamines (
2
-receptors) inhibit adenylate cyclase (with the participation of Gi proteins)
and therefore exert opposite effects. Acetylcholine (parasympathetic stimulation) activate
PLC (with the participation of Gp proteins) which split PIP
3
thus producing IP
3
and DAG.
IP
3
favors the increase in cytosolic Ca (release of Ca from the endoplasmic reticulum stores
or Ca influx from outside the cell) thus activating the CaCalm PK. DAG activates PKC.
The activation of these protein kinases will eventually result in enzyme activation-inhibition.
Note that protein kinases may activate (through phosphorylation) some protein phos-
phatases, resulting in dephosphorylation of some key enzymes. Most key enzymes of anabolic
pathways are active in the dephosphorylated form (example: glycogen synthase), and are acti-
vated by insulin and inhibited by glucagon and catecholamines.Most key enzymes of catabolic

pathways are active in the phosphorylated form (example: glycogen phosphorylase, hormone-
sensitive lipase) and are activated by glucagon or catecholamines and inhibited by insulin.
Abbreviations (alphabetic order): 
2
>
2
-adrenergic receptor; AC>adenylate cyclase;
>-adrenergic receptor; cAMP>cyclic AMP; DAG>1,2-diacylglycerol; G-4>isoform 4
42Belfiore/Iannello
Table 1. Metabolic effects of insulin
Stimulation Inhibition
Synthesis of macromolecules Degradation of macromolecules
(and allied processes): (and allied processes):
Glycogen Glycogen
Glucose transport Proteins
Glucose phosphorylation Amino acid oxidation
Triglycerides Gluconeogenesis
Lipoprotein lipase Lipolysis
Proteins FFA oxidation
Amino acid transport Ketogenesis
Nucleic acids
Other:
Ion transport
Substrate utilization:
Glycolysis
Glucose oxidation
Insulin Resistance
Introduction
Insulin resistance can occur because of defects in insulin action at prere-
ceptor, receptor or postreceptor level. Besides rare cases of abnormal insulins

or presence of receptor antibodies (prereceptor defects), reduction in the insulin
receptor number is a relatively common factor contributing to insulin resist-
ance. In addition, insulin binding (and therefore insulin action) may also be
affected in rare conditions in which qualitative alterations occur in the receptor
(e.g. decrease in the receptor affinity). Postreceptor mechanisms include the
signals triggered by insulin binding to the receptor as well as the resulting
changes in several key steps of intracellular metabolism.
of the glucose transporter; Glg>glucagon receptor; Gq>a further type of G protein; Gs
and Gi> stimulatory and inhibitory G proteins; Ins>insulin receptor; IP3>inositol-1,4,5-
trisphosphate; IRS
1
>insulin receptor substrate-1; IRS
2
>insulin receptor substrate-2;
M>muscarinic receptor; MAP-K>mitogen-activated protein kinase; PC-1>an ecto-protein
kinase probably interfering with insulin receptor tyrosine kinase; PI3-K>phosphatidyl-
inositol-3 kinase; PIP
2
>phosphatidylinositol-4,5-P; PKA>protein kinase A; PKB>protein
kinase B; PKC(, )>protein kinase C, forms  and ;PKC>protein kinase C; PLC>
phospholipase C; Shc>protein containing a single SH2 domain, substrate of insulin receptor
tyrosine kinase; TNF>tumor necrosis factor-; Tyr-K>tyrosine kinase activity of the
insulin receptor.
43Insulin Resistance and Its Relevance to Treatment
Insulin Resistance in Type 2 Diabetes and Obesity
In type 2 diabetic patients, insulin resistance is due to impaired insulin
action either at receptor and postreceptor level, and may result from two
etiological components, the genetic background and some acquired factors,
of which overweight and obesity are certainly the most important ones.
Insulin Receptor Defects

A common cause contributing to decreased insulin action consists of
reduction in the insulin receptor number which, however, most often is second-
ary to insulin resistance and the associated hyperinsulinemia, through the
‘downregulation’ mechanism. In type 2 diabetes, an incomplete activation of
the insulin receptor tyrosine kinase appears to contribute to the pathogenesis
of the signalling defect. Available data suggest that the impaired tyrosine
kinase function of the insulin receptor is not due to an inherited defect but
rather is caused by a modulation of insulin receptor function. The B isoform
is increased in the skeletal muscle in type 2 diabetes, which may not have
significant functional significance.
In this context, it is worthwhile noting that in obese subjects, increased
PTPase activity has been found in the adipose tissue that can dephosphorylate
and inactivate the insulin receptor kinase.
The membrane glycoprotein PC-1 (PC-1) has been proposed to be an
ecto-protein kinase capable of phosphorylating itself as well as exogenous
proteins, and would act as an inhibitor of the tyrosine kinase activity of
the insulin receptor. PC-1 was found to be increased in tissues (muscle and
fibroblasts) of insulin-resistant subjects. Moreover, in transfected cell lines that
overexpress PC-1 there is a reduction in the insulin-stimulated insulin receptor
tyrosine phosphorylation. These and other data raise the possibility that
PC-1 has a role in the insulin resistance of noninsulin-dependent diabetes
mellitus as well as of obesity.
In obese patients, skeletal muscle shows reduction in the phosphorylation
of insulin receptor and IRS-1 and in PI-3-kinase activation. The scarce ex-
pression of these proteins would contribute to determine muscular insulin
resistance.
Hyperglycemia might directly inhibit insulin-receptor tyrosine kinase ac-
tivity and the receptor function. This appears to be mediated by activation
of certain protein kinase C isoforms which form stable complexes with the
insulin receptor and modulate the tyrosine kinase activity of the insulin recep-

tor through serine phosphorylation of the receptor -subunit.
44Belfiore/Iannello
Postreceptor, Metabolic Mechanisms
Key Metabolic Steps Resistant to Insulin. Concerning glucose metabolism,
by means of complex procedures including the glucose clamp technique, la-
belled compound infusion and indirect calorimetry, it has been shown that in
patients with type 2 diabetes there is an impairment of insulin-mediated glucose
utilization by peripheral tissue (muscle) as well as a reduced insulin suppression
of hepatic glucose production. Thus, reduction in insulin sensitivity occurs
for both the peripheral glucose utilization and the hepatic glucose production.
Reduction in the receptor number may play a role, although it is more probable
that this is a secondary phenomenon due to the downregulation of receptors
by the hyperinsulinemia which accompanies insulin resistance. A defect in
intracellular dissociation of the insulin-receptor complex might contribute by
altering the receptor recycling and insulin processing. However, especially in
the most severe cases, reduction in insulin responsiveness of peripheral glucose
utilization does occur, suggesting postbinding defects in insulin action.
Impairment of insulin-mediated glucose utilization by peripheral tissue
(muscle) is due mainly to reduction in nonoxidative glucose utilization (glyco-
gen synthesis) and to a minor extent to reduced glucose oxidation. Concerning
nonoxidative glucose utilization (glycogen synthesis), the defect has been ten-
tatively located at the level of the key enzyme glycogen synthase, or the glyco-
gen-synthase-activating enzyme, protein phosphatase-1. Other data suggest
the implication of earlier steps of glucose utilization, such as glucose transport
and/or glucose phosphorylation to glucose-6-P (effected by the enzyme hexoki-
nase), which may secondarily impair glycogen synthase activation. On the
other hand, the defect in glucose oxidation has been located at the level of
the pyruvate dehydrogenase reaction.
Concerning lipid metabolism, resistance of lipolysis to the antilipolytic ac-
tionofinsulinoftenoccursintype2diabeticpatients,especiallywhenoverweight

or obesity is present, which results in elevated FFA plasma levels. However, it
should be notedthat, when obesity is present, elevationof plasma FFA mayalso
be due to the increased fat mass, even in the presence of normal lipolysis.
Insulin suppresses VLDL production in insulin-sensitive humans partly by
suppressing plasma FFA levels and partly by a non-FFA-mediated, direct he-
patic mechanism (inhibition of ApoB synthesis). Insulin-resistant hyperinsuli-
nemic obese individuals are resistant to this suppressive effect of insulin on
VLDL-ApoBproduction.Resistancetothenormal suppressiveeffectofinsulin,
in addition to other metabolic abnormalities associated with insulin resistance,
may contribute to postprandial and postabsorptive hypertriglyceridemia.
When obesity is present, insulin resistance of fat storage may also be
present, which may be an adaptation limiting further fat deposition, but is
maladaptive in terms of risk factors for atherosclerosis.
45Insulin Resistance and Its Relevance to Treatment
Genetic Factors (Changes in Predisposed Individuals). The genetic compo-
nent of the insulin resistance is suggested by the observation that in first-
degree relatives of type 2 patients, the insulin-stimulated glucose metabolism
was reduced, which was accounted for by a defect in nonoxidative glucose
utilization (glucose storage as glycogen), whereas glucose oxidation rate ap-
peared normal. Consistently, impaired activation of glycogen synthase by
insulin has been reported in these individuals. The glycogen-synthase-activat-
ing enzyme, protein phosphatase-1, also has an abnormally low level of activity
in these subjects. Moreover, in insulin-resistant offspring of parents with nonin-
sulin-dependent diabetes mellitus, after muscle glycogen-depleting exercise,
there is a severely diminished rate of muscle glycogen synthesis during the
recovery period (2–5 h), which is known to be insulin-dependent. These data,
however, do not necessarily mean that the defect is located at the glycogen
synthase level, since defects in the earlier step of glucose metabolism, such as
transport and/or phosphorylation, may also impair glycogen synthase activa-
tion.

Acquired Factors: The Reciprocal Negative Influences of FFA and Glucose
Metabolism. Among the acquired factors favoring insulin resistance, obesity,
which results from absolute or relative hyperphagia and/or hypoactivity, is
certainly the most important one. Obesity is associated with elevated FFA
levels, as well as with enhanced availability of glucose (hyperphagia) and
insulin. In these conditions, there is a competition between FFA and glucose
as energetic fuels. Indeed, reciprocal negative influences between FFA and
glucose metabolism occurs through the mechanisms outlined below (fig. 3).
In fact, the utilization of FFA (tendencially enhanced in obesity, because of
the trend to high plasma FFA levels) leads to the formation of long-chain
CoA or acyl-CoA (LC-CoA) in the cytosol, followed by the entry of LC-CoA
into the mitochondria through the action of the enzyme carnitine palmitoyl
transferase-1 (CPT-1) and by the -oxidation of LC-CoA to acetyl-CoA. Both
these compounds inhibit glucose utilization, thus inducing insulin resistance.
In fact: (a) acetyl-CoA inhibits the key enzyme of glucose utilization, pyruvate
dehydrogenase; the resulting inhibition of oxidative glucose utilization will
maintain saturated the glycogen stores, thus refraining further glucose conver-
sion to glycogen, i.e. the nonoxidative glucose utilization; (b) LC-CoA (directly
or through the generation of regulatory metabolites) exerts complex metabolic
effects, including inhibition of nonoxidative glucose metabolism (fig. 3). On
the other hand, the metabolism of glucose (favored in obesity by dietary
carbohydrates and hyperinsulinism) leads to the formation of malonyl-CoA
(fig. 3), a regulatory compound which inhibits CPT-1, thus reducing the intra-
mitochondrial transport of LC-CoA and therefore its -oxidation. This favors
the accumulation of LC-CoA in the cytosol.
46Belfiore/Iannello
Fig. 3. Reciprocal inhibitory effects between the metabolism of glucose and that of
FFA. Metabolism of FFA (gray background, steps 1–6): The metabolism of FFA leads to
inhibition of glucose oxidation (through the effect of acetyl-CoA on pyruvate dehydrogenase
– step 4) and of nonoxidative glucose utilization (through the complex effects of LC-CoA

– steps 5 and 6). Moreover, glycolysis is also inhibited through the effect of citrate on the
key glycolytic enzyme phosphofructokinase (step 9). The inhibition of pyruvate dehydro-
genase (step 4) by acetyl-CoA is the basis of the Randle’s glucose-FFA cycle. Metabolism
of glucose (white background, steps 4, 7–13): Glucose utilization leads to formation of
malonyl-CoA which, by inhibiting carnitine palmitoyl transferase-1 (step 3), reduces FFA
-oxidation and increases LC-CoA concentration. Dotted arrows indicate the regulatory
effects. From: Belfiore F, Iannello S, Mol Genet Metab 1998;65:121–128.
In summary, when the metabolism of FFA is stimulated (as it occurs in
obesity), the enhanced production of LC-CoA (cytosol) and of acetyl-CoA
(mitochondria) leads to inhibition of glucose utilization, thus inducing insulin
resistance which is often followed by hyperinsulinemia. The latter stimulates
glucose utilization and therefore the production of malonyl-CoA, which in-
hibits CPT-1, thus decreasing intramitochondrial transport and -oxidation
of FFA (and therefore acetyl-CoA formation). This, however, may further
increase the LC-CoA concentration in the cytosol, which may maintain insulin
resistance (fig. 3).
It is noteworthy that obesity, through the increased formation of acetyl-
CoA and LC-CoA (derived from the enhanced availability and oxidation of
FFA), exerts an inhibitory effect on the two metabolic steps (glycogen synthesis
47Insulin Resistance and Its Relevance to Treatment
Fig. 4. Scheme showing the metabolic effects of enhanced availability and oxidation of
FFA, as occurs in obesity. In muscle: reduction of glucose utilization. In liver: reduced
glucose utilization (and insulin uptake) and enhanced gluconeogenesis. In the pancreas:
stimulation of insulin secretion. From: Belfiore F, Iannello S, Mol Genet Metab 1998;65:
121–128.
and glucose oxidation) which are already defective in the type 2 diabetic
patients and in the genetically predisposed subjects.
The metabolic picture resulting from increased FFA availability and oxida-
tion in obesity is outlined in figure 4.
Hormonal Factors

Counterregulatory Hormones. Prevalence of counterregulatory hormones
(stress hormones) over insulin may contribute to the insulin resistance phenom-
enon. Glucagon tends to be elevated because of a decreased -cell/-cell ratio
in the pancreas. This hormone counteracts the insulin actions, especially in
the liver, by favoring glycogenolysis and gluconeogenesis over the opposing
processes of glycogeno-synthesis and glycolysis (promoted by insulin). In occa-
sional patients or under particular circumstances (intercurrent infections or
other illnesses, stress, trauma, etc.), other counterregulatory hormones, prima-
rily cortisol, might participate in opposing the insulin actions.
Tumor Necrosis Factor- (TNF). TNF is one of the proteins formed
by adipocytes, whose production increases with increasing adipocyte mass
(obesity). Indeed, TNF (as well as chronic hyperinsulinemia that induces
insulin resistance) triggers increased Ser/Thr phosphorylation of the insulin
48Belfiore/Iannello
receptor (IR) and of its major insulin receptor substrates, IRS-1 and IRS-2,
which may be a molecular mechanism for uncoupling insulin signaling, as
enhanced Ser/Thr phosphorylation of IRS-1 and IRS-2 impairs their inter-
action with the juxtamembrane region of IR. Thus, the TNF produced
by adipocytes may function as a local ‘adipostat’ to limit fat accumulation.
Increased production of TNF by fat cells stimulates downregulation of the
insulin-sensitive glucose transporter, GLUT-4, in adipocytes. TNF is overex-
pressed in the adipose tissue of obese rodents and humans, and is associated
with insulin resistance. The exact role of TNF, however, remains to be estab-
lished.
Leptin. Leptin is the product of OB gene. This 16-kDa protein is produced
by mature adipocytes and is secreted in the plasma. Its plasma levels are
strongly correlated with adipose mass in rodents as well as in humans. Leptin
inhibits food intake, reduces body weight and stimulates energy expenditure.
Leptin binds to a long-form of leptin receptor in the hypothalamus, thus
stimulating the release of GLP-1 and decreasing the production of neuropep-

tide Y, a neuromediator (stimulator) of food intake. Recent studies have shown
that leptin inhibits insulin secretion and has anti-insulin effects on liver and
adipose tissue. If these effects are confirmed, leptin could play a role similar
to that of TNF and could participate in the insulin resistance of obesity and
type 2 diabetes. Serum leptin is increased in insulin-resistant offspring of type 2
diabetic patients.
Other Factors Contributing to Insulin Resistance
Decreased blood flow and capillary density has been proposed as mecha-
nisms contributing to insulin resistance both in type 2 diabetes and in obese
insulin-resistant Pima Indians.
It has been suggested that insulin action may be modulated by blood
flow. Insulin resistance in moderately obese women was associated with an
abnormal vascular reactivity to stress, entailing exaggerated blood pressure
response; an enhanced vasoconstriction to stress may mediate this response.
Insulin-induced attenuation of noradrenaline-mediated vasoconstriction is
impaired in the obese rats. This defect in insulin action could reside in
the endothelial generation of nitric oxide, as endothelial function is also
abnormal.
A distinct capillary endothelial dysfunction may be involved in the insulin
resistance syndrome. However, the capillary wall crossing is rate-limiting for
muscle glucose uptake (and lactate release) in control subjects but not in
postabsorptive hyperglycemic insulin-resistant subjects.
On the other hand, in a prospective (15 years) study it was found that
capillary density was increased rather than decreased in subjects with impaired
49Insulin Resistance and Its Relevance to Treatment
glucose tolerance who later developed diabetes, a fact that might be regarded
as a compensatory mechanism for intracellular defects in glucose metabolism.
In healthy young men, there is a negative relationship between directly
measured whole-blood viscosity and insulin sensitivity (clamp technique) as
a part of the insulin resistance syndrome, which supports the hypothesis that

insulin resistance has a hemodynamic component.
Insulin-resistant first-degree relatives of type 2 diabetic patients were
shown to have an increased number of the glycolytic, fast-twitch (white),
type IIb muscle fibers compared to the oxidative, slow-twitch (red), type I and
IIa fibers (which are those normally responsive to insulin). Whether this finding
reflects a reduced physical activity level and fitness in the relatives or is of
primary genetic origin remains to be determined.
Insulin Resistance in Type 1 Diabetes
Although type 1 diabetes is due to severe insulin deficiency, it should be
considered that chronic lack of insulin action may produce insulin resistance,
through several mechanisms.
Asalreadypointedout,insulinexertsbothshort-termandlong-termeffects,
the latter consisting of induction or repression of the synthesis of key enzymes.
Therefore, in the tissues of the insulin-deficient subject there will be a decreased
content of the enzymes ‘induced’by insulin (example: hepatic GK) and accumu-
lation of the enzymes repressed by insulin (example: gluconeogenic enzymes).
Upon insulin administration, some degree of ‘resistance’ (reduced effect) may
occur until the enzyme balance is normalized, i.e. until the amount of those
enzymes ‘induced’ by insulin is restored and the accumulation of those enzymes
‘repressed’ by insulin decreases to the normal level, which may takes several
hours or even 1–2 days.Inaddition, severe insulin deficiency is always associated
with active lipolysis and enhanced release of FFA, which counteract insulin ac-
tion through the mechanism of the glucose-FFA cycle, already discussed.
When the type 1 diabetic patient becomes severely decompensated and
ketoacidosis supervenes, the insulin action may be further disturbed by the
interference of the acidosis with insulin binding to its receptor as well as by the
reduced response of the intracellular enzymes caused by the hyperosmolality.
Rare Genetic Forms of Insulin Resistance
These are described in chapter I on Etiological Classification, Pathophysi-
ology and Diagnosis.

50Belfiore/Iannello
Drugs Ameliorating Insulin Resistance
Introduction
The recognized major role played by insulin resistance in the pathogenesis
of type 2 diabetes is the rational basis for the use of drugs capable of improving
insulin sensitivity and consequently enhancing insulin action. The most used
of these drugs today is the biguanide compound metformin (dimethylbiguan-
ide), but other potentially useful agents, today under clinical investigation,
will also be considered, such as the thiazolidinediones (pioglitazone, troglita-
zone and rosiglitazone) and still others.
Metformin
The main biguanides (phenformin and metformin) were first synthesized
in 1929 and were shown to be potent antihyperglycemic agents. They were
rediscovered in 1957 and were widely used in Europe to treat obese type 2
diabetic patients. Phenformin was withdrawn in many countries because of
an association with lactic acidosis, but metformin, which does not bear the
same risk when appropriately prescribed, resurfaced in the 1980s and was
shown to increase insulin sensitivity. This led to its approbation for use in the
USA in 1994.
Actions. Unlike sulfonylureas (and the biguanide phenformin), metformin
does not bind to plasma proteins and is not metabolized by the liver. Metfor-
min has an absolute oral bioavailability of 40–60%, and gastrointestinal
absorption is apparently complete within 6 h of ingestion. An inverse relation-
ship was observed between the dose ingested and the relative absorption with
therapeutic doses ranging from 0.5 to 1.5 g, suggesting the involvement of an
active, saturable absorption process. Metformin is eliminated rapidly by the
kidney and has a mean plasma elimination half-life after oral administration
of between 4.0 and 8.7 h (approx. 6 h). The elimination is prolonged in
patients with impairment of renal function and correlates with creatinine
clearance.

Therapeutic blood levels may be 0.5–1.0 mg/l in the fasting state and
1–2 mg/l after a meal. However, monitoring of blood levels may be useful
only to confirm the diagnosis of lactic acidosis. Metformin has no effect
in the absence of insulin, because the drug seems to act primarily by enhanc-
ing insulin action at postreceptor level. Metformin ameliorates hyperglycemia
by improving peripheral sensitivity to insulin and reducing hepatic glucose
production (via gluconeogenesis) as well as by limiting gastrointestinal glu-
51Insulin Resistance and Its Relevance to Treatment