Mechanisms of arterial hypotension after therapeutic dose of subcutaneous insulin in diabetic autonomic neuropathy

Disruption of circadian blood pressure, heart rate and the impact on glycemic control in type 1 diabetes

Patients with type 1 diabetes (DM-1) have an increased mortality and morbidity risk compared to non-diabetic subjects. Even not recognized clinically at the early period of disease; patients with DM-1 show subtle neurological and cardiovascular abnormalities which is partly responsible for the increased mortality. One of these abnormalities is the disruption of circadian rhythms. Various factors such as autonomic dysfunction, sleep disturbance, smoking, cardiac and kidney function, atherosclerosis, arterial stiffness are suggested to cause these disturbances. Additionally these abnormalities have also implications regarding target organ damage such as microalbuminuria, retinopathy, and structural changes in glomeruli. Surprisingly, there are scarce data regarding the effect of tight blood glucose control and insulin on circadian rhythms in patients with DM-1. By the light of aforementioned data this review will try to summarize causes and consequences of disruption of circadian rhythms and the impact on glycemic control on these issues in patients with DM-1.

Endocrinopathies: The current and changing perspectives in anesthesia practice

The gateways to advancements in medical fields have always been accessed through the coalition between various specialties. It is almost impossible for any specialty to make rapid strides of its own. However, the understanding of deeper perspectives of each specialty or super specialty is essential to take initiatives for the progress of the other specialty. Endocrinology and anesthesiology are two such examples which have made rapid progress in the last three decades. Somehow the interaction and relationship among these medical streams have been only scarcely studied. Diabetes and thyroid pathophysiologies have been the most researched endocrine disorders so far in anesthesia practice but even their management strategies have undergone significant metamorphosis over the last three decades. As such, anesthesia practice has been influenced vastly by these advancements in endocrinology. However, a comprehensive understanding of the relationship between these two partially related specialties is considered to be an essential cornerstone for further progress in anesthesia and surgical sciences. The current review is an attempt to imbibe the current and the changing perspectives so as to make the understanding of the relationship between these two medical streams a little simple and clearer.

Effect of insulin on ACE2 activity and kidney function in the non-obese diabetic mouse

We studied the non-obese diabetic (NOD) mice model because it develops autoimmune diabetes that resembles human type 1 diabetes. In diabetic mice, urinary albumin excretion (UAE) was ten-fold increased at an "early stage" of diabetes, and twenty-fold increased at a "later stage" (21 and 40 days, respectively after diabetes diagnosis) as compared to non-obese resistant controls. In NOD Diabetic mice, glomerular enlargement, increased glomerular filtration rate (GFR) and increased blood pressure were observed in the early stage. In the late stage, NOD Diabetic mice developed mesangial expansion and reduced podocyte number. Circulating and urine ACE2 activity were markedly increased both, early and late in Diabetic mice. Insulin administration prevented albuminuria, markedly reduced GFR, blood pressure, and glomerular enlargement in the early stage; and prevented mesangial expansion and the reduced podocyte number in the late stage of diabetes. The increase in serum and urine ACE2 activity was normalized by insulin administration at the early and late stages of diabetes in Diabetic mice. We conclude that the Diabetic mice develops features of early kidney disease, including albuminuria and a marked increase in GFR. ACE2 activity is increased starting at an early stage in both serum and urine. Moreover, these alterations can be completely prevented by the chronic administration of insulin.

Euglycemic hyperinsulinemia alters the response to orthostatic stress in older adults with type 2 diabetes

OBJECTIVE

Insulin has opposing influences on blood pressure by simultaneously increasing adrenergic activity and vasodilatating peripheral blood vessels. In this study, we sought to determine whether hyperinsulinemia affects tilt table responses in older adults with type 2 diabetes not complicated by orthostatic hypotension.

RESEARCH DESIGN AND METHODS

Twenty-two older adults (mean age 71.7 +/- 1.1) with diet-controlled or oral hypoglycemic drug-controlled type 2 diabetes were recruited. All subjects with orthostatic hypotension, diabetic nephropathy, and sensory neuropathy were excluded. Subjects underwent euglycemic-hyperinsulinemic clamp and placebo "sham clamp" sessions. Sequential euglycemic-hyperinsulinemic clamps were performed for 2 h at 40 mU x m(-2) x min(-1) (low dose) and 2 h at 80 mU x m(-2) x min(-1) (high dose), and each was followed by a head-up tilt table test at 70 degrees C for 10 min.

RESULTS

There were no incidents of presyncope during the sham clamp, whereas there were four presyncopal events during both the low-dose and high-dose tilts. Although the low-dose clamp showed no difference in the response between sessions (two-way ANOVA), subjects demonstrated a significantly larger decrease in mean arterial pressure (P = 0.005) and diastolic blood pressure (P = 0.08) during the high-dose tilt. Doppler measures of middle cerebral artery velocity were no different between the two sessions at either dose.

CONCLUSIONS

The vasodilatory response to insulin can unmask orthostatic intolerance in older adults with type 2 diabetes, resulting in presyncopal symptoms. This could contribute to orthostatic hypotension in combination with other factors such as hyperthermia, hypovolemia, and adverse effects from medications.

Pathophysiology, diagnosis, and treatment of orthostatic hypotension and vasovagal syncope

Orthostatic hypotension (OH) occurs in 0.5% of individuals and as many as 7-17% of patients in acute care settings. Moreover, OH may be more prevalent in the elderly due to the increased use of vasoactive medications and the concomitant decrease in physiologic function, such as baroreceptor sensitivity. OH may result in the genesis of a presyncopal state or result in syncope. OH is defined as a reduction of systolic blood pressure (SBP) of at least 20 mm Hg or diastolic blood pressure (DBP) of at least 10 mm Hg within 3 minutes of standing. A review of symptoms, and measurement of supine and standing BP with appropriate clinical tests should narrow the differential diagnosis and the cause of OH. The fall in BP seen in OH results from the inability of the autonomic nervous system (ANS) to achieve adequate venous return and appropriate vasoconstriction sufficient to maintain BP. An evaluation of patients with OH should consider hypovolemia, removal of offending medications, primary autonomic disorders, secondary autonomic disorders, and vasovagal syncope, the most common cause of syncope. Although further research is necessary to rectify the disease process responsible for OH, patients suffering from this disorder can effectively be treated with a combination of nonpharmacologic treatment, pharmacologic treatment, and patient education. Agents such as fludrocortisone, midodrine, and selective serotonin reuptake inhibitors have shown promising results. Treatment for recurrent vasovagal syncope includes increased salt and water intake and various drug treatments, most of which are still under investigation.

Cerebrovascular reactivity and hypercapnic respiratory drive in diabetic autonomic neuropathy

Because abnormalities in cerebrovascular reactivity (CVR) in subjects with long-term diabetes could partly be ascribed to autonomic neuropathy and related to central chemosensitivity, CVR and the respiratory drive output during progressive hypercapnia were studied in 15 diabetic patients without (DAN-) and 30 with autonomic neuropathy (DAN+), of whom 15 had postural hypotension (PH) (DAN+PH+) and 15 did not (DAN+PH-), and in 15 control (C) subjects. During CO(2) rebreathing, changes in occlusion pressure and minute ventilation were assessed, and seven subjects in each group had simultaneous measurements of the middle cerebral artery mean blood velocity (MCAV) by transcranial Doppler. The respiratory output to CO(2) was greater in DAN+PH+ than in DAN+PH- and DAN- (P < 0.01), whereas a reduced chemosensitivity was found in DAN+PH- (P < 0.05 vs. C). MCAV increased linearly with the end-tidal PCO(2) (PET(CO(2))) in DAN+PH- but less than in C and DAN- (P < 0.01). In contrast, DAN+PH+ showed an exponential increment in MCAV with PET(CO(2)) mainly >55 Torr. Thus CVR was lower in DAN+ than in C at PET(CO(2)) <55 Torr (P < 0.01), whereas it was greater in DAN+PH+ than in DAN+PH- (P < 0.01) and DAN- (P < 0.05) at PET(CO(2)) >55 Torr. CVR and occlusion pressure during hypercapnia were correlated only in DAN+ (r = 0.91, P < 0.001). We conclude that, in diabetic patients with autonomic neuropathy, CVR to CO(2) is reduced or increased according to the severity of dysautonomy and intensity of stimulus and appears to modulate the hypercapnic respiratory drive.

Mechanisms mediating insulin-induced hypotension in rats. A role for nitric oxide and autonomic mediators

The mechanisms associated with insulin-induced cardiovascular inhibitory responses were evaluated in untreated normal rats and in normal rats pretreated with an antagonist of nitric oxide (NO) production (L-NAME), with cholinergic, alpha- and beta-adrenergic antagonists, or after ganglionic blockade. Male Wistar rats were anesthetized with a mixture of urethane and alpha-chloralose and placed on a electric heating pad. The femoral artery and vein were cannulated for measurements of mean arterial pressure (MAP), heart rate, plasma glucose, blood sampling, and intravenous injections. Intravenous injection of insulin (5.0 U/kg) in untreated rats resulted in a significant and sustained decrease in arterial blood pressure (average 24%) and in a slight decrease in heart rate. These cardiovascular responses were blocked by L-NAME and by the cholinergic antagonist atropine, suggesting an involvement of NO and the cholinergic receptors, or an effect of insulin on the central nervous system parasympathetic center. The ganglionic blocker hexamethonium attenuated the insulin-induced response. On the other hand, the hypotensive effect of insulin persisted after sympathetic blockade with the alpha-1 antagonist prazosin and the beta-1 antagonist atenolol. We conclude that the insulin-induced decrease in blood pressure is due to both increased cholinergic outflow and to NO production and that an enhanced sympathetic activity possibly mediated by a reactive release of norepinephrine or epinephrine modulates this response.

Studies on the influence of insulin on cyclic adenosine monophosphate in human vascular smooth muscle cells: dependence on cyclic guanosine monophosphate and modulation of catecholamine effects

Insulin increases both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) in human vascular smooth muscle cells (hVSMC) and attenuates noradrenaline-induced vasoconstriction. In the present study, we aimed at investigation in hVSMC: 1) the interrelationships between insulin-induced increases of cGMP and cAMP; 2) the insulin effect on the catecholamine modulation of cAMP. Catecholamines cause both vasoconstriction and vasodilation. Vasoconstriction is attributable to the reduced synthesis of cAMP in hVSMC through alpha 2-adrenoceptors and to direct effects on calcium fluxes through alpha 1-adrenoceptors; vasodilation is attributable to the increased synthesis of cAMP through beta-adrenoceptors. In the present study, we determined the influence of insulin on cAMP in hVSMC incubated with or without: a) the inhibitor of guanylate cyclase methylene blue or the inhibitor of nitric oxide synthase NG-monomethyl-L-arginine (L-NMMA); b) the beta-adrenergic agonists isoproterenol and salbutamol; c) the physiological catecholamines noradrenaline and adrenaline; d) noradrenaline+the beta-adrenergic antagonist propranolol or the alpha 2-adrenergic antagonist yohimbine; e) noradrenaline+methylene blue of L-NMMA. We demonstrated that: 1) the inhibition of the insulin-induced cGMP synthesis blunts the insulin-induced increase of cAMP; 2) insulin induces a significant increase of cAMP also in the presence of isoproterenol, salbutamol, noradrenaline and adrenaline: the combined effects of insulin and catecholamines were additive in some, but not in all the experiments; 3) insulin enhances the cAMP concentrations induced by noradrenaline also in the presence of alpha 2- or beta-adrenergic antagonists; 4) in the presence of methylene blue or L-NMMA insulin does not modify the noradrenaline effects on cAMP.

Insulin increases cyclic nucleotide content in human vascular smooth muscle cells: a mechanism potentially involved in insulin-induced modulation of vascular tone

It has been suggested that insulin exerts a vasodilating effect, but the mechanisms involved are not completely understood. Since cyclic nucleotides mediate the vasodilation induced by endogenous substances, such as prostacyclin and nitric oxide, we aimed to investigate the influence of insulin (concentration range 240-960 pmol/l) on both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) content in human vascular smooth muscle cells. Insulin dose-dependently increased both nucleotides (cAMP: from 0.7 +/- 0.1 to 2.6 +/- 0.4 pmol/10(6) cells, p = 0.0001; cGMP: from 1.3 +/- 0.2 to 3.4 +/- 0.7 pmol/10(6) cells, p = 0.033). This increase is receptor-mediated, since it was blunted when cells were preincubated with the tyrosine kinase inhibitor genistein. The effect of insulin remained significant (p = 0.0001) when preincubation with the phosphodiesterase inhibitor theophylline prevented cyclic nucleotide catabolism. The increase of cGMP was blunted when the cells were preincubated with the guanylate cyclase inhibitor methylene blue, and with the nitric oxide-synthase inhibitor NG-monomethyl-L-arginine. At all the concentrations tested, insulin potentiated the increase of cAMP induced by the stable prostacyclin analogue Iloprost (p = 0.0001), whereas only at 1920 pmol/l did it potentiate the cGMP increase induced by glyceryltrinitrate (p = 0.05). This study demonstrates that the vasodilating effects exerted by insulin may at least in part be attributable to an increase of both cGMP and cAMP via a receptor-mediated activation of adenylate and guanylate cyclases in human vascular smooth muscle cells and that the insulin effect on cGMP is mediated by nitric oxide.

Vascular actions of insulin in health and disease

Insulin has well known metabolic effects. However, depending on the magnitude and duration of the insulin stimulus, this hormone can also produce vasodilation and vascular smooth muscle growth. The association of hyperinsulinemia with the metabolic disorders of obesity and non-insulin-dependent diabetes, as well as with the cardiovascular pathologies of hypertension and atherosclerosis, has led to suggestions that perhaps elevated insulin levels are causally related to these diseases. Alternatively, insulin resistance may develop following an increase in skeletal muscle vascular resistance, with or without hypertension, such that a reduction in skeletal muscle blood flow leads to an attenuated glucose delivery and uptake. These hypotheses are explored in this review by examining the effects of insulin on vascular smooth muscle tissue during both acute and prolonged exposure. An interaction among hyperinsulinemia, hyperglycemia, and hyperlipidemia associated with the insulin resistant state is described whereby insulin resistance can be both a cause and a result of elevated vascular resistance. The association between blood flow and insulin stimulated glucose uptake suggests that therapeutic intervention against the development of skeletal muscle vascular resistance should occur early in individuals generally predisposed to cardiovascular pathology in order to attenuate, or avoid, insulin resistance and its sequelae.

The splanchnic circulation and postural hypotension in diabetic autonomic neuropathy

Postural hypotension results from sympathetic failure to cause superior peripheral vasoconstriction. The importance of the splanchnic circulation was studied by measuring mesenteric artery blood flow with duplex Doppler scanning. Nine normal and 9 Type 1 diabetic controls were compared to 8 Type 1 patients with autonomic neuropathy whose pressure fell 40-113 mmHg (range) on tilting. Measurements were made supine and after vertical tilt, fasting without insulin and after a 550 kcal meal. Superior mesenteric artery diameter decreased on tilting in normal controls but not in diabetic control or neuropathy groups (supine vs tilted: controls. 6.3 +/- 0.9 to 5 +/- 0.9 mm, p = 0.004, diabetic controls: 6.0 +/- 0.6 to 6.0 +/- 1.0 mm, and neuropathy group: 6.4 +/- 0.9 to 5.6 +/- 0.9 mm), but proportional blood flow changes were similar in all subjects (controls: 407 +/- 154 to 255 +/- 67 ml min-1 (-31%, p = 0.03), diabetic controls: 379 +/- 140 to 306 +/- 149 ml min-1 (-8%, p = 0.28), neuropathy group: 639 +/- 371 to 435 +/- 142 ml min-1 (-23%, p = 0.10). Postprandially supine superior mesenteric artery flow increased in all subjects but this did not affect the degree of systolic blood pressure drop on tilting (fasting vs postprandial blood flow: controls: 407 +/- 154 to 775 +/- 400 ml min-1 (p = 0.04), diabetic controls: 379 +/- 140 to 691 +/- 262 ml min-1 (p = 0.01), neuropathy group: 639 +/- 371 to 943 +/- 468 ml min-1 (p < 0.06)). The similarity of superior mesenteric artery responses to tilting in the three groups, and the lack of exacerbation of postural hypotension in the presence of postprandial hyperaemia indicates that control of splanchnic blood flow is less important in the aetiology of diabetic autonomic postural hypotension than previously thought.

Postural hypotension in diabetic autonomic neuropathy: a review

Postural hypotension is uncommon in diabetes but can occur secondary to autonomic neuropathy. Symptoms are rare and include dizziness, weakness, blurred vision, tiredness, and loss of consciousness. The pathophysiology of postural hypotension is not clear, but changes in intravascular volume, heart rate, cardiac output, and splanchnic vascular resistance are similar in patients and controls. The main factors producing hypotension are a blunted catecholamine response to standing, and failure of lower limb vascular resistance to increase adequately. Treatment for symptomatic postural hypotension includes avoidance of dehydration, adequate salt intake, and fludrocortisone. Other treatments are reviewed but are less helpful. Patients with postural hypotension have intermittent symptoms over the years but rarely become severely disabled. They have a poorer prognosis than patients with symptomatic autonomic neuropathy without postural hypotension.

Cardiovascular response to exercise in diabetic patients: influence of autonomic neuropathy of different severity

We investigated cardiovascular function and plasma catecholamine response during incremental exercise and recovery in diabetic patients with (DAN+) and without autonomic neuropathy (DAN-). The former group was divided according to the presence of parasympathetic (DAN+PH-) or associated parasympathetic and sympathetic (DAN+PH+) damage to the autonomic nervous system. A group of healthy volunteers was studied as a control group. All the patients and control subjects underwent a submaximal or symptom-limited incremental exercise test using a cycle-ergometer. Air flow and respiratory gas fractions were sampled at the level of the mouth allowing a breath-by-breath analysis of oxygen consumption (VO2). Heart rate and systolic blood pressure were recorded and venous blood samples were obtained from the patients at rest and during each minute of exercise and recovery to measure norepinephrine and epinephrine plasma levels. Haemodynamic parameters and plasma catecholamines were computed at rest and at 25, 50, 75 and 100% of the peak VO2 (VO2max). The breath-by-breath relationships among VO2, heart rate and VO2/heart rate against work were assessed during exercise for patients and control subjects. While VO2max in absolute values was not significantly different among the diabetic groups, VO2 max was much less in diabetic patients than in control subjects (p < 0.01). During exercise the rate of heart rate, systolic blood pressure, norepinephrine and epinephrine increase was different among the diabetic groups, being significantly blunted in DAN+PH+. The VO2/work relationship of the three diabetic groups was similar but markedly reduced in respect to that of control subjects (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin blunts sympathetic vasoconstriction through the alpha 2-adrenergic pathway in humans

We investigated the mechanisms underlying the insulin-induced attenuation of sympathetic forearm vasoconstriction in healthy humans. In 5 subjects, we applied 20 mm Hg lower body negative pressure for 30 minutes in control conditions and during a 60-minute infusion of insulin (0.05 mU/kg per minute) in the brachial artery and measured forearm norepinephrine kinetics and hemodynamics. In 11 subjects, we applied graded lower body negative pressure at 5, 10, 15, and 20 mm Hg for 5 minutes each in control conditions and during the simultaneous intrabrachial administration of insulin (0.05 mU/kg per minute) (5 subjects) or insulin plus ouabain (3.5 micrograms/min per liter) (6 subjects) to investigate whether insulin acts through a potentiation of the vascular smooth muscle Na+,K(+)-ATPase. To assess a possible effect of insulin on a specific adrenergic receptor pathway, in a further study group we evaluated (1) the forearm vascular response to intrabrachial infusion of the alpha 1-adrenergic receptor agonist phenylephrine (0.5, 1, and 2 micrograms/kg per minute; n = 7) and of the alpha 2-adrenergic receptor agonist BHT-933 (0.5, 1, 2, and 4 micrograms/kg per minute; n = 9), and (2) the effects of intra-arterial infusion of prazosin (0.5 microgram/100 mL per minute) alone or combined with insulin on the forearm vascular response to graded lower body negative pressure (7 subjects). Insulin blunted the peak increase in forearm vascular resistance (from 13 +/- 2 to 6 +/- 2 U, P < .05) but not the rise in forearm norepinephrine spillover induced by 20 mm Hg lower body negative pressure (from 8.3 +/- 1.8 to 11.1 +/- 3.5 pmol/min per liter, P = NS). Ouabain administration did not prevent the insulin-induced attenuation of the forearm vasoconstrictive response to graded lower body negative pressure. Insulin infusion in the brachial artery did not modify the forearm vasoconstriction induced by intra-arterial infusion of phenylephrine but significantly reduced the increase in forearm vascular resistance induced by BHT-933 (F = 6.111, P < .001). Finally, intra-arterial infusion of prazosin significantly attenuated the forearm vasoconstriction induced by graded lower body negative pressure. The residual vasoconstrictive response was abolished by insulin infusion. Taken together, these findings suggest that insulin interacts with the sympathetic nervous system at the vascular level predominantly through the alpha 2-adrenergic vasoconstrictive pathway.

Relative roles of insulin and hypoglycaemia on induction of neuroendocrine responses to, symptoms of, and deterioration of cognitive function in hypoglycaemia in male and female humans

To assess the relative roles of insulin and hypoglycaemia on induction of neuroendocrine responses, symptoms and deterioration of cognitive function (12 cognitive tests) during progressive decreases in plasma glucose, and to quantitate glycaemic thresholds, 22 normal, non-diabetic subjects (11 males, 11 females) were studied on four occasions: prolonged fast (n = 8, saline euglycaemia study, SA-EU), stepped hypoglycaemia (plasma glucose plateaus of 4.3, 3.7, 3 and 2.3 mmol/l) or euglycaemia during insulin infusion at 1 and 2 mU.kg-1.min-1 (n = 22, high-insulin hypoglycaemia and euglycaemia studies, HI-INS-HYPO and HI-INS-EU, respectively), and stepped hypoglycaemia during infusion of insulin at 0.35 mU.kg-1.min-1 (n = 9, low-insulin hypoglycaemia study, LO-INS-HYPO). Insulin per se (SA-EU vs HI-INS-EU), suppressed plasma glucagon (approximately 20%) and pancreatic polypeptide (approximately 30%), whereas it increased plasma noradrenaline (approximately 10%, p < 0.05). Hypoglycaemia per se (HI-INS-HYPO vs HI-INS-EU) induced responses of counterregulatory hormones (CR-HORM), symptoms and deteriorated cognitive function. With the exception of suppression of endogenous insulin secretion, which had the lowest glycaemic threshold of 4.44 +/- 0.06 mmol/l, pancreatic polypeptide, glucagon, growth hormone, adrenaline and cortisol had similar glycaemic thresholds (approximately 3.8-3.6 mmol/l); noradrenaline (3.1 +/- 0.0 mmol/l), autonomic (3.05 +/- 0.06 mmol/l) and neuroglycopenic (3.05 +/- 0.05 mmol/l) symptoms had higher thresholds. All 12 tests of cognitive function deteriorated at a glycaemic threshold of 2.45 +/- 0.06 mmol/l, but 7 out of 12 tests were already abnormal at a glycaemic threshold of 2.89 +/- 0.06 mmol/l. Although all CR-HORM had a similar glycaemic threshold, the lag time of response (the time required for a given parameter to increase) of glucagon (15 +/- 1 min) and growth hormone (14 +/- 3 min) was shorter than adrenaline (19 +/- 3 min) and cortisol (39 +/- 4 min) (p < 0.05). With the exception of glucagon (which was suppressed) and noradrenaline (which was stimulated), insulin per se (HI-INS-HYPO vs LO-INS-HYPO) did not affect the responses of CR-HORM, and did not influence the symptoms or the cognitive function during hypoglycaemia. Despite lower responses of glucagon, adrenaline and growth hormone (but not thresholds) in females than males, females were less insulin sensitive than males during stepped hypoglycaemia.

Hemodynamic actions of insulin

There is accumulating evidence that insulin has a physiological role to vasodilate skeletal muscle vasculature in humans. This effect occurs in a dose-dependent fashion within a half-maximal response of approximately 40 microU/ml. This vasodilating action is impaired in states of insulin resistance such as obesity, non-insulin-dependent diabetes, and elevated blood pressure. The precise physiological role of insulin-mediated vasodilation is not known. Data indicate that the degree of skeletal muscle perfusion can be an important determinant of insulin-mediated glucose uptake. Therefore, it is possible that insulin-mediated vasodilation is an integral aspect of insulin's overall action to stimulate glucose uptake; thus defective vasodilation could potentially contribute to insulin resistance. In addition, insulin-mediated vasodilation may play a role in the regulation of vascular tone. Data are provided to indicate that the pressor response to systemic norepinephrine infusions is increased in obese insulin-resistant subjects. Moreover, the normal effect of insulin to shift the norepinephrine pressor dose-response curve to the right is impaired in these patients. Therefore, impaired insulin-mediated vasodilation could further contribute to the increased prevalence of hypertension observed in states of insulin resistance. Finally, data are presented to indicate that, via a yet unknown interaction with the endothelium, insulin is able to increase nitric oxide synthesis and release and through this mechanism vasodilate. It is interesting to speculate that states of insulin resistance might also be associated with a defect in insulin's action to modulate the nitric oxide system.(ABSTRACT TRUNCATED AT 250 WORDS)