Dissociation between insulin sensitivity of glucose uptake and endothelial function in normal subjects

Physical activity influences heart rate variability in young adults, regardless of dextrose ingestion

INTRODUCTION

Cardiac autonomic modulation can be altered by carbohydrate ingestion. On the other hand, some of the protective effects of physical activity may be due to its impact on the autonomic nervous system.

PURPOSE

This cross-sectional study was conducted to evaluate whether physical activity level influences heart rate variability (HRV) under fasting and mainly, after dextrose ingestion.

METHODS

Healthy adults aged 18-40 years ( n  = 92; 54.35 % women) were separated into nonactive ( n  = 47) and active groups ( n  = 45), based on the physical activity level determined by International Physical Activity Questionnaire (IPAQ) and then compared.

RESULTS

There were no significant differences ( P  > 0.05) for age, body mass, BMI, abdominal circumference, blood pressure, fasting glucose and blood glucose at 60 min after dextrose ingestion between groups. Body fat (%) was lower in the active group ( P  = 0.02). The time-domain indices (RMSSD, SDNN and pNN50) were higher in the physically active group compared to the nonactive group (main effect of group, P  < 0.01). The SDNN index was higher at 60 min after dextrose ingestion compared to fasting (main effect of time, P  ≤ 0.01). However, no HRV indices showed significantly interaction effect (group x time; P  > 0.05).

CONCLUSION

The magnitude of HRV responses to glucose was not influenced by physical activity status. Young adults who met at least the minimum physical activity recommendations showed a higher cardiac autonomic modulation, regardless of dextrose ingestion.

Differential associations of angiographic extent and severity of coronary artery disease with asymmetric dimethylarginine but not insulin resistance in non-diabetic men with stable angina: a cross-sectional study

Background

Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthesis inhibitor, and insulin resistance (IR) have been implicated in atherogenesis. Our aim was to estimate relations between ADMA, the magnitude of IR and angiographic indices of extent and severity of coronary atherosclerosis in non-diabetic men with stable coronary artery disease (CAD).

Methods

We studied 151 non-diabetic men (mean age 57 ± 11 years) with stable angina, obstructive CAD (at least 1 luminal diameter stenosis of ≥70% in major coronary segments) and without heart failure, and 34 age-matched controls free of ≥50% coronary narrowings. The following CAD indices were computed: the number of major epicardial vessels with ≥70% stenosis, Sullivan extent score representing a proportion of the visible coronary tree with vessel wall irregularities, and Gensini score which reflects both CAD severity and extent, yet assigning a heavier weight to proximal segments and to the more severe narrowings by a non-linear point system. An estimate of IR was derived by homeostasis model assessment (HOMA-IR) from fasting insulin and glucose.

Results

Among the CAD patients, the proportions of subjects with 1-vessel, 2- vessel and 3-vessel CAD were 26%, 25% and 49%, respectively. ADMA levels were higher in patients with obstructive CAD compared to the controls (0.51 ± 0.10 vs. 0.46 ± 0.09 μmol/L [SD], P = 0.01), whereas HOMA-IR was similar (median, 3.2 [interquartile range: 2.4–4.9] vs. 2.9 [2.3–4.7], P = 0.2). Within the CAD group, ADMA increased across ascending quartiles of Sullivan score (Spearman’s rho = 0.23, P = 0.004), but not with Gensini score (rho = 0.12, P = 0.15) or the number of vessels involved (rho = 0.08, P = 0.3). ADMA correlated to log-transformed Sullivan score (Pearson's r = 0.21, P = 0.008), which was only slightly attenuated upon multivariate adjustment (β = 0.19 ± 0.08 [SEM], P = 0.015). HOMA-IR did not differ according to any measure of angiographic CAD (P ≥ 0.2). ADMA and log (HOMA-IR) were mutually unrelated (r = 0.07, P = 0.4).

Conclusions

ADMA is associated with diffuse but not focal coronary atherosclerosis in non-diabetic men with stable CAD irrespectively of the degree of IR. The independent relationship between ADMA and coronary atherosclerotic burden may contribute to the well-recognized prognostic effect of ADMA in CAD.

Dysfunction of endothelium-dependent relaxation to insulin via PKC-mediated GRK2/Akt activation in aortas of ob/ob mice

In diabetic states, hyperinsulinemia may negatively regulate Akt/endothelial nitric oxide synthase (eNOS) activation. Our main aim was to investigate whether and how insulin might negatively regulate Akt/eNOS activities via G protein-coupled receptor kinase 2 (GRK2) in aortas from ob/ob mice. Endothelium-dependent relaxation was measured in aortic rings from ob/ob mice (a type 2 diabetes model). GRK2, β-arrestin2, and Akt/eNOS signaling-pathway protein levels and activities were mainly assayed by Western blotting. Plasma insulin was significantly elevated in ob/ob mice. Insulin-induced relaxation was significantly decreased in the ob/ob aortas [vs. age-matched control (lean) ones]. The response in ob/ob aortas was enhanced by PKC inhibitor or GRK2 inhibitor. Akt (at Thr308) phosphorylation and eNOS (at Ser1177) phosphorylation, and also the β-arrestin2 protein level, were markedly decreased in the membrane fraction of insulin-stimulated ob/ob aortas (vs. insulin-stimulated lean ones). These membrane-fraction expressions were enhanced by GRK2 inhibitor and by PKC inhibitor in the ob/ob group but not in the lean group. PKC activity was much greater in ob/ob than in lean aortas. GRK2 protein and activity levels were increased in ob/ob and were greatly reduced by GRK2 inhibitor or PKC inhibitor pretreatment. These results suggest that in the aorta in diabetic mice with hyperinsulinemia an upregulation of GRK2 and a decrease in β-arrestin2 inhibit insulin-induced stimulation of the Akt/eNOS pathway and that GRK2 overactivation may result from an increase in PKC activity.

Effects of glucose tolerance on the changes provoked by glucose ingestion in microvascular function

AIMS/HYPOTHESIS

Hyperglycaemia and hyperinsulinaemia have opposite effects on endothelium-dependent vasodilatation in microcirculation, but the net effect elicited by glucose ingestion and the separate influence of glucose tolerance are unknown.

METHODS

In participants with normal glucose tolerance (NGT), impaired glucose tolerance (IGT) or diabetic glucose tolerance, multiple plasma markers of both oxidative stress and endothelial activation, and forearm vascular responses (plethysmography) to intra-arterial acetylcholine (ACh) and sodium nitroprusside (SNP) infusions were measured before and after glucose ingestion. In another IGT group, we evaluated the time-course of the skin vascular responses (laser Doppler) to ACh and SNP (by iontophoresis) 1, 2 and 3 h into the OGTT; the plasma glucose profile was then reproduced by means of a variable intravenous glucose infusion and the vascular measurements repeated.

RESULTS

Following oral glucose, plasma antioxidants were reduced by 5% to 10% (p < 0.01) in all patient groups. The response to acetylcholine was not affected by glucose ingestion in any group, while the response to SNP was attenuated, particularly in the IGT group. The ACh:SNP ratio was slightly improved therefore in all groups, even in diabetic participants, in whom it was impaired basally. A time-dependent improvement in ACh:SNP ratio was also observed in skin microcirculation following oral glucose; this improvement was blunted when matched hyperglycaemia was coupled with lower hyperinsulinaemia (intravenous glucose).

CONCLUSIONS/INTERPRETATION

Regardless of glucose tolerance, oral glucose does not impair endothelium-dependent vasodilatation either in resistance arteries or in the microcirculation, despite causing increased oxidative stress; the endogenous insulin response is probably responsible for countering any inhibitory effect on vascular function.

Severe impairment of endothelial function with the HIV-1 protease inhibitor indinavir is not mediated by insulin resistance in healthy subjects

Endothelial dysfunction may contribute to increased cardiovascular events among HIV-1-infected patients receiving antiretroviral therapy. The HIV-1 protease inhibitor indinavir causes both vascular dysfunction and insulin resistance, but the relationship between the two disturbances is not established. Endothelium-dependent vasodilation (EDV), insulin-mediated vasodilation (IMV), and whole body and leg glucose uptake during a euglycemic hyperinsulinemic clamp (40 mU/m(2)/min) were measured before and after four weeks of indinavir in nine healthy men. EDV fell from 270 +/- 67% above basal to 124 +/- 30% (P = 0.04) and IMV from 56 +/- 14% above basal to 8 +/- 8% (P = 0.001) with indinavir. During the clamp, arteriovenous glucose difference and leg glucose uptake were not significantly different after indinavir and whole-body glucose uptake was only modestly reduced (8.0 +/- 0.8 vs. 7.2 +/- 0.8 mg/kg/min, P = 0.04). The change in EDV did not correlate with the change in whole-body glucose uptake after indinavir (r = 0.21, P = 0.6). Despite marked impairment of endothelial function and IMV with indinavir, only modest, inconsistent reductions in measures of insulin-stimulated glucose uptake occurred. This suggests that indinavir's effects on glucose metabolism are not directly related to indinavir-associated endothelial dysfunction. Studies of the vascular effects of newer protease inhibitors are needed.

Serum interleukin-6 correlates with endothelial dysfunction in healthy men independently of insulin sensitivity

OBJECTIVE

Interleukin (IL)-6 is a proinflammatory cytokine that is implicated in the pathogenesis of atherosclerosis and insulin resistance. Both endothelial dysfunction and insulin resistance are among the earliest abnormalities that can be detected in people at risk for cardiovascular events. We aimed to evaluate whether increased serum IL-6 concentrations associated with endothelial dysfunction are independent of insulin sensitivity in apparently healthy men.

RESEARCH DESIGN AND METHODS

Association studies were performed in well-characterized nondiabetic Caucasian men (n = 99) recruited for energy balance studies. Insulin sensitivity (minimal model) and brachial vascular reactivity (high-resolution external ultrasound) were assessed. Circulating IL-6 concentrations were measured by enzyme-linked immunosorbent assay.

RESULTS

Serum IL-6 was an independent contributor to the variance of endothelium-dependent vasodilatation after adjusting for age, BMI, smoking status, LDL cholesterol, systolic blood pressure, diastolic blood pressure, and insulin sensitivity (P = 0.001). In fact, circulating IL-6 was negatively associated with endothelium-dependent vasodilatation (r = -0.247, P = 0.014) and insulin sensitivity (r = -0.262, P = 0.011) and correlated positively with age (r = 0.241, P = 0.016), BMI (r = 0.240, P = 0.017), systolic blood pressure (r = 0.299, P = 0.003), diastolic blood pressure (r = 0.295, P = 0.003), and triglycerides (r = 0.212, P = 0.035). No significant associations were observed between endothelium-independent vasodilatation and serum IL-6 concentrations.

CONCLUSIONS

Circulating IL-6 is linked to endothelial dysfunction independently of insulin sensitivity in apparently healthy men.

Effect of dietary chromium on resistance artery function and nitric oxide signaling in the sucrose-fed spontaneously hypertensive rat

BACKGROUND

Consumption of high-glycemic index foods contributes to the development of hypertension in some patients. Likewise, in spontaneously hypertensive rats (SHR), high sucrose promotes a secondary rise in systolic blood pressure (SBP). Chromium (III) (Cr(3+)) prevents sucrose-induced hypertension, but leaves the basal hypertension that characterizes SHR intact.

METHODS

Since hypertension entails increased peripheral resistance, we compared effects of Cr(3+) on resistance arteries from SHR fed low-glycemic (starch) versus high-glycemic (sucrose) index diets. Subgroups of SHR also received Cr(3+). Structure, stiffness, and vasodilation of mesenteric resistance arteries were studied using pressurized myography.

RESULTS

Sucrose increased SBP in SHR and, exclusively in sucrose-fed SHR, Cr(3+) reduced SBP and augmented acetylcholine or nitroprusside-dependent vasodilation. Neither sucrose nor Cr(3+) affected artery structure or stiffness. Since Cr(3+) enhanced vasodilation, we assessed endothelial NO synthase (eNOS), guanylate cyclase, cGMP-dependent protein kinase (PKG-1alpha and 1beta), and PKG activity by immunoblotting. Sucrose reduced eNOS, PKG-1beta, and PKG activity. Cr(3+) prevented the effects of sucrose on NO signaling.

CONCLUSION

In hypertension exacerbated by high-glycemic index diet, Cr(3+) reduces SBP. The BP-lowering effect of Cr(3+), selectively on sucrose-induced but not basal hypertension in SHR, involves at least in part, improving vasodilatory function vis-à-vis restoration of NO signaling in resistance arteries.

Concomitant Impairment in Endothelial Function and Neural Cardiovascular Regulation in Offspring of Type 2 Diabetic Subjects

Endothelial function is impaired in first-degree relatives (FDRs) of patients with type 2 diabetes. Many states characterized by endothelial dysfunction are associated with increased cardiovascular sympathetic outflow. In this study, we investigated endothelial and autonomic nervous system (ANS) functioning in FDRs and tested the hypothesis that in basal condition, impaired endothelial function is associated with impaired cardiovascular ANS regulation. Flow-mediated endothelium-dependent and -independent vasodilation of the brachial artery was measured with high-resolution ultrasound in 27 otherwise healthy FDRs (14 men and 13 women; mean age 32 years) with normal oral glucose tolerance and in 15 age- and gender-matched control subjects. Cardiovascular ANS regulation was investigated by means of spectral analysis of heart rate and systolic blood pressure (SBP) variability. Baroreflex sensitivity was assessed by the spontaneous baroreflex sequences technique. Flow-mediated endothelium-dependent vasodilation was 9.4+/-1.0% in FDRs and 17.0+/-2.3% in control subjects (P=0.001). Low-frequency oscillations in SBP variability were 8.6+/-2.8 and 2.8+/-0.6 mm Hg in FDRs and controls, respectively (P=0.04). Baroreflex sensitivity was significantly less in FDRs than controls (22.8+/-2.7 versus 37.0+/-5.8, respectively; P=0.01). Change in vessel diameter was inversely correlated with the low-frequency component of SBP variability (r=-0.40; P=0.014). In healthy FDRs of diabetic patients there is a concomitant, possibly related, impairment in endothelial and ANS functioning, which manifests, indirectly, with increase in vascular sympathetic outflow and a depressed baroreflex, vagal, control of heart rate.

Clustering of insulin resistance with vascular dysfunction and low-grade inflammation in type 2 diabetes

Vascular dysfunction, low-grade inflammation, insulin resistance, and impaired fibrinolysis have each been reported to be present in type 2 diabetes, but their relationships, and the role of obesity, have not been investigated. We measured insulin sensitivity (euglycemic clamp), forearm blood flow responses to graded local acetylcholine (Ach) and sodium nitroprusside (SNP) infusions, plasma concentrations of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, von Willebrand factor (vWF), plasminogen activator inhibitor (PAI)-1, tissue plasminogen activator (tPA), and high-sensitivity C-reactive protein (hs-CRP) in 81 diabetic patients. When patients were stratified by insulin resistance, more severe insulin resistance was associated (P < 0.05) with overweight, central fat distribution, hypertension, and dyslipidemia (with similar sex distribution, age, fasting plasma glucose, and HbA1c). With regard to vascular function, both endothelium-dependent (Ach) (-22, -40, and -52%; P < 0.0001) and -independent (SNP) (-3, -7, and -27%; P < 0.02) vasodilatation were progressively reduced across insulin resistance tertiles. In multivariate analysis, inflammatory markers (IL-6, hs-CRP, and TNF-alpha) were independently associated with insulin resistance and fasting glycemia, fibrinolytic markers PAI-1 and tPA with insulin resistance and central fat distribution, and vascular indexes (vWF, Ach, and SNP vasodilation) with insulin resistance and obesity or cytokines (TNF-alpha or IL-6). In type 2 diabetes, insulin resistance is associated with vascular dysfunction/damage, impaired fibrinolysis, and low-grade inflammation independently of obesity and poor glycemic control.

Endothelial function, insulin action and cardiovascular risk factors in young healthy adult offspring of parents with Type 2 diabetes: effect of vitamin E in a randomized double-blind, controlled clinical trial

BACKGROUND AND AIMS

Endothelial dysfunction, insulin resistance and oxidative stress are believed to be central and associated mechanisms in atherogenesis. We aimed to determine the effect of the antioxidant vitamin E on endothelial function, insulin action and cardiovascular risk markers in young healthy adult offspring of parents with Type 2 diabetes.

METHODS

Healthy, glucose-tolerant adults (18-38 years), 14 (12 male/2 female) with at least one parent with Type 2 diabetes, and 14 (12 male/2 female) subjects with no family history of diabetes (controls) were studied. Insulin action was assessed by euglycaemic hyperinsulinaemic clamp (1 mU/kg/min). Endothelial function was assessed by forearm blood flow (FBF) responses to intra-brachial artery infusions of acetylcholine (ACh) (endothelium-dependent vasodilation), sodium nitroprusside (SNP) (endothelium-independent vasodilation) and N(G)-monomethyl L-arginine (LNMMA) (nitric oxide synthase inhibition). Thirteen offspring (18-38 years, 11 male/2 female, BMI < 30 kg/m2) completed a randomized, double-blind, crossover trial (12 weeks vitamin E 800 IU/day or placebo, 6-week washout).

RESULTS

Exogenous glucose infusion rates to maintain euglycaemia were positively associated with response to acetylcholine in offspring (r = 0.61, P < 0.05), and were linked with triglycerides. Vitamin E had no effect on endothelial function, insulin action or cardiovascular risk markers in healthy adult offspring of parents with Type 2 diabetes.

CONCLUSIONS

Our results support a positive association between insulin action and endothelial-dependent vasodilation in young healthy adult offspring of parents with Type 2 diabetes, but indicate no effect of vitamin E on these parameters.

Endothelial dysfunction in obesity and insulin resistance: a road to diabetes and heart disease

Obesity, insulin resistance, and endothelial dysfunction closely coexist throughout the natural history of type 2 diabetes. They all can be identified not only in people with type 2 diabetes, but also in various groups at risk for the disease, such as individuals with impaired glucose tolerance, family history of type 2 diabetes, hypertension, dyslipidemia, prior gestational diabetes, or polycystic ovary syndrome. Whereas their evident association cannot fully establish a cause-effect relationship, fascinating mechanisms that bring them closer together than ever before are rapidly emerging. Central or abdominal obesity leads to insulin resistance and endothelial dysfunction through fat-derived metabolic products, hormones, and cytokines. Insulin resistance leads to endothelial dysfunction through the frequent association with traditional cardiovascular risk factors and through some more direct novel mechanisms. Some specific and shared insulin signaling abnormalities in muscle, fat, and endothelial cells, as well as some new genetic and nontraditional factors, may contribute to this interesting association. Some recent clinical studies demonstrate that nonpharmacological and pharmacological strategies targeting obesity and/or insulin resistance ameliorate endothelial function and low-grade inflammation. All these findings have added a new dimension to the association of obesity, insulin resistance, and endothelial dysfunction that may become a key target in the prevention of type 2 diabetes and cardiovascular disease.

3.5 years of insulin therapy with insulin glargine improves in vivo endothelial function in type 2 diabetes

OBJECTIVE

To determine long-term effects of insulin glargine on vascular function in patients with type 2 diabetes.

METHODS AND RESULTS

A total of 49 in vivo endothelial function tests, intrabrachial artery infusions of endothelium-dependent (acetylcholine [ACh]) and endothelium-independent (sodium nitroprusside [SNP]) vasoactive agents, were performed in 11 patients with type 2 diabetes (age: 59+/-2 years; BMI: 29.7+/-0.9 kg/m2; fasting plasma glucose: 226+/-14 mg/dL) and 16 matched normal subjects. The tests in the type 2 diabetic patients were performed before and after 6 months and 3.5 years of combination therapy with insulin glargine and metformin. A control group of type 2 diabetic patients not treated with insulin was studied twice at 6-month intervals. Before treatment, blood flow during infusions of low and high doses of ACh were significantly lower in the type 2 diabetic patients than in the normal subjects (P=0.021 for ANOVA). In the patients with type 2 diabetes, blood flow during infusion of the low dose of ACh averaged 7.1+/-0.8 mL/dL per minute at baseline, 8.8+/-1.0 mL/dL per minute at 6 months (NS), and then increased compared with baseline by 87+/-29% to 11.6+/-1.4 mL/dL per minute at 3.5 years (P<0.02 versus baseline). Blood flow during infusion of the high dose of ACh increased from 8.8+/-0.9 at baseline to 13.0+/-1.9 mL/dL per minute at 6 months (P<0.05) and by 86+/-25% to 14.7+/-1.6 mL/dL per minute at 3.5 years (P<0.01 versus baseline), which was not different from normal subjects. Blood flow during infusion of low (blood flow at 0 months: 7.7+/-0.5; at 6 months: 9.9+/-0.6; P<0.01 for 6 versus 0 months; and 3.5 years: 11.6+/-1.1 mL/dL per minute; P<0.02 for 3.5 years versus 0 months) and high (blood flow at 0 months: 10.7+/-0.9; 6 months: 13.4+/-1.0; P<0.05 for 6 versus 0 months; and 3.5 years: 16.6+/-1.5 mL/dL per minute; P<0.05 for 3.5 years versus 0 months) doses of SNP also increased significantly during insulin therapy.

CONCLUSIONS

We conclude that insulin glargine therapy improves endothelium-dependent and endothelium-independent vasodilatation. These data support the idea that long-term insulin therapy has beneficial rather than harmful effects on vascular function in type 2 diabetes.

Effect of the alpha-adrenergic blocker, doxazosin, on endothelial function and insulin action

Essential hypertension is associated with impairment of both endothelial function and insulin action, and this has provided rationale for the use of antihypertensive agents that are at least neutral, if not beneficial, in these areas. This study examines the effect of the alpha-adrenergic blocker, doxazosin, on endothelial function and insulin action. Sixteen patients with essential hypertension were recruited with 13 (3 men/10 women; median age, 55 years; range, 38 to 65 years) completing the study. A double-blind, placebo-controlled crossover study design was used. After a 6-week placebo run-in, there were two 12-week treatment periods of either placebo or doxazosin, separated by a 6-week wash out period. Subjects were studied at the end of each treatment period with endothelial function assessed by forearm plethysmography and insulin action by the hyperinsulinemic clamp technique. Blood pressure was significantly lowered by doxazosin (doxazosin 144 +/-3/86 +/- 2 mm Hg; placebo 159 +/- 3/96 +/- 1 mm Hg, P <.005 for both systolic and diastolic pressure; mean +/- SEM). Baseline forearm blood flow (FBF) was unchanged (doxazosin 4.9 +/- 0.9; placebo 4.0 +/- 0.7 mL x 100 mL(-1) x min(-1), P >.05), however, FBF responses (area under dose response curve, percentage change in infused:control arm ratio) to acetylcholine (endothelium-dependent vasodilation) were improved by doxazosin (doxazosin 58.6 +/- 11.7 standard units [SU]; placebo 22.1 +/- 7.0 SU, P =.03) with responses to sodium nitroprusside (endothelium-independent vasodilation) unchanged (doxazosin 40.3 +/- 5.5 SU; placebo 46.3 +/- 8.1 SU, P >.05). Exogenous glucose infusion rates to maintain euglycemia during hyperinsulinemia were not significantly different (doxazosin 30.4 +/- 0.9; placebo 32.3 +/- 1.0 micromol x kg(-1) min(-1), P >.05). Suppression of postabsorptive endogenous glucose production by insulin was also unchanged by treatment (doxazosin 65.6% +/- 7.5% suppression; placebo 68.3% +/- 11.2% suppression, P >.05). Doxazosin has a neutral effect on both peripheral and hepatic insulin action, but improves endothelium-dependent vasodilation. These results indicate that doxazosin can be used safely in patients with insulin resistance, while its positive effect on endothelial function may lessen the subsequent incidence of atherosclerosis.

Vascular responses to alpha-adrenergic stimulation and depolarization are enhanced in insulin-resistant and diabetic Psammomys obesus

Since vascular complications often accompany diabetes, we examined the influence of the endothelial lining on vascular reactivity in Psammomys obesus, a desert gerbil that acquires insulin resistance and diabetes when exposed to a laboratory diet. Vasoconstriction to phenylephrine and depolarizing KCl, as well as carbachol endothelium-dependent relaxation, were assessed in rings of thoracic aortae obtained from three groups: (i) group A, normoglycemic-normoinsulinemic; (ii) group B, normoglycemic-hyperinsulinemic, and (iii) group C, hyperglycemic-hyperinsulinemic animals. As expected, marked hypertriglyceridemia and hypercholesterolemia characterized groups B and C, which developed enhanced contractile responsiveness to phenylephrine and KCl compared with controls (group A). Furthermore, both experimental groups displayed a significant decrease in endothelium-dependent relaxation to carbachol. Altered lipid profiles are considered to play some role in the observed modification of aortic reactivity. Overall, our data indicate that vascular contractile responsiveness is enhanced early in the development of insulin resistance and diabetes in the female P. obesus.

Endothelium-dependent flow-mediated vasodilatation, insulin resistance and the metabolic syndrome in 60-year-old men

OBJECTIVES

To evaluate the endothelium-dependent flow-mediated vasodilatation (FMD) in the brachial artery and to study the relationship to insulin sensitivity and to the metabolic syndrome in 60-year-old clinically healthy men.

SUBJECTS

The men were randomly selected from the general population (n = 55). The subjects with the metabolic syndrome were defined according to a definition proposed by a working group associated with the World Health Organization (WHO).

METHODS

Ultrasound images for measurement of lumen diameter of the brachial artery were recorded before and after reactive hyperaemia induced by occlusion of the artery, both with and without ischaemic hand exercise during the occlusion. Insulin-mediated glucose uptake was determined by euglycaemic hyperinsulinaemic clamp as a measure of insulin sensitivity.

RESULTS

The FMD was in the total group 3.2% when hyperaemia was induced by occlusion only and 8.7% after occlusion plus ischaemic hand exercise (P < 0.001, n = 51). However, no relationship was observed between any measure of FMD and insulin-mediated glucose uptake (r = -0.05 and r = 0.06, n = 47, P > 0.30). Furthermore, subjects with the metabolic syndrome (n = 13) did not differ in any measure of FMD compared with those with no risk factors (n = 11).

CONCLUSION

In this study the ultrasound method to evaluate endothelial function did not show that low insulin sensitivity or the metabolic syndrome were associated with impaired FMD in otherwise clinically healthy 60-year-old men.

Pulse-wave analysis: clinical evaluation of a noninvasive, widely applicable method for assessing endothelial function

Current methods for assessing vasomotor endothelial function are impractical for use in large studies. We tested the hypothesis that pulse-wave analysis (PWA) combined with provocative pharmacological testing might provide an alternative method. Radial artery waveforms were recorded and augmentation index (AIx) was calculated from derived aortic waveforms. Thirteen subjects received sublingual nitroglycerin (NTG), inhaled albuterol, or placebo. Twelve subjects received NTG, albuterol, and placebo separately during an infusion of N(G)-monomethyl-L-arginine (LNMMA) or norepinephrine. Twenty-seven hypercholesterolemic subjects and 27 controls received NTG followed by albuterol. Endothelial function was assessed by PWA and forearm blood flow in 27 subjects. Albuterol and NTG both significantly and repeatably reduced AIx (P<0.001). Only the response to albuterol was inhibited by LNMMA (-9.8+/-5.5% vs -4.7+/-2.7%; P=0.02). Baseline AIx was higher in the hypercholesterolemic subjects, who exhibited a reduced response to albuterol (P=0.02) but not to NTG when compared with matched controls. The responses to albuterol and acetylcholine were correlated (r=0.5, P=0.02). Consistent with an endothelium-dependent effect, the response to albuterol was substantially inhibited by LNMMA. Importantly, the response to albuterol was reduced in subjects with hypercholesterolemia and was correlated to that of intra-arterial acetylcholine. This methodology provides a simple, repeatable, noninvasive means of assessing endothelial function in vivo.

Role of nitric oxide in the regulation of glucose kinetics in response to endotoxin in dogs

The purpose of the present in vivo study was to determine the role of nitric oxide (NO) in the regulation of glucose metabolism in response to endotoxin by blocking NO synthesis with N(G)-monomethyl-L-arginine (L-NMMA). In five dogs, the appearance and disappearance rates of glucose (by infusion of [6,6-(2)H(2)]glucose), plasma glucose concentration, and plasma hormone concentrations were measured on five different occasions: saline infusion, endotoxin alone (E coli, 1.0 microg/kg i.v.), and endotoxin administration plus three different doses of primed, continuous infusion of L-NMMA. Endotoxin increased rate of appearance of glucose from 13.7 +/- 1.6 to 23.6 +/- 3.3 micromol x kg(-1) x min(-1) (P < 0.05), rate of disappearance of glucose from 13.9 +/- 1.1 to 24.8 +/- 3.1 micromol x kg(-1) x min(-1) (P < 0.001), plasma lactate from 0.5 +/- 0.1 to 1.7 +/- 0.1 mmol/l (P < 0.01), and counterregulatory hormone concentrations. L-NMMA did not affect the rise in rate of appearance and disappearance of glucose, plasma lactate, or the counterregulatory hormone response to endoxin. Plasma glucose levels were not affected by endotoxin with or without L-NMMA. In conclusion, in vivo inhibition of NO synthesis by high doses of L-NMMA does not affect glucose metabolism in response to endotoxin, indicating that NO is not a major mediator of glucose metabolism during endotoxemia in dogs.

Improved endothelial function with metformin in type 2 diabetes mellitus

OBJECTIVES

This study was designed to assess the effect of metformin on impaired endothelial function in type 2 diabetes mellitus.

BACKGROUND

Abnormalities in vascular endothelial function are well recognized among patients with type 2 (insulin-resistant) diabetes mellitus. Insulin resistance itself may be central to the pathogenesis of endothelial dysfunction. The effects of metformin, an antidiabetic agent that improves insulin sensitivity, on endothelial function have not been reported.

METHODS

Subjects with diet-treated type 2 diabetes but without the confounding collection of cardiovascular risk factors seen in the metabolic syndrome were treated with metformin 500 mg twice daily (n = 29) or placebo (n = 15) for 12 weeks. Before and after treatment, blood flow responses to intraarterial administration of endothelium-dependent (acetylcholine), endothelium-independent (sodium nitroprusside) and nitrate-independent (verapamil) vasodilators were measured using forearm plethysmography. Whole-body insulin resistance was assessed on both occasions using the homeostasis model (HOMA-IR).

RESULTS

Subjects who received metformin demonstrated statistically significant improvement in acetylcholine-stimulated flows compared with those treated with placebo (p = 0.0027 by 2-way analysis of variance), whereas no significant effect was seen on nitroprusside-stimulated (p = 0.27) or verapamil-stimulated (p = 0.40) flows. There was a significant improvement in insulin resistance with metformin (32.5% reduction in HOMA-IR, p = 0.01), and by stepwise multivariate analysis insulin resistance was the sole predictor of endothelium-dependent blood flow following treatment (r = -0.659, p = 0.0012).

CONCLUSIONS

Metformin treatment improved both insulin resistance and endothelial function, with a strong statistical link between these variables. This supports the concept of the central role of insulin resistance in the pathogenesis of endothelial dysfunction in type 2 diabetes mellitus. This has important implications for the investigation and treatment of vascular disease in patients with type 2 diabetes mellitus.

Evidence for physiological coupling of insulin-mediated glucose metabolism and limb blood flow

We hypothesized that the vasodilation observed during insulin stimulation is closely coupled to the rate of glucose metabolism. Lean (L, n = 13), obese nondiabetic (OB, n = 13), and obese type 2 diabetic subjects (Type 2 DM, n = 16) were studied. Leg blood flow (LBF) was examined under conditions of euglycemic hyperinsulinemia (EH) and hyperglycemic hyperinsulinemia (HH), which produced a steady-state whole body glucose disposal rate (GDR) of approximately 2,000 micromol. m(-2). min(-1). At this GDR, under both conditions, subjects across the range of insulin sensitivity exhibited equivalent LBF (l/min EH: L, 0.42 +/- 0.03; OB, 0.43 +/- 0. 03; Type 2 DM, 0.38 +/- 0.07; P = 0.72 by ANOVA. HH: L, 0.44 +/- 0. 04; OB, 0.39 +/- 0.05; Type 2 DM, 0.41 +/- 0.04; P = 0.71). The continuous relationship between LBF and GDR did not differ across subject groups [slope x 10(-5) l/(micromol. m(-2). min(-1)) by ANOVA. EH: L, 8.6; OB, 9.2; Type 2 DM, 7.9; P = 0.91. HH: L, 4.2; OB, 2.5; Type 2 DM, 4.1; P = 0.77], although this relationship did differ between the EH and HH conditions (P = 0.001). These findings support a physiological coupling of LBF and insulin-mediated glucose metabolism. The mechanism(s) linking substrate delivery and metabolism appears to be intact in insulin-resistant states.

Insulin and the vasculature

Under physiologic conditions, insulin plays a vasculoprotective role by acting as a vasodilator through the stimulation of nitric oxide synthesis and release from the endothelium. In addition, insulin may decrease the contractile response of vascular smooth muscle cells (VSMC) to vasoactive agents by decreasing the intracellular calcium concentration i. However, in the insulin resistance syndrome, the vasodilator effect of insulin may be blunted, an abnormality that may be related to endothelial dysfunction. Insulin resistance correlates with carotid wall thickness and stiffness, but the relationship is modified by sex and ethnic factors. Insulin can act as growth factor stimulating VSMC growth in culture. Insulin-sensitizing agents are efficacious in improving the vascular pathology associated with insulin resistance.

Regulation of endothelial constitutive nitric oxide synthase gene expression in endothelial cells and in vivo : a specific vascular action of insulin

BACKGROUND

The vasodilatory effect of insulin can be acute or increase with time from 1 to 7 hours, suggesting that insulin may enhance the expression of endothelial nitric oxide synthase (eNOS) in endothelial cells. The objective of the present study was to characterize the extent and signaling pathways by which insulin regulates the expression of eNOS in endothelial cells and vascular tissues.

METHODS AND RESULTS

Physiological concentrations of insulin (10(-10) to 10(-7) mmol/L) increased the levels of eNOS mRNA, protein, and activity by 2-fold after 2 to 8 hours of incubation in cultured bovine aortic endothelial cells. Insulin enhanced eNOS gene expression in microvessels isolated from Zucker lean rats but not from insulin-resistant Zucker fatty rats. Inhibitors of phosphatidylinositol-3 kinase (PI-3 kinase) decreased the effect of insulin on eNOS gene expression, but a general protein kinase C (PKC) inhibitor, GF109203X or PKCbeta isoform inhibitor, LY333531 enhanced eNOS expression. In contrast, PKC activators inhibited both the activation by insulin of PI-3 kinase and eNOS mRNA levels. Overexpression of PKCbeta isoform in endothelial cells inhibited the stimulation by insulin of eNOS expression and PI-3 kinase activities in parallel.

CONCLUSIONS

Insulin can regulate the expression of eNOS gene, mediated by the activation of PI-3 kinase, in endothelial cells and microvessels. Thus, insulin may chronically modulate vascular tone. The activation of PKC in the vascular tissues as in insulin resistance and diabetes may inhibit PI-3 kinase activity and eNOS expression and may lead to endothelial dysfunctions in these pathological states.

Endothelial cell dysfunction and the pathogenesis of diabetic macroangiopathy

Type 2 diabetes mellitus (DM) represents a high risk condition for the development of atherosclerotic and thromboembolic macroangiopathy, which make major contributions to diabetic mortality and morbidity. While many cardiovascular risk factors are common to both atherosclerosis and Type 2 DM, the enhanced risk of diabetic macroangiopathy may be attributable to additional pro-atherogenic mediators associated with insulin resistance syndrome. Given the central pathogenic role of endotheliopathy in atherosclerosis, it is likely that this vascular monolayer is the ultimate target of injury in response to such mediators. Furthermore, a pro-oxidative, dysfunctional endothelium may actively contribute to the pro-atherogenic environment through an inappropriate regulation of vascular tone, permeability, coagulation, fibrinolysis, cell adhesion and proliferation. Such dysfunction may mediate hypertension, dyslipidaemia and altered haemostasis, in addition to aggravating in vivo insulin resistance.

Effects of acute euglycemic hyperinsulinemia on urinary nitrite/nitrate excretion and plasma endothelin-1 levels in men with essential hypertension and normotensive controls

Insulin stimulates the production of endothelin-1 (ET-1) and nitric oxide (NO) by isolated endothelial cells. Additionally, insulin-dependent glucose transport and insulin-mediated NO production partially share common elements in signal transduction. There are discordant data on plasma ET-1 levels during acute euglycemic systemic hyperinsulinemia in normotensive men and men with essential hypertension (EH) (known to be insulin-resistant), as well as on the relations between insulin sensitivity and vascular function. Our aim was to assess the response of approximate measures of whole-body generation of NO and ET-1 to acute euglycemic hyperinsulinemia in EH patients and controls. We studied 17 newly diagnosed untreated men with uncomplicated EH and 10 normotensive controls. Plasma ET-1 and urinary excretion of nitrite plus nitrate, stable NO metabolites (Uno(x)), were measured before and during a 3-hour hyperinsulinemic-euglycemic clamp. Both in hypertensives and normotensives, plasma ET-1 levels were reduced after 2 hours of the clamp (EH: baseline, 3.1+/-1.9 pg/mL; 2 hours, 1.9+/-1.2 pg/mL, P = .04 v baseline; controls: baseline, 4.2+/-2.6 pg/mL; 2 hours, 2.8+/-1.4 pg/mL, P = .04 v baseline). No significant changes in Uno(x) during the clamp were observed. Changes in Uno(x) during the clamp (deltaUno(x)) and differences in plasma ET-1 measured before the end and before the beginning of the clamp (deltaET-1) were correlated in the controls (r = .75, P = .01) but not in EH (r = -.01, P = .97). No parameter of glucose metabolism correlated with basal Uno(x), basal plasma ET-1, deltaUno(x), and deltaET-1, whether absolute or percent values, in either group. Thus, acute euglycemic hyperinsulinemia produces a decrease in plasma ET-1 in both EH patients and controls. The lack of correlation between deltaUno(x) and deltaET-1 under these conditions in EH may suggest an impairment of systems governing interactions between the NO-dependent pathway and ET-1. In addition, insulin actions on glucose metabolism and on the endothelial mediators appear dissociated.

Reduced urinary excretion of nitric oxide metabolites and increased plasma levels of asymmetric dimethylarginine in men with essential hypertension

Our aim was to investigate systemic nitric oxide (NO) production and its potential determinants such as insulin resistance, dyslipidemia, and circulating methylated analogs of L-arginine in uncomplicated essential hypertension (EH). Nineteen newly diagnosed, untreated male subjects with mild pure uncomplicated EH and 11 normotensive controls were studied at rest after an overnight fast. The groups had comparable age, body mass index, creatinine clearance, cholesterol, fasting glucose, and insulin. In hypertensives, the urinary excretion rate of nitrite plus nitrate (Unox), an index of endogenous NO production, was depressed (56+/-17 vs. 77+/-23 micromol/mmol creatinine; p < 0.05), whereas plasma levels of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthesis, were increased (2.4+/-1.1 vs. 1.1+/-0.7 microM; p < 0.005). Circulating concentrations of symmetric dimethylarginine were similar in both groups (1.4+/-1.3 vs. 1.5+/-1.1 microM; p = NS). The L-arginine-to-ADMA ratio was reduced in hypertension (3.3+/-0.5 vs. 4.5+/-0.8; p < 0.001 for In-transformed data). There was no correlation between Unox and either the magnitude of insulin resistance or dyslipidemia in EH. Thus in male subjects with EH, endogenous systemic NO formation appears depressed, which is unrelated to accompanying insulin resistance or dyslipidemia. Circulating ADMA levels are increased in uncomplicated EH, which may be of potential relevance.

Microvascular function relates to insulin sensitivity and blood pressure in normal subjects

BACKGROUND

A strong but presently unexplained inverse association between blood pressure and insulin sensitivity has been reported. Microvascular vasodilator capacity may be a common antecedent linking insulin sensitivity to blood pressure. To test this hypothesis, we studied 18 normotensive and glucose-tolerant subjects showing a wide range in insulin sensitivity as assessed with the hyperinsulinemic, euglycemic clamp technique.

METHODS AND RESULTS

Blood pressure was measured by 24-hour ambulatory blood pressure monitoring. Videomicroscopy was used to measure skin capillary density and capillary recruitment after arterial occlusion. Skin blood flow responses after iontophoresis of acetylcholine and sodium nitroprusside were evaluated by laser Doppler flowmetry. Insulin sensitivity correlated with 24-hour systolic blood pressure (24-hour SBP; r=-0.50, P<0.05). Capillary recruitment and acetylcholine-mediated vasodilatation were strongly and positively related to insulin sensitivity (r=0.84, P<0.001; r=0.78, P<0.001, respectively), and capillary recruitment was inversely related to 24-hour SBP (r=-0.53, P<0.05). Waist-to-hip ratio showed strong associations with insulin sensitivity, blood pressure, and the measures of microvascular function but did not confound the associations between these variables. Subsequent regression analysis showed that the association between insulin sensitivity and blood pressure was not independent of the estimates of microvascular function, and part of the variation in both blood pressure (R2=38%) and insulin sensitivity (R2=71%) could be explained by microvascular function.

CONCLUSIONS

Insulin sensitivity and blood pressure are associated well within the physiological range. Microvascular function strongly relates to both, consistent with a central role in linking these variables.

Decreased blood flow but unaltered insulin sensitivity of glucose uptake in skeletal muscle of chronic smokers

Chronic cigarette smoking is associated with dysfunction of the vascular endothelium. Smokers have also been shown to be insulin-resistant, at least in some studies. Since insulin-induced vasodilation is dependent on endothelial cell nitric oxide (NO) synthesis, we tested the hypothesis that decreased skeletal muscle blood flow causes insulin resistance in smokers. We studied 37 young normotensive normolipidemic nondiabetic men, of which 14 were smokers and 23 lifelong nonsmokers. The groups were similar with respect to age, body mass index (BMI), and maximal oxygen uptake (VO2max). Basal and insulin-stimulated femoral muscle blood flow was measured using [(15)O]H2O and insulin-stimulated muscle glucose uptake using [18F]fluoro-2-deoxy-D-glucose ([18F]FDG) and positron emission tomography (PET). Whole-body glucose uptake was measured using the hyperinsulinemic (insulin infusion 5 mU/kg x min)-euglycemic clamp technique. In the basal state, muscle blood flow was 51% lower in smokers (17 +/- 3 mL/kg muscle x min) versus nonsmokers (35 +/- 17 mL/kg x min, P < .0001). Insulin increased muscle blood flow comparably in both groups; the mean rate of insulin-stimulated blood flow was 30 +/- 10 and 55 +/- 38 mL/kg x min (P = .049), respectively. Whole-body and skeletal muscle glucose uptake were similar in both groups during insulin infusion. We conclude that muscle blood flow is lower in chronic smokers compared with nonsmokers under both fasting and hyperinsulinemic conditions. The insulin-induced increase in muscle blood flow and insulin-stimulated glucose uptake appear normal, suggesting that the vasodilatory and metabolic effects of insulin are intact in smokers and the reduced muscle blood flow per se does not cause insulin resistance in these subjects.

Insulin-mediated vasodilation and glucose uptake are functionally linked in humans

Intra-arterial infusion of insulin in physiological doses causes forearm vasodilation which is augmented by co-infusion of D-glucose, leading us to speculate that local insulin-mediated vasodilation may depend on insulin-mediated glucose uptake. We have examined the relationship between whole-body insulin sensitivity and forearm vasodilation in response to local infusion of insulin/glucose, thus avoiding any confounding effects of sympathetic stimulation on peripheral blood flow. Eighteen healthy, normotensive male volunteers (age, 26+/-5.4 years) attended on two separate occasions for measurement of: (1) whole-body insulin sensitivity with use of the hyperinsulinemic euglycemic clamp; (2) forearm vasodilation in response to an intra-arterial infusion of insulin/glucose with use of bilateral venous occlusion plethysmography. Insulin-mediated glucose uptake (M) for the group (mean+/-SD) was 10.0+/-2.2 mg. kg-1. min-1, and the percentage change in forearm blood flow ratio (%FBFR) for the group (median, interquartile range) was 28.2% (13.6, 48.6). In univariate analysis, M was significantly correlated with %FBFR (rs=0.60, P<0.05), but not with body mass index (BMI) (rs=-0. 42), age (r=-0.39) or mean arterial pressure (r=0.13). In multiple regression analysis, %FBFR remained a significant independent predictor of M (R2 (adj)=0.48, t=3.23, P<0.01) in a model involving BMI, age, and blood pressure. These data support the concept of a significant functional relationship between insulin's metabolic and vascular actions, possibly at an endothelial level.

Dyslipoproteinaemia and hyperoxidative stress in the pathogenesis of endothelial dysfunction in non-insulin dependent diabetes mellitus: an hypothesis

Endothelial dysfunction in non-insulin dependent (Type 2) diabetes mellitus (NIDDM) has implications for the pathogenesis of the two major complications, macrovascular disease and microangiopathy. Endothelial dysfunction is a consequence of a disturbance in the L-arginine/nitric oxide pathway. Its occurrence in NIDDM is well supported by both in vitro and in vivo studies. NIDDM results in diverse abnormalities in lipoprotein metabolism, the most significant being hypertriglyceridaemia which is associated with increased plasma concentrations of small dense LDL and low levels of HDL. Dysglycaemia results in hyperoxidative stress and increased formation of advanced-glycosylation endproducts, both of which enhance the oxidative modification of lipoprotein particles. Based on extensive in vitro studies and on human data, we generate the hypothesis that the development of endothelial dysfunction in NIDDM is a consequence of the effect of dyslipoproteinaemia, in particular increased circulatory concentrations of modified small dense LDL and of hyperoxidative stress on the formation, action and disposal of nitric oxide, by diverse molecular mechanisms; HDL is proposed to have a protective effect on these processes through its enzymic antioxidant properties. The hypothesis proposed is simple, testable and consistent with wide sources of evidence. The practical implications of the hypothesis and the existing opportunities for the prevention and reversal of endothelial dysfunction in NIDDM are also reviewed and discussed.

Acute endothelium-mediated vasodilation is not impaired at the onset of diabetes

Vascular injury and impaired vascular function are central to the increased mortality associated with diabetes. Hyperglycemia in diabetes has been suggested to play a role in this process, in part by impairing the function of the vascular endothelium. It has been difficult, however, to isolate the direct effect of glucose in both humans and in animal models of diabetes. This was evaluated in the present study in 7 rats that were chronically instrumented with a Transonic flow probe at the iliac bifurcation of the abdominal aorta, a nonoccluding catheter inserted immediately anterior to the flow probe, and a femoral vein catheter. Acute infusions of acetylcholine and sodium nitroprusside (1 and 10 microg/min IA) increased hindquarter blood flow significantly by approximately 27 and 10 mL/min over baseline, respectively, at the high dose. Streptozotocin (70 mg/kg IV) was administered, but normoglycemia was maintained with continuous intravenous insulin infusion to control for potential streptozotocin side effects. Diabetes was induced 5 to 7 days later by stopping the insulin infusion. Hindlimb blood flow (measured 24 hours per day) decreased during the diabetic period and was accompanied by an increase in mean arterial pressure, suggesting a vasoconstrictor response. However, the responses to acetylcholine and sodium nitroprusside were not altered significantly on either day 2 or day 6 of the diabetic period. This suggests that neither endothelium-mediated vasorelaxation nor responsiveness to nitric oxide is impaired during the initial phase of diabetes and that diabetic hyperglycemia does not have a significant, direct effect to impair endothelium-mediated relaxation in insulin-dependent diabetes mellitus. The mechanism for the change in baseline blood flow and its potential influence on endothelial function, however, are not known.

Insulin as a vascular hormone: implications for the pathophysiology of cardiovascular disease

1. Metabolic disorders, such as obesity and non-insulin-dependent diabetes mellitus, and cardiovascular disorders, such as essential hypertension, congestive cardiac failure and atherosclerosis, have two features in common, namely relative resistance to insulin-mediated glucose uptake and vascular endothelial dysfunction. 2. Significant increases in limb blood flow occur in response to systemic hyperinsulinaemia, although there is marked variation in the results due to a number of confounding factors, including activation of the sympathetic nervous system. Local hyperinsulinaemia has a less marked vasodilator action despite similar plasma concentrations, but this can be augmented by co-infusing D-glucose. 3. Insulin may stimulate endothelial nitric oxide production or may act directly on vascular smooth muscle via stimulation of the Na+-H+ exchanger and Na+/K+-ATPase, leading to hyperpolarization of the cell membrane and consequent closure of voltage-gated Ca2+ channels. 4. There is evidence both for and against the existence of a functional relationship between insulin-mediated glucose uptake (insulin sensitivity) and insulin-mediated vasodilation (which can be regarded as a surrogate measure for endothelial function). 5. If substrate delivery is the rate-limiting step for insulin-mediated glucose uptake (in other words, if skeletal muscle blood flow is a determinant of glucose uptake), then endothelial dysfunction, resulting in a relative inability of mediators, including insulin, to stimulate muscle blood flow, may be the underlying mechanism accounting for the association of atherosclerosis and other cardiovascular disorders with insulin resistance. 6. Glucose uptake may determine peripheral blood flow via stimulation of ATP-dependent ion pumps with consequent vasorelaxation. 7. A 'third factor' may cause both insulin resistance and endothelial dysfunction in cardiovascular disease. Candidates include skeletal muscle fibre type and capillary density, distribution of adiposity and endogenous corticosteroid production. 8. A complex interaction between endothelial dysfunction, abnormal skeletal muscle blood flow and reduced insulin-mediated glucose uptake may be central to the link between insulin resistance, blood pressure, impaired glucose tolerance and the risk of cardiovascular disease. An understanding of the primary mechanisms resulting in these phenotypes may reveal new therapeutic targets in metabolic and cardiovascular disease.

Haemodynamic actions of insulin

Several lines of evidence indicate a significant association between insulin and cardiovascular disease. This association might be explained by direct (cardio) vascular effects of insulin. Two hemodynamic actions of insulin are discussed in this review; it induces direct vasodilation in skeletal muscle and stimulation of the sympathetic nervous system. These closely linked effects normally offset each other. Although more insight has been obtained into responses in insulin-resistant individuals and possible mechanisms, direct evidence to support a causative role for insulin is not yet available.