Insulin resistance and vasodilation in essential hypertension. Studies with adenosine

Endothelial dysfunction of coronary arteries in subjects without diabetes: An association with both insulin resistance and impaired insulin secretion response

AIMS

This study was aimed to compare insulin sensitivity and secretion response, lipoprotein and plasminogen activator inhibitor 1 (PAI-1) levels between the subjects with and without coronary artery endothelial dysfunction (ED).

METHODS

ED was detected by intracoronary injection of acetylcholine (ACh) in 47 nondiabetes subjects without stenotic coronary arteries, selected from 316 consecutive patients with coronary angiography performed for suspected coronary artery disease. The subjects were divided into two groups: presence of ACh-induced coronary spasm (group ED+, N = 30) and absence of ACh-induced coronary spasm (group ED-, N = 17). Insulin sensitivity (Si) was evaluated by frequently sampled intravenous glucose tolerance test (FSIGTT) with minimal model analysis and by HOMA-IR, insulin secretion by acute insulin response (AIR) (calculated from the first 8 min of FSIGTT) and by disposition index (DI) (Si × AIR). Lipids and PAI-1 levels were determined enzymatically, and LDL particle size by gradient gel electrophoresis.

RESULTS

Si was significantly lower (4.22 ± 0.62 vs 6.98 ± 1.47 min^-1/mU/l × 10⁴; p < 0.05) while HOMA-IR was significantly higher in ED + group vs ED- group (2.8 ± 0.3 vs 1.7 ± 0.2; p < 0.05). Simultaneously, AIR and DI was significantly lower in ED + vs ED- groups (p < 0.05 and p < 0.01, respectively). Investigated groups did not differ in fasting lipid levels but ED+ group had significantly smaller LDL particles (p < 0.01) and higher PAI-1 levels (p < 0.05). Regression analysis shown that DI was a strong independent predictor of appearance of ED, together with PAI-1 and LDL particle size.

CONCLUSIONS

Both insulin resistance and impairment in insulin secretion response strongly correlate with coronary ED in subjects without diabetes.

Review: Endothelial dysfunction in type 2 diabetes

Dysfunction of the endothelial cell monolayer leads to increased vascular tone and permeability and a prothrombotic environment. Type 2 diabetes is a state of insulin resistance, hyperglycaemia and dyslipidaemia characterised by high cardiovascular risk and accelerated atherosclerosis. Many mechanisms by which hyperglycaemia can result in endothelial dysfunction have now been identified. However, the presence of endothelial dysfunction in normoglycaemic first-degree relatives and insulin resistant subjects is less well understood and less readily explained by `confounding' variables. We suggest that insulin's effects on glucose transport in classical target tissues and nitric oxide production in the endothelium are fundamentally linked at a molecular level. It is expected that greater understanding of these underlying mechanisms will lead to novel approaches to prevention of cardiovascular disease in both diabetic and non-diabetic subjects.

Adaptation of β-Cell and Endothelial Function to Carbohydrate Loading: Influence of Insulin Resistance

High-carbohydrate diets have been associated with β-cell strain, dyslipidemia, and endothelial dysfunction. We examined how β-cell and endothelial function adapt to carbohydrate overloading and the influence of insulin resistance. On sequential days in randomized order, nondiabetic subjects (classified as insulin-sensitive [IS] [n = 64] or insulin-resistant [IR] [n = 79] by euglycemic clamp) received four mixed meals over 14 h with either standard (300 kcal) or double carbohydrate content. β-Cell function was reconstructed by mathematical modeling; brachial artery flow-mediated dilation (FMD) was measured before and after each meal. Compared with IS, IR subjects showed higher glycemia and insulin hypersecretion due to greater β-cell glucose and rate sensitivity; potentiation of insulin secretion, however, was impaired. Circulating free fatty acids (FFAs) were less suppressed in IR than IS subjects. Baseline FMD was reduced in IR, and postprandial FMD attenuation occurred after each meal, particularly with high carbohydrate, similarly in IR and IS. Throughout the two study days, higher FFA levels were significantly associated with lower (incretin-induced) potentiation and impaired FMD. In nondiabetic individuals, enhanced glucose sensitivity and potentiation upregulate the insulin secretory response to carbohydrate overloading. With insulin resistance, this adaptation is impaired. Defective suppression of endogenous FFA is one common link between impaired potentiation and vascular endothelial dysfunction.

Diabetes and hypertension: the bad companions

High blood pressure is reported in over two-thirds of patients with type 2 diabetes, and its development coincides with the development of hyperglycaemia. Many pathophysiological mechanisms underlie this association. Of these mechanisms, insulin resistance in the nitric-oxide pathway; the stimulatory effect of hyperinsulinaemia on sympathetic drive, smooth muscle growth, and sodium-fluid retention; and the excitatory effect of hyperglycaemia on the renin-angiotensin-aldosterone system seem to be plausible. In patients with diabetes, hypertension confers an enhanced risk of cardiovascular disease. A blood pressure of lower than 140/85 mm Hg is a reasonable therapeutic goal in patients with type 2 diabetes according to clinical trial evidence. People with controlled diabetes have a similar cardiovascular risk to patients without diabetes but with hypertension. A renin-angiotensin system blocker combined with a thiazide-type diuretic might be the best initial antihypertensive regimen for most people with diabetes. In general, the positive effects of antihypertensive drugs on cardiovascular outcomes outweigh the negative effects of antihypertensive drugs on glucose metabolism.

Effects of adenosine, exercise, and moderate acute hypoxia on energy substrate utilization of human skeletal muscle

Glucose metabolism increases in hypoxia and can be influenced by endogenous adenosine, but the role of adenosine for regulating glucose metabolism at rest or during exercise in hypoxia has not been elucidated in humans. We studied the effects of exogenous adenosine on human skeletal muscle glucose uptake and other blood energy substrates [free fatty acid (FFA) and lactate] by infusing adenosine into the femoral artery in nine healthy young men. The role of endogenous adenosine was studied by intra-arterial adenosine receptor inhibition (aminophylline) during dynamic one-leg knee extension exercise in normoxia and acute hypoxia corresponding to ∼3,400 m of altitude. Extraction and release of energy substrates were studied by arterial-to-venous (A-V) blood samples, and total uptake or release was determined by the product of A-V differences and muscle nutritive perfusion measured by positron emission tomography. The results showed that glucose uptake increased from a baseline value of 0.2 ± 0.2 to 2.0 ± 2.2 μmol·100 g(-1)·min(-1) during adenosine infusion (P < 0.05) at rest. Although acute hypoxia enhanced arterial FFA levels, it did not affect muscle substrate utilization at rest. During exercise, glucose uptake was higher (195%) during acute hypoxia compared with normoxia (P = 0.058), and aminophylline had no effect on energy substrate utilization during exercise, despite that arterial FFA levels were increased. In conclusion, exogenous adenosine at rest and acute moderate hypoxia during low-intensity knee-extension exercise increases skeletal muscle glucose uptake, but the increase in hypoxia appears not to be mediated by adenosine.

cGMP phosphodiesterase inhibition improves the vascular and metabolic actions of insulin in skeletal muscle

There is considerable support for the concept that insulin-mediated increases in microvascular blood flow to muscle impact significantly on muscle glucose uptake. Since the microvascular blood flow increases with insulin have been shown to be nitric oxide-dependent inhibition of cGMP-degrading phosphodiesterases (cGMP PDEs) is predicted to enhance insulin-mediated increases in microvascular perfusion and muscle glucose uptake. Therefore, we studied the effects of the pan-cGMP PDE inhibitor zaprinast on the metabolic and vascular actions of insulin in muscle. Hyperinsulinemic euglycemic clamps (3 mU·min(-1)·kg(-1)) were performed in anesthetized rats and changes in microvascular blood flow assessed from rates of 1-methylxanthine metabolism across the muscle bed by capillary xanthine oxidase in response to insulin and zaprinast. We also characterized cGMP PDE isoform expression in muscle by real-time PCR and immunostaining of frozen muscle sections. Zaprinast enhanced insulin-mediated microvascular perfusion by 29% and muscle glucose uptake by 89%, while whole body glucose infusion rate during insulin infusion was increased by 33% at 2 h. PDE2, -9, and -10 were the major isoforms expressed at the mRNA level in muscle, while PDE1B, -9A, -10A, and -11A proteins were expressed in blood vessels. Acute administration of the cGMP PDE inhibitor zaprinast enhances muscle microvascular blood flow and glucose uptake response to insulin. The expression of a number of cGMP PDE isoforms in skeletal muscle suggests that targeting specific cGMP PDE isoforms may provide a promising avenue for development of a novel class of therapeutics for enhancing muscle insulin sensitivity.

HISS, not insulin, causes vasodilation in response to administered insulin

Meal-induced sensitization to the dynamic actions of insulin results from the peripheral actions of a hormone released by the liver (hepatic insulin sensitizing substance or HISS). Absence of meal-induced insulin sensitization results in the pathologies associated with cardiometabolic risk. Using three protocols that have previously demonstrated HISS metabolic action, we tested the hypothesis that HISS accounts for the vasodilation that has been associated with insulin. The dynamic metabolic actions of insulin and HISS were determined using a euglycemic clamp in response to a bolus of 100 mU/kg insulin in pentobarbital-anesthetized Sprague-Dawley rats. Hindlimb blood flow was measured with an ultrasound flow probe on the aorta above the bifurcation of the iliac arteries. Fed rats showed tightly coupled metabolic and vascular responses, which were completed by 35 min after insulin administration. Blocking HISS release, with the use of atropine or hepatic surgical denervation, eliminated the HISS-dependent metabolic and vascular responses to insulin administration. Physiological suppression of HISS release occurs with fasting. In 24-h fasted rats, HISS metabolic and vascular actions were absent, and atropine had no effect on either action. Fed rats with liver denervation did not release HISS, but intraportal venous infusion of acetylcholine, to mimic the permissive parasympathetic nerve signal, restored the ability of insulin to cause HISS release and restored both the metabolic and vascular actions. These studies report vascular actions of HISS for the first time and demonstrate that HISS, not insulin action, results in the peripheral vasodilation generally attributed to insulin.

Contribution of insulin resistance to vascular dysfunction

Insulin is a vascular hormone, able to influence vascular cell responses. In this review, we consider the insulin actions on vascular endothelium and on vascular smooth muscle cells (VSMC) both in physiological conditions and in the presence of insulin resistance. In particular, we focus the relationships between activation of insulin signalling pathways of phosphatidylinositol-3 kinase (PI3-K) and mitogen-activated protein kinase (MAPK) and the different vascular actions of insulin, with a particular attention to the insulin ability to activate the pathway nitric oxide (NO)/cyclic GMP/PKG via PI3-K, owing to the peculiar relevance of NO in vascular biology. We also discuss the insulin actions mediated by the MAPK pathway (such as endothelin-1 synthesis and secretion and VSMC proliferation and migration) and by the interactions between the two pathways, both in insulin-sensitive and in insulin-resistant states. Finally, we consider the influence of free fatty acids, cytokines and endothelin on vascular insulin resistance.

Association between sleep-disordered breathing, aminoterminal pro-brain natriuretic peptide (NT-proBNP) levels and insulin resistance in morbidly obese young women

OBJECTIVE

Sleep-disordered breathing (SDB) is often encountered in morbid obesity (MO) in conjunction with insulin resistance (IR) and several cardio-vascular risk factors. Aminoterminal pro-brain natriuretic peptide (NT-proBNP) is a promising marker for left ventricular dysfunction (LVD) in MO. The aim of this study was to look for possible correlations between SDB, IR, heart structure and function indexes and NT-proBNP levels in MO female subjects.

MATERIALS AND METHODS

Cross-sectional study involving 110 MO (44.5+/-0.7 kg m(-2)) apparently healthy, young (37.8+/-1.0 y.o.) female patients. NT-proBNP was measured using an ELISA kit (Roche). Echo-cardiograms were performed to quantify left ventricular ejection fraction values (LVEF), cardiac output (CO), left ventricular mass (LVM), left atria size (LA) and left ventricular filling pressures (the E/Em ratio). The Berlin Questionnaire (BQ) was used to assess the risk of SDB. IR and sensitivity were assessed using the HOMA index and adiponectin measurements, respectively.

RESULTS

All patients had a normal LVEF (>50%). Hypertension and Type 2 diabetes mellitus prevalences were 34.5 and 4.5% (respectively). Log-transformed NT-proBNP levels correlated with BQ categories (P<0.0005), creatinine (P<0.001), age (P<0.05), LVM (P<0.001), CO (P<0.001), LA (P<0.0005) and E/Em (P<0.01). NT-proBNP levels, LVD and LVM increased significantly along with BQ scores (P<0.0001). Stepwise multiple regression analysis identified BQ and log-transformed HOMA as independent variables predicting as much as 48.0% of log-transformed NT-proBNP's variability (dependent variable).

CONCLUSIONS

NT-proBNP levels are independently predicted by SDB and IR in asymptomatic MO women. Additionally, SDB worsens along with LVH and diastolic dysfunction. Larger prospective studies are warranted.

Impaired microvascular perfusion: a consequence of vascular dysfunction and a potential cause of insulin resistance in muscle

Insulin has an exercise-like action to increase microvascular perfusion of skeletal muscle and thereby enhance delivery of hormone and nutrient to the myocytes. With insulin resistance, insulin's action to increase microvascular perfusion is markedly impaired. This review examines the present status of these observations and techniques available to measure such changes as well as the possible underpinning mechanisms. Low physiological doses of insulin and light exercise have been shown to increase microvascular perfusion without increasing bulk blood flow. In these circumstances, blood flow is proposed to be redirected from the nonnutritive route to the nutritive route with flow becoming dominant in the nonnutritive route when insulin resistance has developed. Increased vasomotion controlled by vascular smooth muscle may be part of the explanation by which insulin mediates an increase in microvascular perfusion, as seen from the effects of insulin on both muscle and skin microvascular blood flow. In addition, vascular dysfunction appears to be an early development in the onset of insulin resistance, with the consequence that impaired glucose delivery, more so than insulin delivery, accounts for the diminished glucose uptake by insulin-resistant muscle. Regular exercise may prevent and ameliorate insulin resistance by increasing "vascular fitness" and thereby recovering insulin-mediated capillary recruitment.

Blocking the renin-angiotensin-aldosterone system to prevent diabetes mellitus

Type 2 diabetes mellitus (DM) is increasing around the world, and the public health impact of DM, driven largely by cardiovascular disease complications, underpins the importance of continued efforts toward primary prevention of DM. Only a few interventions have been shown to prevent DM, with none of them yet proven to improve cardiovascular risk commensurately. Accumulating evidence suggest that drugs that block the renin-angiotensin-aldosterone system (RAAS), many of which have proven cardiovascular disease (CVD) benefit, also have favourable effects on parameters of glucose metabolism and incident diabetes. Here we review the evidence accumulated to date from animal studies, clinical mechanistic studies and clinical trials regarding the effect of RAAS inhibition and incident DM.

Insulin resistance and endothelial dysfunction: the road map to cardiovascular diseases

Cardiovascular disease affects approximately 60% of the adult population over the age of 65 and represents the number one cause of death in the United States. Coronary atherosclerosis is responsible for the vast majority of the cardiovascular events, and a number of cardiovascular risk factors have been identified. In recent years, it has become clear that insulin resistance and endothelial dysfunction play a central role in the pathogenesis of atherosclerosis. Much evidence supports the presence of insulin resistance as the fundamental pathophysiologic disturbance responsible for the cluster of metabolic and cardiovascular disorders, known collectively as the metabolic syndrome. Endothelial dysfunction is an important component of the metabolic or insulin resistance syndrome and this is demonstrated by inadequate vasodilation and/or paradoxical vasoconstriction in coronary and peripheral arteries in response to stimuli that release nitric oxide (NO). Deficiency of endothelial-derived NO is believed to be the primary defect that links insulin resistance and endothelial dysfunction. NO deficiency results from decreased synthesis and/or release, in combination with exaggerated consumption in tissues by high levels of reactive oxygen (ROS) and nitrogen (RNS) species, which are produced by cellular disturbances in glucose and lipid metabolism. Endothelial dysfunction contributes to impaired insulin action, by altering the transcapillary passage of insulin to target tissues. Reduced expansion of the capillary network, with attenuation of microcirculatory blood flow to metabolically active tissues, contributes to the impairment of insulin-stimulated glucose and lipid metabolism. This establishes a reverberating negative feedback cycle in which progressive endothelial dysfunction and disturbances in glucose and lipid metabolism develop secondary to the insulin resistance. Vascular damage, which results from lipid deposition and oxidative stress to the vessel wall, triggers an inflammatory reaction, and the release of chemoattractants and cytokines worsens the insulin resistance and endothelial dysfunction.From the clinical standpoint, much experimental evidence supports the concept that therapies that improve insulin resistance and endothelial dysfunction reduce cardiovascular morbidity and mortality. Moreover, interventional strategies that reduce insulin resistance ameliorate endothelial dysfunction, while interventions that improve tissue sensitivity to insulin enhance vascular endothelial function. There is general agreement that aggressive therapy aimed simultaneously at improving insulin-mediated glucose/lipid metabolism and endothelial dysfunction represents an important strategy in preventing/delaying the appearance of atherosclerosis. Interventions that 1 correct carbohydrate and lipid metabolism, 2 improve insulin resistance, 3 reduce blood pressure and restore vascular reactivity, and 4 attenuate procoagulant and inflammatory responses in adults with a high risk of developing cardiovascular disease reduce cardiovascular morbidity and mortality. Whether these benefits hold when the same prevention strategies are applied to younger, high-risk individuals remains to be determined.

Nutritive blood flow as an essential element supporting muscle anabolism

PURPOSE OF REVIEW

Much of the recent literature concerning hormonal effects on muscle assumes that full perfusion occurs at all times such that nutrient and hormone delivery is complete. New methods to measure the extent of nutritive blood flow in muscle show that this is not the case and that anabolic hormones such as insulin increase nutritive flow and that other agents that increase bulk flow have little effect. This review examines the latest developments concerning insulin action to increase nutritive perfusion of muscle and agents that interact with this effect and which could potentially modulate anabolism.

RECENT FINDINGS

We examine recent attempts to define the anatomical nature of non-nutritive flow route in muscle, the quick onset of insulin action to recruit nutritive blood flow at doses lower than that which activates glucose uptake and bulk blood flow, actions of the inflammatory cytokine tumour necrosis factor alpha TNFalpha to oppose physiologic insulin action, interfibrillar fat depots that grow on the non-nutritive vasculature of muscle and underpin a 'vascrine hypothesis', and drugs that reduce insulin resistance by ameliorating vascular dysfunction.

SUMMARY

Recognition that nutrient and hormone delivery to muscle is controlled by microvascular perfusion and not necessarily by bulk blood flow is the key issue.

Vascular and metabolic effects of methacholine in relation to insulin action in muscle

AIMS/HYPOTHESIS

Methacholine (MC) is a nitric oxide vasodilator, but unlike other vasodilators, it potentiates insulin-mediated glucose uptake by muscle. The present study aimed to resolve whether this action was the result of a vascular effect of MC leading to increased muscle perfusion or a direct effect of MC on the myocytes. We hypothesise that vascular-mediated insulin-stimulated glucose uptake responses to MC occur at lower doses than direct myocyte MC-mediated increases in glucose uptake.

METHODS

The vascular and metabolic effects of this vasodilator were examined in rats in vivo using a novel local infusion technique, and in the pump-perfused rat hindlimb under conditions of constant flow.

RESULTS

Local infusion of low-dose MC (0.3 micromol/l) into the epigastric artery of one leg (test) in vivo markedly increased femoral blood flow and decreased vascular resistance, without effects in the contra-lateral leg. Capillary recruitment, but not glucose uptake, was increased in the test leg. All increases caused by MC were confined to the test leg and blocked by local infusion into the test leg of N-nitro-L-arginine methyl ester (L-NAME), but not by infusion of N-nitro-D-arginine methyl ester (D-NAME). In the constant-flow pump-perfused rat hindlimb, infusion of 0.6 micromol/l MC vasodilated the pre-constriction effected by 70 nmol/l noradrenaline or 300 nmol/l serotonin, and this was blocked by 10 micromol/l L-NAME. 2-Deoxyglucose in muscle was increased by 30 micromol/l MC (p<0.05), but was unaffected by 3 micromol/l MC. All increases in 2-deoxyglucose uptake by 30 micromol/l MC were blocked by 10 micromol/l L-NAME.

CONCLUSIONS/INTERPRETATION

MC has dose-dependent effects both on the vasculature and on muscle metabolism. At low dose (0.3-3 micromol/l), MC is a potent vasodilator in muscle, both in vivo and in vitro, without metabolic effects; at higher doses (> or =30 micromol/l) MC has a direct metabolic effect leading to increased glucose uptake. Both the vascular and metabolic effects are sensitive to L-NAME. The low-dose enhancement of insulin action in vivo by MC, which has been reported previously, thus seems to be attributable to vascular effects.

Relationship between autonomic dysfunction, insulin resistance and hypertension, in diabetes

Sympathovagal imbalance and insulin resistance are the common underlying disorders linking hypertension and diabetes. The role of hyperinsulinemia, however, on sympathovagal balance and blood pressure has never been clearly dissected from that of hyperglycemia. Nevertheless, the study of animal models of hypertension showed that hypertension does not invariably result in the onset of insulin resistance. This suggests that insulin resistance precedes the onset of hypertension and (possibly) contributes to its pathogenesis, mainly through sympathetic activation. To examine this hypothesis, recent studies investigated the relationship between insulin sensitivity and sympathetic activity in subjects with insulin resistance but free of overt hyperglycemia and obesity, i.e., insulin-resistant offspring of type 2 diabetic patients, demonstrating a prevalence of sympathetic over vagal activity. Therefore insulin resistance and sympathovagal imbalance come before hypertension, but a clear causative role cannot be demonstrated since other mechanisms, including an inappropriate lifestyle, must be taken into account to determine clinical hypertension. Finally, several experiments in human healthy volunteers suggest that the modulation of autonomic regulation at the forearm level can regulate insulin sensitivity, tempting us to speculate that it is the primary autonomic imbalance, through vasoconstriction, that results in both insulin resistance and hypertension. In conclusion, the close relationship between autonomic imbalance, insulin resistance and hypertension is unquestionable; although logical hypothesis can be constructed, which of the three is the earliest event is still not understood, and further research is required.

Local methacholine but not bradykinin potentiates insulin-mediated glucose uptake in muscle in vivo by augmenting capillary recruitment

AIMS/HYPOTHESIS

Insulin has nitric-oxide-dependent vasodilatory effects in muscle, including capillary recruitment, that enhance access for itself and glucose. However, nitric-oxide-dependent vasodilators other than methacholine do not enhance insulin action. Our hypothesis is that methacholine, unlike bradykinin, enhances insulin-mediated glucose uptake in muscle by augmenting capillary recruitment.

METHODS

Local infusion of either methacholine or bradykinin into one leg of the anaesthetised rat was made during physiological insulin (3 mU.kg(-1).min(-1)) infusion under euglycaemic conditions and without affecting systemic blood pressure. Whole-body glucose infusion was determined, as was femoral blood flow, 2-deoxyglucose uptake into calf muscles and the metabolism of infused 1-methylxanthine, a measure of capillary recruitment for each leg.

RESULTS

Methacholine alone (0.3 micromol.l(-1)) increased femoral arterial blood flow, increased capillary recruitment but had no effect on 2-deoxyglucose uptake of the test leg relative to the contra-lateral control leg. Insulin alone (systemically) required a glucose infusion rate of 8.7 mg.kg(-1).min(-1) to maintain euglycaemia, increased 2-deoxyglucose uptake and capillary recruitment, but was without effect on femoral blood flow in either leg. Local methacholine with systemic insulin infusion increased femoral blood flow, 2-deoxyglucose uptake and capillary recruitment in the test leg only. Bradykinin (0.07 micromol.l(-1)), alone or with insulin, administered in a manner that increased femoral blood flow similarly to methacholine, did not affect 2-deoxyglucose uptake or capillary recruitment.

CONCLUSIONS/INTERPRETATION

Methacholine but not bradykinin enhances insulin-mediated muscle glucose uptake in vivo. We conclude that methacholine acts at specific sites in the vasculature of muscle to stimulate capillary recruitment and thereby enhance insulin access.

Hypertension and triglyceride catabolism: implications for the hemodynamic model of the metabolic syndrome

OBJECTIVE

We sought to determine if hypertensive adults have a blunted triglyceride catabolic rate (TG K(2)) and if related hemodynamic and vascular alterations are determinants of TG K(2).

METHODS

Fasting levels of insulin, glucose, lipoproteins and plasma catecholamines were measured in 10 normotensive and 10 hypertensive adults. TG K(2) was determined by an intravenous fat tolerance test. Forearm blood flow, maximum forearm blood flow and minimal forearm vascular resistance were determined by strain gauge plethysmography. Vascular compliance and systemic hemodynamics were measured by computerized arterial pulse waveform analysis.

RESULTS

Compared to normotensives, hypertensives had a significantly elevated blood pressure (145 +/- 8/94 +/- 11 versus 111 +/- 15/74 +/- 14 mm Hg, p < 0.001), systemic vascular resistance (1695 +/- 441 versus 1172 +/- 430 dynes x sec x cm(-5), p = 0.02) and reduced large vessel compliance (11.7 +/- 3.6 versus 15.1 +/- 3.1 ml/mm Hg x 100, p = 0.04). There were no significant group differences in TG K(2) (3.07 +/- 2.01 versus 2.88 +/- 2.12 mg/dL/min, p = 0.85) or other metabolic and anthropometric variables. TG K(2) was not predicted by the forearm vascular measures or the hemodynamic variables, but was correlated to waist/hip ratio (r = -0.71, p = 0.001), fasting triglycerides (r = -0.64, p = 0.003), and male gender (r = 0.56, p = 0.012). An enhanced TG K(2) was independently predicted by a reduced small vessel compliance (r = -0.61, p = 0.006).

CONCLUSIONS

Elevated blood pressure per se and hypertension-related hemodynamic and vascular alterations are not associated with reduced TG K(2) or other metabolic abnormalities. Rather, aspects of the insulin resistance syndrome are closely related to abdominal adiposity. The independent association between small vessel compliance and TG K(2) deserves further investigation.

Exogenous nitric oxide increases basal leg glucose uptake in humans

This study addressed the role of blood flow and nitric oxide in leg glucose uptake. Seven subjects (5 men, 2 women) were studied during conditions of resting blood flow and increased blood flow, achieved by infusion of the nitric oxide (NO) donor sodium nitroprusside (SNP) into the femoral artery. Femoral arterial and venous blood samples were obtained and blood flow was determined by infusion of indocyanine green dye. SNP infusion significantly increased leg blood flow (769 +/- 103 v 450 +/- 65 mL. min(-1). leg(-1), P <.001), but did not affect arterial (4.68 +/- 0.13 mmol/L control, 4.63 +/- 0.09 mmol/L SNP) or venous (4.60 +/- 0.14 mmol/L control, 4.54 +/- 0.10 mmol/L SNP) glucose concentrations. Glucose uptake was significantly (P <.01) higher during SNP infusion (65 +/- 6 micromol. min(-1). leg(-1)) than during the basal period (34 +/- 6 micromol. min(-1). leg(-1)), whereas lactate release was unaffected (rest, 45 +/- 11 micromol. min(-1). leg(-1); SNP, 42 +/- 14 micromol. min(-1). leg(-1)). We conclude that blood flow and/or NO increase basal leg glucose uptake.

The role of endothelial insulin signaling in the regulation of vascular tone and insulin resistance

Insulin receptors (IRs) on vascular endothelial cells have been suggested to participate in insulin-regulated glucose homeostasis. To directly address the role of insulin action in endothelial function, we have generated a vascular endothelial cell IR knockout (VENIRKO) mouse using the Cre-loxP system. Cultured endothelium of VENIRKO mice exhibited complete rearrangement of the IR gene and a more than 95% decrease in IR mRNA. VENIRKO mice were born at the expected Mendelian ratio, grew normally, were fertile, and exhibited normal patterns of vasculature in the retina and other tissues. Glucose homeostasis under basal condition was comparable in VENIRKO mice. Both eNOS and endothelin-1 mRNA levels, however, were reduced by approximately 30-60% in endothelial cells, aorta, and heart, while vascular EGF expression was maintained at normal levels. Arterial pressure tended to be lower in VENIRKO mice on both low- and high-salt diets, and on a low-salt diet VENIRKO mice showed insulin resistance. Thus, inactivation of the IR on endothelial cell has no major consequences on vascular development or glucose homeostasis under basal conditions, but alters expression of vasoactive mediators and may play a role in maintaining vascular tone and regulation of insulin sensitivity to dietary salt intake.

Influence of nervous blockade on insulin-mediated glucose uptake in the human forearm

This study was undertaken to investigate if a nonpharmacologic increase in forearm blood flow (FBF) could increase forearm glucose uptake (FGU) during hyperinsulinemia. In 10 young volunteers, FBF and the arterial-venous glucose difference were measured in both arms during a 2-hour euglycemic hyperinsulinemic clamp procedure when 1 of the arms was subjected to axillary plexus nervous blockade with local anesthesia. FBF was measured in both arms by venous occlusion plethysmography. Nervous blockade, increasing FBF by more than 3-fold, did not improve insulin-mediated FGU. On the contrary, a tendency towards a reduced FGU compared with the control arm was seen (P =.07). Furthermore, while insulin increased FBF to a similar degree in both arms (+ 3.0 and 4.4 mL/min/100 mL tissue, P <.01 for both arms), nervous blockade abolished the rapid increase in glucose extraction seen in the control arm when insulin infusion was initiated. The present study showed that an increase in FBF induced by nervous blockade did not increase insulin-mediated FGU. On the contrary, a tendency towards a reduction was seen. Furthermore, insulin induced vasodilation in the blocked arm, but delayed the ability of insulin to promote glucose extraction, suggesting that the well-documented increase in skeletal muscle sympathetic nerve activity seen during acute hyperinsulinemia has metabolic rather than hemodynamic consequences.

Blood flow and muscle metabolism: a focus on insulin action

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.

Caffeine-induced impairment of glucose tolerance is abolished by beta-adrenergic receptor blockade in humans

The caffeine-induced impairment of insulin action is commonly attributed to adenosine receptor (AR) antagonism in skeletal muscle. However, epinephrine, a potent inhibitor of insulin actions, is increased after caffeine ingestion. We tested the hypothesis that the insulin antagonistic effects of caffeine are mediated by epinephrine, and not by AR antagonism, in seven healthy men. On four separate occasions, they received 1) dextrose (placebo, PL), 2) 5 mg/kg caffeine (CAF), 3) 80 mg of propranolol (PR), and 4) 5 mg/kg caffeine + 80 mg of propranolol (CAF + PR) before an oral glucose tolerance test (OGTT). Blood glucose was similar among trials before and during the OGTT. Plasma epinephrine was elevated (P < 0.05) in CAF and CAF + PR. Areas under the insulin and C-peptide curves were 42 and 39% greater (P < 0.05), respectively, in CAF than in PL, PR, and CAF + PR. In the presence of propranolol (CAF + PR), these responses were similar to PL and PR. These data suggest that the insulin antagonistic effects of caffeine in vivo are mediated by elevated epinephrine rather than by peripheral AR antagonism.

Regional myocardial blood flow and glucose utilization during fasting and physiological hyperinsulinemia in humans

We investigated the effect of insulin on total and regional myocardial blood flow (MBF) and glucose uptake (MGU) in healthy subjects (50 +/- 5 yr) by means of positron emission tomography (PET) with oxygen-15-labeled water (H(2)(15)O) and fluorine-18 labeled fluorodeoxyglucose ((18)FDG) before and during physiological hyperinsulinemia (40 mU.min(-1).m(-2)). Twelve male subjects were included in the study. During hyperinsulinemia, MBF increased from 0.91 +/- 0.28 to 1.01 +/- 0.31 ml.min(-1).g(-1) (n = 7 patients, P = 0.05; n = 112 regions, P < 0.005). Intersubject variability ranged from -3.0 to +41%. MGU increased from 0.11 +/- 0.08 (n = 5) to 0.56 +/- 0.08 micromol.min(-1).g(-1) (P < 0.0001, n = 7). MBF and insulin-mediated MGU were higher in the septum and anterior and lateral wall along short-axis regions of the heart. During hyperinsulinemia, MBF was also higher in the apex and midventricle compared with the base. MBF and MGU were positively correlated before (r = 0.66, P < 0.0001) and during hyperinsulinemia (r = 0.24, P < 0.05). These results provide evidence that insulin stimulates MBF in normal human hearts and appears to involve mainly those regions of the heart where insulin-mediated MGU is higher. Furthermore, regional distribution of insulin-stimulated MBF and MGU does not appear to be uniform across the left ventricular wall of healthy subjects.

Use of positron emission tomography for the assessment of skeletal muscle glucose metabolism

Positron emission tomography (PET) is a unique tool for studying regional skeletal muscle glucose metabolism and blood flow in vivo. The application of PET in the assessment of skeletal muscle glucose metabolism depends on recent improvements in instrumentation, data analysis, and production of (18)F-fluorodeoxyglucose (FDG) and (15)O water. The data presented support the validity of the (15)O water model to measure blood flow and the FDG model for the determination of glucose uptake and glucose kinetic constants (influx, efflux, and phosphorylation) in skeletal muscle. However, quantification of absolute glucose transport and backflux rates should be applied with caution because those calculations are based on unproven assumptions such as validity of the lumped constant for these individual processes and constancy of the free and accessible intracellular glucose pool. It is evident that quantification of glucose fluxes using the triple tracer technology generates conflicting data that violate assumptions inherent in triple tracer or PET modeling. Further FDG-PET studies will have to solve those problems to provide more insight into the regulatory processes of glucose transport and phosphorylation of different insulin-resistant disease states. Promising new areas of PET research will include not only detailed study of glucose kinetics but also the measurement of muscle protein synthesis in vivo, which is of interest in a variety of conditions.

Lipid infusion impairs physiologic insulin-mediated capillary recruitment and muscle glucose uptake in vivo

Infusion of triglycerides and heparin causes insulin resistance in muscle. Because the vascular actions of insulin, particularly capillary recruitment, may contribute to the increase in glucose uptake by skeletal muscle, we investigated the effects of Intralipid/heparin infusion on the hemodynamic actions of insulin during clamp conditions. Saline or 10% Intralipid/heparin (33 U/ml) was infused into anesthetized rats at 20 microl/min for 6 h. At 4 h into the saline infusion, a 2-h hyperinsulinemic (3 mU. min(-1).kg(-1))-euglycemic clamp was conducted (Ins group). At 4 h into the lipid infusion, a 2-h saline control (Lip group) or 2-h hyperinsulinemic-euglycemic clamp (Lip + Ins group) was conducted. Arterial blood pressure, heart rate, femoral blood flow (FBF), hindleg vascular resistance, glucose infusion rate (GIR), hindleg glucose uptake (HGU), and muscle 2-deoxyglucose uptake (R'g) were measured. Capillary recruitment, as measured by metabolism of infused 1-methylxanthine (1-MX), was also assessed. When compared with either Lip or Lip + Ins, Ins had no effect on arterial blood pressure, heart rate, FBF, or vascular resistance but increased GIR, HGU, and R'g of soleus, plantaris, extensor digitorum longus, and gastrocnemius red muscles and hindlimb 1-MX metabolism. GIR, HGU, and R'g of soleus, plantaris, gastrocnemius red, and the combined muscles and 1-MX metabolism were less in Lip + Ins than in Ins rats. HGU correlated closely with hindleg capillary recruitment (r = 0.86, P < 0.001) but not total hindleg blood flow. In conclusion, acute elevation of plasma free fatty acids blocks insulin-mediated glucose uptake and capillary recruitment.

Caffeine can decrease insulin sensitivity in humans

OBJECTIVE

Caffeine is a central stimulant that increases the release of catecholamines. As a component of popular beverages, caffeine is widely used around the world. Its pharmacological effects are predominantly due to adenosine receptor antagonism and include release of catecholamines. We hypothesized that caffeine reduces insulin sensitivity, either due to catecholamines and/or as a result of blocking adenosine-mediated stimulation of peripheral glucose uptake.

RESEARCH DESIGN AND METHODS

Hyperinsulinemic-euglycemic glucose clamps were used to assess insulin sensitivity. Caffeine or placebo was administered intravenously to 12 healthy volunteers in a randomized, double-blind, crossover design. Measurements included plasma levels of insulin, catecholamines, free fatty acids (FFAs), and hemodynamic parameters. Insulin sensitivity was calculated as whole-body glucose uptake corrected for the insulin concentration. In a second study, the adenosine reuptake inhibitor dipyridamole was tested using an identical protocol in 10 healthy subjects.

RESULTS

Caffeine decreased insulin sensitivity by 15% (P < 0.05 vs. placebo). After caffeine administration, plasma FFAs increased (P < 0.05) and remained higher than during placebo. Plasma epinephrine increased fivefold (P < 0.0005), and smaller increases were recorded in plasma norepinephrine (P < 0.02) and blood pressure (P < 0.001). Dipyridamole did not alter insulin sensitivity and only increased plasma norepinephrine (P < 0.01).

CONCLUSIONS

Caffeine can decrease insulin sensitivity in healthy humans, possibly as a result of elevated plasma epinephrine levels. Because dipyridamole did not affect glucose uptake, peripheral adenosine receptor antagonism does not appear to contribute to this effect.

Physiologic hyperinsulinemia enhances human skeletal muscle perfusion by capillary recruitment

Despite intensive study, the relation between insulin's action on blood flow and glucose metabolism remains unclear. Insulin-induced changes in microvascular perfusion, independent from effects on total blood flow, could be an important variable contributing to insulin's metabolic action. We hypothesized that modest, physiologic increments in plasma insulin concentration alter microvascular perfusion in human skeletal muscle and that these changes can be assessed using contrast-enhanced ultrasound (CEU), a validated method for quantifying flow by measurement of microvascular blood volume (MBV) and microvascular flow velocity (MFV). In the first protocol, 10 healthy, fasting adults received insulin (0.05 mU. kg(-1). min(-1)) via a brachial artery for 4 h under euglycemic conditions. At baseline and after insulin infusion, MBV and MFV were measured by CEU during continuous intravenous infusion of albumin microbubbles with intermittent harmonic ultrasound imaging of the forearm deep flexor muscles. In the second protocol, 17 healthy, fasting adults received a 4-h infusion of either insulin (0.1 mU. kg(-1). min(-1), n = 9) or saline (n = 8) via a brachial artery. Microvascular volume was assessed in these subjects by an alternate CEU technique using an intra-arterial bolus injection of albumin microbubbles at baseline and after the 4-h infusion. With both protocols, muscle glucose uptake, plasma insulin concentration, and total blood flow to the forearm were measured at each stage. In protocol 2 subjects, tissue extraction of 1-methylxanthine (1-MX) was measured as an index of perfused capillary volume. Caffeine, which produces 1-MX as a metabolite, was administered to these subjects before the study to raise plasma 1-MX levels. In protocol 1 subjects, insulin increased muscle glucose uptake (180%, P < 0.05) and MBV (54%, P < 0.01) and decreased MFV (-42%, P = 0.07) in the absence of significant changes in total forearm blood flow. In protocol 2 subjects, insulin increased glucose uptake (220%, P < 0.01) and microvascular volume (45%, P < 0.05) with an associated moderate increase in total forearm blood flow (P < 0.05). Using forearm 1-MX extraction, we observed a trend, though not significant, toward increasing capillary volume in the insulin-treated subjects. In conclusion, modest physiologic increments in plasma insulin concentration increased microvascular blood volume, indicating altered microvascular perfusion consistent with a mechanism of capillary recruitment. The increases in microvascular (capillary) volume (despite unchanged total blood flow) indicate that the relation between insulin's vascular and metabolic actions cannot be fully understood using measurements of bulk blood flow alone.

Long-term oral L-arginine administration improves peripheral and hepatic insulin sensitivity in type 2 diabetic patients

The aim of this study was to evaluate whether long-term administration of arginine acting through a normalization of NO/cyclic-guanosine-3' 5'-cyclic monophosphate (cGMP) pathway was able to ameliorate peripheral and hepatic insulin sensitivity in 12 lean type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

A double-blind study was performed for 3 months. In the first month, patients were treated with their usual diet. Then they were randomly allocated into to groups. In group 1, patients were treated with diet plus placebo (orally three times per day) for 2 months. In group 2 patients were treated for 1 month with diet plus placebo orally, three times per day) and then for 1 month with diet plus L-arginine (3 g three times per day). At the end of the first and the second month of therapy, patients underwent a euglycemic-hyperinsulinemic clamp combined with [6,6-2H2] glucose infusion. A total of 10 normal subjects underwent the same test as control subjects.

RESULTS

In group 1, no changes in basal cGMP levels, systolic blood pressure, forearm blood flow, glucose disposal, and endogenous glucose production were observed throughout. In group 2, L-arginine normalized basal cGMP levels and significantly increased forearm blood flow by 36% and glucose disposal during the clamp by 34% whereas it decreased systolic blood pressure and endogenous glucose production by 14 and 29%, respectively. However, compared with normal subjects, L-arginine treatment was not able to completely overcome the defect in glucose disposal.

CONCLUSIONS

L-Arginine treatment significantly improves but does not completely normalizc peripheral and hepatic insulin sensitivity in type 2 diabetic patients.

The potential role of adenosine in the pathophysiology of the insulin resistance syndrome

An increased intracellular availability of the co-enzyme A esters of long-chain fatty acids is thought to underlie many aspects of the insulin resistance syndrome. However, the cause of clustering of a hyperdynamic circulation, sympathetic activation, hypertension, hyperuricaemia, and a raised haematocrit in the insulin resistance syndrome remains to be elucidated. We propose a mechanism that expands the etiological role of long-chain fatty acids. By inhibiting adenine nucleotide translocators, elevated intracellular concentrations of the co-enzyme A esters of long-chain fatty acids impair mitochondrial oxidative phosphorylation. This is expected to result in a chronic systemic increase in extracellular adenosine concentrations. As adenosine stimulates the sympathetic nervous system, induces systemic vasodilatation, stimulates erythropoiesis, and induces renal vasoconstriction with renal sodium retention, increased extracellular ADO concentrations may be the common denominator explaining the above-mentioned and still unexplained phenomena associated with the insulin resistance syndrome. Along the same lines, hyperuricaemia can be explained by the fact that adenosine is broken down to urate and because of increased renal urate retention.

Effect of vitamin C on forearm blood flow and glucose metabolism in essential hypertension

In 9 patients with essential hypertension, we tested whether a high-dose (12 mg. min(-1)) vitamin C infusion into the brachial artery, by improving endothelium-dependent vasodilatation, would also attenuate the insulin resistance of deep forearm tissues. We measured the effect of vitamin C on acetylcholine (Ach)-induced vasodilatation and on forearm glucose uptake during systemic hyperinsulinemia; in all studies, the contralateral forearm served as the control. Intrabrachial Ach infusion produced a stable increase in forearm blood flow, from 2.6+/-0.3 to 10.6+/-2.1 mL. min(-1). dL(-1); when vitamin C was added, a further rise in forearm blood flow (to 13.4 mL. min(-1). dL(-1); P<0.03 vs Ach alone) was observed. In response to insulin, blood flow in both the infused and control forearms did not significantly change from baseline values (+10+/-16% and +2+/-11%, respectively). In contrast, when vitamin C was added, blood flow in the infused forearm increased significantly (to 3.7+/-0.7 mL. min(-1). dL(-1); P<0.02 vs 2.8+/-0.6 mL. min(-1). dL(-1) in the control forearm). Insulin stimulated whole-body glucose disposal to 20+/-2 micromol. min(-1). kg(-1), compatible with the presence of marked insulin resistance. Forearm glucose uptake was similarly stimulated after 80 minutes of insulin infusion (to 2.11+/-0.42 and 2.06+/-0.43 micromol. min(-1). dL(-1), infused and control, respectively). When intrabrachial vitamin C was added, no difference in glucose uptake was observed between the 2 forearms (infused, 2.37+/-0.44 micromol. min(-1). dL(-1)and control, 2.36+/-0. 53 micromol. min(-1). dL(-1)). Forearm O(2) uptake at baseline was also similar in the 2 forearms (infused, 9.7+/-0.7 micromol. min(-1). dL(-1) and control, 9.6+/-1.1 micromol. min(-1). dL(-1)) and was not changed by either insulin or vitamin C. We conclude that in the deep forearm tissues of patients with essential hypertension and insulin resistance, an acute improvement in endothelial function, obtained with pharmacological doses of vitamin C, restores insulin-mediated vasodilatation but does not improve insulin-mediated glucose uptake. Thus, the endothelial dysfunction of essential hypertension is unlikely to be responsible for their metabolic insulin resistance.

Cellular compartmentalization in insulin action: altered signaling by a lipid-modified IRS-1

While most receptor tyrosine kinases signal by recruiting SH2 proteins directly to phosphorylation sites on their plasma membrane receptor, the insulin receptor phosphorylates intermediary IRS proteins that are distributed between the cytoplasm and a state of loose association with intracellular membranes. To determine the importance of this distribution to IRS-1-mediated signaling, we constructed a prenylated, constitutively membrane-bound IRS-1 by adding the COOH-terminal 9 amino acids from p21(ras), including the CAAX motif, to IRS-1 (IRS-CAAX) and analyzed its function in 32D cells expressing the insulin receptor. IRS-CAAX migrated more slowly on sodium dodecyl sulfate-polyacrylamide gel electrophoresis than did IRS-1 and demonstrated increased levels of serine/threonine phosphorylation. Insulin-stimulated tyrosyl phosphorylation of IRS-CAAX was slightly decreased, while IRS-CAAX-mediated phosphatidylinositol 3'-kinase (PI3'-kinase) binding and activation were decreased by approximately 75% compared to those for wild-type IRS-1. Similarly, expression of IRS-CAAX desensitized insulin-stimulated [(3)H]thymidine incorporation into DNA by about an order of magnitude compared to IRS-1. By contrast, IRS-CAAX-expressing cells demonstrated increased signaling by mitogen-activated protein kinase, Akt, and p70(S6) kinase in response to insulin. Hence, tight association with the membrane increased IRS-1 serine phosphorylation and reduced coupling between the insulin receptor, PI3'-kinase, and proliferative signaling while enhancing other signaling pathways. Thus, the correct distribution of IRS-1 between the cytoplasm and membrane compartments is critical to the normal balance in the network of insulin signaling.

Sodium nitroprusside increases human skeletal muscle blood flow, but does not change flow distribution or glucose uptake

1. The role of blood flow as a determinant of skeletal muscle glucose uptake is at present controversial and results of previous studies are confounded by possible direct effects of vasoactive agents on glucose uptake. Since increase in muscle blood flow can be due to increased flow velocity or recruitment of new capillaries, or both, it would be ideal to determine whether the vasoactive agent affects flow distribution or only increases the mean flow. 2. In the present study blood flow, flow distribution and glucose uptake were measured simultaneously in both legs of 10 healthy men (aged 29 +/- 1 years, body mass index 24 +/- 1 kg m-2) using positron emission tomography (PET) combined with [15O]H2O and [18F]fluoro-2-deoxy-D-glucose (FDG). The role of blood flow in muscle glucose uptake was studied by increasing blood flow in one leg with sodium nitroprusside (SNP) and measuring glucose uptake simultaneously in both legs during euglycaemic hyperinsulinaemia (insulin infusion 6 pmol kg-1 min-1). 3. SNP infusion increased skeletal muscle blood flow by 86 % (P < 0.01), but skeletal muscle flow distribution and insulin-stimulated glucose uptake (61.4 +/- 7. 5 vs. 67.0 +/- 7.5 micromol kg-1 min-1, control vs. SNP infused leg, not significant), as well as flow distribution between different tissues of the femoral region, remained unchanged. The effect of SNP infusion on blood flow and distribution were unchanged during infusion of physiological levels of insulin (duration, 150 min). 4. Despite a significant increase in mean blood flow induced by an intra-arterial infusion of SNP, glucose uptake and flow distribution remained unchanged in resting muscles of healthy subjects. These findings suggest that SNP, an endothelium-independent vasodilator, increases non-nutritive, but not nutritive flow or capillary recruitment.

Microalbuminuria in essential hypertension: significance, pathophysiology, and therapeutic implications

Some patients with essential hypertension manifest greater than normal urinary albumin excretion (UAE). The significance of this association, which is the object of this review, is not well established. Hypertensive patients with microalbuminuria manifest greater levels of blood pressure, particularly at night, and higher serum levels of cholesterol, triglycerides, and uric acid than patients with normal UAE. Levels of high-density lipoprotein cholesterol, on the other hand, were lower in patients with microalbuminuria than in those with normal UAE. Patients with microalbuminuria manifested greater incidence of insulin resistance and thicker carotid arteries than patients with normal UAE. After a follow-up of 7 years, we observed that 12 cardiovascular events occurred among 54 (21.3%) patients with microalbuminuria and only two such events among 87 patients with normal UAE (P < 0.0002). Stepwise logistic regression analysis showed that UAE, cholesterol level, and diastolic blood pressure were independent predictors of the cardiovascular outcome. Rate of creatinine clearance from patients with microalbuminuria decreased more than that from those with normal UAE. In conclusion, these studies suggest that hypertensive individuals with microalbuminuria manifest a variety of biochemical and hormonal derangements with pathogenic potential, which results in hypertensive patients having a greater incidence of cardiovascular events and a greater decline in renal function than patients with normal UAE.

Insulin: new roles for an ancient hormone

Recent research has greatly expanded the domain of insulin action. The classical action of insulin is the control of glucose metabolism through the dual feedback loop linking plasma insulin with plasma glucose concentrations. This canon has been revised to incorporate the impact of insulin resistance or insulin deficiency, both of which alter glucose homeostasis through maladaptive responses (namely, chronic hyperinsulinaemia and glucose toxicity). A large body of knowledge is available on the physiology, cellular biology and molecular genetics of insulin action on glucose production and uptake. More recently, a number of newer actions of insulin have been delineated from in vitro and in vivo studies. In sensitive individuals, insulin inhibits lipolysis and platelet aggregation. In the presence of insulin resistance, dyslipidaemia, hyper-aggregation and anti-fibrinolysis may create a pro-thrombotic milieu. Preliminary evidence indicates that hyperinsulinaemia per se may be pro-oxidant both in vitro and in vivo. Insulin plays a role in mediating diet-induced thermogenesis, and insulin resistance may therefore be implicated in the defective thermogenesis of diabetes. In the kidney, insulin spares sodium and uric acid from excretion; in chronic hyperinsulinaemic states, these effects may contribute to high blood pressure and hyperuricaemia. Insulin hyperpolarises the plasma membranes of both excitable and non-excitable tissues, with consequences ranging from baroreceptor desensitisation to cardiac refractoriness (prolongation of QT interval). Under some circumstances insulin is vasodilatory-the mechanism involving both the sodium-potassium pump and intracellular calcium transients. Finally, by crossing the blood-brain barrier insulin exerts a host a central effects (sympatho-excitation, vagal withdrawal, stimulation of corticotropin releasing factor), collectively resembling a stress reaction. Description and understanding of these new roles, their interactions, the interplay between insulin resistance and hyperinsulinaemia, and their implications for cardiovascular disease have only begun.

No role of interstitial adenosine in insulin-mediated vasodilation

The mechanisms behind the vasodilatory effect of insulin are not fully understood, but nitric oxide plays an important role. We have investigated the possibility that insulin mediates vasodilatation in the human skeletal muscle via an increase in extracellular adenosine concentrations. In eight healthy subjects (H) and in four subjects with a complete, high (C5-C6/7) spinal cord injury (SCI) a hyperinsulinaemic (480 mU min-1 kg-1), isoglycaemic clamp was performed. SCI subjects were included as it has been proposed that adenosine and adenine nucleotides may be released from nerve endings in the skeletal muscle. Adenosine concentrations in the extracellular fluid (ECF) of skeletal muscle in the thigh were measured by means of the microdialysis technique. Leg blood flow (LBF) was measured by termodilution. In response to insulin infusion, LBF always increased (P < 0.05) (from 228 +/- 25 and 318 +/- 18 mL min-1 to 451 +/- 41 and 530 +/- 29 mL min-1, SCI and H, respectively [mean +/- SEM]). Concentrations of adenosine in the muscle ECF did not change with infusion of insulin and did not differ between groups (before: 147 +/- 55 [SCI] and 207 +/- 108 [H] nmol L-1; during: 160 +/- 36 [SCI] and 165 +/- 74 [H] nmol L-1). No significant correlation between concentrations of adenosine and corresponding LBF rates was achieved (LBF=[-0.0936. Adenosine] + 475. R=-0.092, P=0.22, number of samples=181, number of subjects=12).

CONCLUSION

the mechanism by which insulin mediates an increase in skeletal muscle blood flow is not associated with adenosine in the ECF.

The role of adenosine in insulin-induced vasodilation

It was previously shown that systemic hyperinsulinemia induces vasodilation in human skeletal muscle. The mechanism mediating this vasodilation is not yet completely clarified. Based on data from animal experiments, we hypothesized that stimulation of the adenosine receptor is involved in insulin-induced vasodilation. To test this hypothesis, a 105-min hyperinsulinemic euglycemic clamp was performed in three groups of eight healthy volunteers. In group 1, placebo was infused into the left brachial artery (experimental forearm). In the second and third group, respectively, draflazine (an adenosine-uptake blocker) and theophylline (an adenosine-receptor antagonist) were administered by intrabrachial infusion. Forearm blood flow (FBF) was measured by venous-occlusion plethysmography, both at the experimental and the control forearms. The percentage decrease in flow ratio (FBF experimental arm/control arm) in the draflazine group was significantly less pronounced than that in the placebo group, whereas the percentage decrease in flow ratio was larger in the theophylline group. These results demonstrate that the insulin-induced increase in blood flow in the experimental arm was more pronounced at the site of adenosine-uptake blockade by draflazine, whereas this was reduced during adenosine-receptor antagonism by theophylline. Our observations are compatible with the hypothesis that insulin-induced vasodilation is mediated by the release of adenosine.

Insulin, cation metabolism and insulin resistance

There is considerable evidence that insulin and insulin-like growth factors regulate a number of important physiological functions in a variety of tissues, some not considered to be classically insulin sensitive. Impaired biological responses to insulin and related insulin-like growth factors are referred to as insulin resistance. Persons with insulin resistance often display clinical abnormalities other than impaired glucose tolerance, including central obesity, hypertension, dyslipidemia, microalbuminuria, and abnormal coagulation and fibrinolytic systems. The mechanisms leading to development of insulin resistance are not fully understood. However, in addition to abnormalities of phosphorylation processes, it appears that alterations in cellular cation metabolism contribute to diminished cellular actions of insulin (i.e., glucose transport and hemodynamic actions). This review focuses on known cellular cation abnormalities and associated insulin resistance and cardiovascular disease.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

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.

Vasodilator response to systemic but not to local hyperinsulinemia in the human forearm

Insulin-mediated vasodilation has been proposed as an important determinant of whole-body insulin-stimulated glucose disposal. However, it is not clear whether the vasodilator effect of insulin results from a direct action of the hormone or whether alternative mechanisms are involved. To better characterize the mechanism of insulin-mediated vasorelaxation, we compared forearm blood flow (FBF) responses to local (intra-arterial) and systemic (intravenous, euglycemic clamp) hyperinsulinemia in 10 healthy lean subjects using venous occlusion plethysmography. In addition, we assessed the effect of nitric oxide (NO) synthase inhibition by NG-monomethyl-L-arginine (L-NMMA) on the vasodilator and metabolic responses to hyperinsulinemia. Similar forearm concentrations of insulin were achieved during local and systemic infusion (231+/-39 versus 265+/-22 microU/mL; P=0.54). Of note, FBF did not change significantly in response to local hyperinsulinemia (from 2.6+/-0.3 to 2.4+/-0.3 mL . min-1 . dL-1; P=0.50). In contrast, systemic hyperinsulinemia caused a 52% increase in FBF (from 2.5+/-0.2 to 3. 8+/-0.5 mL . min-1 . dL-1; P<0.004), which was reversed by L-NMMA (FBF decreased from 3.8+/-0.5 to 2.3+/-0.2 mL . min-1 . dL-1; P=0. 004). We conclude that systemic, but not local, hyperinsulinemia induces vasodilation in the forearm. Our findings suggest that insulin-mediated vasodilation is not due solely to a direct stimulatory effect of insulin but involves additional mechanisms activated only during systemic hyperinsulinemia.

Simultaneous gradient-echo/spin-echo EPI of graded ischemia in human skeletal muscle

The goal of this study was to evaluate the usefulness of blood oxygenation level-dependent (BOLD) methodologies to provide temporal and spatial information about skeletal muscle perfusion. A simultaneous gradient echo (GE) and spin-echo (SE) imaging sequence (GE/SE) with alternating TE was used to acquire images of leg skeletal muscle throughout a stepped reactive hyperemia paradigm. The change in both the GE and SE relaxation rates (deltaR2*, deltaR2) measured during ischemia and reactive hyperemia scaled with the duration of cuff inflation (the ischemic period) plateaued for cuff inflations lasting longer than 120 seconds and were greater in soleus muscle than in gastrocnemius. The ratio deltaR2*/deltaR2 was found to be less during the reactive hyperemia period relative to ischemia. Considering that a greater proportion of capillary vessels are perfused during reactive hyperemia than during ischemia, this finding suggests that magnetic susceptibility methodologies, with their dependence on compartment size, may provide a measure of the relative distribution of small and large vessels in skeletal muscle.

IGF-I increases forearm blood flow without increasing forearm glucose uptake

Decreased insulin-mediated muscle glucose uptake is a characteristic feature of non-insulin-dependent diabetes mellitus and other insulin-resistant states. It has been suggested that an impairment in the ability of insulin to augment limb blood flow, resulting in diminished glucose delivery to muscle, may contribute to this abnormality. In this study, we used human insulin-like growth factor (IGF) I in conjunction with the forearm balance technique to determine whether forearm glucose uptake could be stimulated by increasing blood flow without directly stimulating the intrinsic ability of the muscle to extract glucose. IGF-I was infused intra-arterially in healthy controls at a rate of either 0.4 microg . kg-1 . min-1 (high IGF) or 0.04 microg . kg-1 . min-1 (low IGF) for 140 min. With high IGF, forearm blood flow increased approximately twofold (34 +/- 3 vs. 64 +/- 8 ml . min-1 . l forearm volume-1, P < 0.01), and arteriovenous glucose concentration difference (a-v difference) increased modestly (0.19 +/- 0.05 vs. 0.31 +/- 0.08 mM, P = 0.32), resulting in an increased forearm glucose uptake (6.4 +/- 1.7 vs. 21.7 +/- 7.4 micromol . min-1 . l forearm volume-1, P = 0.09 vs. basal). With low IGF, forearm blood flow increased by 59% (29 +/- 4 vs. 46 +/- 9 ml . min-1 . l forearm volume-1, P < 0.05) and was associated with a proportional decrease in the a-v difference (0. 29 +/- 0.04 vs. 0.18 +/- 0.05 mM, P < 0.05). Forearm glucose uptake therefore was not significantly different from basal values (7.6 +/- 0.6 vs. 6.9 +/- 1.8 micromol . min-1 . kg-1). These data demonstrate that increasing blood flow without increasing the intrinsic ability of the muscle to extract glucose does not increase forearm muscle glucose uptake.

Role of tissue-specific blood flow and tissue recruitment in insulin-mediated glucose uptake of human skeletal muscle

BACKGROUND

Conflicting evidence exists concerning whether insulin-induced vasodilation plays a mechanistic role in the regulation of limb glucose uptake. It can be predicted that if insulin augments blood flow by causing tissue recruitment, this mechanism would enhance limb glucose uptake.

METHODS AND RESULTS

Twenty healthy subjects were studied with the forearm perfusion technique in combination with the euglycemic insulin clamp technique. Ten subjects were studied at physiological insulin concentrations (approximately 400 pmol/L) and the other 10 at supraphysiological insulin concentrations (approximately 5600 pmol/L). Four additional subjects underwent a saline control study. Pulse injections of a nonmetabolizable extracellular marker (1-[3H]-L-glucose) were administered into the brachial artery, and its washout curves were measured in one ipsilateral deep forearm vein and used to estimate the extracellular volume of distribution and hence the amount of muscle tissue drained by the deep forearm vein. Both during saline infusion and at physiological levels of hyperinsulinemia we observed no changes in blood flow and/or muscle tissue drained by the deep forearm vein. However, supraphysiological hyperinsulinemia accelerated total forearm blood flow (45.0+/-1.8 versus 36.5+/-1.3 mL x min(-1) x kg(-1), P<0.01) and increased the amount of muscle tissue drained by the deep forearm vein (305+/-46 versus 229+/-32 g, P<0.05). The amount of tissue newly recruited by insulin was strongly correlated to the concomitant increase in tissue glucose uptake (r=0.789, P<0.01).

CONCLUSIONS

Acceleration of forearm blood flow mediated by supraphysiological hyperinsulinemia is accompanied by tissue recruitment, which may be a relevant determinant of forearm (muscle) glucose uptake.

How to measure insulin sensitivity

Insulin resistance is common in the general population and tends to cluster with glucose intolerance, dyslipidaemia and high blood pressure. The importance of the insulin-resistant phenotype for the assessment of cardiovascular risk and response to intervention is increasingly being recognized. Therefore, there is a need for an accurate and reproducible method for measuring insulin resistance in vivo. The euglycaemic insulin clamp is currently the best available standard technique. It provides steady-state measures of insulin action and is easily combined with a number of other investigative methods (tracer dilution, limb catheterization, indirect calorimetry, positron emission tomography and nuclear magnetic resonance scans). Whereas homeostatic model assessment uses fasting plasma glucose and insulin concentrations to derive indices of insulin sensitivity and secretion from a mathematical model, other techniques are based on the exogenous infusion of glucose or insulin, or both, either under steady-state (the insulin suppression test) or under dynamic conditions (insulin tolerance test, intravenous glucose-tolerance test with minimal model analysis, and constant infusion of glucose with model assessment). This article recalls the principles of insulin action, with special reference to the concept of clearance and the equivalence of different approaches to estimating this function. Merits and disadvantages of the various techniques are then concisely reviewed, with emphasis on their relative feasibilities and reliabilities. Recent developments and future trends are mentioned. Criteria for choice and some reference data are given to aid the clinical investigator.

Minimal influence of blood flow on interstitial glucose and lactate-normal and insulin-resistant muscle

To study the regulation of the interstitial glucose concentration in skeletal muscle, nine control subjects and nine older and overweight non-insulin-dependent diabetes mellitus (NIDDM) subjects with extreme insulin resistance were investigated with microdialysis in the medial femoral muscle before and during a euglycemic insulin clamp. After an overnight fast, arterial plasma glucose concentration was 4.9 +/- 0.1 and 8.5 +/- 0.6 mmol/l (P < 0.001), respectively. The arterial-interstitial concentration ([a-i]) differences of glucose and lactate were 0.43 +/- 0.16 (P < 0.05) and -0.13 +/- 0.05 mmol/l, respectively, in normal subjects. In NIDDM subjects, [a-i] differences for glucose and lactate were nonsignificant. Muscle blood flow was similar in controls and NIDDM subjects. During the glucose clamp, the glucose [a-i] differences increased and the lactate [a-i] differences decreased significantly in both groups. The glucose 170 infusion rate was 8.0 +/- 0.77 vs. 3.2 +/- 0.51 mg.kg-1.min-1 (P < 0.001), and blood flow was 9.9 +/- 1.6 vs. 6.7 +/- 0.9 ml.100 g-1.min-1 (P < 0.05) in controls and NIDDM subjects, respectively. These results show that 1) the capillary wall is rate limiting for muscle glucose uptake and lactate release in control subjects but not in postabsorptive hyperglycemic insulin-resistant subjects, 2) vasodilation during insulin infusion does not prevent the increase in [a-i] difference of glucose in normal subjects, and 3) in severely insulin-resistant muscle, the [a-i] difference of glucose is not extended despite lack of vasodilation.

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.