Blood flow and muscle metabolism: a focus on insulin action

Metabolic disturbances linked to obesity: the role of impaired tissue perfusion

Associated with elevated risk of cardiovascular events and cancer, obesity is a worldwide problem affecting developed and developing countries. Microcirculatory vessels, represented by arterioles, capillaries and venules (mean internal diameter < 100 microm), are the place where blood/tissue nutrition and exchange effectively take place. Microvascular dysfunction is an early event in obesity probably secondary to endothelial dysfunction and capillaries rarefaction. New research techniques allow the investigation of the microcirculation in different vascular beds in humans. Studies suggest a link between endothelial dysfunction and visceral obesity. Oxidative stress, inflammation and renin-angiotensin system are among factors considered to be involved on microvascular dysfunction in obesity. Microcirculatory impairment present in obesity suggests that it could be an important causal factor in obesity-related disorders such as insulin resistance and hypertension.

Activation of AMP-activated protein kinase by 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside in the muscle microcirculation increases nitric oxide synthesis and microvascular perfusion

OBJECTIVE

To investigate the effects of activation of the AMP-activated protein kinase (AMPK) on muscle perfusion and to elucidate the mechanisms involved.

METHODS AND RESULTS

In a combined approach, we studied the vasoactive actions of AMPK activator by 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) on rat cremaster muscle resistance arteries ( approximately 100 mum) ex vivo and on microvascular perfusion in the rat hindlimb in vivo. In isolated resistance arteries, AICAR increased Thr172 phosphorylation of AMPK in arteriolar endothelium, which was predominantly located in microvascular endothelium. AICAR induced vasodilation (19+/-4% at 2 mmol/L, P<0.01), which was abolished by endothelium removal, inhibition of NO synthase (with N-nitro-L-arginine), or AMPK (with compound C). Smooth muscle sensitivity to NO, determined by studying the effects of the NO donor S-nitroso-N-acetylpenicillamine (SNAP), was not affected by AICAR except at the highest dose. AICAR increased endothelial nitric oxide synthase activity, as indicated by Ser1177 phosphorylation. In vivo, infusion of AICAR markedly increased muscle microvascular blood volume (approximately 60%, P<0.05), as was evidenced by contrast-enhanced ultrasound, without effects on blood pressure, femoral blood flow, or hind leg glucose uptake.

CONCLUSIONS

Activation of AMPK by AICAR activates endothelial nitric oxide synthase in arteriolar endothelium by increasing its Ser1177 phosphorylation, which leads to vasodilation of resistance arteries and recruitment of microvascular perfusion in muscle.

Diet-induced obesity prevents interstitial dispersion of insulin in skeletal muscle

OBJECTIVE

Obesity causes insulin resistance, which has been interpreted as reduced downstream insulin signaling. However, changes in access of insulin to sensitive tissues such as skeletal muscle may also play a role. Insulin injected directly into skeletal muscle diffuses rapidly through the interstitial space to cause glucose uptake. When insulin resistance is induced by exogenous lipid infusion, this interstitial diffusion process is curtailed. Thus, the possibility exists that hyperlipidemia, such as that seen during obesity, may inhibit insulin action to muscle cells and exacerbate insulin resistance. Here we asked whether interstitial insulin diffusion is reduced in physiological obesity induced by a high-fat diet (HFD).

RESEARCH DESIGN AND METHODS

Dogs were fed a regular diet (lean) or one supplemented with bacon grease for 9-12 weeks (HFD). Basal insulin (0.2 mU x min(-1) x kg(-1)) euglycemic clamps were performed on fat-fed animals (n = 6). During clamps performed under anesthesia, five sequential doses of insulin were injected into the vastus medialis of one hind limb (INJ); the contralateral limb (NINJ) served as a control.

RESULTS

INJ lymph insulin showed an increase above NINJ in lean animals, but no change in HFD-fed animals. Muscle glucose uptake observed in lean animals did not occur in HFD-fed animals.

CONCLUSIONS

Insulin resistance induced by HFD caused a failure of intramuscularly injected insulin to diffuse through the interstitial space and failure to cause glucose uptake, compared with normal animals. High-fat feeding prevents the appearance of injected insulin in the interstitial space, thus reducing binding to skeletal muscle cells and glucose uptake.

Muscle contraction, but not insulin, increases microvascular blood volume in the presence of free fatty acid-induced insulin resistance

OBJECTIVE

Insulin and contraction each increase muscle microvascular blood volume (MBV) and glucose uptake. Inhibiting nitric oxide synthase blocks insulin's but not contraction's effects. We examined whether contraction could augment the MBV increase seen with physiologic hyperinsulinemia and whether free fatty acid (FFA)-induced insulin resistance differentially affects contraction- versus insulin-mediated increases in MBV.

RESEARCH DESIGN AND METHODS

Rats were fasted overnight. Plasma FFAs were increased by intralipid/heparin infusion (3 h), insulin was increased with a euglycemic clamp (3 mU x min(-1) x kg(-1)), and hindlimb muscle contraction was electrically stimulated. Muscle MBV was measured using contrast-enhanced ultrasound. Insulin transport into muscle was measured using (125)I-insulin. BQ-123 (0.4 mg/h) was used to block the endothelin-1 (ET-1) receptor A.

RESULTS

Superimposing contraction on physiologic hyperinsulinemia increased MBV within 10 min by 37 and 67% for 0.1 or 1 Hz, respectively (P < 0.01). FFA elevation alone did not affect MBV, whereas 0.1 Hz stimulation doubled MBV (P < 0.05) and increased muscle insulin uptake (P < 0.05) despite high FFA. Physiologic hyperinsulinemia during FFA elevation paradoxically decreased MBV (P < 0.05). This MBV decrease was reversed by either 0.1 Hz contraction or ET-1 receptor A antagonism, and the combination raised MBV above basal.

CONCLUSIONS

Contraction recruits microvasculature beyond that seen with physiologic hyperinsulinemia by a distinct mechanism that is not blocked by FFA-induced vascular insulin resistance. The paradoxical MBV decline seen with insulin plus FFA may result from differential inhibition of insulin-stimulated nitric oxide-dependent vasodilation relative to ET-1 vasoconstriction. Our results implicate ET-1 as a potential mediator of FFA-induced vascular insulin resistance.

Skeletal muscle microvascular exchange capacity is associated with hyperglycaemia in subjects with central obesity

AIMS

Poor glycaemic control is associated with increased risk of microvascular disease in various organs including the eye and kidney, but the relationship between glycated haemoglobin (HbA(1c)) and microvascular function in skeletal muscle has not been described. We tested the association between HbA(1c) and a measure of microvascular exchange capacity (K(f)) in skeletal muscle in people with central obesity at risk of developing Type 2 diabetes.

METHODS

Microvascular function was measured in 28 women and 19 men [mean (+/- sd) age 51 +/- 9 years] with central obesity who did not have diabetes. We estimated insulin sensitivity by hyperinsulinaemic-euglycaemic clamp, visceral and total fatness by magnetic resonance imaging, fitness (VO(2) max by treadmill testing), physical activity energy expenditure [metabolic equivalents of tasks (METS) by use of the SenseWear Pro armband] and skeletal muscle microvascular exchange capacity (K(f)) by venous occlusion plethysmography.

RESULTS

In regression modelling, age, sex and fasting plasma glucose accounted for 30.5% of the variance in HbA(1c) (r(2) = 0.31, P = 0.001). Adding K(f) to this model explained an additional 26.5% of the variance in HbA(1c) (r(2) = 0.57, P = 0.0001 and K(f) was strongly and independently associated with HbA(1c) (standardized B coefficient -0.45 (95% confidence interval -0.19, -0.06), P = 0.001).

CONCLUSIONS

We found a strong negative independent association between a measure of skeletal muscle microvascular exchange capacity (K(f)) and HbA(1c). K(f) was associated with almost as much of the variance in HbA(1c) as fasting plasma glucose.

Microvascular dysfunction: a direct link among BMI, waist circumference and glucose homeostasis in young overweight/obese normoglycemic women?

OBJECTIVE

Capillary recruitment is impaired in obesity (OB), possibly worsening glucose and insulin availability to target organs. In this study, we investigated whether functional microvascular parameters were correlated with clinical-anthropometrical data and whether these parameters would influence OB-related metabolic disorders, especially glucose homeostasis, in young overweight (OW)/obese women.

DESIGN

Cross-sectional clinical study of microvascular reactivity in young OW/obese women.

SUBJECTS AND METHODS

A total of 10 lean (23.1 + or - 3.2 years, body mass index (BMI) 22.3 + or - 1.6 kg m(-2)) and 42 OW/obese (24.9 + or - 3.5 years; BMI 34.5 + or - 5.7 (25.7-46.5) kg m(-2)) sedentary non-smoking women were evaluated. Lipid profile, fasting plasma glucose (PG), post-load PG (75 g-2 h), insulin, C-reactive protein, HOMA-IR (homeostasis model assessment for insulin resistance) index and anthropometric variables (weight, BMI, waist and hip circumferences, waist-to-hip ratio and blood pressure (BP)) were determined. Functional microvascular parameters (functional capillary density, red blood cell velocity at baseline and peak (RBCV(max)), and time taken to reach RBCV(max) (TRBCV(max)) during post-occlusive reactive hyperemia after 1 min arterial occlusion) were evaluated by nailfold videocapillaroscopy.

RESULTS

The time taken to reach RBCV(max) was significantly longer in OW/obese patients compared with control subjects (8.6 + or - 2.4 versus 5.7 + or - 1.1 s, P<0.001), and its delay was directly associated with adiposity levels, systolic BP and insulin resistance, and inversely related to high-density lipoprotein-cholesterol. Post-load PG could be correlated with TRBCV(max) (R = 0.48, P<0.05) and RBCV(max) (R = -0.29, P<0.05), and it was influenced by weight, waist circumference and TRBCV(max) (adjusted R(2) = 24%) as well.

CONCLUSIONS

In the investigated group of young OW/obese women, the direct correlation between post-load PG and TRBCV(max) links microvascular parameters with metabolic variables and suggests a key role for microcirculation in OB-related metabolic disorders.

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.

Vascular actions of insulin with implications for endothelial dysfunction

Hemodynamic actions of insulin depend largely on the hormone's ability to stimulate synthesis and release of endothelial mediators, whose balanced activity ensures dynamic control of vascular function. Nitric oxide (NO), endothelin-1 (ET-1), and reactive oxygen species (ROS) are important examples of endothelial mediators with opposing properties on vascular tone, hemostatic processes, and vascular permeability. Reduced NO bioavailability, resulting from either insufficient production or increased degradation of NO, characterizes endothelial dysfunction. In turn, endothelial dysfunction predicts vascular complications of metabolic and hemodynamic disorders. In the cardiovascular system, insulin stimulates the production and release of NO, ET-1, and ROS via activation of distinct intracellular signaling pathways. Under insulin-resistant conditions, increased insulin concentrations and/or impaired insulin-signaling pathways in the vasculature may contribute to imbalance in secretion of endothelial mediators that promote pathogenesis of vascular abnormalities. This short review describes signaling pathways involved in insulin-stimulated release of NO, ROS, and ET-1 and suggests possible molecular mechanisms by which abnormal insulin signaling may contribute to endothelial dysfunction.

Experimental hyperlipidemia dramatically reduces access of insulin to canine skeletal muscle

A complex sequence of steps is required for insulin to cause glucose uptake. Impairment of any one of these steps can contribute to insulin resistance. We observed the effect of insulin resistance induced by hyperlipidemia on the dynamics of insulin injected into skeletal muscle. Basal insulin euglycemic clamps (0.2 mU/min/kg) with or without lipid infusions (20% at 1.5 ml/min) were done on anesthetized dogs. Sequential insulin doses were administered by intramuscular injection directly into the vastus medialis of one hindlimb, using the contralateral leg for comparison. Intramuscular insulin injection in normal animals caused a clear dose-dependent increment in interstitial insulin levels, as well as dose-dependent increase in leg glucose uptake. In a second group of animals, lipid was infused before and during intramuscular insulin injection to cause systemic increase in free fatty acids (FFAs). In sharp contrast, systemic lipid infusion caused insulin resistance, indicated by reduced glucose infusion required to maintain euglycemia, and prevented injection-induced increase in lymphatic insulin and leg glucose uptake observed without lipid. The injected insulin was instead detected in the venous outflow from the leg. Lipid infusion caused intramuscular insulin to be diverted from interstitium into the capillary circulation, preventing a rise in interstitial insulin and any increase in local leg glucose uptake. The diversion of insulin from the interstitium under hyperlipidemic conditions may play a role in the insulin resistance observed coincident with elevated nocturnal FFAs as is observed in obesity.

COMP-angiopoietin-1 enhances skeletal muscle blood flow and insulin sensitivity in mice

To test whether chronic enhanced blood flow alters insulin-stimulated glucose uptake, we measured skeletal muscle glucose uptake in chow-fed and high-fat-fed mice injected with adenovirus containing modified angiopoietin-1, COMP-Ang1, via euglycemic-hyperinsulinemic clamp. Blood flow rates and platelet endothelial cell adhesion molecule-1 positive endothelial cells in the hindlimb skeletal muscle were elevated in COMP-Ang1 compared with control LacZ. Whole body glucose uptake and whole body glycogen/lipid synthesis were elevated in COMP-Ang1 compared with LacZ in chow diet. High-fat diet significantly reduced whole body glucose uptake and whole body glycolysis in LacZ mice, whereas high-fat-fed COMP-Ang1 showed a level of whole body glucose uptake that was comparable with chow-fed LacZ and showed increased glucose uptake compared with high-fat-fed LacZ. Glucose uptake and glycolysis in gastrocnemius muscle of chow-fed COMP-Ang1 were increased compared with chow-fed LacZ. High-fat diet-induced whole body insulin resistance in the LacZ was mostly due to approximately 40% decrease in insulin-stimulated glucose uptake in skeletal muscle. In contrast, COMP-Ang1 prevented diet-induced skeletal muscle insulin resistance compared with high-fat-fed LacZ. Akt phosphorylation in skeletal muscle was increased in COMP-Ang1 compared with LacZ in both chow-fed and high-fat-fed groups. These results suggest that increased blood flow by COMP-Ang1 increases insulin-stimulated glucose uptake and prevents high-fat diet-induced insulin resistance in skeletal muscle.

Interleukin-6 attenuates insulin-mediated increases in endothelial cell signaling but augments skeletal muscle insulin action via differential effects on tumor necrosis factor-alpha expression

OBJECTIVE

The cytokine interleukin-6 (IL-6) stimulates AMP-activated protein kinase (AMPK) and insulin signaling in skeletal muscle, both of which result in the activation of endothelial nitric oxide synthase (eNOS). We hypothesized that IL-6 promotes endothelial cell signaling and capillary recruitment in vivo, contributing to increased glucose uptake.

RESEARCH DESIGN AND METHODS

The effect of IL-6 with and without insulin on AMPK, insulin, and eNOS signaling in and nitric oxide (NO) release from human aortic endothelial cells (HAECs) was examined. The physiological significance of these in vitro signaling events was assessed by measuring capillary recruitment in rats during control and euglycemic-hyperinsulinemic clamps with or without IL-6 infusion.

RESULTS

IL-6 blunted increases in insulin signaling, eNOS phosphorylation (Ser1177), and NO production and reduced phosphorylation of AMPK in HAEC in vitro and capillary recruitment in vivo. In contrast, IL-6 increased Akt phosphorylation (Ser473) in hindlimb skeletal muscle and enhanced whole-body glucose disappearance and glucose uptake during the clamp. The differences in endothelial cell and skeletal muscle signaling were mediated by the cell-specific, additive effects of IL-6 and insulin because this treatment markedly increased tumor necrosis factor (TNF)-alpha protein expression in HAECs without any effect on TNF-alpha in skeletal muscle. When HAECs were incubated with a TNF-alpha-neutralizing antibody, the negative effects of IL-6 on eNOS signaling were abolished.

CONCLUSIONS

In the presence of insulin, IL-6 contributes to aberrant endothelial cell signaling because of increased TNF-alpha expression.

Muscle microvascular dysfunction in central obesity is related to muscle insulin insensitivity but is not reversed by high-dose statin treatment

OBJECTIVE

To test the hypotheses that decreased insulin-mediated glucose disposal in muscle is associated with a reduced muscle microvascular exchange capacity (Kf) and that 6 months of high-dose statin therapy would improve microvascular function in people with central obesity.

RESEARCH DESIGN AND METHODS

We assessed skeletal muscle microvascular function, visceral fat mass, physical activity levels, fitness, and insulin sensitivity in skeletal muscle in 22 female and 17 male volunteers with central obesity whose age (mean +/- SD) was 51 +/- 9 years. We tested the effect of atorvastatin (40 mg daily) on muscle microvascular function in a randomized, double-blind, placebo-controlled trial lasting 6 months.

RESULTS

Kf was negatively associated with a measure of glycemia (A1C; r = -0.44, P = 0.006) and positively associated with insulin sensitivity (the ratio of insulin-stimulated glucose effectiveness, or M value, to the mean insulin concentration, or I value; r = 0.39, P = 0.02). In regression modeling, A1C, visceral fat mass, and M:I explained 38% of the variance in Kf (in a linear regression model with Kf as the outcome [R2 = 0.38, P = 0.005]). M:I was associated with Kf independently of visceral fat mass (B coefficient 3.13 [95% CI 0.22-6.02], P = 0.036). Although 6 months' treatment with atorvastatin decreased LDL cholesterol by 51% (P < 0.001) and plasma high-sensitivity C-reactive protein by 75% (P = 0.02), microvascular function was unchanged.

CONCLUSIONS

Decreased insulin-mediated glucose uptake in skeletal muscle is associated with impaired muscle microvascular exchange capacity (Kf), independently of visceral fat mass. Muscle microvascular function is not improved by 6 months of high-dose statin treatment, despite marked statin-mediated improvements in lipid metabolism and decreased inflammation.

Vascular insulin resistance: a potential link between cardiovascular and metabolic diseases

The physiologic actions of insulin in the vasculature serve to couple regulation of metabolic and hemodynamic homeostasis. Insulin activation of the phosphatidylinositol-3-kinase (PI3K) pathway promotes glucose uptake in insulin-responsive tissues and nitric oxide (NO) production in the endothelium. NO induces vasodilation and inhibits platelet aggregation and vascular smooth muscle cell growth. In contrast, insulin activation of the mitogen-activated protein kinase (MAPK) leads to vasoconstriction and pathologic vascular cellular growth. In states of insulin resistance, insulin activation of PI3K is selectively impaired, whereas the MAPK pathway is spared and activated normally. In the endothelium, selective impairment of insulin-mediated NO production may contribute to the development of hypertension, endothelial dysfunction, atherogenesis, and insulin resistance. This article reviews experimental and clinical data elucidating the physiologic and pathophysiologic role of insulin in the vasculature and the mechanisms contributing to the development of vascular and metabolic diseases.

Four grams of glucose

Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle.

Ischaemia and insulin, but not ischaemia and contraction, act synergistically in stimulating muscle glucose uptake in vivo in humans

Ischaemia, like muscle contraction, has been reported to induce skeletal muscle glucose uptake in in vitro models. This stimulating effect appears independent of insulin and is probably mediated by activation of AMPK (AMP-activated protein kinase). In the present study, we hypothesized that in vivo in humans ischaemia- and insulin-induced glucose uptake are additive, and that the combined impact of ischaemia and contraction on glucose uptake is of a similar magnitude when each is applied separately. We assessed the effects of ischaemia with and without euglycaemic–hyperinsulinaemia (clamp; protocol 1) and with and without muscle contraction (protocol 2) on muscle FGU (forearm glucose uptake) in healthy subjects. Furthermore, we assessed the impact of ischaemia on FBF (forearm blood flow; plethysmography). In protocol 1, ischaemia increased FGU from 0.6±0.1 at baseline to 5.5±1.9 μmol·min−1·dl−1, and insulin increased FGU to 1.6±0.3 μmol·min−1·dl−1 (P<0.05 for both). The combination of ischaemia+insulin increased FGU to 15.5±2.2 μmol·min−1·dl−1 (P<0.05 compared with each stimulus alone). Maximal FBF obtained after ischaemia was similar with and without hyperinsulinaemia. In protocol 2, isometric contraction increased FGU from 0.3±0.1 to 2.7±0.8 μmol·min−1·dl−1 (P<0.05), but FGU was not significantly different from ischaemia compared with ischaemia+contraction. However, combined ischaemia+contraction resulted in a greater increase in FBF. In summary, ischaemia and insulin independently stimulate skeletal muscle glucose uptake in vivo in humans, whereas ischaemia and contraction do not. The observed differential effects of these stimuli on glucose uptake appear to be unrelated to changes in muscle blood flow.

Cocoa consumption for 2 wk enhances insulin-mediated vasodilatation without improving blood pressure or insulin resistance in essential hypertension

BACKGROUND

Essential hypertension is characterized by reciprocal relations between endothelial dysfunction and insulin resistance. Cocoa flavanols stimulate production of the vasodilator nitric oxide from vascular endothelium.

OBJECTIVE

The objective was to test the hypothesis that consumption of cocoa may simultaneously lower blood pressure, improve endothelial dysfunction, and ameliorate insulin resistance in subjects with essential hypertension.

DESIGN

We conducted a randomized, placebo-controlled, double-blind, crossover trial of a flavanol-rich cocoa drink (150 mL twice a day, approximately 900 mg flavanols/d) in individuals with essential hypertension (n = 20). Antihypertensive medications were discontinued before study enrollment. After a 7-d cocoa-free run-in period, cocoa or flavanol-poor placebo (approximately 28 mg flavanols/d) treatment for 2 wk was followed by a 1-wk washout and then crossover to the other treatment arm. Blood pressure was measured thrice weekly. At baseline and after each treatment period, we assessed insulin sensitivity (hyperinsulinemic-isoglycemic glucose clamp) and insulin-stimulated changes in brachial artery diameter and forearm skeletal muscle capillary recruitment (Doppler ultrasound with or without microbubble contrast).

RESULTS

Cocoa treatment for 2 wk increased insulin-stimulated changes in brachial artery diameter when compared with placebo [median percentage increase from baseline (25th-75th percentile): 8.3 (4.2-11.3) compared with 5.9 (-0.3 to 9.6); P < 0.04]. Nevertheless, cocoa treatment did not significantly reduce blood pressure or improve insulin resistance and had no significant effects on skeletal muscle capillary recruitment, circulating plasma concentrations of adipocytokines, or endothelial adhesion molecules.

CONCLUSIONS

Daily consumption of flavanol-rich cocoa for 2 wk is not sufficient to reduce blood pressure or improve insulin resistance in human subjects with essential hypertension. This trial was registered at clinicaltrials.gov as NCT00099476.

Skin microcirculatory dysfunction is already present in normoglycemic subjects with metabolic syndrome

The role of microcirculatory dysfunction (MD) in metabolic and cardiovascular diseases is not well established. Considering that metabolic syndrome (MS) is an independent risk factor and diabetic patients have microangiopathy, our aim was to investigate if normoglycemic subjects with MS already have detectable skin MD. Thirty-six subjects with MS (National Cholesterol Education Program-Adult Treatment Panel III criteria) (10 men/26 women, 38.8 +/- 7.9 years, 35.8 +/- 4.9 kg/m(2)) with normal glucose tolerance (American Diabetes Association criteria) and 16 controls (11 men/5 women, 33.6 +/- 8.4 years, 23.9 +/- 3.6 kg/m(2)) were studied using nailfold videocapillaroscopy. Afferent, efferent, and apical capillary diameters; functional capillary density; red blood cell velocity (RBCV) at baseline; and RBCV(max) and time (TRBCV(max)) to reach it during postocclusive reactive hyperemia after 1-minute arterial occlusion were measured. Subjects with MS had smaller afferent, efferent, and apical diameters (4.2 [3.8-4.2] vs 5.6 [4.65-6.25] mum, P < .001; 4.8 [4.2-4.8] vs 6.2 [5.6-7] mum, P < .001; and 5.2 [4.8-5.55] vs 7.4 [6.2-8] mum, P < .001); lower functional capillary density (7.28 [6.37-9.10] vs 10.4 [9.1-11.8] capillaries per square millimeter, P < .001), RBCV (0.62 [0.57-0.65] vs 0.79 [0.76-0.89] mm/s, P < .001], and RBCV(max) (1.14 [1.12-1.210] vs 1.57 [1.45-1.62] mm/s, P < .001); and longer TRBCV(max) (10.0 [10-11] vs 4.5 [4-6] seconds, P < .001) compared with controls. Microcirculatory dysfunction was associated with body mass index. We concluded that subjects with MS already have nutritive skin MD even within the normoglycemic milieu.

Blood viscosity and hematocrit as risk factors for type 2 diabetes mellitus: the atherosclerosis risk in communities (ARIC) study

Several lines of evidence support the notion that elevated blood viscosity may predispose to insulin resistance and type 2 diabetes mellitus by limiting delivery of glucose, insulin, and oxygen to metabolically active tissues. To test this hypothesis, the authors analyzed longitudinal data on 12,881 initially nondiabetic adults, aged 45–64 years, who were participants in the Atherosclerosis Risk in Communities (ARIC) Study (1987–1998). Whole blood viscosity was estimated by using a validated formula based on hematocrit and total plasma proteins at baseline. At baseline, estimated blood viscosity was independently associated with several features of the metabolic syndrome. In models adjusted simultaneously for known predictors of diabetes, estimated whole blood viscosity and hematocrit predicted incident type 2 diabetes mellitus in a graded fashion (P_{trend (linear)} < 0.001): Compared with their counterparts in the lowest quartiles, adults in the highest quartile of blood viscosity (hazard ratio = 1.68, 95% confidence interval: 1.53, 1.84) and hematocrit (hazard ratio = 1.63, 95% confidence interval: 1.49, 1.79) were over 60% more likely to develop diabetes. Therefore, elevated blood viscosity and hematocrit deserve attention as emerging risk factors for insulin resistance and type 2 diabetes mellitus.

Endothelial dysfunction and diabetes: roles of hyperglycemia, impaired insulin signaling and obesity

Endothelial dysfunction comprises a number of functional alterations in the vascular endothelium that are associated with diabetes and cardiovascular disease, including changes in vasoregulation, enhanced generation of reactive oxygen intermediates, inflammatory activation, and altered barrier function. Hyperglycemia is a characteristic feature of type 1 and type 2 diabetes and plays a pivotal role in diabetes-associated microvascular complications. Although hyperglycemia also contributes to the occurrence and progression of macrovascular disease (the major cause of death in type 2 diabetes), other factors such as dyslipidemia, hyperinsulinemia, and adipose-tissue-derived factors play a more dominant role. A mutual interaction between these factors and endothelial dysfunction occurs during the progression of the disease. We pay special attention to the possible involvement of endoplasmic reticulum stress (ER stress) and the role of obesity and adipose-derived adipokines as contributors to endothelial dysfunction in type 2 diabetes. The close interaction of adipocytes of perivascular adipose tissue with arteries and arterioles facilitates the exposure of their endothelial cells to adipokines, particularly if inflammation activates the adipose tissue and thus affects vasoregulation and capillary recruitment in skeletal muscle. Hence, an initial dysfunction of endothelial cells underlies metabolic and vascular alterations that contribute to the development of type 2 diabetes.

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.

Obese but not normal-weight women with polycystic ovary syndrome are characterized by metabolic and microvascular insulin resistance

CONTEXT

Polycystic ovary syndrome (PCOS) and obesity are associated with diabetes and cardiovascular disease, but it is unclear to what extent PCOS contributes independently of obesity.

OBJECTIVE

The objective of the study was to investigate whether insulin sensitivity and insulin's effects on the microcirculation are impaired in normal-weight and obese women with PCOS.

DESIGN AND POPULATION

Thirty-five women with PCOS (19 normal weight and 16 obese) and 27 age- and body mass index-matched controls (14 normal weight and 13 obese) were included. Metabolic Insulin sensitivity (isoglycemic-hyperinsulinemic clamp) and microvascular insulin sensitivity [endothelium dependent (acetylcholine [ACh])] and endothelium-independent [sodium nitroprusside (SNP)] vasodilation with laser Doppler flowmetry was assessed at baseline and during hyperinsulinemia.

MAIN OUTCOME MEASURES

Metabolic insulin sensitivity (M/I value) and the area under the response curves to ACh and SNP curves were measured to assess microcirculatory function at baseline and during insulin infusion (microvascular insulin sensitivity).

RESULTS

Obese women were more insulin resistant than normal-weight women (P < 0.001), and obese PCOS women were more resistant than obese controls (P = 0.02). In contrast, normal-weight women with PCOS had similar insulin sensitivity, compared with normal-weight women without PCOS. Baseline responses to ACh showed no difference in the four groups. ACh responses during insulin infusion were significantly greater in normal-weight PCOS and controls than in obese PCOS and controls. PCOS per se had no significant influence on ACh responses during insulin infusion. During hyperinsulinemia, SNP-dependent vasodilatation did not significantly increase, compared with baseline in the four groups.

CONCLUSION

PCOS per se was not associated with impaired metabolic insulin sensitivity in normal-weight women but aggravates impairment of metabolic insulin sensitivity in obese women. In obese but not normal-weight women, microvascular and metabolic insulin sensitivity are decreased, independent of PCOS. Therefore, obese PCOS women in particular may be at increased risk of metabolic and cardiovascular diseases.

Effects of Intrabrachial metacholine infusion on muscle capillary recruitment and forearm glucose uptake during physiological hyperinsulinemia in obese, insulin-resistant individuals

CONTEXT

Impairment of insulin-mediated capillary recruitment in skeletal muscle contributes to a hampered glucose uptake in obesity.

OBJECTIVE

The objective of this study was to evaluate whether metacholine (MCh), a nitric oxide vasodilator, potentiates muscle capillary recruitment and forearm glucose uptake (FGU) during physiological hyperinsulinemia.

DESIGN

The double-forearm technique [i.e. infused vs. control (Ctrl) forearm] was combined with im microdialysis during an oral glucose tolerance test in 15 nondiabetic, obese subjects divided into a group of insulin-resistant (IR) (n = 7) and insulin-sensitive (n = 8) individuals.

RESULTS

After the oral glucose tolerance test, forearm blood flow in the Ctrl forearm was unchanged, whereas it increased about 3-fold (P < 0.0001 vs. baseline) in response to MCh. Capillary permeability surface area product for glucose (PS(glu)) (capillary recruitment), FGU, and interstitial insulin concentrations increased significantly over time (P < 0.001) in both forearms. Compared with insulin-sensitive, the IR subjects exhibited lower PS(glu) (P < 0.001) and FGU (P < 0.01) in the Ctrl arm, whereas this difference was insignificant in the MCh arm despite the blunted forearm blood flow increase. Moreover, in IR individuals MCh significantly (P < 0.05) ameliorated the delayed onset of insulin action, i.e. the FGU response to hyperinsulinemia. Finally, we found PS(glu) to be a strong and independent predictor of FGU response (adjusted R(2) 0.72; P < 0.0001).

CONCLUSIONS

MCh-induced vasodilation may improve the microvascular and metabolic responses to physiological hyperinsulinemia in obese, IR individuals. Further studies are required to unravel whether stimulation of nitric oxide production in skeletal muscle may represent an attractive therapeutic approach to bypassing cellular resistance to glucose disposal.

Decreased NO signaling leads to enhanced vasoconstrictor responsiveness in skeletal muscle arterioles of the ZDF rat prior to overt diabetes and hypertension

Approximately 40% of patients with type 2 diabetes present with concurrent hypertension at the time of diabetes diagnosis. Increases in peripheral vascular resistance and correspondingly enhanced vasoconstrictor capacity could have profound implications for the development of hypertension and the progression of insulin resistance to overt diabetes. The purpose of this study was to determine whether skeletal muscle arteriolar vasoconstrictor dysfunction precedes or occurs concurrently with the onset of diabetes and hypertension. Male Zucker diabetic fatty (ZDF) rats were studied at 7, 13, and 20 wk of age to represent prediabetic and short-term and long-term diabetic states, respectively. Conscious mean arterial pressure (MAP), fasted plasma insulin and glucose, vasoconstrictor responses, and passive mechanical properties of isolated skeletal muscle arterioles were measured in prediabetic, diabetic, and age-matched control rats. Elevated MAP was manifest in short-term diabetes (control 117 +/- 1, diabetic 135 +/- 3 mmHg) and persisted with long-term diabetes (control 113 +/- 2, diabetic 135 +/- 3 mmHg). This higher MAP was preceded by augmented arteriolar vasoconstrictor responses to norepinephrine and endothelin-1 and followed by diminished beta-adrenergic vasodilation and enhanced myogenic constriction in long-term diabetes. Furthermore, we demonstrate that diminished nitric oxide (NO) signaling underlies the increases in vasoconstrictor responsiveness in arterioles from prediabetic and diabetic rats. Arteriolar stiffness was not different between control and prediabetic or diabetic rats at any time point studied. Collectively, these results indicate that increases in vasoconstrictor responsiveness resulting from diminished NO signaling in skeletal muscle arterioles precede the development of diabetes and hypertension in ZDF rats.

Protein kinase C theta activation induces insulin-mediated constriction of muscle resistance arteries

OBJECTIVE

Protein kinase C (PKC) theta activation is associated with insulin resistance and obesity, but the underlying mechanisms have not been fully elucidated. Impairment of insulin-mediated vasoreactivity in muscle contributes to insulin resistance, but it is unknown whether PKC theta is involved. In this study, we investigated whether PKC theta activation impairs insulin-mediated vasoreactivity and insulin signaling in muscle resistance arteries.

RESEARCH DESIGN AND METHODS

Vasoreactivity of isolated resistance arteries of mouse gracilis muscles to insulin (0.02-20 nmol/l) was studied in a pressure myograph with or without PKC theta activation by palmitic acid (PA) (100 micromol/l).

RESULTS

In the absence of PKC theta activation, insulin did not alter arterial diameter, which was caused by a balance of nitric oxide-dependent vasodilator and endothelin-dependent vasoconstrictor effects. Using three-dimensional microscopy and Western blotting of muscle resistance arteries, we found that PKC theta is abundantly expressed in endothelium of muscle resistance arteries of both mice and humans and is activated by pathophysiological levels of PA, as indicated by phosphorylation at Thr(538) in mouse resistance arteries. In the presence of PA, insulin induced vasoconstriction (21 +/- 6% at 2 nmol/l insulin), which was abolished by pharmacological or genetic inactivation of PKC theta. Analysis of intracellular signaling in muscle resistance arteries showed that PKC theta activation reduced insulin-mediated Akt phosphorylation (Ser(473)) and increased extracellular signal-related kinase (ERK) 1/2 phosphorylation. Inhibition of PKC theta restored insulin-mediated vasoreactivity and insulin-mediated activation of Akt and ERK1/2 in the presence of PA.

CONCLUSIONS

PKC theta activation induces insulin-mediated vasoconstriction by inhibition of Akt and stimulation of ERK1/2 in muscle resistance arteries. This provides a new mechanism linking PKC theta activation to insulin resistance.

The relationship between diabetes mellitus and traffic-related air pollution

OBJECTIVE

Air pollution is associated with an increased risk for cardiovascular events. Many of the biological pathways involved could also promote diabetes mellitus (DM). We therefore investigated the association between DM prevalence and exposure to traffic-related air pollution (nitrogen dioxide [NO 2]).

METHODS

Study participants were patients who attended two respiratory clinics in Hamilton (n = 5228) and Toronto (n = 2406). The diagnosis of DM was ascertained by linkage to administrative databases of the Ontario universal Health Insurance Plan for patients aged 40 years and above. Geographic Information systems methodology was used to assign individual estimates of NO2 based on a network of samplers in each city. Logistic regression was used to estimate the relations between NO2 exposures and the odds of DM diagnosis.

RESULTS

After adjusting for age, body mass index, and neighborhood income there were positive effects in women on the odds ratio for DM for each 1 ppb NO2 exposure in Toronto (OR 1.055, 95% CI: 0.99 to 1.11) and Hamilton (OR 1.029, 95% CI: 0.98 to 1.08). In a meta-analytic model including both cities, there was a significant effect in women (OR = 1.04; 95% CI: 1.00 to 1.08). Across the inter-quartile range (approximately 4 ppb NO2) there was nearly a 17% increase in the odds of DM for women. There were no positive associations among men.

CONCLUSIONS

Exposure to NO2, a marker of traffic-related air pollutants, was associated with DM prevalence among women. Exposure estimate errors in men may explain the apparent gender difference. These results suggest that common air pollutants are associated with DM and warrant more investigation to determine if this is a cause-and-effect relationship.

High-dose ascorbic acid infusion abolishes chronic vasoconstriction and restores resting leg blood flow in healthy older men

Resting whole leg blood flow and vascular conductance decrease linearly with advancing age in healthy adult men. The potential role of age-related increases in oxidative stress in these changes is unknown. Resting leg blood flow during saline and ascorbic acid infusion was studied in 10 young (25 +/- 1 yr) and 11 older (63 +/- 2 yr) healthy normotensive men. Plasma oxidized LDL, a marker of oxidative stress, was greater in the older men (P < 0.05). Absolute resting femoral artery blood flow at baseline (iv saline control infusion) was 25% lower in the older men (238 +/- 25 vs. 316 +/- 38 ml/min; P < 0.05), and it was inversely related to plasma oxidized LDL (r = -0.56, P < 0.01) in all subjects. Infusion of supraphysiological concentrations of ascorbic acid increased femoral artery blood flow by 37% in the older men (to 327 +/- 52 ml/min; P < 0.05), but not in the young men (352 +/- 41 ml/min; P = 0.28), thus abolishing group differences (P = 0.72). Mean arterial blood pressure was greater in the older men at baseline (86 +/- 4 vs. 78 +/- 2 mmHg; P < 0.05), but it was unaffected by ascorbic acid infusion (P >/= 0.70). As a result, the lower baseline femoral artery blood flow in the older men was mediated solely by a 32% lower femoral artery vascular conductance (P < 0.05). Baseline femoral vascular conductance also was inversely related to plasma oxidized LDL (r = -0.65, P < 0.01). Ascorbic acid increased femoral vascular conductance by 36% in the older men (P < 0.05) but not in the young men (P = 0.31). In conclusion, ascorbic acid infused at concentrations known to scavenge reactive oxygen species restores resting femoral artery blood flow in healthy older adult men by increasing vascular conductance. These results support the hypothesis that oxidative stress plays a major role in the reduced resting whole leg blood flow and increased leg vasoconstriction observed with aging in men.

Local nitric oxide synthase inhibition reduces skeletal muscle glucose uptake but not capillary blood flow during in situ muscle contraction in rats

OBJECTIVE

We have previously shown in humans that local infusion of a nitric oxide synthase (NOS) inhibitor into the femoral artery attenuates the increase in leg glucose uptake during exercise without influencing total leg blood flow. However, rodent studies examining the effect of NOS inhibition on contraction-stimulated skeletal muscle glucose uptake have yielded contradictory results. This study examined the effect of local infusion of an NOS inhibitor on skeletal muscle glucose uptake (2-deoxyglucose) and capillary blood flow (contrast-enhanced ultrasound) during in situ contractions in rats.

RESEARCH DESIGN AND METHODS

Male hooded Wistar rats were anesthetized and one hindleg electrically stimulated to contract (2 Hz, 0.1 ms) for 30 min while the other leg rested. After 10 min, the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) (arterial concentration of 5 micromol/l) or saline was infused into the epigastric artery of the contracting leg.

RESULTS

Local NOS inhibition had no effect on blood pressure, heart rate, or muscle contraction force. Contractions increased (P < 0.05) skeletal muscle NOS activity, and this was prevented by L-NAME infusion. NOS inhibition caused a modest significant (P < 0.05) attenuation of the increase in femoral blood flow during contractions, but importantly there was no effect on capillary recruitment. NOS inhibition attenuated (P < 0.05) the increase in contraction-stimulated skeletal muscle glucose uptake by approximately 35%, without affecting AMP-activated protein kinase (AMPK) activation.

CONCLUSIONS

NOS inhibition attenuated increases in skeletal muscle glucose uptake during contraction without influencing capillary recruitment, suggesting that NO is critical for part of the normal increase in skeletal muscle fiber glucose uptake during contraction.

Hyperinsulinemia rapidly increases human muscle microvascular perfusion but fails to increase muscle insulin clearance: evidence that a saturable process mediates muscle insulin uptake

OBJECTIVE

Transport of insulin from the central circulation into muscle is rate limiting for the stimulation of glucose metabolism. By recruiting muscle microvasculature, insulin may promote its own movement into muscle interstitium. We tested whether in humans, as in the rat, insulin exerts an early action to recruit microvasculature within skeletal muscle. We further hypothesized that expansion of the microvascular volume of muscle would enhance muscle insulin clearance.

RESEARCH DESIGN AND METHODS

Microvascular volume, total blood flow, and muscle insulin and glucose uptake (forearm balance method) were measured in 14 lean, healthy volunteers before and during a 2-h hyperinsulinemic-euglycemic clamp (1 mU x kg(-1) x min(-1)). Microvascular volume was measured using contrast-enhanced ultrasound.

RESULTS

Forearm muscle microvascular volume increased within 20 min of insulin infusion (P < 0.01), whereas an effect to increase total forearm flow was not observed until 100 min. Forearm insulin uptake increased with physiological hyperinsulinemia (15 +/- 3 and 87 +/- 13 fmol x min(-1) x 100 ml(-1) basal vs. last 40 min of clamp, P < 0.001). However, the extraction fraction and clearance of insulin declined (P = 0.02, for each), indicating saturability of muscle insulin uptake at physiological hyperinsulinemia.

CONCLUSIONS

Skeletal muscle contributes to peripheral insulin clearance both in the basal state and with physiological hyperinsulinemia. Insulin promptly expands human muscle microvascular volume but only slowly increases blood flow. Despite increased microvascular volume available for insulin uptake, muscle insulin clearance decreases significantly. These findings are consistent with the presence of a saturable transport mechanism facilitating the transendothelial transport of insulin into human muscle.

Contrast-enhanced ultrasound measurement of microvascular perfusion relevant to nutrient and hormone delivery in skeletal muscle: a model study in vitro

Contrast-enhanced ultrasound (CEU) has been used to measure muscle microvascular perfusion in vivo in response to exercise and insulin. In the present study we address whether CEU measurement of capillary volume is influenced by bulk flow and if measured capillary filling rate allows discrimination of different flow pattern changes within muscle. Three in vitro models were used: (i) bulk flow rate was varied within a single length of capillary tubing; (ii) at constant bulk flow, capillary volume was increased 3-fold by joining lengths of capillary in series, and compared to a single length; and (iii) at constant bulk flow, capillary volume was increased by sharing flow between a number of lengths of identical capillaries in parallel. The contrast medium for CEU was gas-filled albumin microbubbles. Pulsing interval (time) versus acoustic-intensity curves were constructed and from these, capillary volume and capillary filling rate were calculated. CEU estimates of capillary volume were not affected by changes in bulk flow. Furthermore, as CEU estimates of capillary volume increased, measures of capillary filling rate decreased, regardless of whether capillaries were connected in series or parallel. Therefore, CEU can detect a change in filling rate of the microvascular volume under measurement, but it can not be used to discriminate between different flow patterns within muscle that might account for capillary recruitment in vivo.

Selective resistance to vasoactive effects of insulin in muscle resistance arteries of obese Zucker (fa/fa) rats

Obesity is related to insulin resistance and hypertension, but the underlying mechanisms are unclear. Insulin exerts both vasodilator and vasoconstrictor effects on muscle resistance arteries, which may be differentially impaired in obesity.

OBJECTIVES

To investigate whether vasodilator and vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats and the roles of Akt and endothelial NO synthase (eNOS).

METHODS/RESULTS

Effects of insulin were studied in resistance arteries isolated from cremaster muscles of lean and obese Zucker rats. In arteries of lean rats, insulin increased activity of both NO and endothelin (ET-1), resulting in a neutral effect under basal conditions. In arteries of obese rats, insulin induced endothelin-mediated vasoconstriction (-15 +/- 5% at 1 nM, P < 0.05 vs. lean). Insulin induced vasodilatation during endothelin receptor blockade in arteries of lean rats (20 +/- 5% at 1 nM) but not in those of obese rats. Inhibition of NO synthesis increased vascular tone (by 12 +/- 2%) and shifted insulin-mediated vasoreactivity to vasoconstriction (-25 +/- 1% at 1 nM) in lean rats but had no effect in arteries of obese rats, indicating reduced NO activity. Protein analysis of resistance arteries revealed that insulin-mediated activation of Akt was preserved in obese rats, whereas expression of eNOS was markedly decreased.

CONCLUSIONS

Vasodilator but not vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats, and this selective impairment is associated with decreased protein levels of eNOS. These findings provide a new mechanism linking obesity to insulin resistance and hypertension.

Microvascular dysfunction in obesity: a potential mechanism in the pathogenesis of obesity-associated insulin resistance and hypertension

Obesity is an important risk factor for insulin resistance and hypertension and plays a central role in the metabolic syndrome. Insight into the pathophysiology of this syndrome may lead to new treatments. This paper has reviewed the evidence for an important role for the microcirculation as a possible link between obesity, insulin resistance and hypertension.

Association of oxidative stress, insulin resistance, and diabetes risk phenotypes: the Framingham Offspring Study

OBJECTIVE

Systemic oxidative stress causes insulin resistance in rodents. We tested the hypothesis that oxidative stress and insulin resistance are associated in humans.

RESEARCH DESIGN AND METHODS

We used cross-sectional data from 2,002 nondiabetic subjects of the community-based Framingham Offspring Study. We measured insulin resistance with the homeostasis model and defined categorical insulin resistance as homeostasis model assessment of insulin resistance (HOMA-IR) > 75th percentile. We measured oxidative stress using the ratio of urine 8-epi-prostaglandin F2alpha (8-epi-PGF2alpha) to creatinine and used age- and sex-adjusted regression models to test the association of oxidative stress with insulin resistance in individuals without diabetes and among subgroups at elevated risk of diabetes.

RESULTS

Across 8-epi-PGF2alpha/creatinine tertiles, the prevalence of insulin resistance increased (18.0, 27.5, and 29.4% for the first, second, and third tertiles, respectively; P < 0.0001), as did mean levels of HOMA-IR (3.28, 3.83, and 4.06 units; P < 0.0001). The insulin resistance-oxidative stress association was attenuated by additional adjustment for BMI (P = 0.06 across tertiles for insulin resistance prevalence; P = 0.004 for mean HOMA-IR). Twenty-six percent of participants were obese (BMI > or = 30 kg/m2), 39% had metabolic syndrome (according to the Adult Treatment Panel III definition), and 37% had impaired fasting glucose (IFG) (fasting glucose 5.6-6.9 mmol/l). Among 528 obese participants, respectively, insulin resistance prevalence was 41.3, 60.6, and 54.2% across 8-epi-PGF2alpha/creatinine tertiles (P = 0.005); among 781 subjects with metabolic syndrome, insulin resistance prevalence was 41.3, 56.7, and 51.7% (P = 0.0025); and among 749 subjects with IFG, insulin resistance prevalence was 39.6, 47.2, and 51.6% (P = 0.04).

CONCLUSIONS

Systemic oxidative stress is associated with insulin resistance in individuals at average or elevated risk of diabetes even after accounting for BMI.

Insulin and contraction increase nutritive blood flow in rat muscle in vivo determined by microdialysis of L-[14C]glucose

In the present study, a mathematical model using the microdialysis outflow: inflow (O/I) ratio of the novel analogue L-[14C]glucose has been developed which allows the calculation of the nutritive (and non-nutritive) flow in muscle as a proportion of total blood flow. Anaesthetized rats had microdialysis probes carrying L-[14C]glucose inserted through a calf muscle group (tibialis/plantaris/gastrocnemius). The nutritive fraction of total blood flow was determined under basal conditions and in response to contraction (electrical field stimulation), insulin (hyperinsulinaemic euglycaemic clamp with 10 mU min(-1) kg(-1) insulin) or saline control from limb blood flow and the microdialysis O/I ratio of L-[14C]glucose. Both contraction and insulin infusion decreased the O/I ratio of L-[14C]glucose and increased total limb blood flow. Calculations based on mathematical models using L-[14C]glucose O/I and limb blood flow revealed that during basal conditions, the nutritive fraction of total flow was 0.38 +/- 0.06, indicating that basal flow was predominantly non-nutritive. Contraction and insulin increased the nutritive fraction to 0.82 +/- 0.24 (P < 0.05) and 0.52 +/- 0.12 (P < 0.05). Thus the increase in limb blood flow from insulin was fully accommodated by nutritive flow, while contraction increased nutritive flow at the expense of non-nutritive flow. This novel method using microdialysis and the O/I ratio of L-[14C]glucose allows the determination of the nutritive fraction of total flow in muscle as well as the proportion of total flow that may be redistributed in response to contraction and insulin.

Contraction stimulates nitric oxide independent microvascular recruitment and increases muscle insulin uptake

We examined whether contraction-induced muscle microvascular recruitment would expand the surface area for insulin and nutrient exchange and thereby contribute to insulin-mediated glucose disposal. We measured in vivo rat hindlimb microvascular blood volume (MBV) using contrast ultrasound and femoral blood flow (FBF) using Doppler ultrasound in response to a stimulation frequency range. Ten minutes of 0.1-Hz isometric contraction more than doubled MBV (P < 0.05; n = 6) without affecting FBF (n = 7), whereas frequencies >0.5 Hz increased both. Specific inhibition of nitric oxide (NO) synthase with N(omega)-l-nitro-arginine-methyl ester (n = 5) significantly elevated mean arterial pressure by approximately 30 mmHg but had no effect on basal FBF or MBV. We next examined whether selectively elevating MBV without increasing FBF (0.1-Hz contractions) increased muscle uptake of albumin-bound Evans blue dye (EBD). Stimulation at 0.1 Hz (10 min) elicited more than twofold increases in EBD content (micrograms EBD per gram dry tissue) in stimulated versus contralateral muscle (n = 8; 52.2 +/- 3.8 vs. 20 +/- 2.5, respectively; P < 0.001). We then measured muscle uptake of EBD and (125)I-labeled insulin (dpm per gram dry tissue) with 0.1-Hz stimulation (n = 6). Uptake of EBD (19.1 +/- 3.8 vs. 9.9 +/- 1; P < 0.05) and (125)I-insulin (5,300 +/- 800 vs. 4,244 +/- 903; P < 0.05) was greater in stimulated muscle versus control. Low-frequency contraction increases muscle MBV by a NO-independent pathway and facilitates muscle uptake of albumin and insulin in the absence of blood flow increases. This microvascular response may, in part, explain enhanced insulin action in exercising skeletal muscle.

Physiological hyperinsulinemia has no detectable effect on access of macromolecules to insulin-sensitive tissues in healthy humans

OBJECTIVE

Physiologically elevated insulin concentrations promote access of macromolecules to skeletal muscle in dogs. We investigated whether insulin has a stimulating effect on the access of macromolecules to insulin-sensitive tissues in humans as well.

RESEARCH DESIGN AND METHODS

In a randomized, controlled trial, euglycemic-hyperinsulinemic clamp (1.2 mU x kg(-1) x min(-1) insulin) and saline control experiments were performed in 10 healthy volunteers (aged 27.5 +/- 4 years, BMI 22.6 +/- 1.6 kg/m(2)). Distribution and clearance parameters of inulin were determined in a whole-body approach, combining primed intravenous infusion of inulin with compartment modeling. Inulin kinetics were measured in serum using open-flow microperfusion in interstitial fluid of femoral skeletal muscle and subcutaneous adipose tissue.

RESULTS

Inulin kinetics in serum were best described using a three-compartment model incorporating a serum and a fast and a slow equilibrating compartment. Inulin kinetics in interstitial fluid of peripheral insulin-sensitive tissues were best represented by the slow equilibrating compartment. Serum and interstitial fluid inulin kinetics were comparable between the insulin and saline groups. Qualitative analysis of inulin kinetics was confirmed by model-derived distribution and clearance parameters of inulin. Physiological hyperinsulinemia (473 +/- 6 vs. 18 +/- 2 pmol/l for the insulin and saline group, respectively; P < 0.001) indicated no effect on distribution volume (98.2 +/- 6.2 vs. 102.5 +/- 5.7 ml/kg; NS) or exchange parameter (217.6 +/- 34.2 vs. 243.1 +/- 28.6 ml/min; NS) of inulin to peripheral insulin-sensitive tissues. All other parameters identified by the model were also comparable between the groups.

CONCLUSIONS

Our data suggest that in contrast to studies performed in dogs, insulin at physiological concentrations does not augment recruitment of insulin-sensitive tissues in healthy humans.

Microvascular function: a potential link between salt sensitivity, insulin resistance and hypertension

OBJECTIVE

Generalized microvascular dysfunction may contribute to the development of salt sensitivity, insulin resistance and hypertension, and may thus link these cardiovascular risk factors. To test this hypothesis, we examined skin microvascular function, salt sensitivity, insulin sensitivity and blood pressure in 27 normotensive and 26 hypertensive individuals.

METHODS

Capillary density was examined by videomicroscopy during venous congestion and postocclusive reactive hyperaemia. Endothelium-(in)dependent vasodilation was assessed by iontophoresis of acetylcholine and sodium nitroprusside and by laser Doppler flowmetry. Salt sensitivity was determined as the difference in mean arterial pressure (MAP) between a 1-week high-salt diet (approximately 235 mmol NaCl/day) versus low-salt diet (approximately 55 mmol NaCl/day). Insulin sensitivity was measured with the hyperinsulinaemic, euglycaemic clamp, and blood pressure was assessed by 24-h ambulatory blood pressure monitoring.

RESULTS

Salt sensitivity of blood pressure was inversely associated with postocclusive capillary recruitment and endothelium-dependent vasodilation (r=-0.67, P<0.001 and r=-0.60, P<0.01, respectively), but not with capillary density during venous congestion or endothelium-independent vasodilation. Salt sensitivity was negatively associated with insulin sensitivity (r=-0.55, P<0.001) and positively with MAP (r=0.58, P<0.001). Multiple regression analyses suggested that associations between salt sensitivity and both insulin sensitivity and MAP were dependent on microvascular function.

CONCLUSION

Our results suggest a close inverse association between skin microvascular function and salt sensitivity and a role for generalized microvascular defects as a link between salt sensitivity, insulin resistance and hypertension.

Metabolic inflexibility in skeletal muscle: a prelude to the cardiometabolic syndrome?

Peripheral insulin resistance, which is largely dependent on skeletal muscle, is closely linked to the development of the cardiometabolic syndrome. Metabolic flexibility is the capacity for skeletal muscle to acutely shift its reliance between lipids or glucose during fasting or postprandial conditions. Obese and insulin-resistant individuals display elevated intramuscular lipids, impaired vasculature function, decreased fatty add oxidation during fasting, and reduced postprandial glucose metabolism. Impairments in metabolic flexibility are linked to physical inactivity, excess energy intake and obesity, and genetic predisposition. Each of these factors precludes the development of insulin resistance and the cardiometabolic syndrome by mechanistic links that are not fully understood.

Impaired local microvascular vasodilatory effects of insulin and reduced skin microvascular vasomotion in obese women

Our study aim is to investigate whether obesity is characterized by an impairment of insulin-mediated vasodilatory effects and by a modification of basal vasomotion in the skin microvasculature. Forty healthy obese and forty healthy lean women were included. Microvascular effects of insulin as compared to a control substance were measured by cathodal iontophoresis combined with laser Doppler flowmetry. Vasomotion was examined by Fourier transform analyses of skin laser Doppler flow at rest. Locally administered insulin, as compared to the control substance, induced a microvascular vasodilatory response in lean (median (interquartile range): 31.6 (17.1-43.9) vs. 22.9 (16.4-36.7) perfusion units, P=0.04), but not in obese women (28.1 (14.4-47.1) vs. 27.5 (17.5-48.2) perfusion units, P=0.7). The relative insulin-induced increase in blood flow corrected for the control substance was higher in lean than obese women (ANOVA for repeated measures F=3.93, P=0.05). The contribution of the total frequency spectrum 0.01-1.6 Hz and of the frequency intervals 0.01-0.02 Hz and 0.02-0.06 Hz (representative of endothelial and neurogenic activity, respectively) to basal microvascular vasomotion was lower in obese than in lean women (P<0.05 for all). These findings show that obesity is characterized by an impaired direct microvascular vasodilatory effect of insulin and by decreased skin microvascular vasomotion in a way that is suggestive for alterations of endothelial and neurogenic activity.

Salt intake and insulin sensitivity in healthy human volunteers

The literature on salt intake and insulin sensitivity presents a mixed picture, as some studies have shown an increase, whereas others have shown a decrease, in insulin action as sodium intake is enhanced. In some cases, this may relate to the study of salt intake in patients with co-morbidities such as hypertension or diabetes. In the present study, we selected healthy normotensive lean volunteers who underwent a euglycaemic clamp following 6 days of a low-salt diet (20 mmol sodium daily) and, subsequently, 6 days of a high-salt diet (200 mmol sodium daily). Our results show an increase in insulin-mediated glucose disposal during euglycaemic clamp conditions that was significantly higher following the high-salt diet compared with the low-salt diet (7.41+/-0.41 compared with 6.11+/-0.40 mg x kg(-1) of body weight x min(-1) respectively; P=0.03). We measured calf blood flow before and during insulin infusion (no significant change after the two dietary salt interventions was detected) and plasma non-esterified fatty acids (also no significant differences were detected). We observed the expected increases in renin concentration and aldosterone activity in subjects on the low-salt diet, and also observed a significantly less increase in plasma noradrenaline concentration during euglycaemic insulin infusion following the high-salt compared with the low-salt diet. We propose that the 4-5-fold increase in serum aldosterone and the greater increase in plasma noradrenaline concentration following the low-salt intervention compared with the high-salt period may have contributed to the differences in insulin sensitivity following the adjustment in dietary sodium intake.

Cardiovascular actions of insulin

Insulin has important vascular actions to stimulate production of nitric oxide from endothelium. This leads to capillary recruitment, vasodilation, increased blood flow, and subsequent augmentation of glucose disposal in classical insulin target tissues (e.g., skeletal muscle). Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways regulating endothelial production of nitric oxide share striking parallels with metabolic insulin-signaling pathways. Distinct MAPK-dependent insulin-signaling pathways (largely unrelated to metabolic actions of insulin) regulate secretion of the vasoconstrictor endothelin-1 from endothelium. These and other cardiovascular actions of insulin contribute to coupling metabolic and hemodynamic homeostasis under healthy conditions. Cardiovascular diseases are the leading cause of morbidity and mortality in insulin-resistant individuals. Insulin resistance is typically defined as decreased sensitivity and/or responsiveness to metabolic actions of insulin. This cardinal feature of diabetes, obesity, and dyslipidemia is also a prominent component of hypertension, coronary heart disease, and atherosclerosis that are all characterized by endothelial dysfunction. Conversely, endothelial dysfunction is often present in metabolic diseases. Insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling that in vascular endothelium contributes to a reciprocal relationship between insulin resistance and endothelial dysfunction. The clinical relevance of this coupling is highlighted by the findings that specific therapeutic interventions targeting insulin resistance often also ameliorate endothelial dysfunction (and vice versa). In this review, we discuss molecular mechanisms underlying cardiovascular actions of insulin, the reciprocal relationships between insulin resistance and endothelial dysfunction, and implications for developing beneficial therapeutic strategies that simultaneously target metabolic and cardiovascular diseases.

Urinary cortisol is inversely associated with capillary recruitment in women: a potential explanation for the cortisol–blood pressure relationship

The relationships of cortisol with elevated blood pressure and insulin resistance are likely to be the result of a complex interplay of different mechanisms. We hypothesize that cortisol is associated with impaired microvascular function and that this contributes to cortisol-associated high blood pressure and insulin resistance. We examined 24 h urinary free cortisol excretion in 56 healthy adults (26 women). Blood pressure was assessed by 24 h ambulatory measurements. Insulin sensitivity was determined using the hyperinsulinaemic euglycaemic clamp technique. Skin capillary recruitment after arterial occlusion was visualized with videomicroscopy and endothelium-(in)dependent vasodilation was evaluated with iontophoresis of acetylcholine and sodium nitroprusside combined with laser Doppler fluxmetry. Men were characterized by higher urinary cortisol excretion [median (interquartile range), 162 (130–194) compared with 118 (99–156) nmol/24 h, P<0.05]. In women, but not in men, urinary cortisol excretion was associated with impaired capillary recruitment (r=−0.66, P<0.001), higher systolic blood pressure (r=0.64, P<0.001) and lower insulin sensitivity (r=−0.43, P<0.05). Urinary cortisol excretion was not associated with endothelium-(in)dependent vasodilation in men or women. Regression analysis demonstrated that capillary recruitment statistically explained 37% of the association between urinary cortisol and blood pressure in women. Capillary recruitment did not explain part of the association between urinary cortisol and insulin sensitivity. In conclusion, urinary cortisol excretion is inversely associated with capillary recruitment in women, but not in men, and capillary recruitment explains part of the cortisol–blood pressure relationship. These data suggest that, in women, impairment of capillary function mediates some of the adverse effects of cortisol and thus may provide a target to prevent such adverse effects.

Chronic treatment with sildenafil improves energy balance and insulin action in high fat-fed conscious mice

Stimulation of nitric oxide-cGMP signaling results in vascular relaxation and increased muscle glucose uptake. We show that chronically inhibiting cGMP hydrolysis with the phosphodiesterase-5 inhibitor sildenafil improves energy balance and enhances in vivo insulin action in a mouse model of diet-induced insulin resistance. High-fat-fed mice treated with sildenafil plus L-arginine or sildenafil alone for 12 weeks had reduced weight and fat mass due to increased energy expenditure. However, uncoupling protein-1 levels were not increased in sildenafil-treated mice. Chronic treatment with sildenafil plus L-arginine or sildenafil alone increased arterial cGMP levels but did not adversely affect blood pressure or cardiac morphology. Sildenafil treatment, with or without l-arginine, resulted in lower fasting insulin and glucose levels and enhanced rates of glucose infusion, disappearance, and muscle glucose uptake during a hyperinsulinemic (4 mU x kg(-1) x min(-1))-euglycemic clamp in conscious mice. These effects occurred without an increase in activation of muscle insulin signaling. An acute treatment of high fat-fed mice with sildenafil plus l-arginine did not improve insulin action. These results show that phosphodiesterase-5 is a potential target for therapies aimed at preventing diet-induced energy imbalance and insulin resistance.

Microdialysis shows metabolic effects in skin during fluid resuscitation in burn-injured patients

INTRODUCTION

Established fluid treatment formulas for burn injuries have been challenged as studies have shown the presence of tissue hypoxia during standard resuscitation. Such findings suggest monitoring at the tissue level. This study was performed in patients with major burn injuries to evaluate the microdialysis technique for the continuous assessment of skin metabolic changes during fluid resuscitation and up to four days postburn.

METHODS

We conducted an experimental study in patients with a burn injury, as represented by percentage of total body surface area burned (TBSA), of more than 25% in a university eight-bed burns intensive care unit serving about 3.5 million inhabitants. Six patients with a median TBSA percentage of 59% (range 33.5% to 90%) and nine healthy controls were examined by intracutaneous MD, in which recordings of glucose, pyruvate, lactate, glycerol, and urea were performed.

RESULTS

Blood glucose concentration peaked on day two at 9.8 mmol/l (6.8 to 14.0) (median and range) and gradually declined on days three and four, whereas skin glucose in MD continued to increase throughout the study period with maximum values on day four, 8.7 mmol/l (4.9 to 11.0). Controls had significantly lower skin glucose values compared with burn patients, 3.1 mmol/l (1.5 to 4.6) (p < 0.001). Lactate from burn patients was significantly higher than controls in both injured and uninjured skin (MD), 4.6 mmol/l (1.3 to 8.9) and 3.8 mmol/l (1.6 to 7.5), respectively (p < 0.01). The skin lactate/pyruvate ratio (MD) was significantly increased in burn patients on all days (p < 0.001). Skin glycerol (MD) was significantly increased at days three and four in burn patients compared with controls (p < 0.01).

CONCLUSION

Despite a strategy that fulfilled conventional goals for resuscitation, there were increased lactate/pyruvate ratios, indicative of local acidosis. A corresponding finding was not recorded systemically. We conclude that MD is a promising tool for depicting local metabolic processes that are not fully appreciated when examined systemically. Because the local response in glucose, lactate, and pyruvate metabolism seems to differ from that recorded systemically, this technique may offer a new method of monitoring organs.