Insulin-mediated Microvascular Recruitment Research Articles

Acute Sympathetic Blockade Improves Insulin-Mediated Microvascular Blood Flow in the Forearm of Adult Human Subjects With Obesity.

Obesity is associated with resistance to the metabolic (glucose uptake) and vascular (nitric-oxide mediated dilation and microvascular recruitment) actions of insulin. These vascular effects contribute to insulin sensitivity by increasing tissue delivery of glucose. Studies by us and others suggest that sympathetic activation contributes to insulin resistance to glucose uptake. Here we tested the hypothesis that sympathetic activation contributes to impaired insulin-mediated vasodilation in adult subjects with obesity. In a randomized crossover study, we used a euglycemic hyperinsulinemic clamp in 12 subjects with obesity to induce forearm arterial vasodilation (forearm blood flow) and microvascular recruitment (contrast-enhanced ultrasonography) during an intrabrachial infusion of saline (control) or phentolamine (sympathetic blockade). Insulin increased forearm blood flow on both study days (from 2.21±1.22 to 4.89±4.21 mL/100 mL per min, P=0.003 and from 2.42±0.89 to 7.19±3.35 mL/100 mL per min, P=0.002 for the intact and blocked day, respectively). Sympathetic blockade with phentolamine resulted in a significantly greater increase in microvascular flow velocity (∆microvascular flow velocity: 0.23±0.65 versus 2.51±3.01 arbitrary intensity units (AIU/s) for saline and phentolamine respectively, P=0.005), microvascular blood volume (∆microvascular blood volume: 1.69±2.45 versus 3.76±2.93 AIU, respectively, P=0.05), and microvascular blood flow (∆microvascular blood flow: 0.28±0.653 versus 2.51±3.01 AIU2/s, respectively, P=0.0161). To evaluate if this effect was not due to nonspecific vasodilation, we replicated the study in 6 subjects with obesity comparing intrabrachial infusion of phentolamine to sodium nitroprusside. At doses that produced similar increases in forearm blood flow, insulin-induced changes in microvascular flow velocity were greater during phentolamine than sodium nitroprusside (%microvascular flow velocity=58% versus 29%, respectively, P=0.031). We conclude that sympathetic activation impairs insulin-mediated microvascular recruitment in adult subjects with obesity.

Pyridoxamine reduces methylglyoxal and markers of glycation and endothelial dysfunction, but does not improve insulin sensitivity or vascular function in abdominally obese individuals: A randomized double-blind placebo-controlled trial.

To investigate the effects of pyridoxamine (PM), a B6 vitamer and dicarbonyl scavenger, on glycation and a large panel of metabolic and vascular measurements in a randomized double-blindplacebo-controlled trial in abdominally obese individuals. Individuals (54% female; mean age 50 years; mean body mass index 32 kg/m2 ) were randomized to an 8-week intervention with either placebo (n=36), 25 mg PM (n=36) or 200 mg PM (n=36). We assessed insulin sensitivity, β-cell function, insulin-mediated microvascular recruitment, skin microvascular function, flow-mediated dilation, and plasma inflammation and endothelial function markers. PM metabolites, dicarbonyls and advanced glycation endproducts (AGEs) were measured using ultra-performance liquid chromatography tandem mass spectrometry. Treatment effects were evaluated by one-way ANCOVA. In the high PM dose group, we found a reduction of plasma methylglyoxal (MGO) and protein-bound Nδ-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine(MG-H1), as compared to placebo. We found a reduction of the endothelial dysfunction marker soluble vascular celladhesion molecule-1 (sVCAM-1) in the low and high PM dose group and of soluble intercellularadhesion molecule-1(sICAM-1) in the high PM dose, as compared to placebo. We found no treatment effects on insulin sensitivity, vascular function or other functional outcome measurements. This study shows that PM is metabolically active and reduces MGO, AGEs, sVCAM-1 and sICAM-1, but does not affect insulin sensitivity and vascular function in abdominally obese individuals. The reduction in adhesion markers is promising because these are important in the pathogenesis of endothelial damage and atherosclerosis.

SKIN MICROVASCULAR FLOWMOTION IS UNALTERED IN ABDOMINALLY OBESE MEN

Objective: Abdominal obesity represents a systemic metabolic pathology, being related to the development of insulin resistance and cardiovascular diseases. Microvascular dysfunction is multifactorial and alters, among other things, insulin-mediated glucose disposal. We have previously shown that insulin-mediated microvascular recruitment in skeletal muscle is impaired in obese men and that weight loss can reverse this impairment. Our objective was to examine whether impairment of insulin-mediated microvascular function and subsequent reversal by weight loss can be detected in skin as well using flowmotion analyses. Design and method: Twenty-five lean and 53 abdominally obese men (waist circumference 102–110 cm; aged18–65 yrs.; no cardiovascular diseases or type 2 diabetes) were recruited. Following baseline measurements, the obese men were randomized to either an 8-week program of low calorie diet (LCD), or usual diet (control). Skin flowmotion was measured by laser-doppler flowmetry and Fourier analyses resulted in the contribution of five frequency domains: endothelial, 0.01–0.02 Hz; neurogenic, 0.02–0.06 Hz; myogenic, 0.06–0.15 Hz; respiratory, 0.15–0.40 Hz and cardiac, 0.40–1.60 Hz. Flowmotion was measured before and during a hyperinsulinaemic euglycaemic clamp. Results: Baseline flowmotion was not different between lean and obese men. In both lean and obese men, hyperinsulinaemia induced a similar increase in total flowmotion (lean: 6.5% (1.5, 11.5); obese: 3.6% (0.6, 6.6); p = 0.309 vs lean) and the endothelial domain (lean: 12.3% (1.7, 22.9); obese: 8.6% (2.5, 14.7); p = 0.538 vs lean), and a decrease in the neurogenic domain (lean: -14.2% (-24.8, -3.7); obese: -4.6% (-13, 3.8); p = 0.149 vs lean). The LCD intervention did not statistically significantly affect flowmotion. Conclusions: In contrast to insulin-mediated microvascular recruitment in skeletal muscle, skin microvascular flowmotion is unaltered in abdominally obese men versus lean. In addition, a weight loss intervention in the obese men did not affect skin microvascular flowmotion. These findings indicate that the sensitivity of the microvasculature to systemic mediators may differ between tissues.

Vasodilatory Actions of Glucagon-Like Peptide 1 Are Preserved in Skeletal and Cardiac Muscle Microvasculature but Not in Conduit Artery in Obese Humans With Vascular Insulin Resistance.

Obesity is associated with microvascular insulin resistance, which is characterized by impaired insulin-mediated microvascular recruitment. Glucagon-like peptide 1 (GLP-1) recruits skeletal and cardiac muscle microvasculature, and this action is preserved in insulin-resistant rodents. We aimed to examine whether GLP-1 recruits microvasculature and improves the action of insulin in obese humans. Fifteen obese adults received intravenous infusion of either saline or GLP-1 (1.2 pmol/kg/min) for 150 min with or without a euglycemic insulin clamp (1 mU/kg/min) superimposed over the last 120 min. Skeletal and cardiac muscle microvascular blood volume (MBV), flow velocity and blood flow, brachial artery diameter and blood flow, and pulse wave velocity (PWV) were determined. Insulin failed to change MBV or flow in either skeletal or cardiac muscle, confirming the presence of microvascular insulin resistance. GLP-1 infusion alone increased MBV by ∼30% and ∼40% in skeletal and cardiac muscle, respectively, with no change in flow velocity, leading to a significant increase in microvascular blood flow in both skeletal and cardiac muscle. Superimposition of insulin to GLP-1 infusion did not further increase MBV or flow in either skeletal or cardiac muscle but raised the steady-state glucose infusion rate by ∼20%. Insulin, GLP-1, and GLP-1 + insulin infusion did not alter brachial artery diameter and blood flow or PWV. The vasodilatory actions of GLP-1 are preserved in both skeletal and cardiac muscle microvasculature, which may contribute to improving metabolic insulin responses and cardiovascular outcomes. In obese humans with microvascular insulin resistance, GLP-1's vasodilatory actions are preserved in both skeletal and cardiac muscle microvasculature, which may contribute to improving metabolic insulin responses and cardiovascular outcomes.

Perivascular Adipose Tissue Controls Insulin-Stimulated Perfusion, Mitochondrial Protein Expression, and Glucose Uptake in Muscle Through Adipomuscular Arterioles.

Insulin-mediated microvascular recruitment (IMVR) regulates delivery of insulin and glucose to insulin-sensitive tissues. We have previously proposed that perivascular adipose tissue (PVAT) controls vascular function through outside-to-inside communication and through vessel-to-vessel, or "vasocrine," signaling. However, direct experimental evidence supporting a role of local PVAT in regulating IMVR and insulin sensitivity in vivo is lacking. Here, we studied muscles with and without PVAT in mice using combined contrast-enhanced ultrasonography and intravital microscopy to measure IMVR and gracilis artery diameter at baseline and during the hyperinsulinemic-euglycemic clamp. We show, using microsurgical removal of PVAT from the muscle microcirculation, that local PVAT depots regulate insulin-stimulated muscle perfusion and glucose uptake in vivo. We discovered direct microvascular connections between PVAT and the distal muscle microcirculation, or adipomuscular arterioles, the removal of which abolished IMVR. Local removal of intramuscular PVAT altered protein clusters in the connected muscle, including upregulation of a cluster featuring Hsp90ab1 and Hsp70 and downregulation of a cluster of mitochondrial protein components of complexes III, IV, and V. These data highlight the importance of PVAT in vascular and metabolic physiology and are likely relevant for obesity and diabetes.

Abstract P1122: Sympathetic Vasodilation Improves Insulin-Mediated Microvascular Recruitment in the Forearm of Obese Subjects

Background: Obesity is associated with sympathetic activation which results in insulin resistance and a blunted insulin-mediated microvascular recruitment. We propose that removal of sympathetic vasoconstriction can result in improvement of insulin-mediated vasodilation and subsequently in sensitivity to insulin-mediated glucose uptake. Methods: We blocked sympathetic vasoconstriction in an isolated forearm model during a hyperinsulinemic euglycemic clamp to determine the effect of insulin recruitment on microvascular circulation using contrast-enhanced ultrasonography (CEU). We studied 9 obese human subjects (age 41±6 years, BMI 37±3 kg*m -2 , SBP 125±6 mm Hg). We assessed the effects of insulin on microvascular recruitment and glucose uptake on two separate occasions, randomly assigned and at least one month apart, during an intrabrachial infusion of the alpha-adrenergic blocker phentolamine (PHE, 25 micrograms/min, blocked day) or saline (SAL, Control day). Subjects were studied at baseline (BSL) and at the end of the clamp (CLAMP). Results: As expected forearm blood flow increased after insulin alone (67±27%, p=0.016) and with alpha blockade and insulin (140± 22%, p=0.0011). Microvascular blood volume (MBV, index of microvascular recruitment), showed a significant increased during PHE compared to SAL (figure). Muscle glucose uptake trended towards increase with PHE but did not reach statistical significance (p=0.0558). There were no systemic effects due to sympathetic blockade. Conclusions: This data shows that sympathetic vasoconstriction contributes to the blunted insulin-mediated microvascular recruitment seen in obesity.

Perivascular Adipose Tissue Controls Insulin-Stimulated Perfusion and Glucose Uptake in Muscle through Adipomuscular Microvascular Anastomoses

Insulin-mediated microvascular recruitment (IMVR) regulates delivery of insulin and glucose to insulin-sensitive tissues. We have previously proposed that perivascular adipose tissue (PVAT) controls glucose metabolism and vascular function through outside-to-inside communication and through vessel-to-vessel, or “vasocrine” signaling. Here, we studied this hypothesis in mice by examining effects of removal of local intramuscular PVAT on muscle blood flow and glucose metabolism. Using the hyperinsulinemic, euglycemic clamp (HEC) in combination with positron emission tomography, we found that local PVAT removal transiently reduces muscle glucose uptake by ±50 percent. Contrast-enhanced ultrasonography and intravital microscopy of the gracilis artery (GA) during the HEC showed that PVAT removal abolishes insulin-induced increases in GA diameter and abrogated insulin-stimulated muscle blood volume (microvascular recruitment or IMVR). The effect of PVAT on IMVR was mediated by distinct microvessels or anastomoses, which we showed using lightsheet microscopy of mice expressing mCherry in endothelial cells. Proteomics analysis revealed that PVAT removal significantly alters expression of 109 of 1719 detected proteins in muscle. Observed changes in protein expression included reduction of a mitochondrial protein cluster and of vesicle-associated membrane protein 5 (Vamp5), involved in Glut4 trafficking. In conclusion, we have found that PVAT within muscle regulates muscle perfusion, glucose uptake and muscle protein expression, communicating with the distal microcirculation via microvascular anastomoses. These data highlight the importance of PVAT in vascular and metabolic physiology, and are relevant for type 2 diabetes and associated muscle dysfunction. Disclosure A.H. Turaihi: None. E.H. Serne: None. C. Molthoff: None. M.T. Goumans: None. C.J. Jimenez: None. J.S. Yudkin: None. Y.M. Smulders: None. V.W. van Hinsbergh: None. E.C. Eringa: None.

Glucagon-Like Peptide-1 (GLP-1) Increases Skeletal and Cardiac Muscle, Microvascular Perfusion, and Improves Metabolic Insulin Action in Obese Humans

Obesity is associated with metabolic and microvascular insulin resistance. The latter is characterized by impaired insulin-mediated microvascular recruitment. GLP-1 engenders vasodilation in addition to its glycemic actions. At physiological concentrations, GLP-1 acutely recruits skeletal and cardiac muscle microvasculature in healthy humans and this action is preserved in the insulin resistant rodents. To examine whether GLP-1 recruits microvasculature and improves insulin’s action in obese humans, 15 obese adults (age 27 ± 2.2, BMI 33.7 ± 1.2) were studied thrice at random order after an overnight fast. Subjects received an intravenous infusion of GLP-1 (1.2 pmol/kg/min) for 150 mins or an intravenous infusion of either normal saline or GLP-1 (1.2 pmol/kg/min) for 150 mins with a euglycemic insulin clamp (1 mU/kg/min) superimposed over the last 120 mins. Skeletal and cardiac muscle microvascular blood volume (MBV), flow velocity (MFV) and blood flow (MBF) were determined at 0, 30 and 150 min. Steady-state glucose infusion rate (GIR) was calculated. Insulin infusion alone did not change MBV or MBF in either skeletal or cardiac muscle, confirming the presence of microvascular insulin resistance. GLP-1 infusion alone increased MBV by ∼30% and ∼40% in skeletal and cardiac muscle respectively (p<0.05) without affecting MFV, leading to a significant increase in MBF in both skeletal and cardiac muscle. Superimposition of insulin infusion to GLP-1 infusion did not further increase MBV or MBF in either skeletal or cardiac muscle, but raised the steady-state GIR from 4.1± 0.4 to 5.0 ± 0.5 mg/kg/min (p<0.05). We conclude that in obese humans with microvascular insulin resistance GLP-1's vasodilatory actions are preserved in both skeletal and cardiac muscle microvasculature. This may contribute to improving metabolic responses to insulin and decreasing cardiovascular morbidity and mortality associated with obesity and diabetes. Disclosure N. Wang: None. A. Tan: None. L. Jahn: None. L. Hartline: None. K.W. Aylor: None. E. Barrett: None. Z. Liu: None.

Liraglutide prevents microvascular insulin resistance and preserves muscle capillary density in high-fat diet-fed rats.

Muscle microvasculature critically regulates endothelial exchange surface area to facilitate transendothelial delivery of insulin, nutrients, and oxygen to myocytes. Insulin resistance blunts insulin-mediated microvascular recruitment and decreases muscle capillary density; both contribute to lower microvascular blood volume. Glucagon-like peptide 1 (GLP-1) and its analogs are able to dilate blood vessels and stimulate endothelial cell proliferation. In this study, we aim to determine the effects of sustained stimulation of the GLP-1 receptors on insulin-mediated capillary recruitment and metabolic insulin responses, small arterial endothelial function, and muscle capillary density. Rats were fed a high-fat diet (HFD) for 4 wk with or without simultaneous administration of liraglutide and subjected to a euglycemic hyperinsulinemic clamp for 120 min after an overnight fast. Insulin-mediated muscle microvascular recruitment and muscle oxygenation were determined before and during insulin infusion. Muscle capillary density was determined and distal saphenous artery used for determination of endothelial function and insulin-mediated vasodilation. HFD induced muscle microvascular insulin resistance and small arterial vessel endothelial dysfunction and decreased muscle capillary density. Simultaneous treatment of HFD-fed rats with liraglutide prevented all of these changes and improved insulin-stimulated glucose disposal. These were associated with a significantly increased AMPK phosphorylation and the expressions of VEGF and its receptors. We conclude that GLP-1 receptor agonists may exert their salutary glycemic effect via improving microvascular insulin sensitivity and muscle capillary density during the development of insulin resistance, and early use of GLP-1 receptor agonists may attenuate metabolic insulin resistance as well as prevent cardiovascular complications of diabetes.

Differential effects of glucagon-like peptide-1 on microvascular recruitment and glucose metabolism in short- and long-term insulin resistance.

Acute glucagon-like peptide-1 (GLP-1) infusion reversed the high fat diet-induced microvascular insulin resistance that occurred after both 5days and 8weeks of a high fat diet intervention. When GLP-1 was co-infused with insulin it had overt effects on whole body insulin sensitivity as well as insulin-mediated skeletal muscle glucose uptake after 5days of a high fat diet, but not after 8weeks of high fat diet intervention. Acute GLP-1 infusion did not have an additive effect to that of insulin on microvascular recruitment or skeletal muscle glucose uptake in the control group. Here we demonstrate that GLP-1 potently increases the microvascular recruitment in rat skeletal muscle but does not increase glucose uptake in the fasting state. Thus, like insulin, GLP-1 increased the microvascular recruitment but unlike insulin, GLP-1 had no direct effect on skeletal muscle glucose uptake. Acute infusion of glucagon-like peptide-1 (GLP-1) has potent effects on blood flow distribution through the microcirculation in healthy humans and rats. A high fat diet induces impairments in insulin-mediated microvascular recruitment (MVR) and muscle glucose uptake, and here we examined whether this could be reversed by GLP-1. Using contrast-enhanced ultrasound, microvascular recruitment was assessed by continuous real-time imaging of gas-filled microbubbles in the microcirculation after acute (5days) and prolonged (8weeks) high fat diet (HF)-induced insulin resistance in rats. A euglycaemic hyperinsulinaemic clamp (3mUmin(-1) kg(-1) ), with or without a co-infusion of GLP-1 (100pmoll(-1) ), was performed in anaesthetized rats. Consumption of HF attenuated the insulin-mediated MVR in both 5day and 8week HF interventions which was associated with a 50% reduction in insulin-mediated glucose uptake compared to controls. Acute administration of GLP-1 restored the normal microvascular response by increasing the MVR after both 5days and 8weeks of HF intervention (P<0.05). This effect of GLP-1 was associated with a restoration of both whole body insulin sensitivity and increased insulin-mediated glucose uptake in skeletal muscle by 90% (P<0.05) after 5days of HF but not after 8weeks of HF. The present study demonstrates that GLP-1 increases MVR in rat skeletal muscle and can reverse early stages of high fat diet-induced insulin resistance in vivo.

A vascular mechanism for high-sodium-induced insulin resistance in rats

High sodium (HS) effects on hypertension are well established. Recent evidence implicates a relationship between HS intake and insulin resistance, even in the absence of hypertension. The aim of the current study was to determine whether loss of the vascular actions of insulin may be the driving factor linking HS intake to insulin resistance. Sprague Dawley rats were fed a control (0.31% wt/wt NaCl) or HS (8.00% wt/wt NaCl) diet for 4 weeks and subjected to euglycaemic-hyperinsulinaemic clamp (10 mU min(-1) kg(-1)) or constant-flow pump-perfused hindlimb studies following an overnight fast. A separate group of HS rats was given quinapril during the dietary intervention and subjected to euglycaemic-hyperinsulinaemic clamp as above. HS intake had no effect on body weight or fat mass or on fasting glucose, insulin, endothelin-1 or NEFA concentrations. However, HS impaired whole body and skeletal muscle glucose uptake, in addition to a loss of insulin-stimulated microvascular recruitment. These effects were present despite enhanced insulin signalling (Akt) in both liver and skeletal muscle. Constant-flow pump-perfused hindlimb experiments revealed normal insulin-stimulated myocyte glucose uptake in HS-fed rats. Quinapril treatment restored insulin-mediated microvascular recruitment and muscle glucose uptake in vivo. HS-induced insulin resistance is driven by impaired microvascular responsiveness to insulin, and is not due to metabolic or signalling defects within myocytes or liver. These results imply that reducing sodium intake may be important not only for management of hypertension but also for insulin resistance, and highlight the vasculature as a potential therapeutic target in the prevention of insulin resistance.

Local NOS inhibition impairs vascular and metabolic actions of insulin in rat hindleg muscle in vivo

Insulin stimulates microvascular recruitment in skeletal muscle, and this vascular action augments muscle glucose disposal by ∼40%. The aim of the current study was to determine the contribution of local nitric oxide synthase (NOS) to the vascular actions of insulin in muscle. Hooded Wistar rats were infused with the NOS inhibitor N(ω)-nitro-L-arginine methylester (L-NAME, 10 μM) retrogradely via the epigastric artery in one leg during a systemic hyperinsulinemic-euglycemic clamp (3 mU·min(-1)·kg(-1) × 60 min) or saline infusion. Femoral artery blood flow, microvascular blood flow (assessed from 1-methylxanthine metabolism), and muscle glucose uptake (2-deoxyglucose uptake) were measured in both legs. Local L-NAME infusion did not have any systemic actions on blood pressure or heart rate. Local L-NAME blocked insulin-stimulated changes in femoral artery blood flow (84%, P < 0.05) and microvascular recruitment (98%, P < 0.05), and partially blocked insulin-mediated glucose uptake in muscle (reduced by 34%, P < 0.05). L-NAME alone did not have any metabolic effects in the hindleg. We conclude that insulin-mediated microvascular recruitment is dependent on local activation of NOS in muscle and that this action is important for insulin's metabolic actions.

Decreased Permeability Surface Area for Glucose in Obese Women with Postprandial Hyperglycemia: No Effect of Phosphodiesterase-5 (PDE-5) Inhibition

Insulin-mediated microvascular recruitment is recognized as a potential mechanism contributing to insulin resistance. In this study, we compared a marker of microvascular function, the permeability surface area for glucose (PS(glu)), and forearm glucose uptake after an OGTT in obese women with impaired glucose metabolism and healthy lean nondiabetic women, with the aim to characterize whether decreased permeability surface area for glucose or decreased glucose uptake may contribute to postprandial hyperglycemia in the obese group. In addition, we evaluated whether the phosphodiesterase-5 (PDE-5) inhibitor tadalafil, in a randomized double blind placebo controlled design, might attenuate postprandial glucose levels in obese women. For these purposes, intramuscular microdialysis, blood sampling from arterial and venous blood of the forearm, and measurements of forearm blood flow were performed. The results showed an impaired permeability surface area for glucose (IAUC PS(glu) 31±13 vs. 124±31; p<0.05) in obese when compared with lean participants, but no differences in forearm glucose uptake appeared between the groups. Furthermore, a single dose of tadalafil 10 mg showed no improvement of the permeability surface area for glucose, glucose uptake, or circulating glucose levels in obese participants. In conclusion, the postprandial PS(glu) response was impaired in obese women showing postprandial hyperglycemia, indicating a compromised microcirculation. However, we were unable to demonstrate any acute effect on either vascular function or glucose uptake of the phosphodiesterase-5 (PDE-5) inhibitor tadalafil.

Globular Adiponectin Enhances Muscle Insulin Action via Microvascular Recruitment and Increased Insulin Delivery

Adiponectin enhances insulin action and induces nitric oxide-dependent vasodilatation. Insulin delivery to muscle microcirculation and transendothelial transport are 2 discrete steps that limit insulin's action. We have shown that expansion of muscle microvascular surface area increases muscle insulin delivery and action. To examine whether adiponectin modulates muscle microvascular recruitment thus insulin delivery and action in vivo. Overnight fasted adult male rats were studied. We determined the effects of adiponectin on muscle microvascular recruitment, using contrast-enhanced ultrasound, on insulin-mediated microvascular recruitment and whole-body glucose disposal, using contrast-enhanced ultrasound and insulin clamp, and on muscle insulin clearance and uptake with (125)I-insulin. Globular adiponectin potently increased muscle microvascular blood volume without altering microvascular blood flow velocity, leading to a significantly increased microvascular blood flow. This was paralleled by a ≈30% to 40% increase in muscle insulin uptake and clearance, and ≈30% increase in insulin-stimulated whole-body glucose disposal. Inhibition of endothelial nitric oxide synthase abolished globular adiponectin-mediated muscle microvascular recruitment and insulin uptake. In cultured endothelial cells, globular adiponectin dose-dependently increased endothelial nitric oxide synthase phosphorylation but had no effect on endothelial cell internalization of insulin. Globular adiponectin increases muscle insulin uptake by recruiting muscle microvasculature, which contributes to its insulin-sensitizing action.

Muscle insulin resistance resulting from impaired microvascular insulin sensitivity in Sprague Dawley rats

Enhanced microvascular perfusion of skeletal muscle is important for nutrient exchange and contributes ∼40% insulin-mediated muscle glucose disposal. High fat-fed (36% fat wt./wt.) rats are a commonly used model of insulin-resistance that exhibit impairment of insulin-mediated microvascular recruitment and muscle glucose uptake, which is accompanied by myocyte insulin-resistance. Distinguishing the contribution of impaired microvascular recruitment and impaired insulin action in the myocyte to decreased muscle glucose uptake in these high-fat models is difficult. It is unclear whether microvascular and myocyte insulin-resistance develop simultaneously. To assess this, we used a rat diet model with a moderate increase (two-fold) in dietary fat. Sprague Dawley rats fed normal (4.8% fat wt./wt., 5FD) or high (9.0% fat wt./wt., 9FD) fat diets for 4 weeks were subject to euglycaemic hyperinsulinemic clamp (10 mU/min/kg insulin or saline) or isolated hindlimb perfusion (1.5 or 15 nM insulin or saline). Body weight, epididymal fat mass, and fasting plasma glucose were unaffected by diet. Fasting plasma insulin and non-esterified fatty acid concentrations were significantly elevated in 9FD. Glucose infusion rate and muscle glucose uptake were significantly impaired during insulin clamps in 9FD. Insulin-stimulated microvascular recruitment was significantly blunted in 9FD. Insulin-mediated muscle glucose uptake between 5FD and 9FD were not different during hindlimb perfusion. Impaired insulin-mediated muscle glucose uptake in vivo can be the direct result of reduced microvascular blood flow responses to insulin, and can result from small (two-fold) increases in dietary fat. Thus, microvascular insulin-resistance can occur independently to the development of myocyte insulin-resistance.

Early Microvascular Recruitment Modulates Subsequent Insulin-Mediated Skeletal Muscle Glucose Metabolism During Lipid Infusion

OBJECTIVETo test whether early, insulin-mediated microvascular recruitment in skeletal muscle predicts steady-state glucose metabolism in the setting of physiological elevation of free fatty acid concentrations.RESEARCH DESIGN AND METHODSWe measured insulin’s microvascular and metabolic effects in 14 healthy young adults during a 2-h euglycemic insulin clamp. Plasma free fatty acid concentrations were raised (Intralipid and heparin infusion) for 3 h before the clamp and maintained at postprandial concentrations during the clamp. Microvascular blood volume (MBV) was measured by contrast-enhanced ultrasound (CEU) continuously from baseline through the first 30 min of the insulin clamp. Muscle glucose and insulin uptake were measured by the forearm balance method.RESULTSThe glucose infusion rate (GIR) necessary to maintain euglycemia during the clamp varied by fivefold across subjects (2.5–12.5 mg/min/kg). The early MBV responses to insulin, as indicated by CEU video intensity, ranged widely, from a 39% decline to a 69% increase. During the clamp, steady state forearm muscle glucose uptake and GIR each correlated significantly with the change in forearm MBV (P < 0.01). To explore the basis for the wide range of vascular and metabolic insulin sensitivity observed, we also measured Vo2max in a subset of eight subjects. Fitness (Vo2max) correlated significantly with the GIR, the forearm glucose uptake, and the percentage change in MBV during the insulin clamp (P < 0.05 for each).CONCLUSIONSEarly microvascular responses to insulin strongly associate with steady state skeletal muscle insulin-mediated glucose uptake. Physical fitness predicts both metabolic and vascular insulin responsiveness.

Insulin‐Induced Microvascular Recruitment in Skin and Muscle are Related and Both are Associated with Whole‐Body Glucose Uptake

Insulin-induced capillary recruitment is considered a determinant of insulin-mediated glucose uptake. Insulin action on the microvasculature has been assessed in skin; however, there is concern as to whether the vascular responses observed in skin reflect those in the muscle. We hypothesized that insulin-induced capillary recruitment in skin would correlate with microvascular recruitment in muscle in a group of subjects displaying a wide variation in insulin sensitivity. Capillary recruitment in skin was assessed using capillary videomicroscopy, and skeletal muscle microvascular recruitment (i.e., increase in MBV) was studied using CEU in healthy volunteers (n = 18, mean age: 30.6 ± 11.1 years). Both microvascular measurements were performed during saline infusion, and during a hyperinsulinemic euglycemic clamp. During hyperinsulinemia, capillary recruitment in skin was augmented from 58.1 ± 18.2% to 81.0 ± 23.9% (p < 0.0001). Hyperinsulinemia increased MBV in muscle from 7.00 (2.66-17.67) to 10.06 (2.70-41.81) units (p = 0.003). Insulin's vascular effect in skin and muscle was correlated (r = 0.57). Insulin's microvascular effects in skin and muscle showed comparable strong correlations with insulin-mediated glucose uptake (r = 0.73 and 0.68, respectively). Insulin-augmented capillary recruitment in skin parallels insulin-mediated microvascular recruitment in muscle and both are related to insulin-mediated glucose uptake.

Microvascular blood flow responses to muscle contraction are not altered by high‐fat feeding in rats

Exercise and insulin each increase microvascular blood flow and enhance glucose disposal in skeletal muscle. We have reported that insulin-mediated microvascular recruitment in a diet-induced model of insulin resistance (high-fat feeding for 4 weeks) is markedly impaired; however, the effect of muscle contraction in this model has not been previously explored. We fed rats either normal (ND, 10% calories from fat) or high-fat (HFD, 60% calories from fat) diets ad libitum for 4-8 weeks. Animals were then anaesthetized and one hindlimb electrically stimulated to contract at 0.05, 0.1 and 2 Hz (field stimulation, 30 V, 0.1 ms duration) in 15 min stepwise increments. Femoral artery blood flow (Transonic flow probe), muscle microvascular blood flow (hindleg metabolism of 1-methylxanthine and contrast-enhanced ultrasound) and muscle glucose disposal (uptake of radiolabelled 2-deoxy-d-glucose and hindleg glucose disappearance) were measured. Both ND and HFD rats received the same voltage across the leg and consequently developed the same muscle tension. Femoral artery blood flow in the contracting leg increased during 2 Hz contraction, but not during the lower frequencies and these effects were similar between ND and HFD rats. Muscle microvascular blood flow significantly increased in a contraction frequency-dependent manner, and preceded increases in total limb blood flow and these effects were similar between ND and HFD rats. Muscle glucose disposal was markedly elevated during 2 Hz contraction and was comparable between ND and HFD rats. Contraction-mediated muscle microvascular recruitment and glucose uptake are not impaired in the HFD insulin resistant rat.

Salsalate Attenuates Free Fatty Acid–Induced Microvascular and Metabolic Insulin Resistance in Humans

OBJECTIVEInsulin recruits muscle microvasculature, thereby increasing endothelial exchange surface area. Free fatty acids (FFAs) cause insulin resistance by activating inhibitor of κB kinase β. Elevating plasma FFAs impairs insulin’s microvascular and metabolic actions in vivo. Whether salsalate, an anti-inflammatory agent, prevents FFA-induced microvascular and/or metabolic insulin resistance in humans is unknown.RESEARCH DESIGN AND METHODSEleven healthy, young adults were studied three times in random order. After an overnight fast, on two occasions each subject received a 5-h systemic infusion of Intralipid ± salsalate pretreatment (50 mg/kg/day for 4 days). On the third occasion, saline replaced Intralipid. A 1 mU/kg/min euglycemic insulin clamp was superimposed over the last 2-h of each study. Skeletal and cardiac muscle microvascular blood volume (MBV), microvascular flow velocity (MFV), and microvascular blood flow (MBF) were determined before and after insulin infusion. Whole body glucose disposal rates were calculated from glucose infusion rates.RESULTSInsulin significantly increased skeletal and cardiac muscle MBV and MBF without affecting MFV. Lipid infusion abolished insulin-mediated microvascular recruitment in both skeletal and cardiac muscle and lowered insulin-stimulated whole body glucose disposal (P < 0.001). Salsalate treatment rescued insulin’s actions to recruit muscle microvasculature and improved insulin-stimulated whole body glucose disposal in the presence of high plasma FFAs.CONCLUSIONSHigh plasma concentrations of FFAs cause both microvascular and metabolic insulin resistance, which can be prevented or attenuated by salsalate treatment. Our data suggest that treatments aimed at inhibition of inflammatory response might help alleviate vascular insulin resistance and improve metabolic control in patients with diabetes.

Rebuttal from Drs. Clark, Rattigan, Barrett, and Vincent

POINT-COUNTERPOINTRebuttal from Drs. Clark, Rattigan, Barrett, and VincentPublished Online:01 Mar 2008https://doi.org/10.1152/japplphysiol.00779.2007bMoreSectionsPDF (39 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat We congratulate David Poole and his associates for a clear and thoughtful argument (6). The accompanying intravital microscopy video of the spinotrapezius muscle is equally instructive where over 80% of the capillaries are supporting RBC flow at rest.However, the data are only as good as the techniques used to acquire them, and the authors themselves (6) acknowledge some of the problems associated with exposing a muscle such as the spinotrapezius for viewing under a microscope. These problems, which could alter the extent of capillary recruitment, include the effect of manipulations required to expose the muscle; the question of whether the thin muscles that can be exposed are representative of the cylindrical load-bearing muscles; the effects of anesthesia; and the superfusion Po2. Since the weight of argument against exercise-mediated capillary recruitment is based on studies using intravital microscopy it is essential that the findings from this system can be extrapolated to in vivo. Not least among the problems is the superfusion Po2 where suffusate buffer for intravital microscopy of isolated muscles is invariably gassed with 5% CO2 in 95% N2 [e.g. tenuissimus (4); spinotrapezius (3); cremaster (2)]. Comparison with a suffusate Po2 dose curve (7) suggests this could have a marked impact to increase the number of capillaries perfused at rest.In contrast, our endeavours in this field have focused on noninvasive estimations of microvascular perfusion in bulk load-bearing muscle in vivo that has not been manipulated nor perturbed by invasive conditions. Ultrasound imaging of a region of interest comprising ∼15 g of human forearm muscle of conscious healthy humans has indicated capillary recruitment to occur in response to insulin infusion under euglycemic conditions, to a mixed meal, and to exercise (8), and that the insulin response is impaired in obese insulin-resistant patients (1).Finally, despite the differences in our two approaches, an impaired blood-muscle exchange has been identified in animal models of type 2 diabetes (5, 9). The percentage of RBC-perfused capillaries is decreased in the Goto-Kakizaki rat (5) and we find that the obese Zucker rat has impaired insulin-mediated capillary recruitment (9). Future experiments may reveal that the vascular dysfunction that is responsible for the impaired insulin response in vivo (9) is also that which prevents capillary recruitment when the muscle is exposed, irrigated with N2 buffer, and viewed microscopically (5). “The voyage of discovery is not in seeking new landscapes but in having new eyes.”—Marcel ProustREFERENCES1 Clerk LH, Vincent MA, Jahn LA, Liu Z, Lindner JR, Barrett EJ. Obesity blunts insulin-mediated microvascular recruitment in human forearm muscle. Diabetes 55: 1436–1442, 2006.Crossref | PubMed | ISI | Google Scholar2 Gorczynski RJ, Klitzman B, Duling BR. Interrelations between contracting striated muscle and precapillary microvessels. Am J Physiol Heart Circ Physiol 235: H494–H504, 1978.Link | ISI | Google Scholar3 Kindig CA, Richardson TE, Poole DC. Skeletal muscle capillary hemodynamics from rest to contractions: implications for oxygen transfer. J Appl Physiol 92: 2513–2520, 2002.Link | ISI | Google Scholar4 Lindbom L. Microvascular blood flow distribution in skeletal muscle. Acta Physiol Scand Suppl 525: 1–40, 1983.PubMed | Google Scholar5 Padilla DJ, McDonough P, Behnke BJ, Kano Y, Hageman KS, Musch TI, Poole DC. Effects of Type II diabetes on capillary hemodynamics in skeletal muscle. Am J Physiol Heart Circ Physiol 291: H2439–H2444, 2006.Link | ISI | Google Scholar6 Poole DC, Brown MD, Hudlicka O.Counterpoint: There is not capillary recruitment in active skeletal muscle during exercise. J Appl Physiol; doi:10.1152/japplphysiol.00779.2007a.Link | ISI | Google Scholar7 Shibata M, Ichioka S, Togawa T, Kamiya A. Arterioles' contribution to oxygen supply to the skeletal muscles at rest. Eur J Appl Physiol 97: 327–331, 2006.Crossref | PubMed | ISI | Google Scholar8 Vincent MA, Clerk LH, Lindner JR, Price WJ, Jahn LA, Leong-Poi H, Barrett EJ. Mixed meal and light exercise each recruit muscle capillaries in healthy humans. Am J Physiol Endocrinol Metab 290: E1191–E1197, 2006.Link | ISI | Google Scholar9 Wallis MG, Wheatley CM, Rattigan S, Barrett EJ, Clark ADH, Clark MG. Insulin-mediated hemodynamic changes are impaired in muscle of Zucker obese rats. Diabetes 51: 3492–3498, 2002.Crossref | PubMed | ISI | Google Scholar Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 104Issue 3March 2008Pages 893-893 Copyright & PermissionsCopyright © 2008 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00779.2007bHistory Published online 1 March 2008 Published in print 1 March 2008 Metrics Downloaded 133 times