Wenhong Cao

Glucagon-Like Peptide 1 Recruits Microvasculature and Increases Glucose Use in Muscle via a Nitric Oxide–Dependent Mechanism

Glucagon-like peptide 1 (GLP-1) increases tissue glucose uptake and causes vasodilation independent of insulin. We examined the effect of GLP-1 on muscle microvasculature and glucose uptake. After confirming that GLP-1 potently stimulates nitric oxide (NO) synthase (NOS) phosphorylation in endothelial cells, overnight-fasted adult male rats received continuous GLP-1 infusion (30 pmol/kg/min) for 2 h plus or minus NOS inhibition. Muscle microvascular blood volume (MBV), microvascular blood flow velocity (MFV), and microvascular blood flow (MBF) were determined. Additional rats received GLP-1 or saline for 30 min and muscle insulin clearance/uptake was determined. GLP-1 infusion acutely increased muscle MBV (P < 0.04) within 30 min without altering MFV or femoral blood flow. This effect persisted throughout the 120-min infusion period, leading to a greater than twofold increase in muscle MBF (P < 0.02). These changes were paralleled with increases in plasma NO levels, muscle interstitial oxygen saturation, hind leg glucose extraction, and muscle insulin clearance/uptake. NOS inhibition blocked GLP-1–mediated increases in muscle MBV, glucose disposal, NO production, and muscle insulin clearance/uptake. In conclusion, GLP-1 acutely recruits microvasculature and increases basal glucose uptake in muscle via a NO-dependent mechanism. Thus, GLP-1 may afford potential to improve muscle insulin action by expanding microvascular endothelial surface area.

Free Fatty Acids Induce Insulin Resistance in Both Cardiac and Skeletal Muscle Microvasculature in Humans

CONTEXTnInsulin recruits microvasculature in both cardiac and skeletal muscle, which increases the endothelial exchange surface area. Plasma concentrations of free fatty acids (FFAs) are elevated in patients with diabetes, which impairs insulin-mediated skeletal muscle microvascular recruitment.nnnOBJECTIVEnThe objective of the study was to examine whether elevated FFAs likewise cause insulin resistance in cardiac muscle microvasculature.nnnSETTINGnThe study was conducted at the General Clinical Research Center at the University of Virginia.nnnMETHODSnTwenty-two healthy, young adults were studied twice in random order after an overnight fast. Each subject received a 5-h systemic infusion of either saline or Intralipid/heparin with a 1 mU/min · kg euglycemic insulin clamp superimposed for the last 2 h. Cardiac and forearm skeletal muscle microvascular blood volume (MBV) and flow velocity were measured and microvascular blood flow (MBF) calculated before and at the end of the insulin infusion.nnnRESULTSnInsulin significantly increased MBV and MBF in both cardiac (P < 0.0001 for both) and skeletal (P = 0.008 and < 0.03, respectively) muscle. Microvascular flow velocity increased slightly but significantly in the skeletal (P = 0.04) but not in cardiac muscle. Lipid infusion lowered insulin-stimulated whole-body glucose disposal and abolished insulin-mediated increases in MBV and MBF in both cardiac and skeletal muscle. Whole-body insulin sensitivity predicted skeletal but not cardiac muscle microvascular responses to insulin. Insulin even decreased skeletal muscle MBV during lipid infusion in subjects who were moderately sensitive to insulin metabolically.nnnCONCLUSIONSnIn conclusion, high plasma concentrations of FFAs cause insulin resistance in cardiac as well as skeletal muscle microvasculature in healthy humans. This may contribute to the association of cardiac complications with metabolic insulin resistance in diabetes.

Angiotensin II Type 1 and Type 2 Receptors Regulate Basal Skeletal Muscle Microvascular Volume and Glucose Use

Angiotensin II causes vasoconstriction via the type 1 receptor (AT1R) and vasodilatation through the type 2 receptor (AT2R). Both are expressed in muscle microvasculature, where substrate exchanges occur. Whether they modulate basal muscle microvascular perfusion and substrate metabolism is not known. We measured microvascular blood volume (MBV), a measure of microvascular surface area and perfusion, in rats during systemic infusion of angiotensin II at either 1 or 100 ng/kg per minute. Each caused a significant increase in muscle MBV. Likewise, administration of the AT1R blocker losartan increased muscle MBV by >3-fold (P<0.001). Hindleg glucose extraction and muscle interstitial oxygen saturation simultaneously increased by 2- to 3-fold. By contrast, infusing AT2R antagonist PD123319 significantly decreased muscle MBV by ≥80% (P<0.001). This was associated with a significant decrease in hindleg glucose extraction and muscle oxygen saturation. AT2R antagonism and inhibition of NO synthase each blocked the losartan-induced increase in muscle MBV and glucose uptake. In conclusion, angiotensin II acts on both AT1R and AT2R to regulate basal muscle microvascular perfusion. Basal AT1R tone restricts muscle MBV and glucose extraction, whereas basal AT2R activity increases muscle MBV and glucose uptake. Pharmacological manipulation of the balance of AT1R and AT2R activity affords the potential to improve glucose metabolism.

Angiotensin II receptors modulate muscle microvascular and metabolic responses to insulin in vivo.

OBJECTIVE Angiotensin (ANG) II interacts with insulin-signaling pathways to regulate insulin sensitivity. The type 1 (AT1R) and type 2 (AT2R) receptors reciprocally regulate basal perfusion of muscle microvasculature. Unopposed AT2R activity increases muscle microvascular blood volume (MBV) and glucose extraction, whereas unopposed AT1R activity decreases both. The current study examined whether ANG II receptors modulate muscle insulin delivery and sensitivity. RESEARCH DESIGN AND METHODS Overnight-fasted rats were studied. In protocol 1, rats received a 2-h infusion of saline, insulin (3 mU/kg/min), insulin plus PD123319 (AT2R blocker), or insulin plus losartan (AT1R blocker, intravenously). Muscle MBV, microvascular flow velocity, and microvascular blood flow (MBF) were determined. In protocol 2, rats received 125I-insulin with or without PD123319, and muscle insulin uptake was determined. RESULTS Insulin significantly increased muscle MBV and MBF. AT2R blockade abolished insulin-mediated increases in muscle MBV and MBF and decreased insulin-stimulated glucose disposal by ~30%. In contrast, losartan plus insulin increased muscle MBV by two- to threefold without further increasing insulin-stimulated glucose disposal. Plasma nitric oxide increased by >50% with insulin and insulin plus losartan but not with insulin plus PD123319. PD123319 markedly decreased muscle insulin uptake and insulin-stimulated Akt phosphorylation. CONCLUSIONS We conclude that both AT1Rs and AT2Rs regulate insulin’s microvascular and metabolic action in muscle. Although AT1R activity restrains muscle metabolic responses to insulin via decreased microvascular recruitment and insulin delivery, AT2R activity is required for normal microvascular responses to insulin. Thus, pharmacologic manipulation aimed at increasing the AT2R-to-AT1R activity ratio may afford the potential to improve muscle insulin sensitivity and glucose metabolism.

Protein kinase A mediates glucagon-like peptide 1-induced nitric oxide production and muscle microvascular recruitment.

Glucagon-like peptide-1 (GLP-1) causes vasodilation and increases muscle glucose uptake independent of insulin. Recently, we have shown that GLP-1 recruits muscle microvasculature and increases muscle glucose use via a nitric oxide (NO)-dependent mechanism. Protein kinase A (PKA) is a major signaling intermediate downstream of GLP-1 receptors. To examine whether PKA mediates GLP-1s microvascular action in muscle, GLP-1 was infused to overnight-fasted male rats for 120 min in the presence or absence of H89, a PKA inhibitor. Hindleg muscle microvascular recruitment and glucose use were determined. GLP-1 infusion acutely increased muscle microvascular blood volume within 30 min without altering microvascular blood flow velocity or blood pressure. This effect persisted throughout the 120-min infusion period, leading to a significant increase in muscle microvascular blood flow. These changes were paralleled with an approximately twofold increase in plasma NO levels and hindleg glucose extraction. Systemic infusion of H89 completely blocked GLP-1-mediated muscle microvascular recruitment and increases in NO production and muscle glucose extraction. In cultured endothelial cells, GLP-1 acutely increased PKA activity and stimulated endothelial NO synthase phosphorylation at Ser(1177) and NO production. PKA inhibition abolished these effects. In ex vivo studies, perfusion of the distal saphenous artery with GLP-1 induced significant vasorelaxation that was also abolished by pretreatment of the vessels with PKA inhibitor H89. We conclude that GLP-1 recruits muscle microvasculature by expanding microvascular volume and increases glucose extraction in muscle via a PKA/NO-dependent pathway in the vascular endothelium. This may contribute to postprandial glycemic control and complication prevention in diabetes.

Hypertension Management and Microvascular Insulin Resistance in Diabetes

Type 2 diabetes is in essence a vascular disease and is frequently associated with hypertension, macrovascular events, and microvascular complications. Microvascular dysfunction, including impaired recruitment and capillary rarefaction, has been implicated in the pathogenesis of diabetic complications. Microvascular insulin resistance and renin-angiotensin system upregulation are present in diabetes, and each contributes to the development of hypertension and microvascular dysfunction. In the insulin-sensitive state, insulin increases microvascular perfusion by increasing endothelial nitric oxide production, but this effect is abolished by insulin resistance. Angiotensin II, acting via the type 1 receptors, induces inflammation and oxidative stress, leading to impaired insulin signaling, reduced nitric oxide availability, and vasoconstriction. Conversely, it acts on the type 2 receptors to cause vasodilatation. Because substrate and hormonal exchanges occur in the microvasculature, antihypertensive agents targeted to improve microvascular insulin sensitivity and function may have beneficial effects beyond their capacity to lower blood pressure in patients with diabetes.

Insulin and Insulin-Like Growth Factor-I Receptors Differentially Mediate Insulin-Stimulated Adhesion Molecule Production by Endothelial Cells

Patients with type 2 diabetes are hyperinsulinemic and insulin resistant and develop premature atherosclerosis. High concentrations of insulin stimulate the production of adhesion molecules by endothelial cells (ECs). ECs express abundant IGF-I receptors as well as insulin receptors. Whether IGF-I receptors contribute to insulin-induced endothelial production of adhesion molecules is unknown. Bovine aortic ECs (BAECs) were incubated with insulin (100 nm) for 24 h. The cellular content of vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) was measured, and monocyte adhesion to ECs was quantified. Insulin increased both VCAM-1 (P < 0.001) and ICAM-1 (P < 0.0002) content, which was accompanied by an increased number of monocytes adherent to BAECs (P = 0.0001). Inhibition of either MAPK kinase-1 or p38 MAPK but not phosphatidylinositol 3-kinase abolished insulin-mediated production of adhesion molecules. Insulin receptor small interfering RNA knockdown abolished insulin-stimulated increases of ICAM-1 but not VCAM-1. Conversely, IGF-I receptor blockade with either a neutralizing antibody or specific small interfering RNA eliminated insulin-induced VCAM-1 but not ICAM-1 production. Blockade of signaling via either the insulin or IGF-I receptors decreased monocyte adherence to BAECs (P < 0.01 for each). We conclude that insulin and IGF-I receptors differentially mediate the production of adhesion molecules by ECs and monocyte adhesion onto the vascular endothelium in response to the hyperinsulinemic state. Dual-receptor activation may most effectively contribute to the pathogenesis of atherosclerotic disease in diabetes.

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

Rationale: 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. Objective: To examine whether adiponectin modulates muscle microvascular recruitment thus insulin delivery and action in vivo. Methods and Results: 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 125I-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. Conclusions: Globular adiponectin increases muscle insulin uptake by recruiting muscle microvasculature, which contributes to its insulin-sensitizing action.

Losartan increases muscle insulin delivery and rescues insulin's metabolic action during lipid infusion via microvascular recruitment

Insulin delivery and transendothelial insulin transport are two discrete steps that limit muscle insulin action. Angiotensin II type 1 receptor (AT1R) blockade recruits microvasculature and increases glucose use in muscle. Increased muscle microvascular perfusion is associated with increased muscle delivery and action of insulin. To examine the effect of acute AT1R blockade on muscle insulin uptake and action, rats were studied after an overnight fast to examine the effects of losartan on muscle insulin uptake (protocol 1), microvascular perfusion (protocol 2), and insulins microvascular and metabolic actions in the state of insulin resistance (protocol 3). Endothelial cell insulin uptake was assessed, using (125)I-insulin as tracer. Systemic lipid infusion was used to induce insulin resistance. Losartan significantly increased muscle insulin uptake (∼60%, P < 0.03), which was associated with a two- to threefold increase in muscle microvascular blood volume (MBV; P = 0.002) and flow (MBF; P = 0.002). Losartan ± angiotensin II had no effect on insulin internalization in cultured endothelial cells. Lipid infusion abolished insulin-mediated increases in muscle MBV and MBF and lowered insulin-stimulated whole body glucose disposal (P = 0.0001), which were reversed by losartan administration. Inhibition of nitric oxide synthase abolished losartan-induced muscle insulin uptake and reversal of lipid-induced metabolic insulin resistance. We conclude that AT1R blockade increases muscle insulin uptake mainly via microvascular recruitment and rescues insulins metabolic action in the insulin-resistant state. This may contribute to the clinical findings of decreased cardiovascular events and new onset of diabetes in patients receiving AT1R blockers.

Ranolazine recruits muscle microvasculature and enhances insulin action in rats

•u2002 Ranolazine, an anti‐anginal compound, improves glycaemic control in clinical trials and increases myocardial perfusion via a direct vasodilatatory effect on coronary arteries. •u2002 Skeletal muscle microvasculature controls the delivery of nutrient and hormones into muscle and their exchanges between plasma and muscle interstitium by providing microvascular exchange surface area. •u2002 In this study we examined whether ranolazine improves glycaemic control via exerting vasodilatatory action on the pre‐capillary arterioles to recruit muscle microvasculature. •u2002 We demonstrate that ranolazine potently recruits muscle microvasculature, which expands microvascular endothelial surface area in muscle and results in increased muscle delivery and action of insulin. •u2002 The results help us better understand the physiological mechanism by which ranolazine improves glycaemic control and its potential as an insulin‐sensitizing agent.