Globular adiponectin enhances muscle insulin action via microvascular recruitment and increased insulin delivery

Abstract

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 (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.

Conclusions: Globular adiponectin increases muscle insulin uptake by recruiting muscle microvasculature, which contributes to its insulin-sensitizing action.

Figures

Figure 1. Experimental protocols

CEU indicates contrast-enhanced ultrasound; fAd, full-length adiponectin; gAd, globular adiponectin; and L-NAME, NG-nitro-l-arginine methyl ester.

Figure 2. Adiponectin increases skeletal muscle microvascular perfusion and plasma NO levels

Each rat was given an intraperitoneal injection of either full-length adiponectin (fAd; 0.2 μg/g body weight), globular adiponectin (gAd; 0.2 or 0.4 μg/g body weight), or saline in the absence or presence of NG-nitro-l-arginine methyl ester (L-NAME) infusion. Contrast-enhanced ultrasound (CEU) measurements were done before and then every 30 minutes after IP injection. A, Microvascular blood volume (MBV). B, Microvascular blood flow velocity (MFV). C, Microvascular blood flow (MBF). D, Plasma NO levels. n=5 to 6. Compared with saline control *P<0.05 (ANOVA).

Figure 3. Globular adiponectin (gAd) augments insulin-stimulated whole-body glucose disposal

Each rat received IP injection of saline or gAd (0.4 μg/g body weight) followed by a 2-hour euglycemic hyperinsulinemic clamp (3 mU/kg per minute) in the presence or absence of NG-nitro-l-arginine methyl ester (L-NAME) infusion. A, Time course of glucose infusion rate (GIR). Compared with saline group, P<0.001 (ANOVA). B, Steady-state GIR. Compared with saline control and L-NAME groups, **P<0.01. n=5 to 6.

Figure 4. Effects of globular adiponectin (gAd) on muscle Akt and ERK1/2 phosphorylation

A, Changes in muscle Akt phosphorylation. B, Changes in muscle ERK phosphorylation. Compared with saline control, *P<0.05, **P<0.01. Compared with insulin #P<0.05. n=3 to 4. gAd 0.2 indicates gAd 0.2 μg/g body weight; gAd 0.4, gAd 0.4 μg/g body weight; insulin, 3 mU/kg per minute; gAd+insulin, gAd 0.4 μg/g body weight + insulin 3 mU/kg per minute.

Figure 5. Globular adiponectin (gAd ) increases muscle 125I-insulin uptake

Blood and skeletal muscle were collected 5 minutes after bolus injection of 125I-insulin (1.5 μCi) administered intravenously. Intact 125I-insulin was determined after trichloroacetic acid (30%) precipitation. A, Muscle clearance of insulin. B, Muscle insulin uptake. C, Fraction of 125I-insulin in blood and in muscle. n=5 to 6. Compared with saline group, *P<0.05. L-NAME indicates NG-nitro-l-arginine methyl ester; and dpm, disintegrations per minute.

Figure 6. Globular adiponectin (gAd) enhances insulin-mediated muscle microvascular recruitment

Each rat received an IP injection of saline or gAd (0.4 μg/g body weight) followed by a 2-hour euglycemic hyperinsulinemic clamp (3 mU/kg per minute). Contrast-enhanced ultrasound measurements were done at the set time points. A, Microvascular blood volume (MBV). B, Microvascular blood flow velocity (MFV). C, Microvascular blood flow (MBF). D, Mean arterial blood pressure (MAP). n=5 to 7. Compared with basal, *P<0.05, **P<0.05; compared with insulin group, #P<0.05. L-NAME indicates NG-nitro-l-arginine methyl ester.

Figure 7. Globular adiponectin (gAd) increases endothelial nitric oxide synthase (eNOS) phosphorylation but not endothelial 125I-insulin uptake in cultured bovine aortic endothelial cells

A, Changes in Akt phosphorylation. B, Changes in AMP-activated protein kinase (AMPK) phosphorylation. C, Changes in eNOS phosphorylation. D, Changes in endothelial 125I-insulin uptake. n=3 to 4. Compared with control, *P<0.05, **P<0.01. Compared with insulin #P<0.05.