Evidence suggests that insulin delivery to skeletal muscle interstitium is the rate-limiting step in insulin-stimulated muscle glucose uptake and that this process is impaired by insulin resistance. In this review we examine the basis for the hypothesis that insulin acts on the vasculature at three discrete steps to enhance its own delivery to muscle: (1) relaxation of resistance vessels to increase total blood flow; (2) relaxation of pre-capillary arterioles to increase the microvascular exchange surface perfused within skeletal muscle (microvascular recruitment); and (3) the trans-endothelial transport (TET) of insulin. Insulin can relax resistance vessels and increase blood flow to skeletal muscle. However, there is controversy as to whether this occurs at physiological concentrations of, and exposure times to, insulin. The microvasculature is recruited more quickly and at lower insulin concentrations than are needed to increase total blood flow, a finding consistent with a physiological role for insulin in muscle insulin delivery. Microvascular recruitment is impaired by obesity, diabetes and nitric oxide synthase inhibitors. Insulin TET is a third potential site for regulating insulin delivery. This is underscored by the consistent finding that steady-state insulin concentrations in plasma are approximately twice those in muscle interstitium. Recent in vivo and in vitro findings suggest that insulin traverses the vascular endothelium via a trans-cellular, receptor-mediated pathway, and emerging data indicate that insulin acts on the endothelium to facilitate its own TET. Thus, muscle insulin delivery, which is rate-limiting for its metabolic action, is itself regulated by insulin at multiple steps. These findings highlight the need to further understand the role of the vascular actions of insulin in metabolic regulation.

We examined whether contraction-induced muscle microvascular recruitment would expand the surface area for insulin and nutrient exchange and thereby contribute to insulinmediated 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 -L-nitro-arginine-methyl ester (n ‫؍‬ 5) significantly elevated mean arterial pressure by ϳ30 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

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

Aims/hypothesis Insulin's rate of entry into skeletal muscle appears to be the rate limiting step for muscle insulin action and is slowed by insulin resistance. Despite its obvious importance, uncertainty remains as to whether the transport of insulin from plasma to muscle interstitium is a passive diffusional process or a saturable transport process regulated by the insulin receptor. Methods To address this, here we directly measure the rate of 125I-insulin uptake by rat hindlimb muscle and examine how that is affected by adding unlabeled insulin at high concentrations. We used mono-iodinated ([125I]TyrA14)insulin and short (5 min) exposure times, combined with trichloroacetic acid precipitation, to trace intact bioactive insulin. Results Compared to saline, high concentrations of unlabeled insulin delivered either continuously (insulin clamp) or as a single bolus significantly raised plasma 125I-insulin, slowed the movement of 125I-insulin from plasma into liver, spleen, and heart (p<0.05, for each) but increased kidney 125I-insulin uptake. High concentrations of unlabeled insulin delivered either continuously (insulin clamp), or as a single bolus significantly decreased skeletal muscle 125I-insulin clearance (p<0.01 for each). Increasing muscle perfusion by electrical stimulation did not prevent the inhibitory effect of unlabeled insulin on muscle 125I-insulin clearance. Conclusions/interpretation This indicates that insulin's trans-endothelial movement within muscle is a saturable process likely involving the insulin receptor. Current findings, together with other recent reports, suggest that trans-endothelial insulin transport may be an important site at which muscle insulin action is modulated in clinical and pathologic settings.