Insulin increases glucose disposal into muscle. In addition, in vivo insulin elicits distinct nitric oxide synthase-dependent vascular responses to increase total skeletal muscle blood flow and to recruit muscle capillaries (by relaxing resistance and terminal arterioles, respectively). In the current study, we compared the temporal sequence of vascular and metabolic responses to a 30-min physiological infusion of insulin (3 mU ⅐ min ؊1 ⅐ kg ؊1 , euglycemic clamp) or saline in rat skeletal muscle in vivo. We used contrast-enhanced ultrasound to continuously quantify microvascular volume. Insulin recruited microvasculature within 5-10 min (P < 0.05), and this preceded both activation of insulin-signaling pathways and increases in glucose disposal in muscle, as well as changes in total leg blood flow. Moreover, L-NAME (N -nitro-L-arginine-methyl ester), a specific inhibitor of nitric oxide synthase, blocked this early microvascular recruitment (P < 0.05) and at least partially inhibited early increases in muscle glucose uptake (P < 0.05). We conclude that insulin rapidly recruits skeletal muscle capillaries in vivo by a nitric oxidedependent action, and the increase in capillary recruitment may contribute to the subsequent glucose uptake.

The vascular system controls the delivery of nutrients and hormones to muscle, and a number of hormones may act to regulate muscle metabolism and contractile performance by modulating blood flow to and within muscle. This review examines evidence that insulin has major hemodynamic effects to influence muscle metabolism. Whole body, isolated hindlimb perfusion studies and experiments with cell cultures suggest that the hemodynamic effects of insulin emanate from the vasculature itself and involve nitric oxide-dependent vasodilation at large and small vessels with the purpose of increasing access for insulin and nutrients to the interstitium and muscle cells. Recently developed techniques for detecting changes in microvascular flow, specifically capillary recruitment in muscle, indicate this to be a key site for early insulin action at physiological levels in rats and humans. In the absence of increases in bulk flow to muscle, insulin may act to switch flow from nonnutritive to the nutritive route. In addition, there is accumulating evidence to suggest that insulin resistance of muscle in vivo in terms of impaired glucose uptake could be partly due to impaired insulin-mediated capillary recruitment. Exercise training improves insulin-mediated capillary recruitment and glucose uptake by muscle.

We have previously shown that skeletal muscle capillaries are rapidly recruited by physiological doses of insulin in both humans and animals. This facilitates glucose and insulin delivery to muscle, thus augmenting glucose uptake. In obese rats, both insulin-mediated microvascular recruitment and glucose uptake are diminished; however, this action of insulin has not been studied in obese humans. Here we used contrast ultrasound to measure microvascular blood volume (MBV) (an index of microvascular recruitment) in the forearm flexor muscles of lean and obese adults before and after a 120-min euglycemic-hyperinsulinemic (1 mU ⅐ min ؊1 ⅐ kg ؊1 ) clamp. We also measured brachial artery flow, fasting lipid profile, and anthropomorphic variables. Fasting plasma glucose (5.4 ؎ 0.1 vs. 5.1 ؎ 0.1 mmol/l, P ‫؍‬ 0.05), insulin (79 ؎ 11 vs. 38 ؎ 6 pmol/l, P ‫؍‬ 0.003), and percent body fat (44 ؎ 2 vs. 25 ؎ 2%, P ‫؍‬ 0.001) were higher in the obese than the lean adults. After 2 h of insulin infusion, whole-body glucose infusion rate was significantly lower in the obese versus lean group (19.3 ؎ 3.2 and 37.4 ؎ 2.6 mol ⅐ min ؊1 ⅐ kg ؊1 respectively, P < 0.001). Compared with baseline, insulin increased MBV in the lean (18.7 ؎ 3.3 to 25.0 ؎ 4.1, P ‫؍‬ 0.019) but not in the obese group (20.4 ؎ 3.6 to 18.8 ؎ 3.8, NS). Insulin increased brachial artery diameter and flow in the lean but not in the obese group. We observed a significant, negative correlation between ⌬MBV and BMI (R ‫؍‬ ؊0.482, P ‫؍‬ 0.027) in response to insulin. In conclusion, obesity eliminated the insulin-stimulated muscle microvascular recruitment and increased brachial artery blood flow seen in lean individuals.

Intense exercise and insulin each increases total limb blood flow and recruits muscle capillaries, presumably to facilitate nutrient exchange. Whether mixed meals or light exercise likewise recruits capillaries is unknown. We fed 18 (9 M, 9 F) healthy volunteers a 480-kcal liquid mixed meal. Plasma glucose, insulin, brachial artery flow, and forearm muscle microvascular blood volume were measured before and after the meal. Brachial artery flow and microvascular volume were also examined with light (25% max), moderate (50%), and heavy (80%) forearm contraction every 20 s in 5 (4 M, 1 F) healthy adults. After the meal, glucose and insulin rose modestly (to ϳ7 mM and ϳ270 pM) and peaked by 30 min, whereas brachial artery blood flow (P Ͻ 0.05) and the microvascular volume (P Ͻ 0.01) each increased significantly by 60 min, and microvascular flow velocity did not change. For exercise, both 50 and 80%, but not 25% maximal handgrip, increased average forearm and brachial artery blood flow (P Ͻ 0.01). Flow increased immediately after each contraction and declined toward basal over 15 s. Exercise at 25% max increased microvascular volume threefold (P Ͻ 0.01) without affecting microvascular flow velocity or total forearm blood flow. Forearm exercise at 80% maximal grip increased both microvascular volume and microvascular flow velocity (P Ͻ 0.05 each). We conclude that light exercise and simple meals each markedly increases muscle microvascular volume, thereby expanding the endothelial surface for nutrient exchange, and that capillary recruitment is an important physiological response to facilitate nutrient/hormone delivery in healthy humans. INSULIN (1, 7, 18) AND EXERCISE (8, 13) each increase total limb blood flow, and each recruits flow to muscle capillaries (13,18). These vascular responses facilitate the delivery of oxygen, insulin, and glucose to muscle cells. Flow to individual capillary networks is controlled by small, ϳ20 m diameter terminal arterioles (21). We previously showed that during insulin infusion, relaxation of larger-resistance arterioles (which contribute to the regulation of total blood flow) and terminal arterioles can occur independently in rat (25, 29) and human (7) muscle.Muscle microvascular responses to physiological stimuli have been difficult to assess in humans due to the lack of noninvasive techniques suitable for assessing microvascular blood volume. We have developed two independent techniques for determining muscle microvascular volume noninvasively.The first technique is based on the metabolism of exogenously infused 1-methylxanthine (1-MX) to 1-methylurate by capillary endothelial xanthine oxidase (18,29). This has worked well in rodent models but has not been adapted to human studies. The second technique, contrast-enhanced ultrasound (CEU), relies on the contrast introduced by intravenously infused microbubbles (27). To measure microvascular volume, CEU takes advantage of the fact that during their continuous infusion it is possible to disrupt microbubbles by a single pulse of high...

OBJECTIVEIngestion of a mixed meal recruits flow to muscle capillaries and increases total forearm blood flow in healthy young lean people. We examined whether these vascular responses are blunted by obesity.RESEARCH DESIGN AND METHODSWe fed eight middle-aged lean and eight obese overnight-fasted volunteers a liquid mixed meal (480 kcal). Plasma glucose and insulin were measured every 30 min, and brachial artery flow and muscle microvascular recruitment (contrast ultrasound) were assessed every 60 min over 2 h after the meal.RESULTSBy 30 min, plasma glucose rose in both the lean (5.1 ± 0.1 vs. 6.7 ± 0.4 mmol/l, P < 0.05) and the obese groups (5.4 ± 0.2 vs. 6.7 ± 0.4 mmol/l, P < 0.05). Plasma insulin rose (28 ± 4 vs. 241 ± 30 pmol/l, P < 0.05) by 30 min in the lean group and remained elevated for 2 h. The obese group had higher fasting plasma insulin levels (65 ± 8 pmol/l, P < 0.001) and a greater postmeal area under the insulin-time curve (P < 0.05). Brachial artery flow was increased at 120 min after the meal in the lean group (38 ± 6 vs. 83 ± 16 ml/min, P < 0.05) but not in the obese group. Muscle microvascular blood volume rose by 120 min in the lean group (14.4 ± 2.2 vs. 24.4 ± 4.2 units, P < 0.05) but not in the obese group.CONCLUSIONSA mixed meal recruits muscle microvasculature in lean subjects, and this effect is blunted by obesity. This impaired vascular recruitment lessens the endothelial surface available and may thereby impair postprandial glucose disposal.