Obesity blunts insulin-mediated microvascular recruitment in human forearm 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 . min(-1) . 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 mumol . min(-1) . 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 DeltaMBV 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.

Insulin sensitivity of muscle capillary recruitment in vivo.

We have reported that insulin exerts two vascular actions in muscle; it both increases blood flow and recruits capillaries. In parallel hyperinsulinemic-euglycemic clamp studies, we compared the insulin dose response of muscle microvascular recruitment and femoral blood flow as well as hindleg glucose uptake in fed, hooded Wistar and fasted Sprague-Dawley rats. Using insulin doses between 0 and 30 mU(-1). min(-1). kg(-1), we measured microvascular recruitment at 2 h by 1-methylxanthine (1-MX) metabolism or contrast-enhanced ultrasound (CEU), and muscle glucose uptake was measured by either arteriovenous differences or using 2-deoxyglucose. We also examined the time course for reversal of microvascular recruitment following cessation of a 3 mU. min(-1). kg(-1) insulin infusion. In both groups, whether measured by 1-MX metabolism or CEU, microvascular recruitment was fully activated by physiologic hyperinsulinemia and occurred at lower insulin concentrations than those that stimulated glucose uptake or hindleg total blood flow. The latter processes were insulin dose dependent throughout the entire dose range studied. Upon stopping the insulin infusion, increases in microvascular volume persisted for 15-30 min after insulin concentrations returned to basal levels. We conclude that the precapillary arterioles that regulate microvascular recruitment are more insulin sensitive than resistance arterioles that regulate total flow.

Microvascular recruitment is an early insulin effect that regulates skeletal muscle glucose uptake in vivo.

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. min(-1). 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(omega)-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 oxide-dependent action, and the increase in capillary recruitment may contribute to the subsequent glucose uptake.

Skeletal muscle capillary responses to insulin are abnormal in late-stage diabetes and are restored by angiotensin-converting enzyme inhibition.

Acute physiological hyperinsulinemia increases skeletal muscle capillary blood volume (CBV), presumably to augment glucose and insulin delivery. We hypothesized that insulin-mediated changes in CBV are impaired in type 2 diabetes mellitus (DM) and are improved by angiotensin-converting enzyme inhibition (ACE-I). Zucker obese diabetic rats (ZDF, n = 18) and control rats (n = 9) were studied at 20 wk of age. One-half of the ZDF rats were treated with quinapril (ZDF-Q) for 15 wk prior to study. CBV and capillary flow in hindlimb skeletal muscle were measured by contrast-enhanced ultrasound (CEU) at baseline and at 30 and 120 min after initiation of a euglycemic hyperinsulinemic clamp (3 mU.min(-1).kg(-1)). At baseline, ZDF and ZDF-Q rats were hyperglycemic and hyperinsulinemic vs. controls. Glucose utilization in ZDF rats was 60-70% lower (P < 0.05) than in controls after 30 and 120 min of hyperinsulinemia. In ZDF-Q rats, glucose utilization was impaired at 30 min but similar to controls at 120 min. Basal CBV was lower in ZDF and ZDF-Q rats compared with controls (13 +/- 4, 7 +/- 3, and 9 +/- 2 U, respectively). With hyperinsulinemia, CBV increased by about twofold in control animals at 30 and 120 min, did not change in ZDF animals, and increased in ZDF-Q animals only at 120 min to a level similar to controls. Anatomic capillary density on immunohistology was not different between groups. We conclude that insulin-mediated capillary recruitment in skeletal muscle, which participates in glucose utilization, is impaired in animals with DM and can be partially reversed by chronic ACE-I therapy.

Contraction stimulates nitric oxide independent microvascular recruitment and increases muscle insulin uptake.

We examined whether contraction-induced muscle microvascular recruitment would expand the surface area for insulin and nutrient exchange and thereby contribute to insulin-mediated 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(omega)-l-nitro-arginine-methyl ester (n = 5) significantly elevated mean arterial pressure by approximately 30 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 contralateral muscle (n = 8; 52.2 +/- 3.8 vs. 20 +/- 2.5, respectively; P < 0.001). We then measured muscle uptake of EBD and (125)I-labeled insulin (dpm per gram dry tissue) with 0.1-Hz stimulation (n = 6). Uptake of EBD (19.1 +/- 3.8 vs. 9.9 +/- 1; P < 0.05) and (125)I-insulin (5,300 +/- 800 vs. 4,244 +/- 903; P < 0.05) was greater in stimulated muscle versus control. Low-frequency contraction increases muscle MBV by a NO-independent pathway and facilitates muscle uptake of albumin and insulin in the absence of blood flow increases. This microvascular response may, in part, explain enhanced insulin action in exercising skeletal muscle.

Mixed meal and light exercise each recruit muscle capillaries in healthy humans.

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 approximately 7 mM and approximately 270 pM) and peaked by 30 min, whereas brachial artery blood flow (P < 0.05) and the microvascular volume (P < 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 < 0.01). Flow increased immediately after each contraction and declined toward basal over 15 s. Exercise at 25% max increased microvascular volume threefold (P < 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 < 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.

Validation and advantages of FAWSETS perfusion measurements in skeletal muscle.

This work discusses the strengths, limitations and validity of a novel arterial spin labeling technique when used specifically to measure perfusion in limb skeletal muscle. The technique, flow-driven arterial water stimulation with elimination of tissue signal (FAWSETS), offers several advantages over existing arterial spin labeling techniques. The primary goal of this study was to determine the perfusion signal response to changes in net hind limb flow that were independently verifiable. The range of perfusate flow was relevant to skeletal muscle during mild to moderate exercise. Localized, single voxel measurements were acquired from a 5 mm-thick slice in the isolated perfused rat hind limb at variable net flow rates. The results show that the perfusion signal is linearly proportional to net hind limb flow with a correlation coefficient of 0.974 (p = 0.0013). FAWSETS is especially well suited for studies of skeletal muscle perfusion, where it eliminates the need to compensate for magnetization transfer and arterial transit time effects. A conceptual discussion of the basic principles underlying these advantages is presented.

Active role for the vasculature in the delivery of insulin to skeletal muscle.

1. In the 80+ years since insulin's discovery, an enormous amount of literature has accumulated relating to its actions on body fat, glucose and protein metabolism. In particular, skeletal muscle has been extensively studied because of its major role as a site of insulin-mediated glucose disposal. Liver and adipose tissue are two other extensively studied sites of insulin action. Much less investigation has been directed towards delineating insulin's actions on cells other than myocytes, adipocytes and hepatocytes. 2. Over the past 5-10 years it has become increasingly evident that insulin exerts important actions on vascular cells. Here, we review evidence that insulin's action within muscle may be very much regulated by its ability to transit the vasculature to access the interstitial fluid (and hence the myocyte insulin receptor). Surprisingly little is known regarding the regulation of vascular events that first bring insulin to the capillary endothelium within muscle, whence presumably it transits from the vascular to the interstitial space. Recent studies suggest that insulin can increase blood flow and also influence the distribution of blood flow within skeletal muscle, potentially therefore regulating its own delivery to the capillary endothelium. Beyond insulin's ability to access the vascular lumen within skeletal muscle microvasculature lies the issue of its passing the endothelial barrier. Even less is known about the processes involved in insulin's actual transit across the endothelium. Available data do not clearly indicate whether this is a saturable, receptor-mediated process or a passive-diffusion pathway. Also, whether insulin in any manner regulates its own transit across the endothelium or its clearance via the lymphatic system is entirely unknown. 3. The aim of the present review is to identify areas where knowledge is deficient and highlight hypotheses which may lead to a better understanding of the coordinated relationship between insulin's vascular actions within muscle and its metabolic actions in that tissue. Even so, there is now sufficient evidence to indicate that insulin's vascular action within skeletal muscle is a major regulatory locus for its insulin mediated glucose disposal.

Molecular and physiologic actions of insulin related to production of nitric oxide in vascular endothelium.

Insulin has important vascular actions that regulate blood flow, in addition to its classical actions to coordinate glucose homeostasis. Insulin-stimulated production of nitric oxide in vascular endothelium results in capillary recruitment and vasodilation that diverts and increases blood flow to skeletal muscle and consequently increases glucose disposal. Thus, vascular actions of insulin may be essential for coupling hemodynamic and metabolic homeostasis. A complete biochemical signaling pathway linking the insulin receptor to activation of endothelial nitric oxide synthase in vascular endothelium has recently been elucidated. Moreover, the time course and dose response for capillary recruitment in response to physiologic concentrations of insulin parallels that of insulin-mediated glucose uptake in vivo. Taken together, these observations suggest a molecular mechanism that may help to explain how insulin resistance contributes to cardiovascular components of the metabolic syndrome and vascular complications of diabetes.

The microvasculature in insulin resistance and type 2 diabetes.

Insulin resistance of muscle has been attributed to impairment of elements of insulin signaling, glucose transport, and/or metabolism within the muscle cells. This article explores the notion that a component of insulin resistance in vivo may result from impaired hemodynamic effects of this hormone to facilitate access to the muscle cells for itself and other nutrients, including glucose. In chronic situations this may manifest as a decreased capillary density of muscle, but in the acute, there may be impaired mechanisms for increasing total limb blood flow or for achieving optimal microvascular perfusion. Newly developed techniques show that insulin acts to recruit muscle capillary flow to enhance microvascular perfusion in animals and humans. This microvascular effect of insulin correlates closely with muscle glucose uptake, is independent of increases in bulk blood flow, and is impaired in obese insulin-resistant patients. Similarly, there are impaired vasodilatory responses in the skin of diabetic subjects.

The vasodilatory actions of insulin on resistance and terminal arterioles and their impact on muscle glucose uptake.

Whether a discrete vascular action of insulin in skeletal muscle integrally participates in insulin-mediated glucose disposal has been extensively examined but remains a contentious issue. Here, we review some of the data both supporting and questioning the role of insulin-mediated increases in limb blood flow in glucose metabolism. We advance the hypothesis that controversy has arisen, at least in part, from a failure to recognize that insulin exerts at least three separate actions on the peripheral vasculature, each with its own characteristic dose and time responsiveness. We summarize how, viewed in this manner, certain points of contention can be resolved. We also advance the hypothesis that an action on the precapillary arteriole may play the dominant role in mediating perfusion-dependent effects of insulin on glucose metabolism in muscle.

Vascular recruitment in skeletal muscle during exercise and hyperinsulinemia assessed by contrast ultrasound.

The purpose of this study was to noninvasively quantify the effects of insulin on capillary blood volume (capBV) and RBC velocity (V(RBC)) in skeletal muscle in vivo with the use of contrast-enhanced ultrasound. We performed contrast ultrasound of the rat hindlimb adductor muscles at baseline and after 2-h infusions of either insulin (3 or 40 mU x kg(-1) x min(-1)) or saline. Saline-treated animals were also studied during contractile exercise. V(RBC) and capBV were calculated from the relation between pulsing interval and video intensity. Femoral artery blood flow, measured by a flow probe, increased with both contractile exercise and insulin. Contractile exercise increased capBV more than twofold and V(RBC) fivefold. Insulin also increased capBV more than twofold in a dose-dependent fashion but did not significantly alter V(RBC). Saline infusion did not significantly alter capBV, V(RBC), or femoral artery blood flow. We conclude that physiological changes in skeletal muscle capillary perfusion can be assessed in vivo with the use of contrast-enhanced ultrasound. Exercise increases both V(RBC) and capBV, whereas hyperinsulinemia selectively increases only capBV, which may enhance skeletal muscle glucose uptake.

Blood flow and muscle metabolism: a focus on insulin action.

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.