Microvascular Recruitment Is an Early Insulin Effect That Regulates Skeletal Muscle Glucose Uptake In Vivo

Abstract

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.

Acute glucosamine-induced insulin resistance in muscle in vivo is associated with impaired capillary recruitment

“…However, in the present studies, the heart rate and blood pressure were normal and other recent studies have shown that insulin-mediated capillary recruitment occurred early, relative to muscle glucose uptake [18] and at physiological doses of insulin [17], indicating that a desensitising effect of the anaesthetic is unlikely. Moreover, studies in human forearm, where the subjects were instructed to remain still, have shown insulin-mediated capillary recruitment, again at physiological insulin levels [44].…”

“…However, there is now indirect evidence making it unlikely that insulinmediated capillary recruitment is regulated by metabolite(s) from muscle. Thus insulin-mediated capillary recruitment, measured by contrast-enhanced ultrasound, is an early event in insulin action in vivo preceding the stimulation of glucose uptake [18]. We have also shown, using both 1-MX metabolism and contrast-enhanced ultrasound, that capillary recruitment occurs at insulin concentrations that do not alter muscle glucose uptake [17].…”

“…In particular, the temporal relationship between insulin signalling and impairment by glucosamine is hard to predict and may depend on duration of exposure and when each is added relative to the other. For the present in vivo experiments our objective was to test whether the early microvascular effect of insulin [18] was impaired by glucosamine. Our approach was to add glucosamine and insulin simultaneously in the belief that the initial site of interaction was the endothelial or vascular smooth muscle cells, from where bulk flow and capillary recruitment are likely to be controlled.…”

“…In Table 1 Arterial and venous plasma 1-MX concentrations, femoral arterial flow rate and calculated hind limb 1-MX metabolism for anaesthetised rats infused with saline, N-glucosamine, insulin or both N-glucosamine and insulin Data were collected 180 min after the commencement of the infusions as shown in Fig. 1 Values are means±SE for n=8 in each group *p<0.05 difference from saline; # p<0.05 difference from insulin contrast, there is growing evidence that insulin-mediated capillary recruitment may play a key role in muscle glucose uptake, particularly in animal models [18] and is impaired in animal models of muscle insulin resistance [12][13][14]41] and in obese humans [42,43]. Capillary recruitment is controlled independently of limb blood flow [17,18,21] and occurs in response to locally administered physiological insulin in human forearm in association with glucose uptake [44].…”

“…For capillary recruitment, the higher sensitivity to insulin [17] and the fact that time course studies show capillary recruitment to be the earliest change caused by insulin in vivo [18] suggest that endothelial cells, in particular endothelial nitric oxide synthase (eNOS), are the source of nitric oxide (NO). Glucosamine inhibits eNOS [19] and the infusion of glucosamine in anaesthetised rats reduced limb blood flow during an insulin clamp where flow was assessed by radioactive microspheres [20], a measure of total flow to specific muscles.…”