Vasopressin is a physiological substrate for the insulin-regulated aminopeptidase IRAP.

Insulin-regulated aminopeptidase (IRAP) is a membrane aminopeptidase and is homologous to the placental leucine aminopeptidase, P-LAP. IRAP has a wide distribution but has been best characterized in adipocytes and myocytes. In these cells, IRAP colocalizes with the glucose transporter GLUT4 to intracellular vesicles and, like GLUT4, translocates from these vesicles to the cell surface in response to insulin. Earlier studies demonstrated that purified IRAP cleaves several peptide hormones and that, concomitant with the appearance of IRAP at the surface of insulin-stimulated adipocytes, aminopeptidase activity toward extracellular substrates increases. In the present study, to identify in vivo substrates for IRAP, we tested potential substrates for cleavage by IRAP-deficient (IRAP(-/-)) and control mice. We found that vasopressin and oxytocin were not processed from the NH(2) terminus by isolated IRAP(-/-) adipocytes and skeletal muscles. Vasopressin was not cleaved from the NH(2) terminus after injection into IRAP(-/-) mice and exhibited a threefold increased half-life in the circulation of IRAP(-/-) mice. Consistent with this finding, endogenous plasma vasopressin levels were elevated twofold in IRAP(-/-) mice, and vasopressin levels in IRAP(-/-) brains, where plasma vasopressin originates, showed a compensatory decrease. We further established that insulin increased the clearance of vasopressin from control but not from IRAP(-/-) mice. In conclusion, we have identified vasopressin as the first physiological substrate for IRAP. Changes in plasma and brain vasopressin levels in IRAP(-/-) mice suggest a significant role for IRAP in regulating vasopressin. We have also uncovered a novel IRAP-dependent insulin effect: to acutely modify vasopressin.

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.

Insulin-mediated hemodynamic changes are impaired in muscle of Zucker obese rats.

Insulin-mediated hemodynamic effects in muscle were assessed in relation to insulin resistance in obese and lean Zucker rats. Whole-body glucose infusion rate (GIR), femoral blood flow (FBF), hindleg glucose extraction (HGE), hindleg glucose uptake (HGU), 2-deoxyglucose (DG) uptake into muscles of the lower leg (R(g)), and metabolism of infused 1-methylxanthine (1-MX) to measure capillary recruitment were determined for isogylcemic (4.8 +/- 0.2 mmol/l, lean; 11.7 +/- 0.6 mmol/l, obese) insulin-clamped (20 mU. min(-1). kg(-1) x 2 h) and saline-infused control anesthetized age-matched (20 weeks) lean and obese animals. Obese rats (445 +/- 5 g) were less responsive to insulin than lean animals (322 +/- 4 g) for GIR (7.7 +/- 1.4 vs. 22.2 +/- 1.1 mg. min(-1). kg(-1), respectively), and when compared with saline-infused controls there was no increase due to insulin by obese rats in FBF, HGE, HGU, and R(g) of soleus, plantaris, red gastrocnemius, white gastrocnemius, extensor digitorum longus (EDL), or tibialis muscles. In contrast, lean animals showed marked increases due to insulin in FBF (5.3-fold), HGE (5-fold), HGU (8-fold), and R(g) ( approximately 5.6-fold). Basal (saline) hindleg 1-MX metabolism was 1.5-fold higher in lean than in obese Zucker rats, and insulin increased in only that of the lean. Hindleg 1-MX metabolism in the obese decreased slightly in response to insulin, thus postinsulin lean was 2.6-fold that of the postinsulin obese. We conclude that muscle insulin resistance of obese Zucker rats is accompanied by impaired hemodynamic responses to insulin, including capillary recruitment and FBF.

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.