Local methacholine but not bradykinin potentiates insulin-mediated glucose uptake in muscle in vivo by augmenting capillary recruitment

Abstract

Aims/hypothesis. Insulin has nitric-oxide-dependent vasodilatory effects in muscle, including capillary recruitment, that enhance access for itself and glucose. However, nitric-oxide-dependent vasodilators other than methacholine do not enhance insulin action. Our hypothesis is that methacholine, unlike bradykinin, enhances insulin-mediated glucose uptake in muscle by augmenting capillary recruitment. Methods. Local infusion of either methacholine or bradykinin into one leg of the anaesthetised rat was made during physiological insulin (3 mU·kg −1 ·min −1 ) infusion under euglycaemic conditions and without affecting systemic blood pressure. Whole-body glucose infusion was determined, as was femoral blood flow, 2-deoxyglucose uptake into calf muscles and the metabolism of infused 1-methylxanthine, a measure of capillary recruitment for each leg. Results. Methacholine alone (0.3 µmol·l -1 ) increased femoral arterial blood flow, increased capillary recruitment but had no effect on 2-deoxyglucose uptake of the test leg relative to the contra-lateral control leg. Insulin alone (systemically) required a glucose infusion rate of 8.7 mg·kg −1 ·min −1 to maintain euglycaemia, increased 2-deoxyglucose uptake and capillary recruitment, but was without effect on femoral blood flow in either leg. Local methacholine with systemic insulin infusion increased femoral blood flow, 2-deoxyglucose uptake and capillary recruitment in the test leg only. Bradykinin (0.07 µmol·l -1 ), alone or with insulin, administered in a manner that increased femoral blood flow similarly to methacholine, did not affect 2-deoxyglucose uptake or capillary recruitment. Conclusions/interpretation. Methacholine but not bradykinin enhances insulin-mediated muscle glucose uptake in vivo. We conclude that methacholine acts at specific sites in the vasculature of muscle to stimulate capillary recruitment and thereby enhance insulin access.Keywords Global vasodilatation in muscle · Insulin and glucose delivery to muscle · Muscle blood flow · Specific regional vasodilatation in muscle.

Vascular and metabolic effects of methacholine in relation to insulin action in muscle

“…The implication of these findings is that MC may influence glucose metabolism in vivo by two discrete effects that are concentration dependent. At low doses the vascular effect of vasodilatation is likely to have beneficial effects on nutrient and hormone delivery by enhancing microvascular perfusion (capillary recruitment), as has been shown by us previously [13]. At the higher dose of MC (30 μmol/l) the two effects would very likely occur concomitantly, with enhanced delivery of nutrients, hormones and MC itself to enhance myocyte glucose uptake.…”

“…The first system, conducted in vivo, assessed the effect of a local low dose of MC, with and without NO synthase inhibitor, on vascular responses and glucose uptake in one hindlimb, using the contra-lateral hindlimb as control. The low dose of MC used in vivo was identical to that found in previous studies [13] to potentiate insulin action on muscle glucose uptake. The second system of experiments used the surgically isolated pump-perfused hindlimb.…”

“…Details for positioning cannulae and flow probes in anaesthetised rats were as described previously [13]. Provision was made for the following: maintenance of anaesthesia by constant trickle infusion of Nembutal; measurement of mean arterial blood pressure (BP), heart rate and femoral arterial blood flow (FBF) in each leg; infusion of MC and/ or NO synthase inhibitors into one (test) leg.…”

“…L-NAME was infused 5 min after MC as indicated. X, time points at which changes in perfusion pressure were analysed arterial BP or heart rate was the same as used previously [13]. As shown in Fig.…”

“…Capillary recruitment was determined by measuring the metabolism of systemically infused 1-methylxanthine (1-MX) as described in detail previously [13]. Plasma (20 μl) from arterial and leg venous blood samples taken at the end of the experiment was mixed with 80 μl perchloric acid (2 mol/l) and centrifuged for 10 min.…”