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

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.

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.

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.

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.