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

The purpose of this study was to non-invasively 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·kg^-1.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.