Skeletal muscle blood flow in vivo: Detection with rubidium-82 and effects of glucose, insulin, and exercise

In order to assess the effects of glucose, insulin, and exercise on skeletal muscle blood flow in vivo, we measured positron emission from the thigh muscle of anesthetized rabbits after simultaneous aortic bolus injection of ⁸²Rb and radiolabeled microspheres (15μm diameter). Estimates of flow with ⁸²Rb were based on first-pass regional extraction of ⁸²Rb by skeletal muscle. Flow estimates were made serially as a function of variations in plasma glucose and insulin and changing the muscle contractile state by electrial stimulation. Flow ranged from 3.1 ml/min/100 g at rest to 71 ml/min/100 g during stimulation. There was good agreement between the two methods of flow measurement over the entire range of flows (r=0.96 at a slope of 0.90). Flow measured by either method did not vary significantly from baseline over a range of plasma glucose from 5 to 30 mM and plasma insulin from 0 to 20 μU/ml. When flow was increased up to 20-fold by electrical stimulation there was a decrease in extraction of ⁸²Rb proportional to the increase in flow. However, at pharmacologic levels of insulin (>150μU/ml) flow was increased twofold as measured by radiolabeled microspheres, but not as measured by rubidium. There was no apparent decrease in extraction of ⁸²Rb with high insulin. The discrepancy between the microsphere measured flow and rubidium measured flow with high plasma insulin levels can be explained by the assumption that the expected decrease in the extraction fraction was counteracted by an increase in Na⁺/K⁺-ATPase activity. It is concluded that the first-pass flow model gives valid estimates of skeletal muscle blood flow in vivo with ⁸²Rb, provided that plasma insulin levels are normal.