Site of enhanced insulin sensitivity in exercise-trained rats at rest

Spontaneously exercised rats show at rest enhanced responsiveness to exogenous insulin and lower plasma insulin levels after oral glucose than sedentary control rats. To assess insulin sensitivity of specific organs, glucose uptake by perfused hindlimb muscle and liver from resting exercise-trained rats was compared with perfused organs from control rats. Glucose uptake, assessed by metabolic clearance formulas, was 17% faster in hindlimbs from exercise-trained rats when perfused without added insulin and 43% faster at perfusate insulin levels of 40 microU/ml. After an overnight fast, glucose clearance in exercise-trained hindlimbs increased over controls by 57% in the basal state and by 97% at low perfusate levels. In contrast, glucose clearance by livers from both fed and fasted exercise-trained rats was less than one-half that of livers from control rats. These results suggest that skeletal muscle, and not liver, is the organ primarily responsible for the increased sensitivity to insulin-induced glucose uptake with exercise training and that this response is enhanced after overnight fasting.