MECHANISM OF ENHANCED INSULIN SENSITIVITY IN ATHLETES - INCREASED BLOOD-FLOW, MUSCLE GLUCOSE-TRANSPORT PROTEIN (GLUT-4) CONCENTRATION, AND GLYCOGEN-SYNTHASE ACTIVITY

We examined the mechanisms of enhanced insulin sensitivity in 9 male h ealthy athletes (age, 25+/-1 yr; maximal aerobic power [VO2max], 57.6/-1.0 ml/kg per min) as compared with 10 sedentary control subjects (a ge, 28+/-2 yr; VO2max, 44.1+/-2.3 ml/kg per min). In the athletes, who le body glucose disposal (240-min insulin clamp) was 32% (P < 0.01) an d nonoxidative glucose disposal (indirect calorimetry) was 62% higher (P < 0.01) than in the controls. Muscle glycogen content increased by 39% in the athletes (P < 0.05) but did not change in the controls duri ng insulin clamp. VO2max correlated with whole body (r = 0.60, P < 0.0 1) and nonoxidative glucose disposal (r = 0.64, P < 0.001). In the ath letes forearm blood flow was 64% greater (P < 0.05) than in the contro ls, whereas their muscle capillary density was normal. Basal blood flo w was related to VO2max (r = 0.63, P < 0.05) and glucose disposal duri ng insulin infusion (r = 0.65, P < 0.05). The forearm glucose uptake i n the athletes was increased by 3.3-fold (P < 0.01) in the basal state and by 73% (P = 0.05) during insulin infusion. Muscle glucose transpo rt protein (GLUT-4) concentration was 93% greater in the athletes than controls (P < 0.01) and it was related to VO2max(r = 0.61, P < 0.01) and to whole body glucose disposal (r = 0.60, P < 0.01). Muscle glycog en synthase activity was 33% greater in the athletes than in the contr ols (P < 0.05), and the basal glycogen synthase fractional activity wa s closely related to blood flow (r = 0.88, P < 0.001). In conclusion: (a) athletes are characterized by enhanced muscle blood flow and gluco se uptake. (b) The cellular mechanisms of glucose uptake are increased GLUT-4 protein content, glycogen synthase activity, and glucose stora ge as glycogen. (c) A close correlation between glycogen synthase frac tional activity and blood flow suggests that they are causally related in promoting glucose disposal.