Insulin and exercise differentially regulate PI3-kinase and glycogen synthase in human skeletal muscle

The purpose of this study was to determine the separate and combined effect s of exercise and insulin on the activation of phosphatidylinositol 3-kinas e (PI3-kinase) and glycogen synthase in human skeletal muscle in vivo. Seve n healthy men performed three trials in random order. The trials included 1 ) ingestion of 2 g/kg body wt carbohydrate in a 10% solution (CHO); 2) 75 m in of semirecumbent cycling exercise at 75% of peak O-2 consumption; follow ed by 5 x 1-min maximal sprints (Ex); and 3) Ex, immediately followed by in gestion of the carbohydrate solution (ExCHO). Plasma glucose and insulin we re increased (P < 0.05) at 15 and 30 (Post-15 and Post-SO) min after the tr ial during CHO and ExCHO, although insulin was lower for ExCHO. Hyperinsuli nemia during recovery in CHO and ExCHO led to an increase (P < 0.001) in PI 3-kinase activity at Post-30 compared with basal, although the increase was lower (P < 0.004) for ExCHO. Furthermore, PI3-kinase activity was suppress ed (P < 0.02) immediately after exercise (Post-0) during Ex and ExCHO. Area under the insulin response curve for all trials was positively associated with PI3-kinase activity (r = 0.66, P < 0.001). Glycogen synthase activity did not increase during CHO but was increased (P < 0.05) at Post-0 and Post -30 during Ex and ExCHO. Ingestion of the drink increased (P < 0.05) carboh ydrate oxidation during CHO and ExCHO, although the increase after ExCHO wa s lower (P < 0.05) than CHO. Carbohydrate oxidation was directly correlated with PI3-kinase activity for all trials (r = 0.63, P < 0.001). In conclusi on, under resting conditions, ingestion of a carbohydrate solution led to a ctivation of the PI3-kinase pathway and oxidation of the carbohydrate. Howe ver, when carbohydrate was ingested after intense exercise, the PI3-kinase response was attenuated and glycogen synthase activity was augmented, thus facilitating nonoxidative metabolism or storage of the carbohydrate. Activa tion of glycogen synthase was independent of PI3-kinase.