FASTING INHIBITS INSULIN-MEDIATED GLYCOLYSIS AND ANAPLEROSIS IN HUMAN SKELETAL-MUSCLE

Euglycemic (approximately 5.5 mM) hyperinsulinemic (60 mU.m-2.min-1) clamps were performed for 2 h after a 10-h fast and after a prolonged (72-h) fast. Biopsies were obtained from the quadriceps femoris muscle before and after each clamp. The rate of whole body glucose disposal was approximately 50% lower during the clamp after the 72-h fast (P less-than-or-equal-to 0.001). The increase in carbohydrate (CHO) oxidation (which is proportional to glycolysis) during the clamp after the 10-h fast (to 13.8 +/- 1.5-mu-mol.kg fat free mass-1.min-1) was completely abolished during the clamp after the 72-h fast (1.7 +/- 0.6; P less-than-or-equal-to 0.001). During the clamp after the 10-h fast, postphosphofructokinase (PFK) intermediates and malate in muscle increased, whereas glutamate decreased (P less-than-or-equal-to 0.05-0.001 vs. basal) and citrate did not change. During the clamp after the 72-h fast, there were no significant changes in post-PFK intermediates or glutamate (P > 0.05 vs. basal), but there was a decrease in citrate (P less-than-or-equal-to 0.01 vs. basal). Euglycemic hyperinsulinemia increased glycogen synthase fractional activity in muscle under both conditions but to a greater extent after the 72-h fast (P less-than-or-equal-to 0.01). It is concluded that insulin (after 10-h fast) increases glycolytic flux and the content of malate in muscle, which is probably due to increased anaplerosis. The decrease in citrate during hyperinsulinemia after the 72-h fast indicates that insulin activated the tricarboxylic acid (TCA) cycle but that the replenishment of TCA cycle intermediates normally observed (10-h fast) was blocked, possibly due to inadequate provision of three-carbon intermediates for anaplerosis. These data demonstrate that prolonged fasting can increase and decrease insulin-mediated activation of processes associated with CHO metabolism in human skeletal muscle.