RAPID ACTIVATION OF GLYCOGEN-SYNTHASE AND PROTEIN PHOSPHATASE IN HUMAN SKELETAL-MUSCLE AFTER ISOMETRIC CONTRACTION REQUIRES AN INTACT CIRCULATION

The effects of isometric contraction (66% of maximal force) and recove ry on glycogen synthase fractional activity (GSF) in human skeletal mu scle have been studied. Biopsies were taken from the quadriceps femori s muscle at rest, at fatigue and 5 min postexercise on two occasions: after one of the contractions, the circulation to the thigh was occlud ed during the 5 min recovery (OCC), and after the other contraction, t he circulation was intact (control, CON). During CON GSF decreased fro m (mean +/- SE) 0.34 +/- 0.05 at rest to 0.24 +/- 0.02 at fatigue and then increased to 0.74 +/- 0.04 at 5 min postexercise; corresponding v alues for OCC were 0.37 +/- 0.04, 0.25 +/- 0.04 and 0.48 +/- 0.05 (P < 0.001 vs. CON for 5 min postexercise only). Compared with the value a t fatigue, protein phosphatase activity (PP) increased by 79 +/- 16% d uring CON recovery (P < 0.01), whereas no change was observed during O CC recovery. Uridine diphosphate glucose increased by approximately 2. 5-fold at fatigue, remained elevated during OCC recovery, but reverted to the preexercise level during CON recovery (P < 0.001 vs. OCC recov ery). Glucose 6-P increased approximately 5-fold at fatigue and was hi gher at 5 min postexercise in OCC vs. CON recovery (8.6 +/- 1.5 vs. 4. 1 +/- 0.9 mmol/kg dry wt; P < 0.01). It is concluded that the rapid in crease in GSF after intense exercise with an intact circulation may be at least partly attributed to an increase in the specific activity of PP. The increase in GSF during recovery in OCC may be at least partly attributed to the high glucose 6-P content in vivo, which enhances th e substrate suitability of GS for PP. Thus, separate mechanisms exist for the activation of PP and GS during recovery from intense short ter m exercise.