IN-VIVO REGULATION OF RAT MUSCLE GLYCOGEN RESYNTHESIS AFTER INTENSE EXERCISE

Time courses of the glycogen synthesis rate and of the glucose 6-phosp hate (G-6-P) concentration after an electrically induced exercise were followed in the anesthetized rat gastrocnemius by in vivo C-13 and P- 31 nuclear magnetic resonance (NMR) spectroscopy, respectively. The ra tio of glygogen synthase I to glycogen synthase I and D (I/I+D) and al losteric activation by G-6-P were also studied in vitro on muscles sam pled at rest and 10 min (early recovery) and 100 min (late recovery) a fter exercise. From early recovery to late recovery, the in vivo glyco gen synthesis rate dropped from 0.46 +/- 0.06 to 0.11 +/- 0.04 mmol.kg wet tissue-1.min-1, the G-6-P concentration from 0.83 +/- 0.08 to 0.3 2 +/- 0.05 mmol/kg wet tissue, and I/I+D from 83 +/- 4 to 47 +/- 1%. T he combination of the changes in G-6-P concentration and in I/I+D quan titatively describes the fourfold decrease in glycogen synthesis rate from early to late recovery. These results demonstrate that phosphoryl ation, determining glycogen synthase I/I+D, and allosteric control of glycogen synthase by G-6-P contribute approximately equally to the reg ulation of the postexercise in vivo glycogen synthesis rate.