Regulation of glycogen breakdown by glycogen level in contracting rat muscle.
The interaction of glycogen concentration, insulin and beta-adrenergic stimulation in the regulation of glycogen breakdown was studied in perfused rat muscles. Rats were pre-conditioned to obtain two groups with either normal (N) or 'supercompensated' (SC) muscle glycogen. The next day their hindlimbs were perfused with a medium containing insulin (0, 40 and 100 microU mL(-1)) and/or isoproterenol (0 and 1.5 nmol L(-1)). Contractions were induced by electrical stimulation of the sciatic nerve. Compared with N, glycogen breakdown in white gastrocnemius during contractions was greater in SC at any hormonal combination (P < 0.05). Conversely, in red gastrocnemius (RG) the higher glycogenolytic rate in SC, compared with N, faded as the insulin concentration was raised from 0 to 100 microU mL(-1). However, isoproterenol restored the higher glycogenolytic rate in SC. In any condition, RG glycogen synthase fractional activity was lower (P < 0.05) during contractions in SC than in N. Furthermore, the percentage of phosphorylase a was higher in SC except when muscles were exposed to insulin alone. In conclusion, high initial glycogen concentration in fast-glycolytic muscle causes high glycogenolytic rate during contractions, irrespective of hormonal stimulation. In contrast, due to down-regulation of phosphorylase activity, such a relationship does not exist in insulin-stimulated fast-oxidative muscle.
Vandenberghe
K
K
Department of Kinesiology, Faculty of Physical Education and Physiotherapy, Katholieke Universiteit Leuven, Heverlee, Belgium.
Richter
E A
EA
Hespel
P
P
D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D004558:Electric Stimulation; D006003:Glycogen / Q000378:metabolism*; D006006:Glycogen Synthase / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018656:Muscle Fibers, Fast-Twitch / Q000187:drug effects / Q000378:metabolism; D018657:Muscle Fibers, Slow-Twitch / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D010101:Oxygen Consumption / Q000187:drug effects; D010477:Perfusion; D006005:Phosphorylases / Q000378:metabolism; D051381:Rats
D000318:Adrenergic beta-Agonists; D007328:Insulin; D006003:Glycogen; D006005:Phosphorylases; D006006:Glycogen Synthase; D007545:Isoproterenol