Regulation of glycogen breakdown by glycogen level in contracting rat muscle

The interaction of glycogen concentration, insulin and beta-adrenergic stim ulation 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 hindlim bs were perfused with a medium containing insulin (0, 40 and 100 mu U 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 brea kdown in white gastrocnemius during contractions was greater in SC at any h ormonal combination (P < 0.05). Conversely, in red gastrocnemius (RG) the h igher glycogenolytic rate in SC, compared with N, faded as the insulin conc entration was raised from 0 to 100 mu U mL(-1). However, isoproterenol rest ored the higher glycogenolytic rate in SC. In any condition, RG glycogen sy nthase fractional activity was lower (P < 0.05) during contractions in SC t han in N. Furthermore, the percentage of phosphorylase a was higher in SC e xcept when muscles were exposed to insulin alone. In conclusion, high initi al glycogen concentration in fast-glycolytic muscle causes high glycogenoly tic rate during contractions, irrespective of hormonal stimulation. in cont rast, due to down-regulation of phosphorylase activity, such a relationship does not exist in insulin-stimulated fast-oxidative muscle.