ROLE OF GLUCOCORTICOID IN THE REGULATION OF GLYCOGEN-METABOLISM IN SKELETAL-MUSCLE

With the use of the hindlimb perfusion technique, the effect of glucocorticoid on the regulation of glycogen metabolism was studied in rat skeletal muscle. Rats were adrenalectomized (ADX) or sham operated (controls) 14 days before the study. The ADX animals were treated with either saline or corticosterone, and the hindlimbs were perfused at rest or during muscle contraction with saline or epinephrine (10(-7) M). In the resting state, the glycogen content was 33.0 +/- 1.9-mu-mol/g in the controls, and the activity ratios of glycogen phosphorylase (GPase) and glycogen synthase (GSase) were 0.27 +/- 0.03 and 0.15 +/- 0.02, respectively. Epinephrine treatment increased GPase activity (0.78 +/- 0.03) and decreased GSase activity (0.05 +/- 0.01), which resulted in decreased glycogen content (25.7 +/- 0.9-mu-mol/g; P < 0.01). Adrenalectomy induced a 35% reduction in glycogen content but had no effect on the activities of basal enzymes. Under these conditions, however, epinephrine had no effect on GPase activity, had a diminished effect on GSase activity (0.11 +/- 0.01), and did not induce further glycogen breakdown. Corticosterone replacement normalized muscle glycogen content in ADX rats as well as the response of the enzymes to epinephrine. Muscle contraction resulted in a decrease in glycogen content (8.9 +/- 1.3-mu-mol/g) and in GPase activity (0.14 +/- 0.02) and an increase in GSase activity (0.25 +/- 0.01); this was not affected by adrenalectomy nor by epinephrine. In conclusion, these data indicate that glucocorticoid is essential for the effects of epinephrine on GPase activation, on GSase inhibition, and consequently on glycogen breakdown in resting muscle. During muscle contraction, however, glycogen breakdown occurs independently of the phosphorylation of the two rate-limiting enzymes and is not affected by adrenalectomy or epinephrine treatment.