Regulation of glycogen synthase activity in muscle by a C-terminal part sequence of human growth hormone

The synthetic peptide hGH 177-191, corresponding to the last 15 residues at the carboxyl terminus of human pituitary growth hormone, promotes the conversion of glycogen synthase a to glycogen synthase b in muscle. When injected, the peptide was found to produce inactivation of glycogen synthase phosphatase activity in rat skeletal muscle. The time course of phosphatase inactivation was closely correlated with that for glycogen synthase. The peptide had no effect either on muscle 3',5'-cyclic AMP levels or on synthase kinase activity. These results can be explained in terms of a dynamic cycle of interconversion of synthase between active and inactive forms, by the simultaneous action of synthase kinases and synthase phosphatases. A decrease in the ratio of phosphatase to kinase activity would result in a decrease in the steady-state level of synthase a activity.