MAPPING THE REGULATORY PATHWAYS OF GLYCOGEN METABOLISM IN MAMMALIAN SKELETAL MUSCLE

Abstract

This chapter elaborates the mapping of regulatory pathways of glycogen metabolism in mammalian skeletal muscle. In resting muscle, phosphorylase and phosphorylase kinase are both in their dephosphorylated b forms and the rate of glycogenolysis is very low. It is found that when a muscle is stimulated electrically, conversion to phosphorylase-a and glycogenolysis takes place. The proportion of phosphorylase-a that is formed increases with increasing frequency of electrical stimulation, until at tetanic frequencies, almost complete conversion takes place within a second. Even at tetanic frequencies, no significant phosphorylation of phosphorylase kinase appears to take place as judged by the pH 6.8/8.2 activity ratio of the enzyme. The stimulation of glycogenolysis during muscle contraction therefore appears to be caused by the appearance of phosphorylase-a, resulting from the activation of phosphorylase kinase b by calcium ions. I-strain mice completely lack phosphorylase kinase and are unable to form phosphorylase-a even when their muscles are stimulated at tetanic frequencies.