The sequence of events set in motion through the activation of adenyl cyclase by E is: increase in cyclic 3',5'-AMP → activation of phosphorylase b kinase → increase in phosphorylase a → increase in glycogen breakdown. Since cyclic 3',5'-AMP is destroyed by a phosphodiesterase, its actual level in the tissues will depend on the relative rates of two opposing reactions, catalyzed by cyclase and diesterase (1, 20). Posner et al. (16) have shown that the concentration of cyclic 3',5'-AMP increases in frog muscle after E administration. Concentrations of the order of 1 x 10-7 to 1 x 10-6 M activate the phosphorylase b kinase of muscle both in vivo and in vitro (10, 16). Phosphofructokinase, on the other hand, is activated in vitro by concentrations of cyclic 3',5'-AMP so much higher than are present in muscle that one wonders whether this activation is of physiological importance (13). In the isolated frog sartorius incubated at 20°C in 5 x 10-7 M E a rise in phosphorylase a can be demonstrated in less than 5 min and this is followed after a lag period of a few minutes by a rise in glucose-6-P, but an increased lactate production through activation of phosphofructokinase does not take place until after 30 min of incubation. Stimulation of a muscle during incubation in E at frequencies as low as 3 shocks per minute, results in an immediate rise in lactate production. Thus, there is no evidence that E has a specific activating effect on phosphofructokinase. Özand and Narahara (15) found that addition of glucagon-free insulin to one of a pair of muscles incubated in E resulted in a smaller accumulation of glucose-6-P and in a larger production of lactate; this result suggests that insulin enhanced the activity of phospho-fructokinase. At present it is not definitely known by what mechanism electrical stimulation or insulin increases phosphofructokinase activity in muscle.
That the problem of activation of glycolytic enzymes in vivo is complex is shown by the fact that according to Lyon and Porter (12) muscle of mice lacking phosphorylase b kinase respond to the injection of epinephrine with increased glycogen breakdown. This suggests that E can increase phosphorylase b activity in an as yet undetermined manner. It would seem that the mechanism of regulation of enzyme activity by hormones requires further study.