Adenosine 3',5'-monophosphate-dependent protein kinase activity decreases in human muscle after insulin infusion
We have previously reported that the increase in glycogen synthase activity in human muscle during a euglycemic clamp was not associated with a measured increase in glycogen synthase phosphatase activity after a 200-min insulin administration. To investigate further the mechanism of the regulation of human muscle glycogen synthase by insulin, we measured the activity of cAMP-dependent protein kinase before and after a 200-min hyperinsulinemic euglycemic clamp in Southwest American Indians. Insulin infusion resulted in a decreased cAMP-dependent protein kinase activity assayed at physiological cAMP concentration with increased glycogen synthase activity in all subjects (n = 5; P < 0.01). No significant change was observed in cAMP-independent protein kinase activity. These results suggest that 200 min of insulin administration during a euglycemic clamp may regulate human muscle glycogen synthase activity by mechanisms other than the stimulation of phosphatase; one probable mechanism is by decreasing the activity of cAMP-dependent protein kinase.