In vivo insulin regulation of skeletal muscle glycogen synthase in calorie-restricted and in ad libitum-fed rhesus monkeys

Chronic calorie restriction in primates has been shown to have profound and unexpected effects on basal and on in vivo insulin action on skeletal musc le glycogen synthase (GS) activity. The decreased ability of insulin to act ivate skeletal muscle GS is a hallmark of insulin resistance and type 2 dia betes. The mechanism and role of in vivo insulin regulation of skeletal mus cle GS are not fully understood. Two pathways for the activation of GS by i nsulin have been described by Larner and others: 1) insulin activates gluco se transport that results in an increase in glucose-6-phosphate (G6P), ther eby activating protein phosphatase-1, which in turn dephosphorylates and ac tivates GS, therefore, pushing substrate into glycogen; and 2) insulin acti vates GS (perhaps by forming low-molecular-weight mediators which may activ ate protein phosphatase-1 and 2C) and activated GS subsequently pulls inter mediates (e.g., G6P and uridine 5'-diphosphoglucose) into glycogen. To dete rmine whether in vivo insulin regulates glycogen synthesis primarily via a push or pull mechanism and how this mechanism might be affected by long-ter m calorie restriction, skeletal muscle samples were obtained before and dur ing a euglycemic hyperinsulinemic clamp from 41 rhesus monkeys. The monkeys varied widely in their degree of insulin sensitivity and age and included chronically calorie-restricted (CR) monkeys and ad libitum-fed monkeys. The ad libitum-fed monkeys included spontaneously type 2 diabetic, prediabetic and clinically normal animals. The apparent affinity of GS for the alloste ric activator G6P (G6P Ka of GS) was measured and compared with G6P content in the muscle samples. Basal G6P Ka of GS was lower in the CR monkeys comp ared with the 3 ad libitum-fed groups (P less than or equal to 0.05). Only the normal ad libitum-fed monkeys had a decrease in the G6P Ka of GS with i nsulin (P < 0.005). The insulin effect (insulin-stimulated minus basal) on the G6P Ka of GS was strongly positively related to the insulin effect on G 6P content (r = 0.80, P < 0.0001) across the entire group of monkeys. This finding supports the hypothesis that activation/dephosphorylation of GS by insulin is related to a decrease in G6P content and that paradoxical inacti vation/phosphorylation of GS by insulin is related to an increase in G6P co ntent las demonstrated in 4 of 6 CR monkeys). Therefore, during a euglycemi c hyperinsulinemic clamp, insulin regulates skeletal muscle glycogen synthe sis primarily via a pull mechanism in both CR and in ad libitum-fed rhesus monkeys.