Insulin action in denervated rat hemidiaphragms. Decreased hormonal stimulation of glycogen synthesis involves both glycogen synthase and glucose transport.

Rat hemidiaphragms were denervated in vivo and then incubated in vitro to assess the ability of insulin to stimulate glycogen synthesis. Denervation for 1 day resulted in 50% decreases in the stimulation by insulin of [U-14C]glucose incorporation into glycogen, and in both the basal and the insulin-stimulated activity ratios (activity minus glucose-6-P/activity plus glucose-6-P) of glycogen synthase; however, the stimulation of 2-deoxyglucose uptake by insulin was not affected by 1 day of denervation. The hormonal stimulation of [U-14C]glucose into glycogen was decreased by 90% after 3 days of denervation. At this time, the stimulation of 2-deoxyglucose uptake by insulin was also reduced and the synthase activity ratios remained depressed. Consistent with its reduced effect on glucose transport, the hormone did not increase glucose-6-P in the 3-day denervated muscles. Furthermore, the Ka for activation of glycogen synthase by glucose-6-P was higher in denervated muscles, and denervation abolished the effect of insulin on decreasing the Ka. The results presented demonstrate that denervation rapidly reduces the extent to which glycogen synthase can be activated by insulin, and has a later effect on decreasing the stimulation of glucose transport. These two effects act synergistically to markedly decrease the hormonal stimulation of glycogen synthesis.