Control of glycogen synthesis by glucose, glycogen, and insulin in cultured human muscle cells.

A key feature of type 2 diabetes is impairment in the stimulation of glycogen synthesis in skeletal muscle by insulin. Glycogen synthesis and the activity of the enzyme glycogen synthase (GS) have been studied in human myoblasts in culture under a variety of experimental conditions. Incubation in the absence of glucose for up to 6 h caused an approximately 50% decrease in glycogen content, which was associated with a small decrease in the fractional activity of GS. Subsequent reincubation with physiological concentrations of glucose led to a dramatic increase in the rate of glycogen synthesis and in the fractional activity of GS, an effect which was both time- and glucose concentration-dependent and essentially additive with the effects of insulin. This effect was seen only after glycogen depletion. Inhibitors of signaling pathways involved in the stimulation of glycogen synthesis by insulin were without significant effect on the stimulatory action of glucose. These results indicate that at least two distinct mechanisms exist to stimulate glycogen synthesis in human muscle: one acting in response to insulin and the other acting in response to glucose after glycogen depletion, such as that which results from exercise or starvation.