Circadian rhythm in sensitivity of glucose metabolism to insulin in rat soleus muscle.

This study determined whether the sensitivity of glucose metabolism to insulin in skeletal muscle varies during a 24-h period. Soleus muscles were isolated from ad libitum-fed rats killed at 0900, 1600, 2100, and 0300. The animal house was illuminated between 0800 and 2000. The sensitivities of glycolysis (which is an excellent index of glucose transport) and glycogen synthesis to insulin were greatest in muscles isolated at 0900 and 2100. Marked decreases in sensitivities of both processes to insulin were observed in muscles isolated at 0300 and 1600, which are times halfway through the feeding and postabsorptive periods, respectively. Hence, this study demonstrates circadian changes in the sensitivity of glucose utilization by skeletal muscle to insulin, which may be important in control of blood glucose concentration. Glycogen levels in skeletal muscles were highest at 0300 and lowest at 2100; hepatic glycogen content reached a peak at 0900, and the lowest content was measured at 2100. The liver glycogen level was increased by only 15% midway into the feeding period (i.e., 0300). This suggests that muscle glycogen may act as a temporary store of glucose residues during the feeding period; it stores glycogen in the first half of the feeding period but during the second half some muscle glycogen is converted to lactate, which acts as a precursor for hepatic gluconeogenesis.