STUDIES ON THE EFFECTS OF GROWTH-HORMONE ADMINISTRATION IN-VIVO ON THE RATES OF GLUCOSE-TRANSPORT AND UTILIZATION IN RAT SKELETAL-MUSCLE

The effects of growth hormone (GH) administration to rats in vivo on the sensitivity of the rate of glucose utilization to insulin were studied in soleus muscles isolated from these rats. A single injection of GH did not increase the rate of glucose transport within 1-2 h. However, 12 h after, the rate of glucose transport was increased at 10 mU insulin l(-1) and was accompanied by a similar increase in the rate of lactate formation but no change in the rate of glycogen synthesis. Prolonged treatment with GH decreased the rate of glucose transport and glycogen synthesis and increased the content of glucose 6-phosphate at physiological levels of insulin but did not affect the rate of lactate formation. These results suggest that: (a) GH does not increase the rate of glucose transport acutely; however, after several hours, the sensitivity of glucose transport and glycolysis to insulin are increased; (b) prolonged elevations of the level of GH in plasma decrease the sensitivity of the rate of glucose transport and glycogen synthesis to insulin. However, redirection of glucose residues away from the pathway of glycogen synthesis towards that of glycolysis and a possible increase in the rate of glycogenolysis maintain a normal rate of lactate formation, although the rate of glucose transport is decreased.