The Action of Insulin on Glycogen Synthesis in Rat Diaphragm. A comparative study of in vitro and in vivo effects

Insulin, administered by intraperitoneal injection in doses which did not significantly alter the blood glucose, caused a 40‐fold increase in the incorporation of 14C‐glucose into glycogen of the diaphragm. To study this effect of insulin, activities of glycogen synthetase and levels of glucose‐6‐P were determined, and compared with the'corresponding values in the isolated diaphragm. Insulin, in vitro, caused a 2‐fold increase in the glucose‐6‐P independent form of glycogen synthetase, while the enzyme activity in the presence of glucose‐6‐P remained unchanged. When glycogen synthetase was studied in vivo, no change of the glucose‐6‐P dependent to the glucose‐6‐P independent form of the enzyme was observed. Insulin, in vitro, did not affect glucose‐6‐P levels when glucose was excluded from the incubation medium. After insulin in vivo glucose‐6‐P levels were increased by 75 per cent. The results suggest that insulin in vivo can markedly stimulate glycogenesis without causing any synthetase D → I conversion. It is suggested that glucose‐6‐P plays an important role in the regulation of muscle glycogen synthesis in vivo.