Effects of insulin on gluconeogenesis and cyclic AMP levels in perfused livers from diabetic rats

Abstract

1. 1. The effects of insulin treatment on hepatic metabolism in diabetic animals were studied using the isolated perfused rat liver preparation. 2. 2. Insulin treatment for 0.5 h in vivo significantly reduced total glucose production and gluconeogenesis from physiological or higher concentrations of lactate in the perfused diabetic liver, but treatment for 3 h was necessary to completely normalize hepatic glucose output and gluconeogenesis. The hormone also produced a small but significantly reduction in glucose output when added to the medium perfusing livers from diabetic rats. It also antagonized in vitro the stimulatory effects of glucagon on glucose production, gluconeogenesis and ureogenesis. 3. 3. Insulin injected in vivo significantly reduced the level of cyclic AMP in diabetic livers after 0.5 h and returned it to normal after 3 h. The time course of the decrease in cyclic AMP closely resembled that of the fall in hepatic gluconeogenesis or glucose output. 4. 4. Diabetes produced changes in the levels of gluconeogenic intermediates in livers perfused with lactate which were consistent with facilitation of reactions located in the gluconeogenic pathway between pyruvate and phosphopyruvate and between glucose-6- P and glucose. These changes were reversed by insulin treatment. Diabetes also increased AMP and ADP. 5. 5. Diabetes did not increase gluconeogenesis from fructose suggesting that the change(s) located between glucose-6- P and glucose in the gluconeogenic pathway is not primarily responsible for the increased rate of gluconeogenesis from lactate. Diabetes also did not alter the high rate of lactate production in livers perfused with fructose suggesting that alterations in pyruvate kinase activity are not responsible for the changes in gluconeogenesis. 6. 6. Diabetes increased the activity of phosphoenolpyruvate carboxykinase in perfused or non-perfused livers several-fold. The increase was significantly reduced by treatment with insulin for 2 or 3 h in vivo. Diabetes also increased pyruvate carboxylase activity slightly, but insulin treatment for up to 6 h did not affect the enzyme. 7. 7. Diabetes increased the level of acetyl-CoA in livers sampled in vivo or perfused with lactase for 40 min. Insulin treatment for 0.5 h in vivo normalized the level of this intermediate in perfused livers and significantly reduced it in non-perfused livers. 8. 8. It is proposed that gluconeogenesis from lactate is increased in diabetic livers because of an increase in the hepatic levels of phosphoenolpyruvate carboxykinase and of acetyl-CoA, the latter promoting an activation of pyruvate carboxylase. The possibility is discussed that these changes may be due in turn to the increase in cyclic AMP in diabetic livers.