Effects of adrenalectomy and glucocorticoid replacement on gluconeogenesis in perfused livers from diabetic rats

Abstract

1. 1. The dependence of the increased rate of hepatic gluconeogenesis in diabetes upon glucocorticoid secretion was studied using the perfused rat liver preparation. Adrenalectomy reduced, and glucocorticoid treatment restored, glucose output, gluconeogenesis and glycogen synthesis from lactate, and ureogenesis in livers from alloxan-diabetic rats. Cortisol treatment for 1 h in vivo or in vitro significantly increased glucose output and gluconeogenesis from high or physiological levels of lactate in livers from adrenalectomized diabetic rats. Cortisol treatment for 2 h in vivo increased ureogenesis and glycogen synthesis in such livers, but cortisol infusion for 2 h in vitro produced no effects on these processes. 2. 2. The in vitro effects of cortisol on glucose output and gluconeogenesis were of similar magnitude to the in vivo effects and were abolished by addition of cycloheximide and actinomycin D. This suggests that the direct effects of glucocorticoids on hepatic glucose production and gluconeogenesis are quantitatively important and probably involved increased mRNA and protein synthesis. 3. 3. Measurements of the levels of gluconeogenic intermediates in perfused livers from adrenalectomized diabetic rats indicated restraint of reactions located between pyruvate and phosphopyruvate and possibly between glucose-6- P and glucose. Glucocorticoid treatment in vivo or in vitro produced changes consistent with facilitation of reactions located at these sites. There was no evidence of glucocorticoid action on fructose diphosphatase or phosphofructokinase and changes in adenine nucleotides and citrate were the opposite to those expected if steroid facilitated the net conversion of fructose-1,6- P 2 to fructose-6- P. 4. 4. Adrenalectomy reduced the large increase in phosphoenolpyruvate carboxykinase activity in livers from diabetic rats, but did not alter the small increase in pyruvate carboxylase. Cortisol treatment for 1 h and longer significantly elevated phosphoenolpyruvate carboxykinase activity in livers from adrenalectomized diabetic rats, but did not change pyruvate carboxylase activity. Adrenalectomy and steroid replacement did not modulate the increased levels of acetyl-CoA and cyclic AMP in livers from diabetic rats. 5. 5. It is concluded that the increase in hepatic gluconeogenesis in diabetes is dependent upon secretion of glucocorticoids and that induction of phosphoenolpyruvate carboxykinase is a major mechanism by which the steroid act. The hormones do not alter cyclics AMP levels nor affect pyruvate carboxylase activity by increasing the level of the enzyme or acetyl-CoA.