Studies on the wAdrenergic Activation of Hepatic Glucose Output I. STUDIES ON THE a-ADRENERGIC ACTIVATION OF PHOSPHORYLASE AND GLUCONEOGENESIS AND INACTIVATION OF GLYCOGEN SYNTHASE IN ISOLATED RAT LIVER PARENCHYMAL

Epinephrine and the a-adrenergic agonist phenylephrine activated phosphorylase, glycogenolysis, and gluconeogenesis from lactate in a dose-dependent manner in isolated rat liver parenchymal cells. The half-maximally active dose of epinephrine was lo-’ M and of phenylephrine was 10m6 M. These effects were blocked by a-adrenergic antagonists including phenoxybenzamine, but were largely unaffected by P-adrenergic antagonists including propranolol. Epinephrine caused a transient 2-fold elevation of adenosine 3’:5’-monophosphate (CAMP) which was abolished by propranolol and other fl blockers, but was unaffected by phenoxybenzamine and other (Y blockers. Phenoxybenzamine and propranolol were shown to be specific for their respective adrenergic receptors and to not affect the actions of glucagon or exogenous CAMP. Neither epinephrine (10e7 M), phenylephrine (10m5 M), nor glucagon (10e7 M) inactivated glycogen synthase in liver cells from fed rats. When the glycogen synthase activity ratio (-glucose 6-phosphate/ +glucose G-‘phosphate) was increased from 0.09 to 0.66 by preincubation of such ceils with 40 mM glucose, these agents substantially inactivated the enzyme. Incubation of hepatocytes from fed rats resulted in glycogen depletion which was correlated with an increase in the glycogen synthase activity ratio and a decrease in phosphorylase a activity. In hepatocytes from fasted animals, the glycogen synthase activity ratio was 0.32 + 0.03, and epinephrine, glucagon, and phenylephrine were able to lower this significantly. The effects of epinephrine and phenylephrine on the enzyme were blocked by phenoxybenzamine, but were largely unaffected by propranolol. Maximal phosphorylase activation in hepatocytes from fasted rats incubated with 10m5 M phenylephrine preceded the maximal inactivation of glycogen synthase. Addition of glucose rapidly reduced, in a dose-dependent manner, both basal and phenylephrine-elevated phosphorylase a activity in hepatocytes prepared from fasted rats. Glucose also increased the glycogen synthase activity ratio, but this effect lagged behind the change in phosphorylase. Phenylephrine (10m5~) and glucagon (5 x lo-” M) decreased by one-half the fall in phosphorylase a activity seen with 40 mM glucose and markedly suppressed the elevation of glycogen synthase activity. The following conclusions are drawn from these findings. (a) The effects of epinephrine and phenylephrine on carbohydrate metabolism in rat liver parenchymal cells are mediated predominantly by a-adrenergic receptors. (6) Stimulation of these receptors by epinephrine or phenylephrine results in activation of phosphorylase and gluconeogenesis and inactivation of glycogen synthase by mechanisms not involving an increase in cellular CAMP. (c) Activation of fl-adrenergic receptors by epinephrine leads to the accumulation of CAMP, but this is associated with minimal activation of phosphorylase or inactivation of glycogen synthase. (d) Glycogen depletion is responsible, in part at least, for the decrease in phosphorylase activity and in’crease in glycogen synthase activity ratio observed in hepatocytes from fasted rats. (e) Glucose can dramatically alter basal phosphorylase a and glycogen synthase activities in hepatocytes and can overcome the effects of glycogenolytic agents on these two enzymes.