THE EFFECTS OF THYROXINE ON RAT HEART ADENOSINE 3', 5'-MONOPHOSPHATE, PHOSPHORYLASE b KINASE AND PHOSPHORYLASE a ACTIVITY

¹ Department of Pharmacology, School of Mediaine, University of Pennsylvania, Philadelphia, Pennsylvania

The effects of chronic treatment with thyroxine or acute injection of epinephrine on cardiac adenosine 3', 5'-monophosphate (cyclic AMP), phosphorylase b kinase and phosphorylase a activity were studied in rats. In agreement with other investigators, we found that epinephrine caused an increase in cyclic AMP, activation of phosphorylase b kinase and increased activity of phosphorylase a. These metabolic effects of the catecholamine were blocked by the beta adrenergic blocking drug, MJ-1999. Although thyroxine increased myocardial phosphorylase a activity, there was neither an increase in cardiac cyclic AMP nor activation of phosphorylase b kinase. Heart homogenates from hyperthyroid rats showed enhanced phosphorylase b kinase activity which was not the result of conversion of the inactive molecular form to the active form of the enzyme. It was concluded that cardiac phosphorylase a activity is elevated by thyroxine through a mechanism different from that of epinephrine.