Abstract
Chronic administration of thyroxine to rats produced an elevation in myocardial phosphorylase a activity, and increased heart weight/body weight ratio, cardiac rate and mean arterial blood pressure. Both tyramine and McNeil-A-343 caused a rise in heart phosphorylase a activity in euthyroid rats. In rats pretreated with thyroxine, the increase in enzyme activity caused by tyramine was similar to that obtained in euthyroid animals. An analysis of variance over the entire dose range of tyramine showed that the elevation in cardiac phosphorylase a observed in thyrotoxic animals was not significantly different from controls. It was concluded that thyroxine does not potentiate the stimulatory action of McNeil-A-343 or tyramine on cardiac phosphorylase a activity.
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