The Effect of Noradrenaline and Tyramine on Cardiac Contractility, Cyclic AMP, and Phosphorylase a in Normal and Hyperthyroid Rats

Abstract

Noradrenaline and tyramine increased contractility, phosphorylase a, and cyclic AMP in a dose-dependent manner in the isolated perfused rat heart. Time–response studies revealed that cyclic AMP was increased by both amines before the increase in contractility. Hearts from hyperthyroid rats responded to noradrenaline in a manner similar to control hearts when contractility or cyclic AMP was measured. The phosphorylase activating effect of noradrenaline was enhanced in the hyperthyroid hearts. It was concluded that a non-cyclic AMP mechanism was involved in the enhancement. The effects of tyramine on contractility and cyclic AMP were decreased in the hearts from hyperthyroid animals when compared with control hearts. Phosphorylase activation by tyramine was not enhanced by the hyperthyroid state and was in fact reduced at higher doses of tyramine.