Response of Rat Atrium to Propranolol and Catecholamines in Various States of Thyroid Action

Abstract

Abstract The effect of propranolol on spontaneous beating, and the response of this to adrenaiinc and noradrenaline, was studied in hypothyroid, euthyroid and thyroxine treated isolated atrium of the rat hcart. A significant negative chronotropic effect was noted in thyroxinized preparations; in other groups this change was less significant. The inotropic property remained unchangcd in control conditions. A propranolol dose of 1 μg/ml blocked the positive chronotropic effect of adrenaline and noradrenaline completely in euthyroid and thyroxinized atria and partially in the hypothyroid ones. A competitive antagonism was observed between propranolol and catecholainines in the various preparations when contraction force and mechanical output werr measured. Thc positive inotropic effect of adrenaline and noradrenaline was highest in the hypothyroid and lowest in thr thyroxinized series. Only quantitative differences were ohserved in the responses of the atria to propranolol and catecholaminrs in various states of thyroid action.