CONTRACTILITY, HEART RATE, AND RESPONSE TO NOREPINEPHRINE OF ISOLATED RAT MYOCARDIUM FOLLOWING I¹³¹-INDUCED HYPOTHYROIDISM

¹ Department of Pharmacology, State University of New York at Buffalo, Buffalo, New York

A single intraperitoneal injection of 300 microcuries of I¹³¹ into 100-g rats produced a severe hypothyroidism as evidenced by loss of body weight, bradycardia and a 35% reduction in the BMR. The bradycardia persists under in vitro conditions and is therefore not entirely vagal in origin. Right ventricle strips of hypothyroid animals are less resistant to stretch than controls but are able to increase the active tension developed with stretch over a greater range than controls. In such an in vitro system it seems probable that the effect is not due to a greater efficiency of hypothyroid myocardium but that it is a consequence of decreased oxygen needs in the face of a limited oxygen supply imposed by problems of diffusion. No difference in the frequency-tension relationship was observed. The hypothyroid strips were not significantly less sensitive to the automaticity-producing or chronotropic effects of norepinephrine.