COMPARISON OF CARDIAC AND METABOLIC ACTIONS OF THYROXINE, THYROXINE DERIVATIVES AND DINITROPHENOL IN THYROIDECTOMIZED RATS

Abstract

[male] rats weighing 130 to 150 g. were thyroidectomized and treated with various thyroid compounds or dinitrophenol. Three wks. after operation, the heart rate of the untreated animals had decreased on the avg. from 431-339 beats/min. in fully conscious, restrained animals, and from 379-265 beats/min. in anesthetized animals. A dose of 3 [gamma] of thyroxine daily restored the heart rate almost to normal, while doses of 30, 300 and 3000 [gamma] gave values above normal, reaching in the case of the highest dose 647 in conscious and 574 in nem-butal-treated rats. Diiodothyronine (412 [mu] daily) raised the heart rate to the normal range. Thyronine, diiodotyrosine, and KI appeared to have no effect on the heart of the thyroidectomized rat. In general, thyroidectomy reduced, and graded doses of thyroxine increased in a parallel fashion heart rate, body temp., breathing rate and O2 consumption. Dinitrophenol stimulated breathing rate and body temp. along with O2 consumption, but it affected the heart rate only slightly. This discrepancy suggested that the effect of thyroxine on the heart rate is not the consequence of general metabolic stimulation but is due to a direct effect on the heart. Thyroxine appears to be the specific heart-stimulating agent in thyroid preps.