Action of Thyroxin on Tissue Respiration. ∗

Abstract

Experiments in which thyroxin, or thyroid substance, has been added directly to surviving animal tissues in vitro have given, almost without exception, negative results. This might have been expected since the maximum action from a single dose of thyroxin in vivo is reached only after several days while the tissue survival is for about an equal number of hours. Respiration experiments on tissues taken from cretin and thyroxinized animals have given variable results. The reports of several workers,^{1, 7} however, offer rather convincing proof of a direct thermogenic action of thyroxin on the body tissues. The present work was begun to study the nature of this thermogenic action.

The oxygen consumption (QO₂) of muscle fasciculi was measured volumetrically by means of differential microrespirometers (Fenn). The animals were routinely anesthetized with amytal for 30 minutes after which they were bled from the carotid artery. The left semimembranous muscle was quickly removed and the muscle strips carefully prepared according to the method of Richardson, Shorr, and Loebel.⁸ These were suspended in the microrespirometer flasks in buffered saline, pH 7.35, or in the same solution with 0.9 mg. of lactic acid per cc. The air in the flasks was replaced with oxygen. The CO₂ was absorbed by 5 drops of N NaOH solution. The time interval before the microrespirometers were placed in the water bath was 35 minutes. A temperature equilibration of 30 minutes was always allowed (bath T. 38°C). The tests usually ran for 7 hours. Bacteriological precautions were carefully taken. The tissues were later dried to constant weight at 110°C.

A 36-hour fast in normal pups diminishes the QO₂ of surviving muscle strips by about 21% when compared with that of non-fasting animals.