α-Glycerophosphate Dehydrogenase Response to Thyroxine in Thyroidectomized, Thiouracil-Fed and Temperature-Adapted Rats

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Abstract
The large increase in liver and kidney α-glycerophosphate dehydrogenase (GPD) activity following the administration of thyroxine (T4) was used as a tool for the study of a variety of problems in thyroid metabolism. Changes in liver and kidney GPD paralleled the changes in metabolic rate and liver slice QO2) but the magnitude of the alteration in enzyme activity was much greater and more sensitive. The GPD activity in liver and kidney increased in proportion to the amount of T4 administered, and a determination of such dose-response curves under a variety of experimental conditions showed that thyroidectomized rats were more sensitive to T4 than were intact rats. Similarly, rats maintained at an environmental temperature of 5 C were more sensitive to T4 than were rats kept at room temperature (23 C), and the latter were more sensitive than those kept at 35 C. The endogenous secretion of thyroxinelike activity by the normal euthyroid was estimated by this enzyme procedure to be approximately 1 μg L-T4/100 g body wt/day. An estimate of T4 output by the antigoitrogenic procedure was too high (3.3 μg). Thyroxine output was increased approximately 50% when the rats were kept at 5 C, and practically ceased in rats maintained at 35 C. (Endocrinology75: 908, 1964)