EXTRATHYROIDAL ACTIONS OF PITUITARY THYROTROPIN: EFFECTS ON THE CARBOHYDRATE, LIPID AND RESPIRATORY METABOLISM OF RAT ADIPOSE TISSUE*

Abstract

The actions of pituitary thyrotropin (TSH) upon the metabolism of rat epididymal adipose tissue have been examined. Commercial as well as the most highly purified available preparations of TSH were employed. Addition of all types of TSH to the incubation mixture resulted in an increase in the tissue stores of free fatty acids, and a release of fatty acids into the suspending medium in the presence of extracellular albumin. In contradistinction to the lipolytic effects of equivalent quantities of corticotropin (ACTH), the action of TSH was not contingent upon the presence of extracellular ionic calcium. Concomitant with the induction of lipolysis, TSH effected an enhancement of the oxygen consumption of adipose tissue which was maximal in the presence of extracellular glucose. Assimilation of glucose from all types of suspending media was also promoted by TSH. Heating of TSH for 3 minutes at pH 2 caused profound reduction in its effects upon lipolysis, respiration and glucose assimilation. Such mild acid-heating did not diminish the similar activities of ACTH in adipose tissue. Differentially-labeled glucose-C14 was employed to assess the effects of TSH upon the disposition of assimilated glucose. TSH caused a disparate augmentation in the oxidation of glucose carbon-6. Simple supplementation of albumin and glucose-containing media with larger quantities of palmitate or oleate effected comparable alterations in oxygen consumption and glucose oxidation. The parallelisms prompted the suggestion that lipolysis may constitute the progenitor for some of the TSH-induced changes in the metabolism of surviving adipose tissue. Adipose tissue has been analyzed as a model system wherein stimulation of respiratory activity and assimilative capacity may occur coincident with mobilization of preformed endogenous substrate.