Dependence of the lipolytic action of epinephrine in vitro upon thyroid hormone*†

Abstract

The influence of thyroid hormone on the epinephrine-induced release of free fatty acids (FFA) from rat epididymal adipose tissue was studied in vitro. Untreated tissue from euthyroid animals released only small amounts of FFA. However, the tissues responded to treatment with epinephrine with a significant increase in the rate of release of FFA, confirming observations previously reported by others. In the case of fat pads removed from hypothyroid animals, no increase in the rate of release of FFA was observed after epinephrine; in the case of fat pads removed from hyperthyroid animals, the epinephrine-induced release of FFA was markedly exaggerated. This thyroidal enhancement of epinephrine action on the fat pad was not evident when tissues were removed from euthyroid animals 3 hours after intraperitoneal injection of triiodothyronine, but it was maximal in tissues removed after 15 hours from animals that had received repeated intraperitoneal injections of triiodothyronine. When triiodothyronine was added in vitro to fat pads from euthyroid rats, the basal release of FFA was not affected nor was the normal response to epinephrine altered. These studies show that the thyroid hormone is essential for the epinephrine-induced release of FFA from adipose tissue.