Evidence that the effect of triiodothyronine on epinephrine-stimulated lipolysis in white adipose tissue is mediated via nuclear receptors.

The decreased basal and catecholamine stimulated lipolysis in adipocytes from hypothyroid rats can be increased by thyroid hormone (TH) treatment. In the present study it was investigated whether this TH effect is mediated via nuclear receptors and synthesis of new proteins or via other mechanisms. Epinephrine stimulated lipolysis was measured in vitro in isolated white adipocytes. Decreased lipolysis in adipocytes from hypothyroid rats was increased to euthyroid levels when the animals received one dose of 15 micrograms triiodothyronine (T3)/100 g body weight (BW) iv 15 h before the adipocytes were prepared for the in vitro measurement of lipolysis. No effect of T3 on lipolysis could be seen when the T3 was given only 10 min. before obtaining adipose tissue for the in vitro measurements. The stimulation of lipolysis several hours after the administration of T3 could be abolished by the protein synthesis blocker cycloheximide. These data and our previous demonstration of nuclear T3 receptors in rat adipocytes are compatible with the assumption that the enhancement of lipolysis by TH is mediated via a mechanism involving nuclear TH receptors and synthesis of new proteins.