-Adrenergic Receptors in Rat Fat Cells and Their Relationship with Lipolysis

Norepinephrine stimulated lipolysis in rat fat cells while (—) -alprenolol completely inhibited this lipolysis. (—)-Alprenolol competed for (—)-[3H]dihydroalprenolol(DHA) binding sites on rat fat cells. The specific (—)-[3H]DHA binding sites identified by competition with (—)-alprenolol were found to be transferred to the solubilized supernatant during preparation of endogenous lipid droplets from the fat cells. Although the lipid droplets did not exhibit specific (—)-[3H]DHA binding, norepinephrine induced lipolysis in a cell-free system consisting of the lipid droplets and hormone-sensitive lipase (HSL). Norepinephrine-induced lipolysis in the cell-free system was inhibited by pro-pranolol and (—)-alprenolol, but not by phenoxybenzamine. The lipolytic action of norepinephrine and the anti-lipolytic actions of propranolol and (—)-alprenolol disappeared after sonication of the lipid droplets in the cell-free system. These results suggest that the adrenergic receptor concerned with lipolysis in fat cells may not be a specific (—)- [3H]DHA binding site, but may be closely related to the lipid droplets.