The interaction of hCG with rat adipose tissue: apparent lack of hCG--LH receptors.

The interaction of human chorionic gonadotropin (hCG) with rat adipose tissue was investigated by both metabolic and binding studies. Highly purified preparations of hCG did not affect the adenylate cyclase activity nor the lipolysis of rat adipocytes in the presence or in the absence of GTP. However, it was demonstrated that (a) the hCGs used were biologically active since they stimulated cAMP and testosterone production by rat Leydig cells, and (b) there are receptor sites on the rat ovary that bind [125I]hCG and recognize rat luteinizing hormone (LH). The lack of response cannot then be attributed to a loss of activity of the hormone preparation tested nor to a failure of the rat tissues to recognize an hormone of human origin, but rather to an absence of hCG--LH receptors on the fat cell membrane surface. It is suggested that results previously reported in other laboratories could be explained by the presence of contaminating amounts of lipolytic hormones in their preparations.