Luteinizing hormone and its agonist human chorionic gonadotropin belong to the glycoprotein hormone family which also includes thyroid stimulating hormone and follicle stimulating hormone (1). LH and hCG share a common receptor designated the LH/CG receptor; it is found exclusively in gonadal cells. In the testes it is present in Leydig cells, and in the ovaries in theca, granulosa, luteal, and interstitial glandular cells. The receptor is located at the target cell surface and predominantly on cell surface regions abutting upon the capillary spaces (Fig. 1) (2, 3).
Biophysically the LH/CG receptor is a typical amphiphilic membrane protein which can be dissolved with nonionic detergents like Triton X-100. An immediate response of target cells to hormone binding to the receptor is a G protein-mediated activation of plasma membrane adenylate cyclase (e.g. 4,5). The resulting increase in intracellular cAMP ultimately stimulates steroid biosynthesis and secretion. This cAMP-mediated signal transduction pathway classifies