MELATONIN RECEPTORS ON HUMAN GRANULOSA-CELL MEMBRANES

Putative melatonin binding sites were detected in the membrane fractio n of gonadotropin-stimulated human granulosa cells using the melatonin analogue 2-[I-125]-iodomelatonin (I-125-IML). Saturation studies and Scatchard analysis revealed the presence of a major binding site with a K-d of 99 pM. Guanosine triphosphate shifted the receptor affinity t o 380 pM. In competition studies, the rank order of potency of indoles for inhibition of I-125-IML binding at these sites was typical of mel atonin receptors : 2-iodomelatonin > melatonin > N-acetylserotonin > 5 -methoxytryptamine > serotonin. Culture of cells for 7 days in vitro i ncreased receptor density but not the affinity. These findings strongl y suggest that melatonin found in follicular fluid may have a physiolo gical role.