RESPONSIVENESS OF ADENYLATE-CYCLASE TO PITUITARY GONADOTROPINS AND EVIDENCE OF A HORMONE-INDUCED DESENSITIZATION IN THE LIZARD OVARY

Gonadotropins (FSH and LH) affect several mammalian gonadal, functions . In particular FSH stimulates oogonial proliferation and oocyte growt h, while LH regulates ovulation and progesterone secretion. In lacerti lian reptiles, gonadal function is also regulated by pituitary gonadot ropins, but which hormone controls ovarian activities and the mechanis ms of action are unknown. The present study aimed to clarify mechanism s of action of pituitary gonadotropins on the ovary of Podarcis sicula (Lacertilia). The data demonstrate that mammalian gonadotropins FSH a nd LH produce a threefold stimulation of adenylate cyclase activity in follicular membranes, while hCG and TSH are less effective, causing a twofold increase in adenylate cyclase activity. Neurotrasmitters such as dopamine, serotonin, and catecholamines have no effect on enzyme a ctivity. The action of mammalian FSH and LH on the ovary mimics the ef fect of homologous hormones: in lizard ovaries incubated in vitro in t he presence of isolated homologous pituitary glands, the intracellular cAMP level increased by 50% with respect to control ovaries. Mammalia n gonadotropins appear homologous to lizard gonadotropin(s): Southern blot analyses show that the lizard genome contains nucleotide sequence s homologous to those encoding for mammalian beta FSH and beta LH. Bot h homologous and heterologous desensitization of adenylate cyclase act ivity occurs in the lizard ovary. In fact, responsiveness of adenylate cyclase to gonadotropin stimulation is abolished in animals 2 hr afte r in vivo treatment with FSH. Sensitivity to gonadotropin stimulation is restored 2 weeks after the beginning of the in vivo treatment. Dese nsitization was also observed in ovaries incubated in vitro with mamma lian FSH or with isolated pituitary glands. (C) 1997 Academic Press.