No abstract

No abstract

Highly purified pregnant mares serum gonadotropin (PMSG) was nearly as potent as ovine luteinizing hormone (LH) and human follicle stimulating hormone (hFSH) when bioassayed in vitro in systems known to respond primarily to LH or FSH. An analogue of human chorionic gonadotropin treated with neuraminidase, galactosidase, beta-N-acetylglucosaminidase, and mannosidase (hCG) inhibited the stimulatory effects of hCG, LH, and PMGS on cAMP accumulation in rat Leydig cells but did not inhibit the stimulatory effects of FSH or PMSG on cAMP accumulation in ovarian granulosa cells obtained from immature rats fed diethylstillbestrol. Thus PMSG appeared to form functional complexes with both LH and FSH receptors and may be unique among mammalian gonadotropins. Treatment of PMGS with neuraminidase increased its potency nearly tenfold in vitro apparently by increasing its affinity for both LH and FSH receptors. Although the kinetics of PMSG binding were not investigated with radiolabeled materials, indirect functional binding studies are described that suggest that hCG more rapidly forms stable hormone-receptor complexes than PMSG, asialo-PMSG, FSH, and LH when all hormones are incubated under the same conditions.

A new general method is proposed for quantifying ligand-receptor interactions using the biological response induced by the ligand as an index of ligand binding. With this method the binding of human chorionic gonadotropin (hCG), several hCG derivatives, and luteinizing hormone (LH) to rat Leydig cells was measured by analysis of the ability of these materials to stimulate testosterone formation. As applied here, hormone dose-response curves were generated in the presence of increasing numbers of cells incubated in vitro in a successful attempt to alter the concentrations of bound and free hormone in the incubation mixture. Measurements of testosterone synthesis as a function of the total amounts of hormone and numbers of cells enabled us to evaluate the concentrations of both bound and free hormone at any constant fractional response (i.e. quarter-, half-, or three-quarter-maximal). We were thus able to measure hormone binding at the extremely low hormone concentrations (1 pM) within the steroidogenic dose-response range under conditions that would not have been possible using currently available radioiodinated hCG preparations. The results obrained confirmed the presence of spare functional receptors. Specific quantitative results are discussed in the text.

No abstract