Human chorionic gonadotrophin and immature fish ovary: characterization and mechanism of the in vitro stimulation of cyclic adenosine monophosphate accumulation.

Human chorionic gonadotrophin (hCG) increased cyclic adenosine monophosphate (cAMP) in pieces of eel ovary in vitro. Although the ED50 (about 0.1 micrograms/ml) was the same as for carp gonadotrophin (cGTH), the maximal stimulation (close to 3 times basal level) was about 10 times lower than that for the fish hormone. The mechanism of this atypical action of hCG was studied using different approaches. Human CG did not inhibit the action of cGTH. Desensitization experiments (cGTH or hCG being injected in vivo 18 hr before the sampling of the ovary) were carried out; the in vitro action of hCG was suppressed by both pretreatments; the in vitro action of cGTH was largely reduced by in vivo cGTH and to a much lesser extent by in vivo hCG. Together with other data the results indicated that hCG acts via binding to adenylate cyclase-coupled receptors which also recognize cGTH; moreover, they suggested that among those systems only a fraction can be activated by hCG. We propose that in teleosts the pool of GTH receptors is not homogeneous but contains classes of "isoreceptors" differing by their recognition specificity at least. Parallel studies with ovine lutrophin (oLH) indicated that this hormone binds to the same adenylate cyclase-coupled receptors which are sensitive to hCG but that it does it with a much lower affinity. Other characteristics of oLH/hCG-stimulated receptor-adenylate cyclase systems include an important release of cAMP into the incubation medium. The precise physiological significance of GTH "isoreceptors" still requires clarification. The ability of hCG to induce desensitization must be considered in fish culture experiments using this hormone.