Gonadal Receptors: Upregulation in Response to Elevated Serum Gonadotropin Levels1

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Abstract
After a single i.p. injection of human chorionic gonadotropin (hCG) (CR 121;6.4 µg/animal), the number of gonadotropin receptors in the rat testis has been found to increase as a function of time. The number of receptors in the rat testis was maximal at 3 h and remained elevated for at least 5 to 6 h before dropping to values below nontreated control samples. During this period of intense activity in the testis, the basal cyclic AMP (cAMP) and testosterone levels were increased. Despite high levels of biologically and immunologically active hCG in circulation, the receptor occupancy was minimal [max)]; the ability of the intact testis to produce/release cAMP into the medium in response to minute quantities of added human luteinizing hormone (hLH) in vitro was greater. At 3 h, soluble receptors accumulated in the interstitial fluid and the testis was highly vascular. These results suggest that the levels of receptors fluctuate in the testis, and their levels in the tissue dictate the biologic response.

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