LH Receptors and Leydig Cell Responsiveness to hCGin vitro

Abstract

The in vitro responsiveness of isolated Leydig cells from 30-day-old rats to hCG was assessed in terms of androgen and cAMP production, under conditions where the quantity of LH/hCG receptors on the Leydig cell membranes was reduced to varying degrees. Reduction in the receptor population was achieved by a single, in vivo injection of 75 IU hCG 3 and 5 days prior to incubation. When receptor binding was reduced by approximately 30%, neither the sensitivity (dose of hCG needed to give a half maximum response) nor the magnitude of the Leydig cell response to hCG in terms of androgen production was affected. However, a reduction in [125I]-hLH receptor binding by 80% was associated with a decrease in the in vitro sensitivity of Leydig cells to hCG, resulting in a displacement of dose-response curve to the right. Maximal quantity of androgen produced was not reduced. In the case of cAMP production, the same reduction in LH binding was associated with a decrease in both sensitivity and magnitude of the maximal response. The number of LH/hCG receptors has to be reduced more than 30% in order to have significant effects on the steroidogenic response. Immunofluorescence studies of LH/hCG receptor in a normal Leydig cell population revealed a striking intercellular variation in intensity of peripheral membrane staining, indicating great differences in number of LH receptors on individual cells. Three days after injection of hCG (75 IU), this variation in staining intensity was even more pronounced. The injection caused a generalized decrease in staining intensity and reduced the number of cells with distinct membrane fluorescence by 60% despite the fact that the number of Leydig cells identified by morphology and by the presence of 3β hydroxy steroid dehydrogenase was not changed. The reduction in [125I]-hLH binding 3 days after the injection of hCG was not the result of endogenous occupancy (due to the injected hCG), but a result of disappearance of LH/hCG receptor from Leydig cell surface. The effect of hCG on membrane staining of Leydig cells indicates a general loss of LH/hCG receptors rather than a selective loss from a subpopulation of cells.