Regulation of immunoreactive inhibin A and B secretion in cultured human granulosa-luteal cells by gonadotropins, activin A and insulin-like growth factor type-1 receptor

Inhibins are gonadal glycoproteins with endocrine effects on pituitary FSH secretion and para/autocrine effects on ovarian and testicular function. Th e purpose of this study was to investigate the endocrine and para/autocrine regulation of inhibin A and inhibin B secretion in human ovarian granulosa -luteal cells. The cells were obtained from women undergoing in vitro ferti lization, and the primary cultures were treated with FSH, LH, human chorion ic gonadotropin (hCG), activin A, 8-bromo cyclic AMP (8-BrcAMP), staurospor ine (a protein kinase C inhibitor) and an antagonist of IGF action (type-l IGF receptor antibody alpha IR3). The secretion of inhibins was measured by ELISA assays capable of reliably distinguishing between inhibin A and B. FSH, LH, hCG and 8-BrcAMP increased inhibin A secretion on average up to 18 0% (P<0.01), 192% (P<0.05), 210% (P<0.01) and 243% (P<0.01) respectively of the control level, while their stimulatory effect on inhibin B secretion w as less pronounced (up to 167%, P<0.01; 139%, P<0.05; 127%, P>0.05; 133%, P >0.05 of the controls respectively). alpha IR3 decreased inhibin A and B se cretion down to 70% (P<0.01) and 50% (P<0.01) respectively of the control. Staurosporine decreased inhibin B secretion down to 49% (P<0.01) of the con trol; its effect on inhibin A secretion was not significant. Activin A incr eased inhibin B secretion up to fourfold of the control (P<0.05) while its effect on inhibin A secretion was insignificant. We conclude that gonadotropins via the protein kinase A signal transduction pathway are the main positive regulators of inhibin A and B secretion in h uman granulosa-luteal cells. The protein kinase C signal transduction pathw ay seems to be important especially for inhibin B secretion. Locally produc ed IGFs are probably important inducers of the production of both forms of inhibin in human ovaries while activins seem to upregulate inhibin B secret ion.