Antigonadotropic effects of the bovine ovarian gonadotropin-releasing hormone-binding inhibitor/histone H2A in rat luteal and granulosal cells

A gonadotropin-releasing hormone (GnRH)-binding inhibitor (GnRH-BI) was purified from bovine ovaries and identified as histone H2A. In the present studies, the biological effects of partially purified and purified ovarian GnRH-BI, as well as calf thymus histone H2A, were examined in rat ovarian cells. Since GnRH has direct antigonadotropic actions on these cells, the effects on luteinizing hormone-stimulated cAMP accumulation in luteal cells and follicle stimulating hormone-induced cAMP and progesterone production in granulosal cells were evaluated. Antigonadotropic activity in both luteal and granulosal cells coeluted directly with GnRH-BI activity during purification from bovine ovaries, and the antigonadotropic effects were dose dependent and reversible. In contrast to GnRH, GnRH-BI maximally inhibited gonadotropin responses and the effects of GnRH-BI were not blocked by a GnRH antagonist. The purified ovarian GnRH-BI and calf thymus histone H2A had identical antigonadotropic properties, and the half-maximal concentrations for inhibiting the gonadotropin responses of granulosal and luteal cells was 2 and 5 microM, respectively. In conclusion, the ovarian GnRH-binding inhibitor, identified as histone H2A, not only inhibits the high affinity binding of GnRH to rat ovarian membranes but also evokes GnRH-like antigonadotropic responses in rat ovarian cells that do not appear to be mediated by binding to GnRH receptors.