Effects of Luteinizing Hormone Receptor-Binding Inhibitor on the in Vitro Steroidogenesis by Rat Ovary and Testis

An inhibitor for LH receptor site binding (LHRBI) was tested for its effect on in vitro LH-stimulated progesterone biosynthesis by an ovarian slice preparation from pseudopregnant rats and for its effect on testosterone biosynthesis by a testicular mince preparation. LH-RBI was extracted from pseudopregnant rat ovaries stored at -20 C for 10 weeks or longer after excision. There was a >100% increase of progesterone production in the 2-h incubated control compared to the unincubated control. LH (1.7 μg/ml) further stimulated progesterone production (>100% over the incubated control). LH-RBI preparation in quantities of 10–100 mg tissue equivalent significantly inhibited the LH- and hCG-stimulated progesterone biosynthesis but did not affect basal progesterone biosynthesis. In contrast to the effect on ovarian progesterone biosynthesis, a similar preparation of LH-RBI, alone or in conjunction with LH, significantly stimulated testosterone biosynthesis in the testicular mince preparation. Extracts prepared from freshly excised ovaries of pseudopregnant rats failed to show the inhibition of LH binding to ovarian receptor or LH-stimulated progesterone biosynthesis in ovarian slices; however, these extracts stimulated testosterone biosynthesis in testicular minces almost as well as the ovarian extracts from stored ovaries. We conclude that 1) LH-RBI acts as an antagonist to LH in ovarian tissue in vitro, which not only inhibits the binding of LH to ovarian LH receptors but also blocks the LH-stimulated progesterone biosynthesis; and 2) an unidentified substance in the ovarian extract stimulates testosterone biosynthesis in testicular tissue in vitro but has different activity properties than LH-RBI or has different extractability as a function of frozen storage.