Progesterone antagonizes the ability of porcine ovarian inhibin to sensitize ovine pituitary cell culture to luteinizing hormone-releasing hormone: Dependence on ovaries in vivo

Progesterone (P₄) and a porcine follicular preparation of inhibin (MGRA-IV) have opposite actions on regulation of the ability of LHRH to release LH in ovine pituitary cell culture. Both P₄ and inhibin change the response to LHRH. The ability of inhibin to sensitize cultures to LHRH (126–273%) was greatly inhibited (up to 100%) in the presence of P₄ (10⁻⁷ m). The inhibitory action of P4 on LHRH-stimulated and inhibin-sensitized LHRH-stimulated LH secretion in ovine pituitary cell culture was dependent on the presence of ovaries in vivo. P₄ inhibited 68% of LHRH-stimulated LH secretion in pituitary cultures from intact ewes. However, when cultures were prepared from pituitaries collected on days 9, 21, and 42 after ovariectomy, P₄ inhibited LHRH-stimulated LH secretion by only 36%, 13%, and 0%, respectively. Ovariectomy had no effect (P > 0.05) on the sensitizing action of inhibin on LHRH-stimulated LH secretion, but ovariectomy did cause a time-dependent decline in the inhibitory action of P₄ on inhibin-sensitized LHRH-stimulated LH secretion. Furthermore, when cultures were prepared from pituitaries collected from ewes ovariectomized for 35 days but treated with estradiol implants, both LHRH-stimulated and inhibin-sensitized LHRH-stimulated LH secretion were inhibited as well by P₄ as in pituitary cultures from intact ewes. These results suggest that although P₄ can completely inhibit the sensitizing action of inhibin on LHRH-stimulated LH secretion, its inhibitory action is dependent on the presence of ovaries or estradiol in vivo.