EQUINE CHORIONIC-GONADOTROPIN REGULATES LUTEAL STEROIDOGENESIS IN PREGNANT MARES

The onset of eCG secretion in pregnant mares coincides with an increas e in luteal steroid production and a relative shift toward androgen an d estrogen synthesis. However, a cause-effect relationship between eCG and the shift in luteal steroidogenesis has not been demonstrated. In this study, we have investigated the effect of eCG on steroid product ion by the corpus luteum (CL) during equine pregnancy. All mares were supplemented with 44 mg altrenogest (a progestogen) per day on Days 18 -50. Increasing doses of eCG were administered on Days 26-28, before t he onset of endogenous eCG secretion, to four mares with and four mare s without a functional CL (prostaglandin F-2 alpha administered on Day 18). Four mares with a functional CL received no exogenous eCG. In eC G-treated mares without a functional CL, progestin, androstenedione, a nd estrogen concentrations did not significantly increase after exogen ous eCG administration or endogenous eCG secretion. In eCG-treated mar es with a functional CL, progestin and estrogen production increased s ignificantly after exogenous eCG administration and endogenous eCG sec retion, whereas androstenedione concentrations tended to increase foll owing exogenous eCG and increased significantly following endogenous e CG secretion. In mares with a functional CL that did not receive exoge nous eCG, progestin and estrogen concentrations increased and androste nedione concentrations tended to increase only after the onset of endo genous eCG secretion. These data demonstrate that the increase in lute al steroidogenesis that coincides with the onset of eCG secretion is i nduced by eCG and results in an increase in luteal androgen and estrog en synthesis. Our findings support the hypothesis that eCG has a luteo tropic action in pregnant mares.