Steroid control of gonadotropin secretion and ovarian function in heifers

The failure of anti-steroid treatments to induce multiple ovulations in cattle in a repeatable way, prompted us to examine the role of the gonadal steroids in the control of gonadotropin secretion in this species. A better understanding of the control of gonadotropin secretion in the cow would assist in the development of treatments to control prolificacy. Groups of six heifers were implanted with one of three sizes of estradiol (E2) implant on day 9 of a synchronized estrous cycle, and five control heifers received empty implants. All heifers were ovariectomized during the luteal phase of the subsequent estrous cycle and given progesterone-releasing intravaginal devices (PRID). Blood samples were taken every 10 min for 12 h with PRIDs and for 6 h after PRID withdrawal, for the measurement of LH and FSH concentrations. The two larger implant sizes (increasing E2 concentrations to 7.4 and 18.7 pg/ml plasma during the luteal phase of the cycle) decreased ovulation rate, the number of large follicles, and luteal weight. After ovariectomy, the three implant sizes produced E2 concentrations comparable with those during the luteal and follicular phases of the estrous cycle and at estrus (1.5, 4.4, and 10.5 pg/ml, respectively). E2 alone decreased mean LH and FSH concentrations and LH pulse amplitude, whereas progesterone alone reduced mean gonadotropin concentrations and LH pulse frequency. Only in the presence of progesterone did E2 decrease LH pulse frequency. Steroid concentrations which mimicked those of the luteal and follicular phases of the cycle produced luteal- and follicular-phase patterns of LH and FSH secretion. These results confirm that E2 and progesterone are important regulators of gonadotropin secretion in cattle, and question the role of inhibin in this respect.