Immunoneutralization of inhibin and estradiol during the follicular phase of the estrous cycle in cows

To investigate the physiological importance of inhibin and estradiol in the regulation of FSH secretion during the follicular phase of the estrous cycle in cows, animals were passively immunized against the two hormones. Sixteen cows were divided into four equal groups and given injections of prostaglandin F2 alpha (PG) i.m. twice at 8-h intervals on Day 10 of the estrous cycle (Day 0 = day of estrus) to induce luteal regression. At 48 h after the first PG injection, each group of four cows received injections of one of the following: 100 ml castrated goat serum (control serum), 100 ml antiserum against inhibin, 100 ml antiserum against estradiol, or a combination of 100 ml inhibin antiserum and 100 ml estradiol antiserum. The LH surge occurred within 2 days after injection of the control serum or inhibin antiserum, whereas it was not detected in either of the groups passively immunized against estradiol, indicating that passive immunization against estradiol blocks a positive feedback effect of estradiol on LH secretion. There was no clear difference in basal concentrations of LH among the four groups. Injection of the inhibin antiserum resulted in a marked increase (p < 0.01) in plasma concentrations of FSH compared with values in the control group, while there were no significant changes in concentrations of plasma FSH after injection of the estradiol antiserum. Combined administration of the inhibin and estradiol antisera also produced a marked increase (p < 0.01) in plasma concentrations of FSH. The FSH response to the combined immunization was longer in duration than the FSH response to immunization with inhibin alone. Treatment with the inhibin antiserum, either alone or in combination with the estradiol antiserum, induced the growth of a large number of small (greater than or equal to 4 < 7 mm in diameter), medium (greater than or equal to 7 < 10 mm), and large (greater than or equal to 10 mm) follicles compared with numbers in the controls. No increase in the number of follicles was noted in the estradiol-immunized animals. The present results provide strong evidence that inhibin is an important factor in the inhibitory regulation of FSH secretion during the follicular phase in cows, suggesting also that estradiol has a synergistic effect with inhibin on FSH secretion.