Evidence that inhibin is an important factor in the regulation of FSH secretion during the mid-luteal phase in cows

Abstract

To investigate the physiological importance of inhibin in the regulation of FSH secretion in cows, seven cyclic cows were treated with an inhibin antiserum raised against bovine 32 kDa inhibin in a castrated goat. The same animals treated with a castrated goat serum (control serum) served as controls. On day 12 of the oestrous cycle (day 0 = day of oestrus), four of seven cows were injected with 100 ml inhibin antiserum first, and the remaining three cows with 100 ml control serum first. Twelve days after the second oestrus following the first serum injection (42–46 days after the first serum injection), the former four cows were injected with control serum and the latter three with inhibin antiserum. Follicular development after the injections of control serum or inhibin antiserum was assessed by daily ultrasonographic examination. Treatment with inhibin antiserum resulted in a marked increase (P < 0·01) in plasma concentrations of FSH and oestradiol-17β but not LH or progesterone, compared with those after treatment with control serum. Plasma concentrations of FSH increased significantly (P < 0·01) at 8 h after injection of antiinhibin serum when compared with the control value. Concentrations of FSH in the plasma remained high for 72 h, then declined to the control level by 84 h, concomitant with an abrupt decrease in the titre of free inhibin antibody in the plasma. High concentrations of oestradiol-17β were observed between 36 and 96 h after treatment. Treatment with inhibin antiserum markedly increased the number of small (≥ 4 < 7 mm in diameter), medium (≥ 7 < 10 mm) and large (≥ 10 mm) follicles by 48, 72 and 96 h after treatment when compared with the value before treatment. The number of large follicles returned to the pretreatment value at 168 h, whereas the number of small and medium follicles remained increased. The present results provide strong evidence that inhibin is an important factor in the inhibitory regulation of FSH secretion during the mid-luteal phase of cows, and demonstrate that an increase in endogenous FSH secretion after immunoneutralization of circulating inhibin stimulates the rapid growth of a large number of follicles. Journal of Endocrinology (1993) 136, 35–41