The ability of steroid-free bovine follicular fluid to suppress FSH secretion and delay ovulation persists in heifers actively immunized against inhibin

Abstract

ABSTRACT Previous reports indicate that administration of steroid-free bovine follicular fluid (bFF) to intact heifers suppresses plasma FSH levels and delays the timing of ovulation. In addition, cessation of bFF treatment is associated with a rebound hypersecretion of FSH. To test the hypothesis that these effects of bFF are mediated by inhibin, we have compared the responses to bFF treatment in heifers actively immunized against the N-terminal sequence of inhibin α subunit bIα(1–29)Tyr30-ovalbumin) with those in ovalbumin-immunized controls. Oestrous cycles were synchronized, and inhibin-immune (n = 10) and control (n = 10) heifers were subdivided into two groups which received either 5 ml bFF (n = 5) or 5 ml bovine serum (n = 5) every 4 h for a 60 h period starting 8 h before prostaglandin (PG)-induced luteolysis. Blood was withdrawn every 4 h for 10 days starting 30 h before luteolysis and ovaries were examined daily by ultrasonography. Overall, mean ovulation rate in bIα(1–29)Tyr30-immunized heifers was 44% higher (P < 0·02) than in controls. Inhibin antibody titres tested in bIα(1–29)Tyr30-immunized heifers before (19 ± 3%), during (19 ± 3%) and after (20 ± 3%) bFF treatment did not differ. In the pretreatment period (i.e. mid-luteal phase), plasma FSH levels were 32% (P < 0·05) higher in inhibin-immunized than in control heifers. During administration of bFF to control heifers, plasma FSH was suppressed to a level 40% lower than in serum-treated heifers (P < 0·02). Unexpectedly, bFF suppressed FSH to a similar extent in inhibin-immunized heifers (37% lower than in the serum-treated group; P < 0·025). Similarly, a post-bFF rebound hypersecretion of FSH was observed in both control (P < 0·05) and inhibin-immunized (P < 0·05) heifers, although the FSH rebound lasted about 24 h longer in the latter group (P < 0·001). The timing of the preovulatory LH surge in control (86 ± 7 h post-PG) and immunized (81 ± 6 h post-PG) groups treated with serum was similar as was the timing of the preovulatory rise in plasma oestradiol and the subsequent rise in plasma progesterone. However, bFF treatment delayed (P < 0·001) the preovulatory surges of LH and oestradiol and the subsequent rise in plasma progesterone to a similar extent (> 4 days) in both control and inhibin-immunized groups. Ultrasonography confirmed that bFF delayed the emergence of the wave of dominant ovulatory follicles by 5 days and also showed that inhibin immunization and bFF treatment were both effective in promoting the development of more follicles during the preovulatory period. These results lead us to conclude that a non-steroidal factor(s) in bFF other than inhibin is responsible for the observed delay in ovulation. Like inhibin, this factor acts to suppress pituitary FSH secretion and this could fully account for its ability to delay the onset of preovulatory follicular development and thus delay ovulation. However, the further possibility of a direct inhibitory action of certain bFF components on the ovary cannot be ruled out at this stage. Journal of Endocrinology (1993) 136, 137–148