FOLLICULAR DEVELOPMENT AND OVARIAN HORMONE-SECRETION FOLLOWING PASSIVE-IMMUNIZATION OF EWES AGAINST INHIBIN OR ESTRADIOL

Immunization against inhibin or oestradiol causes an increase in the peripheral plasma concentration of FSH. In this study we have investigated the effect of this post-immunization rise in FSH concentration on follicular development by means of real-time ultrasonography, in ewes in which an ovary and its vascular pedicle had been autotransplanted to a site in the neck. Groups of ewes on day 10 of the luteal phase of the oestrous cycle were injected with a single 10 ml i.v. bolus of plasma from normal ewes (control; n = 4), antiserum to the 1-26 peptide fragment of the N-terminus of the alpha chain of porcine inhibin (n = 5) or antiserum to oestradiol-17beta (n = 4). The plasma concentration of FSH was unaffected by treatment in the control group but showed a significant (P < 0.001) rise following treatment in both immunized groups (inhibin-immunized 175% over 19 h; oestradiol-immunized 138% over 22 h as a per cent of the original value). This rise in FSH concentration was accompanied by a significant (P < 0.001) rise in the total number of follicles >2.0 mm per ovary in both immunized groups (inhibin-immunized 5.5 +/- 1.0 to 13.6 +/- 1.4; oestradiol-immunized 4.6 +/- 0.5 to 11.5 +/- 1.0). In the oestradiol-immunized group, this rise was due to an increase (P < 0.001) in the number of small follicles (<3.5 mm) alone while, in the inhibin-immunized group, there was a rise in both the number of small follicles (P < 0.001) and the number of medium-sized follicles of 3.5-4.5 mm (1.3 +/- 0.5 to 3.2 +/- 1.0, P < 0.05). During the period from 12 to 20 h after immunization, when FSH concentrations were falling following the post-immunization rise, there was a significant (P < 0.05) rise in the ovarian secretion rate of inhibin in the oestradiol-immunized group, and in the inhibin-immunized group the mean secretion rate of oestradiol was double that in the control group (2.7 +/- 0.5 vs 1.3 +/- 0.3 pmol/min, P < 0.06). The temporal relationships suggest that the rise in FSH induced by passive immunization against oestradiol or inhibin stimulates the development of many antral follicles which contribute to the increased ovarian secretion of inhibin and oestradiol.