EFFECTS OF PASSIVELY IMMUNIZING EWES AGAINST INHIBIN AND ESTRADIOL DURING THE FOLLICULAR PHASE OF THE ESTROUS-CYCLE

Passive immunization was used to investigate the importance of inhibin and oestradiol in the control of FSH production during the follicular phase of the oestrous cycle in the sheep. Four groups of five mature Scottish Blackface ewes were injected with normal sheep plasma (control), antiserum to the 1-26α peptide fragment of porcine inhibin, antiserum to oestradiol-17β, or a combination of the two antisera, 24 h following cloprostenol-induced luteal regression. There was no difference in the concentration of LH in jugular venous plasma between the control and inhibin-immunized groups following the injection of normal sheep plasma or inhibin anti-serum, with both groups exhibiting normal LH surges. In both the groups immunized against oestradiol, the basal concentration of LH rose by 25-30% (P < 0.05) during the 96-h period following injection, while the LH surge and consequent formation of a corpus luteum was inhibited. In all three immunized groups there was a significant (P < 0.001) rise in the concentration of FSH starting 3.8-4.8 h after the injection of antiserum. The duration of the rise was similar in the groups injected with oestradiol anti-serum alone (43.6 ± 12.8 h) or in combination with inhibin antiserum (40.6 ± 11.7 h), but was significantly (P < 0.05) shorter in the group immunized against inhibin alone (17.0 ± 0.5 h). The rise in FSH was similar in the groups immunized against inhibin (142 ± 6%) or oestradiol (143 ± 4%) alone, and was significantly (P < 0.01) greater in the group injected with both antisera (195 ± 17%). These results provide evidence that both oestradiol and inhibin play a role in regulating the concentration of FSH during the follicular phase of the oestrous cycle, while reinforcing the hypothesis that inhibin is not involved in the regulation of LH production.