Effects of passive immunization of ewes against an inhibin-peptide on gonadotropin levels, ovulation rate, and prolificacy

The experimental objectives were to determine whether injection of semi-purified (sp; ammonium sulfate-precipitated) and highly purified (hp; immunoaffinity-purified) ovine antibody (Ab) against an inhibin-peptide fragment (alpha-IF) before the preovulatory period would 1) stimulate FSH secretion in a dose-response manner, 2) induce an increase in ovulation rate, and 3) affect pregnancy rate and prolificacy (lambs born alive per ewe lambing). During the early breeding season, estrus was synchronized in 30 2-yr old crossbred ewes through use of progesterone-releasing pessaries (CIDR-G). Two doses (330 and 660 laboratory reference preparation [RP-2] kU) of sp- and hp-alpha-IF-Ab were injected i.m. 48 h before CIDR-G removal (6 ewes per group). Six other ewes received control solution. Plasma alpha-IF-Ab titers peaked at 12 h postinjection. Plasma FSH levels were higher (p < 0.02) in alpha-IF-Ab-treated ewes than in control ewes from 12 to 24 h postimmunization. Magnitudes of FSH increases were similar in ewes administered sp- and hp-alpha-IF-Ab and were greater (p < 0.05) in ewes receiving 660 than in those receiving 330 RP-2 kU. Compared to control values, the higher alpha-IF-Ab dose increased FSH levels by 44 +/- 5% and the lower dose increased the levels by 22 +/- 3%. Plasma LH levels were similar among passively immunized and control sheep. Ovulation rate was increased (p < 0.0005) by alpha-IF-Ab treatment. Higher ovulation rates were induced by sp- than by hp-alpha-IF-Ab (3.1 +/- 0.1 vs. 2.7 +/- 0.2; p < 0.05) and by 660 than by 330 RP-2 kU (3.3 +/- 0.1 vs. 2.5 +/- 0.2; p < 0.01). Control ewes had an ovulation rate of 1.8 +/- 0.3. Positive associations existed between alpha-IF-Ab titers, FSH levels, and ovulation rates. Number of lambs born per ewe lambing tended (p = 0.07) to be higher for alpha-IF-Ab-treated than for control ewes, but number of lambs born alive was similar. In conclusion, passive immunization against alpha-IF prior to CIDR-G withdrawal stimulated FSH secretion and increased ovulation rate in a dose-related manner. Semi- and highly purified alpha-IF-Ab preparations were equally effective in stimulating FSH secretion, while sp-alpha-IF-Ab was more effective than hp-alpha-IF-Ab in increasing ovulation rate. Passive immunization against alpha-IF did not affect pregnancy rate nor prolificacy.