Two experiments were conducted to determine effects of active and passive immunoneutralization of inhibin on FSH secretion and ovulation rate. A synthetic peptide (alpha-IF) matching the N-terminus of the alpha-subunit of ovine inhibin was coupled to human alpha-globulin (h alpha-G) and used as an immunogen. In experiment 1, estrus was synchronized in 10 sheep that had been actively immunized against alpha-IF-h alpha-G or h alpha-G. Plasma FSH levels were similar in the two groups of ewes at -52 and -48 h (0 h = onset of estrus). In alpha-IF-h alpha-G-immunized ewes, FSH increased from -48 to -44 h (18.8-22.1 ng/ml), and then fell to 16.2 ng/ml by 0 h. In h alpha-G-immunized ewes, FSH decreased from -48 to 0 h (17.6-7.2 ng/ml). Ovulation rate was higher in alpha-IF-h alpha-G- than h alpha-G-immunized ewes (9.4 vs. 2.4). In experiment 2, antibodies (Ab) were extracted from sera obtained from experiment 1 ewes and then were injected i.v. into 12 other ewes. Estrus was synchronized twice during the breeding season using progesterone-releasing pessaries (CIDR-G). One day before CIDR-G withdrawal, alpha-IF-h alpha-G and h alpha-G Ab were administered in a crossover design. After injection of Ab against alpha-IF-h alpha-G, plasma FSH increased from 0 to 24 h post-injection (10.9-21.5 ng/ml), after which levels fell to 14.2 ng/ml by onset of the preovulatory LH surge.(ABSTRACT TRUNCATED AT 250 WORDS)
A synthetic fragment representing the N-terminal 25 amino acid residues of the alpha-subunit of ovine inhibin (alpha-IF) was coupled to human alpha-globulin (h alpha-G) and used as an antigen. In Exp. 1, ovine antiserum generated against alpha-IF-h alpha-G was shown in vitro to neutralize inhibin bioactivity contained in ovine follicular fluid. In Exp. 2, 18 lambs were immunized with .3, .6 and 1.2 mg alpha-IF-h alpha-G or equivalent doses of h alpha-G. Antibody titer to alpha-IF was detected only in serum from lambs immunized against alpha-IF-h alpha-G and was first detected 27 +/- 2 d after primary immunization. Thereafter, antibody titers increased steadily. The degree of antibody responses was unrelated to antigen dose and differed among lambs. Plasma FSH concentrations were unchanged, whereas LH concentrations were lower (P less than .001) in sheep immunized against alpha-IF-h alpha-G. Ovulation rate was increased (3.5 +/- .5 vs 1.5 +/- .1; P less than .01) in lambs immunized against alpha-IF-h alpha-G. Ovulation rate was similar among animals receiving different antigen doses and increased with time after primary immunization (P less than .01). At estrous periods occurring approximately 34, 50, 74 and 107 d after primary immunization, respective ovulation rates were 157, 169, 207 and 450% of control values. Ovulation rate and antibody titer were correlated positively (pooled r = .95; P less than .01) within lambs. In Exp. 3, three lambs were immunized with .25 mg unconjugated alpha-IF; this was nonantigenic. In conclusion, the use of a synthetic fragment of the alpha-subunit of ovine inhibin as a hapten elicits an antibody capable of neutralizing inhibin bioactivity in vitro and increasing ovulation rate in vivo.
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