Abstract
Cyclic Merino ewes were immunized three times against 5α−androstane-3α,17β-diol (3α-diol) at three week intervals (weeks 0,3 and 6). After the third immunization, estrous cycles were synchronized by a series of PG injections and intensive blood samples were obtained for gonadotropin analyses during the mid-luteal phase and during the early follicular phase. Ovulation rates were determined after each immunization and subsequent cloprostenol (PG)-induced luteolysis (weeks 3, 6, 8 and 11). After the second immunization, daily blood samples were obtained and concentrations of plasma progesterone (P) were measured. Immunization against 3α-diol significantly increased the ovulation rate after the second and third immunizations and at week 10 (P<0.01, P<0.01 and P<0.05 respectively). It also induced anovulation in several ewes. Plasma concentrations of P were significantly increased by 3α-diol immunization (P<0.01). Pulse frequency of LH and concentrations of FSH were also increased during the luteal phase (P<0.01 and P<0.05 respectively). There was a significant correlation between the mean concentrations of FSH during the luteal phase and the subsequent ovulation rates (P<0.01). These results indicate that immunization against 3α-diol increases ovulation rate in ewes and its effect was, at least partially, exerted by increased plasma concentrations of FSH.
