Plasma concentrations of FSH, LH and progesterone in sheep immunized against an androstenedione-protein conjugate

A significant increase in the plasma concentrations of FSH (P less than 0.05) and LH (P less than 0.001) was observed during the luteal (Days 9-11) phase but not during the subsequent cloprostenol-induced follicular phase in androstenedione-immunized ewes compared to those in control ewes. Over the same time period the geometric mean (and 95% confidence limits) androstenedione antibody titres in the immunized ewes was 1/305 (1/158, 1/590) whereas they were not detectable in the controls. In the subsequent cycle, the ovulation rates were 1.6 +/- 0.2 for the immunized ewes and 1.1 +/- 0.1 for the control ewes (P less than 0.05) and the luteal progesterone concentrations were significantly higher in the immunized ewes compared to the controls (P less than 0.01). Collectively, these results suggest that active immunization against androstenedione leads to an increase in the plasma concentrations of both FSH and LH. The results are consistent with the hypothesis that FSH plays a central role in determining the ovulation rate in sheep.