Date of this Version
Mature Suffolk ewes were either actively or passively immunized against the synthetic fragment of porcine inhibin alpha, pIα(1-30),to determine the effects on gonadotrophin secretion and ovulation rate. Thirteen control ewes were immunized against human serum albumin, 12 ewes were actively immunized against pIα(1-30)and 36 ewes were passively immunized with pIα(1-30)antiserum. Blood samples were collected at 4-h intervals for 72 h from oestrus-synchronized ewes following the withdrawal of the progestagen pessaries. Mean gonadotrophin concentrations measured during the oestrous cycle of control ewes, ewes actively immunized against pIα(1-30) and ewes passively immunized against pIα(1-30)were similar, but their secretory profiles differed. Serum concentrations of follicle-stimulating hormone (FSH) were highest in ewes which had received antiserum at the time of pessary withdrawal; FSH concentrations did not decrease during the follicular phase of the oestrous cycle in ewes given antiserum 24 h after pessary withdrawal. Subtle but significant increments in serum FSH concentrations were observed in all passively immunized ewes in which sampling commenced at the time of treatment. The amplitude of the preovulatory luteinizing hormone (LH) peak, but not of the FSH peak, and the postovulatory secondary rise in FSH were lower (P < 0·05)in actively immunized ewes than in control ewes. The mean ( ±s.e.) ovulation rate for actively immunized ewes (6·6±1·0) was 3 times higher (P < 0·05)than that for control ewes (2·0±0·2),but was unaffected by passive immunization (range, 1·8-2·3).The divergent results involving serum gonadotrophins and ovulation rate following active or passive immunization to the same pIα(1-30)immunogen suggest that changes in circulating gonadotrophins alone may not account for the increased ovulation rate in immunized Suffolk ewes.