Effect of immunization against synthetic peptide sequences of bovine inhibin α-subunit on gonadotrophin concentrations in heifers

Taxa de ovulação e concentração plasmática de progesterona em fêmeas bovinas imunizadas com líquido folicular suíno

O presente estudo objetivou avaliar os efeitos da imunização ativa contra proteínas do líquido folicular suíno sobre a taxa de ovulação, duração do ciclo estral e concentração plasmática de progesterona em vacas e novilhas da raça Limousin. Realizaram-se duas imunizações com 15 dias de intervalo, aplicando-se um imunógeno composto do conteúdo protéico de 25ml de fluido folicular suíno adicionados de 1ml de gel de hidróxido de alumínio, via subcutânea. O número de ovulações foi mensurado por palpação retal e ultra-sonografia sete a nove dias após cada período de estro. Após a última imunização, os animais apresentaram maior incidência de ovulações duplas (41,7%). As médias de ovulações pré e pós-imunização foram 1,00+0,00 e 1,40+0,31 ovulações/ciclo, respectivamente (P<0,01;chi2). A imunização foi efetiva em aumentar o número de ovulações. Não foram observadas diferenças na duração do ciclo estral e na concentração de progesterona nos diferentes dias do ciclo. A manipulação das ações fisiológicas da inibina pode ser utilizada como alternativa para indução de ovulações múltiplas em bovinos.

Plasma inhibin A in heifers: relationship with follicle dynamics, gonadotropins, and steroids during the estrous cycle and after treatment with bovine follicular fluid

The relationship between follicle growth and plasma inhibin A, FSH, LH, estradiol (E), and progesterone was investigated during the normal bovine estrous cycle and after treatment with steroid-free bovine follicular fluid (bFF) to arrest follicle development. In the first study, four heifers were monitored over three prostaglandin (PG)-synchronized cycles. Blood was collected every 2-8 h, and ovaries were examined daily by ultrasonography. Inhibin A was measured using a modified enzyme-linked immunosorbent assay that employed a new monoclonal antibody against the alpha subunit of bovine inhibin. Plasma inhibin A ( approximately 50 pg/ml before luteolysis) rose steadily during the induced follicular phase (P < 0.05) to a peak ( approximately 125 pg/ml) coincident with the preovulatory E/LH/FSH surge. After ovulation, inhibin A fell sharply (P < 0.05) to a nadir ( approximately 55 pg/ml) coincident with the secondary FSH rise. During the next 3 days, inhibin A increased to approximately 90 pg/ml in association with growth of the new dominant follicle (DF). Plasma E also rose twofold during this period, whereas FSH fell by approximately 50%. Inhibin A was negatively correlated with FSH (r = -0.37, P < 0.001) and positively correlated with E (r = 0.49, P < 0.0001). Observations on eight cycles (two cycles/heifer), in which growth of the ovulatory DF was monitored from emergence to ovulation, showed that the first-wave DF (DF1) ovulated in three cycles and the second-wave DF (DF2) in five cycles. After PG, plasma inhibin A and E increased similarly in both groups, with concomitant falls in FSH. In the former group, the restricted ability of DF1 to secrete both inhibin A and E was restored after luteolysis. Results indicate that dynamic changes in the secretion of both E and inhibin A from the DF contribute to the fall in FSH during the follicular phase and to the generation and termination of the secondary FSH surge, both of which play a key role in follicle selection. In the second study, bFF (two dose levels) was administered to heifers (n = 3-4) for 60 h starting from the time of DF1 emergence. Both doses suppressed FSH (P < 0.05) and blocked DF1 growth to the same extent (P < 0.01), although inhibin A levels were only marginally raised by the lower dose (not significant compared to controls). The high bFF dose raised (P < 0.001) inhibin A to supraphysiological levels ( approximately 1 ng/ml). A large "rebound" rise in FSH occurred within 1 day of stopping both treatments, even though the inhibin A level in the high-dose bFF group was still approximately threefold higher than that in controls. This indicates that desensitization of gonadotropes to inhibin negative feedback is a contributory factor, together with reduced ovarian output of E, in generation of the post-bFF rebound in FSH.

Effect of peptide to carrier ratio on the immune and ovarian response to inhibin immunization in cattle

We report on the effects of the peptide to carrier ratio on the immune and biological response to inhibin immunization in cattle. A peptide sequence from the alpha C-subunit of bovine inhibin was synthesized and conjugated to human serum albumin (HSA) at ratios of 4.3 moles (L) and 13.1 moles (M) of peptide per mole of HSA. Hereford-cross heifers (n = 6 per group) were injected with 3 mg of one of the peptide conjugates at primary, followed by a booster injection (1.5 mg) 11 weeks later. Control heifers (n = 6) were injected with HSA only. Blood samples were taken at regular intervals to measure antibody titre. Ovulation rate was measured by ultrasonography. Antibodies were generated in both peptide immunized groups. Control heifers and group L heifers had 1 ovulation at all ovulatory cycles monitored. Ovulation rate was increased (P < 0.05) in group M immunized heifers, with four of six heifers having twin ovulations in the first cycle following boost. These data support those of previous studies which indicated that immunization against the alpha C-subunit of bovine inhibin significantly disrupted the mechanism(s) controlling ovulation rate in cattle. It also indicates that both the immune and associated biological response is dependent on the nature of the conjugate used for immunization, specifically the ratio of peptide to carrier.