EFFECTS OF SUPPLEMENTARY TREATMENT WITH BOVINE GROWTH-HORMONE ON HORMONAL AND OVULATORY RESPONSES TO INHIBIN IMMUNIZATION IN EWES

The aim of this study was to determine whether supplementary treatment with recombinant bovine growth hormone(rbGH) can enhance the ovulator y response of ewes to inhibin immunization. Crossbred ewes (n = 20) we re actively immunized against bovine inhibin alpha 1-29 peptide conjug ate while 20 ewes served as controls. Oestrus was synchronized using p rogestagen sponges and ewes were allocated to four groups: control ewe s (n = 10); control ewes given rbGH (n = 10); inhibin-immunized ewes ( n = 10) and inhibin-immunized ewes given rbGH (n = 10). A single s.c. dose of rbGH (50 mg) was given 7 days before sponge removal. Blood was collected for measurement of inhibin antibody titre, and concentratio ns of insulin-like growth factor I (IGF-I), FSH, oestradiol and proges terone. Ovulation, pregnancy and lambing rates were also recorded. All inhibin-immunized ewes produced antibodies that bound I-125-labelled (32 kDa) inhibin. The concentration of FSH in the plasma of the ewes a fter the second booster inhibin immunization was higher than that in c ontrol ewes (P < 0.005). Treatment with rbGH promoted a 2-3-fold incre ase in plasma concentration of IGF-I (P < 0.001); the response was les s (P < 0.01) in immunized compared with control ewes. Treatment with r bGH alone had no significant effect on the concentration of FSH or oes tradiol or on ovulation rate or litter size. Overall, inhibin-immunize d ewes had higher mean FSH concentrations (P< 0.002), higher preovulat ory oestradiol surges (P<0.05) and higher progesterone concentrations in the luteal phase (P<0.0001). Treatment with rbGH reduced the effect s of immunization on FSH (P<0.01) and progesterone (P < 0.02) concentr ations. Immunized ewes showed a threefold increase in ovulation rate ( P < 0.001) and a 1.8-fold increase in litter size (P < 0.05) compared with control ewes. In immunized ewes given rbGH, ovulation rate was in creased by a factor of 2.2 and litter size by a factor of 1.8. In conc lusion, these data do not support the hypothesis that supplementary tr eatment of ewes with rbGH to raise plasma IGF-I concentrations (and pr esumably intraovarian IGF-I) can enhance the ovulatory response to inh ibin immunization.