Effects of supplementary treatment with bovine growth hormone on hormonal and ovulatory responses to inhibin immunization in ewes
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The aim of this study was to determine whether supplementary treatment with recombinant bovine growth hormone(rbGH) can enhance the ovulatory response of ewes to inhibin immunization. Crossbred ewes (n = 20) were actively immunized against bovine inhibin a1–29 peptide conjugate while 20 ewes served as controls. Oestrus was synchronized using progestagen sponges and ewes were allocated to four groups: control ewes (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 concentrations of insulin-like growth factor I (IGF-I), FSH, oestradiol and progesterone. Ovulation, pregnancy and lambing rates were also recorded. All inhibin-immunized ewes produced antibodies that bound125I-labelled (32 kDa) inhibin. The concentration of FSH in the plasma of the ewes after the second booster inhibin immunization was higher than that in control ewes (P < 0.005). Treatment with rbGH promoted a 2–3-fold increase in plasma concentration of IGF-I (P < 0.001); the response was less (P < 0.01) in immunized compared with control ewes. Treatment with rbGH alone had no significant effect on the concentration of FSH or oestradiol or on ovulation rate or litter size. Overall, inhibin-immunized ewes had higher mean FSH concentrations (P < 0.002), higher preovulatory oestradiol surges (P <0.05) and higher progesterone concentrations in the luteal phase (P < 0.0001). Treatment with rbGH reduced the effects of immunization on FSH (P < 0.01) and progesterone (P < 0.02) concentrations. 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 increased by a factor of 2.2 and litter size by a factor of 1.8. In conclusion, these data do not support the hypothesis that supplementary treatment of ewes with rbGH to raise plasma IGF-I concentrations (and presumably intraovarian IGF-I) can enhance the ovulatory response to inhibin immunization.
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