| Summary: | Previous studies in this and other laboratories have, to date, failed to result in the development of a commercially useful technique for the reliable induction of twinning in cattle. The use of methods known to be efficacious in sheep have not resulted in repeatable effects in the cow. It would appear that the cow differs from the sheep in some important aspect of the control of the reproductive processes, particularly those governing follicular growth and dominance. The aim of these studies was to clarify the effects of follicular fluid proteins on follicular growth and gonadotrophin secretion. A previous study in this laboratory (Price, 1987) demonstrated that LH and FSH concentrations were grossly elevated following immunisation against a porcine follicular fluid preparation. We sought to confirm this observation and extend our understanding of the underlying processes involved. Heifers immunised against an ovine follicular fluid preparation displayed abnormally elevated peripheral FSH and LH concentrations. However, no difference in the response of heifers to an exogenous bolus of GnRH during the luteal phase of the oestrous cycle was observed. Analysis of the patterns of gonadotrophin secretion during the luteal and follicular phases of the cycle suggested that the immunised animals were unresponsive to endogenous oestradiol concentrations. Following ovariectomy, differences between immunised and control heifers were abolished, but the differences between treatment groups were restored following the insertion of a s.c. oestradiol implant, confirming our hypothesis. It would appear that the normal functioning of the oestradiol-mediated negative feedback control of gonadotrophin is dependent on the action of a follicular fluid protein. Having demonstrated the existence of such a follicular protein, it was of interest to examine the effects of the direct administration of follicular fluid proteins on gonadotrophin secretion and ovulation rate. Treatment failed to reduce peripheral FSH concentrations, although a significant dose-dependenthypersecretion of FSH was observed following the cessation of treatment. In addition, treatment with bovine or ovine follicular fluid proteins led to a significant increase in LH pulse amplitude. These effects on gonadotrophin secretion again occurred in the absence of any change in oestradiol concentrations, suggesting that the effects of treatment were most probably mediated via direct pituitary or hypothalamic effects. Collectively, we have demonstrated that follicular fluid proteins are important components in the control of the reproductive system of the cow. Such proteins are involved in the oestradiol-mediated negative feedback regulation of FSH and LH, the determination of LH pulse amplitude and the regulation of follicular growth. This latter role may represent the mechanism by which follicular dominance is effected and may present a useful target for future research attempting to develop techniques for the induction of twinning in cattle.
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