LEVEL OF NUTRITION MODULATES THE DYNAMICS OF ESTRADIOL FEEDBACK ON PLASMA FSH IN OVARIECTOMIZED EWES

The frequency of multiple ovulations in mature, cyclic ewes is strongl y influenced by the level of nutrition, but it is difficult to demonst rate concurrent changes in plasma concentrations of gonadotropins. The failure to do so might be a consequence of rapid compensation by the homeostatic feedback mechanism linking secretion by the hypothalamus/p ituitary gland and ovarian hormones. Most experimental models have exa mined the components of the homeostatic feedback system after steady s tate relationships had been established, We hypothesised that the effe cts of nutrition might be observed more readily if the system were dis rupted and then examined while equilibrium was being re-established. T his hypothesis was tested in three experiments in Merino ewes by allow ing gonadotropin secretion to escape feedback for 5 days after ovariec tomy and then replacing ovarian hormones and examining effects of feed ing regimen on the return of plasma concentrations of FSH to baseline values. In all three experiments, oestrogen replacement caused plasma concentrations of FSH to decline more rapidly (P < 0.05) in ewes fed a t 0.5 x maintenance, than in ewes fed at 1.4 x maintenance, with group s fed at maintenance being intermediate. No effect of diet was observe d on plasma FSH concentrations in the absence of oestradiol, and neith er progesterone nor charcoal-treated bovine follicular fluid influence d the effect of nutrition. Plasma concentrations of oestradiol were 9. 8% lower on average (NS) in ewes fed above maintenance than in the she ep fed below maintenance over the three experiments, suggesting that t here may have been a reduced clearance of oestradiol which contributed to the result. We conclude that feeding regimen affects the secretion or clearance of gonadotropins in mature ewes, as in the mature ram, a nd that this is one mechanism by which ovulation rate may be affected. (C) 1997 Published by Elsevier Science B.V.