Effect of small doses of bovine follicular fluid on the tonic secretion of gonadotrophins in the ewe

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Abstract
Previous work has shown that treatment of ewes with steroid-free bovine follicular fluid (bFF), a rich source of inhibin, partially inhibits the increase in mean plasma concentrations of LH induced by ovariectomy. The present experiment was designed to test the hypothesis that this effect was a reflection of reduced LH pulse amplitude which would only be expressed at high (pharmacological) doses of bFF. To do this, we assessed the dose–response to bFF of the secretion of FSH and LH pulses in intact and acutely ovariectomized ewes. In intact ewes, a low dose of bFF (0·2 ml s.c. every 8 h) had no detectable effect on the secretion of FSH, an intermediate dose (0·6 ml s.c. every 8 h) depressed FSH concentrations for about 24 h and a high dose (1·8 ml s.c. every 8 h) reduced FSH concentrations to undetectable levels. In ewes treated with 1·8 ml bFF, FSH concentrations also remained undetectable after ovariectomy and did not increase until treatment was withdrawn. In ewes treated with 0·6 ml bFF, FSH concentrations were maintained at normal intact levels for about 32 h following ovariectomy but then rose to normal ovariectomized levels. In ewes treated with 0·2 ml bFF, FSH concentrations increased immediately after ovariectomy but more slowly than in control ovariectomized ewes. Profiles of LH pulses were recorded after ovariectomy, during and after the withdrawal of bFF treatment. In ewes treated with the highest dose (1·8 ml s.c. every 8 h), mean LH levels and pulse amplitude were lower than in control ewes and increased significantly following withdrawal of treatment. None of the other pulse variables measured (apparent half-life, pulse interval, nadir) were significantly affected. The lower doses did not significantly affect LH secretion. It was concluded that FSH secretion can be inhibited for short periods by low doses of inhibin and that a dose of 0·6 ml bFF every 8 h is approximately equivalent to normal ovarian output. However, another factor, possibly oestrogen, is also involved in the long-term regulation of plasma FSH concentrations. The inhibitory effect of bFF on LH pulse amplitude was only observed at the highest dose, suggesting that it is a pharmacological effect and that it is unlikely that inhibin plays a major role in the control of tonic LH secretion. J. Endocr. (1987) 114, 73–79

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