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Ovulation rate, follicle growth, serum FSH and oestradiol concentrations were studied after a single intraperitoneal injection of inhibin antiserum in 5-day-cyclic rats. Control rats received (non-immune) serum from castrated sheep or saline. Rats were injected at 10.00 h on dioestrus-1 (D1), i.e. the day following the day of oestrus, or at 17.00 h on dioestrus-2 (D2). The ovaries were excised at necropsy 48 h after injection, or at first or second oestrus after injection. After routine histology fresh corpora lutea were counted and/or differential follicle counts were made.

Results from rats injected with either (non-immune) serum from castrated sheep or with saline were not different and were therefore combined to form the control group. The activity of inhibin-neutralizing antibodies in the circulation of antiserum-treated rats was reduced by approximately 39% between 8 h and second oestrus after injection, as determined by the binding of purified bioactive radioiodinated 31 kDa bovine inhibin.

Rats were injected on D1 and killed at first oestrus. The number of fresh corpora lutea was significantly higher in antiserum-treated rats than in controls (13·9±0·4 vs 11·8±0·4; P < 0·05). Other rats injected on D1 were killed either 48 h or at the second oestrus after injection. Blood was collected 8, 16, 24 and 48 h and at first and second oestrus after injection. At 48 h after injection differential follicle counts showed that the ovaries of antiserum-treated rats contained approximately 32 more healthy follicles and 11 fewer atretic follicles than controls (both P < 0·05 vs control; data for follicles with volume > 100 x 10⁵µm³ and diameter > 260 µm). The ovaries of the antiserum-treated group collected at second oestrus contained more corpora lutea than controls (17·5±0·5 vs 13·6±0·4; P < 0·001). Serum FSH levels at 8, 16, 24 and 48 h after antiserum injection were elevated (P < 0·05). Overall oestradiol levels in antiserum-treated rats were increased from 8 to 24 h and at first oestrus (P < 0·05) as compared with control rats. Further rats were injected on D2 and necropsied at first or second oestrus which caused ovulation rate to almost double at first oestrus (antiserum 23·7±1·4 vs control 12·4±0·4; P < 0·01), while at second oestrus there was no difference between antiserum-treated and control rats.

The rise in FSH level after injection of antiserum on D1 caused follicle recruitment in addition to that normally occurring on the morning of oestrus (36 h earlier) and reduced atresia, resulting in a moderately increased ovulation rate on the first and second oestrus after injection. If the interval between antiserum injection and the next oestrus was shortened (injection on D2), ovulation rate was doubled, while on the next oestrus (second) there was no difference compared with controls. It is concluded that inhibition is progressively involved in the control of follicle growth and ovulation rate via its effect on serum FSH levels during the oestrus cycle of the rat.

Journal of Endocrinology (1991) 130, 297–303

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