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  • Author: R. J. Medhamurthy x
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R. Medhamurthy, T. D. Carruthers and J. G. Manns

Summary. Experiments were conducted with ewes to investigate the effects of an enriched bovine follicular fluid inhibin preparation (INH) on gonadotrophin secretion after the onset of oestrus. Administration of INH (10 mg) 1 h after the onset of oestrus did not significantly alter the preovulatory FSH and LH surges or the second FSH peak. To determine the effects of INH on the second FSH surge, ewes were treated with saline (N = 7) or INH (N = 10) at 4 h (10 mg) and 24 h (5 mg) after the peak of the preovulatory LH surge. The second FSH surge was delayed about 24 h (P < 0·05) in ewes treated with INH; however, the delay did not alter the interval to the next oestrus.

In a third experiment, 16 ewes were assigned to 4 groups in a 2 × 2 factorial with the main effects being ovariectomy at 4 h and INH treatment (10 mg) at 4, 20 and 36 h after the peak of the LH surge. Controls received sham ovariectomy and saline injection as appropriate. Ovariectomy resulted in a rapid increase in serum FSH but not LH and this was delayed (P < 0·05) by INH treatment. These results indicate that inhibin has a selective inhibitory action on FSH secretion in ewes and suggests that the second FSH surge results from increased basal FSH secretion due to decreased endogenous inhibin levels.

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W. D. Currie, R. J. Medhamurthy, S. J. Cook and N. C. Rawlings

Diurnal and seasonal fluctuations were detected in luteinizing hormone (LH) interpulse interval, but not amplitude, in ewes examined during the mid-luteal phase of an oestrous cycle at five stages of the breeding season. Daytime and night-time LH interpulse intervals were greater in the early and late than in the mid-breeding season (P < 0.05). During the early and late breeding season, LH interpulse interval was less during daylight than during darkness (P < 0.05). Toward the mid-breeding season, interpulse interval decreased during daytime earlier in the season than the night-time decrease. It was concluded that the diurnal fluctuations observed are a component of a circannual rhythm in LH secretion resulting from gradual seasonal transitions in photoperiodic drive to, or an endogenous rhythm in, the hypothalamic–pituitary axis in ewes.