Episodic ovarian inhibin secretion is not due to LH pulses in anoestrous ewes

Abstract

In sheep, secretion of oestradiol by the ovary is stimulated by pulses of LH but the factors controlling ovarian inhibin secretion are not well understood. We have investigated the effect of a single injection of LH on the ovarian secretion of inhibin. Six anoestrous Finn-Merino ewes which had one ovary autotransplanted to a site in the neck had jugular and timed ovarian venous blood samples collected at 10-min intervals for a total of 5 h. The secretion rates of both inhibin (1-3 ng/min) and oestradiol (0.5-8 ng/min) were similar to those observed during the breeding season indicating significant follicular development in these animals. After injection of 2.5 micrograms NIH-LH-S25 intravenously the concentration of LH in plasma rose from a baseline of 1.8 +/- 0.1 (S.E.M.) microgram/l to a peak of 3.9 +/- 0.3 micrograms/l (P less than 0.01). This LH pulse stimulated a corresponding increase (P less than 0.01) in oestradiol secretion from a basal level of 0.9 +/- 0.2 ng/min to a peak of 4.6 +/- 0.6 ng/min that occurred within 30 min of injection. Although inhibin secretion was episodic in nature, increases were not related to either exogenous or endogenous LH pulses. We conclude that, in contrast to oestradiol, the secretion of inhibin by the ovary is not controlled acutely by changes in plasma levels of LH during anoestrus.