The effect of a potent gonadotrophin-releasing hormone antagonist on ovarian secretion of oestradiol, inhibin and androstenedione and the concentration of LH and FSH during the follicular phase of the sheep oestrous cycle.

By selective removal and replacement of LH stimulation we sought to examine the relative importance of inhibin and oestradiol in controlling FSH secretion, and the role of LH in the control of ovarian hormone secretion, during the follicular phase of the oestrous cycle. Eight Finn-Merino ewes which had one ovary removed and the other autotransplanted to a site in the neck were given two injections of a gonadotrophin-releasing hormone (GnRH) antagonist (50 micrograms/kg s.c.) in the follicular phase of the cycle 27 h and 51 h after luteal regression had been induced by cloprostenol (100 micrograms i.m.). Four of the ewes received, in addition, i.v. injections of 2.5 micrograms LH at hourly intervals for 23 h from 42 to 65 h after GnRH antagonist treatment. Ovarian jugular venous blood samples were taken at 10-min intervals for 3 h before and 5 h after the injection of antagonist (24-32 h after cloprostenol) and from 49 to 53 h after antagonist (74-78 h after cloprostenol). Additional blood samples were taken at 4-h intervals between the periods of intensive blood sampling. The GnRH antagonist completely inhibited endogenous pulsatile LH secretion within 1 h of injection. This resulted in a marked decrease in the ovarian secretion of oestradiol and androstenedione (P less than 0.001), an effect that was reversible by injection of exogenous pulses of LH (P less than 0.001). The pattern of ovarian inhibin secretion was episodic, but removal or replacement of stimulation by LH had no effect on the pattern or level of inhibin secretion.(ABSTRACT TRUNCATED AT 250 WORDS)