An experiment was conducted in order to deter-mine the pattern of, and the relationships between, the secretion of inhibin, estradiol, and androstenedione by the ovary and the concentration of LH, FSH, and PRL during the estrous cycle of sheep. The estrous cycles of 6 Finn-Merino ewes in which the left ovary had been autotransplanted to the neck were synchronized by two injections of cloprostenol (100 μg im) a potent analog of prostaglandin F2α (PG) given 14 days apart. The ewes had ovarian and jugular venous blood samples taken at four hourly intervals from 42 h before the second PG injection until day 6 of the following cycle. All animals responded to PG with the preovulatory LH surge occurring within 58 ± 2 h (mean ± SEM). The concentration of FSH in jugular venous plasma fell (P < 0.001) after the induction of luteolysis and then exhibited 3 peaks, the first coincident with the LH surge, the second on day 1, and the third on day 6. After injection of PG the secretion rates of inhibin, estradiol, and androstenedione increased (P < 0.05) within 4-8 h. After this increase in the early follicular phase the secretion rate of estradiol continued to rise until the time of the LH surge (P < 0.001). Although the secretion of androstenedione and inhibin increased in the 36 h before the LH surge the magnitude of this rise was less marked than for estradiol and was not statistically significant. Within 4-8 h of the start of the LH surge the secretion of estradiol and androstenedione declined rapidly reaching barely detectable levels within 16 h (P < 0.001). In contrast the secretion of inhibin increased after the LH surge reaching a broad peak (P < 0.05) of approximately 16-h duration, coincident with the second peak of FSH. From days 2-6 mean secretion of inhibin remained relatively stable at 2-6 ng/min although considerable variation was observed in individual profiles. The rate of estradiol secretion increased steadily from its nadir on day 1 to a broad peak centered around day 3 (3-6 ng/min, P < 0.001) followed by a decline until by day 6 the estradiol secretion rate was less than 1 ng/min (P < 0.01). The secretory profile for PRL showed a close relationship with estradiol secretion. The concentrations of FSH in jugular venous blood and ovarian estradiol secretion rates were negatively correlated during the follicular phase (P < 0.05) and between days 1-6 of the subse-quent luteal phase (P < 0.01). Inhibin secretion was also weakly negatively correlated with FSH but at no stage of the cycle was this relationship statistically significant. Furthermore, while the secretory profiles for estradiol and androstenedione in individual animals were very similar, this was not the case for inhibin as there was a great deal of variation between animals in secretory profiles. We conclude that the fluctuations in the pattern of secretion of FSH throughout the sheep estrous cycle are mainly under the control of estradiol while inhibin contributes to the overall level of negative feedback. © 1990 by The Endocrine Society.
