An inverse relationship between inhibin and follicle-stimulating hormone during the period of ovulation induced by human chorionic gonadotrophin in dioestrous rats.

To investigate the mechanism of the selective surge of FSH during the period of ovulation induced by human chorionic gonadotrophin (hCG) in dioestrous rats, inhibin activity in ovarian vein plasma was determined at varying time-intervals after treatment with hCG using the primary monolayer culture system of anterior pituitary cells. Inhibin activity in ovarian vein plasma had already decreased 6 h after injection of hCG, when concentrations of FSH in the plasma were still low in three of four animals. Inhibin activity further decreased 12-18 h after hCG, when a selective surge of FSH occurred. Inhibin activity increased to the level before hCG treatment 24 h after the treatment, when ovulation was completed and the FSH surge terminated. These results suggest that the selective surge of FSH occurs as a consequence of the decrease in inhibin secretion from the ovary, which is perhaps due to the ovulation dose of hCG altering the functional activity of the granulosa cells in the large Graafian follicles.