Bio- and immunoactive luteinizing hormone in negative feedback action of gonadal steroids

Abstract

To clarify the possible intervention of gonadal steroids on the secretion of bioactive LH from the pituitary, the levels of plasma LH were measured by in vitro bioassay as well as radioimmunoassay, and their changes in specific endocrine conditions were investigated. The ratio of bioactivity to immunoactivity (B/I ratio) of LH in patients with hypergonadotropic hypogonadism and in postmenopausal women was significantly greater than that in women during the follicular phase of the menstrual cycle, and estradiol levels in the former groups were significantly lower than those of the latter group. In cases of gonadotropin-secreting pituitary tumor with high LH and normal estradiol levels, B/I ratio was found at a level similar to that during the follicular phase. The administration of combined pills decreased the high B/I ratio of oophorectomized women to levels comparable to those in the normal follicular phase. Although bioactive LH responded in all groups to LH-RH with a similar pattern to that observed with immunoactive LH with a peak value 15 approximately 30 min. after the stimulation, followed by a gradual decrease thereafter, the B/I ratio of women during the follicular phase was the lowest at 15 min. and returned up to the initial value within 60 min. after the injection of LH-RH. In contrast, this drop of the B/I ratio did not occur in 7 out of 15 cases composed either of hypergonadotropic hypogonadism or postmenopausal women. The peak values of LH in patients without decrease in B/I ratios were higher than those of women showing decreased B/I ratios. In addition, circulating levels of gonadotropin alpha-subunit did not affect the B/I ratio. These results suggest that circulating levels of gonadal steroids regulate the secretion and synthesis of LH quantitatively as well as qualitatively.