Analysis of initial and delayed effects of orchidectomy and ovariectomy on pituitary and serum LH levels in adult and immature rats.

Abstract

The initial LH secretory responses to castration were examined in adult and immature rats of both sexes, using an 0:0 radioimmunoassay for rat LH. The patterns of response differed markedly between males and females. Adult males exhibited a dramatic increase in serum LH levels, which was detectable by 8 hr after gonadectomy, and reached a plateau at about 20 times pre-castration levels by 18 hr. This plateau was sustained through the fourth day; serum LH levels then increased about 4-fold further over the next month. The initial response in females was less marked and no plateau phase was evident. Although serum LH levels did not match those of comparable males during the first 10 post-castration days, they did reach or surpass them later. The pattern of pituitary LH changes also differed: in males, LH stores initially fell, reaching a nadir at about day 4; females exhibited no such fall. This sex difference in the pattern of the initial LH secretory responses to castration was evident in prepubertal castrates as well, although immature males did not respond as markedly as adult males. The weaker initial response of the female was evident in rats castrated at various times during the estrus cycle. Serum LH appeared to rise more rapidly in rats ovariectomized during diestrus than in rats spayed at other times, but no female castrate ever exhibited the very marked initial serum LH increment characteristic of the adult male. Analysis of concomitant changes in serum and pituitary LH levels suggest that LH net synthesis increased quite soon after castration, perhaps even as early as LH release (i.e., within the first 8–18 hr after gonadectomy). However, it should be emphasized that studies such as these may not accurately define the time lags in the responses of the LH secretory system to removal of steroid negative feedback signals, since surgical gonadectomy may itself influence the LH secretory response(s). (Endocrinology86: 1102, 1970)