The effects of oestradiol, 5α-dihydrotestosterone (DHT) and oestradiol plus DHT on pituitary responsiveness to LHRH were studied. Rats ovariectomized for 2 weeks were infused s.c. (by osmotic minipump) with LHRH at 250 ng/h for 6 days. Control rats received a sham s.c. pump. On day 3, silicone elastomer implants containing oestradiol or DHT were implanted s.c., and on day 6 the effects of these in-vivo treatments on pituitary LH and FSH content and on in-vitro (perifusion) LH and FSH secretion following maximal LHRH stimulation (1 μg/ml perifusion medium) were assessed.
Luteinizing hormone-releasing hormone alone decreased pituitary LH/FSH content and, in response to acute LHRH challenge in vitro, the absolute rate of LH/FSH release, but not LH/FSH release expressed as a fraction of pituitary content. Oestradiol alone increased pituitary LH/FSH content and LHRH-induced LH/FSH release in vitro, both absolutely and as a fraction of pituitary LH/FSH. Oestradiol exacerbated the decrease in pituitary LH/FSH caused by LHRH pretreatment in vivo, and decreased the absolute rate of LHRH-stimulated LH/FSH release in vitro, but increased this rate when it was expressed as a fraction of pituitary LH/FSH. In both LHRH-treated and control rats, DHT increased pituitary LH/FSH content, did not change the absolute rate of LH/FSH release in response to acute LHRH challenge in vitro, but decreased the rate of LH/FSH release expressed as a fraction of pituitary LH/FSH content. Combined pretreatment with oestradiol and DHT resulted in both control and LHRH-treated rats having pituitary contents comparable with those produced by DHT, similar fractions of pituitary LH/FSH release to those produced by oestradiol, and absolute rates of LH and FSH release following LHRH stimulation that were generally greater than those of the respective controls and those following other steroid pretreatment regimens. It is suggested that depletion of pituitary gonadotrophin stores caused by LHRH pretreatment in vivo, not desensitization, is responsible for the reduced LH/FSH release to an LHRH challenge in vitro and that, in the female, because of the presence of oestrogens, androgens do not exert a negative feedback on the pituitary gland as they do in the male, but, on the contrary, contribute to a high level of pituitary LHRH responsiveness.
Journal of Endocrinology (1989) 123, 257–262
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