Stimulation of FSH and LH Releasein Vitroby Natural and Synthetic LH and FSH Releasing Hormone*

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Abstract
FSH-releasing hormone (FSH-RH) and LH-releasing hormone (LH-RH) activities of three synthetic preparations of (pyro) Glu-His- Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 were compared with that of a homogeneous preparation of natural LH-releasing hormone (LH-RH) of porcine origin. An in vitro system based on release of FSH and LH from normal male rat pituitaries was used to assay FSH-RH and LH-RH. LH-RH activity was also determined in vivo by stimulation of the release of LH in ovariectomized rats pretreated with estrogen and progesterone. Released FSH was measured by specific bioassay as well as by radioimmunoassay for rat FSH. Released LH was measured by radioimmunoassay for rat LH. Quantitatively, the FSH-RH activity and the LH-RH activity of all three synthetic preparations of LH-RH/FSH-RH were identical with those of the natural material. The time courses of the in vitro release of LH and FSH, induced by the synthetic and natural materials, appeared to be identical, suggesting that one hypothalamic hormone may be responsible for the release of both gonadotropins. These results confirm and extend our earlier observations on the inherent FSH-RH activity of LH-RH. Addition of ethinyl estradiol (0.5 μg/ml) to the incubation medium greatly inhibited the stimulation of FSH and LH release by the hypothalamic hormone suggesting a direct effect of this steroid on pituitary cells. (Endocrinology90: 1561, 1972)