GnRH Interaction with Anterior Pituitary. IV. Effect of Estradiol-17β on GnRH-Mediated Release of LH from Ovine Pituitary Cells Obtained During the Breeding Season, Anestrous Season, and Period of Transition Into or Out of the Breeding Season1

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Abstract
The effect of estradiol-17.beta. (estradiol) on gonadotropin releasing hormone (GnRH)[gonadoliberin]-mediated release of luteinizing hormone (LH) [lutropin] was investigated in pituitary cells obtained from ovariectomized ewes during the breeding season, anestrous season and periods of transition into and out of the breeding season. Of the enzymatically dispersed cells, 1/2 were cultured for 18 h with estradiol (10-9 M); the remainder served as controls. After the cells had been washed 6 times with estradiol-free buffer, they were resuspended and incubated in estradiol-free media for 2 h at 37.degree. C with GnRH concentrations ranging from 2 .times. 10-12-3.2 .times. 10-8 M. The quantity of LH released into the media and that retained in the cells were determined by radioimmunoassay. The concentration of GnRH required to elicit half maximal (ED50) release of LH was calculated for each ewe. Although the magnitude of the effect varied between sheep, estradiol treatment of pituitary cells obtained during the breeding season (no. = 5) decreased the ED50 (P < 0.05), increased total LH/100,000 cells by 10.8 .+-. 3.2% (P < 0.05) and increased the quantity of LH released after GnRH administration (P < 0.05). Estradiol treatment of pituitary cells obtained during the anestrous season (n = 7) tended to increase (P < 0.1) the ED50, but total LH/100,000 cells and the quantity of LH released after GnRH administration were not altered (P > 0.80, P > 0.82, respectively). In pituitary cells obtained from ewes during the period of transition into or out of the breeding season (no. = 7) estradiol treatment increased the ED50 in 4 ewes, decreased the ED50 in 2 ewes and had no effect in 1 ewe. Total LH/100,000 cells and the quantity of LH released after GnRH administration were depressed (P < 0.05) compared with nonestradiol-treated controls. Apparently in pituitary cells obtained from castrate ewes, treatment with estradiol alters GnRH-mediated release of LH during the breeding season but does not enhance GnRH-mediated release of LH during the anestrous season or period of transition into or out of the breeding season. Estradiol may depress LH release during the latter 2 periods.

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