Biology of Reproduction, Vol 23, 398-403, Copyright © 1980 by Society for the Study of Reproduction

GnRH Interaction with Anterior Pituitary. IV. Effect of Estradiol-17 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 Season

¹ Department of Physiology and Biophysics, Colorado State University, Fort Collins, Colorado 80523

The effect of estradiol-17 (estradiol) on gonadotropin releasing hormone (GnRH)-mediated release of luceinizing hormone (LH) 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. Half of the enzymatically dispersed cells were cultured for 18 h with estradiol (10^-9 M), and the remainder served as controls. After the cells had been washed six times with estradiol-free buffer, they were resuspended and incubated in estradiol-free media for 2 h at 37°C with concentrations of GnRH ranging from 2 x 10^-12 to 3.2 x 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 (n = 5) decreased the ED₅₀ ’s (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 ’s, 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 (n = 7), estradiol treatment increased the ED₅₀ in four ewes, decreased the ED₅₀ ’s in two ewes, and had no effect in one ewe. Both total LH/100,000 cells and the quantity of LH released after GnRH administration were depressed (P<0.05) compared with nonestradiol-treated controls. These data indicate that in pituitary cells obtained from castrate ewes treatment with estradiol alters GnRH-mediated release of LH in a manner that varies with season. It is apparent that estradiol enhances 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. In addition, the data suggest that estradiol may depress release of LH during these latter two periods.

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ACKNOWLEDGMENTS The technical assistance of Mr. J. Parfet and advice of Dr. T. E. Adams during the course of this study are greatly appreciated.