MODULATION OF GONADOTROPIN-RELEASING HORMONE-STIMULATED LUTEINIZING-HORMONE RELEASE IN CULTURED MALE EQUINE ANTERIOR-PITUITARY-CELLS BY GONADAL-STEROIDS

The objective of the present study was to determine whether the testic ular steroids, i.e., testosterone (T), dihydrotestosterone (DHT), estr adiol (E2), estrone (E1), and estrone sulfate (E1S04), play a physiolo gical role in regulating LH release in the male horse by direct action s at the anterior pituitary gland. Enzymatically dispersed anterior pi tuitary cells from stallions (n = 4) or geldings (n = 3) were cultured for 48 h in alpha-modified Eagle's medium containing 10% steroid-free horse medium. To determine the effects of the steroids on the LH resp onse to GnRH, the cells were incubated for 24 h in fresh media with or without 10(-10) M E2 or 10(-8) M T or DHT followed by a 4-h incubatio n +/- GnRH (10(-11) to 10(-7) M). media and cells were analyzed for LH by RIA. In the stallion, GnRH increased LH release (p < 0.001) in a d ose-dependent manner (ED50 GnRH = 4.5 x 10(-9) M), and this response w as unaltered by T or DHT but greatly enhanced by E2 (p < 0.001). E2 lo wered the ED50 for GnRH to 5 x 10(-10) M and increased the maximum LH response to GnRH by 350%. The LH release in response to a constant dos e of 1 nM GnRH was unaltered by varying doses of T, DHT, or E1SO4 (10( -11) to 10(-7) M). In contrast, E2 increased GnRH-stimulated LH releas e maximally at 10(-10) M with an ED50 of 2.9 x 10(-11) M, whereas E1 w as less effective with an ED(max) of 10(-8) M and an ED50 of 3 x 10(-9 ) M, respectively. In the gelding, the responsiveness to GnRH (i.e., E D50 = 2.8 +/- 0.8 x 10(-10) M) was enhanced (P < 0.01) compared to tha t in the stallion; however, none of the steroids, i.e., T, DHT, E2, E1 , or E1S04, altered GnRH-stimulated LH release. In conclusion, in the male horse, the negative feedback of the testicular androgens on LH re lease appears to be due to an action on the hypothalamus by inhibition of GnRH secretion rather than a direct effect at the pituitary. Furth ermore, it appears that E, may be important for maintaining pituitary responsiveness to GnRH.