Effects of steroids on basal and GnRH-induced LH and FSH secretion by dispersed pituitary cells of immature female lambs.

The effects of 17 beta-estradiol (E2), progesterone (P) and their combination on the basal and GnRH-induced LH and FSH release were studied in immature female lamb pituitary cells. Lambs born in February (slaughtered at 12--13 weeks of age) (Y) or older (O) out of season born in the previous year and carried over to January and April (hoggets) were used as pituitary donors. The cells were preincubated for 24 h in the presence or absence of steroids, then washed and incubated for 2 h with or without GnRH. E2 (100 pg ml-1) depressed the basal and GnRH-induced LH and FSH release and P at the highest dose used (10 ng ml-1) decreased significantly the basal and GnRH-induced LH release, whereas it did not affect these parameters of FSH release from pituitary cells of Y. In contrast to these effects of steroids, E2 (10--1000 pg ml-1) stimulated the basal and GnRH-induced LH and FSH release from pituitary cells of O. P (0.1--10 ng ml-1), in a way inversely related to the concentration, stimulated the basal LH and FSH release; the higher P concentration did not affect the GnRH-induced FSH, while it permitted a small but significant increase of the GnRH-induce LH release in O (43% above the GnRH-treated level). Taking in account the basal release, the sensitivity of LH and FSH response to GnRH was increased by E2. In E2 + P treated cells, P did not modify the E2 effect on the LH responsiveness to GnRH in pituitaries of Y or O. On the other hand, P at the higher dose (10 ng ml-1) potentiated the stimulatory effect of E2 on basal and GnRH-induced FSH release in O. The results suggest that E2 and P can exert negative or positive effects on gonadotropin release from pituitary cells of immature female lambs depending on the developmental stage of the pituitary donors and the steroid concentration. P depressed the sensitivity of response to GnRH induced by E2.