In pituitary gonadotropes, estrogens have biphasic actions to cause an initial negative feedback followed by a positive feedback on LH secretion, but the mechanisms involved are not clearly understood. To investigate the feedback effects of estrogen, we used mixed ovine pituitary cell cultures (48-72h) which were treated with 10(-9)M estradiol-17beta (E2) or vehicle followed by a pulse of 10(-9)M GnRH. Medium was collected for LH assay and cells extracted to determine activation of mitogen-activated protein kinase (MAPK) (phosphorylated ERK-1/2). E2 treatment for 5 min reduced GnRH-induced LH release and caused phosphorylation of ERK-1/2. E2 alone also caused phosphorylation of ERK-1/2, similar to the response evoked by GnRH alone. GnRH increased cytoplasmic [Ca(2+)]i and this was abolished by 2 min pretreatment with E2 or E-bovine serum albumen conjugate. Blockade of Ca(2+) channels with nifedipine had no effect on the initial peak of GnRH-induced increase in [Ca(2+)]i, but reduced its duration by 27 +/- 6 . Depletion of intracellular Ca(2+) stores with thapsigargin prevented GnRH-induced increase in [Ca(2+)]i. Thapsigargin (10(-7)M) or nifedipine (10(-5)M) pretreatment (15 min) of cells lowered GnRH-induced LH secretion by 30 +/- 6 and 50 +/- 4 , respectively. We conclude that inhibition of the GnRH-induced increase in [Ca(2+)]i in gonadotropes by E2 is a likely mechanism for the negative feedback effect of E2 on LH secretion involving a rapid non-genomic effect of E2. Activation of the MAPK pathway by E2 may be the mechanism for the time-delayed positive feedback effect on LH secretion at the level of the gonadotrope.