title: Estradiol-17beta inhibits gonadotropin-releasing hormone-induced Ca2+ in gonadotropes to regulate negative feedback on luteinizing hormone release.
abstract: 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 (48-72 h), which were treated with 10(-9) M estradiol-17beta (E(2)) or vehicle followed by a pulse of 10(-9) M GnRH. Medium was collected for LH assay and cells extracted to determine activation of MAPK (phosphorylated ERK-1/2). E(2) treatment for 5 min reduced GnRH-induced LH release and caused phosphorylation of ERK-1/2. E(2) alone also caused phosphorylation of ERK-1/2, similar to the response evoked by GnRH alone. GnRH increased cytoplasmic intracellular free calcium concentration ([Ca(2+)](i)) and this was abolished by 2 min pretreatment with E(2) 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 E(2) is a likely mechanism for the negative feedback effect of E(2) on LH secretion involving a rapid nongenomic effect of E(2). Activation of the MAPK pathway by E(2) may be the mechanism for the time-delayed positive feedback effect on LH secretion at the level of the gonadotrope.
authors.affiliation: Department of Physiology, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: Javed
authors.initials: J
authors.lastname: Iqbal
authors.forename: Olivier
authors.initials: O
authors.lastname: Latchoumanin
authors.forename: Ika P
authors.initials: IP
authors.lastname: Sari
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Lang
authors.forename: Harold A
authors.initials: HA
authors.lastname: Coleman
authors.forename: Helena C
authors.initials: HC
authors.lastname: Parkington
authors.forename: Iain J
authors.initials: IJ
authors.lastname: Clarke
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004958:Estradiol / Q000494:pharmacology*; D005246:Feedback; D005260:Female; D052681:Gonadotrophs / Q000187:drug effects / Q000378:metabolism*; D007987:Gonadotropin-Releasing Hormone / Q000037:antagonists & inhibitors*; D007986:Luteinizing Hormone / Q000378:metabolism*; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D009543:Nifedipine / Q000494:pharmacology; D010766:Phosphorylation; D012756:Sheep; D019284:Thapsigargin / Q000494:pharmacology
chemical_list: D007987:Gonadotropin-Releasing Hormone; D004958:Estradiol; D019284:Thapsigargin; D007986:Luteinizing Hormone; D020929:Mitogen-Activated Protein Kinase Kinases; D009543:Nifedipine; D002118:Calcium