Gonadotropin-releasing hormone-induced Ca2+ transients in single identified gonadotropes require both intracellular Ca2+ mobilization and Ca2+ influx.
We examined the effects of gonadotropin-releasing hormone (GnRH) on the intracellular free Ca2+ concentration ([Ca2+]i) in single rat anterior pituitary gonadotropes identified by a reverse hemolytic plaque assay. Concentrations of GnRH greater than 10 pM elicited increases in [Ca2+]i in identified cells but not in others. In contrast, depolarization induced by 50 mM K+ increased [Ca2+]i in all cells. Ca2+ transients induced by GnRH exhibited a complex time course. After an initial rapid rise, the [Ca2+]i fell to near basal levels only to be followed by a secondary extended rise and fall. Analysis of the Ca2+ transients on a rapid time base revealed that responses frequently consisted of several rapid oscillations in [Ca2+]i. Removal of extracellular Ca2+ or addition of the dihydropyridine Ca2+-channel blocker nitrendipine completely blocked the secondary rise in [Ca2+]i but had no effect whatsoever on the initial spike. Nitrendipine also blocked 50 mM K+-induced increases in [Ca2+]i in identified gonadotropes. The secondary rise induced by GnRH could be enhanced by a phorbol ester in a nitrendipine-sensitive fashion. Multiple spike responses to GnRH stimulation of the same cell could only be obtained if subsequent Ca2+ influx was permitted either by allowing a secondary rise to occur or by producing a Ca2+ transient by depolarizing the cells with 50 mM K+. It therefore appears that the response to GnRH consists of an initial phase of Ca2+ mobilization, probably mediated by inositol trisphosphate, and a subsequent phase of Ca2+ influx through nitrendipine-sensitive Ca2+ channels that may be activated by protein kinase C. The relative roles of these phases in the control of gonadotropin secretion are discussed.
D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D002478:Cells, Cultured; D004095:Dihydropyridines / Q000494:pharmacology; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D009568:Nitrendipine / Q000494:pharmacology; D010903:Pituitary Gland, Anterior / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains
D004095:Dihydropyridines; D007987:Gonadotropin-Releasing Hormone; C038806:1,4-dihydropyridine; D009568:Nitrendipine; D011188:Potassium; D002118:Calcium