Na+ dependence of gonadotropin-releasing hormone action: characterization of the Na+/H+ antiport in pituitary gonadotropes.

GnRH stimulates LH release from gonadotropes in a Ca2(+)-dependent manner. Because of the apparent relationship between cellular Ca2+ metabolism and Na(+)-driven antiports, we investigated their influence on GnRH action. We also assessed the influence of bicarbonate, because its transport may alter effects of Na+/H+ exchange on intracellular pH. In pituitary cell cultures without bicarbonate, GnRH-stimulated LH release was reduced by Na+ omission, by amiloride, and by amiloride analogs that selectively block Na+/H+ exchange. The Na+ dependence of amiloride action (EC50, 14 and 100 microM in medium with 20 and 135 mM NaCl, respectively, and no effect in Na(+)-free medium) and the order of potency of these analogs, indicated specific inhibition of Na+/H+ exchange. 5-(N,N-Di-methyl)amiloride (DMA; a potent Na+/H+ exchange inhibitor) reduced GnRH-stimulated LH release but not GnRH receptor binding or Ca2+ ionophore (A23187)-stimulated LH release, suggesting inhibition at a locus beyond receptor occupancy but before exocytosis. Amiloride analogs that selectively inhibit Na+/Ca2+ exchange also modestly reduced GnRH-stimulated LH release. Bicarbonate (10 mM) reduced the inhibitory effects of DMA and Na+ omission (but not the effects of the Na+/Ca2+ exchange inhibitors or of a Ca2+ channel antagonist), and the effect of bicarbonate was inhibited by a blocker of bicarbonate-dependent antiports. These observations reveal the Na+ dependence of GnRH action and that gonadotropes possess a Na+/H+ exchanger. The Na+ dependence of GnRH-stimulated LH release appears to reflect at least in part dependence upon this antiport. Prevention of the Na+/H+ exchange inhibitor effects by bicarbonate supports the specificity of their action, but suggests regulation of this antiport as an unlikely means of controlling LH release in vivo.