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.
McArdle
C A
CA
Institute for Hormone and Fertility Research, Hamburg, Germany.
Cragoe
E J
EJ
Poch
A
A
D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001491:Basophils / Q000378:metabolism*; D001639:Bicarbonates / Q000494:pharmacology; D002064:Buserelin / Q000378:metabolism; D000001:Calcimycin / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000502:physiology*; D006062:Gonadotropins / Q000378:metabolism*; D007986:Luteinizing Hormone / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D012964:Sodium / Q000502:physiology*; D017923:Sodium-Hydrogen Exchangers
D001639:Bicarbonates; D002352:Carrier Proteins; D006062:Gonadotropins; D017923:Sodium-Hydrogen Exchangers; D007987:Gonadotropin-Releasing Hormone; D000001:Calcimycin; C039465:5-dimethylamiloride; D000584:Amiloride; D007986:Luteinizing Hormone; D012964:Sodium; D002064:Buserelin