Different acid secretagogues activate different Na+/H+ exchanger isoforms in rabbit parietal cells

Rabbit parietal cells express three Na+/H+ exchanger isoforms (NHE1, NHE2, and NHE4). We investigated the effects of carbachol, histamine, and forskolin on Na+/H+ exchange activity and acid formation in cultured rabbit parietal cells and tested the effect of NHE isoform-specific inhibition on agonist-induced Na+/H+ exchange. Carbachol (10(-4) M) was the weakest acid secretagogue but caused the strongest Na+/H+ exchange activation, which was completely blocked by 1 microM HOE-642 (selective for NHE1); histamine (10(-4) M) and forskolin (10(-5) M) were stronger stimulants of [14C]aminopyrine accumulation but weaker stimulants of Na+/H+ exchange activity. HOE-642 (1 microM) reduced forskolin-stimulated Na+/H+ exchange activity by 35%, and 25 microM HOE-642 (inhibits NHE1 and -2) inhibited an additional 13%, but 500 microM dimethyl amiloride (inhibits NHE1, -2, and -4) caused complete inhibition. The presence of 5% CO2-HCO-3 markedly reduced agonist-stimulated H+ efflux rates, suggesting that the anion exchanger is also activated. Hyperosmolarity also activated Na+/H+ exchange. Our data suggest that, in rabbit parietal cells, Ca2+-dependent stimulation causes a selective activation of NHE1, whereas cAMP-dependent stimulation activates NHE1, NHE2, and more strongly NHE4. Because intracellular pH (pHi) did not change in the presence of CO2-HCO-3 and concomitant activation of Na+/H+ and anion exchange is one of the volume regulatory mechanisms, we speculate that the physiological significance of secretagogue-induced Na+/H+ exchange activation may not be related to pHi but to volume regulation during acid secretion.