Influence of acid secretory state on Cl(-)-base and Na(+)-H+ exchange and pHi in isolated rabbit parietal cells

Parietal cell apical proton secretion is accompanied by apical Cl- secretion, K+ cycling, and the generation of intracellular base. We examined the ability of the parietal cell to maintain intracellular pH (pHi) during its transformation from the resting to the stimulated state and evaluated the ion transport mechanisms involved in the maintenance of ionic equilibrium. Isolated rabbit parietal cells were loaded with the pH-sensitive fluorescent dye BCECF, and pHi was monitored in maximally stimulated, resting (absence of a secretagogue), and omeprazole-inhibited cells. Although [14C]aminopyrine (AP) accumulation increased up to 30-fold above basal during stimulation, the mean pHi of maximally stimulated cells was not different from that of inhibited cells both immediately and late after stimulation in an extracellular pH range from 6.2 to 7.8 in either HEPES or CO2/HCO3- buffer. When the stilbene DIDS was added 2 min after stimulation of acid formation, pHi rapidly increased (0.09 +/- 0.02 vs. 0.04 +/- 0.02 pH units in unstimulated cells in 6 min), indicating an increased base efflux through a DIDS-sensitive transporter (most likely the Cl(-)-base exchanger). Stimulation of acid secretion did not change the transport capacity, apparent affinity for extracellular Cl-, or dependency of the anion flux rate on pHi of the DIDS-sensitive base exporter. For a given pHi, amiloride-inhibitable proton efflux rates during pHi recovery from an acid load were identical in resting and stimulated cells, suggesting that neither the transport capacity not the pHi set point of the parietal cell Na(+)-H+ exchanger is altered by cAMP-dependent stimulation of acid formation. We conclude that, during the cAMP-mediated stimulation of acid formation in isolated rabbit parietal cells, the pHi remains constant, but an increased base efflux occurs without a change in the transport capacity of the involved base extrusion mechanisms or an inhibition of the parietal cell base loading mechanisms. Whether changes in the intracellular Cl- concentration on stimulation of acid formation initiate the increased base efflux and whether additional ion transporters are involved in the maintenance of ion homeostasis during acid secretion remain to be determined.