Intracellular pH dependence of buffer capacity and anion exchange in the parietal cell

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Abstract
When parietal cells (PC) are stimulated with histamine, the anion exchanger rate increase three to five times to compensate for alkaline loading induced by H+-K+ adenosinetriphosphatase (ATPase) and to provide Cl for acid secretion. It has been hypothesized that this increased activity is caused by the increase in intracellular pH (pHi) that often occurs in stimulated PC (from 7.1 to a maximum of 7.3). The dependence of the anion exchanger on pHi was studied using microspectrofluorimetry of the pH-sensitive dye 2'',7''-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). N-2-hydroxethylpiperazine-N''-2-ethanesulfonic acid (HEPES)-buffered solutions were used because the anion exchanger can transport OH- (or HCO3) in exchange for Cl- even with [HCO3]o = 200 .mu.M. It was found that when solutios were changed either from NaCl to Cl- free or Cl- free to NaCl, rates of change of pHi (.delta. pH/.delta.t) were strongly dependent on pHi; nearly 0 at pHi 6.6 and 1.25 pH/min at pHi 8.0. To convert these pHi changes into anion flux rates, the intrinsic buffer capacity (.beta.i) was determined over the same pHi range by making small changes of [NH4]o to determine the resulting changes of [NH4]i and pHi (i.e., .beta.1 = .DELTA. [NH4]i/.DELTA.pHi) in PC that had been pretreated with 1 mM amiloride and 200 .mu.M [H2]4,4''-diisothiocyanostilbene-2,2''-disulfonic acid (DIDS) [to block Na+-H+ and Cl--OH-(HCO3-] exchange]. .beta.i was also strongly dependent on pHi; at pHi 6.5 .beta.i = 48 mM/pH, and this decreased as pHi increased; at pHi 7.75 .beta.i = 8 mM/pH. The derived anion fluxes (i.e., JOH = .beta.i .times. .delta.pH/.delta.t) were roughly linearly related to pHi between 6.6 (JOH near 0) and 8.1 (JOH = 13 mM/min). Between pHi 7.1 and 7.3, the range normally observed during stimulation of PC, rates of anion exchange increased by 75%. This pHi sensitivity cannot explain the 300-500% increase in anion exchanger activity observed during secretagogue stimulation of PC.

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