Intracellular pH-regulatory mechanisms in pancreatic acinar cells. II. Regulation of H+ and HCO3- transporters by Ca2(+)-mobilizing agonists.

Pancreatic acini loaded with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein were used to examine the effect of Ca2(+)-mobilizing agonists on the activity of acid-base transporters in these cells. In the accompanying article (Muallen, S., and Loessberg, P. A. (1990) J. Biol. Chem. 265, 12813-12819) we showed that in 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid (HEPES)-buffered medium the main pHi regulatory mechanism is the Na+/H+ exchanger, a while in HCO3(-)-buffered medium pHi is determined by the combined activities of a Na+/H+ exchanger, a Na(+)-HCO3- cotransporter and a Cl-/HCO3- exchanger. In this study we found that stimulation of acini with Ca2(+)-mobilizing agonists in HEPES or HCO3(-)-buffered media is followed by an initial acidification which is independent of any identified plasma membrane-located acid-base transporting mechanism, and thus may represent intracellularly produced acid. In HEPES-buffered medium there was a subsequent large alkalinization to pHi above that in resting cells, which could be attributed to the Na+/H+ exchanger. Measurements of the rate of recovery from acid load indicated that the Na+/H+ exchanger was stimulated by the agonists. In HCO3(-)-buffered medium the alkalinization observed after the initial acidification was greatly attenuated. Examination of the activity of each acid-base transporting mechanism in stimulated acini showed that in HCO3(-)-buffered medium: (a) recovery from acid load in the presence of H2-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (H2DIDS) (Na+/H+ exchange) was stimulated similar to that found in HEPES-buffered medium; (b) recovery from acid load in the presence of amiloride and acidification due to removal of external Na+ in the presence of amiloride (HCO3- influx and efflux, respectively, by Na(+)-HCO3- cotransport) were inhibited; and (c) HCO3- influx and efflux due to Cl-/HCO3- exchange, which was measured by changing the Cl- or HCO3- gradients across the plasma membrane, were stimulated. Furthermore, the rate of Cl-/HCO3- exchange in stimulated acini was higher than the sum of H+ efflux due to Na+/H+ exchange and HCO3- influx due to Na(+)-HCO3- cotransport. Use of H2DIDS showed that the latter accounted for the attenuated changes in pHi in HCO3(-)-buffered medium, as much as treating the acini with H2DIDS resulted in similar agonist-mediated pHi changes in HEPES- and HCO3(-)-buffered media. The effect of agonists on the various acid-base transporting mechanisms is discussed in terms of their possible role in transcellular NaCl transport, cell volume regulation, and cell proliferation in pancreatic acini.