pH heterogeneity at intracellular and extracellular plasma membrane sites in HT29-Cl cell monolayers

In the colonic mucosa, short-chain fatty acids change intracellular pH (pH( i)) and extracellular pH (pH(e)). In this report, confocal microscopy and d ual-emission ratio imaging of carboxyseminaphthorhodofluor-1 were used for direct evaluation of pH(i) and pH(e) in a simple model epithelium, HT29-C1 cells. Live cell imaging along the apical-to-basal axis of filter-grown cel ls allowed simultaneous measurement of pH in the aqueous environment near t he apical membrane, the lateral membrane, and the basal membrane. Subapical cytoplasm reported the largest changes in pH(i) after isosmotic addition o f 130 mM propionate or 30 mM NH4Cl. In resting cells and cells with an impo sed acid load, lateral membranes had pH(i) values intermediate between the relatively acidic subapical region (pH 6.3-6.9) and the relatively alkaline basal pole of the cells (pH 7.4-7.1). Transcellular pH(i) gradients were d iminished or eliminated during an induced alkaline load. Propionate differe ntially altered pH(e) near the apical membrane, in lateral intracellular sp aces between adjacent cells, and near the basal membrane. Luminal or serosa l propionate caused alkalinization of the cis compartment (where propionate was added) but acidification of the trans compartment only in response to luminal propionate. Addition of NH4Cl produced qualitatively opposite pH(e) excursions. The microscopic values of pH(i) and pH(e) can explain a portio n of the selective activation of polarized Na/H exchangers observed in HT29 -C1 cells in the presence of transepithelial propionate gradients.