Effect of intracellular acidification on colonic NaCl absorption

CO2 stimulates Na+ and Cl- absorption in rat distal colon. This is most likely due to intracellular generation of H+ and HCO3- and stimulation of apical Na(+)-H+ and Cl(-)-HCO3- exchangers. We examined whether intracellular acidification by means other than CO2 would also stimulate Na+ absorption. Stripped segments of distal colon from male Sprague-Dawley rats were studied under short-circuit conditions in Ussing chambers. Identically prepared tissues were used for intracellular pH (pHi) measurements with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. When the Ringer PCO2 was increased from 20 to 34 mmHg, pHi decreased from 7.50 +/- 0.04 to 7.35 +/- 0.04 and net Na absorption increased from 2.4 +/- 0.7 to 3.7 +/- 0.7 mu eq.cm-2 x h-1. A similar degree of intracellular acidification was obtained with 2.6 microM nigericin, but no stimulation of Na+ absorption was seen. When Ringer-HCO3- concentration was reduced from 39 to 11 mM at constant PCO2 = 35 mmHg, pHi decreased from 7.55 +/- 0.02 to 7.11 +/- 0.02 with no effect on net Na+ absorption. A similar reduction in pHi in a CO2-HCO3(-)-free, N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered Ringer also did not stimulate Na+ absorption. Methazolamide had no effect on steady-state pHi at any given PCO2 but caused marked reductions in net Na+ absorption (9.6 +/- 2.4 to 5.2 +/- 1.2 mu eq.cm-2 x h-1 at PCO2 = 70 mmHg). We conclude that Na+ absorption in rat distal colon is not stimulated by intracellular acidification per se but rather has an absolute requirement for CO2 and carbonic anhydrase activity.