Dissociation of colonic apical Na/H exchange activity from bulk cytoplasmic pH

Intracellular acidification by stimuli other than CO2 fails to stimulate colonic apical Na/H exchange and Na absorption. We examined whether Na absorption could be stimulated in the absence of changes in cytoplasmic pH (pH(i)). Distal colon of male Sprague-Dawley rats was used for pH(i) measurements with 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and for flux measurements in Ussing chambers. In 21 mM HCO3-Ringer, increasing PCO2 from 20 to 70 mmHg decreased pH(i) from 7.51 to 7.03 and increased net Na flux (J(net)(Na)) from 4.2 +/- 0.4 to 6.8 +/- 0.6 mu eq . cm(-2). h(-1). Similar increases in J(net)(Na) occurred in the absence of mucosal Cl and in the presence of phalloidin to inhibit microfilaments or benzolamide to inhibit membrane-bound carbonic anhydrase. Isohydric increases in PCO2 did not alter pH(i) but stimulated J(net)(Na) from 5.1 +/- 0.6 to 7.2 +/- 0.8 mu eq . cm(-2). h(-1). Carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreased pH(i) from 7.45 to 7.35 but did not stimulate J(net)(Na). Butyrate (25 mM) decreased pH(i) from 7.15 to 7.02 with recovery to baseline within 6 min; however, J(net)(Na) increased by 2.2 mu eq . cm(-2). h(-1) for 60 min. We conclude that apical Na/H exchange activity is unresponsive to changes in bulk pH(i) and is independent of Cl/HCO3 exchange, microfilaments, and membrane-bound carbonic anhydrase. The presence of an H-tight, CO2, and butyrate-permeable subapical domain is postulated.