title: Dissociation of colonic apical Na/H exchange activity from bulk cytoplasmic pH.
abstract: Intracellular acidification by stimuli rather than CO2 fails to stimulate colonic apical Na/H ex-change and Na absorption. We examined whether Na absorption could be stimulated in the absence of changes in cytoplasmic pH (pHi). Distal colon of male Sprague-Dawley rats was used for pHi 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 pHi from 7.51 to 7.03 and increased net Na flux (JnetNa) from 4.2 +/- 0.4 to 6.8 +/- 0.6 mu eq.cm-2.h-1. Similar increases in JnetNa occurred in the absence of mucosal CI and in the presence of phalloidin to inhibit microfilaments or penzolamide to inhibit membrane-bound carbonic anhydrase. sohydric increases in Pco2 did not alter pHi but stimulated JnetNa from 5.1 +/- 0.6 to 7.2 +/- 0.8 mu eq.cm-2.h-1. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) decreased pHi from 7.45 to 7.35 but did not stimulate JnetNa. Butyrate (25 mM) decreased pHi from 7.15 to 7.02 with recovery to baseline within 6 min; however, JnetNa 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 pHi 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.
authors.affiliation: Nephrology Section, Veterans Affairs Medical Center, New York, New York 10010, USA.
authors.forename: P C
authors.initials: PC
authors.lastname: Dagher
authors.forename: T
authors.initials: T
authors.lastname: Behm
authors.forename: A
authors.initials: A
authors.lastname: Taglietta-Kohlbrecher
authors.forename: R W
authors.initials: RW
authors.lastname: Egnor
authors.forename: A N
authors.initials: AN
authors.lastname: Charney
mesh_terms: D000042:Absorption; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002087:Butyrates / Q000494:pharmacology; D020148:Butyric Acid; D002245:Carbon Dioxide / Q000494:pharmacology; D002256:Carbonic Anhydrases / Q000378:metabolism; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D002462:Cell Membrane / Q000378:metabolism; D003106:Colon / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism*; D005452:Fluoresceins; D005456:Fluorescent Dyes; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008297:Male; D010590:Phalloidine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*
chemical_list: D002087:Butyrates; D005452:Fluoresceins; D005456:Fluorescent Dyes; D017923:Sodium-Hydrogen Exchangers; D020148:Butyric Acid; D002245:Carbon Dioxide; D010590:Phalloidine; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; D006859:Hydrogen; C043829:2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; D012964:Sodium; D002256:Carbonic Anhydrases