Acid-base effects on ileal sodium chloride absorption in vitro

Previous work from this laboratory has shown that in the intact animal, ileal sodium chloride absorption is responsive to alterations in systemic acid-base balance. In vitro studies were undertaken to determine the nature of the active transport process affected. Paired, unstripped ileal tissues from Sprague-Dawley rats were mounted in modified Ussing chambers, and unidirectional 22Na and 36Cl fluxes were measured under short-circuited conditions (Isc). Acid-base effects were examined by changing either bathing solution partial pressure of CO2 (PCO2) or HCO3 concentration or by the addition of HCl or NaOH to a non-HCO3 N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer. When pH was decreased from 7.6 to 7.1, net sodium absorption increased twofold regardless of the means by which pH was changed. This increase was primarily accounted for by an increase in mucosal-to-serosal flux. Net chloride absorption was affected in a similar way, but the increase in absorption was primarily accounted for by an increase in mucosal-to-serosal flux only in the HEPES buffer. These changes were accompanied by reductions in Isc and residual flux. When all groups were considered, mucosal-to-serosal Na (JNam----s) and net Na (JNanet) fluxes correlated strongly with bathing solution pH (r = 0.84 and 0.94, respectively, P less than 0.01). These data indicate that an active ileal sodium chloride absorptive process is specifically responsive to changes in bathing solution pH. In addition, pH may affect ileal electrogenic chloride, HCO3, and/or H secretion.