NaCl absorption in the rabbit ileum. Effect of acid-base variables

In vivo and in vitro studies suggest that acid-base variables regulate ion transport in the rabbit ileum. The relative importance of these variables on active Na+ and Cl- absorption has not been defined. Isolated, stripped ileal segments were studied under short-circuited conditions in the Ussing flux chamber. Unidirectional 22Na and 36Cl fluxes were measured after changes in bathing solution pH, PCO2, and/or [HCO3-]. When pH was decreased from 7.6 to 7.1, net flux of Na+ increased from 0.1 +/- 0.7 to 2.6 +/- 0.7 mu Eq/cm2 per hour and net flux of Cl- increased from -2.0 +/- 0.9 to 1.3 +/- 0.6 mu Eq/cm2 per hour. These changes were rapid in onset, completely reversible, and accounted for by changes in mucosal-to-serosal fluxes of these ions. They were accompanied by small decreases in short-circuit current, but there were no changes in residual flux. These pH effects were not inhibited by the presence of luminal bumetanide (1 mmol/L), furosemide (1 mmol/L), hydroflumethiazide (1 mmol/L), or 4,4'-diisothiocyanostilbene-2,2'-disulfonate (1 mmol/L), or by the carbonic anhydrase inhibitor methazolamide (1 mmol/L). When data from all combinations of acid-base conditions were combined and analyzed by linear regression, pH was the only variable that correlated with mucosal-to-serosal fluxes (r = -0.84) and net flux (r = -0.85) for Na+, mucosal-to-serosal fluxes (r = -0.96) and net flux (r = -0.99) for Cl-, and short-circuit current (r = 0.97). These findings suggest that extracellular pH modulates active Na+ and Cl- absorption in the rabbit ileum.