Na depletion inhibits electroneutral Na-Cl absorption in intact tissues and Na/H exchange in apical membrane vesicles (AMV) of rat distal colon. Two anion (Cl/HCO₃ and Cl/OH) exchanges have been identified in AMV from surface cells of rat distal colon. To determine whether Cl/HCO₃ and/or Cl/OH exchange is responsible for vectorial Cl movement, this study examined the spatial distribution and the effect of Na depletion on anion-dependent ³⁶Cl uptake by AMV in rat distal colon. These studies demonstrate that HCO₃ concentration gradient-driven ³⁶Cl uptake (i.e., Cl/HCO₃ exchange) is 1) primarily present in AMV from surface cells and 2) markedly reduced by Na depletion. In contrast, OH concentration gradient-driven ³⁶Cl uptake (i.e., Cl/OH exchange) present in both surface and crypt cells is not affected by Na depletion. In Na-depleted animals HCO₃ also stimulates ³⁶Cl via Cl/OH exchange with low affinity. These results suggest that Cl/HCO₃ exchange is responsible for vectorial Cl absorption, whereas Cl/OH exchange is involved in cell volume and/or cell pH homeostasis.

aldosterone; cell functions; sodium depletion; vectorial ion movement; vesicle uptake

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