Chloride activities in epithelia

In Cl-absorbing epithelia, intracellular Cl activities have only been determined in tissues possessing a NaCl cotransport process at the apical (lumen-facing) membrane, which appears to be responsible for active Cl absorption. In these tissues, cell Cl activities average 2-4 times the values predicted for a passive distribution of Cl across the apical membrane, but this cellular Cl accumulation is abolished if Na in the luminal solution is replaced by non-transported cations. Thus, the energy for cell Cl accumulation and transepithelial transport appears to be derived from the interaction between Na and Cl entry into the cell and the electrochemical potential difference for Na across the apical membrane. In epithelia that actively secrete Cl, cell Cl activities are also significantly higher than the predicted equilibrium values, so that the uphill step in transepithelial Cl transport lies at the basolateral (plasma-facing) membrane. Available evidence suggests that NaCl cotransport may also be responsible for cell Cl accumulation by Cl-secreting tissues. In both absorbing and secreting epithelia, the exit of Cl from the cell is directed down a favorable electrochemical potential difference for the anion, but the mechanisms responsible for Cl exit require further study.