Intracellular Cl activity: evidence of dual mechanisms of cl absorption in sweat duct.

The human sweat duct (SD) reabsorbs NaCl from lumen to blood over a wide range of luminal concentrations. The physiological strategies employed by the SD to cope with such extreme transport loads remain elusive. When we employed intracellular Cl-sensitive microelectrodes, we found that at high (150 mM) luminal NaCl concentrations ([NaCl]) transcellular Cl absorption occurs through passive diffusion that is evidenced by a large Cl conductance (GCl) in both cell membranes and by a favorable electrochemical driving force for Cl (delta psi Cl) across the apical and basolateral membranes. However, lowering the luminal [NaCl] to 15 mM markedly altered the electrochemical gradient for Cl and reversed the direction of delta psi Cl. Under these conditions, passive absorption of Cl was not feasible, so that Cl can only be absorbed by a nonconductive transport carrier. We surmise that, in the face of such changes in delta psi Cl as a function of luminal [NaCl], continuous transcellular Cl transport in SD could only be sustained if both electroconductive and carrier-mediated Cl transport are present in the SD.