Apical membrane sodium and chloride entry during osmotic swelling of renal (A6) epithelial cells

To assess the role of chloride in cell volume and sodium transport regulation, we measured cell height changes (CH), transepithelial chloride and sodium fluxes, and intracellular chloride content during challenge with hyposmotic solutions under open circuit (OC) conditions. CH maximally increased following hyposmotic challenge within 5 minutes. The change in CH was smaller under short circuit (SC) conditions or following replacement of chloride in the mucosal solution by gluconate or cyclamate (Cl^–-free _m ). When corrected for the osmotically inactive cell volume (30 ± 2%), CH for controls (OC) were greater than predicted for an ideal osmometer. In contrast, ACH for Cl^–-free _m or SC conditions were similar to that predicted for an ideal osmometer.Na⁺ and Cl^– mucosa-to-serosa fluxes increased following hyposmotic challenge. Chloride fluxes increased maximally within 5 min, then decreased. In contrast, the Na⁺ flux increased slowly and reached a steady state after 25 min. Under isosmotic conditions, exposure to Cl^–-free _m solutions led to decreases in the transepithelial conductance, Na⁺ flux, and CH. Chloride permeabilities in the apical and basolateral membranes were detected using the fluorescent intracellular chloride indicator MQAE.