Sodium Chloride Cotransport at the Basolateral Membrane of Type II Pneumocytes

Current evidence indirectly supports the hypothesis that Na⁺/Cl⁻ cotransport occurs at basolateral membranes of type II pneumocytes. To test this hypothesis, enriched apical and basolateral plasma membrane vesicles were prepared from adult bovine type II pneumocytes. Uptake of ²²Na⁺ or ³⁶Cl⁻ by these vesicle populations was monitored over time. Using enriched basolateral vesicles, substitution of formate^- for Cl⁻ nearly eliminated ²²Na⁺ uptake, and substitution of Tris⁺ for Na⁺ significantly reduced ³⁶Cl⁻ uptake. These observations of Cl⁻-dependent ²²Na⁺ uptake and Na⁺-dependent ³⁶Cl⁻ uptake demonstrated coupling between Na⁺ and Cl⁻ absorption at the basolateral membrane. Na⁺/Cl⁻ cotransport inhibitors, furosemide (1 mM) and bumetanide (0.1 mM), also reduced ²²Na⁺ and ³⁶Cl⁻ uptakes by enriched basolateral vesicles. By contrast, furosemide and bumetanide did not affect ²²Na⁺ and ³⁶Cl⁻ uptakes by enriched apical vesicles. Collectively, these observations support localization of Na⁺/Cl⁻ cotransport to the basolateral membranes of mature type II pneumocytes.