Chloride transport across the basolateral cell membrane of the Necturus proximal tubule: Dependence on bicarbonate and sodium

The transport of chloride across the Necturus proximal tubule cell was studied in the doubly-perfused kidney using conventional, chloride-sensitive and pH-sensitive microelectrodes. Lowering chloride activity in the basolateral solution results in a reduction in intracellular Cl^- activity (a_Clⁱ). This reduction in a_Clⁱ is inhibited by removing either HCO₃^- or Na⁺ from the perfusion solution, indicating that both HCO₃^- and Na⁺ are required for Cl^- movement across the basolateral cell membrane. Reducing either HCO₃^- or Na⁺ in the basolateral solution causes an increase in a_Clⁱ. Thus changes in either Na⁺ or HCO₃^- chemical gradients across the basolateral cell membrane significantly affect chloride movement. Changing intracellular pH by means of NH₄Cl exposure results in an increase in a_Clⁱ followed by a sharp decrease when NH₄Cl is removed. These changes in intracellular chloride do not occur in the absence of HCO₃^-. Likewise, the decrease in a_Clⁱ following NH₄Cl treatment requires the presence of Na⁺ in the basolateral solution. We conclude that chloride is transported across the basolateral cell membrane in exchange for both Na⁺ and HCO₃^-. Our results also support the presence of a Na⁺/Cl^- contransport mechanism on the apical cell membrane.