Volume regulation in the early proximal tubule of the Necturus kidney

The ability of early proximal tubule cells of the Necturus kidney to regulate volume was evaluated using light microscopy, video analysis and conventional microelectrodes. Necturus proximal tubule cells regulate volume in both hyperand hyposmotic solutions. Volume regulation in hyperosmotic fluids is HCO₃^- dependent and is associated with a decrease in the relative K⁺ conductance of the basolateral cell membrane and a decrease in the resistance ratio, R_a/R_bl. Volume regulation in hyposmotic solutions is also dependent upon the presence of HCO₃^- but is also inhibited by 2 mm Ba²⁺ in the basolateral solution. Hyposmotic regulation is accompanied by an increase in the relative K⁺ conductance of the basolateral cell membrane and an increase in R_a/R_bl. Neither hypo- nor hyposmotic regulation have any affect on the depolarization of the basolateral cell membrane potential induced by HCO₃^- removal. We conclude that volume regulation in the early proximal tubule of the kidney involves both HCO₃^--dependent transport systems and the basolateral K⁺ conductance.