Axial heterogeneity of sodium-bicarbonate cotransport in proximal straight tubule of rabbit kidney

Intracellular microelectrodes were used to investigate rheogenic Na⁺(HCO ₃ ^– ) _n cotransport in different segments of isolated proximal straight tubule (PST) of rabbit kidney. In the first portion (S2 segment) the peritubular cell membrane potentialV _b averaged –46.0, SE±1.3 mV (n=20), while in the terminal portion (S3 segment) it averaged –68.3, SE±2.5 mV (n=10). This difference may reflect different modes of anion permeation across the peritubular cell membrane. In S2 segments, sudden 101 reduction of bath HCO ₃ ^– concentration caused a fast transient cell depolarization, V _b=–45.8, SE±1.2 mV (n=33) as expected from the presence of Na⁺(HCO ₃ ^– ) _n contransport. As the puncture site moved further distally, V _b declined and gradually changed its time course by superposition of a slower secondary depolarization. In this region the transient cell depolarization could be recuperated by inhibiting the peritubular K⁺ conductance with Ba²⁺ (1 mmol/l). In S3 segments, however, the HCO ₃ ^– -dependent transient cell depolarization was completely lost both in the absence and presence of Ba²⁺. In addition, sudden reduction of bath Na⁺ concentration did not acidify the cell, as it did in the S2 segment. The data indicate that the expression of Na⁺(HCO ₃ ^– ) _n cotransport in the peritubular cell membrane gradually diminishes towards the end of the S2 segment and is lost in the S3 segment.