Microelectrode characterization of the basolateral membrane of rabbit S3 proximal tubule

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Abstract
Summary The purpose of this study was to characterize the basolateral membrane of the S3 segment of the rabbit proximal tubule using conventional and ion-selective microelectrodes. When compared with results from S1 and S2 segments, S3 cells under control conditions have a more negative basolateral membrane potential (V bl=−69 mV), a higher relative potassium conductance (t K=0.6), lower intracellular Na+ activity (A Na=18.4mm), and higher intracellular K+ activity (A K=67.8mm). No evidence for a conductive sodium-dependent or sodium-independent HCO −3 pathway could be demonstrated. The basolateral Na−K pump is inhibited by 10−4 m ouabain and bath perfusion with a potassium-free (0-K) solution. 0-K perfusion results inA Na=64.8mm,A K=18.5mm, andV bl=−28 mV. Basolateral potassium channels are blocked by barium and by acidification of the bathing medium. The relative K+ conductance, as evaluated by increasing bath K+ to 17mm, is dependent upon the restingV bl in both S2 and S3 cells. In summary, the basolateral membrane of S3 cells contains a pump-leak system with similar properties to S1 and S2 proximal tubule cells. The absence of conductive bicarbonate pathways results in a hyperpolarized cell and larger Na+ and K+ gradients across the cell borders, which will influence the transport properties and intracellular ion activities in this tubule segment.