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Abstract

The mechanism of HCO⁻₃ exit from rabbit renal proximal tubule S3 segments was investigated. Isolated tubules were perfused luminally and peritubularly with test solutions and cell pH (pH_i), cell Cl⁻ activity ([Cl⁻]_i) and cell Na⁺ activity ([Na⁺]_i) were measured with ion-selective microelectrodes. From the response of pH_i and [Cl⁻]_i to changes in bath Cl⁻ or HCO⁻₃ concentrations a Cl⁻/HCO⁻₃ exchanger was identified in the basolateral cell membrane. It was reversibly inhibited by millimolar concentrations of the disulfonic stilbene SITS (4-acetamido-4′-isothiocyanato-stilbene-2,2′-disulfonic acid). Cell potential measurements and preliminary determinations of initial ion flux rates suggested a stoichiometry of Cl⁻ to HCO⁻₃ flux near 1.0. The transport rate appeared to saturate already at low bath Cl⁻ concentrations (≈30 mmol/l), but it was independent of bath pH in the range of 7.4-6.4. Cl⁻/HCO⁻₃ exchange was not directly coupled to Na⁺ flux although in approximately half of the experiments long-term incubation in Na⁺-free solutions indirectly inhibited the exchanger. Sudden application of SITS under control conditions revealed that the exchanger normally facilitates the exit of HCO⁻₃ from cell to interstitium at the expense of Cl⁻ uptake into the cell. How Cl⁻ ions recirculate towards the peritubular surface is presently not known.