Electrical effects of potassium and bicarbonate on proximal tubule cells ofNecturus

Summary The effects of stepwise concentration changes of K⁺ and HCO₃^– in the basolateral solution on the basolateral membrane potential (V_bl) of proximal tubule cells of the doubly-perfusedNecturus kidney were examined using conventional microelectrodes. Apparent transference numbers were calculated from changes inV_bl after alterations in external K⁺ concentration from 1.0 to 2.5mm (t_{K, 1.0–2.5}), 2.5 to 10, and in external HCO₃^– concentration (at constant pH) from 5 to 10mm (t_{HCO3, 5–10}), 10 to 20, or 10 to 50.t_{K, 2.5–10} was 0.38±0.02 under control conditions but was sharply reduced to 0.08±0.03 (P>0.001) by 4mm Ba⁺⁺. This concentration of Ba⁺⁺ reducedV_bl by 9±1 mV (at 2.5 external K⁺). Perfusion with SITS (5×10^–4m) for 1 hr hyperpolarizedV_bl by 10±3 mV and increasedt_{K, 2.5–10} significantly to 0.52±0.01 (P<0.001). Ba⁺⁺ application in the presence of SITS depolarizedV_bl by 22±3 mV. In control conditionst_{HCO3, 10–50} was 0.63±0.05 and was increased to 0.89±0.07 (P<0.01) by Ba⁺⁺ but was decreased to 0.14±0.02 (P<0.001) by SITS. In the absence of apical and basolateral chloride, the response ofV_bl to bicarbonate was diminished but still present (t_{HCO3, 10–20} was 0.35±0.03). Intracellular pH, measured with liquid ion-exchange microelectrodes, increased from 7.42±0.19 to 7.57±0.17 (P<0.02) when basolateral bicarbonate was increased from 10 to 20mm at constant pH. These data show that the effects of bicarbonate onV_bl are largely independent of effects on the K⁺ conductance and that there is a significant current-carrying bicarbonate pathway in the basolateral membrane. Hence, both K⁺ and HCO₃^– gradients are important in the generation ofV_bl, and their relative effects vary reciprocally.