Electrical effects of potassium and bicarbonate on proximal tubule cells of Necturus

The effects of stepwise concentration changes of K+ and HCO3- in the basolateral solution on the basolateral membrane potential (Vbl) of proximal tubule cells of the doubly-perfused Necturus kidney were examined using conventional microelectrodes. Apparent transference numbers were calculated from changes in Vb1 after alterations in external K+ concentration from 1.0 to 2.5 mM (tK, 1.0-2.5), 2.5 to 10, and in external HCO3- concentration (at constant pH) from 5 to 10 mM (tHCO3, 5-10), 10 to 20, or 10 to 50. tK, 2.5-10 was 0.38 +/- 0.02 under control conditions but was sharply reduced to 0.08 +/- 0.03 (P greater than 0.001) by 4 mM Ba++. This concentration of Ba++ reduced Vb1 by 9 +/- 1 mV (at 2.5 external K+). Perfusion with SITS (5 X 10(-4)M) for 1 hr hyperpolarized Vb1 by 10 +/- 3 mV and increased tK, 2.5-10 significantly to 0.52 +/- 0.01 (P less than 0.001). Ba++ application in the presence of SITS depolarized Vb1 by 22 +/- 3 mV. In control conditions tHCO3, 10-50 was 0.63 +/- 0.05 and was increased to 0.89 +/- 0.07 (P less than 0.01) by Ba++ but was decreased to 0.14 +/- 0.02 (P less than 0.001) by SITS. In the absence of apical and basolateral chloride, the response of Vb1 to bicarbonate was diminished but still present (tHO3, 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 less than 0.02) when basolateral bicarbonate was increased from 10 to 20 mM at constant pH. These data show that the effects of bicarbonate on Vb1 are largely independent of effects on the K+ conductance and that there is a significant current-carrying bicarbonate pathway in the basolateral membrane.(ABSTRACT TRUNCATED AT 250 WORDS)