Ba2+-sensitive potassium permeability of the apical membrane in newt kidney proximal tubule.

The apical membrane K+ permeability of the newt proximal tubular cells was examined in the doubly perfused isolated kidney by measuring the apical membrane potential change (Va change) during alteration of luminal K+ concentration and resultant voltage deflections caused by current pulse injection into the lumen. Va change/decade for K+ was 50 mV at K+ concentration higher than 25 mM, and the resistance of the apical membrane decreased by 58% of control when luminal K+ concentration was increased from 2.5 to 25 mM. Ba2+ (1 mM in the lumen) reduced Va change/decade to 24 mV and increased the apical membrane resistance by 70%. These data support the view that Ba2+-sensitive K+ conductance exists in the apical membrane of the newt proximal tubule. Furthermore, intracellular K+ activity measured by K+-selective electrode was 82.4 +/- 3.6 meq/liter, which was higher than that predicted from the Nernst equation for K+ across both cell membranes. Thus, it is concluded that cell K+ passively diffuses, at least in part, through the K+ conductive pathway of the apical membrane.