Barium blocks cell membrane and tight junction conductances in Necturus gallbladder epithelium. Experiments with an extended impedance analysis technique.

The site and concentration dependence of the blocking effect of Ba2+ on Necturus gallbladder epithelium has been investigated. A new approach was used which combines time-dependent electrical cell coupling analysis with intermittently performed measurements of transepithelial and apparent intracellular impedance. From the coupling pulse data the sum of apical and basolateral membrane conductances is obtained, which is then held constant during fitting of the impedance data. This combination technique yields more reliable estimates of apical and basolateral membranes resistances (Ra, Rbl) and of tight junction resistance (Rj) than our previous impedance analysis technique. Using the new approach we have found that luminal Ba2+ concentrations between 0.5 and 1.0 mmol/l increase Ra with saturation-type kinetics without affecting Rbl and Rj, while higher luminal Ba2+ concentrations progressively increase Rj. Corresponding effects were observed under serosal Ba2+. The results validate the new impedance analysis approach and demonstrate that millimolar concentrations of Ba2+ block tight junction conductances. Accordingly, Ba2+ can no longer be considered a tool to exclusively alter cell membrane resistances in epithelia.