Ba²⁺-inhibitable⁸⁶Rb⁺ fluxes across membranes of vesicles from toad urinary bladder

Summary

⁸⁶Rb⁺ fluxes have been measured in suspensions of vesicles prepared from the epithelium of toad urinary bladder. A readily measurable barium-sensitive, ouabain-insensitive component has been identified; the concentration of external Ba²⁺ required for half-maximal inhibition was 0.6mm. The effects of externally added cations on⁸⁶Rb⁺ influx and efflux have established that this pathway is conductive, with a selectivity for K⁺, Rb⁺ and Cs⁺ over Na⁺ and Li⁺. the Rb⁺ uptake is inversely dependent on external pH, but not significantly affected by internal Ca²⁺ or external amiloride, quinine, quinidine or lidocaine. It is likely, albeit not yet certain, that the conductive Rb⁺ pathway is incorporated in basolateral vesicles oriented right-side-out. It is also not yet clear whether this pathway comprises the principle basolateral K⁺ channel in vivo, and that its properties have been unchanged during the preparative procedures. Subject to these caveats, the data suggest that the inhibition by quinidine of Na⁺ transport across toad bladder does not arise primarily from membrane depolarization produced by a direct blockage of the basolateral channels. It now seems more likely that the quinidine-induced elevation of intracellular Ca²⁺ activity directly blocks apical Na⁺ entry.