The mechanism of electrodiffusive K+ transport in leaky epithelia and some of its consequences for anion transport

The ventricular membrane of the epithelium from the choroid plexus ofNecturus maculosus was probed with double-barrelled ion-selective microelectrodes while the ventricular bathing solution was changed abruptly. The transient states induced by increasing the external concentration of K⁺ or by the application of ouabain showed that the passive movements of K⁺ across the ventricular membrane were electro-diffusive and could be described by the Goldmann equation and one constant permeability (P_K) of 24×10^–6 cm s^–1. The passive efflux was balanced by a ouabain-sensitive influx.P_K was different in different steady states; when the cell was acidified by half a pH unit by increasing CO₂ in the bathing solutions from 1 to 5%, thenP_K decreased to 13×10^–6 cm s^–1. Removal of Cl^– from the bathing solution increasedP_K by 50% and reduction of Cl^– transport by furosemide did not alterP_K, consequently major movements of K⁺ were independent of Cl^– movements. Depolarizations of the cell caused an increase in the cellular HCO₃^– concentration due to an electrodiffusive permeability (P_HCO3), the value of which was estimated to 17×10^–6 cm s^–1.