Ba2+‐induced changes in the Na+‐ and K+‐permeability of the isolated frog skin

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Abstract
Addition of the K+‐channel blocking agent Ba2+ to the basolateral solution (in a concentration which is assumed to block the K+‐flux via the K+‐channels completely) resulted initially in a two‐thirds reduction in the short‐circuit current (SCC), followed by a complete recovery of the SCC. To examine the reason for this recovery, experiments were carried out which made it possible to calculate the Na+‐permeability of the apical membrane (PaNa) and the K+‐permeability of the basolateral membrane (PbK). The presence of Ba2+ had no significant effect on the cell volume and the cellular Na+‐ and K+‐concentration. Addition of Ba2+ resulted in a depolarization of the intracellular potential (Vscc) from a control value of ‐ 76.3 ± 2.8 mV to ‐ 15.1 ± 1.7 mV. Although a complete recovery in the SCC was observed, Vscc did not recover. The K+‐flux across the basolateral membrane was estimated from washout experiments. The washout of 42K+ (the K+‐efflux) could be described by a single exponential component with a half time of 30–70 min. The addition of Ba2+ during the washout resulted in a transient decrease in 42K+‐efflux from the epithelium. From Vscc and the cellular K+ and Na+‐concentration and the coupling ratio of the Na‐K pump, it was found that Na+‐permeability of the apical membrane was 6.5.10‐7 cm.s‐1 before the addition of Ba2+ and 1.7.10‐6 cm.s‐1 when the SCC had recovered after the addition of Ba2+ and PbK changed from 8.8.10‐6 cm.s‐1 to 1.5.10‐6 cm.s‐1. Thus, the observed recovery in SCC was due to a considerable increase in Na+‐permeability of the apical membrane and the presence or appearance of a small Ba2+‐insensitive K+‐permeability in the basolateral membrane.

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