Swelling-activated cation-selective channels in A6 epithelia are permeable to large cations

Effects of basolateral monovalent cation replacements(Na⁺ byLi⁺,K⁺,Cs⁺, methylammonium, andguanidinium) on permeability to⁸⁶Rb of volume-sensitive cationchannels (VSCC) in the basolateral membrane and on regulatory volumedecrease (RVD), elicited by a hyposmotic shock, were studied in A6epithelia in the absence of apicalNa⁺ uptake. A complete and quickRVD occurred only when the cells were perfused withNa⁺ orLi⁺ saline. With both cations,hypotonicity increased basolateral ⁸⁶Rb release(R^bl_Rb), which reached a maximum after 15 min and declined back to control level. When the major cation wasK⁺,Cs⁺, methylammonium, orguanidinium, the RVD was abolished. Methylammonium induced a biphasictime course of cell thickness(T_c), with an initial decline ofT_c followed by a gradual increase.With K⁺,Cs⁺, or guanidinium,T_c increased monotonously afterthe rapid initial rise evoked by the hypotonic challenge. In thepresence of K⁺,Cs⁺, or methylammonium,R^bl_Rb remained high during most of thehypotonic period, whereas with guanidinium blockage of R^bl_Rb was initiated after 6 min ofhypotonicity, suggesting an intracellular location of the site ofaction. With all cations, 0.5 mM basolateralGd³⁺ completely blocked RVD andfully abolished the R^bl_Rb increaseinduced by the hypotonic shock. The lanthanide also blocked theadditional volume increase induced byCs⁺,K⁺, guanidinium, ormethylammonium. When pH was lowered from 7.4 to 6.0, RVD andR^bl_Rb were markedly inhibited. This studydemonstrates that the VSCCs in the basolateral membrane of A6 cells arepermeable to K⁺,Rb⁺,Cs⁺, methylammonium, andguanidinium, whereas a marked inhibitory effect is exerted byGd³⁺, protons, and possiblyintracellular guanidinium.