Stimulation of transepithelial Na+ current by extracellular Gd3+ in Xenopus laevis alveolar epithelium

In the present study we investigated the effect of extracellular gadolinium on amiloride-sensitive Na+ current across Xenopus alveolar epithelium by Ussing chamber experiments and studied its direct effect on epithelial Na+ channels with the patch-clamp method. As observed in various epithelia, the short-circuit current (I-SC) and the amiloride-sensitive Na+ current l across Xenopus alveolar epithelium was downregulated by high apical Na+ concentrations. Apical application of gadolinium (Gd3+) increased I-SC in a dose-dependent manner (EC50 23.5 muM). The effect of Gd3+ was sensitive to amiloride, which indicated the amiloride-sensitive transcellular Na+ transport to be upregulated. Benzimidazolyl-guanidin (BIG) and p-hydroxy-mercuribenzonic-acid (PHMB) probably release apical Na+ channels from Na+-dependent autoregulating mechanisms. BIG did not stimulate transepithelial Na+ currents across Xenopus lung epithelium but, interestingly, it prevented the stimulating effect of Gd3+ on transepithelial Na+ transport. PHMB increased I, and this stimulation was similar to the effect of Gd3+. Co-application of PHMB and Gd3+ had no additive effects on I-SC. In cell-attached patches on Xenopus oocytes extracellular Gd3+ increased the open probability (NPO) of Xenopus epithelial sodium channels (ENaC) from 0.72 to 1.79 and decreased the single-channel conductance from 5.5 to 4.6 pS. Our data indicate that Xenopus alveolar epithelium exhibits Na+-dependent non-hormonal control of transepithelial Na+ transport and that the earth metal gadolinium interferes with these mechanisms. The patch-clamp experiments indicate that Gd3+ directly modulates the activity of ENaCs.