, Volume 195, Issue 1, pp 43-51

Stimulation of Transepithelial Na+ Current by Extracellular Gd3+ in Xenopus laevis Alveolar Epithelium

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Abstract

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 (I ami) across Xenopus alveolar epithelium was downregulated by high apical Na+ concentrations. Apical application of gadolinium (Gd3+) increased I sc in a dose-dependent manner (EC 50 = 23.5 µM). The effect of Gd3+ was sensitive to amiloride, which indicated the amiloride-sensitive transcellular Na+ transport to be upregulated. Benz-imidazolyl-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 sc 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 (NP o) 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.