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Stimulation of transepithelial Na(+) current by extracellular Gd(3+) in Xenopus laevis alveolar epithelium.
Fronius M, Clauss W, Schnizler M.
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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 (Gd(3+)) increased I(sc) in a dose-dependent manner ( EC(50) = 23.5 microM). The effect of Gd(3+) 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 lungepithelium but, interestingly, it prevented the stimulating effect of Gd(3+) on transepithelial Na(+) transport. PHMB increased I(sc) and this stimulation was similar to the effect of Gd(3+). Co-application of PHMB and Gd(3+) had no additive effects on I(sc). In cell-attached patches on Xenopus oocytes extracellular Gd(3+) 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 Gd(3+) directly modulates the activity of ENaCs.
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