Short-term exposure to waterborne free silver has acute effects on membrane current of Xenopus oocytes

Waterborne free silver can cause osmo- and ionoregulatory disturbances in freshwater organisms. The effects of a short-term exposure to extracellular Ag+ ions on membrane currents were investigated in voltage-clamped defolliculated Xenopus oocytes. At a holding potential of − 60 mV, ionic silver (1 μM Ag+) increased inward currents (=IAg) from − 8 ± 2 nA to − 665 ± 41 nA (n = 74; N = 27). IAg activated within 2 min of silver exposure and then rose impetuously. This current was largely reversible by washout and repeatable. IAg reversed around − 30 mV and rectified slightly at more positive potentials. Na+-free bath conditions reduced the silver-induced current to a smaller but sustained current. The response to silver was abolished by the Cl− channel blockers DIDS and SITS, whereas niflumic acid strongly potentiated IAg. Intraoocyte injection of AgNO3 to about 1 mM [Ag]i strongly potentiated IAg. Extracellular application of either dithiothreitol (DTT), a compound known to reduce disulfide bridges, or l-cysteine abolished Ag+-activated increase of membrane current. In contrast, n-ethylmaleimide (NEM) which oxidizes SH-groups potentiated IAg. Hypoosmotic bath solution significantly increased IAg whereas hyperosmolar conditions attenuated IAg. The activation of IAg was largely preserved after chelation of cytosolic Ca2+ ions with BAPTA/AM. Taken together, these data suggest that Xenopus oocytes are sensitive to short-term exposure to waterborne Ag+ ions and that the elicited membrane currents result from extra- and intracellular action of Ag+ ions on peptide moieties at the oocyte membrane but may also affect conductances after internalization.