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

摘要

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 mu M Ag+) increased inward currents (=I-Ag) from -8 +/- 2 nA to -665 +/- 41 nA (n=74; N=27). I-Ag activated within 2 min of silver exposure and then rose impetuously. This current was largely reversible by washout and repeatable. I-Ag 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 I-Ag. Intraoocyte injection of AgNO3 to about 1 mM [Ag]; strongly potentiated I-Ag. Extracellular application of either dithiothreitol (DT'T), 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 I-Ag. Hypoosmotic bath solution significantly increased I-Ag whereas hyperosmolar conditions attenuated I-Ag. The activation of I-Ag. 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. (c) 2006 Elsevier B.V. All rights reserved.