Abstract
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 (=
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 AgNO
3 to about 1 mM [Ag]
i strongly potentiated
I
Ag. 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
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 Ca
2+ 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.