Highly selective blockade of the frog skin sodium channels by monovalent copper cations

.5 mM Cu+ added to the mucosal side of frog skin caused rapid reversible inhibition of short-circuit current while no effect of Cu+ could be observed at the serosal side. In both cases Cu2+ was reduced to Cu+ by adding 10 mM ascorbic acid. Cu+ being similar to Na+ both in charge and crystal radius (0.096 and 0.095 nm, respectively) appears to block Na+ channels in the apical membrane. Cu2+ being of a smaller size (crystal radius 0.072 nm) was ineffective at the mucosal side causing only a rather slow irreversible inhibition of Na+ transport when added to the serosal bathing solution.