Paradoxical stimulation of a DEG/ENaC channel by amiloride

Extracellular amiloride inhibits all known DEG/ENaC ion channels, including BNC1, a proton-activated human neuronal cation channel. Earlier studies showed that protons cause a conformational change that activates BNC1 and exposes residue 430 to the extracellular solution. Here we demonstrate that, in addition to blocking BNC1, amiloride also exposes residue 430. This result suggested that, like protons, amiloride might be capable of activating the channel. To test this hypothesis, we introduced a mutation in the BNC1 pore that reduces amiloride block, and found that amiloride stimulated these channels. Amiloride inhibition was voltage-dependent, suggesting block within the pore, whereas stimulation was not, suggesting binding to an extracellular site. These data show that amiloride can have two distinct effects on BNC1, and they suggest two different interaction sites. The results suggest that extracellular amiloride binding may have a stimulatory effect similar to that of protons in BNC1 or extracellular ligands in other DEG/ENaC channels.