Selectivity of ASIC1a mutants toward alkali metals. The symbols represent relative permeability toward alkali metals of C70L channels (black circles), G438C (white circles), and G438A channels (gray circles). Whole-cell currents were elicited by a change in extracellular pH from 8.0 to 5.0 with solutions containing Na+, Li+, K+, Rb+, or Cs+ as the main permeable cations. The peak current evoked by extracellular acidification with a defined alkali metal as the main permeable cation was normalized to the peak current evoked by extracellular acidification with Na+ as the main permeable cation. The relative permeability (X+/Na+) is plotted as a function of the diameter of the nonhydrated cations (n = 8–15).