Asymmetry in the ability of low concentrations of K to support Na conduction gives rise to a Mg-independent form of inward rectification. (A) Unblocked HCN2 currents were collected and processed as described in Fig. 8 except the major charge carrier in both internal and external solutions was Na with K included at only 5 mM. Records are averages of 10 “active” sweeps before subtraction of the averaged cognate “leak” sweeps (not shown for simplicity). (B) Leak-subtracted records obtained in response to steps from −140 mV to potentials ranging from −200 to +200 mV with the later sweep highlighted in blue. (C) Plateau tail current amplitudes (measured 1 ms after the beginning of each test step) plotted versus the corresponding step potential.