The S664 and S667 phosphorylation sites regulate the voltage dependence of current activation, whereas the S671 phosphorylation site regulates the peak I_Na density. Currents were recorded as described in the legend to Fig. 3. (A–D) The voltage dependence of current activation is shifted toward more depolarized potentials in cells expressing Na_V1.5-S664A (A), Na_V1.5-S664E (A), Na_V1.5-S667A (B), or Na_V1.5-S667E (B) than in cells expressing Na_V1.5-WT, whereas no significant differences are observed with the Na_V1.5-T670 (C) or Na_V1.5-S671 (D) phosphomutants. (E–H) The mean ± SEM peak I_Na densities are plotted as a function of test potential. The peak I_Na density is reduced in cells expressing Na_V1.5-S671E (H), compared with cells expressing Na_V1.5-WT, whereas no significant differences are observed with the other phosphomutants. Current densities, time- and voltage-dependent properties, and statistical comparisons across groups are provided in Table 4. G_Na, sodium conductance.