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Figure 4 Macroscopic Na⁺ Currents of M1875T Channels

(A) Representative whole-cell current traces of wild-type (WT) and M1875T sodium (Na⁺) channels. Cells were transfected with human β1-subunit (protocol shown as an inset). (B) Voltage dependence of inactivation time constants. The time course of inactivation was fit with a 2 exponential equation: I/I_max = A_f x exp(–t/_f) + A_s x exp(–t/_s). Lower and upper bundles of symbols indicate fast (_f) and slow (_s) time constant values, respectively. Statistically significant differences are indicated (*p < 0.05, **p < 0.01). (C) Average current-voltage relationship for WT and M1875T channels. The current is normalized to cell capacitance to give a measure of Na⁺ current density. Asterisks indicate the voltages at which the current density was statistically different (*p < 0.05). (D) Average peak Na⁺ current density of WT and M1875T channels. The peak current density was significantly larger in M1875T (WT at –20 mV, 326.2 ± 28.2 pA/pF, n = 23; M1875T at –30 mV, 484.6 ± 49.6 pA/pF, n = 31, p < 0.05). (E) Representative Na⁺ current traces recorded in the absence or presence of 30 µmol/l tetrodotoxin. Tetrodotoxin-sensitive persistent currents were calculated by digital subtraction. M1875T channels showed no persistent inward Na⁺ currents.