Figure 3

Slow inactivation properties of WT, R1451C and R1451L channels. (A) Onsets of slow inactivation of WT (n = 10), R1451C (n = 7), and R1451L (n = 8) channels. The entry into slow inactivation was measured using a double-pulse protocol. The cells were first depolarized to −10 mV for a variable duration (1 ms to 30 s) from a holding potential of −120 mV. Following a 20-ms interpulse at −120 mV to allow complete recovery from fast inactivation, a brief 5-ms test pulse to −10 mV was applied to measure the available Na⁺ current (fraction not slow inactivated). The resulting curves were fitted to a double exponential equation, which yielded two time constants: τ_fast and τ_slow. There was no significant difference in the time constants of WT, R1451C, and R1451L channels. (B,C) Recovery from slow inactivation of WT (n = 7), R1451C (n = 6), and R1451L (n = 7) channels. The cells were depolarized to −10 mV for 4 s (B) or 30 s (C) from a holding potential of −120 mV to inactivate all Na⁺ channels. Test pulses were then applied for 20 ms to −10 mV to measure current amplitudes, with an interval ranging from 0.5 ms to 10 s. The resulting curves were fitted to a double exponential equation, which yielded two time constants: τ_fast and τ_slow. After a short (4 s) conditioning pulse, there was no significant difference in the time constants of WT, R1451C and R1451L channels. After a longer (30 s) conditioning pulse, τ_slow of R1451C (726 ± 97 ms) and R1451L (1978 ± 206 ms) is larger than τ_slow of WT (382 ± 64 ms).