Figure 2. BTX prevents the slow decay of hNav1.5-CW mutant Na⁺ currents. (A) A family of superimposed hNav1.5-CW Na⁺ current traces was recorded with 4-second test pulses, ranging from −120 to +50 mV. Individual current traces at −80, 0 and +50 mV are labeled. The Na⁺ reversal potential is near −10 mV. (B) A family of superimposed hNav1.5-CW Na⁺ current traces were recorded with 4-second test pulses ranging from −140 to +50 mV in the presence of 5 μM BTX within the pipette solution. Notice that BTX-modified Na⁺ currents were maintained throughout the duration of the pulse. Individual current traces at −130, −100, −50, 0 and +50 mV are labeled. Before this record was taken, 400 repetitive 20-ms pulses at +50 mV were applied in a frequency of 2Hz to facilitate BTX binding with the open channel. The Na⁺ reversal potential is near −20 mV. Different cells were used in each panel. (C) With 5 μM BTX within the pipette, the Na⁺ conductance (closed circles) was clearly shifted to the hyperpolarizing direction as compared with that without BTX presence (dashed line as from Fig. 1B). The midpoint voltage (V_0.5) and slope (k) of the function were −109.7 ± 1.0 mV and 7.6 ± 0.9 mV, respectively (n = 5). Currents were recorded as shown in Figure 2B and the conductance was calculated as described Figure 1B. Holding potential was set at −140 mV.