Figure 10. Comparison of the voltage dependence of activation and inactivation of I_Na and I_DA.

A and B, families of superimposed current traces and voltage protocols used to study the voltage dependence of activation (A) and inactivation (B). Measurements were made of I_Na at its peak and activation of I_DA 30 ms after stepping to various potentials from a holding potential of −100 mV; at 30 ms I_Na is largely inactivated (cf. Fig. 3) but I_DA is close to its peak value. In B the test potential was +10 mV, close to the reversal potential (V_rev) for I_DA, thus minimizing its contribution during measurements of I_Na inactivation. Again, a 30 ms pulse allows I_Na to inactivate leaving I_DA close to its peak value. C, open and filled circles show, respectively, normalized I_Na amplitude from B (h_Na) and relative Na⁺ conductance calculated from I_Na amplitudes in A (m_Na) as described for Fig. 3C, respectively. Filled and open triangles show relative tail current amplitudes from A (m_Cat) and B (h_Cat), respectively, measured by fitting single exponential functions and extrapolating to time zero at the end of each test pulse. Data points were fitted by Boltzmann functions with the following best fit parameters: I_Na activation: V_1/2 = −11.1 mV; k = −8.3 mV; I_Na inactivation: V_1/2 = −74.5 mV, k = 8.8 mV; I_DA activation: V_1/2 = −45.8 mV, k = −5.0 mV; I_DA inactivation: V_1/2 = −69.2 mV, k = 8.1 mV. This cell was in 130 mm Na⁺, divalent cation-free solution; I_HA was small (-145 and +155 pA at −100 and +90 mV, respectively).