D1 agonist-induced suppression of Na⁺ current occurred without alterations in the voltage dependence of activation or inactivation. A, The voltage-dependence of activation of whole-cell Na⁺current was not altered by SKF 38393. Relative conductance is plotted as a function of test potential. Activation curves were determined by voltage-clamping cells at −70 mV and applying voltage steps to potentials of −70 to +60 mV. To ensure that channel inactivation was removed completely, a lower frequency (0.1–0.2 Hz) was used. Peak currents were obtained at every test potential. Conductance was derived according to the equation G_Na =I_Na,peak/(V −V_Na), whereG_Na is conductance,I_Na,peak is peak Na⁺current, V is test potential, andV_Na is the Na⁺ reversal potential. The steady-state activation curves (m-curve) were determined by plottingG_Na/G_Na,maxversus membrane potential as the Boltzmann equationm = [1 + exp (V − V_½)/k] − 1 whereV_½ is the midpoint of them-curves and k is the curve slope factor.B, The voltage dependence of inactivation was not altered by SKF 38393. Relative current is plotted as a function of prepulse potential. Steady-state inactivation curves were determined with a conditioning prepulse protocol. Cells were voltage-clamped at −70 mV. Membrane potential was first stepped (prepulse) to potentials from −120 to −40 mV (200 msec duration) before evoking Na⁺ currents with a step to −20 mV. The steady-state inactivation curves (h-curves;h = I_{Na, peak}/I_Na,max vs conditioning potential) were fitted to a Boltzmann equation h = [1 + exp (V −V_½)/k] − 1, whereV_½ is the half-point of the inactivation curve and k is the slope factor. Time constants for inactivation were obtained by fitting traces to a single exponential.