A quantitative description of the sodium current in the rat sympathetic neurone

The somata of rat sympathetic neurones were voltage-clamped in vitro at 27 degrees C using separate intracellular voltage and current micro-electrodes. Na currents were isolated from other current contributions by using: Cd to block the Ca current (ICa) and the related Ca-dependent K current (IK(Ca)), and external tetraethylammonium to suppress the delayed rectifier current (IK(V) ). The holding potential was maintained at -50 mV to inactivate the fast transient K current (IA) when the IA contamination was unacceptable. Step depolarizations beyond -30 mV activated a fast, transient inward current carried by Na ions; it was suppressed by tetrodotoxin and was absent in Na-free solution. Once activated, INa declined exponentially to zero with a voltage-dependent time constant. The underlying conductance, gNa, showed a sigmoidal activation between -30 and +10 mV, with half-activation at -21.1 mV and a maximal value (mean gNa) of 4.44 microS per neurone. The steady-state inactivation level, h infinity, varied with membrane potential, ranging from complete inactivation at -30 mV to minimal inactivation at about -90 mV with a midpoint at -56.2 mV. Double-pulse experiments showed that development and removal of inactivation followed a single-exponential time course; tau h was markedly voltage-dependent and ranged from 46 ms at -50 mV to 2.5 ms at -100 mV. Besides the fast inactivation, the Na conductance showed a slow component of inactivation. The steady-state value, s infinity, was maximal at -80 mV and minimal at -40 mV. The removal of slow inactivation is a two-time-constant process, the first with a time constant in the order of hundreds of milliseconds and the second with a time constant of seconds. Slow inactivation onset appeared to be a faster process than its removal. When slow inactivation was fully removed the peak INa increased by a factor of 1.8. INa was well described by assuming it to be proportional to m3h. The temperature dependence of peak INa, tau m and tau h was studied in the temperature range 17-27 degrees C and found similar to that reported for other preparations. The Q10 of these parameters allowed the reconstruction of the INa kinetic properties at 37 degrees C.