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

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Abstract
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.