The interactions between potassium and sodium currents in generating action potentials in the rat sympathetic neurone.

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Abstract
1. Membrane conductance parameters for the rat sympathetic neurone in vitro at 37.degree.C have been determined by two-electrode voltage-clamp analysis. The activation kinetics of two ionic currents, IA and IK(V), has been considered. Data for both currents are expressed in terms of Hodgkin-Huxley equations. 2. The isolated IA developed following third-order kinetics. The activation time constant, .tau.a, was estimated from the current time-to-peak and, for V .ltoreq. -40 mV, from the IA tail current analysis upon membrane repolarization to various potentials. The maximum .tau.a occurred at -55mV and varied from 0.26 to 0.82 ms in the range of potentials between -100 and +10 mV. The steady-state value of the variable a, corrected for inactivation, was evaluated in the voltage range from -60 to 0 mV; 14.4 mV are required to change a.infin. e-fold. Steady-state gA was voltage dependent, increasing with depolarization to a maximum of 1.40 .mu.S at +10 mV. 3. IK(V)was similarly analysed in isolation. The current proved to develop as a first-order process. .tau.n was determined by fitting a single exponential to the IK(V) rising phase and to the tail currents at the end of short depolarizing pulses. The bell-shaped voltage dependence of .tau.n exhibited a maximum (25.5 ms) at -30 mV, becoming minimal (1.8 ms) at -80 and +20 mV. The n.infin. curve was obtained (N.infin. = 0.5 at -6.54 mV; k = 8.91 mV). The mean maximum conductance, .hivin.g mV gK(V), was 0.33 .mu.S per neurone at +10 mV. 4. Single spikes have been elicited by brief current pulses at membrane potentials from -40 to -100 mV under two-electrode current-clamp conditions in normal saline and in the presence of blockers of the ICa-IK(Ca) (Cd2+) and/or IK(V) (TEA, tetraethylammonium) systems. Spike repolarization was affected by the suppression of either current in the depolarized neurone, but was insensitive to both treatments when the spike arose from holding levels negative to -75 to -80 mV, indicating that at these membrane potentials that IA current mainly, if not exclusively, contributes to the action potential falling phase. 5. The basic features of the sympathetic neurone action potential were reconstructed by simulations based on present and previous voltage-clamp characterization of the IA,IK(V) and INa conductances. These computations simulated reasonably well the various types of neuronal electrical behaviour detected experimentally under current-clamp conditions, and confirmed the increasing importance of IA in sustaining the fast spike repolarization at high membrane potential levels.