Resting membrane potential and potassium currents in cultured parasympathetic neurones from rat intracardiac ganglia.

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Abstract
1. Whole‐cell K+ currents contributing to the resting membrane potential and repolarization of the action potential were studied in voltage‐clamped parasympathetic neurones dissociated from neonatal rat intracardiac ganglia and maintained in tissue culture. 2. Rat intracardiac neurones had a mean resting membrane potential of ‐52 mV and mean input resistance of 850 M omega. The current‐voltage relationship recorded during slow voltage ramps indicated the presence of both leakage and voltage‐dependent currents. The contribution of Na+, K+ and Cl‐ to the resting membrane potential was examined and relative ionic permeabilities PNa/PK = 0.12 and PCl/PK < 0.001 were calculated using the Goldman‐Hodgkin‐Katz voltage equation. Bath application of the potassium channel blockers, tetraethylammonium ions (TEA; 1 mM) or Ba2+ (1 mM) depolarized the neurone by approximately 10 mV. Inhibition of the Na(+)‐K+ pump by exposure to K(+)‐free medium or by the addition of 0.1 mM ouabain to the bath solution depolarized the neurone by 3‐5 mV. 3. In most neurones, depolarizing current pulses (0.5‐1 s duration) elicited a single action potential of 85‐100 mV, followed by an after‐hyperpolarization of 200‐500 ms. In 10‐15% of the neurones, sustained current injection produced repetitive firing at maximal frequency of 5‐8 Hz. 4. Tetrodotoxin (TTX; 300 nM) reduced, but failed to abolish, the action potential. The magnitude and duration of the TTX‐insensitive action potential increased with the extracellular Ca2+ concentration, and was inhibited by bath application of 0.1 mM Cd2+. The repolarization rate of the TTX‐insensitive action potential was reduced, and after‐hyperpolarization was replaced by after‐depolarization upon substitution of internal K+ by Cs+. The after‐hyperpolarization of the action potential was reduced by bath application of Cd2+ (0.1 mM) and abolished by the addition of Cd2+ and TEA (10 mM). 5. Depolarization‐activated outward K+ currents were isolated by adding 300 nM TTX and 0.1 mM Cd2+ to the external solution. The outward currents evoked by step depolarizations increased to a steady‐state plateau which was maintained for > 5 s. The instantaneous current‐voltage relationship, examined under varying external K+ concentrations, was linear, and the reversal (zero current) potential shifted in accordance with that predicted by the Nernst equation for a K(+)‐selective electrode. The shift in reversal potential of the tail currents as a function of the extracellular K+ concentration gave a relative permeability, PNa/PK = 0.02 for the delayed outward K+ channel(s).(ABSTRACT TRUNCATED AT 400 WORDS)