Resting membrane potential and potassium currents in cultured parasympathetic neurones from rat intracardiac ganglia
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 MΩ. 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 P(Na)/P(K) = 0.12 and P(Cl)/P(K) < 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 ~ 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 m V, 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 potentia] 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, P(Na)/P(K) = 0.02 for the delayed outward K+ channel(s). 6. The outward K+ current was reduced by 35% when extracellular Ca2+ was replaced by Mg2+, or when Cd2+ (0.1 mM) was added to the external solution. These data suggest that the outward K+ current in rat parasympathetic cardiac neurones is comprised of at least two components: a voltage-dependent delayed rectifying K+ current (I(Kv)) and a Ca2+-activated K+ current (I(KCa)) The macroscopic K+ current showed no rapidly activating and inactivating component. 7. The activation kinetics of the outward K+ current were voltage dependent, with the rate of activation increasing at more depolarized potentials. The half-time to peak K+ current amplitude was reduced from 4.7 ms at OmV to 3.2 ms at + 60 mV. The slow decline of the outward current amplitude during prolonged depolarization followed an exponential time course with a time constant of decay of approximately 10 s at + 60 mV. This slow 'inactivation' of K+ currents was accelerated by increasing depolarization.