Voltage‐dependent sodium and calcium currents in cultured parasympathetic neurones from rat intracardiac ganglia.

1. Depolarization‐activated Na+ and Ca2+ currents underlying the rising phase of the action potential in mammalian parasympathetic ganglion cells were investigated in voltage‐clamped neurones dissociated from neonatal rat intracardiac ganglia and maintained in tissue culture. 2. A current component isolated by replacing intracellular K+ with Cs+ or arginine and adding 0.1 mM Cd2+ to the external solution was dependent on extracellular [Na+] and reversibly blocked in the presence of 300 nM tetrodotoxin (TTX). Peak amplitudes of Na+ currents elicited by step depolarization from a holding potential of ‐100 mV were 351 +/‐ 18 pA/pF (140 mM extracellular Na+). 3. The sodium current‐voltage (I‐V) curve exhibited a threshold for activation at ‐40 mV and reached a maximum at ‐10 mV. The Na+ conductance increased sigmoidally with increasing depolarization reaching half‐maximal activation at ‐25 mV, with a maximum slope corresponding to 7.5 mV per e‐fold change in conductance. 4. During a maintained depolarization, Na+ currents turned on and then decayed (inactivated) with an exponential time course. The time constant of inactivation was voltage dependent decreasing from 0.85 ms at ‐20 mV to 0.3 ms at +60 mV (23 degrees C). The steady‐state inactivation of the Na+ conductance was voltage‐dependent with half‐inactivation occurring at ‐61 mV and near‐complete inactivation at ‐20 mV. Recovery from inactivation also followed an exponential time course with a time constant that increased at depolarized membrane potentials. 5. A voltage‐ and Ca(2+)‐dependent current was isolated by replacement of intracellular K+ with either Cs+ or arginine and of extracellular Na+ with tetraethylammonium and the addition of TTX. Extracellular Ba2+ or Na+ (in the absence of external divalent cation) could substitute for Ca2+. Peak Ca2+ current increased with increasing extracellular [Ca2+] and above 10 mM (Kd approximately 4 mM) approached saturation. The peak Ca2+ current density was 45 +/‐ 4 pA/pF (2.5 mM‐extracellular Ca2+). 6. The Ca2+ I‐V relation exhibited a high threshold for activation (‐20 mV) and reached a maximum at +20 mV. Changing the holding potential from ‐100 to ‐40 mV did not alter the I‐V relationship. Peak Ca2+ conductance increased sigmoidally with increasing depolarization reaching half‐maximal activation at ‐4 mV, with a maximal slope of 4 mV per e‐fold change in Ca2+ conductance. 7. The kinetics of activation and inactivation of the Ca2+ current were voltage dependent and the time course of inactivation was fitted by the sum of two exponentials.(ABSTRACT TRUNCATED AT 400 WORDS)