A fast transient outward current in the rat sympathetic neurone studied under voltage‐clamp conditions.

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Abstract
Post-ganglionic neurons of the isolated rat superior cervical ganglion were voltage clamped at 37.degree. C using separate intracellular voltage and current micro-electrodes. Control experiments in current clamp suggested that the neuron is electronically compact, the soma and the proximal dendritic membranes being under good spatial voltage uniformity. Depolarizing voltage steps from membrane potentials near -50 mV evoked a voltage-dependent inward Na+ current, an inward Ca2+ current, a voltage-dependent outward K+ current and a Ca2+-activated K+ outward current. Depolarizations from holding potentials more negative than -60 mV elicited, besides the currents mentioned above, a fast transient inactivating outward current IA which peaked in 1-2.5 ms and then decayed to zero following an exponential time course. The IA current was primarily, if not exclusively, carried by K+. It was unaffected by removal of external Ca2+ or addition of Cd2+ and was weakly blocked by tetraethylammonium ions and partially by 4-aminopyridine. The IA current showed a linear instantaneous current-voltage relationship. Its activation ranged from -60 to 0 mV with a midpoint at -30 mV. The A conductance could be described in terms of a simple Boltzmann distribution for a single gating particle with a valency of +3. Both the development and removal of inactivation followed a single exponential time course with a voltage-dependent time constant which was large near the resting potential (42 ms at -70 mV) and small (11 ms) near -100 and -40 mV. Steady-state inactivation h.infin. ranged from -100 to -50 mV, with a midpoint at -78 mV, suggesting that .apprx. 50% of the IA channels are available at the physiological resting potential. Action potentials elicited from various holding potentials showed maximal repolarization rates dependent on the holding potential itself. This voltage dependence was in reasonably good agreement with that of h.infin. curve. In the rat sympathetic neuron, under physiological conditions, it is the IA current rather than the delayed outward current that is responsible for the fast action potential.