A muscarine-sensitive, slow, transient outward current in frog autonomic neurones

A slow, 4-aminopyridine-sensitive K+ current was observed in Rana pipiens autonomic neurones when they were studied under whole-cell patch-clamp recording conditions. This 'slow-A' current (ISA), which was independent of extracellular Ca2+, exhibited a similar voltage dependence to a classical A current (IA) yet inactivated with an 80-fold slower time course. Although ISA is difficult to distinguish from the delayed rectifier K+ current (IK), muscarine enhanced the current in sympathetic neurones and either enhanced or suppressed the current in parasympathetic neurones. Effects on slow transient outward currents must therefore be considered when attempting to understand cholinergic modulation of repetitive discharge in autonomic neurones.