Action potential repolarization may involve a transient, Ca2+-sensitive outward current in a vertebrate neurone.

Repolarization of the action potential in squid axon1 and several types of neurones2-4 involves a voltage-activated potassium (K+) current. Voltage clamp analysis has demonstrated that this current has rapid activation kinetics1,3-5. In several neuronal types, the same technique has also revealed a slowly activated K+ current that is calcium (Ca2+)-sensitive3,5-10. This slow Ca2+-activated K+ current is the major current underlying the late, slower portion of the after-hyperpolarization following an action potential11-14. In several muscle types, fast, transient Ca2+-dependent K+ currents have been described15-17 which may contribute to repolarization of the action potential. Rapidly activating, Ca2+-dependent K+ currents have been observed in sympathetic neurones of the bullfrog and it has been suggested that they contribute to action potential repolarization of those neurones8,9,18. We have studied the membrane currents in bullfrog sympathetic neurones using voltage clamp methods and report here a transient outward current that appears to be composed of two separate currents. One of those currents is a transient, Ca2+-sensitive outward current as indicated by a significant reduction of the current by treatments that reduce or block Ca2+ entry (Mn2+, Cd2+, Co2+, Mg2+ or Ca2+-free Ringer). Such treatments also decreased the rate of action potential repolarization. The results suggest that this current is involved in repolarization of the action potential and consequently may regulate Ca2+ entry into the neurone during spike activity.