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

Abstract

Repolarization of the action potential in squid axon¹ and several types of neurones^2–4 involves a voltage-activated potassium (K⁺) current. Voltage clamp analysis has demonstrated that this current has rapid activation kinetics^1,3–5. In several neuronal types, the same technique has also revealed a slowly activated K⁺ current that is calcium (Ca²⁺)-sensitive^3,5–10. This slow Ca²⁺-activated K⁺ current is the major current underlying the late, slower portion of the after-hyperpolarization following an action potential^11–14. In several muscle types, fast, transient Ca²⁺-dependent K⁺ currents have been described^15–17 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 neurones^8,9,18. 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.