The Rapid Repolarization of the Action Potential in Twitch Skeletal Muscle Fibers from Frog is due Almost Entirely to a Calcium-Activated K Current

Abstract

Action potentials (aps) in cut fibers mounted in a double Vaseline-gap chamber were generated by short current-clamp pulses superimposed on a holding current that maintained the resting potential at −90 mV. Ca2+ release from the sarcoplasmic reticulum (SR) was assessed with either the EGTA/phenol red method or the SR [Ca2+] indicator, tetramethylmurexide. In contrast to the rapid rate of repolarization of the ap with normal SR Ca contents, in the near absence of Ca, the time course of repolarization was approximately exponential with an average exponential time constant of 12.5 ms. Removal of K from the internal and external solutions eliminated the effect of SR Ca2+ release on the ap indicating that the current is carried by K+ ions. The time course of total current during the repolarization was assessed from the rate of change of voltage. Taking into account a small Ca-insensitive background current and the relationship between K current and permeability (PK) on voltage using the Goldman-Hodgkin-Katz equation, the time course of PK is seen to closely match that of the myoplasmic Ca transient. At physiological SR Ca load, the Ca-activated K current, denoted IK(Ca), at its peak was ∼80% of the total current. Since almost all of the background current was associated with recharging the membrane capacitance via the holding current passing through ohmic pathways in the cut-fiber preparation, the peak percentage of current responsible for repolarization in an intact fiber carried by IK(Ca) should be significantly greater than 90%. While results with voltage-clamp stimulation indicate a complex dependence of IK(Ca) on calcium and voltage similar to that displayed by BKCa channels, the IK(Ca) current was insensitive to the BKCa channel inhibitor charybdotoxin.