Existence of a transient outward K+ current in guinea pig cardiac myocytes

Abstract

A novel transient outward K+ current that exhibits inward-going rectification (I(to.ir)) was identified in guinea pig atrial and ventricular myocytes. I(to.ir) was insensitive to 4-aminopyridine (4-AP) but was blocked by 200 μmol/1 Ba2+ or removal of external K+. The zero current potential shifted 51-53 mv/decade change in external K+. I(to.ir) density was twofold greater in ventricular than in atrial myocytes, and biexponential inactivation occurs in both types of myocytes. At -20 mv, the fast inactivation time constants were 7.7 ± 1.8 and 6.1 ± 1.2 ms and the slow inactivation time constants were 85.1 ± 14.8 and 77.3 ± 10.4 ms in ventricular and atrial cells, respectively. The midpoints for steady-state inactivation were -36.4 ± 0.3 and -51.6 ± 0.4 mV, and recovery from inactivation was rapid near the resting potential (time constants = 7.9 ± 1.9 and 8.8 ± 2.1 ms, respectively). I(to.ir) was detected in Na+-containing and Na+-free solutions and was not blocked by 20 nmol/1 saxitexin. Action potential clamp revealed that I(to.ir) contributed an outward crescent that activated rapidly on depolarization and inactivated by early phase 2 in both tissues. Although it is well knower, that 4-AP-sensitive transient outward current is absent in guinea pig, this Ba2+-sensitive and 4-AP-insensitive K+ current has been overlooked.