Existence of a transient outward K+ current in guinea pig cardiac myocytes
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/l 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/l saxitoxin. Action potential clamp revealed that I to.ir contributed an outward current that activated rapidly on depolarization and inactivated by early phase 2 in both tissues. Although it is well known that 4-AP-sensitive transient outward current is absent in guinea pig, this Ba2+-sensitive and 4-AP-insensitive K+ current has been overlooked.