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Am J Physiol Heart Circ Physiol 279: H130-H138, 2000;
0363-6135/00 $5.00
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Vol. 279, Issue 1, H130-H138, July 2000

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

Gui-Rong Li1,2, Baofeng Yang1, Haiying Sun1, and Clive M. Baumgarten2

1 Department of Medicine, Montreal Heart Institute and University of Montreal, Montreal, Quebec, Canada H3C 3J7; and 2 Department of Physiology, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298

A novel transient outward K+ current that exhibits inward-going rectification (Ito.ir) was identified in guinea pig atrial and ventricular myocytes. Ito.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+. Ito.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). Ito.ir was detected in Na+-containing and Na+-free solutions and was not blocked by 20 nmol/l saxitoxin. Action potential clamp revealed that Ito.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.

transient outward potassium current; whole cell patch clamp; repolarization; excitability; action potential configuration


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