Characterization of a transient outward K+ current with inward rectification in canine ventricular myocytes

The threshold potential for the classical depolarization-activated transient outward K+ current and Cl- current is positive to -30 mV. With the whole cell patch technique, a transient outward current was elicited in the presence of 5 mM 4-aminopyridine (4-AP) and 5 mu M ryanodine at voltages positive to the K+ equilibrium potential in canine ventricular myocytes. The current was abolished by 200 mu M Ba2+ or omission of external K+ (K-o(+)) and showed biexponential inactivation. The current-voltage relation for the peak of the transient outward component showed moderate inward rectification. The transient outward current demonstrated voltage-dependent inactivation (half-inactivation voltage: -43.5 +/- 3.2 mV) and rapid, monoexponential recovery from inactivation (time constant: 13.2 +/- 2.5 ms). The reversal potential responded to the changes in K-o(+) concentration. Action potential clamp revealed two phases of Ba2+-sensitive current during the action potential, including a large early transient component after the upstroke and a later outward component during phase 3 repolarization. The present study demonstrates that depolarization may elicit a Ba2+- and K-o(+)-sensitive, 4-AP-insensitive, transient outward current with inward rectification in canine ventricular myocytes. The properties of this K+ current suggest that it may carry a significant early outward current upon depolarization that may play a role in determining membrane excitability and action potential morphology.