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 μM ryanodine at voltages positive to the K⁺ equilibrium potential in canine ventricular myocytes. The current was abolished by 200 μM 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.