Background potassium current active during the plateau of the action potential in guinea pig ventricular myocytes

Background outward K+ currents in guinea pig ventricular myocytes were characterized over a broad range of membrane potentials, including those corresponding to the plateau of the action potential. The background current that is blocked by 1 mM Ba2+ (IK,p) activates within 5 msec at positive potentials, does not inactivate, and deactivates very rapidly on repolarization. IK,p is insensitive to Cl- channel blockers, internal or external [Cl-], dihydropyridines, and sulfonylureas. In contrast, the delayed rectifier K+ current (IK) was not completely blocked even by 30 mM Ba2+. Ba(2+)-sensitive current density increased progressively from 0.16 +/- 0.04 pA/pF at 0 mV to 0.52 +/- 0.21 pA/pF at +80 mV (n = 13, mean +/- SEM). The background current remains present when [K+]o is reduced to 0 mM, which suppresses the inward rectifier K+ current (IK1). These and other features suggest that IK,p is generated by K+ channels that are distinct from IK1 or IK. The kinetics and voltage dependence of IK,p render it capable of modulating both the height and duration of the cardiac action potential.