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+ (I(K,p)) activates within 5 msec at positive potentials, does not inactivate, and deactivates very rapidly on repolarization. I(K,p) is insensitive to Cl- channel blockers, internal or external [Cl-], dihydropyridines, and sulfonylureas. In contrast, the delayed rectifier K+ current (I(K)) was not completely blocked even by 30 mM Ba2+. Ba2+-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 (I(K1)). These and other features suggest that I(K,p) is generated by K+ channels that are distinct from I(KI) or I(K). The kinetics and voltage dependence of I(K,p) render it capable of modulating both the height and duration of the cardiac action potential.