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_K1 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.