The pacemaker current in cardiac Purkinje myocytes.
The actions of cesium (Cs) on intracellular sodium activity (aiNa), membrane potentials, and force were studied in sheep cardiac Purkinje and myocardial fibers superfused in vitro. In Purkinje fibers, Cs (2 mM) decreased diastolic depolarization, aiNa (-6.7%, p less than 0.005), and force (-28.0%, p less than 0.01). The effects of 4 and 8 mM Cs were more pronounced. In quiescent fibers, Cs (2-4 mM) also decreased aiNa (-17.3%, p less than 0.005) and induced an initial hyperpolarization (+5.6 +/- 1.3%, p less than 0.005) followed by a return toward control. Diastolic depolarization was almost abolished by driving the fibers at 180/min (diastole was very short) but still Cs decreased aiNa (-15.4%). Tetrodotoxin decreased aiNa (-16.2%, p less than 0.025) and reduced the Cs-induced fall in aiNa (-2.2%, p less than 0.05). In zero [K]o, Cs decreased aiNa and caused repolarization. In 0.1 mM strophanthidin, Cs did not decrease aiNa any longer and affected the membrane potential little. In quiescent myocardial fibers, Cs (4 mM) decreased aiNa (-12.6%, p less than 0.05) and transiently hyperpolarized (+2.1%). Rubidium (2 mM) decreased aiNa and resting potential in Purkinje fibers and in myocardial fibers and also decreased diastolic depolarization in Purkinje fibers. Thus, cesium and rubidium decrease aiNa and modify the membrane potential but not through a block of the inward pacemaker current If.