The interrelationship of cesium, intracellular sodium activity, and pacemaker potential in cardiac Purkinje fibers

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