Electrophysiological Effects of Ionophore-induced Increases in Intracellular Na⁺ in Cardiomyocytes

Na ionophores increase intracellular Na+ ([Na+]i). Membrane potentials and currents were measured using microelectrode and whole-cell patch-clamp techniques. Monensin (10−6-3×10−5 M) reduced the slope of the pacemaker potentials and shortened the action potential duration (APD) in sino-atrial nodal and Purkinje cells. Monensin (10−5 M) shortened the APD and reduced the amplitude of the plateau phase in ventricular myocytes. Monensin decreased the hyperpolarization-activated inward current (If), and it increased the transient outward potassium current (Ito) in Purkinje cells. In addition, monensin decreased the sodium current (INa), shifting the inactivation curve to the hyperpolarized direction. Moreover, monensin decreased the L-type calcium current (ICa) in ventricular myocytes. The Na+-Ca2+ exchange current (INa-Ca) was augmented particularly in the reverse mode, and the Na+-K+ pump current (INa-K) was also activated by monensin in cardiomyocytes. The ATP-activated potassium current (IK,ATP) could be induced by monensin. Notably, the inward rectifying K+ current (IK1), and the slow delayed outward K+ current (IKs) were not affected evidently by monensin. Collectively, alteration of [Na+]i can influence the activities of various ion channels and transporters. Thus, the significance of altered [Na+]i should be taken into consideration in the action of drugs affecting [Na+]i such as digitalis, Na+ channel blockers, and Na+ channel activating agents.