Transmembrane action potential of a Purkinje fiber as recorded with an intracellular microelec-trode. When the electrode tip penetrates the fiber, a resting membrane potential of -90 mV is recorded. The application of a subthreshold stimulus (#1) produces a depolarizing current that fails to result in excitation of the myocardial cell. The application of a threshold stimulus (#2) reaches the threshold potential (TP) and results in an inward current and an action potential. Major transmembrane currents carried by specific ions entering the cell through selective ion channels are depicted to the right. Antiarrhythmic agents alter the electrophysiologic properties of the cardiac cells by modulating one or more of the transmembrane currents, especially the fast inward sodium current and the transmembrane currents carried by the potassium ion (IK and IK-ATP). I na — fast inward sodium current; Ica — "L"-type calcium current; Ito — transient outward current; INa-Ca — sodium-calcium exchange current; IK-ATP — adenosine triphosphate-sensitive potassium current; IK — inward rectifying potassium current; IK — delayed rectifying potassium current.
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