The mechanism thought responsible for generation of delayed afterdepolarizations is illustrated in this figure. Intracellular calcium is increased via the L type calcium inward current (1) and by the Na/Ca exchange mechanism (2). This triggers the release of calcium from the sarcolplasmic reticulum (3) which leads to a further increase in intracellular calcium. This increase in intracellular calcium then activates the Na/Ca exchange mechanism, but in the reverse direction. This results in the extrusion of calcium and a transient inward current carried by sodium ions (4). It is this transient inward current that is believed to be the ultimate cause of the afterdepolariztion (5).
This is one of the mechanisms thought responsible for the abnormal beats and tachyarrhythmias caused by digitalis.