On the characteristics of the inward tail current induced by calcium overload.

The characterization of an inward tail current (Iex) induced by calcium overload and often associated with the oscillatory current (Ios) was studied by means of a two-microelectrode voltage clamp method in sheep cardiac Purkinje fibers. It was found that: (1) the inward tail current requires a prior depolarization to about -25 mV (or less negative values) in order to appear; (2) after depolarization to those values, the membrane needs to be repolarized to about -45 mV (or to more negative values) for the inward tail current to be initiated; (3) the inward tail current becomes larger after larger or longer clamp steps and increases progressively at more negative potentials; (4) the inward tail current can be separated from other known currents, notably from the pacemaker current; (5) the inward tail current is reduced by quinacrine, a blocker of Na-Ca exchange; (6) the inward tail current does not reverse at less negative potentials; (7) the membrane conductance does not increase during the inward tail current. Thus, the inward tail current shows similarities and dissimilarities with the oscillatory current, probably in relation to a common final pathway and to a different mode of onset of the two currents, respectively.