Direct measurement of L-type Ca2+ window current in heart cells.

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Abstract
The activation and inactivation relations of several ion channel currents overlap, suggesting the existence of a steady-state or "window" current. We studied L-type Ca2+ channel window current in single cardiac Purkinje cells using a voltage-clamp protocol by which channels were first inactivated nearly completely during a long-duration depolarizing step, and then the recovery of Ca2+ current was observed during repolarizing steps into the L-type Ca2+ window voltage range. With these conditions, a small-amplitude inward Ca2+ current gradually developed after repolarization to voltages within the window but not after steps to voltages positive or negative to it. Window current was suppressed by Cd2+ (50 microM), nifedipine (1 microM), and nicardipine (1 microM), and it was augmented by isoproterenol (5 microM) and Bay K 8644 (1 microM). At voltages at which window current developed, L-type Ca2+ channels also recovered to a closed state from which they could be reopened by an additional depolarizing step. A...