Calcium currents of ventricular cell pairs during action potential conduction.

We have studied the L-type calcium current that occurs during action potential conduction between an isolated pair of guinea pig ventricular cells. To accomplish this, we first recorded action potentials from the leader cell (stimulated cell, cell 1) and the follower cell (nonstimulated cell, cell 2) with a fixed coupling resistance between the cells supplied by a coupling clamp circuit. We then applied these recorded action potentials as command potential waveforms for other cells studied in the voltage-clamp mode in which internal and external solutions that isolated the L-type calcium current were used. The action potential waveform of the leader cell had a rapid upstroke and then a partial repolarization during the conduction delay before activation of the follower cell. The L-type calcium current occurred with a large magnitude during the conduction delay for the leader cell but not for the follower cell. This leads to an asymmetry of calcium current for the two cells, with greater calcium current for the leader cell than for the follower cell. When we reversed the direction of conduction for cell 1 and cell 2 by stimulating cell 2, we found that application of these recorded waveforms for the action potentials for cell 1 and cell 2 to the voltage-clamped cells also reversed the asymmetry of the magnitude of the calcium current. We conclude that discontinuous conduction in cardiac tissue is associated with a directionally determined asymmetry in the magnitude of the calcium current, with the leader cell experiencing a greater peak calcium current than the follower cell.