CALCIUM-DEPENDENT POTENTIATION AND INACTIVATION OF SINGLE L-TYPE CALCIUM-CHANNEL CURRENT IN FURA-2-LOADED GUINEA-PIG VENTRICULAR MYOCYTES

Modulations of cardiac L-type Ca channel by tonic changes in intracellular Ca2+ concentration ([Ca2+](i)) were investigated. Unitary currents were recorded from fura-2-loaded myocytes in the cell-attached configuration simultaneously with the measurement of fura-2 fluorescence signals. [Ca2+](i) was controlled through changes in bath Ca2+ concentration during K+-depolarization. When [Ca2+](i) exceeded similar to 2 times the resting level, the activity of Ca channels was reversibly potentiated. This was due to the increased ''availability'' and increased channel open probability (P-o) during non-blank sweeps. Enhancement of sweeps with unusually long openings also contributed to increase P-o during Ca2+-dependent potentiation. When [Ca2+](i) was further increased (estimated [Ca2+](i) above 600 nM), Ca-channel activity was suppressed. Our results provide direct evidence that [Ca2+](i) modulates the activity of Ca channel in two opposite ways, depending on its concentration.