Measurements of calcium transients in ventricular cells during discontinuous action potential conduction

The L-type calcium current (I-Ca) is important in sustaining propagation du ring discontinuous conduction. In addition, I-Ca is altered during disconti nuous conduction, which may result in changes in the intracellular calcium transient. To study this, we have combined the ability to monitor intracell ular calcium concentration ([Ca2+](i)) in an isolated cardiac cell using co nfocal scanning laser fluorescence microscopy with our "coupling clamp" tec hnique, which allows action potential propagation from the real cell to a r eal-time simulation of a model cell. Coupling a real cell to a model cell w ith a value of coupling conductance (G(C) = 8 nS) just above the critical v alue for action potential propagation results in both an increased amplitud e and an increased rate of rise of the calcium transient. Similar but small er changes in the calcium transient are caused by increasing G(C) to 20 nS. The increase of [Ca2+](i) by discontinuous conduction is less than the inc rease of I-Ca, which may indicate that much of [Ca2+](i) is the result of c alcium released from the sarcoplasmic reticulum rather than the integration of I-Ca.