The mechanism that links membrane potential changes to the release of calcium from internal stores to cause contraction of cardiac cells is unclear. By using the calcium indicator fura-2 under voltage-clamp conditions, changes in intracellular calcium could be monitored in single rat ventricular cells while controlling membrane potential. The voltage dependence of the depolarization-induced increase in intracellular calcium was not the same as that of the calcium current (Isi), which suggests that only a small fraction of Isi is required to trigger calcium release from the sarcoplasmic reticulum. In addition, sarcoplasmic reticulum calcium release may be partly regulated by membrane potential, since repolarization could terminate the rise in intracellular calcium. Thus, changes in the action potential will have immediate effects on the time course of the calcium transient beyond those associated with its effects on Isi.
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