The role of intracellular Ca buffers in determining the shape of the systolic Ca transient in cardiac ventricular myocytes

We have studied the kinetics of decay of cardiac Ca transients elicited by either caffeine or electrical stimulation. The decay of the caffeine-evoked increase of intracellular Ca concentration ([Ca2+](i)) could not be fit by a single exponential. A two exponential fit showed an initial rapid component. The rate of decay of total Ca was calculated from measured free Ca and the buffering properties of the cell. There was no initial rapid phase of decay of total Ca. We conclude that the rapid phase of decay of free Ca is due to a decrease of Ca buffering power at elevated [Ca2+](i). In contrast, the decay of the Ca transient produced by voltage-clamp depolarization or field stimulation was fit by a single exponential. We suggest that these apparently simple kinetics arise because a tendency to saturation at elevated [Ca2+](i) of the sarcoplasmic reticulum Ca-ATPase offsets the decrease of Ca buffering power. These data show the importance of Ca buffers as well as transporters in determining the kinetics of changes of [Ca2+](i).