The calcium transients associated with contraction in human working myocardium were recorded by use of the bioluminescent protein, aequorin, a substance that emits light when it combines with calcium ion (Ca++). Small amounts of aequorin were microinjected into superficial cells of human atrial and ventricular muscle obtained from tissue routinely excised and discarded at the time of cardiac surgery. Light output, an index of intracellular Ca++, and isometric tension development were recorded at 37.5°C at 1 to 5 second intervals of stimulation. Light increases much more quickly than tension and decreases toward basal levels by the time that peak tension is reached. The configuration and time course of the aequorin signal in human myocardium and its responses to inotropic interventions are similar to those recorded in lower mammalian species. The calcium transient appears to be dominated by the release and uptake of Ca++ from intracellular stores under all conditions studied. These results indicate that aequorin is a useful tool for studying the effects of drugs and disease states on cardiac excitation-contraction coupling in human beings as well as in lower animals.
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