Calcium transients in aequorin-injected frog cardiac muscle

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Abstract
The Ca2+-sensitive bioluminescent protein aequorin was microinjected into cells of frog [Rana pipiens] atrial trabeculae to study intracellular Ca transients associated with excitation-contraction coupling. The amplitude of the aequorin signal increased with extracellular Ca2+ concentration and stimulus frequency, but decreased with stretch. Isoprenaline and acetylstrophanthidin both increased the amplitude, but had strikingly different effects on the time course of the signal.