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The calcium-sensitive photoprotein aequorin was microinjected into cells of rat and cat ventricular muscle. During subsequent stimulation of the muscle light emission could be detected and this signal is a function of the intracellular Ca ++ . The time course and amplitude of the intracellular Ca ++ transient occurring during contraction is described. The effects on tension and light emission of changing external Ca ++ and stimulus frequency and of adding adrenaline and caffeine to the bathing solution are described. These results show that changes in external calcium and stimulus frequency alter tension by means of changes in the intracellular Ca ++ whereas adrenaline in addition alters the sensitivity of the contractile system to intracellular Ca ++ . The results also suggest that although a fall in intracellular Ca ++ always precedes relaxation, the time course of the fall of Ca ++ is not generally a rate limiting step in the lime course of relaxation.
Allen et al. (1980) studied this question.