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Characterization of sarcoplasmic reticulum in skinned heart muscle cultures

The plasma membranes of rat heart muscle, grown in cell culture, were made permeable with saponin in a Ca‐free solution. The cells were then supplied with a medium resembling the cytosol, and the adenosine triphosphate (ATP)‐dependent Ca2+ sequestration was measured in the presence of oxalate. The nonmitochondrial component accounts for about 50% of the total Ca2+ uptake. The nonmitochondrial accumulation of Ca2+ within myocardial cells was found to be reversible by addition of the Ca2+ ionophore A23187. On the other hand, the Ca2+ antagonist D‐600 (50 μM) had almost no effect on Ca2+ accumulation. Caffeine reduced Ca2+ accumulation in the skinned cardiomyocytes in a concentration‐dependent manner. In addition, the anticalmodulin drug trifluo‐perazine (TFP) reduced Ca2+ accumulation in the skinned cells. Because of the analogy between nonmitochondrial ATP‐dependent Ca2+ accumulation and the sarcoplasmic reticulum (SR) function with regard to the influence of various agents, it is assumed that we actually measure Ca2+ accumulation in the SR. The rate of Ca2+ accumulation into the SR measured during the development of the cardiomyocytes in culture shows an almost linear increase as a function of culture age. Amiodarone, a potent antiarrhythmic agent, and its metabolite, desethyl‐amiodarone, inhibited Ca2+ accumulation into SR, which may explain their therapeutic effect.