Intracellular Ca2+ storage sites in the carp heart: comparison with the rat heart

The Ca(2+)-releasing mechanisms of the sarcoplasmic reticulum responsible for cardiac muscle contraction in carp were examined and compared with these mechanisms in rats. Morphologically, the ventricular muscles of the carp heart are composed of an outer compact and an inner spongy layer. In the present study, ventricular muscle preparations were obtained from the compact layer of the carp heart, because the spongy layer does not contribute significantly to the overall force of contraction. Electron microscopic observations showed that the sarcoplasmic reticulum in the carp ventricular muscle, compared to that in the rat ventricular muscle, was poorly developed. Consistent with this finding, specific [3H]ryanodine binding to partially purified sarcoplasmic reticulum preparations obtained from carp ventricular muscle as compared with the preparations isolated from the rat ventricular muscle showed a lower affinity and a smaller number of binding sites. Additionally, a higher Ca2+ concentration was required to cause a half maximal stimulation of [3H]ryanodine binding in the carp heart. In skinned ventricular muscle fibers isolated from carp hearts, the caffeine-induced contracture was significantly weaker than that observed in rat hearts. These results suggest that, in carp hearts, the sarcoplasmic reticulum has an important role as a supply source of Ca2+ for muscle contraction, though the storage capacity and/or amount of Ca2+ release in carp was significantly smaller than that in rats.