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

The Ca2+-releasing mechanisms of the sarcoplasmic reticulum responsible for cardiac muscle contraction in carp were examined and compared with these m echanisms in rats. Morphologically, the ventricular muscles of the carp hea rt are composed of an outer compact and an inner spongy layer. In the prese nt study, ventricular muscle preparations were obtained from the compact la yer of the carp heart, because the spongy layer does not contribute signifi cantly to the overall force of contraction. Electron microscopic observatio ns showed that the sarcoplasmic reticulum in the carp ventricular muscle, c ompared to that in the rat ventricular muscle, was poorly developed. Consis tent with this finding, specific [H-3]ryanodine binding to partially purifi ed sarcoplasmic reticulum preparations obtained from carp ventricular muscl e as compared with the preparations isolated from the rat ventricular muscl e showed a lower affinity and a smaller number of binding sites. Additional ly, a higher Ca2+ concentration was required to cause a half maximal stimul ation of [H-3]ryanodine binding in the carp heart. In skinned ventricular m uscle fibers isolated from carp hearts, the caffeine-induced contracture wa s significantly weaker than that observed in rat hearts. These results sugg est that, in carp hearts, the sarcoplasmic reticulum has an important role as a supply source of Ca2+ for muscle contraction, though the storage capac ity and/or amount of Ca2+ release in carp was significantly smaller than th at in rats. (C) 1999 Elsevier Science Inc. All rights reserved.