Changes in subcellular calcium transport in rat hearts during the calcium paradox

Changes in the transport of calcium in the sarcoplasmic reticulum (SR) and mitochondria (MT), of the rat heart during calcium paradox were investigated. Calcium binding and uptake by SR in the paradox hearts were from about 1.5 to 2 times greater than in normal hearts, whereas in the MT of the paradox hearts they were about half those of normal hearts. There was no difference between paradox and normal hearts in calcium stimulated ATPase activity in the SR. The ultrastructure of the MT was disrupted in calcium paradox, but the SR was essentially the same as in normal hearts. We propose that during calcium paradox, the intracellular calcium overload damages both calcium transport and the ultrastructure of the MT, but that calcium transport by the SR is accelerated to compensate for the calcium overload.