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Calcium transport and release by the sarcoplasmic reticulum.

The slow rate of calcium transport by cardiac sarcoplasmic reticulum vesicles, compared to those prepared from skeletal muscle, is due mainly to a lesser density of transport sites and a lower Ca2+ affinity. Because the turnover rate of the cardiac sarcoplasmic reticulum calcium pump is similar to that of comparable skeletal muscle preparations, the cardiac sarcoplasmic reticulum seems able to effect the slower relaxation of the heart. Calcium permeability of skeletal sarcoplasmic reticulum vesicles is increased when the Ca2+ concentration outside the vesicles is increased, or when that inside the vesicle is decreased.