Competition Between Oxalate and Phosphate During Active Calcium Accumulation by Sarcoplasmic Vesicles

1. During ATP supported active calcium uptake oxalate as well as phosphate are accumulated with calcium. The uptake of calcium exceeds that of both anions by a small quantity--accounting for calcium binding to vesicular proteins and lipids. 2. From assay media containing phosphate and oxalate--nearly exclusively either oxalate or phosphate are taken up together with calcium by the sarcoplasmic reticulum vesicles. The mutual exclusion occurs in a very narrow concentration range of the anions. 3. In solutions containing phosphate and oxalate, calcium phosphate or calcium oxalate precipitates are formed according to their solubility properties. 4. When phosphate prevents oxalate from being taken up, calcium transport is inhibited. Inhibition occurs, because the concentration of ionized calcium inside the vesicles rises approximately 100-fold when oxalate is replaced by phosphate. The activity of the calcium dependent ATPase parallels the calcium uptake activity. 5. It is excluded that the inhibition of calcium uptake produced by phosphate is caused by an enhanced permeability of the sarcoplasmic reticulum membranes for calcium in the presence of phosphate.