Competition between Oxalate and Phosphate during Active Calcium Accumulation by Sarcoplasmic Vesicles

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Abstract
During ATP-supported active Ca uptake [by rabbit skeletal muscle], oxalate and phosphate were accumulated with Ca. The uptake of Ca exceeded that of both anions by a small quantity, accounting for Ca binding to vesicular proteins and lipids. From assay media containing phosphate and oxalate, nearly exclusively either oxalate or phosphate were taken up together with Ca by the sarcoplasmic reticulum vesicles. Mutual exclusion occurred in a very narrow concentration range of the anions. In solutions containing phosphate and oxalate, calcium phosphate or calcium oxalate precipitates were formed according to their solubility properties. When phosphate prevented oxalate from being taken up, Ca transport was inhibited. Inhibition occurred because the concentration of ionized Ca inside the vesicles rose approximately 100-fold when oxalate was replaced by phosphate. The activity of the Ca dependent ATPase paralleled the Ca uptake activity. The inhibition of Ca uptake produced by phosphate was caused by an enhanced permeability of the sarcoplasmic reticulum membranes for Ca in the presence of phosphate.

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