A.: Ion association. III. The effect of sulfate infusion on calcium excretion

In the course of a study directed towards re-placing extracellular chloride with sulfate in dogs, it was noted that sulfate infusion led to a con-siderable increase in the urinary excretion of calcium and signs of calcium deficiency (1, 2). The formation of a complex between calcium and sulfate was considered as a possible explanation of this observation, since it is well known that organic anions which complex calcium, including citrate (3) and ethylenediaminetetracetic acid (EDTA) (4), also augment calcium excretion. However, the nature of the association between calcium and sulfate in aqueous solution differs fundamentally from the covalent complexes formed with citrate and EDTA. It is believed that electrostatic forces alone are responsible for the apparently in-complete dissociation of calcium sulfate (5, 6). The physiological properties of this type of com-plex lhave not been previously studied. This paper is one in a series directed towards this general problem. In the first paper (7) it was shown that the addition of multivalent anions (sulfate, phosphate or ferrocyanide) to serum led to an increased amount of complexed calcium in serum ultrafiltrate, determined by the difference between free calcium ion concentration (measured by the purpurate method) and total ultrafiltrable calcium. The results were consistent with the interpretation that calcium, when electrostatically paired with multivalent anions, does not combine witlh purpurate. In the second paper it was shown that calcium ions, when associated wit