Calcium phosphate-containing precipitate and the carcinogenicity of sodium salts in rats

Sodium saccharin, ascorbate and other sodium salts fed at high doses to rats produce urinary bladder urothelial cytotoxicity with consequent regenerative hyperplasia. For sodium salts that have been tested, tumor activity is enhanced when administered either alone or after a brief exposure to a known genotoxic bladder carcinogen. These sodium salts alter urinary composition of rats resulting in formation of an amorphous precipitate. We examined the precipitate to ascertain its composition and further delineate the basis for its formation in rat urine. Using scanning electron microscopy with attached X-ray energy dispersive spectroscopy, the principal elements present were calcium, phosphorus, minor amounts of silicon and sulfur. Smaller elements are not detectable by this method. Infrared analyses demonstrated that calcium phosphate was in the tribasic form and silicon was most likely in the form of silica. Small amounts of saccharin were present in the precipitate from rats fed sodium saccharin (< 5%), but ascorbate was not detectable in the precipitate from rats fed similar doses of sodium ascorbate. Large amounts of urea and mucopolysaccharide, apparently chondroitin sulfate, were detected in the precipitate by infrared analysis. Chemical analyses confirmed the presence of large amounts of calcium phosphate with variably small amounts of magnesium, possibly present as magnesium ammonium phosphate crystals, present in urine even in controls. Small amounts of protein, including albumin and α(2u)-globulin, were also detected (< 5% of the precipitate). Calcium phosphate is an essential ingredient of the medium for tissue culture of epithelial cells, but when present at high concentrations (>5 mM) it precipitates and becomes cytotoxic. The nature of the precipitate reflects the unique composition of rat urine and helps to explain the basis for the species specificity of the cytotoxic and proliferative effects of high doses of these sodium salts.