Comparative metabolism of calcium and strontium in the rat

This is a report on the comparative metabolism of calcium and strontium in the rat. Animals were injected intraperitoneally, under varying conditions, with solutions containing Sr 89 and Ca 45. The appearance of these isotopes, with the passage of time, was observed in the tibia, dentin, plasma, and urine by periodic sacrifice from experimental groups. After 10 days of daily injection, the bone and dentin of the rats contained 30% more Ca 45 than Sr 89. Time sequence retention studies also showed that the Sr 89:Ca 45 ratio of the plasma remained near unity during the experiments, while the urine ratio continuously fell with the passage of time. Examination of the feces 24 hr. after injection revealed a reduced ratio as in the case of urine. Ultrafiltration experiments showed that bound Ca 45 and Sr 89 in the plasma were essentially the same 1 hr. and 12 hr. after injection. Thus, the continuous drop in ratio in urine could not be explained by preferential binding of Ca 45 in the plasma. Synthetic hydroxyapatite samples tagged with Sr 89 and Ca 45 exhibited a preferential loss of Sr 89 when the samples were shaken with calcium acetate solution. The apatites were prepared by ( a) hydrolysis of CaHPO 4 and ( b) direct precipitation from solution. The differences between these preparations are discussed. The discrimination of the animal against Ca 45 in the urine and feces is probably regulated on the bone level as well as by the kidney and gut function.