Discrimination of Calcium and Strontium by the Kidney

Skeletal retention and excretion of strontium were directly compared with those of calcium, by injecting solutions containing both radioisotopes, Ca45 and Sr90, into rats and rabbits. In normal animals, a greater fraction of the injected dose of Ca45 was retained in the skeleton than was observed for Sr90. In complementary fashion, the fraction of the injected Sr90 which appeared in the urine was greater than the fraction of the dose of Ca45 which was excreted. However, in rats which were actively calcifying tibial fractures, this difference disappeared. The enhanced accumulation of radioactivity in deposits of new bone possessed the same ratio of Sr90: Ca45 as the solution injected. Furthermore, when the functions of the kidneys of normal rabbits were impaired by poisoning with mercuric chloride or were completely extinguished by nephrectomy, again the usual differences in Ca45 and Sr90 deposition in bone tissue disappeared. When plasma containing both Ca45 and Sr90 was shaken with powdered bone, both radioisotopes were extracted in equivalent amounts. It was concluded that the avidity of bone tissue, per se, for strontium ions is not discernably different from that for calcium ions. The retention by the skeleton of a larger fraction of an injected quantity of calcium than of simultaneously administered strontium was attributed, in part, to a renal discrimination causing a greater relative loss of strontium to the urine.