Exchange of Bone Calcium and Phosphorus in vivo

Abstract

THE value of the radioactive isotopes of calcium and phosphorus in studies of bone growth has been called in question¹ owing to the difficulty of distinguishing between newly deposited bone and old bone which has become labelled by exchange. This view originated as a result of short-term experiments with rats, in which the activity of the calcium or phosphorus in portions of bone was compared with the activity of the serum at varying times after the administration of a single dose of calcium-45 or phosphorus-32 either by injection or by stomach tube. Tomlin, Henry and Kon² have criticized these experiments on the grounds that the specific activity of the serum is unreliable as a standard owing to the rapid variations which it undergoes after a single dose of an isotope, and because of the impossibility of discriminating between new and old bone by the method used. They have shown that, when calcium-45 is fed to rats continuously for several weeks, it is possible by means of an autoradiographic technique to distinguish effectively between new and old bone and that, in animals initially 30 days of age, the exchange of calcium is approximately 15 per cent after treatment for 30 days with the isotope. In vitro experiments with powdered bone using both radioactive calcium and phosphorus have given exchange percentages of a similar order³.