Calcium Exchange; the Mechanism of Adsorption by Bone of Ca46*

Until recently, the difficulties of obtaining and of measuring the radioactive isotopes of calcium have kept all but a few investigators from examining the distribution of calcium in the hard tissues of the body. Using Ca45 as a tool, Campbell and Greenberg (l), Pecher (2), and Greenberg (3) found between 15 and 28 per cent of a dose in the skeleton of mice or rats within short periods of time. More recently, Armstrong and Barnum (4) administered Ca46 and P32 simultaneously to rats and, after 5 days, compared the percentages of each isotope in various tooth and bone tissues. The total isotope content of the entire skeleton was not estimated, but similar percentages of the Ca46 and p2 doses were reported in a number of bone samples. All of these workers agree that a substantial part of a Ca46 dose appears promptly in the skeleton. Since the time periods of the studies in viva were too short to permit growth and calcification to be major factors, the experiments in vitro reported below are believed to give grounds for a hypothesis that can be applied directly to the rapid phase of Ca46 deposition in the living animal. Previous work in this laboratory has shown (a) that P32 also appears promptly in considerable proportions in the bone (5) and (5) that the rapid phase of the radiophosphorus adsorption is adequately described as a reversible exchange reaction (6). The incorporation of phosphorus into bone glycol ash in vitro over a period of 7 days reaches a steady state; at this time about 20 per cent of the bone phosphorus has exchanged with solution phosphorus. The magnitude of the exchangeable phosphate fraction of the bone makes this exchange reaction important physiologically. Since the principal mineral constituent of the bone has long been described as a calcium phosphate, it is natural to inquire whether calcium