Equilibration between tissue calcium and injected radiocalcium in the rat

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Abstract
The exchange of calcium in soft tissues was studied in normal and slightly hyperparathyroid, adult rats. Both chemical and radiocalcium assays were performed on a variety of tissues at periods from 10 min to 1 hr and also 1 to 4 days after Ca45 injections. Bone, brain, tendon, and muscle were found to exchange quite slowly, requiring from several hours to several days to equilibrate with the circulation. On the other hand, skin, lung, heart, liver, uterus, and kidney showed specific activities equal to or greater than that of the circulating blood within 10 min. Thereafter, the specific activities of these tissues fell rapidly, approximating the rapidly decreasing specific activity of serum. No significant alterations in these patterns were caused by parathyroid extract. The general significance of this rapid exchange of calcium in many soft tissues is discussed.