Retention and Turnover of Radiocalcium by the Skeleton of Large Rats.

The calcium specific activities of the femoral epiphyses, femoral diaphyses, humeri, lumbar vertebrae and the remainder of the skeleton of large rats differ soon after the administration of radiocalcium but decrease to a common value after 52 days, following which the specific activities decline at equal but much reduced rates. The fraction of the injected dose of radiocalcium retained in the animals declines to 42-45% on the 52nd day, after which only small quantities of radiocalcium are excreted. The results indicate the existence in large rats of two kinds of skeletal calcium which differ markedly in their rate of turnover and in the lenght of time over which they fix radiocalcium. The apparent reduction in skeletal calcium specific activity occurring in large rats after the 52nd day following the injection of radiocalcium is due to accretion of calcium to the skeleton. Part of the decline in skeletal specific activities which takes place in rats before radiocalcium excretion ceases is also due to skeletal growth.