Comparative Kinetics of Ca 47 and Sr 85 in Man

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Abstract
The kinetics of skeletal metabolism of Sr85 and Ca47 in man over a 30-day period are characterized in terms of accretion, exchange and biological loss (absorption and long-term exchange). From a kinetic point of view, bone must be considered to be composed of a number of compartments with time constants ranging from minutes to years. To represent the data in the present study, a 4-compartment model of the kinetic metabolism system, as developed on an analog computer, was found to be adequate. Computer analysis yielded estimates of compartment sizes and flow rates between compartments. The models derived from the Ca47 and Sr85 data differ from each other only in the greater rate of urinary excretion of Sr85 and in the slightly smaller size of the readily exchangeable extracellular compartment of Sr85 compared to that of Ca47. Initially the skeletal accretion rate and exchange capacity are very similar for Sr and Ca. Differences in the retention begin to appear only after 10 days, when the metabolic response is characterized chiefly by binding of Ca and Sr tracers in the crystal lattice and long-term exchange. The data suggest that identical roles in skeletal metabolism for Ca and Sr tracers over long periods of time cannot be postulated on the basis of the metabolic similarities observed in the short interval after administration. The difficulty in making long-term predictions on Sr and Ca kinetics from this short-term study lies in the fact that the kinetic model represents less than 1% of the total body Ca. The amount of metabolically active Ca with which this single tracer injection of radioactive Ca or Sr interacts is very small indeed; the vast body Ca store with its very slow turnover rate is not reflected in the model.

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