G. D. McPherson: Estimation of the 24 Hour Exchangeable Calcium Pool 189 



Summary 



The kinetics of calcium have been analysed in the wall of the aorta of the living 

 rat. The calcium content of the aorta in the physiological state is surprisingly high 

 (19.2 //M/g). It exchanges with blood calcium according to the same modalities as 

 bone calcium and includes three fractions: rapidly exchangeable (3.5 /<M/g), slowly 

 exchangeable (6.7 //M/g) and non exchangeable calcium (9.0 //M/g). 



References 



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Estimation of the 24 Hour Exchangeable Calcium Pool in 

 Children Using ^^Ca 



G. D. McPherson 



The Hospital for Special Surgery — Affiliated with the New York Hospital — 

 Cornell University Medical College, New York City, U.S.A. 



Stable •^^Ca is now produced in enrichments sufficiently high in relation of its low 

 natural abundance to permit its use as a non-radioactive tracer in dilution studies 

 of calcium metabolism. Changes in the abundance of •'^Ca may be determined 

 precisely by either mass spectrometry or neutron activation analysis. 



The ten milligram samples of calcium required for •'^Ca abundance estimation by 

 neutron activation analysis are isolated from timed urine collections by oxalate 

 precipitation. Sodium, which is an important source of interference in activation 

 analysis of biological samples, is removed to a considerable extent by the simple 

 isolation procedure. The induced -'^Na does not interfere with the counting of ■'"Ca 

 because of the high energy of •'^Ca (3.1 MeV). As the specific activity of urine and 

 serum are similar, the abundance of a stable tracer in samples of calcium isolated 

 from urine collections reflects the specific activity of the isotope in serum. 



This report describes the use of stable ^^Ca in the estimation of the 24 hour 

 exchangeable calcium pool in children. 



