Skeletal retention of 45Ca and 85Sr as a tracer for skeletal calcium

The physical characteristics of 47Ca and 45Ca make them unsuitable for studying the longterm (> 30 days) handling of skeletal calcium in patients. Therefore a study was conducted in 3 normal subjects in which the skeletal uptakes and subsequent retentions of simultaneously administered intravenous doses of 45Ca and 85Sr were compared. Specific activities of 45Ca (relative to 40Ca) were then observed in plasma and prolonged urine collections for up to 5 months. The whole-body retention of 85Sr was followed simultaneously in a whole-body bounter. In the hypothesis that the single passage retention of both tracers in the skeleton was identical, after the effects of second and subsequent tracer recirculations the skeleton had been removed by deconvolution analysis, it was possible to calculate from the 85Sr data and the excretory clearance of 40Ca the predicted 45Ca specific activity for each subject as a function of time. Within the limits imposed by data scatter and measurement uncertainties, the predicted and observed long-term 45Ca specific activity curves appeared identical. However, in a separate study on approximately 40 patients, 85Sr was shown to underestimate the size of the exchangeable pools of bone calcium by about 20%. (orig./AJ)