Skeletal retention of45Ca and85Sr compared: Further studies on intravenously injected85Sr as a tracer for skeletal calcium

Abstract

The physical characteristics of47Ca and45Ca make them unsuitable for studying the long-term (>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 of45Ca and85Sr were compared. Specific activities of45Ca (relative to40Ca) were then observed in plasma and prolonged urine collections for up to 5 months. The whole-body retention of85Sr was followed simultaneously in a whole-body counter. On 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 the85Sr data and the excretory clearance of40Ca the predicted45Ca 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-term45Ca 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%. In the same group of patients, a close relationship was observed between the endogenous fecal calcium excretion rates, as calculated, respectively, from data obtained after simultaneous injections of85Sr and47Ca. Furthermore, the47Ca accretion rate to bone could be predicted from the85Sr data with a coefficient of variation of 15%.85Sr is not the preferred tracer for studying the short-term handling of calcium by the skeleton. However, long-term “exchange” processes lead to prolonged (>30 days) tracer retention at surfaces where new bone is not being formed. When it is necessary to separate these processes from true bone formation,85Sr is a practical and suitable tracer for patient studies.