The study of skeletal calcium metabolism with Ca-41 and Ca-45

The living skeleton can be labeled for life by the administration of radiol ogically trivial amounts of Ca-41 tracer. After initial elimination of trac er from the readily exchangeable calcium pools subsequent skeletal calcium turnover maintains and modulates the urine Ca-41 content. Uniquely, bone ca lcium metabolism may then be studied with tracer in near equilibrium with t he body's calcium and resorbing calcium directly measured by accelerator ma ss spectrometry (AMS) of excreta. Our experiments with 25 Ca-41 labeled sub jects demonstrate excellent diurnal stability and remarkable response to in tervention of the urine signal. Thus the tracer method may prove a competit ive means of measuring the effects of antiresorptive osteoporosis treatment s, for therapy development or even clinical monitoring. Novel studies of lo ng-term skeletal evolution are also possible. We realize that routinely adm inistered short-lived calcium radiotracers contain Ca-41 impurities and tha t thousands of experimental participants have been historically inadvertent ly Ca-41 labeled. The Ca-41 urine index might now rapidly further be charac terized by contemporary measurements of these onetime subjects, and with th eir by now thoroughly skeleton-equilibrated tracer they might be ideal part icipants in other new experiments. We are also investigating Ca-45 AMS. It may prove preferable to label the skeleton with this radiotracer already fa miliar to bioscientists, but new to AMS, (C) 2000 Elsevier Science B.V. All rights reserved.