Comparative Kinetics of Ca 47 and Sr 85 in Man

Abstract

In studies of skeletal metabolism, the radionuclide Sr85 is frequently employed as a tracer for calcium on the basis that the chemical similarity of these two elements produces correspondingly similar metabolic responses. In certain processes, however, the two elements are handled discriminatively. For example, there is preferential absorption of calcium over strontium by the intestinal wall, and preferential elimination of Sr over Ca by the kidneys. Further, some evidence has been adduced which indicates that discrimination may also occur at the level of bone. Although this has not been established conclusively, crystallographic studies do indicate that Sr cannot easily substitute for Ca in the bone lattice because of the size of the Sr ion (1). The conditions under which Sr may be used as a tracer in order to characterize Ca metabolism in bone remains to be determined more precisely. Such a study necessitates a delineation of both the qualitative and the quantitative differences in the skeletal metabolism of Ca and Sr. The problem may be divided into two parts. The first situation is the short-term study (up to approximately 30 days) which is characterized by the rapid equilibration of Sr and Ca among blood, soft tissue, and bone, and by the subsequent rapid loss of Ca and Sr from bone which must follow the loss from the body. This study is the subject of the present paper. The second is the long-term study, which is characterized chiefly by the very slow loss of Ca and Sr from bone that occurs in remodeling as a result of resorption, and by slow exchange between bone and plasma. This long-term study is the subject of a separate paper (2). Measurements of whole-body retention of Sr85 and Ca47 were made with a wholebody gamma spectrometer. Simultaneous measurements of the tracers made on the excreta and on samples of plasma yielded data necessary for the determination of the skeletal accretion rate and exchange capacity. The kinetics of skeletal metabo-