Microscopic Metabolism of Calcium in Bone: V. The Paradox of Diffuse Activity and Long-Term Exchange

Abstract

By quantitative autoradiography and quantitative microradiography the in vivo diffuse uptake of Ca/sup 45/ in highly mineralized canine bone many years old has been measured and identified primarily with long-term exchange of calcium. By long-term exchange we mean an equal rate of transfer of calcium atoms in opposite directions between blood and bone in which the mean residence time of an individual calcium atom in bone is long, here of the order of a month or longer. In microscopic regions of canine cortical bone with a calcium density of 0.60 plus or minus 0.03 gm/sub Ca//cm/sup 3/, the rate of transfer of calcium atoms between blood and bone in this exchange is 0.05 to 0.20 gm/sub Ca/ per year per gram of calcium in bone. This rate of calcium transfer represents a significant fraction of the total calcium turnover in the skeleton of an adult dog and may play an important part in the calcium regulation of the blood. The possible implications for the distribution of longterm radiation dose to the skeleton due to internally deposited Sr/sup 90/ or Ra/sup 226/ are being examined. The observed rate of calcium transfer suggests that loss of Ca/sup 45/ activity from highly mineralizedmore » canine bone by a process other than resorption should be just detectable over a period of a year. (auth)« less