Steady-State Relationship of Calcium-45 between Bone and Blood: Differences in Growing Dogs, Chicks, and Rats

In young animals that had received multiple doses of calcium-45, a constant ratio of calcium-45 specific activity in blood to that in bone was found in growing dogs and chicks but not in rats. This steady-state relationship of calcium-45 between bone and blood suggests that during growth in dogs and chicks most of the skeletal calcium is in an active state of turnover. In growing rats, after the first 2 weeks of life, the blood/bone ratio of calcium-45 decreases due to a decrease in bone resorption.