Assay of bone resorption in vivo with 3H-tetracycline.

3H-Tetracycline (3H-TC) was used to quantify resorption in whole bones of growing rats and dogs. After repeated isotopic labeling of actively growing embryos or neonates, 3H-TC was observed to be distributed homogeneously and in equilibrium with 45Ca. A rapid and large loss of 3H-TC and a small loss of 45Ca occurred during the early weeks of rapid bone growth, suggesting that absolute amounts of 45Ca resorbed from bone, as reflected by losses of 3H-TC, are five to ten times greater than the net amounts of 45Ca lost from bone. Minimal loss of 3H-TC occurred due to nonspecific physicochemical exchange in vivo or in vitro (5%) except with nonradioactive tetracycline, and 3H-TC was not greatly exchanged or reused (10%) in vivo. The data are considered in terms of local and systemic conservation of calcium.