Tetracycline and Calcium Kinetics Are Comparable for Estimating Bone Resorption in Rats

Abstract

³H-tetracycline (³H-TC) is thought to be superior to calcium (Ca) isotopic tracers for estimating bone resorption rates due to the less redeposition upon release in animal models. However, these 2 tracers have not been compared directly using complete kinetic studies with sampling of blood, urine, feces, and bone. Our goal was to compare the 2 isotopes for evaluating bone turnover. We firstly developed a model for ³H-TC kinetics in 4-mo-old female rats (n = 3) by measuring the tracer in serum, urine, and feces over 4 d. Then, 9-mo-old ovariectomized (OVX) rats (n = 6) were given both ⁴⁵Ca and ³H-TC subcutaneously. Urine was collected in 24-h pools and assayed for both tracers. Rats were killed 7 and 46 d after the dose and whole skeleton was harvested. We calculated bone resorption rates by modeling the ⁴⁵Ca and ³H-TC data in urine and bone. ³H-TC kinetics revealed that, like Ca, there are 2 exchangeable compartments between serum and bone. An additional pool was required to account for bone mass of Ca. Bone resorption rates determined from urinary ⁴⁵Ca and ³H-TC did not differ significantly. The tracers ⁴⁵Ca and ³H-TC can be used interchangeably to determine bone resorption rates in OVX rats. Thus, both labels can be used to screen dietary and other interventions for beneficial effects on bone.