Effects of age on cell size and ion uptake in canine cortical bone

In cortical bone from dogs of various ages, the uptakes of two bone-seeking isotopes, 85Sr and 99mTc-methylene diphosphonate (MDP), were compared with that of the intracellular cation 42K by using the volume of distribution (VD) technique. The rate of bone remodeling and the osteocyte volume were estimated by morphometric techniques. The percentage cell volume was related to the VD of 42K (r = 0.83; P less than 0.001) and decreased with increasing age. The decline in the VD of 85Sr and 99mTc-MDP with aging was related to the decrease in bone formation (r = 0.90; P less than 0.001). On a logarithmic plot, the cell volume and the VD of 42K had a linear relationship with the rate of new bone formation, the VD of 85Sr, and the VD of 99mTc-MDP. Our results demonstrate that, with increasing age in dogs, the bone formation rate and cell size decrease, as does the uptake of 42K, 99mTc-MDP, and 85Sr.