Comparative deposition of 45Ca, 65Zn, and 91Y in bone.

Autoradiographic studies of bone were made after intravenous injections of three radioisotopes-45 Ca, ${}^{91}{\rm Y}$, and65 Zn-into rats, dogs, and a monkey. Counting techniques were used to measure the amount of each of these radioisotopes retained in the concentrated areas of new bone formation compared with that retained diffusely in the fully calcified bone. It was found that the ratio of concentrated to diffuse concentrations differed considerably for the three radioisotopes studied. For45 Ca the mean ratio was 8.9; for65 Zn the ratio was 40.1; and for ${}^{91}{\rm Y}$ the ratio was 4.4. Some attempts were made to discover the mechanism of uptake of these radioisotopes in bone tissue. Both calcium and yttrium were taken up in association with the mineral phase of bone but in a different way, while the mechanism of the uptake of65 Zn remains obscure.