A comparative study of the exchange in vivo of major constituents of bone mineral

Groups of young and old rats were injected with a variety of labelled substanzes (urea, Cl-, K+, Na+, HCO3-, PO43-, Ca++). Data for Mg++ were taken from the literature. One and a half hours later, compact shafts of long bones were removed and cleaned scrupulously, and analyses were performed for both "cold" and isotopic concentrations of substances. This time point was chosen to insure equilibration of the aqueous phase of bone while minimizing contributions from surface exchange, recrystallization, solid diffusion, growth or resorption. With fixed variables of time, species, bone specimen, and methodology, uambiguous comparisons of the exchange in bone could be made between the many substances studied. The exchange data could be divided into three categories: a) complete exchange (urea Cl-, and K+); b) partial exchange, decreasing variably with age (Na+, CO2, and Mg++); and c) minimal exchange (Ca++ and PO43-). Clearly the traditional classification of "available" and "unavailable" skeleton is ambiguous and determined by the conditions and the ion or substance chosen for study. Clearly also, a new overall concept of bone exchange in vivo is badly needed. Calculations of the apparent concentration of the various electrolytes in bone water reveal that the aqueous phase of bone has a composition markedly different from plasma water. In particular, the concentration of potassium in bone water was found to be remarkably high. © 1968 Springer-Verlag.