Two compartments of slowly exchangeable sodium in bone

Abstract

The commonly accepted concept of sodium kinetics is a two-compartment system where total sodium (NaT) is the sum of bone sodium (NaB) and soft tissue sodium (NaST). On the basis of the experiments described, the authors propose that the slowly exchangeable sodium (NaSE) is equated with NaB and the quickly exchangeable sodium (NaQE) with NaST. They propose that NaB be subdivided into two compartments, NaB=NaSEI+NSEII, where I refers to sodium on the bone mineral crystal surface (NaBI=NaBMCS) and II refers to sodium in the bone mineral crystal interior (NaSEII=NaBMCI). The above experiment was repeated with ashed samples of the rabbit femur (Pilecka and Rakovic, 1991). In this case, the release of the radiotracer on stirring was nearly abolished, which is in perfect agreement with a known concept of the thermal recrystallization of the bone mineral (Neuman and Neuman, 1958). Further evidence about the heteroionic exchange can be acquired by in vitro experiments performed under different conditions, with different types of bones and with different ions.