ON NATURE OF EXCHANGEABLE SODIUM IN BONE

While earlier studies in the literature have established that some skeletal sodium resides in intracrystalline lattice positions and is non-exchangeable, some questions remained as to the location of that sodium which is readily exchanged. A kinetic study was made of a model system (hydroxyapatite crystals)in vitro. Only under special conditions could lattice-bound surface sites be demonstrated. Under physiological conditions, the exchange was nearly instantaneous; 99% of the final equilibrium was attained in two minutes or less. Such a rapid exchange is consistent only with an aqueous reaction. The surface sodium is, therefore, present in the hydration layers of the individual crystals. The amount of sodium associated with the crystal surface itself is, for practical purposes, insignificant. © 1969 Springer-Verlag.