Ionic interaction with bone mineral I. Evidence for an isoionic calcium exchange with hydroxyapatite

Exchange of calcium with hydroxyapatite crystals in aqueous suspension was measured with ⁴⁵Ca. The results were analyzed by fitting to a curve characterized by four exponential terms, of the form: Isotope uptake (or release) = σ i=I i=4A_i^e-α_i^t Total exchange was also calculated from measurements of non-isotopic calcium. Isotopic steady state was reached by 65 h. Uptake and release were characterized by similar A and α values. Release in 1 h was the same whether uptake had been carried out for 1 or 65 h. This, with the high values of α₁ and α₂, indicates that early exchange is quantitatively accounted for largely by the contribution of two 'fast' compartments. As further evidence for exchange, the uptake of isotope by hydroxyapatite was completely reversible by a wash-out procedure. Release was largely prevented by omission of Ca²⁺ from solution, whether or not they were replaced with Na⁺; this indicates that calcium uptake is an isoionic exchange process.