pH Dependency of Free Ca²⁺ in Adult and Child Saliva

Objectives: One role of saliva in enamel homeostasis is to provide a source of calcium ions. At normal resting oral pH, many calcium ions are complexed to either inorganic ions, or bound to salivary proteins. Only unbound calcium ions contribute to enamel homeostasis, and therefore calculation of degree of saturation of bathing solutions with respect to enamel mineral is difficult. Many conventional methods of measuring [Ca²⁺] cannot give a true value of the free ion concentration because they cannot discriminate between free and bound ions. Novel solid state ion selective electrodes (ISEs) can measure free ion concentrations without contributions from bound ions. ISEs were used to compare the free [Ca²⁺] in the saliva of children and adults, and, to measure changes on reducing pH during a simulated oral challenge.

Methods: Stimulated and unstimulated saliva was collected from 19 adults and 19 children. ISE electrodes were used to measure free [Ca²⁺] in each. In 3 children and 4 adults, pH was reduced to 4 by addition of HCl, and [Ca²⁺] re-measured.

Results: The average free [Ca²⁺] for unstimulated and stimulated in adults was 0.33 and 0.83 ppm, and children were 0.79 and 0.89 ppm. These values were much lower than previously reported. Free [Ca²⁺] in the selected group increased by a factor of 9 in children and by a factor of 2.9 in adults when the pH was reduced to 4.

Conclusions: At normal oral pH, although values of free [Ca²⁺] in saliva measured using ISEs were lower than previously reported, saliva is still supersaturated with respect to enamel mineral in adults and children. However, as the pH is reduced, the there is release of calcium ions(presumably from the salivary proteins) which will further assist in the protection of enamel.