Salivary Free Ca²⁺ Concentration During a Simulated Stephan pH Cycle

Objectives: During a Stephan cycle, oral pH drops, then gradually returns to physiological pH. During the acidic challenge, the dental hard tissues are exposed to demineralising conditions. Our hypothesis is that one cariostatic function of saliva is to increase the free calcium ion concentration only during the period of low pH, reducing the driving force for demineralisation, but preventing unwanted precipitation at physiological pH. The interaction of calcium ions and salivary proteins in relation to this calcium management mechanism is of interest to understand the cariostatic functions of saliva. The objective was to measure the free calcium ion concentration during a complete simulated Stephan cycle.

Methods: Twenty ml of chewing gum base stimulated whole saliva was collected from 32 adults, aged 21-49 years. Saliva specimens were titrated to pH 5 by dropwise addition of 0.1 M acetic acid, then returned to their initial pH by dropwise addition of 0.1 M sodium bicarbonate. The free calcium ion concentration was recorded throughout using an ion selective electrode. The percentage change in free calcium ion concentration during the complete Stephan cycle was calculated for each individual. All changes were compared by the Wilcoxon matched pair sign rank test using a significance level of 0.05.

Results: The mean of the free calcium ion concentration in the saliva of all individuals increased by 190% (s.e.= 30% ) when the pH was decreased to pH 5. This subsequently decreased to 113% (s.e.= 30% ) of the original value when the pH was subsequently returned to physiological value.

Conclusion: Saliva protects dental hard tissues by releasing free calcium ions during an acidic challenge reducing the driving force for demineralisation. The partial reversibility of this process may prevent unwanted precipitation at physiological pH.