Effects of salivary bicarbonate content and film velocity on pH changes in an artificial plaque containing Streptococcus oralis, after exposure to sucrose.

Chewing-gum stimulation of salivary flow (at the time of the pH minimum following exposure of plaque to carbohydrate) has been shown to cause a rapid increase in plaque pH. The objective of this study was to determine whether the rise in plaque pH is primarily due to the increased buffering capacity of stimulated saliva, or to the fact that an increased flow rate increases the concentration gradient for acid to diffuse from the plaque into the overlying salivary film, which will be moving at a higher velocity. This was investigated with an in vitro technique in which artificial plaque (0.5 or 1.5 mm deep) containing S. oralis cells was exposed to 10% sucrose for one min. The pH values at the proximal and distal undersurfaces of the plaque were then monitored during the passage of a 0.1-mm-thick film of a sucrose-free artificial saliva over the surface, at a range of film velocities (0.8-8 mm/min) that have been estimated to occur in vivo. When a minimum plaque pH had been achieved, the salivary film velocity was either (a) kept the same, with or without 15 mmol/L HCO3 (the concentration measured in chewing-gum-stimulated saliva), (b) increased to 86.2 mm/min, or (c) increased to 86.2 mm/min with 15 mmol/L HCO3 added to the artificial saliva. The findings suggest that after sucrose ingestion, the rapid rise from minimum plaque pH values, which can occur with gum-chewing stimulation of salivary flow, is due to the combined effects of the increase in salivary film velocity, and of a greater availability of bicarbonate.(ABSTRACT TRUNCATED AT 250 WORDS)