Effect of pH and calcium concentration on calcium diffusion in streptococcal model-plaque biofilms

Demineralization during a cariogenic episode is affected by storage and transport in dental plaque of ions released from enamel, and by the effect on both of plaque fluid pH and ion concentrations. To investigate this, 45Ca effusion from a condensed film of streptococci was measured at pH 7, 6 and 5, and 0-20 mmol/l calcium. Cells were loaded into effusion chambers and the appearance of 45Ca and [3H]-inulin in carrier-containing but initially tracer-free buffer was measured. Ratios of 45Ca and [3H]-inulin activity in the initial suspending solution and at equilibrium in the clearance solution, permitted calculation of extracellular volume and bound calcium. The rate of Ca appearance was proportional to the retarded diffusion coefficient (rDe), which was related to the effective diffusion coefficient (De) by: rDe = De/(1 + R) in which R is the ratio of bound to free Ca2+. The rate of Ca2+ effusion increased with calcium concentration, converging on a value of 2.8 x 10(-10) m2/sec. At low pH it reached convergence at a lower [Ca]. This demonstrates that calcium effusion is dependent on binding, so a high proportion of binding sites in plaque will reduce mineral loss in vivo. Loss of binding sites at low pH will increase mineral loss.