Kinetic Consideration of Ca Transport and Associated Crystal Precipitation in an in vitro Precipitation Model Using Dialysis Membranes: Fluoride-dependent Regulation of Ca Concentration in Liquid Environment

To elucidate how the fluid environment in forming enamel is modified by ion transport through the ameloblasts and associated extracellular events, we studied precipitation reactions in an experimental model utilizing a dialysis chamber. The experimental set-up comprised of upper and lower compartments, which were separated by a dialysis membrane of various cut-off sizes. Synthetic hydroxyapatite and pig enamel proteins were used as seeds and adsorbates, respectively. Seed crystals with or without proteincoating were placed in the lower compartment. Upon feeding the supersaturated solution (containing 1mM or 0.5mM Ca) in the upper compartment, Ca ion was allowed to diffuse into the lower compartment according to the existing concentration gradients. By monitoring time-sequential changes in Ca concentration in the lower solution, we found that Ca supply through the membrane was a rate-determining step, so the Ca concentration in the medium surrounding seed crystals became in proximity to the steady-state condition, under which the Ca concentration ([Ca] s) was maintained at lower levels than that of the supersaturated solution filling the upper compartment. The establishment of [Ca] s in the mineralizing milieu was speculated to be due to the kinetic difference between Ca supply and consumption for precipitation because [Ca] s levels were affected markedly by the presence of regulators (i. e., enamel proteins and fluoride) for precipitation. Notably, fluoride at 1ppm or lower levels exerted decreasing effects on [Ca] s, due to accelerating the precipitation kinetics and lowering the solubility of precipitated fluoridated crystals. The overall results obtained support the importance of the multitudinous physico-chemical processes in determin- ing the fluid composition surrounding forming enamel mineral.