Fluctuations in surface pH of maturing rat incisor enamel are a result of cycles of H+-secretion by ameloblasts and variations in enamel buffer characteristics

It is disputed if ameloblasts in the maturation zone of the enamel organ mainly buffer protons released by hydroxyapatite (HA) crystal growth or if they periodically secrete protons to create alternating acidic and alkaline conditions. The latter hypothesis predicts alternating pH regimes in maturing enamel, which would be affected by pharmacological interference with ameloblast H+-secretion. This study tests these predictions. Colorimetric pH-indicators and ratiometric fluorometry were used to measure surface pH in maturation zone enamel of rat incisors. Alternating acidic (down to pH 624 +/- 0.06) and alkaline zones (up to pH 7.34 +/- 0.08) were found along the tooth coinciding with ameloblast morphological cycles. Underlying the cyclic pattern, a gradual decrease in pH towards the incisal edge was seen. Vinblastine or FR167356 (H+-ATPase-inhibitor) disturbed ameloblast acid-secretion, especially in the early parts of acidic zones. Enamel surface pH reflects the titration state of surface PO43--ions. At the pH-values observed, PO43- would be protonated (pKa > 12) and HA dissolved. However, by molecular dynamics simulations we estimate the pKa of HP0(4)(2-) at an ideal HA surface to be 4.3. The acidic pH measured at the enamel surface may thus only dissolve non-perfect domains of HA crystals in which PO43- is less electrostatically shielded. During repeated alkaline/ acidic cycles, near-perfect HA-domains may therefore gradually replace less perfect HA-domains resulting in near-perfect HA-crystals. In conclusion, cyclic changes in ameloblast H+-secretion and the degree of enamel maturation determine enamel surface pH. This is in accordance with a hypothesis implicating H+-ATPase mediated acid-secretion by ameloblasts. (C) 2013 Elsevier Inc. All rights reserved.