Calcium transport in dentinogenesis. An experimental study in the rat incisor odontoblast

Since cellular calcium transport mechanisms during biological calcification are less known, a series of experiments were performed by in vivo as well as in vitro methodologies in the dentinogenically active rat incisor. By means of micro-electrode technique, the pH and pCa (calcium ion activity) in predentin in situ were found to be 7.0 and 2.9, respectively. It was concluded that there exists a Ca2+ion concentrating mechanism over the odontoblast layer in direction towards the mineralization front. The kinetics of this calcium flow was determined in vivo by radiotracer technique. The time for 45Ca2+ uptake into the dentin mineral phase was determined to 10-15 min. Transmembraneous Ca2+ ion pumps and channels in odontoblasts were further analyzed. The resting membrane potential of rat incisor odontoblasts was determined to -24 mV. Using ion-specific mini-electrode technique as well as fluorescence spectrophotometry, calcium channels, Ca(2+)-ATPase and Na+/Ca2+ antiports, responsible for cellular Ca2+ uptake and extrusion, were identified in the odontoblast plasma membrane. Dissected odontoblasts were subjected to subcellular fractionation. An electrophoretic uniporter and a Na2+/Ca2+ exchanger, for Ca2+ release and uptake, respectively, were demonstrated in mitochondria, whereas a Ca(2+)-ATPase was present in the microsomal fraction. Mitochondria, microsomes and whole, digitonin-permeabilized odontoblasts, were able to maintain a steady state Ca2+ activity at pCa = 6.4-6.6 in vitro. In rats treated with colchicine, the incorporation of 45Ca2+ into dentin mineral was severely altered. Similarly, administration to rats of specific calcium channel blockers strongly inhibited 45Ca2+ incorporation. Together, the results indicate that a transcellular pathway is a major route for Ca2+ ion transport during dentinogenesis, and that this may be under a relatively strict cellular control.