Mechanical distortion influences on transjunctional Ca spreading in human odontoblasts

Objectives: Odontoblasts (OBs) are well-known to be associated with dentinification. They have a monopolar processus in dentinal tubule and align in the outermost dentin. Thus, they seem suitable to be a possible sensor even though clear evidence for a role of OBs as sensory receptor cells has not been clarified. But, technical difficulties have hampered approach to them. We have demonstrated cell-to-cell communication between odontoblasts. Many important cell functions are controlled by Ca²⁺ release from intracellular stores via the inositol 1,4,5-trisphosphate receptor (IP₃R). We designed the present study to determine intra-/intercellular Ca²⁺ propagation in mechanically stimulated and IP3-injected human OB and surrounding OBs.

Methods: Six freshly extracted premolars for an orthodontic reason from informed patients were used. Cells with a monopolar processus (>100mm) were plating HEPE-based buffer with fura-2 acetoxymethyl ester (3 µM). Intracellular Ca²⁺ concentrations were monitored when mechanically stimulated with a fire-polished glass micropipette by a micromanipulator or when IP3 was intracellularly applied either with or without gap-junction blocker (25mM 2-bromo-2-chloro-1,1,1-trifluiroethane) and extracellular Ca²⁺.

Results: (1) Mechanical stimulation of a single OB resulted in an significant (p<0.05) increase of cytosolic Ca²⁺ in the stimulated OB that propagated from the point of stimulation throughout the cell cytoplasm. (2) Mechanical stimulation to OB cell membrane significantly increased intracellular Ca²⁺ in surrounding OBs. (3) Intracellularly injected IP₃ also increased intracellular Ca²⁺ significantly. The IP₃ dependent-intracellular Ca²⁺ propagation to adjacent OBs was inhibited by gap-junction blocker.

Conclusions: (1) Intracellular Ca²⁺ signaling system exists in the human OB layer and may play an important role in mechano-transduction mechanism triggered by deformation of cell membrane. (2) Influx of extracellular Ca²⁺ into the mechanically stimulated OB may contribute to intracellular increase of Ca²⁺ propagation. (3) Cell-to-cell Ca²⁺ propagation may be mainly mediated IP₃ dependent Ca²⁺ release from intracellular pool.