Ca2+ signaling mediated by IP3-dependent Ca2+ releasing and store-operated Ca2+ channels in rat odontoblasts

In the phospholipase-C (PLC) signaling system, Ca2+ is mobilized from intracellular Ca2+ stores by an action of inositol 1,4,5-trisphosphate (IP3). The depletion of IP3-sensitive Ca2+ stores activates a store-operated Ca2+ entry (SOCE). However, no direct evidence has been obtained about these signaling pathways in odontoblasts. In this study, we investigate the characteristics of the SOCE and IP3-mediated Ca2+ mobilizations in rat odontoblasts using fura-2 microfluorometry and a nystatin-perforated patch-clamp technique. In the absence of extracellular Ca2+ ([Ca2+]o), thapsigargin (TG) evoked a transient rise in intracellular Ca2+ concentration ([Ca2+]i). After TG treatment to deplete the store, the subsequent application of Ca2+ resulted in a rapid rise in [Ca2+]i caused by SOCE. In the absence of TG treatment, no SOCE was evoked. The Ca2+ influx was dependent on [Ca2+]o (KD = 1.29 mM) and was blocked by an IP3 receptor inhibitor, 2-aminoethoxydiphenyl borate (2-APB), as well as La3+ in a concentration-dependent manner (IC50 = 26 microM). In TG-treated cells, an elevation of [Ca2+]o from 0 to 2.5 mM elicited an inwardly rectifying current at hyperpolarizing potentials with a positive reversal potential. The currents were selective for Ca2+ over the other divalent cations (Ca2+ > Ba2+ > Sr2+ >> Mn2+). In the absence of [Ca2+]o, carbachol, bradykinin, and 2-methylthioadenosine 5'triphosphate activated Ca2+ release from the store; these were inhibited by 2-APB. These results indicate that odontoblasts possessed Ca2+ signaling pathways through the activation of store-operated Ca2+ channels by the depletion of intracellular Ca2+ stores and through the IP3-induced Ca2+ release activated by PLC-coupled receptors.