Na+/Ca2+ exchanger isoforms of rat odontoblasts and osteoblasts

In odontoblasts as well as osteoblasts, a number of mechanisms for the inflow and extrusion of Ca2+ have been demonstrated. The entrance of Ca2+ ions into odontoblasts occurs mainly through voltage-gated calcium channels. Extrusion of Ca2+ is found to be an ATP-dependent process and, in addition, Na+/Ca2+-antiports exist, which are provoked by extracellular Na+. The aim of this study was to identify the Na+/Ca2+-antiport isoforms expressed in dentinogenically active rat incisor odontoblasts and to make a comparison with different osteoblastic cells. Using RT-PCR and RNAse protection assay, we demonstrated the expression of three different isoforms, NaCa 3, 7, and 10, of the NCX1-encoded antiport in odontoblasts and osteoblastic cells. When incubated in the presence of Na+, dissected rat incisor odontoblasts as well as the osteoblastic cells extruded Ca2+ ions, as detected by chlorotetracycline and Fura-2 fluorometry, thus supporting a physiological role for the detected isoform expression. Odontoblasts and rat calvarial osteoblasts, as well as osteoblast-like cell lines UMR-106.01 and Saos-2, were shown to exhibit identical phenotypes of Na+/Ca2+-antiport isoform expression, different from the expression patterns of other tissues. The significance of this specific expression pattern is unknown, but there is a possibility that it is in some way related to the unique demands on these cell types to produce mineralized connective tissue.