Ca2+ Extrusion via Na+-Ca2+ Exchangers in Rat Odontoblasts

Introduction: Intracellular Ca2+ is essential to many signal transduction pathways, and its level is tightly regulated by the Ca2+ extrusion system in the plasma membrane, which includes the Na+-Ca2+ exchanger (NCX). Although expression of NCX1 isoforms has been demonstrated in odontoblasts, the detailed properties of NCX remain to be clarified. In this study, we investigated localization and ion-transporting/pharmacologic properties of NCX isoforms in rat odontoblasts. Methods: We characterized both the reverse and forward modes of NCX activity in odontoblasts in a dental pulp slice preparation. Ca2+ influx by reverse NCX activity was measured by fura-2 fluorescence. Ca2+ efflux by forward NCX activity elicited inward Na+ current as measured by perforated-patch clamp recording. For immunohistochemical analysis, cryostat sections of incisors were incubated with antibodies against NCX. Results: Immunohistochemical observation revealed localization of NCX1 and NCX3 in the distal membrane of odontoblasts. Inward currents by forward NCX activity showed dependence on external Na+. Fura-2 fluorescence measurement revealed that Ca2+ influx by reverse NCX activity depended on extracellular Ca2+ concentration, and that this influx was blocked by NCX inhibitor KB-R7943 in a concentration-dependent manner. However, Ca2+ influx by NCX showed a slight sensitivity to SEA0400 (a potent NCX1 inhibitor), indicating that expression potencies in odontoblasts were NCX3 > NCX1. Conclusions: These results suggest that odontoblasts express NCX1 and NCX3 at the distal membrane, and that these isoforms play an important role in the Ca2+ extrusion system as well as in the directional Ca2+ transport pathway from the circulation to the dentin-mineralizing front. (1 Endod 2010;36:668-674)