Ca2+ extrusion via Na+-Ca2+ exchangers in rat odontoblasts

Intracellular Ca(2+) is essential to many signal transduction pathways, and its level is tightly regulated by the Ca(2+) extrusion system in the plasma membrane, which includes the Na(+)-Ca(2+) 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.

We characterized both the reverse and forward modes of NCX activity in odontoblasts in a dental pulp slice preparation. Ca(2+) influx by reverse NCX activity was measured by fura-2 fluorescence. Ca(2+) 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.

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 Ca(2+) influx by reverse NCX activity depended on extracellular Ca(2+) concentration, and that this influx was blocked by NCX inhibitor KB-R7943 in a concentration-dependent manner. However, Ca(2+) influx by NCX showed a slight sensitivity to SEA0400 (a potent NCX1 inhibitor), indicating that expression potencies in odontoblasts were NCX3 > NCX1.

These results suggest that odontoblasts express NCX1 and NCX3 at the distal membrane, and that these isoforms play an important role in the Ca(2+) extrusion system as well as in the directional Ca(2+) transport pathway from the circulation to the dentin-mineralizing front.