Modulation of Ca2+ signaling by Na+/Ca2+ exchangers in mast cells

Mast cells rely on Ca2+ signaling to initiate activation programs leading to release of proinflammatory mediators. The interplay between Ca2+ release from internal stores and Ca2+ entry through store-operated Ca2+ channels has been extensively studied. Using rat basophilic leukemia (RBL) mast cells and murine bone marrow-derived mast cells, we examine the role of Na+/Ca2+ exchangers. Calcium imaging experiments and patch clamp current recordings revealed both K+-independent and K+-dependent components of Na+/Ca2+ exchange. Northern blot analysis indicated the predominant expression of the K+-dependent sodium-calcium exchanger NCKX3. Transcripts of the exchangers NCX3 and NCKX1 were additionally detected in RBL cells with RT-PCR. The Ca2+ clearance via Na+/Ca2+ exchange represented similar to50% of the total clearance when Ca2+ signals reached levels greater than or equal to200 nM. Ca2+ signaling and store-operated Ca2+ entry were strongly reduced by inverting the direction of Na+/Ca2+ exchange, indicating that Na+/Ca2+ exchangers normally extrude Ca2+ ions from cytosol and prevent the Ca2+-dependent inactivation of store-operated Ca2+ channels. Working in the Ca2+ efflux mode, Na+/Ca2+ exchangers such as NCKX3 and NCX3 might, therefore, play a role in the Ag-induced mast cell activation by controlling the sustained phase of Ca2+ mobilization.