Na + /Ca 2+ exchange in rat osteoblast‐like UMR 106 cells

Ca 2+ efflux from osteoblasts is thought to be mediated by Na + /Ca 2+ exchange and by a plasma membrane Ca 2+ ‐ATPase. The presence of plasma membrane Na + /Ca 2+ exchange was determined in rat UMR 106 osteosarcoma cells by functional and molecular studies. Na + /Ca 2+ exchange activity was tested by measuring changes of [Ca 2+ ] i in single cells. After Na + loading the cells and removing extracellular Na + , the direction of exchange was reversed and [Ca 2+ ] i increased by 100%. Multiple isoforms of the NCX 1 gene product, encoding plasma membrane Na + /Ca 2+ exchangers, were cloned from UMR 106 cells and a sample of primary human osteoblasts using homology‐based RT‐PCR. Isoforms NACA3, NACA7, and NACA10 were found in UMR 106 cells, whereas human osteoblasts expressed NACA3 and NACA7. Transcripts for NCX 2 and the Na + /Ca 2+ , K + exchanger were not detected. Northern analysis of UMR 106 cells with a probe to the NCX 1 gene product revealed the presence of a transcript of 7 kb, the size of the exchanger message. Western analysis of UMR 106 cell membrane preparations with a polyclonal antibody specific for the NCX 1 exchanger showed the presence of reacting proteins consistent with the reported masses of the exchanger at 125 and 85 kD. These results demonstrate Na + ‐dependent Ca 2+ efflux from UMR 106 cells and the presence of several NACA isoforms in UMR 106 and primary human osteoblasts. (J Bone Miner Res 1996;11:1666‐1675)