Targeted Disruption of Na+/Ca2+Exchanger Gene Leads to Cardiomyocyte Apoptosis and Defects in Heartbeat*

Ca2+, which enters cardiac myocytes through voltage-dependent Ca2+channels during excitation, is extruded from myocytes primarily by the Na+/Ca2+exchanger (NCX1) during relaxation. The increase in intracellular Ca2+concentration in myocytes by digitalis treatment and after ischemia/reperfusion is also thought to result from the reverse mode of the Na+/Ca2+exchange mechanism. However, the precise roles of the NCX1 are still unclear because of the lack of its specific inhibitors. We generated Ncx1-deficient mice by gene targeting to determine the in vivofunction of the exchanger. Homozygous Ncx1-deficient mice died between embryonic days 9 and 10. Their hearts did not beat, and cardiac myocytes showed apoptosis. No forward mode or reverse mode of the Na+/Ca2+exchange activity was detected in null mutant hearts. The Na+-dependent Ca2+exchange activity as well as protein content of NCX1 were decreased by ∼50% in the heart, kidney, aorta, and smooth muscle cells of the heterozygous mice, and tension development of the aortic ring in Na+-free solution was markedly impaired in heterozygous mice. These findings suggest that NCX1 is required for heartbeats and survival of cardiac myocytes in embryos and plays critical roles in Na+-dependent Ca2+handling in the heart and aorta.