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 vivo function 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.