Molecular cloning and characterization of the human cardiac Na+/Ca2+ exchanger cDNA

The Na+/Ca2+ exchanger plays important roles in Ca2+ handling in many excitable cells. In particular, the Na+/Ca2+ exchanger is expressed at high levels in the cardiac sarcolemma and is the dominant mechanism of Ca2+ extrusion from the cells. In addition, the exchanger has been suggested to play key roles in digitalis action and in postischemic reperfusion injury of cardiac myocytes. We report here the isolation and characterization of the cDNA encoding the human cardiac Na+/Ca2+ exchanger. Twelve overlapping clones corresponding to 5.6 kilobases of the exchanger cDNA sequence were isolated from 5 x 10(5) phage plaques screened. The sequence predicted a 973-amino acid polypeptide with a putative leader peptide, 11 potential membrane-spanning regions, and one large putative cytoplasmic loop between the fifth and sixth transmembrane helices. When RNA was synthesized in vitro from the cloned cDNA and injected into Xenopus oocytes, it induced expression of Na+/Ca2+ exchange activity at high levels, confirming that this clone encodes the functional Na+/Ca2+ exchanger. Southern blot analysis indicated that the cardiac exchanger gene exists as a single copy in the human genome, although existence of other related genes cannot be ruled out. Northern blot and S1 mapping analyses revealed that the cardiac type exchanger mRNA is expressed most abundantly in the heart and next in the brain. The cardiac-type exchanger mRNA was also expressed in the retina and in skeletal and smooth muscles at very low levels. The levels of mRNA encoding the exchanger were significantly lower in fetal hearts than in adult hearts but were unchanged in the myocardium from patients with end-stage heart failure.