Cloning, expression, and characterization of the trout cardiac Na+/Ca2+exchanger.

Isoform 1 of the cardiac Na+/Ca2+exchanger (NCX1) is an important regulator of cytosolic Ca2+ concentration in contraction and relaxation. Studies with trout heart sarcolemmal vesicles have shown NCX to have a high level of activity at 7°C, and this unique property is likely due to differences in protein structure. In this study, we describe the cloning of an NCX (NCX-TR1) from a Lambda ZAP II cDNA library constructed from rainbow trout ( Oncorhynchus mykiss) heart RNA. The NCX-TR1 cDNA has an open reading frame that codes for a protein of 968 amino acids with a deduced molecular mass of 108 kDa. A hydropathy plot indicates the protein contains 12 hydrophobic segments (of which the first is predicted to be a cleaved leader peptide) and a large cytoplasmic loop. By analogy to NCX1, NCX-TR1 is predicted to have nine transmembrane segments. The sequences demonstrated to be the exchanger inhibitory peptide site and the regulatory Ca2+ binding site in the cytoplasmic loop of mammalian NCX1 are almost completely conserved in NCX-TR1. NCX-TR1 cRNA was injected into Xenopus oocytes, and after 3-4 days currents were measured by the giant excised patch technique. NCX-TR1 currents measured at ∼23°C demonstrated Na+-dependent inactivation and Ca2+-dependent activation in a manner qualitatively similar to that for NCX1 currents.

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