Molecular Determinants of Na+/Ca2+ Exchange (NCX1) Inhibition by SEA0400

SEA0400 is a potent and selective Na+/Ca2+ exchanger (NCX) inhibitor. We evaluated the inhibitory effects of SEA0400 on Na+i-dependent 45Ca2+ uptake and whole-cell Na+/Ca2+ exchange currents in NCX-transfected fibroblasts. SEA0400 preferentially inhibited 45Ca2+ uptake by NCX1 compared with inhibitions by NCX2, NCX3, and NCKX2. SEA0400 also selectively blocked outward exchange currents from NCX1 transfectants. We searched for regions that may form the SEA0400 receptor in the NCX1 molecule by NCX1/NCX3 chimeric analysis. The results suggest that the first intracellular loop and the fifth transmembrane segment are mostly responsible for the differential drug responses between NCX1 and NCX3. Further site-directed mutagenesis revealed that multiple mutations at Phe-213 markedly reduced sensitivity to SEA0400 without affecting that to KB-R7943. We also found that Gly-833-to-Cys mutation (within the α-2 repeat) greatly reduced the inhibition by SEA0400, but unexpectedly the NCX1 chimera with an α-2 repeat from NCKX2 possessed normal drug sensitivity. In addition, exchangers with mutated exchanger inhibitory peptide regions, which display either undetectable or accelerated Na+-dependent inactivation, had a markedly reduced sensitivity or hypersensitivity to SEA0400, respectively. To verify the efficacy of the NCX inhibitor, we examined the renoprotective effect of SEA0400 in a hypoxic injury model using porcine renal tubular cells. SEA0400 protected against hypoxia/reoxygenation-induced cell damage in tubular cells expressing wild-type NCX1 but not in cells expressing SEA0400-insensitive mutants. These results suggest that Phe-213, Gly-833, and residues that eliminate Na+-dependent inactivation are critical determinants for the inhibition by SEA0400, and their mutants are very useful for checking the pharmacological importance of NCX inhibition by SEA0400.