Inhibitory effect of aprindine on Na + /Ca 2+ exchange current in guinea‐pig cardiac ventricular myocytes

Using the whole‐cell voltage clamp technique, the effect of aprindine on Na + /Ca 2+ exchange current ( I NCX ) was examined in guinea‐pig single cardiac ventricular myocytes and CCL39 fibroblasts expressing a dog cardiac Na + /Ca 2+ exchanger (NCX1).I NCX was recorded by ramp pulses from the holding potential of −60 mV with the external solution containing 140 m M Na + and 1 m M Ca 2+ , and the pipette solution containing 20 m M Na + , 20 m M BAPTA and 13 m M Ca 2+ (433 n M free Ca 2+ ). External application of aprindine suppressed I NCX in a concentration‐dependent manner. The IC 50 values of outward (measured at 50 mV) and inward (measured at −100 mV) I NCX components were 48.8 and 51.8 μ M with Hill coefficients of 1.3 and 1, respectively. Intracellular application of trypsin via the pipette solution did not change the blocking effect of aprindine, suggesting that aprindine does not affect the exchanger from the cytoplasmic side. Aprindine inhibited I NCX of a mutant NCX1 with a deletion of amino acids 247 – 671 in the large intracellular domain between the transmembrane segments 5 and 6 in a similar manner to that of the wild‐type, suggesting that the site of aprindine inhibition is not in the large intracellular domain of NCX1. A kinetic study indicated that aprindine was cooperatively competitive with KB‐R7943, another inhibitor of NCX and that aprindine was a competitive inhibitor with respect to external Ca 2+ . We conclude that aprindine may modestly inhibit I NCX in a therapeutic range of concentrations (around 2.5∼6.9 μ M ) possibly at an external or intra‐membranous site of the exchanger.British Journal of Pharmacology (2002) 136 , 361–366; doi: 10.1038/sj.bjp.0704721