Transport and regulation of the cardiac Na+-Ca2+ exchanger, NCX1 - Comparison between Ca2+ and Ba2+

Cardiac muscle fails to relax upon replacement of extracellular Ca2+ with Ba2+. Among the manifold consequences of this intervention, one major possibility is that Na+-Ba2+ exchange is inadequate to support normal relaxation. This could occur due to reduced transport rates of Na+-Ba2+ exchange and/or by failure of Ba2+ to activate the exchanger molecule at the high affinity regulatory Ca2+ binding site. In this study, we examined transport and regulatory properties for Na+-Ca2+ and Na+-Ba2+ exchange. Inward and outward Na+-Ca2+ or Na+-Ba2+ exchange currents were examined at 30 degrees C in giant membrane patches excised from Xenopus oocytes expressing the cloned cardiac Na+-Ca2+ exchanger, NCX1. When excised patches were exposed to either cytoplasmic Ca2+ or Ba2+, robust inward Na+-Ca2+ exchange currents were observed, whereas Na+-Ba2+ currents were absent or barely detectable. Similarly, outward currents were greatly reduced when pipette solutions contained Ba2+ rather than Ca2+. However, when solution temperature was elevated from 30 degrees C to 37 degrees C, a substantial increase in outward Na+-Ba2+ exchange currents was observed, but not so for inward currents. We also compared the relative abilities of Ca2+ and Ba2+ to activate outward Na+-Ca2+ exchange currents at the high affinity regulatory Ca2+ binding site. While Ba2+ was capable of activating the exchanger, it did so with a much lower affinity (K-D similar to 10 mu M) compared with Ca2+ (K-D similar to 0.3 mu M). Moreover, the efficiency of Ba2+ regulation of Na+-Ca2+ exchange is also diminished relative to Ca2+, supporting similar to 60% of maximal currents obtainable with Ca2+. Ba2+ is also much less effective at alleviating Na-i(+)-induced inactivation of NCX1. These results indicate that the reduced ability of NCX1 to adequately exchange Na+ and Ba2+ contributes to failure of the relaxation process in cardiac muscle.