Transport and Regulation of the Cardiac Na(+)-Ca(2+) Exchanger, NCX1 : Comparison between Ca(2+) and Ba(2+)

Cardiac muscle fails to relax upon replacement of extracellular Ca(2+) with Ba(2+). Among the manifold consequences of this intervention, one major possibility is that Na(+)-Ba(2+) exchange is inadequate to support normal relaxation. This could occur due to reduced transport rates of Na(+)-Ba(2+) exchange and/or by failure of Ba(2+) to activate the exchanger molecule at the high affinity regulatory Ca(2+) binding site. In this study, we examined transport and regulatory properties for Na(+)-Ca(2+) and Na(+)-Ba(2+) exchange. Inward and outward Na(+)-Ca(2+) or Na(+)-Ba(2+) exchange currents were examined at 30°C in giant membrane patches excised from Xenopus oocytes expressing the cloned cardiac Na(+)-Ca(2+) exchanger, NCX1. When excised patches were exposed to either cytoplasmic Ca(2+) or Ba(2+), robust inward Na(+)-Ca(2+) exchange currents were observed, whereas Na(+)-Ba(2+) currents were absent or barely detectable. Similarly, outward currents were greatly reduced when pipette solutions contained Ba(2+) rather than Ca(2+). However, when solution temperature was elevated from 30°C to 37°C, a substantial increase in outward Na(+)-Ba(2+) exchange currents was observed, but not so for inward currents. We also compared the relative abilities of Ca(2+) and Ba(2+) to activate outward Na(+)-Ca(2+) exchange currents at the high affinity regulatory Ca(2+) binding site. While Ba(2+) was capable of activating the exchanger, it did so with a much lower affinity (K (D) ∼ 10 μM) compared with Ca(2+) (K (D) ∼ 0.3 μM). Moreover, the efficiency of Ba(2+) regulation of Na(+)-Ca(2+) exchange is also diminished relative to Ca(2+), supporting ∼60% of maximal currents obtainable with Ca(2+). Ba(2+) 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 Ba(2+) contributes to failure of the relaxation process in cardiac muscle.