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°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°C to 37°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 ∼ 10 μM) compared with Ca²⁺ (K_D ∼ 0.3 μM). Moreover, the efficiency of Ba²⁺ regulation of Na⁺-Ca²⁺ exchange is also diminished relative to Ca²⁺, supporting ∼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.