Membrane current evoked by mitochondrial Na⁺-Ca²⁺ exchange in mouse heart

The electrogenicity of mitochondrial Na⁺-Ca²⁺ exchange (NCXm) had been controversial and no membrane current through it had been reported. We succeeded for the first time in recording NCXm-mediated currents using mitoplasts derived from mouse ventricle. Under conditions that K⁺, Cl^-, and Ca²⁺ uniporter currents were inhibited, extra-mitochondrial Na⁺ induced inward currents with 1 μM Ca²⁺ in the pipette. The half-maximum concentration of Na⁺ was 35.6 mM. The inward current was diminished without Ca²⁺ in the pipette, and was augmented with 10 μM Ca²⁺. The Na⁺-induced inward currents were largely inhibited by CGP-37157, an NCXm blocker. However, the reverse mode of NCXm, which should be detected as an outward current, was hardly induced by extra-mitochondrial application of Ca²⁺ with Na⁺ in the pipette. It was concluded that NCXm is electrogenic. This property may be advantageous for facilitating Ca²⁺ extrusion from mitochondria, which has large negative membrane potential.