Electrogenic property of mitochondrial Na⁺-Ca²⁺ exchange in rat ventricular myocyte

Abstract

To study electrogenecity and voltage-dependence of mitochondrial Na⁺-Ca²⁺ exchange (NCX_mit), we measured mitochondrial membrane potential (ΔΨ_mit) and mitochondrial Ca²⁺ (Ca²⁺_mit) using TMRE and Rhod-2, respectively, in permeabilized rat ventricular myocytes. The induction of forward mode of NCX_mit depolarized ΔΨ_mit in the mitochondria preloaded with 300 nM Ca²⁺, when the respiratory chain was suppressed. On the other hand, ΔΨ_mit hyperpolarized when the reverse mode of NCX_mit was induced by applying 600 nM Ca²⁺ under the conditions that mitochondrial Ca²⁺ uniporter was suppressed by ruthenium red and that ΔΨ_mit was depolarized by FCCP. These results indicated that NCX_mit is electrogenic. In agreement with the above findings, applying 600 nM Ca²⁺ augmented Ca²⁺_mit by about 500% when ΔΨ_mit was depolarized by FCCP + oligomycin or antimycin A + oligomycin, while it did not induce significant change in Ca²⁺_mit when ΔΨ_mit was intact. However, the ΔΨ_mit depolarization did not significantly affect Ca²⁺ efflux rate via the forward mode of NCX_mit in mitochondria preloaded with 300 nM Ca²⁺. Our computer simulation supported the electrogenic and voltage-dependent property of NCX_mit and suggested that the net Ca²⁺ efflux via the forward mode largely saturated at negative ΔΨ_mit when Ca²⁺_mit is relatively low. It was concluded that NCX_mit is electrogenic and voltage-dependent. [J Physiol Sci. 2008;58 Suppl:S185]