Mitochondrial Ca2+ flux through Na+/Ca2+ exchange

To clarify the property of mitochondrial Na+/Ca2+ exchange in situ, we measured mitochondrial Ca2+ using Rhod-2 in permeabilized rat ventricular myocytes. Cytoplasmic 300 nM Ca2+ (Ca-c(2+)) augmented the Rhod-2 intensity by similar to ninefolds without cytoplasmic Na+ (Na-c(+)). Increasing Na-c(+) attenuated the maximum level of Rhod-2 fluorescence, probably due to the activation of forward mode of mitochondrial Na+/Ca2+ exchange. The Rhod-2 intensity decayed upon removing Ca-c(2+). The decay was dependent on Na-c(+) (K-1/2 = similar to 1 mM) and largely abolished by an inhibitor of mitochondrial Na+/Ca2+ exchange, CGP-37157. It was suggested that Na+ binding to the mitochondrial Na+/Ca2+ exchange is saturated in the physiological concentration of Na-c(+).