Previously, we showed rapid mitochondrial Ca2+ transients in adult rabbit cardiac myocytes during the contractile cycle. Ruthenium red, an inhibitor of mitochondrial Ca2+ uptake by the Ca2+ uniporter, inhibits mitochondrial Ca2+ transients in adult rabbit cardiac myocytes during electrical stimulation. Here, we extend this finding to show that ruthenium red inhibition is specific for mitochondrial Ca2+ transients and not cytosolic Ca2+ transients.

Ca 2+-tolerant adult rabbit cardiac myocytes were isolated by collagenase and hyaluronidase digestion and loaded in the cold with 2.5 μM Rhod 2-AM for 30 minutes. Subsequently, the cells were incubated in nutrient medium at 37°C for 4-6 hours during which time cytosolic but not mitochondrial Rhod 2 was lost. Subsequently, the myocytes were loaded with 10 μM Fluo 3-AM for 15 minutes at 37°C, conditions that lead to predominantly cytosolic localization of Fluo 3. In sequential scans, the red fluorescence of Rhod 2 and the green fluorescence of Fluo 3 were imaged using a Zeiss LSM 410 laser scanning confocal microscope.