Mitochondrial calcium transients in adult rabbit cardiac myocytes: Inhibition by ruthenium red and artifacts caused by lysosomal loading of Ca2+-indicating fluorophores

A cold/warm loading protocol was used to ester-load Rhod 2 into mitochondri a and other organelles and Flue 3 into the cytosol of adult rabbit cardiac myocytes for confocal fluorescence imaging. Transient increases in both cyt osolic Flue 3 and mitochondrial Rhod 2 fluorescence occurred after electric al stimulation. Ruthenium red, a blocker of the mitochondrial Ca2+ uniporte r, inhibited mitochondrial Rhod 2 fluorescence transients but not cytosolic Flue 3 transients. Thus the ruthenium red-sensitive mitochondrial Ca2+ uni porter catalyzes Ca2+ uptake during beat-to-beat transients of mitochondria l free Ca2+, which in turn may help match mitochondrial ATP production to m yocardial ATP demand. After ester loading, substantial amounts of Ca2+-indi cating fluorophores localized into an acidic lysosomal/endosomal compartmen t. This lysosomal fluorescence did not respond to electrical stimulation. B ecause fluorescence arose predominantly from lysosomes after the cold loadi ng/warm incubation procedure, total cellular fluorescence failed to track b eat-to-beat changes of mitochondrial fluorescence. Only three-dimensionally resolved confocal imaging distinguished the relatively weak mitochondrial signal from the bright lysosomal fluorescence.