Ruthenium red-sensitive and -insensitive release of Ca2+ from uncoupled heart mitochondria. 
The uncoupler-induced release of accumulated Ca2+ from heart mitochondria can be separated into two components, one sensitive and one insensitive to ruthenium red.  In mitochondria maintaining reduced NAD(P)H pools and adequate levels of endogenous adenine nucleotides, the release of Ca2+ following addition of an uncoupler is virtually all inhibited by ruthenium red and can be presumed to occur via reversal of the Ca2+ uniporter.  When ruthenium red is added to block efflux via this pathway, high rates of Ca2+ efflux can still be induced by an uncoupler, provided either NADH is oxidized or mitochondrial adenine nucleotide pools are depleted by prior treatment.  This ruthenium red-insensitive Ca2+-efflux pathway is dependent on the level of Ca2+ accumulated and is accompanied by swelling of the mitochondria and loss of endogenous Mg2+.  Loss of Ca2+ by this relatively nonspecific pathway is strongly inhibited by Sr2+ and by nupercaine, as well as by oligomycin and exogenous adenine nucleotides.  The loss of Ca2+ from uncoupled heart mitochondria occurs via a combination of these two mechanisms except under conditions chosen specifically to limit efflux to one or the other pathway.