Calcium-induced Cytochrome c release from rat brain mitochondria is altered by digitonin

To determine if calcium could release Cytochrome c (Cyt c) from brain mitochondria without activating the permeability transition (mPT), brain mitochondria were prepared in two different ways. Digitonin was used to lyse synaptosomes and release synaptosomal mitochondria or a Percoll gradient was used to separate non-synaptosomal mitochondria from the synaptosomes. In gradient-purified mitochondria, low levels of added digitonin produced swelling and Cyt c release. Digitonin augmented Ca²⁺-induced Cyt c release that was insensitive to the mPT inhibitors, cyclosporin A CsA and ADP. Similarly, in mitochondria prepared with digitonin, these inhibitors also failed to prevent Ca²⁺-induced Cyt c release. Thus the mPT-independent, Ca²⁺-induced Cyt c release pathway was attributable to alteration of the permeability properties of the outer mitochondrial membrane by digitonin.