Differential activities of putative subsarcolemmal and interfibrillar mitochondria from cardiac muscle

Mitochondria were isolated from rat hearts by Polytron treatment in a low ionic strength medium. The mitochondria that remained in the tissue after Polytron treatment were then isolated by Nagarse treatment in a high ionic strength medium. Properties of these two preparations were compared. The state 3 rate of respiration was not significantly different between Polytron and Nagarse prepared mitochondria if the assay was carried out in a low ionic strength sucrose medium. On the other hand, when the assay was carried out in a high ionic strength KCl medium, a significant difference in state 3 rate was observed between these two preparations of mitochondria, mainly due to the decreased state 3 rate of Polytron mitochondria. The total cytochrome c and a + a₃ content and enzyme activities were similar in Polytron and Nagarse mitochondria. Although no release of cytochrome c was observed in low ionic strength assay medium, about 25% of the total cytochrome c was released from Polytron mitochondria in the high ionic strength assay medium. It was, therefore, concluded that the difference in state 3 rate of respiration in KCl assay medium is probably due to the loss of cytochrome c into the medium and may not represent any physiological difference between them.