Early ischemia-induced alterations of the outer mitochondrial membrane and the intermembrane space: a potential cause for altered energy transfer in cardiac muscle?

Our aim was to carefully analyse the time-dependent changes that affect the mitochondrial function of myocardial cells during and after an ischemic episode. To this end, variables characterizing mitochondrial function have been evaluated on myocardial samples from isolated rat hearts subjected to different conditions of ischemia. The technique of permeabilized fibers was used in order to evaluate the mitochondrial function whilst retaining intracellular structure. The earliest alteration that could be detected was a decrease in the stimulatory effect of creatine on mitochondrial respiration. This alteration became more pronounced as the severity (or duration) of the ischemia increased. Afterwards, a significant decrease in the apparent Km of mitochondrial respiration for ADP also appeared, followed by a diminution of the maximal respiration rate which was partly restored by adding cytochrome c. Finally, for the most severe conditions of ischemia, the basal respiratory rate also increased. These observations are indicative of a sequence of alterations affecting first the intermembrane space, then the outer mitochondrial membrane, and finally the inner membrane. The discussion is focused on the very early alterations, that could not be detected using the conventional techniques of isolated mitochondria. We postulate that these alterations to the intermembrane space and outer mitochondrial membrane can induce disturbances both in the channelling of energy from the mitochondria, and on the signalling towards the mitochondria. The potential consequences on the regulation of the production of energy (ATP, PC) by the mitochondria are evoked.