Cyclosporin A-sensitive and insensitive mechanisms produce the permeability transition in mitochondria

Mitochondria, oxygen and reperfusion damage

Reperfusion of the ischaemic or hypoxic heart elicits a number of oxygen dependent processes such as cell lysis and Ca2+ uptake. It is known that the energisation of mitochondria, which requires oxygen, plays a key role in these processes and that the organelle actively sequesters Ca2+ under these circumstances. In this brief review we discuss how oxidants derived from mitochondrial electron transport may perturb mitochondrial calcium handling on reoxygenation of the hypoxic myocardium. In addition we show that the immunosuppressive agent cyclosporin has little or no effect on the oxygen dependent increase in total cell Ca2+ which occurs when hypoxic myocytes are reoxygenated. This result suggests that the Ca2+ dependent mitochondrial pore, which is known to function under conditions of oxidative stress, does not play a major role in the perturbation of Ca2+ homeostasis which occurs on reoxygenation of hypoxic hearts.

Effect of cyclosporine A and oligomycin on non-specific permeability of the inner mitochondrial membrane

The effect of oligomycin and cyclosporine A on the induction of non-specific permeability of the inner mitochondrial membrane by Ca2+ was under study. Both oligomycin and cyclosporine A were able to prevent the activation of non-specific permeability, but cyclosporine A was the only agent which could restore initial permeability of the inner mitochondrial membrane. The effect of cyclosporine A was shown not to be mediated through redistribution of Ca2+ between different mitochondrial subpopulations.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.