Cyclosporine A protects mitochondria in an in vitro model of hypoxia/reperfusion injury

Hypoxia/reperfusion injury is a major clinical problem. One of its hallmarks is an increased cytosolic Ca2+ content and an increased generation of reactive oxygen species in the cytosol and in mitochondria. In the present study of an in vitro model of hypoxia/reperfusion injury, mitochondria are exposed to Ca2+ in combination with extra- and intramitochondrially acting prooxidants. In this model mitochondria are damaged in a Ca(2+)-dependent manner. The extent and the site(s) of damage depend on both the kind of respiratory substrate and prooxidant used. The major damage occurs specifically at site I of the respiratory chain, and is due to hydrolysis of oxidized pyridine nucleotides and Ca2+ release followed by Ca2+ re-uptake (Ca2+ 'cycling'). Cyclosporine A completely protects against this damage. The protection is due to inhibition of pyridine nucleotide hydrolysis, an obligatory step in the sequence of events that links prooxidants to Ca2+ release from intact mitochondria.