The study of mammalian ischemia tolerance has focused for many years on preconditioning, in which a brief, sub-lethal period without blood flow or oxygen protects tissues against a later, longer ischemic period that would otherwise prove fatal. Preconditioning studies focus primarily on the heart,and to a lesser extent on the brain and other tissues. A number of compounds have been shown to be related to this protection, including adenosine,ATP-dependent potassium channels, reactive oxygen species (ROS), and a variety of upregulated molecular pathways (heat shock proteins, mitogen activated protein kinases) that induce the preconditioned phenotype. The goal, of course, is to exploit this natural protection in clinical applications, such as myocardial infarctions or stroke, and thus the elucidation of underlying mechanisms is critical to future treatment options.
Much recent research has demonstrated the critical importance of mitochondrial ATP-dependent potassium channels (KATP) in preconditioning. These channels, present in many cell types, remain closed as long as cellular energy is adequate, but open in response to falling ATP levels. Open KATP channels, also present in the plasma membrane,thus allow a temporary cellular hyperpolarization, and play a critical protective role in neurons by suppressing the release of excitatory neurotransmitters. In preconditioning, mitochondrial KATP channel protection is thought to be related to the generation of ROS. However,mitochondria also play a critical role in cell death when oxygen levels are inadequate, as the loss of mitochondrial membrane potential triggers the cascade of caspase proteins that induce apoptosis.
Because of these mitochondrial roles in cell death and cell survival, Sven Vetter and colleagues, associated with Achim Vogt's group in Heidelberg, have developed a novel model of ischemic preconditioning utilizing isolated mitochondria from the rat heart to examine two key questions: Do isolated mitochondria themselves show the preconditioning phenomenon? And does that protection involve the KATP channels? Preconditioning the mitochondria with a 4-minute anoxic exposure, the team then exposed the mitochondria to complete anoxia for 14 minutes under argon before reoxygenation. The team then monitored the release of mitochondrial enzymes into the medium, which provided a measure of structural damage, while oxygen consumption was monitored continuously to determine mitochondrial function. To test if mitochondrial KATP channels were involved in the preconditioning response, the experiments were repeated using the specific KATP channel opener diazoxide or a KATP blocker to determine if the drugs would either mimic or abrogate the effect.
Following the anoxic/reoxygenation exposure, the mitochondria were no longer able to carry out respiration; however, this loss of function was prevented by anoxic preconditioning or exposure to diazoxide. The team also found that preconditioning was prevented by administration of the KATP blocker, indicating that mitochondrial preconditioning is directly related to the opening of KATP channels. Neither ATP-hydrolysis nor mitochondrial enzyme loss differed with anoxic preconditioning or the experimental treatment, indicating that anoxic preconditioning represents a functional adaptation rather than a preservation of mitochondrial structural integrity.
The authors' findings thus support in vivo work suggesting that mitochondria are at the heart of natural cardioprotection achieved through preconditioning, and suggest that their model may help resolve the many remaining questions surrounding preconditioning, including the actual mechanisms by which KATP channels help protect tissues.