The role of reactive free radicals in ischemic preconditioning--clinical and evolutionary implications.

Ischemic preconditioning is a condition of reduced sensitivity to ischemic damage. This protective state can be induced by exposure to periods of brief, sublethal, ischemia prior to a protracted ischemic event, but, more interestingly, also by administration of specific drugs. Recent studies have emphasized the central role of free radicals (including superoxide anion and nitric oxide) in this process. In line with these observations, studies have demonstrated that also drugs such as organic nitrates, which are able to release nitric oxide (but also the highly oxidant superoxide anion) can induce preconditioning. Starting from our observations made in human experimental models regarding the effects of chronic therapy with organic nitrates, we criticize the hypothesis whereby nitrates might be used to induce a state similar to preconditioning upon chronic exposure. As well, we propose a theory for the evolutionary meaning of ischemic preconditioning based on the hypothesis that, while protective over short periods of time, continuous exposure to oxidant free radicals might be associated with a loss of this protective effect and, in certain cases, with an increased oxidative damage.