Diving seals: are they a model for coping with oxidative stress?

The diving lifestyle of seals depends upon cardiovascular adjustments that result in frequent vasoconstriction of numerous organs. With the first post-dive breath, reperfusion allows for eliminating accumulated carbon dioxide (CO(2)) and reloading oxygen (O(2)) stores. Reintroduction of oxygenated blood raises the potential for production of reactive oxygen species (ROS) and the possibility that they may overwhelm the antioxidant defenses. This study addresses the question of possible adaptive responses that allow ringed seal (Phoca hispida) tissues to tolerate repeated cycles of ischemia and reperfusion, and thus protect them from oxidative insult. We obtained samples of ringed seal heart, muscle and kidney through the cooperation of native subsistence hunters at Barrow, Alaska. Samples were subjected to oxidative stress by addition of xanthine oxidase. Production of superoxide radical (O(2)(.-)), lipid peroxidation (as determined by the presence of thiobarbituric acid reactive substances, TBARS) and antioxidant capacity (AOX) were quantified by spectrophotometric analysis. Similarly treated pig tissues were anticipated to be more susceptible to oxidative stress. Contrary to expectations, pig tissues revealed less O(2)(.-) and TBARS compared with ringed seal tissues. These results show that ringed seal muscle, heart and kidney can be induced in vitro to generate ROS, and suggest that the living seal's protective defenses may depend upon O(2)(.-) production, similar to the protective effect of experimental preconditioning, or on enhanced intermediate scavenging, as evidenced by the larger AOX found in ringed seal tissues.