Pulmonary Bypass Shunt Reduces Oxidative Stress in the American Alligator

RM 217

There have been several hypotheses proposed discussing the evolutionary preservation of cardiac shunting among some of the vertebrates; specifically the right-to-left (R-L) pulmonary bypass shunt seen in reptiles. We hypothesized that the R-L shunt mitigates the amount of oxidative stress imposed upon the tissues of vertebrates exposed to atmospheric hyperoxia. In order to test this hypothesis, we eliminated the R-L shunting ability in juvenile American alligators (Alligator mississippiensis) by surgical ligation of the left aorta (LAo), effectively preventing their circulatory system to no longer function in-parallel but in-series. Experimental animals (no R-L shunt; n=8) and shamoperated controls (shunt intact; n=8) were housed under normoxia (21% O2) and hyperoxia (35% O2) conditions at 30°C for 25 days. Whole blood and plasma samples collected after each exposure were assayed for lipid peroxidation and antioxidant activity. We found significantly higher (+13%) malondialdehyde concentrations in response to hyperoxia in experimental animals, and no differences in catalase concentration between treatment groups. This suggests alligators without shunting ability suffered greater oxidative damage than those who maintained the shunt, and were also unable to mount a sufficient cellular antioxidant defenses to protect against the influx in reactive oxygen species. We suggest the pulmonary bypass shunt, by admixture of deoxygenated and oxygenated blood, reduces blood oxygen tension and limits oxidative stress upon the systemic tissues. Palaeoatmospheric oxygen fluctuations would have had limited effect on contemporary vertebrate taxa with in-parallel circulation. Evolution of in-series circulation in ancestors of mammals and bird must have necessitated upregulation of antioxidant expression.