Hypoxic incubation at 50% of atmospheric levels shifts the cardiovascular response to acute hypoxia in American alligators, Alligator mississippiensis

We designed a series of studies to investigate whether hypoxia (10% O 2 ) from 20% of incubation to hatching, or from 20 to 50% of incubation, affects cardiovascular function when juvenile American alligators reached an age of 4–5 years compared to juveniles that were incubated in 21% O 2 . At this age, we measured blood flows in all the major arteries as well as heart rate, blood pressure, and blood gases in animals in normoxia and acute hypoxia (10% O 2 and 5% O 2 ). In all three groups, exposure to acute hypoxia of 10% O 2 caused a decrease in blood O 2 concentration and an increase in heart rate in 4–5-year-old animals, with limited effects on blood flow in the major outflow vessels of the heart. In response to more acute hypoxia (5% O 2 ), where blood O 2 concentration decreased even further, we measured increased heart rate and blood flow in the right aorta, subclavian artery, carotid artery, and pulmonary artery; however, blood flow in the left aorta either decreased or did not change. Embryonic exposure to hypoxia increased the threshold for eliciting an increase in heart rate indicative of a decrease in sensitivity. Alligators that had been incubated in hypoxia also had higher arterial PCO 2 values in normoxia, suggesting a reduction in ventilation relative to metabolism.