Pulmonary bypass shunt is turned off during walking in the American alligator (Alligator mississippiensis)
Authors: Madrigal Manuel , De La Paz Kayla , Owerkowicz Tomasz 
Published: May 2023

In non-avian reptiles, the pulmonary bypass (right-to-left) shunt directs blood away from lungs via the left aorta (LAo). At times of elevated oxygen demand, such as exercise or digestion, animals can boost systemic oxygen delivery by either increasing pulmonary bypass shunting to augment cardiac output or reducing the shunt to maximise arterial oxygen content. Previous studies on squamates and turtles showed the shunt is reduced when oxygen demand rises. In contrast, a recent study showed increased shunting during digestion in the American alligator, but whether a similar response occurs during exercise has not been reported to date. We investigated the effects of terrestrial locomotion on blood flow and shunting pattern in female juveniles of the American alligator (n=6; body mass 700-1100g). Perivascular probes were chronically implanted on the major vessels of the outflow tract: the left aorta (LAo), the left pulmonary artery (LPA), the common carotid artery (CCA), and the right aortic arch jointly with the right subclavian artery (RAo+SCA). Shunt fraction was calculated as LAo flow relative to the right ventricular output (= LAo + 2✕LPA). Experiments were performed in a walk-in environmental chamber on animals fasted for a week and equilibrated to 30°C. Upon instrumentation and recovery from surgery, animals were exercised on a level treadmill (0.5 km/h) until exhaustion. Blood flows were recorded continuously before, during and after (10+ min) exercise. Systolic blood flow in the LAo was in the forward direction (shunt on) in alligators at rest, became retrograde (shunt off) immediately upon onset of exercise, and turned anterograde (shunt on again) during recovery. Compared with flow rate values obtained at rest (1.64±0.30 ml/kg/min), average (systolic and diastolic) flow in the LAo decreased significantly during exercise (0.65±0.46 ml/kg/min), and gradually increased with time during recovery (to 1.52±0.28 ml/kg/min in the 10th min post-exercise). In contrast, blood flow in other instrumented vessels increased during exercise, and gradually decreased during recovery. Shunt fraction was highly variable in resting animals (0.4 – 11.5%), decreased significantly during exercise, remained depressed in the 1st min post-exercise and increased gradually during recovery to (p<0.05, mixed model ANOVA, post-hoc Dunnett’s test with rest as control). This suggests that alligators turn the pulmonary bypass shunt off during acute bouts of terrestrial exercise. According to the Fick Principle, crocodilians favour increasing arterial oxygen content, by reducing admixture of oxygenated and deoxygenated bloodstreams, over augmenting cardiac output with shunt flow. Thus, despite profound differences in their heart morphology, crocodilian and non-crocodilian “reptiles” exhibit the same response – turning the shunt off – at times of elevated oxygen demand. Whether this is modulated by exercise-induced sympathetic stimulation remains to be tested. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.