Digestion is unaffected by surgical elimination of the right-to-left cardiac shunt in American alligators (Alligator mississippiensis)

Abstract

Crocodilian cardiac anatomy is characterized by a four-chambered heart with complete septal division of ventricles, with the pulmonary artery and systemic left aorta (LAo) originating from the right ventricle (RV). This morphology allows for right-to-left (R-L) cardiac shunting, in which a fraction of oxygen-poor blood can be recirculated from the RV to the systemic circulation through the LAo, bypassing the pulmonary circulation. The R-L shunt can be eliminated through surgical occlusion of the LAo, which permits investigation into its physiological and adaptive significance. As the LAo delivers blood to the gut, it has been suggested that desaturated blood supplied by R-L shunting delivers higher levels of CO2 and H+, ultimately increasing gastric acid secretion and facilitating digestion. Consequently, we hypothesized that chronic surgical occlusion would negatively impact digestive performance of lab-raised American alligators (Alligator mississippiensis). In this study, two groups of alligators (9 years old; 7 years post-operation) with and without the capacity for R-L shunting (recipients of sham surgery or surgery to occlude LAo, respectively) were fed rodent meals of known mass and energy content to investigate digestive performance. There were no differences in apparent digestive efficiency or transit time between groups. We demonstrated that there were no underlying compensatory changes in mass or length of digestive or accessory digestive organs and no change in relative surface area or digestive enzyme activity in any intestinal region. Surgical elimination of the R-L cardiac shunt did not affect digestion in alligators, refuting the hypothesis that the R-L shunt is advantageous for crocodilian digestion.

Keywords: Crocodilian; Digestibility; Heart; Intestine; Pulmonary bypass shunt; Reptile.