In situ cardiac perfusion reveals interspecific variation of intraventricular flow separation in reptiles

The ventricles of non-crocodilian reptiles are incompletely dividedand provide an opportunity for mixing of oxygen-poor blood andoxygen-rich blood (intracardiac shunting). However, both cardiacmorphology and in vivo shunting patterns exhibit considerableinterspecific variation within reptiles. In the present study, wedevelop an in situ double-perfused heart approach to characterisethe propensity and capacity for shunting in five reptile species: theturtle Trachemys scripta, the rock python Python sebae, the yellowanaconda Eunectes notaeus, the varanid lizard Varanusexanthematicus and the bearded dragon Pogona vitticeps. Tosimulate changes in vascular bed resistance, pulmonary andsystemic afterloads were independently manipulated and changesin blood flow distribution amongst the central outflow tracts weremonitored. As previously demonstrated in Burmese pythons, rockpythons and varanid lizards exhibited pronounced intraventricularflow separation. As pulmonary or systemic afterload was raised, flowin the respective circulation decreased. However, flow in the othercirculation, where afterload was constant, remained stable. Thiscorrelates with the convergent evolution of intraventricular pressureseparation and the large intraventricular muscular ridge, whichcompartmentalises the ventricle, in these species. Conversely, inthe three other species, the pulmonary and systemic flows werestrongly mutually dependent, such that the decrease in pulmonaryflow in response to elevated pulmonary afterload resulted inredistribution of perfusate to the systemic circuit (and vice versa).Thus, in these species, the muscular ridge appeared labile and bloodcould readily transverse the intraventricular cava. We conclude thatrelatively minor structural differences between non-crocodilianreptiles result in the fundamental changes in cardiac function.Further, our study emphasises that functionally similar intracardiacflow separation evolved independently in lizards (varanids) andsnakes (pythons) from an ancestor endowed with the capacity forlarge intracardiac shunts.Funding agencies: Danish Research Council (Det Frie Forskningsrad \ Natur og Univers); Swedish Research Council (Vetenskapsradet)