Poster U9 - Comparative biomechanical analysis of ventricular passive mechanical properties in amphibians: anurans and pre- and post-metamorphic salamanders (Ambystoma andersoni)

Introduction: Amphibians are the ancestors of existing mammals and were the first vertebrates to land. Adaptation to changes in oxygen levels, gravity, and other factors was essential for the transition from aquatic to terrestrial habitats. The function and structure of the heart, which is responsible for delivering oxygen throughout the body and circulating blood against gravity, may have changed during the process of terrestrial adaptation. In this study, we examined differences in ventricular stiffness by habitat by comparing the ventricles of aquatic and terrestrial frogs. Additionally, by comparing the ventricles of Ambystoma andersoni before and shortly after metamorphosis, we examined the differences in ventricular stiffness associated with terrestrial adaptation that occur during the lifetime of an individual.

Materials and

Methods: We performed pressure loading tests on the ventricles of amphibians from different habitats to evaluate the passive mechanical properties of their ventricles. Aquatic (Xenopus laevis) and terrestrial (Bufo japonicus formosus) anurans, juvenile (aquatic) and adult (terrestrial) neotenic Ambystoma andersoni were used in the pressure-load tests. The adult Ambystoma andersoni specimens were collected shortly after metamorphosis. Histological analysis focused on ventricular wall thickness and collagen content as factors contributing to the passive mechanical properties of the ventricle.

Results, Conclusions, and Discussions: Pressure-load tests showed that the ventricles of terrestrial anurans were stiffer than those of aquatic anurans. Histological analysis revealed that terrestrial anurans had a thick layer of dense cardiac muscle. Second harmonic generation (SHG) imaging revealed an abundance of collagen fibers in terrestrial anurans. This suggests that the amphibian ventricles may have become stiffer during the process of landing and terrestrial adaptation due to ventricular wall thickening and collagen accumulation. A comparison of the mechanical properties of the ventricles before and shortly after metamorphosis in Ambystoma andersoni showed that the stiffness of the ventricles was similar. This suggests that the transition from an aquatic to a terrestrial habitat during an individual’s lifetime may not affect the mechanical properties of the ventricles. In conclusion, while ventricular stiffening does not occur immediately upon landing, it may evolve as a part of the terrestrial adaptation process in vertebrates over time.

Acknowledgements (Optional): This study was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (JP22K20506 and JP23H03855) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Nikki Saneyoshi Foundation, and the Nitto Foundation. The authors acknowledge Amphibian Research Center of Hiroshima University for providing Bufo japonicus formosus.