2021 年 59 巻 6 号 p. 162-168
During the course of evolution, the vertebrate heart has undergone many changes and adaptations. Transition from aquatic to terrestrial environments required the ability to circulate blood against the force of gravity;therefore, it is likely that the structure and function of the ventricles changed during this transition. In the present study, we focused on pressure resistance in amphibians which were the first vertebrates to migrate to land. We investigated the pressure resistance and histology of the ventricles of three Anuran species (frogs and toads) from different habitats:the African clawed frog (aquatic), dark-spotted frog (semiaquatic), and Japanese common toad (terrestrial). To analyze the pressure resistance of the ventricles, intraventricular pressure was recorded during continuous injection of cardioplegic solution into the ventricles. The results showed that maximum stiffness, the pressure at which the solution leaked from the ventricle, and the maximum ventricular pressure were the highest in the Japanese common toad among the three species, demonstrating that the ventricles of the Japanese common toad were the stiffest and had the highest pressure resistance. To determine the factors responsible for the differences in pressure resistance, we measured the thickness of ventricle walls and the amount of collagen fibers in ventricle walls. The wall of the ventricles of the Japanese common toad and dark-spotted frog were thicker than that of the African clawed frog. The isolated cardiomyocyte size was similar for all the species, suggesting that the ventricular wall of the Japanese common toad and dark-spotted frog became thicker not by hypertrophying cardiomyocytes, but by increasing the number of cardiomyocytes. The collagen fibers in the ventricular wall of the Japanese common toad were richer compared to those of the African clawed frog and dark-spotted frog. Taken together, these results suggest that the pressure resistance in Anura increased through the increase in collagen. Vertebrate ventricles may have become resistant to higher pressure as an adaptive process to terrestrial life that facilitates blood circulation against the force of gravity.