Decrease in Ca2+ Concentration in Quail Cardiomyocytes Is Faster than That in Rat Cardiomyocytes

Mammals and birds have quicker heart rates compared to other species. Mammalian cardiomyocytes have T-tubule membranes that facilitate rapid changes in Ca2+ concentrations. In contrast, bird cardiomyocytes do not possess T-tubule membranes, which raises the question of how birds achieve fast heartbeats. In this study, we compared the changes in Ca2+ concentration in cardiomyocytes isolated from adult quails and rats to elucidate the mechanism resulting in rapid heart rates in birds. Cardiomyocytes isolated from quails were significantly narrower than those isolated from rats. When Ca2+ concentration changes in the entire cardiomyocytes were measured using Fura-2 acetoxymethyl ester (AM), the time to peak was statistically longer in quails than in rats. In contrast, the decay time was markedly shorter in quails than in rats. As a result, the total time of Ca2+ concentration change was shorter in quails than in rats. A spatiotemporal analysis of Ca2+ concentration changes in quail cardiomyocytes showed that the decrease in Ca2+ concentration was faster in the center of the cell than near the cell membrane. These results suggest that avian cardiomyocytes achieve rapid changes in Ca2+ concentration by increasing the Ca2+ removal capacity in the central part of the cell compared to mammalian cardiomyocytes.