Difference in calcium content of atrial and ventricular muscle

The calcium content and inulin space of the atrial and ventricular muscle were determined in isolated perfused heart of the toad, bullfrog, guinea pig, rat, and cat. In all species studied, the total and "cellular" calcium content of the atrium were higher than those of the ventricle. A high calcium content of the atrial muscle was also observed in fresh unperfused hearts. The total calcium content of atrial and ventricular muscles increased when extracellular phosphate concentration was increased. When severe ventricular arrhythmia such as fibrillation occurred during perfusion, the total calcium content of the ventricle increased. The decay of tissue calcium content with calcium washout was examined in the toad, guinea pig, and rat heart. It was revealed that about 80 percent of total calcium existed as "exchangeable" calcium in both atrial and ventricular muscles. When the extracellular calcium concentration ([Ca]o) was altered in the toad, guinea pig, and rat heart preparation, the cellular calcium content of atrial muscle varied in proportion to [Ca]o, whereas that of ventricular muscle remained fairly constant at a higher [Ca]o value. The development of contractile tension in the atrial and ventricular muscles at various [Ca]o values corresponded well to these changes in the cellular calcium content except for the rat ventricle in which the contractile tension was almost proportional to [Ca]o. The relationship between the development of contractile tension and the level of cellular calcium content or [Ca]o was discussed.