Intracellular Ca
2+ ([Ca
2+]
i) dynamics in isolated myocytes differ between the atria and ventricles due to the distinct t-tubular distributions. Although cellular aspects of ventricular [Ca
2+]
i dynamics in the heart have been extensively studied, little is known about those of atrial myocytes
in situ. Here we visualized precise [Ca
2+]
i dynamics of atrial myocytes in Langendorff-perfused rat hearts by rapid-scanning confocal microscopy. Of 16 fluo-4-loaded hearts imaged during pacing up to 4-Hz, five hearts showed spatially uniform Ca
2+ transients on systole among individual cells, whereas no discernible [Ca
2+]
i elevation developed during diastole. In contrast, the remaining hearts showed non-uniform [Ca
2+]
i dynamics within and among the cells especially under high-frequency (4 Hz) excitation, where subcellular cluster-like [Ca
2+]
i rises or wave-like [Ca
2+]
i propagation occurred on excitation. Such [Ca
2+]
i inhomogeneity was more pronounced at high-frequency pacing, showing beat-to-beat Ca
2+ transient alternans. Despite such non-uniform dynamics, cessation of burst pacing of the atria was not followed by emergence of spontaneous Ca
2+ waves, indicating minor Ca
2+-releasing potentials of the sarcoplasmic reticulum (SR). In summary, rat atria display a propensity to show non-uniform [Ca
2+]
i dynamics on systole due to impaired Ca
2+-release from the SR and paucity of t-tubules. Our results provide an important basis for understanding atrial pathophysiology.
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