Abstract 17552: Variable T-Tubule Density and Calcium Homeostasis Across the Atria
Although t-tubules have traditionally been thought to be absent in atrial cardiomyocytes, recent studies suggest that t-tubules exist in the atria of large mammals, albeit at lower density than in the ventricles. We postulated that cardiomyocyte function is regionally defined by variations in t-tubule density across the atria. This hypothesis was investigated by examining isolated cardiomyocytes and intact atrial tissue from rat and pig. In contrast to previous reports that t-tubules are absent in rat atria, we observed t-tubules in approximately one third of isolated rat atrial cardiomyocytes (di-8-ANEPPS staining). In a minority (≈10%) of atrial cardiomyocytes the t-tubular network was well organized, with a transverse structure resembling that of ventricular cardiomyocytes. Immunostaining showed the presence of L-type Ca2+ channels in these t-tubules, and fast Fourier transforms revealed clear peaks indicative of transverse organization. Consistent with high between-cell variability in the distribution of t-tubules and Ca2+ channels, L-type Ca2+ current amplitude were also highly variable (SD = 1.33 in atrial vs. 0.84 in ventricular cells, P<0.05), and steeply dependent on capacitance and t-tubule density. Accordingly, confocal linescan images of Ca2+ transients (field stimulation, Fluo-4AM) showed that Ca2+ release synchrony varied greatly between atrial cells (SD of dyssynchrony index measurements = 3.48 in atrial vs. 1.36 in ventricular cells, P<0.05). Simultaneous imaging of the cell membrane and Ca2+ transients confirmed t-tubule functionality. In rat and pig atrial tissue cryosectioned and stained with dystrophin or wheat germ agglutinin, we observed higher t-tubule density in the epicardium than endocardium (% cellular area occupied=1.51±0.06% vs. 1.08±0.06% for rat and 1.72±0.06% vs. 1.18±0.05% for pig, P<0.05). We propose that resulting transmural differences in Ca2+ release kinetics may support synchronization of contraction across the atrial wall. We additionally observed regional differences in t-tubule density between right and left atria, and between main chamber and appendage, which may have consequences for arrhythmogenesis.
- © 2013 by American Heart Association, Inc.