Regional distribution of T-tubule density in left and right atria in dogs

Background The peculiarities of transverse tubule (T-tubule) morphology and distribution in the atrium—and how they contribute to excitation-contraction coupling—are just beginning to be understood. Objectives The objectives of this study were to determine T-tubule density in the intact, live right and left atria in a large animal and to determine intraregional differences in T-tubule organization within each atrium. Methods Using confocal microscopy, T-tubules were imaged in both atria in intact, Langendorf-perfused normal dog hearts loaded with di-4-ANEPPS. T-tubules were imaged in large populations of myocytes from the endocardial surface of each atrium. Computerized data analysis was performed using a new MatLab (Mathworks, Natick, MA) routine, AutoTT. Results There was a large percentage of myocytes that had no T-tubules in both atria with a higher percentage in the right atrium (25.1%) than in the left atrium (12.5%) (P < .02). The density of transverse and longitudinal T-tubule elements was low in cells that did contain T-tubules, but there were no significant differences in density between the left atrial appendage, the pulmonary vein-posterior left atrium, the right atrial appendage, and the right atrial free wall. In contrast, there were significant differences in sarcomere spacing and cell width between different regions of the atria. Conclusion There is a sparse T-tubule network in atrial myocytes throughout both dog atria, with significant numbers of myocytes in both atria—the right atrium more so than the left atrium—having no T-tubules at all. These regional differences in T-tubule distribution, along with differences in cell width and sarcomere spacing, may have implications for the emergence of substrate for atrial fibrillation.