T-Tubules In Myocytes Of Intact Dog Left And Right Atria

The cardiac ventricular T-tubule (TT) system has a major role in normal and abnormal E-C coupling. TT allow for balanced calcium cycling and synchronous contraction throughout the myocyte. However, there is some evidence that T-tubules are absent from atrial myocytes of large animals. The goal of this project was to analyze TT organization in normal intact dog atria. Whole left (LA) and right (RA) atria from dog hearts were perfused and maintained in a chamber on the stage of a confocal microscope. After loading with di-4-ANEPPS, we imaged endocardial myocytes from different atrial regions which were then analyzed using computerized analysis software. Many but not all myocytes had some TT although they tended to be arranged in clusters along one or both sides of the cell at very low density. In cells with TT, there was no difference in TT density between any regions of either atria. However, we did find RA had twice as many myocytes without TT than LA (25.1% of cells compared to 12.5%). These features of TT density and organization in dog atrium are dramatically different from ventricle (rat) where every cell has a highly dense and organized TT system. We also found that TT spacing was greater in LA than RA. Finally RA myocytes are wider than in LA so there is no correlation between cell width and TT density. We conclude that there are many cells in both dog atria with TT although many cells do not (more in RA than LA) and there are regional differences in TT spacing but not density in TT-containing cells within the atria. Thus, even though there is an organized TT system in dog atria, it is far less organized and dense compared to ventricle.