Introduction: The cellular membrane of ventricular myocytes is invaginated by t- tubules, which extend in a predominantly transverse orientation. Excitation–contraction coupling is initiated in close proximity of this network. Recent data indicate that a fairly extensive t-tubule network is present in atrial cells of large mammals however, it is still thought to be virtually absent from the atria of rodents.

Objective: 

We aimed to compare the t-tubule network of atrial and ventricular cardiomyocytes in 6 week-old Wistar rats.

Methods: 

Atrial and ventricular myocytes were isolated by Langendorf perfusion and atrial tissue was subsequently diced and agitated in the presence of collagenase. Following di-8- ANEPPS (Molecular Probes, USA) staining of the cell membranes, t-tubules were imaged by confocal microscopy. Ca²⁺-transients were recorded in field stimulated cells loaded with Fluo-4 (Invitrogen, USA). Selected cells were also stained with RH 237, to simultaneously visualize cell membrane and Ca²⁺.

Results: 

We observed that a significant proportion (32%) of rat atrial cardiomyocytes possesses a t-tubule network. In a minority of atrial cardiomyocytes (approximately 10%), the t- tubular network is well organized into a predominately transverse structure. All ventricular cells exhibited an extensive t- tubule network and synchronous Ca²⁺ transients (dyssynchrony index (DI) <4.25). About 35% of atrial cardiomyocytes exhibited a DI within the range of ventricular cells, suggesting the presence of a functional t-tubule network. This finding was supported by simultaneous imaging of t-tubules and Ca²⁺ transients.

Conclusion: 

A fraction ([asymp]35%) of rat atrial cardiomyocytes possesses functional t-tubules, which are surprisingly well organized in a smaller proportion ([asymp]10%) of cells.