Variable T-Tubule Density and Organization in Rat Atrial Cardiomyocytes

Abstract

T-tubules are invaginations in the cellular membrane of the myocyte. In ventricular cardiomyocytes initiation of excitation-contraction coupling is highly localized near the t-tubules, which span the cell in a predominantly transverse orientation. While t-tubules have traditionally been thought to be virtually absent from atrial cells, recent data indicate that a fairly substantial network is present in the atria of large mammals. We presently compared the t-tubule network of atrial and ventricular cardiomyocytes in 6 week-old Wistar rats. Atrial myocytes were isolated by dicing and agitating tissue in the presence of collagenase, and ventricular cells were isolated by Langendorf perfusion. Cell membrane and t-tubules were stained with di-8-ANEPPS and imaged by confocal microscopy. 100% of ventricular and 37% of atrial cardiomyocytes possessed a measurable t-tubular density (defined as occupying >5% of the cytosolic lumen). In the tubulated atrial cells, t-tubular density was similar to that observed in ventricular myocytes (% cross-sectional area = 16% in atrial vs. 17% in ventricular cardiomyocytes, P=0.76). Surprisingly, 10% of atrial myocytes exhibited an organized, largely transverse structure. Power spectral analysis revealed that this organization was somewhat less homogeneous than that observed in ventricular cells, as atrial myocytes exhibited a higher proportion of longitudinal elements (ratio of peak power to between-peak power = 3.15 in atrial vs. 5.13 in ventricular cells, P=0.02). Neither density nor organization of t-tubules was correlated to atrial cardiomyocyte size. Thus, in contrast to several previous reports, a significant proportion of rat atrial cardiomyocytes possess a t-tubule network which is as extensive as that observed in ventricular cells. Furthermore, in a minority of atrial cardiomyocytes (approximately 10%) the t-tubular network is well organized into a predominately transverse structure.