The clinical significance of the atrial subendocardial smooth muscle layer and cardiac myofibroblasts in human atrial tissue with valvular atrial fibrillation

BackgroundThe existence of myofibroblasts (MFBs) and the role of subendocardial smooth muscle (SSM) layer of human atrial tissue in atrial fibrillation (AF) have not yet been elucidated. We hypothesized that the SSM layer and MFB play some roles in atrial structural remodeling and maintenance of valvular AF in patients who undergo cardiac surgery.MethodsWe analyzed immunohistochemical staining of left atrial (LA) appendage tissues taken from 17 patients with AF and 15 patients remaining in sinus rhythm (SR) who underwent cardiac surgery (male 50.0%, 54.1±14.2 years old, valve surgery 87.5%). SSM was quantified by α-smooth muscle actin (α-SMA) stain excluding vascular structure. MFB was defined as α-SMA + cells with disorganized Connexin 43-positive gap junctions in Sirius red-positive fibrotic area.ResultsThe SSM layer of atrium was significantly thicker in patients with AF than in those with SR (P=.0091). Patients with SSM layer ≥ 14 μm had a larger LA size (P=.0006) and greater fibrotic area (P=.0094) than those patients whose SSM layer < 14 μm. MFBs were found in 7 of 17 (41.2%) patients with AF and 2 of 15 (13.3%) in SR group (P=.0456) in SSM area, colocalized with Periodic Acid-Schiff (PAS) stain-positive glycogen storage cells (95.5%).ConclusionSSM layer was closely related to the existence of AF, degrees of atrial remodeling, and fibrosis in patients who underwent open heart surgery. We found that MFB does exist in SSM layer of human atrial tissue co-localized with PAS-positive cells.