Atrial fibrillation (AF) is often due to a structural substrate, caused by co-morbidity like hypertension. Cardiac fibroblasts, activated by stretch, play an important role in creating this substrate by secreting extracellular matrix proteins. The objective of this study is to investigate stretch-induced effects of atrial myocytes and fibroblasts. We hypothesized that stretching cardiomyocytes and fibroblasts leads to myofibroblast differentiation and expression of fibrosis-related genes, contributing to the AF substrate.Methods
Neonatal rat atrial myocytes and fibroblasts were subjected to 24 hours (myocytes) or 48 hours (fibroblasts) cyclic mechanical stretch (1Hz, 15%). Conditioned medium from stretched cardiomyocytes was used to stimulate cardiac fibroblasts to study paracrine effects. Direct stretched fibroblasts were subjected to Ki67 and alpha smooth muscle actin (α-SMA) immunofluorescent (IF) staining to assess proliferation and differentiation, respectively. RNA was isolated for rt-PCR analysis of α-SMA and other fibrotic markers. Incorporation of 3H-proline was measured to assess collagen synthesis by stretched fibroblasts.Results
Stretch-conditioned medium did not influence the proliferation rate of atrial fibroblasts, but mildly increased myofibroblast differentiation. 3H-proline incorporation was not affected by stretch conditioned medium. Direct stretch on the fibroblasts did not induce proliferation either. In contrast, myofibroblast differentiation was significantly increased by stretch. IF staining showed a 1.8 fold increase (p < 0.01) in the number of α-SMA positive cells after 48 hours stretch. PCR confirmed this result on mRNA level. Furthermore, PCR analysis showed a trend towards higher collagen-I and PAI-1 expression after stretch. Expression of the type 1 angiotensin II receptor decreased in stretched fibroblasts. Type 2 angiotensin II receptor expression was significantly upregulated upon stretch. Interestingly, comparative analysis in ventricular fibroblasts did not show this angiotensin II type 2 receptor upregulation upon stretch.Conclusion
Stretch of the atria can lead to direct and indirect changes in cardiac fibroblasts leading to structural atrial remodeling, preceding AF. Results of this study could help to identify pathways by which increased atrial pressure leads to fibrosis. The RAS, and specifically the type 2 angiotensin II receptor is a first potential target for further studies.