Stretch Reduces Interfilament Spacing And Triggers Alterations In Thick-filament Structure In Healthy Left-atrial Fibers

Introduction: Left-atrial dilatation is thought to play a key pathological role in facilitating remodeling in both genetic and acquired cardiac myopathies, as well as enabling the onset and maintenance of atrial fibrillation. However, despite its likely role in disease, the structural responses of the atria to stretch remain understudied. The myofilament responses to stretch have been extensively studied in ventricular tissue, where strain-dependent recruitment of reserve, OFF-state, myosin heads from the thick-filament contribute to length dependent activation (LDA, i.e., greater forcer at longer sarcomere lengths). In these experiments, the effects of stretch in both atrial sarcomere and thick-filament structure were evaluated. Methods: Myofilament structure was evaluated via x-ray diffraction at two sarcomere lengths (SL, SHORT: 2.1 μm, and LONG: 2.3 μm) in relaxed left-atrial and left-ventricular myocardial skinned fibers from Yucatan mini-pigs. In these experiments, both equatorial and meridional reflections reflecting sarcomeric structure and ordering were evaluated. In addition, the biomechanical effects of stretch were also evaluated in both skinned porcine fibers and engineered heart tissues (EHT) from atrial and ventricular hIPSc-derived myocytes via a proprietary system (MyoPod; Propria Biosciences). Results: In both atrial skinned fibers and EHTs, stretch triggered moderate increases in force, demonstrating LDA. LA fibers, stretch decreased lattice spacing but failed to alter the I 11 /I 10 equatorial intensity ratio, suggesting negligible length-dependent recruitment of OFF-state myosin heads from the thick-filament. LA fibers, however, tended to have higher I 11 /I 10 values at short SL (vs. LV fibers) suggesting an intrinsically higher population of ON-state myosin heads. In addition, stretch triggered alterations in thick filament ordering in both LA and LV fibers, as reflected by changes in both the 3 rd and 6 th order meridional reflections (M3 and M6), suggesting effects in both myosin heads and thick-filament backbone. Conclusion: Atrial stretch results in marked sarcomere structural alterations, reducing both inter-filament spacing and thick-filament ordering that likely contribute to length dependent activation. The role of these alteration in atrial pathophysiology remains to be studied