Numerous pathologies lead to remodelling of the mammalian ventricle, often associated with fibrosis. Recent work in fish has shown that fibrotic remodelling of the ventricle is ‘reversible’, changing seasonally as temperature-induced changes in blood viscosity alter hemodynamic load on the heart. The atrial response to varying hemodynamic load is less understood in mammals and completely unexplored in non-mammalian vertebrates. To investigate atrial remodelling, rainbow trout were chronically cooled (from 10 ± 1 °C to 5 ± 1 °C) and chronically warmed (from 10 ± 1 °C to 18 ± 1 °C) for a minimum of 8 weeks. We assessed the functional effects on compliance using ex vivo heart
preparations and atomic force microscopy nano-indentation and found chronic cold increased passive stiffness of the whole atrium and micromechanical stiffness of tissue sections. We then performed histological, biochemical and molecular assays to probe the mechanisms underlying functional remodeling of the atrial tissue. We found cooling resulted in collagen deposition which was associated with an up-regulation of collagen promoting genes, including the fish specific collagen I alpha 3 chain, and a reduction in gelatinase activity of collagen degrading matrix metalloproteinase (MMPs). Finally, we found that cooling reduced mRNA expression of cardiac growth factors and hypertrophic markers. Following long term warming there was an opposing response to that seen with cooling, however these changes were more moderate. Our findings suggest that chronic cooling causes atrial dilation and increased myocardial stiffness in trout atria analogous to pathological states defined by changes
in preload or afterload of the mammalian atria. The reversal of this phenotype following chronic warming is particularly interesting as it suggests that typically pathological features of mammalian atrial remodelling may oscillate seasonally in the fish, revealing a more dynamic and plastic atrial remodelling response.