Abstract
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 haemodynamic load on the heart. The atrial response to varying haemodynamic 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 to 5 ± 1 °C) and chronically warmed (from 10 ± 1 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 remodelling of the atrial tissue. We found cooling resulted in collagen deposition which was associated with an upregulation of collagen-promoting genes, including the fish-specific collagen I alpha 3 chain, and a reduction in gelatinase activity of collagen-degrading matrix metalloproteinases (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.
Highlights
Chronic changes in pressure or volume load can cause the vertebrate heart to change in size, form and function [57, 46]
Based on our recent work on the trout ventricle [35], we hypothesised that chronic cooling would increase atrial stiffness and fibrosis and upregulate growth factors associated with pathological remodelling in mammals
Transcript abundance of genes associated with muscle growth, hyperplasia, angiogenesis, collagen I (Col1a1, Col1a2 and Col1a3), connective tissue regulators (MMP2, MMP9, MMP13 and TIMP2), stretch and heart failure markers (ANP and brain natriuretic peptide (BNP)) and a pro-hypertrophic nuclear factor of activating T (NFAT) signalling mediator were quantified in the atria of fish from cold-acclimated, control and warm-acclimated groups (n = 7 atria for each temperature)
Summary
Chronic changes in pressure or volume load can cause the vertebrate heart to change in size, form and function [57, 46] This cardiac remodelling response is often compensatory and maintains optimal cardiac function under conditions of increased haemodynamic preload or afterload. Proteins involved in excitation-contraction coupling like SERCA, phospholamban, Ca2+-binding proteins and the Na+-Ca2+ exchanger show similar changes in gene expression following cold acclimation in the atrium compared with the ventricle [42, 43, 30]. We know of no studies to directly investigate atrial remodelling following both warming and cooling in any fish species. Based on our recent work on the trout ventricle [35], we hypothesised that chronic cooling would increase atrial stiffness and fibrosis and upregulate growth factors associated with pathological remodelling in mammals. MO, USA) before being processed and embedded in paraffin wax
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