Abstract
Rainbow trout remain active in waters that seasonally change between 4°C and 20°C. To explore how these fish are able to maintain cardiac function over this temperature range we characterized changes in cardiac morphology, contractile function, and the expression of contractile proteins in trout following acclimation to 4°C (cold), 12°C (control), and 17°C (warm). The relative ventricular mass (RVM) of the cold acclimated male fish was significantly greater than that of males in the control group. In addition, the compact myocardium of the cold acclimated male hearts was thinner compared to controls while the amount of spongy myocardium was found to have increased. Cold acclimation also caused an increase in connective tissue content, as well as muscle bundle area in the spongy myocardium of the male fish. Conversely, warm acclimation of male fish caused an increase in the thickness of the compact myocardium and a decrease in the amount of spongy myocardium. There was also a decrease in connective tissue content in both myocardial layers. In contrast, there was no change in the RVM or connective tissue content in the hearts of female trout with warm or cold acclimation. Cold acclimation also caused a 50% increase in the maximal rate of cardiac AM Mg2+-ATPase but did not influence the Ca2+ sensitivity of this enzyme. To identify a mechanism for this change we utilized two-dimensional difference gel electrophoresis to characterize changes in the cardiac contractile proteins. Cold acclimation caused subtle changes in the phosphorylation state of the slow skeletal isoform of troponin T found in the heart, as well as of myosin binding protein C. These results demonstrate that acclimation of trout to warm and cold temperatures has opposing effects on cardiac morphology and tissue composition and that this results in distinct warm and cold cardiac phenotypes.
Highlights
The hypertrophic response, and the resulting change in cardiac function, differs if the increased load is due to a pathological condition or a physiological requirement [1,2]
We have previously demonstrated that cardiac myosin binding protein C (cMyBP-C), regulatory light chain (RLC), TnT and TnI can be phosphorylated in trout cardiac muscle [21]
There was no change in relative ventricular mass (RVM) of the female fish with either cold or warm acclimation (Fig. 1B)
Summary
The hypertrophic response, and the resulting change in cardiac function, differs if the increased load is due to a pathological condition or a physiological requirement [1,2]. A physiological hypertrophic response results in improved contractile function associated with an increase in the Ca2+ sensitivity of the myocardium as well as an increase in the activity of AM Mg2+ATPase [1,3].There are clear differences in the structure and function of hearts exhibiting pathological and physiological cardiac hypertrophy, which are the outcome of distinct molecular signaling pathways (reviewed in [1]). The hypertrophic response is thought to be triggered by the coldinduced increase in blood viscosity, which increases stress on the heart [9] This change in morphology is associated with a downregulation of collagen genes (protein was not investigated) [10] and an increase in contractility, with the rate of AM Mg2+-ATPase increasing [11]. The molecular basis of this latter change in function is unknown
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