Mechanical efficiency of the trout heart during volume and pressure-loading: metabolic implications of the stiffness of the ventricular tissue

Is the teleost heart oxygen limited? - Insights using "hyperoxic" incubations of contracting cardiac tissue from rainbow trout

Considerable effort has been devoted to understanding the negative effects of reduced PO₂ on cardiac function. Much less is known about the impacts of elevated PO₂ (hyperoxia) on cardiac performance and energetics, especially in fishes. The fish heart is of particular interest because cardiac dependence on oxygen is extremely variable between species and the early evolution of fish occurred when atmospheric PO₂ was higher than current conditions. Although extracellular PO₂ is variable and normally does not exceed 21 kPa, recent evidence suggests that teleost cardiac function is stimulated at supraphysiological PO₂ values. The purpose of this study was to address whether cardiac contractility and energy metabolism is responsive to elevated PO₂ values in sexually-immature female rainbow trout. Isometric force development (contractility) and oxygen consumption (V̇O₂) were recorded in electrically-paced ventricular preparations. Contractility and V̇O₂decreased when superfusate PO₂ was decreased from ~70 kPa to 57 kPa or 45 kPa. However, PO₂ calculated at the preparation core was always above 18 kPa. This estimate, along with complete recovery of contractility and V̇O₂ at ~70 kPa, suggests that decreases observed in cardiac performance were not due to tissue hypoxia at the lower PO₂ levels. In conclusion, the heart of female rainbow trout may be oxygen-limited in vitro and this study raises new questions about the choice of appropriate PO₂ for experimentation, the relevance of elevated and varying PO₂ to measurements of cardiac performance, and the possible existence of an oxygen sensor within rainbow trout cardiomyocytes.

Macro- and micromechanical remodelling in the fish atrium is associated with regulation of collagen 1 alpha 3 chain expression

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.

Mechano-electric feedback in the fish heart

Background

Mechanoelectric feedback (MEF) describes the modulation of electrical activity by mechanical activity. This may occur via the activation of mechanosensitive ion channels (MSCs). MEF has not previously been investigated in fish ventricular tissue even though fish can greatly increase ventricular end diastolic volume during exercise which should therefore provide a powerful mechanical stimulus for MEF.

Methodology/Principal Finding

When the ventricles of extrinsically paced, isolated working trout hearts were dilated by increasing afterload, monophasic action potential (MAP) duration was significantly shortened at 25% repolarisation, unaltered at 50% repolarisation and significantly lengthened at 90% repolarisation. This observation is consistent with the activation of cationic non-selective MSCs (MSC_NSs). We then cloned the trout ortholog of TRPC1, a candidate MSC_NS and confirmed its presence in the trout heart.

Conclusions/Significance

Our results have validated the use of MAP technology for the fish heart and suggest that, in common with amphibians and mammals, MEF operates in fish ventricular myocardium, possibly via the activation of mechanosensitive TRPC1 ion channels.

The effect of adrenaline and high Ca2+ on the mechanical performance and oxygen consumption of the isolated perfused trout (Oncorhynchus mykiss) heart

In heart muscle from mammals, catecholamines frequently evoke an oxygen waste and reduce efficiency. It was examined if this also applies to fish in which heart muscle activity is often restricted by oxygen availability. In the isolated perfused heart from rainbow trout, adrenaline (0.5 microM) increased power output by approximately 97%, when afterload was adjusted to maximum both before and after adrenaline addition, and by approximately 68%, when afterload remained at the maximum obtained before adrenaline addition. Oxygen consumption was enhanced by a similar amount (approximately 70%) in both situations. Hence, efficiency, i.e. power output/oxygen consumption, increased significantly from 25 to 30% for the heart always exposed to maximal afterload, whereas it stayed unchanged at 24% for the heart exposed to control afterload only. Adrenaline increases the Ca2+ activity participating in activation, but doing this by elevating perfusate Ca2+ from 1.5 to 5 mM instead of applying adrenaline did not change the mechanical efficiency, although afterload was maximized. The results suggest that catecholamines enhance heart pump activity in the living trout without any oxygen waste. On the contrary, the oxygen budget may even improve when the peripheral resistance is increased by catecholamines.