Taurine depletion impairs cardiac function and affects tolerance to hypoxia and high temperatures in brook char (Salvelinus fontinalis)

Amino Acids as Dietary Additives for Enhancing Fish Welfare in Aquaculture

The interest in fish welfare within aquaculture facilities has significantly increased over the past decade, recognizing the fundamental role of animal welfare in the quality of aquaculture products. It has been shown that stress in fish can affect their health, causing pathologies and immune failures, while stress-free fish grow faster and healthier. This has prompted aquaculture farmers to adopt strategies that reduce stress, improve water quality, and optimize stocking densities, thereby enhancing fish welfare. A key area is the role of amino acids in improving fish welfare. Amino acids, such as histidine, isoleucine, leucine, and tryptophan, are essential for various physiological processes, including neurotransmitter formation, energy metabolism, and immune function. Amino acids like tryptophan, arginine, and methionine play a crucial role in mitigating the effects of stress, improving immune function, and reducing oxidative stress. In the present review, the main roles of those amino acids related to fish stress have been shown, analyzing the physiological pathways involved in the link between amino acid ingestion and metabolization and stress responses.

Taurine efflux counters the hydrodynamic impact of anaerobic metabolism to protect cardiorespiratory function under acute thermal stress in brook char (Salvelinus fontinalis)

Upper thermal tolerance may be limited by convective oxygen transport in fish, but the mechanisms constraining heart function remain elusive. The activation of anaerobic metabolism imposes an osmotic stress on cardiomyocytes at high temperatures that must be countered to prevent swelling and cardiac dysfunction. We tested the hypothesis that cardiac taurine efflux is required to counter the osmotic impact of anaerobic end product accumulation in brook char, Salvelinus fontinalis. Fish were fed a diet enriched in β-alanine, a competitive inhibitor of the taurine transporter, to induce taurine deficiency and inhibit transporter function. In vivo, stroke volume increased by 60% and cardiac output doubled in control fish during a 2°C h-1 thermal ramp. Stroke volume was temperature insensitive in taurine-deficient (TD) fish, so cardiac output was 30% lower at high temperatures. The thermal sensitivity of aerobic metabolism did not differ, and lactate accumulated to a similar degree in the two diet treatment groups, indicating that taurine deficiency does not impact energy metabolism. Heart taurine efflux was absent and ventricular muscle osmolality was 40 mOsmol kg-1 higher in TD brook char following thermal stress. Swelling and decreased ventricular compliance likely impair diastolic filling to constrain stroke volume in TD fish. The adrenaline sensitivity of cardiac contractility and the regulation of intracellular pH in the brain and liver were also impacted in TD brook char. Taurine efflux appears necessary to counteract the hydrodynamic impact of activating anaerobic metabolism and this process may limit heart function under acute thermal stress.

The impacts of diet on cardiac performance under changing environments

Natural and anthropogenic stressors are dramatically altering environments, impacting key animal physiological traits, including cardiac performance. Animals require energy and nutrients from their diet to support cardiac performance and plasticity; however, the nutritional landscape is changing in response to environmental perturbations. Diet quantity, quality and options vary in space and time across heterogeneous environments, over the lifetime of an organism and in response to environmental stressors. Variation in dietary energy and nutrients (e.g. lipids, amino acids, vitamins, minerals) impact the heart's structure and performance, and thus whole-animal resilience to environmental change. Notably, many animals can alter their diet in response to environmental cues, depending on the context. Yet, most studies feed animals ad libitum using a fixed diet, thus underestimating the role of food in impacting cardiac performance and resilience. By applying an ecological lens to the study of cardiac plasticity, this Commentary aims to further our understanding of cardiac function in the context of environmental change.

Measuring maximum heart rate to study cardiac thermal performance and heat tolerance in fishes

The thermal sensitivity of heart rate (fH) in fishes has fascinated comparative physiologists for well over a century. We now know that elevating fH is the primary mechanism through which fishes increase convective oxygen delivery during warming to meet the concomitant rise in tissue oxygen consumption. Thus, limits on fH can constrain whole-animal aerobic metabolism. In this Review, we discuss an increasingly popular methodology to study these limits, the measurement of pharmacologically induced maximum fH (fH,max) during acute warming of an anaesthetized fish. During acute warming, fH,max increases exponentially over moderate temperatures (Q10∼2-3), but this response is blunted with further warming (Q10∼1-2), with fH,max ultimately reaching a peak (Q10≤1) and the heartbeat becoming arrhythmic. Because the temperatures at which these transitions occur commonly align with whole-animal optimum and critical temperatures (e.g. aerobic scope and the critical thermal maximum), they can be valuable indicators of thermal performance. The method can be performed simultaneously on multiple individuals over a few hours and across a broad size range (<1 to >6000 g) with compact equipment. This simplicity and high throughput make it tractable in lab and field settings and enable large experimental designs that would otherwise be impractical. As with all reductionist approaches, the method does have limitations. Namely, it requires anaesthesia and pharmacological removal of extrinsic cardiac regulation. Nonetheless, the method has proven particularly effective in the study of patterns and limits of thermal plasticity and holds promise for helping to predict and mitigate outcomes of environmental change.

Diet effects on ectotherm thermal performance

The environment is changing rapidly, and considerable research is aimed at understanding the capacity of organisms to respond. Changes in environmental temperature are particularly concerning as most animals are ectothermic, with temperature considered a key factor governing their ecology, biogeography, behaviour and physiology. The ability of ectotherms to persist in an increasingly warm, variable, and unpredictable future will depend on their nutritional status. Nutritional resources (e.g. food availability, quality, options) vary across space and time and in response to environmental change, but animals also have the capacity to alter how much they eat and what they eat, which may help them improve their performance under climate change. In this review, we discuss the state of knowledge in the intersection between animal nutrition and temperature. We take a mechanistic approach to describe nutrients (i.e. broad macronutrients, specific lipids, and micronutrients) that may impact thermal performance and discuss what is currently known about their role in ectotherm thermal plasticity, thermoregulatory behaviour, diet preference, and thermal tolerance. We finish by describing how this topic can inform ectotherm biogeography, behaviour, and aquaculture research.

Mechanisms of PVP-functionalized silver nanoparticle toxicity in fish: Intravascular exposure disrupts cardiac pacemaker function and inhibits Na+/K+-ATPase activity in heart, but not gill

Polyvinylpyrrolidone-functionalized silver nanoparticles (nAgPVP) are popular in consumer products for their colloidal stability and antimicrobial activity. Whole lake additions of nAgPVP cause long term, ecosystem-scale changes in fish populations but the mechanisms underlying this effect are unclear. We have previously shown that in fish, nAgPVP impairs cardiac contractility and Na+/K+-ATPase (NKA) activity in vitro, raising the possibility that heart dysfunction could underlie population-level exposure effects. The goal of this study was to determine if nAgPVP influences the control of heart rate (fh), blood pressure, or cardiac NKA activity in vivo. First, a dose-response curve for the effects of 5 nm nAgPVP on contractility was completed on isometrically contracting ventricular muscle preparations from Arctic char (Salvelinus alpinus) and showed that force production was lowest at 500 μg L-1 and maximum pacing frequency increased with nAgPVP concentration. Stroke volume, cardiac output, and power output were maintained in isolated working heart preparations from brook char (Salvelinus fontinalis) exposed to 700 μg L-1 nAgPVP. Both fh and blood pressure were elevated after 24 h in brook char injected with 700 μg kg body mass-1 nAgPVP and fh was insensitive to modulation with blockers of β-adrenergic and muscarinic cholinergic receptors. Na+/K+-ATPase activity was significantly lower in heart, but not gill of nAgPVP injected fish. The results indicate that nAgPVP influences cardiac function in vivo by disrupting regulation of the pacemaker and cardiomyocyte ionoregulation. Impaired fh regulation may prevent fish from appropriately responding to environmental or social stressors and affect their ability to survive.

Transcriptome-based nutrigenomics analysis reveals the roles of dietary taurine in the muscle growth of juvenile turbot (Scophthalmus maximus)

The present study explored transcriptomics and gene regulation variations in the muscle of turbot fed with dietary taurine. A 70-day feeding trial was conducted using turbot (initial body weight: 3.66 ± 0.02 g) fed with different levels of dietary taurine: 0 % (C), 0.4 % (T2), 1.2 % (T4) and 2.0 % (T6). Two methods were used to analyze and verify the taurine effects on muscle growth: (1) real-time quantitative PCR (qRT-PCR) for the key muscle growth-related genes and (2) transcriptomic analysis by next-generation sequencing (NGS). The results showed that 1.2 % of dietary taurine supplementation significantly increased the expression of muscle growth stimulatory genes, including TauT, myoD, Myf5, myogenin and follistatin. And also, the 1.2 % level significantly decreased the expression of the muscle growth-restricting gene (myostatin). Meanwhile, transcriptomics analysis found that 1.2 % dietary taurine supplementation significantly increased the number of up-regulated genes linked to metabolic pathways. In contrast, taurine significantly enriched the actin cytoskeleton and metabolic pathways in the T4 and T2 groups, respectively. These findings align with the gene ontology (GO) analysis, which indicated a higher number of cellular component (CC) gene expressions at a 1.2 % of dietary taurine compared to a 0.4 % of dietary taurine supplementation. In conclusion, dietary taurine had positive impacts on the growth-stimulatory genes. Moreover, 1.2 % of dietary taurine supplementation is important to the metabolic pathway enrichment.