Mechanisms of enhanced cardiorespiratory performance under hyperoxia differ with exposure duration in yellowtail kingfish.

Hyperoxia has been shown to expand the aerobic capacity of some fishes, although there have been very few studies examining the underlying mechanisms and how they vary across different exposure durations. Here, we investigated the cardiorespiratory function of yellowtail kingfish (Seriola lalandi) acutely (~20 h) and chronically (3-5 weeks) acclimated to hyperoxia (~200% air saturation). Our results show that the aerobic performance of kingfish is limited in normoxia and increases with environmental hyperoxia. The aerobic scope was elevated in both hyperoxia treatments driven by a ~33% increase in maximum O2 uptake (MO2max), although the mechanisms differed across treatments. Fish acutely transferred to hyperoxia primarily elevated tissue O2 extraction, while increased stroke volume-mediated maximum cardiac output was the main driving factor in chronically acclimated fish. Still, an improved O2 delivery to the heart in chronic hyperoxia was not the only explanatory factor as such. Here, maximum cardiac output only increased in chronic hyperoxia compared with normoxia when plastic ventricular growth occurred, as increased stroke volume was partly enabled by an ~8%-12% larger relative ventricular mass. Our findings suggest that hyperoxia may be used long term to boost cardiorespiratory function potentially rendering fish more resilient to metabolically challenging events and stages in their life cycle.

Beating the heart failure odds: long-term survival after myocardial ischemia in juvenile rainbow trout.

Salmonid fish include some of the most valued cultured fish species worldwide. Unlike most other fish, the hearts of salmonids, including Atlantic salmon and rainbow trout, have a well-developed coronary circulation. Consequently, their hearts' reliance on oxygenation through coronary arteries leaves them prone to coronary lesions, believed to precipitate myocardial ischemia. Here, we mimicked such coronary lesions by subjecting groups of juvenile rainbow trout to coronary ligation, assessing histomorphological myocardial changes associated with ischemia and scarring in the context of cardiac arrhythmias using electrocardiography (ECG). Notable ECG changes resembling myocardial ischemia-like ECG in humans, such as atrioventricular blocks and abnormal ventricular depolarization (prolonged and fragmented QRS complex), as well as repolarization (long QT interval) patterns, were observed during the acute phase of myocardial ischemia. A remarkable 100% survival rate was observed among juvenile trout subjected to coronary ligation after 24 wk. Recovery from coronary ligation occurred through adaptive ventricular remodeling, coupled with a fast cardiac revascularization response. These findings carry significant implications for understanding the mechanisms governing cardiac health in salmonid fish, a family particularly susceptible to cardiac diseases. Furthermore, our results provide valuable insights into comparative studies on the evolution, pathophysiology, and ontogeny of vertebrate cardiac repair and restoration.NEW & NOTEWORTHY Juvenile rainbow trout exhibit a remarkable capacity to recover from cardiac injury caused by myocardial ischemia. Recovery from cardiac damage occurs through adaptive ventricular remodeling, coupled with a rapid cardiac revascularization response. These findings carry significant implications for understanding the mechanisms governing cardiac health within salmonid fishes, which are particularly susceptible to cardiac diseases.

Sea water acclimation of rainbow trout (Oncorhynchus mykiss) modulates the mucosal transcript immune response induced by Vibrio anguillarum and Aeromonas salmonicida vaccine, and prevents further transcription of stress-immune genes in response to acute stress.

Mucosal tissues appear to be more important in fish than in mammals due to living in a microbial-rich aquatic milieu, yet the complex interaction between the immune and the neuroendocrine system in these tissues remains elusive. The aim of this work was to investigate the mucosal immune response in immunized rainbow trout vaccinated with Alpha ject vaccine (bivalent), kept in fresh water (FW) or transferred to seawater (SW), and to evaluate their response to acute stress (chasing). Acute stress resulted in higher levels of plasma cortisol (Sham + Stress and Vaccine + Stress). A similar response was observed in skin mucus, but it was lower in Vaccine + Stress compared with stressed fish. With a few exceptions, minimal alterations were detected in the transcriptomic profile of stress-immune gene in the skin of vaccinated and stressed fish in both FW and SW. In the gills, the stress elicited activation of key stress-immune components (gr1, mr, ß-ar, hsp70, c3, lysozyme, α-enolase, nadph oxidase, il1ß, il6, tnfα, il10 and tgfß1) in FW, but fewer immune changes were induced by the vaccine (nadph oxidase, il6, tnfα, il10 and igt) in both SW and FW. In the intestine, an array of immune genes was activated by the vaccine particularly those related with B cells (igm, igt) and T cells (cd8α) in FW with no stimulation observed in SW. Therefore, our survey on the transcriptomic mucosal response demonstrates that the immune protection conferred by the vaccine to the intestine is modulated in SW. Overall, our results showed: i) plasma and skin mucus cortisol showed no additional stress effect induced by prolonged SW acclimation, ii) the stress and immune response were different among mucosal tissues which indicates a tissue-specific response to specific antigens/stressor. Further, the results suggest that the systemic immune organs may be more implicated in infectious events in SW (as few changes were observed in the mucosal barriers of immunized fish in SW) than in FW.

Increased reliance on coronary perfusion for cardiorespiratory performance in seawater-acclimated rainbow trout.

Salmonid ventricles are composed of spongy and compact myocardium, the latter being perfused via a coronary circulation. Rainbow trout (Oncorhynchus mykiss) acclimated to sea water have higher proportions of compact myocardium and display stroke volume-mediated elevations in resting cardiac output relative to freshwater-acclimated trout, probably to meet the higher metabolic needs of osmoregulatory functions. Here, we tested the hypothesis that cardiorespiratory performance of rainbow trout in sea water is more dependent on coronary perfusion by assessing the effects of coronary ligation on cardiorespiratory function in resting and exhaustively exercised trout acclimated to fresh water or sea water. While ligation only had minor effects on resting cardiorespiratory function across salinities, cardiac function after chasing to exhaustion was impaired, presumably as a consequence of atrioventricular block. Ligation reduced maximum O2 consumption rate by 33% and 17% in fish acclimated to sea water and fresh water, respectively, which caused corresponding 41% and 17% reductions in aerobic scope. This was partly explained by different effects on cardiac performance, as maximum stroke volume was only significantly impaired by ligation in sea water, resulting in 38% lower maximum cardiac output in seawater compared with 28% in fresh water. The more pronounced effect on respiratory performance in sea water was presumably also explained by lower blood O2 carrying capacity, with ligated seawater-acclimated trout having 16% and 17% lower haemoglobin concentration and haematocrit, respectively, relative to ligated freshwater trout. In conclusion, we show that the coronary circulation allows seawater-acclimated trout to maintain aerobic scope at a level comparable to that in fresh water.

Roles of leptin in initiation of acquired growth hormone resistance and control of metabolism in rainbow trout.

Catabolic conditions often induce concomitant changes in plasma leptin (Lep), growth hormone (GH), and insulin growth factor I (IGF-I) levels in teleost fish, but it is unclear whether these parts of the endocrine system are responding independently or functionally linked. In this study, fasted rainbow trout was used to study the effects of Lep on the GH-IGF-I system and metabolism. Fish were implanted intraperitoneally with recombinant rainbow trout Lep pellets and remained unfed. After 4 days, plasma GH levels were elevated in the Lep-treated fish in a dose-dependent manner; the expression of hepatic igf1 and plasma IGF-I levels were suppressed accordingly. In vitro Lep treatment reversed ovine GH (oGH)-stimulated expression of igf1 and igf2 in hepatocytes isolated from fasted fish, similar to the inhibitory effects of the MEK1/2 inhibitor U0126 treatment. However, Lep treatment alone had no effect on the expression of igfs or oGH-stimulated ghr2a expression in the hepatocytes. These results demonstrate an additive effect of Lep on suppression of IGF-I under catabolic conditions, indicating that Lep is likely involved in initiation of acquired GH resistance. Although the Lep-implant treatment had no effect on standard metabolic rate, it significantly suppressed gene expression of hepatic hydroxyacyl-CoA dehydrogenase, phosphoenolpyruvate carboxykinase, and glucose 6-phosphatase, which are key enzymes in lipid utilization and gluconeogenesis, in different patterns. Overall, this study indicates that the Lep increase in fasting salmonids is an important regulatory component for physiological adaptation during periods of food deprivation, involved in suppressing growth and hepatic metabolism to spare energy expenditure.

Impairment of branchial and coronary blood flow reduces reproductive fitness, but not cardiac performance in paternal smallmouth bass (Micropterus dolomieu).

The capacity to extract oxygen from the water, and the ability of the heart to drive tissue oxygen transport, are fundamental determinants of important life-history performance traits in fish. Cardiac performance is in turn dependent on the heart's own oxygen supply, which in some teleost species is partly delivered via a coronary circulation originating directly from the gills that perfuses the heart, and is crucial for cardiac, metabolic and locomotory capacities. It is currently unknown, however, how a compromised branchial blood flow (e.g., by angling-induced hook damage to the gills), constraining oxygen uptake and coronary blood flow, affects the energetically demanding parental care behaviours and reproductive fitness in fish. Here, we tested the hypothesis that blocking » of the branchial blood flow and abolishing coronary blood flow would negatively affect parental care behaviours, cardiac performance (heart rate metrics, via implanted Star-Oddi heart rate loggers) and reproductive fitness of paternal smallmouth bass (Micropterus dolomieu). Our findings reveal that branchial/coronary ligation compromised reproductive fitness, as reflected by a lower proportion of broods reaching free-swimming fry and a tendency for a higher nest abandonment rate relative to sham operated control fish. While this was associated with a tendency for a reduced aggression in ligated fish, parental care behaviours were largely unaffected by the ligation. Moreover, the ligation did not impair any of the heart rate performance metrics. Our findings highlight that gill damage may compromise reproductive output of smallmouth bass populations during the spawning season. Yet, the mechanism(s) behind this finding remains elusive.

Thermal acclimation and acute temperature effects on cardiac barostatic reflexes in rainbow trout (Oncorhynchus mykiss).

Acute warming raises the metabolism of fish, which is matched by increased heart rate. However, thermal acclimation may reduce heart rate through combinations of lowered intrinsic pacemaker rate and increased inhibitory vagal tone. This could affect the baroreflex, which regulates arterial blood pressure through heart rate changes via altered vagal tone. Using pharmacological tools, we assessed autonomic tones and baroreflex regulation of heart rate in rainbow trout (Oncorhynchus mykiss) at 11 °C, and after acute (24 h, 17°Cacute) or chronic (>7 weeks, 17°Cchronic) warming to 17 °C. We hypothesised that warm acclimation would manifest as reduced heart rate and elevated vagal tone in 17°Cchronic trout relative to 11 °C and 17°Cacute trout, which would increase baroreflex gain and the scope for fH increase through vagal release during hypotension. Compared to 11 °C, the 17°Cacute group exhibited slightly higher heart rate (Q10 = 1.5) and a strong trend for elevated vagal tone (54%). Surprisingly, however, routine heart rate was unaltered by warm acclimation (Q10 = 1.6), while intrinsic heart rate and vagal tone (22%) declined. Consequently, baroreflex sensitivity to reduced blood pressure was elevated in the 17°Cacute group but returned towards 11 °C conditions in the 17°Cchronic group. Atropine abolished nearly all chronotropic changes. Bradycardic responses to hypertension and cardiac adrenergic tone were unaltered across temperature treatments. The lack of a clear acclimation effect on routine heart rate in the present study is likely explained by a seasonal effect as the experiments were performed in early winter. Nonetheless, we conclude that baroreflex sensitivity in trout is thermally plastic; the heightened baroreflex sensitivity to hypotension following acute warming likely serves to safeguard tissue oxygen delivery as metabolism is elevated, while the reduced baroreflex sensitivity observed with warm acclimation may be linked to a pronounced metabolic down-regulation.

Coronary blood flow influences tolerance to environmental extremes in fish.

Approximately half of all fishes have, in addition to the luminal venous O2 supply, a coronary circulation supplying the heart with fully oxygenated blood. Yet, it is not fully understood how coronary O2 delivery affects tolerance to environmental extremes such as warming and hypoxia. Hypoxia reduces arterial oxygenation, while warming increases overall tissue O2 demand. Thus, as both stressors are associated with reduced venous O2 supply to the heart, we hypothesised that coronary flow benefits hypoxia and warming tolerance. To test this hypothesis, we blocked coronary blood flow (via surgical coronary ligation) in rainbow trout (Oncorhynchus mykiss) and assessed how in vivo cardiorespiratory performance and whole-animal tolerance to acute hypoxia and warming was affected. While coronary ligation reduced routine stroke volume relative to trout with intact coronaries, cardiac output was maintained by an increase in heart rate. However, in hypoxia, coronary-ligated trout were unable to increase stroke volume to maintain cardiac output when bradycardia developed, which was associated with a slightly reduced hypoxia tolerance. Moreover, during acute warming, coronary ligation caused cardiac function to collapse at lower temperatures and reduced overall heat tolerance relative to trout with intact coronary arteries. We also found a positive relationship between individual hypoxia and heat tolerance across treatment groups, and tolerance to both environmental stressors was positively correlated with cardiac performance. Collectively, our findings show that coronary perfusion improves cardiac O2 supply and therefore cardiovascular function at environmental extremes, which benefits tolerance to natural and anthropogenically induced environmental perturbations.

It takes time to heal a broken heart: ventricular plasticity improves heart performance after myocardial infarction in rainbow trout, Oncorhynchus mykiss.

Coronary arteriosclerosis is a common feature of both wild and farmed salmonid fishes and may be linked to stress-induced cardiac pathologies. Yet, the plasticity and capacity for long-term myocardial restructuring and recovery following a restriction in coronary blood supply are unknown. Here, we analyzed the consequences of acute (3 days) and chronic (from 33 to 62 days) coronary occlusion (i.e. coronary artery ligation) on cardiac morphological characteristics and in vivo function in juvenile rainbow trout, Oncorhynchus mykiss. Acute coronary artery occlusion resulted in elevated resting heart rate and decreased inter-beat variability, which are both markers of autonomic dysfunction following acute myocardial ischemia, along with severely reduced heart rate scope (maximum-resting heart rate) relative to sham-operated trout. We also observed a loss of myocardial interstitial collagen and compact myocardium. Following long-term coronary artery ligation, resting heart rate and heart rate scope normalized relative to sham-operated trout. Moreover, a distinct fibrous collagen layer separating the compact myocardium into two layers had formed. This may contribute to maintain ventricular integrity across the cardiac cycle or, alternatively, demark a region of the compact myocardium that continues to receive oxygen from the luminal venous blood. Taken together, we demonstrate that rainbow trout may cope with the aversive effects caused by coronary artery obstruction through plastic ventricular remodeling, which, at least in part, restores cardiac performance and myocardium oxygenation.

Normoxic limitation of maximal oxygen consumption rate, aerobic scope and cardiac performance in exhaustively exercised rainbow trout (Oncorhynchus mykiss).

In fish, maximum O2 consumption rate (MO2,max) and aerobic scope can be expanded following exhaustive exercise in hyperoxia; however, the mechanisms explaining this are yet to be identified. Here, in exhaustively exercised rainbow trout (Oncorhynchus mykiss), we assessed the influence of hyperoxia on MO2,max, aerobic scope, cardiac function and blood parameters to address this knowledge gap. Relative to normoxia, MO2,max was 33% higher under hyperoxia, and this drove a similar increase in aerobic scope. Cardiac output was significantly elevated under hyperoxia at MO2,max because of increased stroke volume, indicating that hyperoxia released a constraint on cardiac contractility apparent with normoxia. Thus, hyperoxia improved maximal cardiac performance, thereby enhancing tissue O2 delivery and allowing a higher MO2,max. Venous blood O2 partial pressure (PvO2) was elevated in hyperoxia at MO2,max, suggesting a contribution of improved luminal O2 supply in enhanced cardiac contractility. Additionally, despite reduced haemoglobin and higher PvO2, hyperoxia treated fish retained a higher arterio-venous O2 content difference at MO2,max. This may have been possible because of hyperoxia offsetting declines in arterial oxygenation that are known to occur following exhaustive exercise in normoxia. If this occurs, increased contractility at MO2,max with hyperoxia may also relate to an improved O2 supply to the compact myocardium via the coronary artery. Our findings show MO2,max and aerobic scope may be limited in normoxia following exhaustive exercise as a result of constrained maximal cardiac performance and highlight the need to further examine whether or not exhaustive exercise protocols are suitable for eliciting MO2,max and estimating aerobic scope in rainbow trout.

Cardiorespiratory adjustments to chronic environmental warming improve hypoxia tolerance in European perch (Perca fluviatilis).

Aquatic hypoxia will become increasingly prevalent in the future as a result of eutrophication combined with climate warming. While short-term warming typically constrains fish hypoxia tolerance, many fishes cope with warming by adjusting physiological traits through thermal acclimation. Yet, little is known about how such adjustments affect tolerance to hypoxia. We examined European perch (Perca fluviatilis) from the Biotest enclosure (23°C, Biotest population), a unique ∼1 km2 ecosystem artificially warmed by cooling water from a nuclear power plant, and an adjacent reference site (16-18°C, reference population). Specifically, we evaluated how acute and chronic warming affect routine oxygen consumption rate (MO2,routine) and cardiovascular performance in acute hypoxia, alongside assessment of the thermal acclimation of the aerobic contribution to hypoxia tolerance (critical O2 tension for MO2,routine: Pcrit) and absolute hypoxia tolerance (O2 tension at loss of equilibrium; PLOE). Chronic adjustments (possibly across lifetime or generations) alleviated energetic costs of warming in Biotest perch by depressing MO2,routine and cardiac output, and by increasing blood O2 carrying capacity relative to reference perch acutely warmed to 23°C. These adjustments were associated with improved maintenance of cardiovascular function and MO2,routine in hypoxia (i.e. reduced Pcrit). However, while Pcrit was only partially thermally compensated in Biotest perch, they had superior absolute hypoxia tolerance (i.e. lowest PLOE) relative to reference perch irrespective of temperature. We show that European perch can thermally adjust physiological traits to safeguard and even improve hypoxia tolerance during chronic environmental warming. This points to cautious optimism that eurythermal fish species may be resilient to the imposition of impaired hypoxia tolerance with climate warming.

Hemodynamic responses to warming in euryhaline rainbow trout: implications of the osmo-respiratory compromise.

In seawater, rainbow trout (Oncorhynchus mykiss) drink and absorb water through the gastrointestinal tract to compensate for water passively lost to the hyperosmotic environment. Concomitantly, they exhibit elevated cardiac output and a doubling of gastrointestinal blood flow to provide additional O2 to the gut and increase convective flux of absorbed ions and water. Yet, it is unknown how warming waters, which elevate tissue O2 demand and the rate of diffusion of ions and water across the gills (i.e. the osmo-respiratory compromise), affects these processes. We measured cardiovascular and blood variables of rainbow trout acclimated to freshwater and seawater during acute warming from 11 to 17°C. Relative to freshwater-acclimated trout, cardiac output was 34% and 55% higher in seawater-acclimated trout at 11 and 17°C, respectively, which allowed them to increase gastrointestinal blood flow significantly more during warming (increases of 75% in seawater vs. 31% in freshwater). These adjustments likely served to mitigate the impact of warming on osmotic balance, as changes in ionic and osmotic blood composition were minor. Furthermore, seawater-acclimated trout seemingly had a lower tissue O2 extraction, explaining why trout acclimated to freshwater and seawater often exhibit similar metabolic rates, despite a higher cardiac output in seawater. Our results highlight a novel role of gastrointestinal blood perfusion in the osmo-respiratory compromise in fish, and improve our understanding of the physiological changes euryhaline fishes must undergo when faced with interacting environmental challenges such as transient warming events.

In vivo aerobic metabolism of the rainbow trout gut and the effects of an acute temperature increase and stress event.

The fish gut is responsible for numerous potentially energetically costly processes, yet little is known about its metabolism. Here, we provide the first in vivo measurements of aerobic metabolism of the gut in a teleost fish by measuring gut blood flow, as well as arterial and portal venous oxygen content. At 10°C, gut oxygen uptake rate was 4.3±0.5 ml O2 h-1 kg-1 (∼11% of whole-animal oxygen uptake). Following acute warming to 15°C, gut blood flow increased ∼3.4-fold and gut oxygen uptake rate increased ∼3.7-fold (16.0±3.3 ml O2 h-1 kg-1), now representing ∼25% of whole-animal oxygen uptake. Although gut blood flow decreased following an acute stress event at 15°C, gut oxygen uptake remained unchanged as a result of a ∼2-fold increase in oxygen extraction. The high metabolic thermal sensitivity of the gut could have important implications for the overall aerobic capacity and performance of fish in a warming world and warrants further investigation.

Importance of the coronary circulation for cardiac and metabolic performance in rainbow trout (Oncorhynchus mykiss).

Cardiac oxygenation is achieved via both coronary arterial and luminal venous oxygen supply routes in many fish species. However, the relative importance of these supplies for cardiac and aerobic metabolic performance is not fully understood. Here, we investigated how coronary artery ligation in rainbow trout (Oncorhynchus mykiss), implanted with heart rate loggers, affected cardiorespiratory performance in vivo While coronary ligation significantly elevated resting heart rate, the standard metabolic rate was unchanged compared to sham-treated controls. However, coronary ligation reduced the maximum metabolic rate while heart rate remained unchanged following enforced exercise. Thus, coronary ligation reduced metabolic and heart rate scopes by 29% and 74%, respectively. Our findings highlight the importance of coronary oxygen supply for overall cardiorespiratory performance in salmonid fish, and suggest that pathological conditions that impair coronary flow (e.g. coronary arteriosclerosis) constrain the ability of fish to cope with metabolically demanding challenges such as spawning migrations and environmental warming.

Effects of coeliacomesenteric blood flow reduction on intestinal barrier function in rainbow trout Oncorhynchus mykiss.

The aim of the current work was to elucidate if there is a connection between stress-induced decrease in coeliacomesenteric artery blood flow (i.e. gastrointestinal blood flow; GBF) and disruption of the intestinal primary barrier in rainbow trout Oncorhynchus mykiss. Upon initiation of a 15 min acute chasing stress, the GBF decreased instantly by c. 92%. The GBF then slowly increased and reached c. 28% of resting values at the end of the stress protocol. After the stress was ceased, the GBF slowly increased and returned to resting values within c. 45 min. Intestinal permeability assessment in an Ussing-chambers set-up revealed impaired intestinal barrier function 24 h after stress. When the stress-induced GBF reduction was mimicked by an experimental occlusion of the coeliacomesenteric artery for 15 min followed by 24 h recovery, no effect on intestinal barrier function was observed. These results suggest that no direct causal relationship can be found between the GBF reduction and development of intestinal barrier dysfunction following periods of acute stress in this species of fish.

Influence of the coronary circulation on thermal tolerance and cardiac performance during warming in rainbow trout.

Thermal tolerance in fish may be related to an oxygen limitation of cardiac function. While the hearts of some fish species receive oxygenated blood via a coronary circulation, the influence of this oxygen supply on thermal tolerance and cardiac performance during warming remain unexplored. Here, we analyzed the effect in vivo of acute warming on coronary blood flow in adult sexually mature rainbow trout (Onchorhynchus mykiss) and the consequences of chronic coronary ligation on cardiac function and thermal tolerance in juvenile trout. Coronary blood flow at 10°C was higher in females than males (0.56 ± 0.08 vs. 0.30 ± 0.08 ml·min-1·g ventricle-1), and averaged 0.47 ± 0.07 ml·min-1·g ventricle-1 across sexes. Warming increased coronary flow in both sexes until 14°C, at which it peaked and plateaued at 0.78 ± 0.1 and 0.61 ± 0.1 ml·min-1·g ventricle-1 in females and males, respectively. Thus, the scope for increasing coronary flow was 101% in males, but only 39% in females. Coronary-ligated juvenile trout exhibited elevated heart rate across temperatures, reduced Arrhenius breakpoint temperature for heart rate (23.0 vs. 24.6°C), and reduced upper critical thermal maximum (25.3 vs. 26.3°C). To further analyze the effects of coronary flow restriction on cardiac rhythmicity, electrocardiogram characteristics were determined before and after coronary occlusion in anesthetized trout. Occlusion resulted in reduced R-wave amplitude and an elevated S-T segment, indicating myocardial ischemia, while heart rate was unaffected. This suggests that the tachycardia in ligated trout across temperatures in vivo was mainly to compensate for reduced cardiac contractility to maintain cardiac output. Moreover, our findings show that coronary flow increases with warming in a sex-specific manner. This may improve whole animal thermal tolerance, presumably by sustaining cardiac oxygenation and contractility at high temperatures.

Cardiac remodeling and increased central venous pressure underlie elevated stroke volume and cardiac output of seawater-acclimated rainbow trout.

Substantial increases in cardiac output (CO), stroke volume (SV), and gastrointestinal blood flow are essential for euryhaline rainbow trout (Oncorhyncus mykiss) osmoregulation in seawater. However, the underlying hemodynamic mechanisms responsible for these changes are unknown. By examining a range of circulatory and cardiac morphological variables of seawater- and freshwater-acclimated rainbow trout, the present study revealed a significantly higher central venous pressure (CVP) in seawater-acclimated trout (~0.09 vs. -0.02 kPa). This serves to increase cardiac end-diastolic volume in seawater and explains the elevations in SV (~0.41 vs. 0.27 ml/kg) and CO (~21.5 vs. 14.2 ml·min-1·kg-1) when compared with trout in freshwater. Furthermore, these hemodynamic modifications coincided with a significant increase in the proportion of compact myocardium, which may be necessary to compensate for the increased wall tension associated with a larger stroke volume. Following a temperature increase from 10 to 16.5°C, both acclimation groups exhibited similar increases in heart rate (Q10 of ~2), but SV tended to decrease in seawater-acclimated trout despite the fact that CVP was maintained in both groups. This resulted in CO of seawater- and freshwater-acclimated trout stabilizing at a similar level after warming (~26 ml·min-1·kg-1). The consistently higher CVP of seawater-acclimated trout suggests that factors other than compromised cardiac filling constrained the SV and CO of these individuals at high temperatures. The present study highlights, for the first time, the complex interacting effects of temperature and water salinity on cardiovascular responses in a euryhaline fish species.

Dynamic changes in cardiac mitochondrial metabolism during warm acclimation in rainbow trout.

Although the mitochondrial metabolism responses to warm acclimation have been widely studied in fish, the time course of this process is less understood. Here, we characterized the changes of rainbow trout (Oncorhynchus mykiss) cardiac mitochondrial metabolism during acute warming from 10 to 16°C, and during the subsequent warm acclimation for 39 days. We repeatedly measured mitochondrial oxygen consumption in cardiac permeabilized fibers and the functional integrity of mitochondria (i.e. mitochondrial coupling and cytochrome c effect) at two assay temperatures (10 and 16°C), as well as the activities of citrate synthase (CS) and lactate dehydrogenase (LDH) at room temperature. LDH and CS activities significantly increased between day 0 (10°C acclimated fish) and day 1 (acute warming to 16°C) while mitochondrial oxygen consumption measured at respective in vivo temperatures did not change. Enzymatic activities and mitochondrial oxygen consumption rates significantly decreased by day 2, and remained stable during warm acclimation (days 2-39). The decrease in rates of oxygen between day 0 and day 1 coincided with an increased cytochrome c effect and a decreased mitochondrial coupling, suggesting a structural/functional impairment of mitochondria during acute warming. We suggest that after 2 days of warm acclimation, a new homeostasis is reached, which may involve the removal of dysfunctional mitochondria. Interestingly, from day 2 onwards, there was a lack of differences in mitochondrial oxygen consumption rates between the assay temperatures, suggesting that warm acclimation reduces the acute thermal sensitivity of mitochondria. This study provides significant knowledge on the thermal sensitivity of cardiac mitochondria that is essential to delineate the contribution of cellular processes to warm acclimation.

Thermal sensitivity and phenotypic plasticity of cardiac mitochondrial metabolism in European perch, Perca fluviatilis.

Cellular and mitochondrial metabolic capacity of the heart has been suggested to limit performance of fish at warm temperatures. We investigated this hypothesis by studying the effects of acute temperature increases (16, 23, 30, 32.5 and 36°C) on the thermal sensitivity of 10 key enzymes governing cardiac oxidative and glycolytic metabolism in two populations of European perch (Perca fluviatilis) field-acclimated to 15.5 and 22.5°C, as well as the effects of acclimation on cardiac lipid composition. In both populations of perch, the activity of glycolytic (pyruvate kinase and lactate dehydrogenase) and tricarboxylic acid cycle (pyruvate dehydrogenase and citrate synthase) enzymes increased with acute warming. However, at temperatures exceeding 30°C, a drastic thermally induced decline in citrate synthase activity was observed in the cold- and warm-acclimated populations, respectively, indicating a bottleneck for producing the reducing equivalents required for oxidative phosphorylation. Yet, the increase in aspartate aminotransferase and malate dehydrogenase activities occurring in both populations at temperatures exceeding 30°C suggests that the malate-aspartate shuttle may help to maintain cardiac oxidative capacities at high temperatures. Warm acclimation resulted in a reorganization of the lipid profile, a general depression of enzymatic activity and an increased fatty acid metabolism and oxidative capacity. Although these compensatory mechanisms may help to maintain cardiac energy production at high temperatures, the activity of the electron transport system enzymes, such as complexes I and IV, declined at 36°C in both populations, indicating a thermal limit of oxidative phosphorylation capacity in the heart of European perch.

Bigger is not better: cortisol-induced cardiac growth and dysfunction in salmonids.

Stress and elevated cortisol levels are associated with pathological heart growth and cardiovascular disease in humans and other mammals. We recently established a link between heritable variation in post-stress cortisol production and cardiac growth in salmonid fish too. A conserved stimulatory effect of the otherwise catabolic steroid hormone cortisol is probably implied, but has to date not been established experimentally. Furthermore, whereas cardiac growth is associated with failure of the mammalian heart, pathological cardiac hypertrophy has not previously been described in fish. Here, we show that rainbow trout (Oncorhynchus mykiss) treated with cortisol in the diet for 45 days have enlarged hearts with lower maximum stroke volume and cardiac output. In accordance with impaired cardiac performance, overall circulatory oxygen-transporting capacity was diminished as indicated by reduced aerobic swimming performance. In contrast to the well-known adaptive/physiological heart growth observed in fish, cortisol-induced growth is maladaptive. Furthermore, the observed heart growth was associated with up-regulated signature genes of mammalian cardiac pathology, suggesting that signalling pathways mediating cortisol-induced cardiac remodelling in fish are conserved from fish to mammals. Altogether, we show that excessive cortisol can induce pathological cardiac remodelling. This is the first study to report and integrate the etiology, physiology and molecular biology of cortisol-induced pathological remodelling in fish.