Blood oxygenation and cardiorespiratory function in steelhead trout (Oncorhynchus mykiss) challenged with an acute temperature increase and zatebradine-induced bradycardia

Hyperoxia disproportionally benefits the aerobic performance of large fish at elevated temperature

Increasing evidence shows that larger fish are more vulnerable to acute warming than smaller individuals of the same species. This size-dependency of thermal tolerance has been ascribed to differences in aerobic performance, largely owing to a decline in oxygen supply relative to demand. To shed light on these ideas, we examined metabolic allometry in 130 rainbow trout ranging from 12 to 358 g under control conditions (17°C) and in response to acute heating (to 25°C), with and without supplemental oxygen (100% versus 150% air saturation). Under normoxia, high temperature caused an average 17% reduction in aerobic scope compared with 17°C. Aerobic performance disproportionally deteriorated in bigger fish as the scaling exponent (b) for aerobic scope declined from b=0.87 at 17°C to b=0.74 at 25°C. Hyperoxia increased maximum metabolic rate and aerobic scope at both temperatures and disproportionally benefited larger fish at 25°C as the scaling exponent for aerobic scope was reestablished to the same level as at 17°C (b=0.86). This suggests that hyperoxia may provide metabolic refuge for larger individuals, allowing them to sustain aerobic activities when facing acute warming. Notably, the elevated aerobic capacity afforded by hyperoxia did not appear to improve thermal resilience, as mortality in 25°C hyperoxia (13.8%, n=4) was similar to that in normoxia (12.1%, n=4), although we caution that this topic warrants more targeted research. We highlight the need for mechanistic investigations of the oxygen transport system to determine the consequences of differential metabolic scaling across temperature in a climate warming context.

Comparing methods for determining the metabolic capacity of lumpfish (Cyclopterus lumpus Linnaeus 1758)

Lumpfish (Cyclopterus lumpus) mortalities have been reported during the summer at some North Atlantic salmon cage-sites where they serve as "cleaner fish." To better understand this species' physiology and whether limitations in their metabolic capacity and thermal tolerance can explain this phenomenon, we compared the aerobic scope (AS) of 6°C-acclimated lumpfish (~50 g and 8.8 cm in length at the beginning of experiments) when all individuals (N = 12) were given a chase to exhaustion, a critical swim speed (Ucrit) test, and a critical thermal maximum (CTMax) test (rate of warming 2°C h-1). The Ucrit and CTMax of the lumpfish were 2.36 ± 0.08 body lengths per second and 20.6 ± 0.3°C. The AS of lumpfish was higher during the Ucrit test (206.4 ± 8.5 mg O2 kg-1 h-1) versus that measured in either the CTMax test or after the chase to exhaustion (141.0 ± 15.0 and 124.7 ± 15.5 mg O2 kg-1 h-1, respectively). Maximum metabolic rate (MMR), AS, and "realistic" AS (ASR) measured using the three different protocols were not significantly correlated, indicating that measurements of metabolic capacity using one of these methods cannot be used to estimate values that would be obtained using another method. Additional findings include that (1) the lumpfish's metabolic capacity is comparable to that of Atlantic cod, suggesting that they are not as "sluggish" as previously suggested in the literature, and (2) their CTMax (20.6°C when acclimated to 6°C), in combination with their recently determined ITMax (20.6°C when acclimated to 10°C), indicates that high sea-cage temperatures are unlikely to be the primary cause of lumpfish mortalities at salmon sea-cages during the summer.

The upper temperature and hypoxia limits of Atlantic salmon (Salmo salar) depend greatly on the method utilized

In this study, Atlantic salmon were: (i) implanted with heart rate (fH) data storage tags (DSTs), pharmacologically stimulated to maximum fH, and warmed at 10°C h-1 (i.e. tested using a 'rapid screening protocol'); (ii) fitted with Doppler® flow probes, recovered in respirometers and given a critical thermal maximum (CTmax) test at 2°C h-1; and (iii) implanted with fH DSTs, recovered in a tank with conspecifics for 4 weeks, and had their CTmax determined at 2°C h-1. Fish in respirometers and those free-swimming were also exposed to a stepwise decrease in water oxygen level (100% to 30% air saturation) to determine the oxygen level at which bradycardia occurred. Resting fH was much lower in free-swimming fish than in those in respirometers (∼49 versus 69 beats min-1) and this was reflected in their scope for fH (∼104 versus 71 beats min-1) and CTmax (27.7 versus 25.9°C). Further, the Arrhenius breakpoint temperature and temperature at peak fH for free-swimming fish were considerably greater than for those tested in the respirometers and given a rapid screening protocol (18.4, 18.1 and 14.6°C; and 26.5, 23.2 and 20.2°C, respectively). Finally, the oxygen level at which bradycardia occurred was significantly higher in free-swimming salmon than in those in respirometers (∼62% versus 53% air saturation). These results: highlight the limitations of some lab-based methods of determining fH parameters and thermal tolerance in fishes; and suggest that scope for fH may be a more reliable and predictive measure of a fish's upper thermal tolerance than their peak fH.

Cardiorespiratory physiology and swimming capacity of Atlantic salmon (Salmo salar) at cold temperatures

We investigated how acclimation to 8, 4 and 1°C, and acute cooling from 8  to 1°C, affected the Atlantic salmon's aerobic and anaerobic metabolism, and cardiac function, during a critical swim speed (Ucrit) test. This study revealed several interesting temperature-dependent effects. First, while differences in resting heart rate (fH) between groups were predictable based on previous research (range ∼28-65 beats  min-1), with values for 1°C-acclimated fish slightly higher than those of acutely exposed conspecifics, the resting cardiac output () of 1°C-acclimated fish was much lower and compensated for by a higher resting blood oxygen extraction (ṀO2/). In contrast, the acutely exposed fish had a ∼2-fold greater resting stroke volume (VS) compared with that of the other groups. Second, increases in fH (1.2- to 1.4-fold) contributed little to during the Ucrit test, and the contributions of (VS) versus ṀO2/ to aerobic scope (AS) were very different in the two groups tested at 1°C (1°C-acclimated and 8-1°C fish). Finally, Ucrit was 2.08 and 1.69 body lengths (BL) s-1 in the 8 and 4°C-acclimated groups, but only 1.27 and 1.44 BL s-1 in the 1°C-acclimated and 8-1°C fish, respectively - this lower value in 1°C versus 8-1°C fish despite higher values for maximum metabolic rate and AS. These data: support recent studies which suggest that the capacity to increase fH is constrained at low temperatures; show that cardiorespiratory function at cold temperatures, and its response to increased demands, depends on exposure duration; and suggest that AS does not constrain swimming capacity in salmon when chronically exposed to temperatures approaching their lower limit.

Effects of dietary Hyssop, Hyssopus officinalis, extract on physiological and antioxidant responses of rainbow trout, Oncorhynchus mykiss, juveniles to thermal stress

The present study aimed at assessing the effects of dietary Hyssop, Hyssopus officinalis, extract on rainbow trout, Oncorhynchus mykiss, responses to thermal stress. The juveniles (69.8 ± 0.38 g) were stocked in 12 through-flow tanks at a density of 12 fish per tank. Methanolic extract of Hyssop (HME) was added to diet at 0, 100, 250, and 500 mg/kg and the fish were fed (3% of biomass) over a 70-d period: 62 d at 13.3 ± 0.08°C and 7 d at 21–22°C. At the end of the trial, the plasma alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), triiodothyronine (T3), thyroxin (T4), cortisol, glucose, lactate, total antioxidant capacity (TAC), ascorbate, and the gill glutathione peroxidase (GPx), glutathione reductase (GR), glutathione-S-transferase (GST), and malondialdehyde (MDA). The results showed that HME had no significant effects on fish growth performance, survival, and feed efficiency. Dietary 250 mg/kg HME significantly decreased plasma ALT activity (P < 0.001), but showed no significant effects on plasma AST) (P = 0.106) activity, T3 (P = 0.992), and T4 (P = 0.070) levels. Thermal stress significantly (P < 0.001) increased plasma ALT and AST activities, but lowered plasma T3 and T4 levels. Dietary HME and thermal stress had interaction effects on plasma cortisol (P < 0.001), glucose (P = 0.007), lactate (P = 0.010), LDH (P = 0.005), TAC (P = 0.038), ascorbate (P < 0.001), and the gill GPx (P = 0.001), GR (P < 0.001), GST (P < 0.001), and MDA (P = 0.001). Thermal stress significantly increased plasma cortisol, glucose, lactate, and LDH, the gill GPX, GR, and GST, but dietary HME supplementation significantly reduced such elevations, particularly at 250 mg/kg level. Dietary HME significantly increased plasma TAC before the thermal stress and mitigated the stress-induced decreased in TAC, particularly at 250 mg/kg level. Dietary HME significantly decreased the gill MDA before and after the thermal stress, and lowest MDA was observed in 250 mg/kg HME level. Based on the present results, 250 mg/kg HME is recommended as suitable dose to improve antioxidative responses and hepatoprotection in rainbow trout under heat stress.

Can temperature-dependent changes in myocardial contractility explain why fish only increase heart rate when exposed to acute warming?

Fish increase heart rate (f_H), not stroke volume (V_S), when acutely warmed as a way to increase cardiac output (Q). To assess whether aspects of myocardial function may have some basis in determining temperature-dependent cardiac performance, we measured work and power (shortening, lengthening and net) in isolated segments of steelhead trout (Oncorhynchus mykiss) ventricular muscle at the fish's acclimation temperature (14°C), and at 22°C, when subjected to increased rates of contraction (30–105 min⁻¹, emulating increased f_H) and strain amplitude (8–14%, mimicking increased V_S). At 22°C, shortening power (indicative of Q) increased in proportion to f_H, and the work required to re-lengthen (stretch) the myocardium (fill the heart) was largely independent of f_H. In contrast, the increase in shortening power was less than proportional when strain was augmented, and lengthening work approximately doubled when strain was increased. Thus, the derived relationships between f_H, strain and myocardial shortening power and lengthening work, suggest that increasing f_H would be preferable as a mechanism to increase Q at high temperatures, or in fact may be an unavoidable response given constraints on muscle mechanics as temperatures rise. Interestingly, at 14°C, lengthening work increased substantially at higher f_H, and the duration of lengthening (i.e. diastole) became severely constrained when f_H was increased. These data suggest that myocardial contraction/twitch kinetics greatly constrain maximal f_H at cool temperatures, and may underlie observations that fish elevate V_S to an equal or greater extent than f_H to meet demands for increased Q at lower temperatures.

Summary: Myocardial contraction and twitch kinetics provide mechanistic explanations as to why heart rate, but not stroke volume, increases in fish with temperature, and why maximal heart rate is constrained at cool/cold temperatures.

Effects of hypoxic acclimation, muscle strain, and contraction frequency on nitric oxide-mediated myocardial performance in steelhead trout (Oncorhynchus mykiss)

Whether hypoxic acclimation influences nitric oxide (NO)-mediated control of fish cardiac function is not known. Thus, we measured the function/performance of myocardial strips from normoxic- and hypoxic-acclimated (40% air saturation; ∼8 kPa O₂) trout at several frequencies (20-80 contractions·min^-1) and two muscle strain amplitudes (8% and 14%) when exposed to increasing concentrations of the NO donor sodium nitroprusside (SNP) (10^-9 to 10^-4 M). Further, we examined the influence of 1) nitric oxide synthase (NOS) produced NO [by blocking NOS with 10^-4 M N^G-monomethyl-l-arginine (l-NMMA)] and 2) soluble guanylyl cyclase mediated, NOS-independent, NO effects (i.e., after blockade with 10^-4 M ODQ), on myocardial contractility. Hypoxic acclimation increased twitch duration by 8%-10% and decreased mass-specific net power by ∼35%. However, hypoxic acclimation only had minor impacts on the effects of SNP and the two blockers on myocardial function. The most surprising finding of the current study was the degree to which contraction frequency and strain amplitude influenced NO-mediated effects on myocardial power. For example, at 8% strain, 10^-4 SNP resulted in a decrease in net power of ∼30% at 20 min^-1 but an increase of ∼20% at 80 min^-1, and this effect was magnified at 14% strain. This research suggests that hypoxic acclimation has only minor effects on NO-mediated myocardial contractility in salmonids, is the first to report the high frequency- and strain-dependent nature of NO effects on myocardial contractility in fishes, and supports previous work showing that NO effects on the heart (myocardium) are finely tuned spatiotemporally.

Research on sablefish (Anoplopoma fimbria) suggests that limited capacity to increase heart function leaves hypoxic fish susceptible to heat waves

Studies of heart function and metabolism have been used to predict the impact of global warming on fish survival and distribution, and their susceptibility to acute and chronic temperature increases. Yet, despite the fact that hypoxia and high temperatures often co-occur, only one study has examined the effects of hypoxia on fish thermal tolerance, and the consequences of hypoxia for fish cardiac responses to acute warming have not been investigated. We report that sablefish (Anoplopoma fimbria) did not increase heart rate or cardiac output when warmed while hypoxic, and that this response was associated with reductions in maximum O2 consumption and thermal tolerance (CTmax) of 66% and approximately 3°C, respectively. Further, acclimation to hypoxia for four to six months did not substantially alter the sablefish's temperature-dependent physiological responses or improve its CTmax. These results provide novel, and compelling, evidence that hypoxia can impair the cardiac and metabolic response to increased temperatures in fish, and suggest that some coastal species may be more vulnerable to climate change-related heat waves than previously thought. Further, they support research showing that cross-tolerance and physiological plasticity in fish following hypoxia acclimation are limited.

The effects of acute temperature change and digestive status on in situ cardiac function in mahi-mahi (Coryphaena hippurus)

In this study, we investigated the effect of acute increases in temperature on cardiovascular function of mahi-mahi (Coryphaena hippurus). We also describe, for the first time, an artery that supplies the gastrointestinal tract that originates from the fourth branchial artery. We used vascular casting to verify the anatomical location of this unique celiaco-mesenteric artery. We predicted that blood flow in this vessel would be correlated with the digestive state of the animal. Increasing water temperature from 25.0 to 30.5 °C resulted in a linear increase in heart rate (f_H) from 165 ± 4 beats∙min^-1to 232 ± 7 beats∙min^-1. Over this temperature range, f_H strongly correlated with water temperature (R² = 0.79). At 31 °C f_H no longer correlated with water temperature, and at 34 °C f_H had dropped to 114 ± 19 beats∙min^-1. Furthermore, we found that mahi are capable of maintaining constant cardiac output over a temperature range from 25 to 31 °C. Cardiac function appeared to be compromised at temperatures >31 °C. In fed anesthetized fish, blood flow was pulsatile in the celiaco-mesenteric artery and was not in fasted fish. In fed fish, blood flow in the left celiaco-mesenteric artery was 1.99 ± 0.78 ml·min^-1·kg^-1 compared to the total cardiac output of 168.6 ± 12.7 ml·min^-1·kg^-1. The data suggest that mahi can differentially regulate gastric blood flow based on feeding state, which may explain the high digestive efficiency and very high growth rates of these pelagic predators.

A rapid intrinsic heart rate resetting response with thermal acclimation in rainbow trout, Oncorhynchus mykiss

We examined cardiac pacemaker rate resetting in rainbow trout following a reciprocal temperature transfer. In the original experiment, performed in winter, 4°C-acclimated fish transferred to 12°C reset intrinsic heart rate after just 1 h (from 56.8±1.2 to 50.8±1.5 beats min-1); 12°C-acclimated fish transferred to 4°C reset intrinsic heart rate after 8 h (from 33.4±0.7 to 37.7±1.2 beats min-1). However, in a replicate experiment, performed in the summer using a different brood year, intrinsic heart rate was not reset, even after 10 weeks at a new temperature. Using this serendipitous opportunity, we compared mRNA expression changes of a suite of proteins in sinoatrial node (SAN), atrial and ventricular tissues after both 1 h and longer than 3 weeks for both experimental acclimation groups to identify those changes only associated with pacemaker rate resetting. Of the changes in mRNA expression occurring after more than 3 weeks of warm acclimation and associated with pacemaker rate resetting, we observed downregulation of NKA α1c in the atrium and ventricle, and upregulation of HCN1 in the ventricle. However, in the SAN there were no mRNA expression changes unique to the fish with pacemaker rate resetting after either 1 h or 3 weeks of warm acclimation. Thus, despite identifying changes in mRNA expression of contractile cardiac tissues, there was an absence of changes in mRNA expression directly involved with the initial, rapid pacemaker rate resetting with warm acclimation. Importantly, pacemaker rate resetting with thermal acclimation does not always occur in rainbow trout.

Resting time before slaughter restores homeostasis, increases rigor mortis time and fillet quality of surubim Pseudoplatystoma spp

This study assesses the respiratory dynamics related to stress parameters and resting time before slaughter, in the quality of surubim (Pseudopatystoma spp.) fillets. A completely randomized design was conducted using five treatments: resting time before slaughter of 0, 2, 4, 8 and 24 hours, with 15 fish sampled per treatment. Time 0 corresponded to the treatment without resting time, where the fish were slaughtered immediately after arriving at the processing plant. The resting time did not affect the electrolyte balance, hemoglobin, plasma, hepatic glycogen, myofibrillar fragmentation index (MFI) and water holding capacity (WHC) of surubins. However, with increased resting time, there was a significant decrease in muscle glycogen and an increase in blood pH and blood bicarbonate levels. Additionally, respiratory parameters showed an increase in pO₂ and, consequently, in O₂ saturation and a decrease in pCO₂.The hematocrit and MCV values of the surubins after 24 hours of resting decreased significantly. In the first hours of resting, the highest values of erythrocytes and CHCM were observed. The lowest level of stress was observed for fish having 24 hours of resting. Fish having longer resting periods (8 and 24 hours) presented fillets with a higher pH (P <0.05) and the rigor mortis establishment time was shorter for the first 2 hours and 24 hours of resting time. There was a linear decrease in fillet lightness and an increase in the intensity of red (CIE a*) color up to 24 hours when resting was increased. In CIE b*, a linear decrease (P <0.05) of the yellow intensity of the fillets was observed as the surubim resting time increased. A resting time of 4 to 8 hours before slaughter is effective in reestablishing homeostasis after transporting surubim, providing fillets with higher quality and a greater length of the pre-rigor mortis period.

What determines systemic blood flow in vertebrates?

In the 1950s, Arthur C. Guyton removed the heart from its pedestal in cardiovascular physiology by arguing that cardiac output is primarily regulated by the peripheral vasculature. This is counterintuitive, as modulating heart rate would appear to be the most obvious means of regulating cardiac output. In this Review, we visit recent and classic advances in comparative physiology in light of this concept. Although most vertebrates increase heart rate when oxygen demands rise (e.g. during activity or warming), experimental evidence suggests that this tachycardia is neither necessary nor sufficient to drive a change in cardiac output (i.e. systemic blood flow, Q̇ _sys) under most circumstances. Instead, Q̇ _sys is determined by the interplay between vascular conductance (resistance) and capacitance (which is mainly determined by the venous circulation), with a limited and variable contribution from heart function (myocardial inotropy). This pattern prevails across vertebrates; however, we also highlight the unique adaptations that have evolved in certain vertebrate groups to regulate venous return during diving bradycardia (i.e. inferior caval sphincters in diving mammals and atrial smooth muscle in turtles). Going forward, future investigation of cardiovascular responses to altered metabolic rate should pay equal consideration to the factors influencing venous return and cardiac filling as to the factors dictating cardiac function and heart rate.

Membrane and calcium clock mechanisms contribute variably as a function of temperature to setting cardiac pacemaker rate in zebrafish Danio rerio

Here, we show that heart rate in zebrafish Danio rerio is dependent upon two pacemaking mechanisms and it possesses a limited ability to reset the cardiac pacemaker with temperature acclimation. Electrocardiogram recordings, taken from individual, anaesthetised zebrafish that had been acclimated to 18, 23 or 28°C were used to follow the response of maximum heart rate (f_Hmax ) to acute warming from 18°C until signs of cardiac failure appeared (up to c. 40°C). Because f_Hmax was similar across the acclimation groups at almost all equivalent test temperatures, warm acclimation was limited to one significant effect, the 23°C acclimated zebrafish had a significantly higher (21%) peak f_Hmax and reached a higher (3°C) test temperature than the 18°C acclimated zebrafish. Using zatebradine to block the membrane hyperpolarisation-activated cyclic nucleotide-gated channels (HCN) and examine the contribution of the membrane clock mechanisms to cardiac pacemaking, f _Hmax was significantly reduced (by at least 40%) at all acute test temperatures and significantly more so at most test temperatures for zebrafish acclimated to 28°C vs. 23°C. Thus, HCN channels and the membrane clock were not only important, but could be modified by thermal acclimation. Using a combination of ryanodine (to block sarcoplasmic calcium release) and thapsigargin (to block sarcoplasmic calcium reuptake) to examine the contribution of sarcoplasmic reticular handling of calcium and the calcium clock, f _Hmax was again consistently reduced independent of the test temperature and acclimation temperature, but to a significantly lesser degree than zatebradine for zebrafish acclimated to both 28 and 18°C. Thus, the calcium clock mechanism plays an additional role in setting pacemaker activity that was independent of temperature. In conclusion, the zebrafish cardiac pacemaker has a limited temperature acclimation ability compared with known effects for other fishes and involves two pacemaking mechanisms, one of which was independent of temperature.

Echocardiography and electrocardiography reveal differences in cardiac hemodynamics, electrical characteristics, and thermal sensitivity between northern pike, rainbow trout, and white sturgeon

Doppler and B-mode ultrasonography and electrocardiography (ECG) were used to determine cardiac hemodynamics and electrical characteristics in 12°C-acclimated and metomidate-anesthetized northern pike, rainbow trout and white sturgeon (7-9 per species) at 12°C and 20°C, and at a comparable heart rate (f_H , ~60 beats/min). Despite similar relative ventricle masses and cardiac output (Q), interspecific differences were observed at 12°C in f_H , ventricular filling and ejection, stroke volume, the duration ECG intervals, and cardiac valve cross-sectional areas. Vis-a-fronte filling of the atrium due to ventricular contraction was observed in all species. However, biphasic ventricular filling (i.e., due to central venous pressure and then atrial contraction) was only observed in rainbow trout and white sturgeon. Changes in atrial and ventricular performance varied between the species as temperature increased from 12°C to 20°C. Rainbow trout had the highest thermal sensitivity for f_H (Q₁₀  = 3.73), which doubled Q, and the largest increase in transvalvular blood velocity during ventricular filling. Conversely, northern pike had the lowest Q₁₀ for f_H (1.58) and did not increase Q. At ~60 beats/min, the rainbow trout heart had the shortest period of electrical activity, which also resulted in the longest recovery period (TP interval) between successive beats. The QT interval at ~60 beats/min was also longer in the white sturgeon versus the other species. These results suggest that interspecific differences in fish cardiac hemodynamics may be related to cardiac morphology, the duration of electrical impulses through the heart, cardiac thermal sensitivity, and valve dimensions.

Physiological and metabolic responses of juvenile Lophiosilurus alexandri catfish to air exposure

The present study aimed to evaluate the physiological and metabolic stress responses of juvenile Lophiosilurus alexandri submitted to an air exposure test. The subjects consisted of 72 juveniles. Blood samples were taken at: 0 h-fish not exposed to air; 0.5 h-fish shortly after exposure to air for 30 min (prior to returning to the tank); 1.5 h (90 min), 24, 48, and 96 h after the initiation of exposure to air for 30 min. After 96 h, survivorship was 100%. Cortisol and glucose levels were higher at 0.5 h, returning to baseline at 48 and 24 h, respectively. Lactate dehydrogenase levels were highest at 1.5 h after exposure to air, returning to normal values in 24 h. Several changes were recorded in gasometric blood values and electrolytes. With regard to hematology and blood chemistry, exposure to air did not affect globular volume and AST throughout the 96 h of the experiment. The values for alkaline phosphatase were highest at 0, 1.5, and 24 h. Total protein was similar between 0 and 1.5 h and lowest at 96 h, while ALT was highest at 0.5 h. Leukocytes were highest at 0.5, 1.5, 48, and 96 h, while erythrocytes were highest at 96 h. After 96 h, juvenile L. alexandri were able to reestablish the main indicators of stress (cortisol, glucose and lactate dehydrogenase), while other indicators (hematological, biochemical, and gasometric) exhibited compensatory variation for normal physiological re-establishment.

IMPLICAÇÕES DE EXTRATO DE Erythrina crista-galli COMO ANSIOLÍTICO PARA Carassius auratus

Resumo A fitoterapia vem sendo utilizada em criação de peixes a fim de promover estabilidade no ambiente de cultivo e na profilaxia de doenças, contribuindo para a melhoria do bem-estar animal. Dentre os fitoterápicos, Erythrina crista-galli tem efeito sedativo e atividade antioxidante e antimicrobiana, além de funcionar como calmante natural. Na ausência de formulações para uso em peixes cultivados, surgem protocolos terapêuticos repletos de incertezas quanto à sua eficácia, ao impacto ambiental e ao perfil hematológico dos peixes. No ensaio de tolerância aguda da Erythrina crista-galli, foram utilizados exemplares de Carassius auratus, expostos às concentrações de controle (zero), 50, 100 e 200 mgL-1, com três repetições, por 96 horas. Foi observada a sobrevivência de 100% em todos os tratamentos. Estes resultados revelam que o extrato de Erythrina crista-galli não apresenta toxicidade para peixes. O balanço eletrolítico plasmático não apresentou mudanças. A utilização de até 100 mgL-1 não promoveu mudanças na trocas gasosas. O pH e a concentração de bicarbonato e glicose foram crescentes até a concentração de 100 mgL-1. Conclui-se que a utilização de Erythrina crista-galli para exposição aguda de Carassius auratus não promove mortalidade, mas provoca alterações indesejáveis nos parâmetros fisiológicos sanguíneos quando utilizadas doses acima de 100 mgL-1.

Maximum cardiac performance of Antarctic fishes that lack haemoglobin and myoglobin: exploring the effect of warming on nature's natural knockouts

Antarctic notothenioids, some of which lack myoglobin (Mb) and/or haemoglobin (Hb), are considered extremely stenothermal, which raises conservation concerns since Polar regions are warming at unprecedented rates. Without reliable estimates of maximum cardiac output ([Formula: see text]), it is impossible to assess their physiological scope in response to warming seas. Therefore, we compared cardiac performance of two icefish species, Chionodraco rastrospinosus (Hb^-Mb⁺) and Chaenocephalus aceratus (Hb^-Mb^-), with a related notothenioid, Notothenia coriiceps (Hb⁺Mb⁺) using an in situ perfused heart preparation. The maximum [Formula: see text], heart rate (f _H), maximum cardiac work (W _C) and relative ventricular mass of N. coriiceps at 1°C were comparable to temperate-water teleosts, and acute warming to 4°C increased f _H and W _C, as expected. In contrast, icefish hearts accommodated a higher maximum stroke volume (V _S) and maximum [Formula: see text] at 1°C, but their unusually large hearts had a lower f _H and maximum afterload tolerance than N. coriiceps at 1°C. Furthermore, maximum V _S, maximum [Formula: see text] and f _H were all significantly higher for the Hb^-Mb⁺ condition compared with the Hb^-Mb^- condition, a potential selective advantage when coping with environmental warming. Like N. coriiceps, both icefish species increased f _H at 4°C. Acutely warming C. aceratus increased maximum [Formula: see text], while C. rastrospinosus (like N. coriiceps) held at 4°C for 1 week maintained maximum [Formula: see text] when tested at 4°C. These experiments involving short-term warming should be followed up with long-term acclimation studies, since the maximum cardiac performance of these three Antarctic species studied seem to be tolerant of temperatures in excess of predictions associated with global warming.

Reduced thermal tolerance during salinity acclimation in brook trout (Salvelinus fontinalis) can be rescued by prior treatment with cortisol

The aims of this study were to assess whether thermal tolerance of brook trout (Salvelinus fontinalis) is affected during seawater (SW) acclimation and to investigate the role of cortisol in osmoregulation and thermal tolerance during SW acclimation. Freshwater (FW)-acclimated brook trout at 18 °C (Tacc) were exposed to SW for 16 d, whilst maintaining a FW control. Fish were examined for critical thermal maximum (CTmax) 0 (before), 2, 5, and 16 d after SW exposure, and sampled at Tacc and CTmax for analysis of plasma cortisol, glucose, and Cl−, gill Na+/K+-ATPase (NKA) activity and heat shock protein 70 (HSP70) abundance, and white muscle water content. At 2 d in SW, CTmax was significantly reduced (from 31 to 26 °C), then recovered by 16 d. This transient decrease in thermal tolerance coincided with a transient increase in plasma Cl− and decrease in muscle moisture content. Salinity itself had no effect on gill HSP70 abundance compared to the large and immediate effects of high temperature exposure during CTmax testing. To examine the role of cortisol in osmoregulation, brook trout were administered a cortisol implant (5 and 25 µg/g CORT) prior to SW exposure. Both CORT doses significantly increased their capacity to maintain plasma Cl− during SW acclimation. Treatment with the 25 µg/g CORT dose was shown to significantly improve CTmax after 2 d in SW, and CTmax was associated with plasma Cl−. These findings indicate that brook trout are sensitive to temperature during SW acclimation and that thermal tolerance is associated with ion and water balance during SW acclimation.

Toxicokinetic models and related tools in environmental risk assessment of chemicals

Environmental risk assessment of chemicals for the protection of ecosystems integrity is a key regulatory and scientific research field which is undergoing constant development in modelling approaches and harmonisation with human risk assessment. This review focuses on state-of-the-art toxicokinetic tools and models that have been applied to terrestrial and aquatic species relevant to environmental risk assessment of chemicals. Both empirical and mechanistic toxicokinetic models are discussed using the results of extensive literature searches together with tools and software for their calibration and an overview of applications in environmental risk assessment. These include simple tools such as one-compartment models, multi-compartment models to physiologically-based toxicokinetic (PBTK) models, mostly available for aquatic species such as fish species and a number of chemical classes including plant protection products, metals, persistent organic pollutants, nanoparticles. Data gaps and further research needs are highlighted.

Metabolismo respiratório e da glicose de Carassius auratus submetidos à concentrações de eugenol

RESUMO Este trabalho tem como finalidade avaliar o uso do eugenol como anestésico para Carassius auratus mensurando o tempo de indução à anestesia em relação a diferentes concentrações e seus efeitos nas trocas gasosas respiratórias. Os peixes foram expostos as concentrações de 20, 40, 80, 120, 150mg L-1 de eugenol. Os resultados foram analisados segundo um delineamento inteiramente casualisado (DIC) com seis tratamentos e cinco repetições cada um, submetidos à análise de variância e quando significativos, as médias foram comparadas pelo teste t de Student (α = 0,05). Os valores de PaO2e PaCO2 foram submetidos à regressão polinomial (p > 0,05). Foram avaliados o tempo de sedação e recuperação. O sangue foi retirado para analise parâmetros hematológicos, pH, pressão parcial de oxigênio (PaO2), pressão parcial de dióxido de carbono (PaCO2), bicarbonato (HCO-3) e glicose. As concentrações acima de 80 mg.L-1 apresentaram sedação com 62 seg. A recuperação possui correlação inversa ao tempo de anestesia. O aumento das concentrações de eugenol proporcionou elevação de 27,5% nos hematócrito e de 37% no eritrócitos e houve aumento na glicose plasmática. Ademais, apresentaram diminuição na pressão de oxigênio no sangue (PaO2), e aumento na pressão de dióxido de carbono (PaCO2). A utilização do eugenol possibilitaria e melhoria o manejo nas pisciculturas sendo a dose recomendada de 40mg. L-1 para Carassius auratus pois minimiza os riscos inerentes a processos anestésicos rotineiros, com menor comprometimento cardiovascular e, consequentemente, redução do risco de óbito.

Long-term hypoxia exposure alters the cardiorespiratory physiology of steelhead trout (Oncorhynchus mykiss), but does not affect their upper thermal tolerance

It has been suggested that exposure to high temperature or hypoxia may confer tolerance to the other oxygen-limited stressor (i.e., 'cross-tolerance'). Thus, we investigated if chronic hypoxia-acclimation (>3 months at 40% air saturation) improved the steelhead trout's critical thermal maximum (CT_Max), or affected key physiological variables that could impact upper thermal tolerance. Neither CT_Max (24.7 vs. 25.3°C) itself, nor oxygen consumption ( [Formula: see text] ), haematocrit, blood haemoglobin concentration, or heart rate differed between hypoxia- and normoxia-acclimated trout when acutely warmed. However, the cardiac output (Q̇) of hypoxia-acclimated fish plateaued earlier compared to normoxia-acclimated fish due to an inability to maintain stroke volume (S_V), and this resulted in a ~50% lower maximum Q̇. Despite this reduced maximum cardiac function, hypoxia-acclimated trout were able to consume more O₂ per volume of blood pumped as evidenced by the equivalent [Formula: see text] . These results provide additional evidence that long-term hypoxia improves tissue oxygen utilization, and that this compensates for diminished cardiac pumping capacity. The limited S_V in hypoxia-acclimated trout in vivo was not associated with changes in cardiac morphology or in vitro maximum S_V, but the affinity and density of myocardial ß-adrenoreceptors were lower and higher, respectively, than in normoxia-acclimated fish. These data suggest that alterations in ventricular filling dynamics or myocardial contractility constrain cardiac function in hypoxia-acclimated fish at high temperatures. Our results do not support (1) 'cross-tolerance' between high temperature and hypoxia when hypoxia is chronic, or (2) that cardiac function is always the determinant of temperature-induced changes in fish [Formula: see text] , and thus thermal tolerance, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) theory.

Effects of Loma morhua (Microsporidia) infection on the cardiorespiratory performance of Atlantic cod Gadus morhua (L)

The microsporidian Loma morhua infects Atlantic cod (Gadus morhua) in the wild and in culture and results in the formation of xenomas within the gill filaments, heart and spleen. Given the importance of the two former organs to metabolic capacity and thermal tolerance, the cardiorespiratory performance of cod with a naturally acquired infection of Loma was measured during an acute temperature increase (2 °C h(-1)) from 10 °C to the fish's critical thermal maximum (CT(Max)). In addition, oxygen consumption and swimming performance were measured during two successive critical swimming speed (U(crit)) tests at 10 °C. While Loma infection had a negative impact on cod cardiac function at warm temperatures, and on metabolic capacity in both the CT(Max) and U(crit) tests (i.e. a reduction of 30-40%), it appears that the Atlantic cod can largely compensate for these Loma-induced cardiorespiratory limitations. For example, (i) CT(Max) (21.0 ± 0.3 °C) and U(crit) (~1.75 BL s(-1)) were very comparable to those reported in previous studies using uninfected fish from the same founder population; and (ii) our data suggest that tissue oxygen extraction, and potentially the capacity for anaerobic metabolism, is enhanced in fish infected with this microsporidian.

The beat goes on: Cardiac pacemaking in extreme conditions

In order for an animal to survive, the heart beat must go on in all environmental conditions, or at least restart its beat. This review is about maintaining a rhythmic heartbeat under the extreme conditions of anoxia (or very severe hypoxia) and high temperatures. It starts by considering the primitive versions of the protein channels that are responsible for initiating the heartbeat, HCN channels, divulging recent findings from the ancestral craniate, the Pacific hagfish (Eptatretus stoutii). It then explores how a heartbeat can maintain a rhythm, albeit slower, for hours without any oxygen, and sometimes without autonomic innervation. It closes with a discussion of recent work on fishes, where the cardiac rhythm can become arrhythmic when a fish experiences extreme heat.

Temperature and sex dependent effects on cardiac mitochondrial metabolism in Atlantic cod (Gadus morhua L.)

To test the hypothesis that impaired mitochondrial respiration limits cardiac performance at warm temperatures, and examine if any effect(s) are sex-related, the consequences of high temperature on cardiac mitochondrial oxidative function were examined in 10°C acclimated, sexually immature, male and female Atlantic cod. Active (State 3) and uncoupled (States 2 and 4) respiration were measured in isolated ventricular mitochondria at 10, 16, 20, and 24°C using saturating concentrations of malate and pyruvate, but at a submaximal (physiological) level of ADP (200µM). In addition, citrate synthase (CS) activity was measured at these temperatures, and mitochondrial respiration and the efficiency of oxidative phosphorylation (P:O ratio) were determined at [ADP] ranging from 25-200µM at 10 and 20°C. Cardiac morphometrics and mitochondrial respiration at 10°C, and the thermal sensitivity of CS activity (Q10=1.51), were all similar between the sexes. State 3 respiration at 200µM ADP increased gradually in mitochondria from females between 10 and 24°C (Q10=1.48), but plateaued in males above 16°C, and this resulted in lower values in males vs. females at 20 and 24°C. At 10°C, State 4 was ~10% of State 3 values in both sexes [i.e. a respiratory control ratio (RCR) of ~10] and P:O ratios were approximately 1.5. Between 20 and 24°C, State 4 increased more than State 3 (by ~70 vs. 14%, respectively), and this decreased RCR to ~7.5. The P:O ratio was not affected by temperature at 200μM ADP. However, (1) the sensitivity of State 3 respiration to increasing [ADP] (from 25 to 200μM) was reduced at 20 vs. 10°C in both sexes (Km values 105±7 vs. 68±10μM, respectively); and (2) mitochondria from females had lower P:O values at 25 vs. 100μM ADP at 20°C, whereas males showed a similar effect at 10°C but a much more pronounced effect at 20°C (P:O 1.05 at 25μM ADP vs. 1.78 at 100μMADP). In summary, our results demonstrate several sex-related differences in ventricular mitochondrial function in Atlantic cod, and suggest that myocardial oxidative function and possibly phosphorylation efficiency may be limited at temperatures of 20°C or above, particularly in males. These observations could partially explain why cardiac function in Atlantic cod plateaus just below this species׳ critical thermal maximum (~22°C) and may contribute to yet unidentified sex differences in thermal tolerance and swimming performance.

Increased ventricular stiffness and decreased cardiac function in Atlantic cod (Gadus morhua) at high temperatures

We employed the work loop method to study the ability of ventricular and atrial trabeculae from Atlantic cod to sustain power production during repeated contractions at acclimation temperatures (10°C) and when acutely warmed (20°C). Oxygen tension (Po2) was lowered from 450 to 34% air saturation to augment the thermal stress. Preparations worked under conditions simulating either a large stroke volume (35 contractions/min rate, 8-12% muscle strain) or a high heart rate (70 contractions/min, 2-4% strain), with power initially equal under both conditions. The effect of declining Po2 on power was similar under both conditions but was temperature and tissue dependent. In ventricular trabeculae at 10°C (and atria at 20°C), shortening power declined across the full range of Po2 studied, whereas the power required to lengthen the muscle was unaffected. Conversely, in ventricular trabeculae at 20°C, there was no decline in shortening power but an increase in lengthening power when Po2 fell below 100% air saturation. Finally, when ventricular trabeculae were paced at rates of up to 115 contractions/min at 20°C (vs. the maximum of 70 contractions/min in vivo), they showed marked increases in both shortening and lengthening power. Our results suggest that although elevated heart rates may not impair ventricular power as they commonly do isometric force, limited atrial power and the increased work required to expand the ventricle during diastole may compromise ventricular filling and hence, stroke volume in Atlantic cod at warm temperatures. Neither large strains nor high contraction rates convey an apparent advantage in circumventing this.

Cardiorespiratory collapse at high temperature in swimming adult sockeye salmon

Elevated summer river temperatures are associated with high in-river mortality in adult sockeye salmon (Oncorhynchus nerka) during their once-in-a-lifetime spawning migration up the Fraser River (British Columbia, Canada). However, the mechanisms underlying the decrease in whole-animal performance and cardiorespiratory collapse above optimal temperatures for aerobic scope (T opt) remain elusive for aquatic ectotherms. This is in part because all the relevant cardiorespiratory variables have rarely been measured directly and simultaneously during exercise at supra-optimal temperatures. Using the oxygen- and capacity-limited thermal tolerance hypothesis as a framework, this study simultaneously and directly measured oxygen consumption rate (MO2), cardiac output [Formula: see text], heart rate (f H), and cardiac stroke volume (V s), as well as arterial and venous blood oxygen status in adult sockeye salmon swimming at temperatures that bracketed T opt to elucidate possible limitations in oxygen uptake into the blood or internal delivery through the oxygen cascade. Above T opt, the decline in MO2max and aerobic scope was best explained by a cardiac limitation, triggered by reduced scope for f H. The highest test temperatures were characterized by a negative scope for f H, dramatic decreases in maximal [Formula: see text] and maximal V s, and cardiac dysrhythmias. In contrast, arterial blood oxygen content and partial pressure were almost insensitive to supra-optimal temperature, suggesting that oxygen delivery to and uptake by the gill were not a limiting factor. We propose that the high-temperature-induced en route mortality in migrating sockeye salmon may be at least partly attributed to physiological limitations in aerobic performance due to cardiac collapse via insufficient scope for f H. Furthermore, this improved mechanistic understanding of cardiorespiratory collapse at high temperature is likely to have broader application to other salmonids and perhaps other aquatic ectotherms.

Thermal dependence of cardiac function in Arctic fish: implications of a warming world

With the Arctic experiencing one of the most rapid and greatest increases in sea temperatures in modern time, predicting how Arctic marine organisms will respond to elevated temperatures has become critical for conservation biology. Here we examined the thermal sensitivity of cardiorespiratory performance for three closely related species of sculpins that inhabit the Arctic waters, two of which, Gymnocanthus tricuspis and Myoxocephalus scorpioides, have adapted to a restricted range within the Arctic, whereas the third species, Myoxocephalus scorpius, has a wider distribution. We tested the hypothesis that the fish restricted to Arctic cold waters would show reduced cardiorespiratory scope in response to an increase in temperature, as compared with the more eurythermal M. scorpius. As expected from their biogeography, M. scorpioides and G. tricuspis maximised cardiorespiratory performance at temperatures between 1-4oC, whereas M. scorpius maximised performance over a wider range of temperatures (1-10oC). Furthermore, factorial scope for cardiac output collapsed at elevated temperature for the two high-latitude species, negatively impacting their ability to support aerobically driven metabolic processes. Consequently, these results concurred with our hypothesis, suggesting that the sculpin species restricted to the Arctic will likely be negatively impacted by increases in ocean temperatures.

Aerobic scope and cardiovascular oxygen transport is not compromised at high temperatures in the toad Rhinella marina

Numerous recent studies convincingly correlate the upper thermal tolerance limit of aquatic ectothermic animals to reduced aerobic scope, and ascribe the decline in aerobic scope to failure of the cardiovascular system at high temperatures. In the present study we investigate whether this 'aerobic scope model' applies to an air-breathing and semi-terrestrial vertebrate Rhinella marina (formerly Bufo marinus). To quantify aerobic scope, we measured resting and maximal rate of oxygen consumption at temperatures ranging from 10 to 40°C. To include potential effects of acclimation, three groups of toads were acclimated chronically at 20, 25 and 30°C, respectively. The absolute difference between resting and maximal rate of oxygen consumption increased progressively with temperature and there was no significant decrease in aerobic scope, even at temperature immediately below the lethal limit (41-42°C). Haematological and cardiorespiratory variables were measured at rest and immediately after maximal activity at benign (30°C) and critically high (40°C) temperatures. Within this temperature interval, both resting and active heart rate increased, and there was no indication of respiratory failure, judged from high arterial oxygen saturation, P(O2) and [Hb(O2)]. With the exception of elevated resting metabolic rate for cold-acclimated toads, we found few differences in the thermal responses between acclimation groups with regard to the cardiometabolic parameters. In conclusion, we found no evidence for temperature-induced cardiorespiratory failure in R. marina, indicating that maintenance of aerobic scope and oxygen transport is unrelated to the upper thermal limit of this air-breathing semi-terrestrial vertebrate.