Cardiac Form, Function and Physiology

Effects of in vitro cytochalasin D and hypoxia on mitochondrial energetics and biogenesis, cell signal status and actin/tubulin/Hsp/MMP entity in air-breathing fish heart

The cardiac actin cytoskeleton has a dynamic pattern of polymerisation. It is uncertain how far actin destabilisation impacts mitochondrial energetics and biogenesis, cell signal status, and structural entities in cardiomyocytes, particularly in hypoxic conditions. We thus tested the in vitro action of cytochalasin D (Cyt D), an inhibitor of actin polymerisation, in hypoxic ventricular explants to elucidate the role of the actin in mitochondrial energetics and biogenesis, cell signals and actin/tubulin/hsps/MMPs dynamics in hypoxic air-breathing fish hearts. The COX activity increased upon Cyt D exposure, whereas hypoxia lowered COX and SDH activities but increased LDH activity. The ROS increased, and NO decreased by Cyt D. COX and LDH activities, and NO content reversed after Cyt D exposure in hypoxic hearts. Cyt D exposure upregulated actin isoform expression (Actc1 and Actb1) but downregulated tubulin isoform (Tedc1). Hypoxia upregulated actin (Acta1a, Actb1, Actb2, Actc1a) tubulin (Tuba, Tubb5, Tedc1, Tubd1) and hsp (Hspa5, Hspa9, Hspa12a, Hspa14, Hspd1, Hsp90) isoform transcript expression and Cyt D in hypoxic hearts reversed these isoform's expression. Hypoxia upregulated Mmp2 and 9 transcript expressions but downregulated Mfn1, Fis1, Nfkb1, Prkacaa, and Aktip expressions, and Cyt D exposure reversed almost all these markers in hypoxic hearts. The data provide novel evidence for the mechanistic role of actin in integrating mitochondrial energetics and biogenesis, cell signal status and actin/tubulin/Hsp/MMP entity, indicating its critical cardioprotective role in defending against hypoxia. Besides proposing an air-breathing fish heart as a model, the study further brings the therapeutic potential of Cyt D towards hypoxia intervention.

Exploring the ventricular morphology of the heart of Brycon amazonicus (Agassiz, 1829) (Teleostei, Characiformes)

Brycon amazonicus, a fish species widely distributed along freshwater rivers in Latin America, is important in maintaining ecological balance. Although some morphological/physiological aspects of B. amazonicus have been reported, the cardiac histomorphology of this species remains poorly understood. Therefore, this study aimed to describe the heart, emphasizing coronary distribution and ventricular myoarchitecture, correlating its structure with its functional activity. Ten adult specimens were anesthetized and perfused with Trident® ink to highlight the coronary branching. Subsequently, the animals were euthanized to collect the hearts, which were collected and processed for histological and histochemical techniques and scanning electron microscopy. We observed that the heart of B. amazonicus has a pyramidal ventricle, mixed myocardium, and coronary vessels restricted to the compact layer, according to classifications in the literature. The compact layer comprises two types of muscle bundles, arranged in longitudinal and circular orientations. The coronary circulation, which originates in the lateral cephalic region, starts from the heart and extends to the ventricular myocardium. Therefore, B. amazonicus has a cardiac morphology similar to that of other teleost species already described in the literature, expanding information on the Characiformes group. The description obtained of the heart of this species may contribute to further studies that seek a better understanding of the ecophysiology and evolution of teleost fish in the freshwaters of the Neotropical region.

Echocardiographic evaluation in koi carp (Cyprinus carpio) under manual restraint compared to anesthesia with isoeugenol

OBJECTIVE

To establish an echocardiographic technique in koi carp (Cyprinus carpio), compare cardiopulmonary parameters under manual restraint versus anesthesia, and provide a gross anatomical and histologic cardiac description.

METHODS

A randomized, crossover echocardiography study was performed in 40 clinically healthy adult, unknown sex, privately owned koi carp on May 10 and 11 through June 26 and 27, 2021. Echocardiography was examined for each koi under manual restraint and isoeugenol at 50 ppm, with 3 measurements per examination performed by a radiologist and cardiologist. Two koi were euthanized for gross anatomic and histologic cardiac evaluation.

RESULTS

Mean ejection fraction (EF), stroke volume (SV), and cardiac output (CO) were significantly lower, mean heart rate (HR) was significantly higher, and opercular rate (OPR) was decreased significantly in anesthetized compared to manually restrained koi. Poor reproducibility for EF and SV was observed.

CONCLUSIONS

Echocardiography was feasible in both manually restrained and anesthetized koi; however, this technique may best be applied to monitoring trends over time in individual fish due to low reproducibility. Significant differences in multiple cardiopulmonary parameters, including HR, EF, SV, CO, and OPR, were present between manually restrained and anesthetized koi. A gross anatomic and histologic cardiac description is provided for this species to pair with the echocardiographic images.

CLINICAL RELEVANCE

This study provides the first description of echocardiography, cardiac gross anatomy, and histology in koi. The results support echocardiography as a safe and practical noninvasive diagnostic for cardiac assessment in koi under both manual restraint and anesthesia.

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.

Impairing cardiac oxygen supply in swimming coho salmon compromises their heart function and tolerance to acute warming

Climatic warming elevates mortality for many salmonid populations during their physically challenging up-river spawning migrations, yet, the mechanisms underlying the increased mortality remain elusive. One hypothesis posits that a cardiac oxygen insufficiency impairs the heart's capacity to pump sufficient oxygen to body tissues to sustain up-river swimming, especially in warm water when oxygen availability declines and cardiac and whole-animal oxygen demand increases. We tested this hypothesis by measuring cardiac and metabolic (cardiorespiratory) performance, and assessing the upper thermal tolerance of coho salmon (Oncorhynchus kisutch) during sustained swimming and acute warming. By surgically ligating the coronary artery, which naturally accumulates arteriosclerotic lesions in migrating salmon, we partially impaired oxygen supply to the heart. Coronary ligation caused drastic cardiac impairment during swimming, even at benign temperatures, and substantially constrained cardiorespiratory performance during swimming and progressive warming compared to sham-operated control fish. Furthermore, upper thermal tolerance during swimming was markedly reduced (by 4.4 °C) following ligation. While the cardiorespiratory capacity of female salmon was generally lower at higher temperatures compared to males, upper thermal tolerance during swimming was similar between sexes within treatment groups. Cardiac oxygen supply is a crucial determinant for the migratory capacity of salmon facing climatic environmental warming.

The biology and ecology of the basking shark: A review

Here we review the literature on the basking shark (Cetorhinus maximus, Gunnerus, 1765), well known as the second largest extant shark (and fish) species globally. Previous reviews were published by Kunzlik in 1988 and Sims in 2008, but in the last 15 years modern electronic and DNA sequencing technologies have resulted in considerable advances in our knowledge of the species' behaviour and ecology. Basking sharks are planktivores and under appropriate conditions spend prolonged periods at the ocean surface feeding on copepod prey that primarily make up their diet, the behaviour that gave rise to their common name. In general, they are migratory and move into higher latitude waters during the summer months, when loose surface-feeding aggregations may form at favoured sites, the best known of which at present occur at hotspots on the west coasts of Britain and Ireland. The species is found circumglobally in temperate waters, but they are also now known on occasion to migrate at depth between northern and southern hemispheres, as well as across oceans within the northern hemisphere. In the past basking shark were more abundant across much of their range, but, consequent on targeted fisheries and in some places intentional eradication, became everywhere scarce, with recent population recovery in the north-east Atlantic being the result of protective measures initiated in the 1990s. Despite their charismatic nature, some of their most fundamental biological processes including copulation, gestation and birth remain largely unknown, due to their migratory and often deep-water lifestyle. In contrast, the deployment of small-scale archival and satellite tags has revealed the details of both broadscale migratory movements and horizontal and vertical foraging behaviours. Recent genetic studies support evidence suggesting a degree of site fidelity in relation to seasonal feeding grounds, which likely explains why in the past local populations have collapsed following periods of intensive fishing. Other recent research using aerial drones and towed cameras has revealed within loose feeding aggregations elements of social behaviour that may have a courtship function as well as enhance feeding efficiency.

Sinus venosus adaptation models prolonged cardiovascular disease and reveals insights into evolutionary transitions of the vertebrate heart

How two-chambered hearts in basal vertebrates have evolved from single-chamber hearts found in ancestral chordates remains unclear. Here, we show that the teleost sinus venosus (SV) is a chamber-like vessel comprised of an outer layer of smooth muscle cells. We find that in adult zebrafish nr2f1a mutants, which lack atria, the SV comes to physically resemble the thicker bulbus arteriosus (BA) at the arterial pole of the heart through an adaptive, hypertensive response involving smooth muscle proliferation due to aberrant hemodynamic flow. Single cell transcriptomics show that smooth muscle and endothelial cell populations within the adapting SV also take on arterial signatures. Bulk transcriptomics of the blood sinuses flanking the tunicate heart reinforce a model of greater equivalency in ancestral chordate BA and SV precursors. Our data simultaneously reveal that secondary complications from congenital heart defects can develop in adult zebrafish similar to those in humans and that the foundation of equivalency between flanking auxiliary vessels may remain latent within basal vertebrate hearts.

Getting to the heart of anatomical diversity and phenotypic plasticity: fish hearts are an optimal organ model in need of greater mechanistic study

Natural selection has produced many vertebrate 'solutions' for the cardiac life-support system, especially among the approximately 30,000 species of fishes. For example, across species, fish have the greatest range for central arterial blood pressure and relative ventricular mass of any vertebrate group. This enormous cardiac diversity is excellent ground material for mechanistic explorations. Added to this species diversity is the emerging field of population-specific diversity, which is revealing that cardiac design and function can be tailored to a fish population's local environmental conditions. Such information is important to conservation biologists and ecologists, as well as physiologists. Furthermore, the cardiac structure and function of an individual adult fish are extremely pliable (through phenotypic plasticity), which is typically beneficial to the heart's function when environmental conditions are variable. Consequently, exploring factors that trigger cardiac remodelling with acclimation to new environments represents a marvellous opportunity for performing mechanistic studies that minimize the genetic differences that accompany cross-species comparisons. What makes the heart an especially good system for the investigation of phenotypic plasticity and species diversity is that its function can be readily evaluated at the organ level using established methodologies, unlike most other organ systems. Although the fish heart has many merits as an organ-level model to provide a mechanistic understanding of phenotypic plasticity and species diversity, bringing this potential to fruition will require productive research collaborations among physiologists, geneticists, developmental biologists and ecologists.

Gill surface area allometry does not constrain the body mass scaling of maximum oxygen uptake rate in the tidepool sculpin, Oligocottus maculosus

The gill oxygen limitation hypothesis (GOLH) suggests that hypometric scaling of metabolic rate in fishes is a consequence of oxygen supply constraints imposed by the mismatched growth rates of gill surface area (a two-dimensional surface) and body mass (a three-dimensional volume). GOLH may, therefore, explain the size-dependent spatial distribution of fish in temperature- and oxygen-variable environments through size-dependent respiratory capacity, but this question is unstudied. We tested GOLH in the tidepool sculpin, Oligocottus maculosus, a species in which body mass decreases with increasing temperature- and oxygen-variability in the intertidal, a pattern consistent with GOLH. We statistically evaluated support for GOLH versus distributed control of [Formula: see text] allometry by comparing scaling coefficients for gill surface area, standard and maximum [Formula: see text] ([Formula: see text]_,Standard and [Formula: see text]_,Max, respectively), ventricle mass, hematocrit, and metabolic enzyme activities in white muscle. To empirically evaluate whether there is a proximate constraint on oxygen supply capacity with increasing body mass, we measured [Formula: see text]_,Max across a range of Po₂s from normoxia to P_crit, calculated the regulation value (R), a measure of oxyregulatory capacity, and analyzed the R-body mass relationship. In contrast with GOLH, gill surface area scaling either matched or was more than sufficient to meet [Formula: see text] demands with increasing body mass and R did not change with body mass. Ventricle mass (b = 1.22) scaled similarly to [Formula: see text]_,Max (b = 1.18) suggesting a possible role for the heart in the scaling of [Formula: see text]_,Max. Together our results do not support GOLH as a mechanism structuring the distribution of O. maculosus and suggest distributed control of oxyregulatory capacity.

High temperature and hyperkalemia cause exit block of action potentials at the atrioventricular junction of rainbow trout (Oncorhynchus mykiss) heart

At critically high temperatures, atrioventricular (AV) block causes ventricular bradycardia and collapse of cardiac output in fish. Here, the possible role of the AV canal in high temperature-induced heart failure was examined. To this end, optical mapping was used to measure action potential (AP) conduction in isolated AV junction preparations of the rainbow trout (Oncorhynchus mykiss) heart during acute warming/cooling in the presence of 4 or 8 mM external K⁺ concentration. The preparation included the AV canal and some atrial and ventricular tissue at its edges, and it was paced either from atrial or ventricular side at a frequency of 0.67 Hz (40 beats min^-1) to trigger forward (anterograde) and backward (retrograde) conduction, respectively. The propagation of AP was fast in atrial and ventricular tissues, but much slower in the AV canal, causing an AV delay. Acute warming from 15 °C to 27 °C or cooling from 15 °C to 5 °C did not impair AP conduction in the AV canal, as both anterograde and retrograde excitations propagated regularly through the AV canal. In contrast, anterograde conduction through the AV canal did not trigger ventricular excitation at the boundary zone between the AV canal and the ventricle when extracellular K⁺ concentration was raised from 4 mM to 8 mM at 27 °C. Also, the retrograde conduction was blocked at the border between the AV canal and the atrium in high K⁺ at 27 °C. These findings suggest that the AV canal is resistant against high temperatures (and high K⁺), but the ventricular muscle cannot be excited by APs coming from the AV canal when temperature and external K⁺ concentration are simultaneously elevated. Therefore, bradycardia at high temperatures in fish may occur due to inability of AP of the AV canal to trigger ventricular AP at the junctional zone between the AV canal and the proximal part of the ventricle.

Ontogeny of cardiomyocytes: ultrastructure optimization to meet the demand for tight communication in excitation-contraction coupling and energy transfer

The ontogeny of the heart describes its development from the fetal to the adult stage. In newborn mammals, blood pressure and thus cardiac performance are relatively low. The cardiomyocytes are thin, and with a central core of mitochondria surrounded by a ring of myofilaments, while the sarcoplasmic reticulum (SR) is sparse. During development, as blood pressure and performance increase, the cardiomyocytes become more packed with structures involved in excitation–contraction (e-c) coupling (SR and myofilaments) and the generation of ATP (mitochondria) to fuel the contraction. In parallel, the e-c coupling relies increasingly on calcium fluxes through the SR, while metabolism relies increasingly on fatty acid oxidation. The development of transverse tubules and SR brings channels and transporters interacting via calcium closer to each other and is crucial for e-c coupling. However, for energy transfer, it may seem counterintuitive that the increased structural density restricts the overall ATP/ADP diffusion. In this review, we discuss how this is because of the organization of all these structures forming modules. Although the overall diffusion across modules is more restricted, the energy transfer within modules is fast. A few studies suggest that in failing hearts this modular design is disrupted, and this may compromise intracellular energy transfer.

This article is part of the theme issue ‘The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease’.

DIAGNOSIS AND MANAGEMENT OF SUSPECTED CONGESTIVE HEART FAILURE SECONDARY TO DILATED CARDIOMYOPATHY IN A SAND TIGER SHARK (CARCHARIAS TAURUS) WITH ESTABLISHMENT OF PRELIMINARY NORMAL ECHOCARDIOGRAPHIC INDICES

Elasmobranch cardiac anatomy and physiology has been well described; however, there is a dearth of information regarding cardiac disease. In support of a clinical case of suspected congestive heart failure in a 22-yr-old male sand tiger shark (Carcharias taurus), a study was undertaken to identify feasible echocardiographic imaging planes and preliminary indices for this species. Eleven echocardiograms were performed on six apparently healthy sand tiger sharks. Echocardiographic parameters are presented using descriptive statistics, including mean, median, standard deviation (SD), minimum and maximum values. These data were utilized for the diagnosis and clinical management of the affected shark. The shark initially presented with increased respiratory effort, dependent, peripheral edema, and anemia. Echocardiography revealed atrial, ventricular, and sinus venosus dilation. As congestive heart failure secondary to dilated cardiomyopathy was strongly suspected, therapy was initiated with oral benazepril and torsemide, and later pimobendan. After a year of therapy, clinical signs resolved. Cardiac size and function improved on echocardiography with a reduction in sinus venosus dilation, maximum and minimum atrial and ventricular inner diameters, and an increase in atrial and ventricular fractional shortening. Cardiac disease in elasmobranchs may be underdiagnosed, so it may be necessary to develop standardized ultrasound techniques and cardiac measurements for each species of elasmobranch managed within zoos and aquaria.

Inducible Nitric Oxide Synthase/Nitric Oxide System as a Biomarker for Stress and Ease Response in Fish: Implication on Na+ Homeostasis During Hypoxia

The cellular and organismal response to stressor-driven stimuli evokes stress response in vertebrates including fishes. Fishes have evolved varied patterns of stress response, including ionosmotic stress response, due to their sensitivity to both intrinsic and extrinsic stimuli. Fishes that experience hypoxia, a detrimental stressor that imposes systemic and cellular stress response, can evoke disturbed ion homeostasis. In addition, like other vertebrates, fishes have also developed mechanisms to recover from the impact of stress by way of shifting stress response into ease response that could reduce the magnitude of stress response with the aid of certain neuroendocrine signals. Nitric oxide (NO) has been identified as a potent molecule that attenuates the impact of ionosmotic stress response in fish, particularly during hypoxia stress. Limited information is, however, available on this important aspect of ion transport physiology that contributes to the mechanistic understanding of survival during environmental challenges. The present review, thus, discusses the role of NO in Na⁺ homeostasis in fish particularly in stressed conditions. Isoforms of nitric oxide synthase (NOS) are essential for the synthesis and availability of NO at the cellular level. The NOS/NO system, thus, appears as a unique molecular drive that performs both regulatory and integrative mechanisms of control within and across varied fish ionocytes. The activation of the inducible NOS (iNOS)/NO system during hypoxia stress and its action on the dynamics of Na⁺/K⁺-ATPase, an active Na⁺ transporter in fish ionocytes, reveal that the iNOS/NO system controls cellular and systemic Na⁺ transport in stressed fish. In addition, the higher sensitivity of iNOS to varied physical stressors in fishes and the ability of NO to lower the magnitude of ionosmotic stress in hypoxemic fish clearly put forth NO as an ease-promoting signal molecule in fishes. This further points to the signature role of the iNOS/NO system as a biomarker for stress and ease response in the cycle of adaptive response in fish.

Acute and chronic cold exposure differentially affect cardiac control, but not cardiorespiratory function, in resting Atlantic salmon (Salmo salar)

No studies have examined the effects of cold temperatures (∼0-1 °C) on in vivo cardiac function and control, and metabolism, in salmonids. Thus, we examined: 1) how acclimation to 8 °C vs. acclimation (>3 weeks) or acute exposure (8-1 °C at 1 °C h-1) to 1 °C influenced cardiorespiratory parameters in resting Atlantic salmon; and 2) if/how the control of cardiac function was affected. Oxygen consumption ( M ˙ O 2 ) and cardiac function [i.e., heart rate (f H) and cardiac output (Q ˙ ) ] were 50% lower in the acutely cooled and 1oC-acclimated salmon as compared to 8 °C fish, whereas stroke volume (VS) was unchanged. Intrinsic f H was not affected by whether the fish were acutely exposed or acclimated to 1 °C (values ∼51, 24 and 21 beats min-1 in 8 and 1 °C-acclimated fish, and 8-1 °C fish, respectively), and in all groups f H was primarily under adrenergic control/tone (cholinergic tone 13-18%; adrenergic tone 37-70%). However, β-adrenergic blockade resulted in a 50% increase in VS in the 1oC-acclimated group, and this was surprising as circulating catecholamine levels were ∼1-3 nM in all groups. Overall, the data suggest that this species has a limited capacity to acclimate to temperatures approaching 0 °C. However, we cannot exclude the possibility that cardiac and metabolic responses are evoked when salmon are cooled to ∼ 0-1 °C, and that this prevented further declines in these parameters (i.e., they 'reset' quickly). Our data also provide further evidence that VS is temperature insensitive, and strongly suggest that changes in adrenoreceptor mediated control of venous pressure/capacitance occur when salmon are acclimated to 1 °C.

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 ¼ 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.

Hypoxic and Thermal Stress: Many Ways Leading to the NOS/NO System in the Fish Heart

Teleost fish are often regarded with interest for the remarkable ability of several species to tolerate even dramatic stresses, either internal or external, as in the case of fluctuations in O2 availability and temperature regimes. These events are naturally experienced by many fish species under different time scales, but they are now exacerbated by growing environmental changes. This further challenges the intrinsic ability of animals to cope with stress. The heart is crucial for the stress response, since a proper modulation of the cardiac function allows blood perfusion to the whole organism, particularly to respiratory organs and the brain. In cardiac cells, key signalling pathways are activated for maintaining molecular equilibrium, thus improving stress tolerance. In fish, the nitric oxide synthase (NOS)/nitric oxide (NO) system is fundamental for modulating the basal cardiac performance and is involved in the control of many adaptive responses to stress, including those related to variations in O2 and thermal regimes. In this review, we aim to illustrate, by integrating the classic and novel literature, the current knowledge on the NOS/NO system as a crucial component of the cardiac molecular mechanisms that sustain stress tolerance and adaptation, thus providing some species, such as tolerant cyprinids, with a high resistance to stress.

Does hydrostatic pressure influence lumpfish (Cyclopterus lumpus) heart rate and its response to environmental challenges?

Studies on the effects of environmental changes with increasing depth (e.g. temperature and oxygen level) on fish physiology rarely consider how hydrostatic pressure might influence the observed responses. In this study, lumpfish (Cyclopterus lumpus, 200-400 g), which can exhibit vertical migrations of over 100 m daily and can be found at depths of 500 m or more, were implanted with Star-Oddi micro-HRT loggers. Then, their heart rate (f _H) was measured in a pressure chamber when exposed to the following: (i) increasing pressure (up to 80 bar; 800 m in depth) at 10°C or (ii) increasing temperature (12-20°C), decreasing temperature (12 to 4°C) or decreasing oxygen levels (101-55% air saturation at 12°C) in the absence or presence of 80 bar of pressure. Additionally, we determined their f _H response to chasing and to increasing temperature (to 22°C) at atmospheric pressure. Pressure-induced increases in f _H (e.g. from 48 to 61 bpm at 12°C) were associated with hyperactivity. The magnitude of the rise in f _H with temperature was greater in pressure-exposed vs. control fish (i.e. by ~30 bpm vs. 45 bpm between 5°C and 20°C). However, the relative increase (i.e. slope of the relationship) was not different between groups. In contrast, 80 bar of pressure eliminated the small (5 bpm) increase in f _H when control fish were exposed to hypoxia. Exhaustive exercise and increasing temperature to 22°C resulted in a maximum f _H of 77 and 81 bpm, respectively. Our research shows that pressure influences the f _H response to environmental challenges and provides the first evidence that lumpfish have a limited capacity to increase f _H.

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.

Adrenergic tone benefits cardiac performance and warming tolerance in two teleost fishes that lack a coronary circulation

Tolerance to acute environmental warming in fish is partly governed by the functional capacity of the heart to increase systemic oxygen delivery at high temperatures. However, cardiac function typically deteriorates at high temperatures, due to declining heart rate and an impaired capacity to maintain or increase cardiac stroke volume, which in turn has been attributed to a deterioration of the electrical conductivity of cardiac tissues and/or an impaired cardiac oxygen supply. While autonomic regulation of the heart may benefit cardiac function during warming by improving myocardial oxygenation, contractility and conductivity, the role of these processes for determining whole animal thermal tolerance is not clear. This is in part because interpretations of previous pharmacological in vivo experiments in salmonids are ambiguous and were confounded by potential compensatory increases in coronary oxygen delivery to the myocardium. Here, we tested the previously advanced hypothesis that cardiac autonomic control benefits heart function and acute warming tolerance in perch (Perca fluviatilis) and roach (Rutilus rutilus); two species that lack coronary arteries and rely entirely on luminal venous oxygen supplies for cardiac oxygenation. Pharmacological blockade of β-adrenergic tone lowered the upper temperature where heart rate started to decline in both species, marking the onset of cardiac failure, and reduced the critical thermal maximum (CT_max) in perch. Cholinergic (muscarinic) blockade had no effect on these thermal tolerance indices. Our findings are consistent with the hypothesis that adrenergic stimulation improves cardiac performance during acute warming, which, at least in perch, increases acute thermal tolerance.

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.

Heart development in two populations of Atlantic killifish (Fundulus heteroclitus) following exposure to a polycyclic aromatic hydrocarbon mixture

Historic industrial pollution of the Elizabeth River, Virginia resulted in polycyclic aromatic hydrocarbon (PAH) contamination in sediments. Atlantic killifish (Fundulus heteroclitus) inhabiting the Atlantic Wood (AW) industrial site adapted to complex PAH mixture at this Superfund site. Their embryos have proved highly resistant to cardiac abnormalities indicative of PAH toxicity. In this study, embryos spawned from adults collected at AW and King's Creek (KC), a reference site, were exposed at 24 h post fertilization (hpf) to Elizabeth River Sediment Extract (ERSE), a complex PAH mixture, in a range of concentrations (0, 5.04, 50.45, 100.90, 151.35, or 252.25 µg/L total PAHs). Embryos were processed for histology at 144 hpf to enable evaluations of hearts at tissue and cellular levels. Morphometry and severity scoring were used to evaluate the extent of alterations. Unexposed embryos were similar in both populations. ERSE exposure resulted in multiple changes to hearts of KC embryos but not AW. Alterations were particularly evident in KC embryos exposed to concentrations above 1% ERSE (50.45 µg/L), which had thinner ventricular walls and larger pericardial edema. Individuals with moderate pericardial edema maintained arrangement and proximity of heart chambers, but changes were seen in ventricular myocytes. Severe pericardial edema was prevalent in exposed KC embryos and typically resulted in tube heart formation. Ventricles of tube hearts had very thin walls composed of small, basophilic cells and lacked trabeculae. Edematous pericardial fluid contained small amounts of proteinaceous material, as did controls, and was free of cells. This fluid was primarily unstained, suggesting water influx due to increased permeability. The use of histological approaches provided more specific detail for tissue and cellular effects in hearts of embryos exposed to PAHs and enabled understanding of potential links to later life effects of early life exposure.

Effects of hypoxic acclimation on contractile properties of the spongy and compact ventricular myocardium of steelhead trout (Oncorhynchus mykiss)

The trout ventricle has an outer compact layer supplied with well-oxygenated arterial blood from the coronary circulation, and an inner spongy myocardium supplied with oxygen poor venous blood. It was hypothesized that: (1) the spongy myocardium of steelhead trout (Oncorhynchus mykiss), given its routine exposure to low partial pressures of oxygen (PO₂), would be better able to maintain contractile performance (work) when exposed to acute hypoxia (100 to 10% air saturation) relative to the compact myocardium, and would show little benefit from hypoxic acclimation; and (2) the compact myocardium from hypoxia-acclimated (40% air saturation) fish would be better able to maintain work during acute exposure to hypoxia relative to normoxia-acclimated individuals. Consistent with our expectations, when PO₂ was acutely lowered, net work from the compact myocardium of normoxia-acclimated fish declined more (by ~ 73%) than the spongy myocardium (~ 50%), and more than the compact myocardium of hypoxia-acclimated fish (~ 55%), and hypoxic acclimation did not benefit the spongy myocardium in the face of reduced PO₂. Further, while hypoxic acclimation resulted in a 25% (but not significant) decrease in net work of the spongy myocardium, the performance of the compact myocardium almost doubled. This research suggests that, in contrast to the spongy myocardium, performance of the compact myocardium is improved by hypoxic acclimation; and supports previous research suggesting that the decreased contractile performance of the myocardium upon exposure to lowered PO₂ may be adaptive and mediated by mechanisms within the muscle itself.

Postprandial cardiorespiratory responses and the regulation of digestion-associated tachycardia in Nile tilapia (Oreochromis niloticus)

Cardiorespiratory adjustments that occur after feeding are essential to supply the demands of digestion in vertebrates. The well-documented postprandial tachycardia is triggered by an increase in adrenergic activity and by non-adrenergic non-cholinergic (NANC) factors in mammals and crocodilians, while it is linked to a withdrawal of vagal drive and NANC factors in non-crocodilian ectotherms-except for fish, in which the sole investigation available indicated no participation of NANC factors. On the other hand, postprandial ventilatory adjustments vary widely among air-breathing vertebrates, with different species exhibiting hyperventilation, hypoventilation, or even no changes at all. Regarding fish, which live in an environment with low oxygen capacitance that requires great ventilatory effort for oxygen uptake, data on the ventilatory consequences of feeding are also scarce. Thus, the present study sought to investigate the postprandial cardiorespiratory adjustments and the mediation of digestion-associated tachycardia in the unimodal water-breathing teleost Oreochromis niloticus. Heart rate (f_H), cardiac autonomic tones, ventilation rate (f_V), ventilation amplitude, total ventilation and f_H/f_V variability were assessed both in fasting and digesting animals under untreated condition, as well as after muscarinic cholinergic blockade with atropine and double autonomic blockade with atropine and propranolol. The results revealed that digestion was associated with marked tachycardia in O. niloticus, determined by a reduction in cardiac parasympathetic activity and by circulating NANC factors-the first time such positive chronotropes were detected in digesting fish. Unexpectedly, postprandial ventilatory alterations were not observed, although digestion triggered mechanisms that were presumed to increase oxygen uptake, such as cardiorespiratory synchrony.

What do warming waters mean for fish physiology and fisheries?

Environmental signals act primarily on physiological systems, which then influence higher-level functions such as movement patterns and population dynamics. Increases in average temperature and temperature variability associated with global climate change are likely to have strong effects on fish physiology and thereby on populations and fisheries. Here we review the principal mechanisms that transduce temperature signals and the physiological responses to those signals in fish. Temperature has a direct, thermodynamic effect on biochemical reaction rates. Nonetheless, plastic responses to longer-term thermal signals mean that fishes can modulate their acute thermal responses to compensate at least partially for thermodynamic effects. Energetics are particularly relevant for growth and movement, and therefore for fisheries, and temperature can have pronounced effects on energy metabolism. All energy (ATP) production is ultimately linked to mitochondria, and temperature has pronounced effects on mitochondrial efficiency and maximal capacities. Mitochondria are dependent on oxygen as the ultimate electron acceptor so that cardiovascular function and oxygen delivery link environmental inputs with energy metabolism. Growth efficiency, that is the conversion of food into tissue, changes with temperature, and there are indications that warmer water leads to decreased conversion efficiencies. Moreover, movement and migration of fish relies on muscle function, which is partially dependent on ATP production but also on intracellular calcium cycling within the myocyte. Neuroendocrine processes link environmental signals to regulated responses at the level of different tissues, including muscle. These physiological processes within individuals can scale up to population responses to climate change. A mechanistic understanding of thermal responses is essential to predict the vulnerability of species and populations to climate change.

Cardiac mitochondrial function, nitric oxide sensitivity and lipid composition following hypoxia acclimation in sablefish

In fishes, the effect of O₂ limitation on cardiac mitochondrial function remains largely unexplored. The sablefish (Anoplopoma fimbria) encounters considerable variations in environmental oxygen availability, and is an interesting model for studying the effects of hypoxia on fish cardiorespiratory function. We investigated how in vivo hypoxia acclimation (6 months at 40% then 3 weeks at 20% air saturation) and in vitro anoxia-reoxygenation affected sablefish cardiac mitochondrial respiration and reactive oxygen species (ROS) release rates using high-resolution fluorespirometry. Further, we investigated how hypoxia acclimation affected the sensitivity of mitochondrial respiration to nitric oxide (NO), and compared mitochondrial lipid and fatty acid (FA) composition between groups. Hypoxia acclimation did not alter mitochondrial coupled or uncoupled respiration, or respiratory control ratio, ROS release rates, P ₅₀ or superoxide dismutase activity. However, it increased citrate synthase activity (by ∼20%), increased the sensitivity of mitochondrial respiration to NO inhibition (i.e., the NO IC₅₀ was 25% lower), and enhanced the recovery of respiration (by 21%) and reduced ROS release rates (by 25-30%) post-anoxia. In addition, hypoxia acclimation altered mitochondrial FA composition [increasing arachidonic acid (20:4ω6) and eicosapentaenoic acid (20:5ω3) proportions by 11 and 14%, respectively], and SIMPER analysis revealed that the phospholipid:sterol ratio was the largest contributor (24%) to the dissimilarity between treatments. Overall, these results suggest that hypoxia acclimation may protect sablefish cardiac bioenergetic function during or after periods of O₂ limitation, and that this may be related to alterations in mitochondrial sensitivity to NO and to adaptive changes in membrane composition (fluidity).

Can´t beat the heat? Importance of cardiac control and coronary perfusion for heat tolerance in rainbow trout

Coronary perfusion and cardiac autonomic regulation may benefit myocardial oxygen delivery and thermal performance of the teleost heart, and thus influence whole animal heat tolerance. Yet, no study has examined how coronary perfusion affects cardiac output during warming in vivo. Moreover, while β-adrenergic stimulation could protect cardiac contractility, and cholinergic decrease in heart rate may enhance myocardial oxygen diffusion at critically high temperatures, previous studies in rainbow trout (Oncorhynchus mykiss) using pharmacological antagonists to block cholinergic and β-adrenergic regulation showed contradictory results with regard to cardiac performance and heat tolerance. This could reflect intra-specific differences in the extent to which altered coronary perfusion buffered potential negative effects of the pharmacological blockade. Here, we first tested how cardiac performance and the critical thermal maximum (CTmax) were affected following a coronary ligation. We then assessed how these performances were influenced by pharmacological cholinergic or β-adrenergic blockade, hypothesising that the effects of the pharmacological treatment would be more pronounced in coronary ligated trout compared to trout with intact coronaries. Coronary blockade reduced CTmax by 1.5 °C, constrained stroke volume and cardiac output across temperatures, led to earlier cardiac failure and was associated with reduced blood oxygen-carrying capacity. Nonetheless, CTmax and the temperatures for cardiac failure were not affected by autonomic blockade. Collectively, our data show that coronary perfusion improves heat tolerance and cardiac performance in trout, while evidence for beneficial effects of altered cardiac autonomic tone during warming remains inconclusive.

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.

Autonomic cardiac regulation facilitates acute heat tolerance in rainbow trout: in situ and in vivo support

Acute warming in fish increases heart rate (f _H) and cardiac output to peak values, after which performance plateaus or declines and arrhythmia may occur. This cardiac response can place a convective limitation on systemic oxygen delivery at high temperatures. To test the hypothesis that autonomic cardiac regulation protects cardiac performance in rainbow trout during acute warming, we investigated adrenergic and cholinergic regulation during the onset and progression of cardiac limitations. We explored the direct effects of adrenergic stimulation by acutely warming an in situ working perfused heart until arrhythmia occurred, cooling the heart to restore rhythmicity and rewarming with increasing adrenergic stimulation. Adrenergic stimulation produced a clear, dose-dependent increase in the temperature and peak f _H achieved prior to the onset of arrhythmia. To examine how this adrenergic protection functions in conjunction with cholinergic vagal inhibition in vivo, rainbow trout fitted with ECG electrodes were acutely warmed in a respirometer until they lost equilibrium (CT_max) with and without muscarinic (atropine) and β-adrenergic (sotalol) antagonists. Trout exhibited roughly equal and opposing cholinergic and adrenergic tone on f _H that persisted up to critical temperatures. β-Adrenergic blockade significantly lowered peak f _H by 14-17%, while muscarinic blockade significantly lowered the temperature for peak f _H by 2.0°C. Moreover, muscarinic and β-adrenergic blockers injected individually or together significantly reduced CT_max by up to 3°C, indicating for the first time that cardiac adrenergic stimulation and cholinergic inhibition can enhance acute heat tolerance in rainbow trout at the level of the heart and the whole animal.

Heart structure in the Amazonian teleost Arapaima gigas (Osteoglossiformes, Arapaimidae)

The fish heart ventricle has varied morphology and may have a specific morpho-functional design in species adapted to extreme environmental conditions. In general, the Amazonian ichthyofauna undergoes constant variations in water temperature, pH and oxygen saturation, which makes these species useful for investigations of cardiac morphology. Arapaima gigas, a member of the ancient teleost group Osteoglossomorpha, is one of the largest freshwater fish in the world. This species has a specific heart metabolism that uses fat as the main fuel when O2 supplies are abundant but also can change to glycogen fermentation when O2 content is limiting. However, no information is available regarding its heart morphology. Here, we describe the heart of A. gigas, with emphasis on the ventricular anatomy and myoarchitecture. Specimens of A. gigas weighing between 0.3 and 4040 g were grouped into three developmental stages. The hearts were collected and the anatomy analyzed with a stereomicroscope, ultrastructure with a scanning electron microscope, and histology using toluidine blue, Masson's trichrome and Sirius red stains. The ventricle undergoes morphological changes throughout its development, from the initial saccular shape with a fully trabeculated myocardium and coronary vessel restricted to the subepicardium (Type I) (group 1) to a pyramidal shape with mixed myocardium and coronary vessels that penetrate only to the level of the compact layer (Type II) (groups 2 and 3). The trabeculated myocardium has a distinct net-like organization in all the specimens, differing from that described for other teleosts. This arrangement delimits lacunae with a similar shape and distribution, which seems to allow a more uniform blood distribution through this myocardial layer.

The environmental tolerances and metabolic physiology of sablefish (Anoplopoma fimbria)

Given the potential impacts of global warming, such as increases in temperature and the frequency/severity of hypoxia in marine ecosystems, it is important to study the impacts of these environmental challenges on sea-cage reared aquaculture species. This study focuses on the sablefish (Anoplopoma fimbria), an emerging aquaculture species that has a unique ecology in the wild. For instance, adults inhabit oxygen minimum zones and cool waters at depths up to 1500 m. Using Atlantic salmon (Salmo salar) (~1132 g adults) as a comparative species, we used intermittent-flow respirometry to characterize the tolerance and metabolic response of sablefish (~10 g juveniles and ~675 g adults) to acute increases in temperature (2 °C h^-1) and decreases in oxygen level (~10% air saturation h^-1). Adult sablefish were much more hypoxia tolerant than adult salmon [O₂ level at loss of equilibrium ~5.4% vs. ~24.2% air saturation, respectively]. In addition, sablefish could withstand upper temperatures only slightly lower than salmon [critical thermal maximum (CT_max) ~24.9 °C vs. ~26.2 °C, respectively]. Sablefish juveniles were both less hypoxia and thermally tolerant than adults [critical O₂ tension ~18.9% vs. ~15.8% air saturation; CT_max ~22.7 vs. ~24.9 °C, respectively]. Interestingly, many of these differences in environmental tolerance could not be explained by differences in metabolic parameters (aerobic scope or routine metabolic rate). Our findings show that sablefish are tolerant of high temperatures, and very tolerant of hypoxia, traits that are advantageous for an aquaculture species in the era of climate change.

Contractile function of the excised hagfish heart during anoxia exposure

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia and their systemic hearts continue to work throughout the exposure. Recent work demonstrates that glycogen stores are utilized in the E. stoutii heart during anoxia but that these are not sufficient to support the measured rate of ATP production. One metabolic fuel that could supplement glycogen during anoxia is glycerol. This substrate can be derived from lipid stores, stored in the heart, or delivered via the blood. The purpose of this study was to determine the effect of glycerol on the contractile function of the excised E. stoutii heart during anoxia exposure. When excised hearts, perfused with metabolite free saline (mf-saline), were exposed to anoxia for 12 h, there was no difference in heart rate, pressure generation (max-dP), rate of contraction (max-dP/dt_sys), or rate of relaxation (max-dP/dt_dia) compared to hearts perfused with mf-saline in normoxia. However, hearts perfused with saline containing glycerol (gly-saline) in anoxia had higher max-dP, max-dP/dt_sys, and max-dP/dt_dia than hearts perfused with mf-saline in anoxia. Tissue levels of glycerol increased when hearts were perfused with gly-saline in normoxia, but not when perfused with gly-saline in anoxia. Anoxia exposure did not affect the activities of triglyceride lipase, glycerol kinase, or glycerol-3-phosphate dehydrogenase. This study suggests that glycerol stimulates cardiac function in the hagfish but that it is not derived from stored lipids. How glycerol may stimulate contraction is not known. This could be as an energy substrate, as an allosteric factor, or a combination of the two.

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.

Examples of Weak, If Not Absent, Form-Function Relations in the Vertebrate Heart

That form and function are related is a maxim of anatomy and physiology. Yet, form-function relations can be difficult to prove. Human subjects with excessive trabeculated myocardium in the left ventricle, for example, are diagnosed with non-compaction cardiomyopathy, but the extent of trabeculations may be without relation to ejection fraction. Rather than rejecting a relation between form and function, we may ask whether the salient function is assessed. Is there a relation to electrical propagation, mean arterial blood pressure, or propensity to form blood clots? In addition, how should the extent of trabeculated muscle be assessed? While reviewing literature on trabeculated muscle, we applied Tinbergen's four types of causation-how does it work, why does it work, how is it made, and why did it evolve-to better parse what is meant by form and function. The paper is structured around cases that highlight advantages and pitfalls of applying Tinbergen's questions. It further uses the evolution of lunglessness in amphibians to argue that lung reduction impacts on chamber septation and it considers the evolution of an arterial outflow in fishes to argue that reductions in energy consumption may drive structural changes with little consequences to function. Concerning trabeculations, we argue they relate to pumping function in the embryo in the few weeks before the onset of coronary circulation. In human fetal and postnatal stages, a spectrum of trabeculated-to-compact myocardium makes no difference to cardiac function and in this period, form and function may appear unrelated.

Exploring nature's natural knockouts: in vivo cardiorespiratory performance of Antarctic fishes during acute warming

We tested the hypothesis that blackfin icefish (Chaenocephalus aceratus), one of the six species in the family Channichthyidae (the icefishes) that do not express haemoglobin and myoglobin, lack regulatory cardiovascular flexibility during acute warming and activity. The experimental protocols were designed to optimize the surgical protocol and minimize stress. First, minimally invasive heart rate (f_H) measurements were made during a thermal ramp until cardiac failure in C. aceratus and compared with those from the closely related red-blooded black rockcod (Notothenia coriiceps). Then, integrative cardiovascular adjustments were more extensively studied using flow probes and intravascular catheters in C. aceratus during acute warming (from 0 to 8°C) at rest and after imposed activity. Chaenocephalus aceratus had a lower routine f_H than N. coriiceps (9 beats min^-1 versus 14 beats min^-1) and a lower peak f_H during acute warming (38 beats min^-1 versus 55 beats min^-1) with a similar cardiac breakpoint temperature (13 and 14°C, respectively). Routine cardiac output (Q̇) for C. aceratus at ∼0°C was much lower (26.6 ml min^-1 kg^-1) than previously reported, probably because fish in the present study had a low f_H (12 beats min^-1) indicative of a high routine vagal tone and low stress. Chaenocephalus aceratus increased oxygen consumption during acute warming and with activity. Correspondingly, Q̇ increased considerably (maximally 86.3 ml min^-1 kg^-1), as did vascular conductance (5-fold). Thus, unlike earlier suggestions, these data provide convincing evidence that icefish can mount a well-developed cardiovascular regulation of heart rate, cardiac output and vascular conductance, and this regulatory capacity provides flexibility during acute warming.

Specialized impulse conduction pathway in the alligator heart

Mammals and birds have a specialized cardiac atrioventricular conduction system enabling rapid activation of both ventricles. This system may have evolved together with high heart rates to support their endothermic state (warm-bloodedness) and is seemingly lacking in ectothermic vertebrates from which first mammals then birds independently evolved. Here, we studied the conduction system in crocodiles (Alligator mississippiensis), the only ectothermic vertebrates with a full ventricular septum. We identified homologues of mammalian conduction system markers (Tbx3-Tbx5, Scn5a, Gja5, Nppa-Nppb) and show the presence of a functional atrioventricular bundle. The ventricular Purkinje network, however, was absent and slow ventricular conduction relied on trabecular myocardium, as it does in other ectothermic vertebrates. We propose the evolution of the atrioventricular bundle followed full ventricular septum formation prior to the development of high heart rates and endothermy. In contrast, the evolution of the ventricular Purkinje network is strongly associated with high heart rates and endothermy.

Mammals and birds are referred to as ‘warm-blooded’ animals, because they maintain a constant high body temperature. This requires a lot of energy, so their bodies need to be well supplied with blood at all times. The hearts of mammals and birds contain two important structures that help them do this. The first is a full wall of muscle – called the ventricular septum – that divides the heart into left and right sides. The second is an electrical circuit made of specialized muscle cells that ensures that the heart beats fast enough by sending rapid electrical signals to the rest of the heart muscle. The circuit contains one group of cells in the ventricular septum, called the bundle of His, and another group termed the Purkinje network.

Reptiles, however, do not maintain high body temperatures and are instead often thought of as ‘cold-blooded’ animals. The hearts of reptiles do not need to pump blood around the body as quickly and have different structures from warm-blooded animals. For example, most reptile hearts do not have a fully developed ventricular septum. The only exceptions are crocodiles, alligators and their relatives (the ‘crocodilians’), which do.

Jensen, Boukens et al. therefore wanted to determine if a crocodilian heart also contained a specialized electrical circuit like those of birds and mammals. Previous studies that attempted to answer this question using only anatomical and electrical methods had yielded ambiguous results. As such, Jensen, Boukens et al. combined these methods with genetic techniques for a more detailed study.

First, the ventricular septum of American alligators, a species of crocodilian, was examined, and found to contain a narrow tissue structure that strongly resembled the bundle of His. Indeed, if this presumptive bundle of His was cut, the electrical circuit was broken. Additional genetic analysis of this structure confirmed that genes similar to those active in the mammalian bundle of His were also switched on in alligators. However, recordings of heart activity showed that heart rates and the spread of electrical signals were both slower in alligators than in warm-blooded animals. This suggests that, although alligators have evolved some specialized muscle cells (in the form of a bundle of His), their electrical circuit is still ‘incomplete’. The lack of a Purkinje network, for example, would explain why their heart rates remain slow like other reptiles’.

Together these findings add to the current understanding of how the heart works in different animals with varying requirements for energy and blood flow. Also, since crocodiles and warm-blooded birds both evolved from ancient reptiles, detailed descriptions of their heart structures could shed more light on how warm-bloodedness first developed.