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

Acquirement of the autonomic nervous system modulation evaluated by heart rate variability in medaka (Oryzias latipes)

Small teleosts have recently been established as models of human diseases. However, measuring heart rate by electrocardiography is highly invasive for small fish and not widely used. The physiological nature and function of vertebrate autonomic nervous system (ANS) modulation of the heart has traditionally been investigated in larvae, transparent but with an immature ANS, or in anesthetized adults, whose ANS activity may possibly be disturbed under anesthesia. Here, we defined the frequency characteristics of heart rate variability (HRV) modulated by the ANS from observations of heart movement in high-speed movie images and changes in ANS regulation under environmental stimulation in unanesthetized adult medaka (Oryzias latipes). The HRV was significantly reduced by atropine (1 mM) in the 0.25-0.65 Hz and by propranolol (100 μM) at 0.65-1.25 Hz range, suggesting that HRV in adult medaka is modulated by both the parasympathetic and sympathetic nervous systems within these frequency ranges. Such modulations of HRV by the ANS in adult medaka were remarkably suppressed under anesthesia and continuous exposure to light suppressed HRV only in the 0.25-0.65 Hz range, indicating parasympathetic withdrawal. Furthermore, pre-hatching embryos did not show HRV and the power of HRV developed as fish grew. These results strongly suggest that ANS modulation of the heart in adult medaka is frequency-dependent phenomenon, and that the impact of long-term environmental stimuli on ANS activities, in addition to development of ANS activities, can be precisely evaluated in medaka using the presented method.

Warm acclimation alters antioxidant defences but not metabolic capacities in the Antarctic fish, Notothenia coriiceps

The Southern Ocean surrounding the Western Antarctic Peninsula region is rapidly warming. Survival of members of the dominant suborder of Antarctic fishes, the Notothenioidei, will likely require thermal plasticity and adaptive capacity in key traits delimiting thermal tolerance. Herein, we have assessed the thermal plasticity of several cellular and biochemical pathways, many of which are known to be associated with thermal tolerance in notothenioids, including mitochondrial function, activities of aerobic and anaerobic enzymes, antioxidant defences, protein ubiquitination and degradation in cardiac, oxidative skeletal muscles and gill of Notothenia coriiceps warm acclimated to 4°C for 22 days or 5°C for 42 days. Levels of triacylglycerol (TAG) were measured in liver and oxidative and glycolytic skeletal muscles, and glycogen in liver and glycolytic muscle to assess changes in energy stores. Metabolic pathways displayed minimal thermal plasticity, yet antioxidant defences were lower in heart and oxidative skeletal muscles of warm-acclimated animals compared with animals held at ambient temperature. Despite higher metabolic rates at elevated temperature, energy storage depots of TAG and glycogen increase in liver and remain unchanged in muscle with warm acclimation. Overall, our studies reveal that N. coriiceps displays thermal plasticity in some key traits that may contribute to their survival as the Southern Ocean continues to warm.

Antarctic teleosts with and without hemoglobin behaviorally mitigate deleterious effects of acute environmental warming

Recent studies forecast that many ectothermic animals, especially aquatic stenotherms, may not be able to thrive or even survive predicted climate change. These projections, however, generally do not call much attention to the role of behavior, an essential thermoregulatory mechanism of many ectotherms. Here we characterize species-specific locomotor and respiratory responses to acute ambient warming in two highly stenothermic Antarctic Notothenioid fishes, one of which (Chaenocephalus aceratus) lacks hemoglobin and appears to be less tolerant to thermal stress as compared to the other (Notothenia coriiceps), which expresses hemoglobin. At the onset of ambient warming, both species perform distinct locomotor maneuvers that appear to include avoidance reactions. In response to unavoidable progressive hyperthermia, fishes demonstrate a range of species-specific maneuvers, all of which appear to provide some mitigation of the deleterious effects of obligatory thermoconformation and to compensate for increasing metabolic demand by enhancing the efficacy of branchial respiration. As temperature continues to rise, Chaenocephalus aceratus supplements these behaviors with intensive pectoral fin fanning which may facilitate cutaneous respiration through its scaleless integument, and Notothenia coriiceps manifests respiratory-locomotor coupling during repetitive startle-like maneuvers which may further augment gill ventilation. The latter behaviors, found only in Notothenia coriiceps, have highly stereotyped appearance resembling Fixed Action Pattern sequences. Altogether, this behavioral flexibility could contribute to the reduction of the detrimental effects of acute thermal stress within a limited thermal range. In an ecologically relevant setting, this may enable efficient thermoregulation of fishes by habitat selection, thus facilitating their resilience in persistent environmental change.

Resilience of cardiac performance in Antarctic notothenioid fishes in a warming climate

Warming in the region of the Western Antarctic Peninsula is occurring at an unprecedented rate, which may threaten the survival of Antarctic notothenioid fishes. Herein, we review studies characterizing thermal tolerance and cardiac performance in notothenioids - a group that includes both red-blooded species and the white-blooded, haemoglobinless icefishes - as well as the relevant biochemistry associated with cardiac failure during an acute temperature ramp. Because icefishes do not feed in captivity, making long-term acclimation studies unfeasible, we focus only on the responses of red-blooded notothenioids to warm acclimation. With acute warming, hearts of the white-blooded icefish Chaenocephalus aceratus display persistent arrhythmia at a lower temperature (8°C) compared with those of the red-blooded Notothenia coriiceps (14°C). When compared with the icefish, the enhanced cardiac performance of N. coriiceps during warming is associated with greater aerobic capacity, higher ATP levels, less oxidative damage and enhanced membrane integrity. Cardiac performance can be improved in N. coriiceps with warm acclimation to 5°C for 6-9 weeks, accompanied by an increase in the temperature at which cardiac failure occurs. Also, both cardiac mitochondrial and microsomal membranes are remodelled in response to warm acclimation in N. coriiceps, displaying homeoviscous adaptation. Overall, cardiac performance in N. coriiceps is malleable and resilient to warming, yet thermal tolerance and plasticity vary among different species of notothenioid fishes; disruptions to the Antarctic ecosystem driven by climate warming and other anthropogenic activities endanger the survival of notothenioids, warranting greater protection afforded by an expansion of marine protected areas.

Homeoviscous adaptation occurs with thermal acclimation in biological membranes from heart and gill, but not the brain, in the Antarctic fish Notothenia coriiceps

As temperatures continue to rise, adjustments to biological membranes will be key for maintenance of function. It is largely unknown to what extent Antarctic notothenioids possess the capacity to remodel their biological membranes in response to thermal change. In this study, physical and biochemical properties were examined in membranes prepared from gill epithelia (plasma membranes), cardiac ventricles (microsomes, mitochondria), and brains (synaptic membranes, myelin, mitochondria) from Notothenia coriiceps following acclimation to 5 °C (or held at ambient temperature, 0 °C) for a minimum of 6 weeks. Fluidity was measured between 0 and 30 °C in all membranes, and polar lipid compositions and cholesterol contents were analyzed in a subset of biological membranes from all tissues. Osmotic permeability was measured in gills at 0 and 4 °C. Gill plasma membranes, cardiac mitochondria, and cardiac microsomes displayed reduced fluidity following acclimation to 5 °C, indicating compensation for elevated temperature. In contrast, no fluidity changes with acclimation were observed in any of the membranes prepared from brain. In all membranes, adjustments to the relative abundances of major phospholipid classes, and to the extent of fatty acid unsaturation, were undetectable following thermal acclimation. However, alterations in cholesterol contents and acyl chain length, consistent with the changes in fluidity, were observed in membranes from gill and cardiac tissue. Water permeability was reduced with 5 °C acclimation in gills, indicating near-perfect homeostatic efficacy. Taken together, these results demonstrate a homeoviscous response in gill and cardiac membranes, and limited plasticity in membranes from the nervous system, in an Antarctic notothenioid.

Regulation of splenic contraction persists as a vestigial trait in white-blooded Antarctic fishes

In fishes, the spleen can function as an important reservoir for red blood cells (RBCs), which, following splenic contraction, may be released into the circulation to increase haematocrit during energy-demanding activities. This trait is particularly pronounced in red-blooded Antarctic fishes in which the spleen can sequester a large proportion of RBCs during rest, thereby reducing blood viscosity, which may serve as an adaptation to life in cold environments. In one species, Pagothenia borchgrevinki, it has previously been shown that splenic contraction primarily depends on cholinergic stimulation. The aim of the present study was to investigate the regulation of splenic contraction in five other Antarctic fish species, three red-blooded notothenioids (Dissostichus mawsoni Norman, 1937, Gobionotothen gibberifrons Lönnberg, 1905, Notothenia coriiceps Richardson 1844) and two white-blooded "icefish" (Chaenocephalus aceratus Lönnberg, 1906 and Champsocephalus gunnari Lönnberg, 1905), which lack haemoglobin and RBCs, but nevertheless possess a large spleen. In all species, splenic strips constricted in response to both cholinergic (carbachol) and adrenergic (adrenaline) agonists. Surprisingly, in the two species of icefish, the spleen responded with similar sensitivity to red-blooded species, despite contraction being of little obvious benefit for releasing RBCs into the circulation. Although the icefish lineage lost functional haemoglobin before diversifying over the past 7.8-4.8 millions of years, they retain the capacity to contract the spleen, likely as a vestige inherited from their red-blooded ancestors.

Characterization of the hypoxia-inducible factor-1 pathway in hearts of Antarctic notothenioid fishes

The ability of Antarctic notothenioid fishes to mount a robust molecular response to hypoxia is largely unknown. The transcription factor, hypoxia-inducible factor-1 (HIF-1), a heterodimer of HIF-1α and HIF-1β subunits, is the master regulator of oxygen homeostasis in most metazoans. We sought to determine if, in the hearts of Antarctic notothenioids, HIF-1 is activated and functional in response to either an acute heat stress or hypoxia. The red-blooded Notothenia coriiceps and the hemoglobinless icefish, Chaenocephalus aceratus, were exposed to their critical thermal maximum (CT_MAX) or hypoxia (5.0 ± 0.3 mg of O₂ L^-1) for 2 h. Additionally, N. coriiceps was exposed to 2.3 ± 0.3 mg of O₂ L^-1 for 12 h, and red-blooded Gobionotothen gibberifrons was exposed to both levels of hypoxia. Levels of HIF-1α were quantified in nuclei isolated from heart ventricles using western blotting. Transcript levels of genes involved in anaerobic metabolism, and known to be regulated by HIF-1, were quantified by real-time PCR, and lactate levels were measured in heart ventricles. Protein levels of HIF-1α increase in nuclei of hearts of N. coriiceps and C. aceratus in response to exposure to CT_MAX and in hearts of N. coriiceps exposed to severe hypoxia, yet mRNA levels of anaerobic metabolic genes do not increase in any species, nor do lactate levels increase, suggesting that HIF-1 does not stimulate metabolic remodeling in hearts of notothenioids under these conditions. Together, these data suggest that Antarctic notothenioids may be vulnerable to hypoxic events, which are likely to increase with climate warming.

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.

Mitochondrial membranes in cardiac muscle from Antarctic notothenioid fishes vary in phospholipid composition and membrane fluidity

Antarctic notothenioid fishes are highly stenothermal, yet their tolerance for warming is species-dependent. Because a body of literature points to the loss of cardiac function as underlying thermal limits in ectothermic animals, we investigated potential relationships among properties of ventricular mitochondrial membranes in notothenioids with known differences in both cardiac mitochondrial metabolism and organismal thermal tolerance. Fluidity of mitochondrial membranes was quantified by fluorescence depolarization for the white-blooded Chaenocephalus aceratus and the red-blooded Notothenia coriiceps. In these same membranes, lipid compositions and products of lipid peroxidation, the latter of which can disrupt membrane order, were analyzed in both species and in a second icefish, Pseudochaenichthys georgianus. Mitochondrial membranes from C. aceratus were significantly more fluid than those of the more thermotolerant species N. coriiceps (P < .0001). Consistent with this, ratios of total phosphatidylethanolamine (PE) to total phosphatidylcholine (PC) were lower in membranes from both species of icefishes, compared to those of N. coriiceps (P < .05). However, membranes of N. coriiceps displayed a greater unsaturation index (P < .0001). No differences among species were found in membrane products of lipid peroxidation. With rising temperatures, greater contents of PC in mitochondrial membranes from ventricles of icefishes are likely to promote membrane hyperfluidization at a lower temperature than for cardiac mitochondrial membranes from the red-blooded notothenioid. We propose that physical and chemical properties of the mitochondrial membranes may contribute to some of the observed differences in thermal sensitivity of physiological function among these species.

Weighing the evidence for using vascular conductance, not resistance, in comparative cardiovascular physiology

Vascular resistance and conductance are reciprocal indices of vascular tone that are often assumed to be interchangeable. However, in most animals in vivo, blood flow (i.e. cardiac output) typically varies much more than arterial blood pressure. When blood flow changes at a constant pressure, the relationship between conductance and blood flow is linear, whereas the relationship between resistance and blood flow is non-linear. Thus, for a given change in blood flow, the change in resistance depends on the starting point, whereas the attendant change in conductance is proportional to the change in blood flow regardless of the starting conditions. By comparing the effects of physical activity at different temperatures or between species – concepts at the heart of comparative cardiovascular physiology – we demonstrate that the difference between choosing resistance or conductance can be marked. We also explain here how the ratio of conductance in the pulmonary and systemic circulations provides a more intuitive description of cardiac shunt patterns in the reptilian cardiovascular system than the more commonly used ratio of resistance. Finally, we posit that, although the decision to use conductance or resistance should be made on a case-by-case basis, in most circumstances, conductance is a more faithful portrayal of cardiovascular regulation in vertebrates.

Extreme blood boosting capacity of an Antarctic fish represents an adaptation to life in a sub-zero environment

Blood doping, the practice of boosting the oxygen carrying capacity of blood, is an illegal strategy used by human athletes to enhance aerobic capacity and athletic performance. Interestingly, the practice of boosting blood oxygen carrying capacity is also naturally prevalent in the animal kingdom via the splenic release of stored erythrocytes. Here we demonstrate that an Antarctic notothenioid fish, the bald notothen (Pagothenia borchgrevinki), is a master of this practice. Due to the sub-zero environment these fish inhabit, they sequester a large proportion of erythrocytes in the spleen during times of inactivity to reduce the energetic and physiological costs associated with continuously pumping highly viscous blood around the body. However, in response to metabolically demanding situations (i.e. exercise and feeding), these fish contract the spleen to eject stored erythrocytes into circulation, which boosts blood oxygen carrying capacity by up to 207% (c.f. exercise-induced increases of ∼40-60% in a range of other vertebrates and ∼5-25% in blood-doping athletes). By evaluating cardiorespiratory differences between splenectomized (unable to release erythrocytes from the spleen) and sham-operated individuals, we demonstrate the metabolic benefits (i.e. aerobic scope increased 103%) and the cardiovascular trade-offs (i.e. ventral aortic blood pressure and cardiac workload increased 12% and 30%, respectively) associated with the splenic blood boosting strategy. In conclusion, this strategy provides bald notothens with an extraordinary facultative aerobic scope that enables an active lifestyle in the extreme Antarctic marine environment, while minimizing the energetic and physiological costs of transporting highly viscous blood during times of reduced energetic demand.

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.

A solution to Nature's haemoglobin knockout: a plasma-accessible carbonic anhydrase catalyses CO2 excretion in Antarctic icefish gills

In all vertebrates studied to date, CO₂ excretion depends on the enzyme carbonic anhydrase (CA) that catalyses the rapid conversion of HCO₃ ^- to CO₂ at the gas-exchange organs. The largest pool of CA is present within red blood cells (RBCs) and, in some vertebrates, plasma-accessible CA (paCA) isoforms participate in CO₂ excretion. However, teleost fishes typically do not have paCA at the gills and CO₂ excretion is reliant entirely on RBC CA - a strategy that is not possible in icefishes. As the result of a natural knockout, Antarctic icefishes (Channichthyidae) are the only known vertebrates that do not express haemoglobin (Hb) as adults, and largely lack RBCs in the circulation (haematocrit <1%). Previous work has indicated the presence of high levels of membrane-bound CA activity in the gills of icefishes, but without determining its cellular orientation. Thus, we hypothesised that icefishes express a membrane-bound CA isoform at the gill that is accessible to the blood plasma. The CA distribution was compared in the gills of two closely related notothenioid species, one with Hb and RBCs (Notothenia rossii) and one without (Champsocephalus gunnari). Molecular, biochemical and immunohistochemical markers indicate high levels of a Ca4 isoform in the gills of the icefish (but not the red-blooded N. rossii), in a plasma-accessible location that is consistent with a role in CO₂ excretion. Thus, in the absence of RBC CA, the icefish gill could exclusively provide the catalytic activity necessary for CO₂ excretion - a pathway that is unlike that of any other vertebrate.

Cardiac mitochondrial metabolism may contribute to differences in thermal tolerance of red- and white-blooded Antarctic notothenioid fishes

Studies in temperate fishes provide evidence that cardiac mitochondrial function and the capacity to fuel cardiac work contribute to thermal tolerance. Here, we tested the hypothesis that decreased cardiac aerobic metabolic capacity contributes to the lower thermal tolerance of the haemoglobinless Antarctic icefish, Chaenocephalus aceratus, compared with that of the red-blooded Antarctic species, Notothenia coriiceps. Maximal activities of citrate synthase (CS) and lactate dehydrogenase (LDH), respiration rates of isolated mitochondria, adenylate levels and changes in mitochondrial protein expression were quantified from hearts of animals held at ambient temperature or exposed to their critical thermal maximum (CTmax). Compared with C. aceratus, activity of CS, ATP concentration and energy charge were higher in hearts of N. coriiceps at ambient temperature and CTmax While state 3 mitochondrial respiration rates were not impaired by exposure to CTmax in either species, state 4 rates, indicative of proton leakage, increased following exposure to CTmax in C. aceratus but not N. coriiceps The interactive effect of temperature and species resulted in an increase in antioxidants and aerobic metabolic enzymes in N. coriiceps but not in C. aceratus Together, our results support the hypothesis that the lower aerobic metabolic capacity of C. aceratus hearts contributes to its low thermal tolerance.

The effects of thermal acclimation on cardio-respiratory performance in an Antarctic fish (Notothenia coriiceps)

The Southern Ocean has experienced stable, cold temperatures for over 10 million years, yet particular regions are currently undergoing rapid warming. To investigate the impacts of warming on cardiovascular oxygen transport, we compared the cardio-respiratory performance in an Antarctic notothenioid (Notothenia coriiceps) that was maintained at 0 or 5°C for 6.0-9.5 weeks. When compared at the fish's respective acclimation temperature, the oxygen consumption rate and cardiac output were significantly higher in 5°C-acclimated than 0°C-acclimated fish. The 2.7-fold elevation in cardiac output in 5°C-acclimated fish (17.4 vs. 6.5 ml min^-1 kg^-1) was predominantly due to a doubling of stroke volume, likely in response to increased cardiac preload, as measured by higher central venous pressure (0.15 vs. 0.08 kPa); tachycardia was minor (29.5 vs. 25.2 beats min^-1). When fish were acutely warmed, oxygen consumption rate increased by similar amounts in 0°C- and 5°C-acclimated fish at equivalent test temperatures. In both acclimation groups, the increases in oxygen consumption rate during acute heating were supported by increased cardiac output achieved by elevating heart rate, while stroke volume changed relatively little. Cardiac output was similar between both acclimation groups until 12°C when cardiac output became significantly higher in 5°C-acclimated fish, driven largely by their higher stroke volume. Although cardiac arrhythmias developed at a similar temperature (~14.5°C) in both acclimation groups, the hearts of 5°C-acclimated fish continued to pump until significantly higher temperatures (CT_max for cardiac function 17.7 vs. 15.0°C for 0°C-acclimated fish). These results demonstrate that N. coriiceps is capable of increasing routine cardiac output during both acute and chronic warming, although the mechanisms are different (heart rate-dependent versus primarily stroke volume-dependent regulation, respectively). Cardiac performance was enhanced at higher temperatures following 5°C acclimation, suggesting cardiovascular function may not constrain the capacity of N. coriiceps to withstand a warming climate.