Hagfish: Champions of CO2 tolerance question the origins of vertebrate gill function

An evolutionary timeline of the oxytocin signaling pathway

Oxytocin is a neuropeptide associated with both psychological and somatic processes like parturition and social bonding. Although oxytocin homologs have been identified in many species, the evolutionary timeline of the entire oxytocin signaling gene pathway has yet to be described. Using protein sequence similarity searches, microsynteny, and phylostratigraphy, we assigned the genes supporting the oxytocin pathway to different phylostrata based on when we found they likely arose in evolution. We show that the majority (64%) of genes in the pathway are 'modern'. Most of the modern genes evolved around the emergence of vertebrates or jawed vertebrates (540 - 530 million years ago, 'mya'), including OXTR, OXT and CD38. Of those, 45% were under positive selection at some point during vertebrate evolution. We also found that 18% of the genes in the oxytocin pathway are 'ancient', meaning their emergence dates back to cellular organisms and opisthokonta (3500-1100 mya). The remaining genes (18%) that evolved after ancient and before modern genes were classified as 'medium-aged'. Functional analyses revealed that, in humans, medium-aged oxytocin pathway genes are highly expressed in contractile organs, while modern genes in the oxytocin pathway are primarily expressed in the brain and muscle tissue.

Evolution of the glomerulus in a marine environment and its implications for renal function in terrestrial vertebrates

Nearly a century ago, Homer Smith proposed that the glomerulus evolved to meet the challenge of excretion of water in freshwater vertebrates. This hypothesis has been repeatedly restated in the nephrology and renal physiology literature, even though we now know that vertebrates evolved and diversified in marine (saltwater) environments. A more likely explanation is that the vertebrate glomerulus evolved from the meta-nephridium of marine invertebrates, with the driving force for ultrafiltration being facilitated by the apposition of the filtration barrier to the vasculature (in vertebrates) rather than the coelom (in invertebrates) and the development of a true heart and the more complex vertebrate vascular system. In turn, glomerular filtration aided individual regulation of divalent ions like magnesium, calcium, and sulfate compatible with the function of cardiac and skeletal muscle required for mobile predators. The metabolic cost, imposed by reabsorption of the small amounts of sodium required to drive secretion of these over-abundant divalent ions, was small. This innovation, developed in a salt-water environment, provided a preadaptation for life in freshwater, in which the glomerulus was co-opted to facilitate water excretion, albeit with the additional metabolic demand imposed by the need to reabsorb the majority of filtered sodium. The evolution of the glomerulus in saltwater also provided preadaptation for terrestrial life, where the imperative is conservation of both water and electrolytes. The historical contingencies of this scenario may explain why the mammalian kidney is so metabolically inefficient, with ∼80% of oxygen consumption being used to drive reabsorption of filtered sodium.

Dining on the dead in the deep: Active NH4 + excretion via Na+ /H+ (NH4 + ) exchange in the highly ammonia tolerant Pacific hagfish, Eptatretus stoutii

AIM

Pacific hagfish are exceptionally tolerant to high environmental ammonia (HEA). Here, we elucidated a cellular mechanism that enables hagfish to actively excrete ammonia against steep ammonia gradients expected to be found inside a decomposing whale carcass.

METHODS

Hagfish were exposed to varying concentrations of HEA in the presence or absence of environmental Na⁺ , while plasma ammonia levels were tracked. ¹⁴ C-methylammonium was used as a proxy for NH₄ ⁺ to measure efflux in whole animals and in isolated gill pouches; the latter allowed us to assess the effects of amiloride specifically on Na⁺ /H⁺ exchangers (NHEs) in gill cells. Western blotting and immunohistochemistry were utilized to evaluate the abundance and sub-cellular localization of Rhesus glycoprotein (Rh) channels in the response to HEA.

RESULTS

Hagfish actively excreted NH₄ ⁺ against steep inwardly directed E_NH4 ⁺ (ΔE_NH4 ⁺  ~ 35 mV) and pNH₃ (ΔpNH₃  ~ 2000 μtorr) gradients. Active NH₄ ⁺ excretion and plasma ammonia hypo-regulation were contingent on the presence of environmental Na⁺ , indicating a Na⁺ /NH₄ ⁺ exchange mechanism. Active NH₄ ⁺ excretion across isolated gill pouches was amiloride-sensitive. Exposure to HEA resulted in decreased abundance of Rh channels in the apical membrane of gill ionocytes.

CONCLUSIONS

During HEA exposure, hagfish can actively excrete ammonia against a steep concentration gradient using apical NHEs energized by Na⁺ -K⁺ -ATPase in gill ionocytes. Additionally, apical Rh channels are removed from the apical membrane, presumably to reduce ammonia loading from the environment. We suggest that this mechanism allows hagfish to maintain tolerable ammonia levels while feeding inside decomposing carrion, allowing them to exploit nutrient-rich food-falls.

Rapid embryonic development supports the early onset of gill functions in two coral reef damselfishes

The gill is one of the most important organs for growth and survival of fishes. Early life stages in coral reef fishes often exhibit extreme physiological and demographic characteristics that are linked to well-established respiratory and ionoregulatory processes. However, gill development and function in coral reef fishes is not well understood. Therefore, we investigated gill morphology, oxygen uptake and ionoregulatory systems throughout embryogenesis in two coral reef damselfishes, Acanthochromis polyacanthus and Amphiprion melanopus (Pomacentridae). In both species, we found key gill structures to develop rapidly early in the embryonic phase. Ionoregulatory cells appear on gill filaments 3-4 days post-fertilization and increase in density, whilst disappearing or shrinking in cutaneous locations. Primary respiratory tissue (lamellae) appears 5-7 days post-fertilization, coinciding with a peak in oxygen uptake rates of the developing embryos. Oxygen uptake was unaffected by phenylhydrazine across all ages (pre-hatching), indicating that haemoglobin is not yet required for oxygen uptake. This suggests that gills have limited contribution to respiratory functions during embryonic development, at least until hatching. Rapid gill development in damselfishes, when compared with that in most previously investigated fishes, may reflect preparations for a high-performance, challenging lifestyle on tropical reefs, but may also make reef fishes more vulnerable to anthropogenic stressors.

Understanding ventilation and oxygen uptake of Pacific hagfish (Eptatretus stoutii), with particular emphasis on responses to ammonia and interactions with other respiratory gases

The hagfishes are an ancient and evolutionarily important group, with breathing mechanisms and gills very different from those of other fishes. Hagfish inhale through a single nostril via a velum pump, and exhale through multiple separate gill pouches. We assessed respiratory performance in E. stoutii (31 ppt, 12 ºC, 50-120 g) by measuring total ventilatory flow ([Formula: see text]) at the nostril, velar (respiratory) frequency (fr), and inspired (P_IO₂) and expired (P_EO₂) oxygen tensions at all 12 gill pouch exits plus the pharyngo-cutaneous duct (PCD) on the left side, and calculated ventilatory stroke volume (S[Formula: see text]), % O₂ utilization, and oxygen consumption (ṀO₂). At rest under normoxia, spontaneous changes in [Formula: see text] ranged from apnea to > 400 ml kg^-1 min^-1, due to variations in both fr and S[Formula: see text]; "normal" [Formula: see text] averaged 137 ml kg^-1 min^-1, ṀO₂ was 718 µmol kg^-1 h^-1, so the ventilatory convection requirement for O₂ was about 11 L mmol^-1. Relative to anterior gill pouches, lower P_EO₂ values (i.e. higher utilization) occurred in the more posterior pouches and PCD. Overall, O₂ utilization was 34% and did not change during hyperventilation but increased to > 90% during hypoventilation. Environmental hypoxia (P_IO₂ ~ 8% air saturation, 1.67 kPa, 13 Torr) caused hyperventilation, but neither acute hyperoxia (P_IO₂ ~ 275% air saturation, 57.6 kPa, 430 Torr) nor hypercapnia (P_ICO₂ ~ 1% CO₂, 1.0 kPa, 7.5 Torr) significantly altered [Formula: see text]. ṀO₂ decreased in hypoxia and increased in hyperoxia but did not change in hypercapnia. Acute exposure to high environmental ammonia (HEA, 10 mM NH₄HCO₃) caused an acute decrease in [Formula: see text], in contrast to the hyperventilation of long-term HEA exposure described in a previous study. The hypoventilatory response to HEA still occurred during hypoxia and hyperoxia, but was blunted during hypercapnia. Under all treatments, ṀO₂ increased with increases in [Formula: see text]. Overall, there were lower convection requirements for O₂ during hyperoxia, higher requirements during hypoxia and hypercapnia, but unchanged requirements during HEA. We conclude that this "primitive" fish operates a flexible respiratory system with considerable reserve capacity.

Evolution of epithelial sodium channels: current concepts and hypotheses

The conquest of freshwater and terrestrial habitats was a key event during vertebrate evolution. Occupation of low-salinity and dry environments required significant osmoregulatory adaptations enabling stable ion and water homeostasis. Sodium is one of the most important ions within the extracellular liquid of vertebrates, and molecular machinery for urinary reabsorption of this electrolyte is critical for the maintenance of body osmoregulation. Key ion channels involved in the fine-tuning of sodium homeostasis in tetrapod vertebrates are epithelial sodium channels (ENaCs), which allow the selective influx of sodium ions across the apical membrane of epithelial cells lining the distal nephron or the colon. Furthermore, ENaC-mediated sodium absorption across tetrapod lung epithelia is crucial for the control of liquid volumes lining the pulmonary surfaces. ENaCs are vertebrate-specific members of the degenerin/ENaC family of cation channels; however, there is limited knowledge on the evolution of ENaC within this ion channel family. This review outlines current concepts and hypotheses on ENaC phylogeny and discusses the emergence of regulation-defining sequence motifs in the context of osmoregulatory adaptations during tetrapod terrestrialization. In light of the distinct regulation and expression of ENaC isoforms in tetrapod vertebrates, we discuss the potential significance of ENaC orthologs in osmoregulation of fishes as well as the putative fates of atypical channel isoforms in mammals. We hypothesize that ancestral proton-sensitive ENaC orthologs might have aided the osmoregulatory adaptation to freshwater environments whereas channel regulation by proteases evolved as a molecular adaptation to lung liquid homeostasis in terrestrial tetrapods.

Specialized adaptations allow vent-endemic crabs (Xenograpsus testudinatus) to thrive under extreme environmental hypercapnia

Shallow hydrothermal vent environments are typically very warm and acidic due to the mixing of ambient seawater with volcanic gasses (> 92% CO₂) released through the seafloor making them potential ‘natural laboratories’ to study long-term adaptations to extreme hypercapnic conditions. Xenograpsus testudinatus, the shallow hydrothermal vent crab, is the sole metazoan inhabitant endemic to vents surrounding Kueishantao Island, Taiwan, where it inhabits waters that are generally pH 6.50 with maximum acidities reported as pH 5.50. This study assessed the acid–base regulatory capacity and the compensatory response of X. testudinatus to investigate its remarkable physiological adaptations. Hemolymph parameters (pH, [HCO₃⁻], \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2, [NH₄⁺], and major ion compositions) and the whole animal’s rates of oxygen consumption and ammonia excretion were measured throughout a 14-day acclimation to pH 6.5 and 5.5. Data revealed that vent crabs are exceptionally strong acid–base regulators capable of maintaining homeostatic pH against extreme hypercapnia (pH 5.50, 24.6 kPa \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2) via HCO₃⁻/Cl⁻ exchange, retention and utilization of extracellular ammonia. Intact crabs as well as their isolated perfused gills maintained \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2tensions below environmental levels suggesting the gills can excrete CO₂ against a hemolymph-directed \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2 gradient. These specialized physiological mechanisms may be amongst the adaptations required by vent-endemic animals surviving in extreme conditions.

Preferential intracellular pH regulation is a common trait amongst fishes exposed to high environmental CO2

Acute (<96 h) exposure to elevated environmental CO2 (hypercarbia) induces a pH disturbance in fishes that is often compensated by concurrent recovery of intracellular and extracellular pH (pHi and pHe, respectively; coupled pH regulation). However, coupled pH regulation may be limited at CO2 partial pressure (PCO2 ) tensions far below levels that some fishes naturally encounter. Previously, four hypercarbia-tolerant fishes had been shown to completely and rapidly regulate heart, brain, liver and white muscle pHi during acute exposure to >4 kPa PCO2  (preferential pHi regulation) before pHe compensation was observed. Here, we test the hypothesis that preferential pHi regulation is a widespread strategy of acid-base regulation among fish by measuring pHi regulation in 10 different fish species that are broadly phylogenetically separated, spanning six orders, eight families and 10 genera. Contrary to previous views, we show that preferential pHi regulation is the most common strategy for acid-base regulation within these fishes during exposure to severe acute hypercarbia and that this strategy is associated with increased hypercarbia tolerance. This suggests that preferential pHi regulation may confer tolerance to the respiratory acidosis associated with hypercarbia, and we propose that it is an exaptation that facilitated key evolutionary transitions in vertebrate evolution, such as the evolution of air breathing.

Hypoxia induces selective modifications to the acetylome in the brain of zebrafish (Danio rerio)

Reversible protein acetylation is an important regulatory mechanism for modulating protein function. The cellular protein acetylome is in large part dictated by the cellular redox balance, and in particular [NAD⁺]. While the relationship between hypoxia, redox balance, energy charge and resulting mitochondrial dysfunction has been examined in the context of hypoxia-linked pathologies, little is known about the direct effects of decreases in environmental oxygen on reversible lysine acetylation, and the resulting modifications to mitochondrial metabolism. To address this knowledge gap, we exposed zebrafish (Danio rerio) to 16 h of hypoxia (2.21 kPa) and quantified acetylation levels of 1220 proteins using whole-cell proteomics in samples of brain taken from normoxic and hypoxic zebrafish. In addition, we examined the effects of hypoxia on cytoplasmic and mitochondrial redox status, whole-cell energetics, the activity of the mitochondrial NAD⁺-dependent deacetylase SIRT3, and electron transport chain complex activities to determine if there is an association between hypoxia-induced metabolic disturbances, protein acetylation, and mitochondrial function. Our results (1) reveal several key changes in the acetylation status of proteins in the brain, primarily within the mitochondria; (2) show significant fluctuations in cytoplasmic and mitochondrial redox status within the brain during hypoxia exposure; and (3) provide evidence that lysine acetylation may be related to large changes in electron transport and ATP-synthase complex activities and adenylate status in zebrafish exposed to hypoxic stress. Together, these data provide new insights into the role of protein modifications in mitochondrial metabolism during hypoxia.

Preferential intracellular pH regulation: hypotheses and perspectives

The regulation of vertebrate acid-base balance during acute episodes of elevated internal PCO2  is typically characterized by extracellular pH (pHe) regulation. Changes in pHe are associated with qualitatively similar changes in intracellular tissue pH (pHi) as the two are typically coupled, referred to as 'coupled pH regulation'. However, not all vertebrates rely on coupled pH regulation; instead, some preferentially regulate pHi against severe and maintained reductions in pHe Preferential pHi regulation has been identified in several adult fish species and an aquatic amphibian, but never in adult amniotes. Recently, common snapping turtles were observed to preferentially regulate pHi during development; the pattern of acid-base regulation in these species shifts from preferential pHi regulation in embryos to coupled pH regulation in adults. In this Commentary, we discuss the hypothesis that preferential pHi regulation may be a general strategy employed by vertebrate embryos in order to maintain acid-base homeostasis during severe acute acid-base disturbances. In adult vertebrates, the retention or loss of preferential pHi regulation may depend on selection pressures associated with the environment inhabited and/or the severity of acid-base regulatory challenges to which they are exposed. We also consider the idea that the retention of preferential pHi regulation into adulthood may have been a key event in vertebrate evolution, with implications for the invasion of freshwater habitats, the evolution of air breathing and the transition of vertebrates from water to land.

White sturgeon (Acipenser transmontanus) acid-base regulation differs in response to different types of acidoses

White sturgeon (Acipenser transmontanus) completely protect intracellular tissue pH (pH_i) despite large reductions in extracellular (blood) pH (pH_e), termed preferential pH_i regulation, in response to elevated environmental PCO₂ (hypercarbia) and in general appear to be relatively resilient to stressors. Preferential pH_i regulation is thought to be associated with hypercarbia tolerance in general, but has also recently been observed to protect pH_i against metabolic acidoses induced by exhaustive exercise and anoxia in a tropical air breathing catfish. We hypothesized that preferential pH_i regulation may also be a general strategy of acid-base regulation in sturgeon. To address this hypothesis, severe acidoses were imposed to reduce pH_e, and the presence or absence of preferential pH_i regulation was assessed in red blood cells (RBC), heart, brain, liver and white muscle. A respiratory acidosis was imposed using hyperoxia, while metabolic acidoses were induced by exhaustive exercise, anoxia or air exposure. Reductions in pH_e occurred following hyperoxia (0.15 units), exhaustive exercise (0.30 units), anoxia (0.10 units) and air exposure (0.35 units); all acidoses reduced RBC pH_i. Following hyperoxia, heart, brain and liver pH_i were preferentially regulated against the reduction in pH_e, similar to hypercarbia exposure. Following all metabolic acidoses heart pH_i was protected and brain pH_i remained unchanged following exhaustive exercise and air exposure, however, brain pH_i was reduced following anoxia. Liver and white muscle pH_i were reduced following all metabolic acidoses. These results suggest preferential pH_i regulation may be a general strategy during respiratory acidoses but during metabolic acidoses, the response differs between source of acidoses and tissues.

Altered brain ion gradients following compensation for elevated CO2 are linked to behavioural alterations in a coral reef fish

Neurosensory and behavioural disruptions are some of the most consistently reported responses upon exposure to ocean acidification-relevant CO2 levels, especially in coral reef fishes. The underlying cause of these disruptions is thought to be altered current across the GABAA receptor in neuronal cells due to changes in ion gradients (HCO3(-) and/or Cl(-)) that occur in the body following compensation for elevated ambient CO2. Despite these widely-documented behavioural disruptions, the present study is the first to pair a behavioural assay with measurements of relevant intracellular and extracellular acid-base parameters in a coral reef fish exposed to elevated CO2. Spiny damselfish (Acanthochromis polyacanthus) exposed to 1900 μatm CO2 for 4 days exhibited significantly increased intracellular and extracellular HCO3(-) concentrations and elevated brain pHi compared to control fish, providing evidence of CO2 compensation. As expected, high CO2 exposed damselfish spent significantly more time in a chemical alarm cue (CAC) than control fish, supporting a potential link between behavioural disruption and CO2 compensation. Using HCO3(-) measurements from the damselfish, the reversal potential for GABAA (EGABA) was calculated, illustrating that biophysical properties of the brain during CO2 compensation could change GABAA receptor function and account for the behavioural disturbances noted during exposure to elevated CO2.

Introducing a novel mechanism to control heart rate in the ancestral Pacific hagfish

Although neural modulation of heart rate is well established among chordate animals, the Pacific hagfish (Eptatretus stoutii) lacks any cardiac innervation, yet it can increase its heart rate from the steady, depressed heart rate seen in prolonged anoxia to almost double its normal normoxic heart rate, an almost fourfold overall change during the 1-h recovery from anoxia. The present study sought mechanistic explanations for these regulatory changes in heart rate. We provide evidence for a bicarbonate-activated, soluble adenylyl cyclase (sAC)-dependent mechanism to control heart rate, a mechanism never previously implicated in chordate cardiac control.

Embryonic common snapping turtles (Chelydra serpentina) preferentially regulate intracellular tissue pH during acid-base challenges

The nests of embryonic turtles naturally experience elevated CO2 (hypercarbia), which leads to increased blood PCO2 and a respiratory acidosis resulting in reduced blood pH [extracellular pH (pHe)]. Some fishes preferentially regulate tissue pH [intracellular pH (pHi)] against changes in pHe; this has been proposed to be associated with exceptional CO2 tolerance and has never been identified in amniotes. As embryonic turtles may be CO2 tolerant based on nesting strategy, we hypothesized that they preferentially regulate pHi, conferring tolerance to severe acute acid-base challenges. This hypothesis was tested by investigating pH regulation in common snapping turtles (Chelydra serpentina) reared in normoxia then exposed to hypercarbia (13kPa PCO2) for 1h at three developmental ages, 70 and 90% of incubation, and in yearlings. Hypercarbia reduced pHe but not pHi, at all developmental ages. At 70% of incubation, pHe was depressed by 0.324 pH units while pHi of brain, white muscle, and lung increased; heart, liver, and kidney pHi remained unchanged. At 90% of incubation, pHe was depressed by 0.352 pH units but heart pHi increased with no change in pHi of other tissues. Yearling exhibited a pHe reduction of 0.235 pH units but had no changes in pHi of any tissues. The results indicate common snapping turtles preferentially regulate pHi during development, but the degree of the response is reduced throughout development. This is the first time preferential pHi regulation has been identified in an amniote. These findings may provide insight into the evolution of acid-base homeostasis during development of amniotes, and vertebrates in general.

Characterizing the metabolic capacity of the anoxic hagfish heart

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia at 10°C. Such anoxia tolerance demands the mobilization of anaerobic fuels and the removal of metabolic wastes--processes that require a functional heart. The purpose of this study was to measure the metabolic response of the excised, cannulated hagfish heart to anoxia using direct calorimetry. These experiments were coupled with measurements of cardiac pH and metabolite concentrations, at multiple time points, to monitor acid-base balance and anaerobic ATP production. We also exposed hagfish to anoxia to compare the in vitro responses of the excised hearts with the in vivo responses. The calorimetry results revealed a significant reduction in the rate of metabolic heat production over the first hour of anoxia exposure, and a recovery over the subsequent 6 h. This response is likely attributable to a rapid anoxia-induced depression of aerobic ATP-production pathways followed by an upregulation of anaerobic ATP-production pathways such that the ATP production rate was restored to that measured in normoxia. Glycogen-depletion measurements suggest that metabolic processes were initially supported by glycolysis but that an alternative fuel source was used to support the sustained rates of ATP production. The maintenance of intracellular pH during anoxia indicates a remarkable ability of the myocytes to buffer/regulate protons and thus protect cardiac function. Altogether, these results illustrate that the low metabolic demand of the hagfish heart allows for near-routine levels of cardiac metabolism to be supported anaerobically. This is probably a significant contributor to the hagfish's exceptional anoxia tolerance.

Characterizing the metabolic capacity of the anoxic hagfish heart

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia at 10°C. Such anoxia tolerance demands the mobilization of anaerobic fuels and the removal of metabolic wastes, processes that require a functional heart. The purpose of this study was to measure the metabolic response of the excised, cannulated hagfish heart to anoxia using direct calorimetry. These experiments were coupled with measurements of cardiac pH and metabolite concentrations, at multiple time points, to monitor acid-base balance and anaerobic ATP-production. We also exposed hagfish to anoxia to compare the in vitro responses of the excised hearts with the in vivo responses. The calorimetry results revealed a significant reduction in the rate of metabolic heat production over the first hour of anoxia exposure, and a recovery over the subsequent 6 h. This response was likely attributable to a rapid anoxia-induced depression of aerobic ATP-production pathways followed by an up-regulation of anaerobic ATP-production pathways such that the ATP production rate was restored to that measured in normoxia. Glycogen-depletion measurements suggest that metabolic processes were initially supported by glycolysis but that an alternate fuel source was used to support the sustained rates of ATP production. The maintenance of intracellular pH during anoxia indicates a remarkable ability of the myocytes to buffer/regulate protons and thus protect cardiac function. Altogether, these results illustrate that the low metabolic demand of the hagfish heart allows for near-routine levels of cardiac metabolism to be supported anaerobically. This is likely a significant contributor to the hagfish's exceptional anoxia tolerance.

Characterizing the metabolic capacity of the anoxic hagfish heart

Pacific hagfish, Eptatretus stoutii, can recover from 36 h of anoxia at 10°C. Such anoxia tolerance demands the mobilization of anaerobic fuels and the removal of metabolic wastes, processes that require a functional heart. The purpose of this study was to measure the metabolic response of the excised, cannulated hagfish heart to anoxia using direct calorimetry. These experiments were coupled with measurements of cardiac pH and metabolite concentrations, at multiple time points, to monitor acid-base balance and anaerobic ATP-production. We also exposed hagfish to anoxia to compare the in vitro responses of the excised hearts with the in vivo responses. The calorimetry results revealed a significant reduction in the rate of metabolic heat production over the first hour of anoxia exposure, and a recovery over the subsequent 6 h. This response was likely attributable to a rapid anoxia-induced depression of aerobic ATP-production pathways followed by an up-regulation of anaerobic ATP-production pathways such that the ATP production rate was restored to that measured in normoxia. Glycogen-depletion measurements suggest that metabolic processes were initially supported by glycolysis but that an alternate fuel source was used to support the sustained rates of ATP production. The maintenance of intracellular pH during anoxia indicates a remarkable ability of the myocytes to buffer/regulate protons and thus protect cardiac function. Altogether, these results illustrate that the low metabolic demand of the hagfish heart allows for near-routine levels of cardiac metabolism to be supported anaerobically. This is likely a significant contributor to the hagfish's exceptional anoxia tolerance.