Development of aerobic and anaerobic metabolism in cardiac and skeletal muscles from harp and hooded seals

Consistent changes in muscle metabolism underlie dive performance across multiple lineages of diving ducks

Diving animals must sustain high activity with limited O2 stores to successfully capture prey. Studies suggest that increasing body O2 stores supports breath-hold diving, but less is known about metabolic specializations that underlie underwater locomotion. We measured maximal activities of 10 key enzymes in locomotory muscles (gastrocnemius and pectoralis) to identify biochemical changes associated with diving in pathways of oxidative and substrate-level phosphorylation and compared them across three groups of ducks-the longest diving sea ducks (eight spp.), the mid-tier diving pochards (three spp.) and the non-diving dabblers (five spp.). Relative to dabblers, both diving groups had increased activities of succinate dehydrogenase and cytochrome c oxidase, and sea ducks further showed increases in citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HOAD). Both diving groups had relative decreases in capacity for anaerobic metabolism (lower ratio of lactate dehydrogenase to CS), with sea ducks also showing a greater capacity for oxidative phosphorylation and lipid oxidation (lower ratio of pyruvate kinase to CS, higher ratio of HOAD to hexokinase). These data suggest that the locomotory muscles of diving ducks are specialized for sustaining high rates of aerobic metabolism, emphasizing the importance of body O2 stores for dive performance in these species.

Myoglobin-mediated lipid shuttling increases adrenergic activation of brown and white adipocyte metabolism and is as a marker of thermogenic adipocytes in humans

BACKGROUND

Recruitment and activation of brown adipose tissue (BAT) results in increased energy expenditure (EE) via thermogenesis and represents an intriguing therapeutic approach to combat obesity and treat associated diseases. Thermogenesis requires an increased and efficient supply of energy substrates and oxygen to the BAT. The hemoprotein myoglobin (MB) is primarily expressed in heart and skeletal muscle fibres, where it facilitates oxygen storage and flux to the mitochondria during exercise. In the last years, further contributions of MB have been assigned to the scavenging of reactive oxygen species (ROS), the regulation of cellular nitric oxide (NO) levels and also lipid binding. There is a substantial expression of MB in BAT, which is induced during brown adipocyte differentiation and BAT activation. This suggests MB as a previously unrecognized player in BAT contributing to thermogenesis.

METHODS AND RESULTS

This study analyzed the consequences of MB expression in BAT on mitochondrial function and thermogenesis in vitro and in vivo. Using MB overexpressing, knockdown or knockout adipocytes, we show that expression levels of MB control brown adipocyte mitochondrial respiratory capacity and acute response to adrenergic stimulation, signalling and lipolysis. Overexpression in white adipocytes also increases their metabolic activity. Mutation of lipid interacting residues in MB abolished these beneficial effects of MB. In vivo, whole-body MB knockout resulted in impaired thermoregulation and cold- as well as drug-induced BAT activation in mice. In humans, MB is differentially expressed in subcutaneous (SC) and visceral (VIS) adipose tissue (AT) depots, differentially regulated by the state of obesity and higher expressed in AT samples that exhibit higher thermogenic potential.

CONCLUSIONS

These data demonstrate for the first time a functional relevance of MBs lipid binding properties and establish MB as an important regulatory element of thermogenic capacity in brown and likely beige adipocytes.

Pinniped Ontogeny as a Window into the Comparative Physiology and Genomics of Hypoxia Tolerance

Diving physiology has received considerable scientific attention as it is a central element of the extreme phenotype of marine mammals. Many scientific discoveries have illuminated physiological mechanisms supporting diving, such as massive, internally bound oxygen stores and dramatic cardiovascular regulation. However, the cellular and molecular mechanisms that support the diving phenotype remain mostly unexplored as logistic and legal restrictions limit the extent of scientific manipulation possible. With next-generation sequencing (NGS) tools becoming more widespread and cost-effective, there are new opportunities to explore the diving phenotype. Genomic investigations come with their own challenges, particularly those including cross-species comparisons. Studying the regulatory pathways that underlie diving mammal ontogeny could provide a window into the comparative physiology of hypoxia tolerance. Specifically, in pinnipeds, which shift from terrestrial pups to elite diving adults, there is potential to characterize the transcriptional, epigenetic, and posttranslational differences between contrasting phenotypes while leveraging a common genome. Here we review the current literature detailing the maturation of the diving phenotype in pinnipeds, which has primarily been explored via biomarkers of metabolic capability including antioxidants, muscle fiber typing, and key aerobic and anaerobic metabolic enzymes. We also discuss how NGS tools have been leveraged to study phenotypic shifts within species through ontogeny, and how this approach may be applied to investigate the biochemical and physiological mechanisms that develop as pups become elite diving adults. We conclude with a specific example of the Antarctic Weddell seal by overlapping protein biomarkers with gene regulatory microRNA datasets.

Diving deep: understanding the genetic components of hypoxia tolerance in marine mammals

Marine mammals have highly specialized physiology, exhibited in many species by extreme breath-holding capabilities that allow deep dives and extended submergence. Cardiovascular control and cell-level hypoxia tolerance are key features of this phenotype. Identifying genomic signatures tied to physiology will be valuable in understanding these natural model species, which may generate translational opportunities to human diseases arising from hypoxic stress or tissue injury. Genomic analyses have now been conducted in dolphins, river dolphins, minke whales, bowhead whales, and polar bears, with multispecies studies exploring evolutionary signals across marine mammal lineages, encompassing extinct and extant divers. Single-species genome studies for sirenians do not yet exist. Extant marine mammals arose in three lineages from separate aquatic recolonizations. Their physiological specializations, along with these independent origins create an interesting case to examine convergent evolution. Although molecular mechanisms of hypoxia tolerance are not universally apparent across marine mammal genomic studies, altered evolutionary rates have been identified for genes linked to oxygen binding and transport (e.g., MB, HBA, and HBB), blood pressure control (e.g., endothelin pathway genes), and cell protection in multiple species. Despite convergent phenotypes across clades, instances of identical molecular convergence have been uncommon. Given the inherent logistical and regulatory difficulties associated with functional genetic experiments in marine mammals, several avenues of further investigation are suggested to enable validation of candidate genes for hypoxia tolerance: leveraging phylogeny to better understand convergent phenotypes; ontogenic studies to identify regulation of key genes underlying the elite, adult, hypoxia-tolerant physiology; and cell culture manipulations to understand gene function.

microRNA profiling in the Weddell seal suggests novel regulatory mechanisms contributing to diving adaptation

Background

The Weddell Seal (Leptonychotes weddelli) represents a remarkable example of adaptation to diving among marine mammals. This species is capable of diving > 900 m deep and remaining underwater for more than 60 min. A number of key physiological specializations have been identified, including the low levels of aerobic, lipid-based metabolism under hypoxia, significant increase in oxygen storage in blood and muscle; high blood volume and extreme cardiovascular control. These adaptations have been linked to increased abundance of key proteins, suggesting an important, yet still understudied role for gene reprogramming.

In this study, we investigate the possibility that post-transcriptional gene regulation by microRNAs (miRNAs) has contributed to the adaptive evolution of diving capacities in the Weddell Seal.

Results

Using small RNA data across 4 tissues (brain, heart, muscle and plasma), in 3 biological replicates, we generate the first miRNA annotation in this species, consisting of 559 high confidence, manually curated miRNA loci. Evolutionary analyses of miRNA gain and loss highlight a high number of Weddell seal specific miRNAs.

Four hundred sixteen miRNAs were differentially expressed (DE) among tissues, whereas 80 miRNAs were differentially expressed (DE) across all tissues between pups and adults and age differences for specific tissues were detected in 188 miRNAs. mRNA targets of these altered miRNAs identify possible protective mechanisms in individual tissues, particularly relevant to hypoxia tolerance, anti-apoptotic pathways, and nitric oxide signal transduction. Novel, lineage-specific miRNAs associated with developmental changes target genes with roles in angiogenesis and vasoregulatory signaling.

Conclusions

Altogether, we provide an overview of miRNA composition and evolution in the Weddell seal, and the first insights into their possible role in the specialization to diving.

Low guanylyl cyclase activity in Weddell seals: implications for peripheral vasoconstriction and perfusion of the brain during diving

Nitric oxide (NO) is a potent vasodilator, which improves perfusion and oxygen delivery during tissue hypoxia in terrestrial animals. The vertebrate dive response involves vasoconstriction in select tissues, which persists despite profound hypoxia. Using tissues collected from Weddell seals at necropsy, we investigated whether vasoconstriction is aided by downregulation of local hypoxia signaling mechanisms. We focused on NO-soluble guanylyl cyclase (GC)-cGMP signaling, a well-known vasodilatory transduction pathway. Seals have a lower GC protein abundance, activity, and capacity to respond to NO stimulation than do terrestrial mammals. In seal lung homogenates, GC produced less cGMP (20.1 ± 3.7 pmol·mg protein-1·min-1) than the lungs of dogs (-80 ± 144 pmol·mg protein-1·min-1 less than seals), sheep (-472 ± 96), rats (-664 ± 104) or mice (-1,160 ± 104, P < 0.0001). Amino acid sequences of the GC enzyme α-subunits differed between seals and terrestrial mammals, potentially affecting their structure and function. Vasoconstriction in diving Weddell seals is not consistent across tissues; perfusion is maintained in the brain and heart but decreased in other organs such as the kidney. A NO donor increased median GC activity 49.5-fold in the seal brain but only 27.4-fold in the kidney, consistent with the priority of cerebral perfusion during diving. Nos3 expression was high in the seal brain, which could improve NO production and vasodilatory potential. Conversely, Pde5a expression was high in the seal renal artery, which may increase cGMP breakdown and vasoconstriction in the kidney. Taken together, the results of this study suggest that alterations in the NO-cGMP pathway facilitate the diving response.

Preparing Muscles for Diving: Age-Related Changes in Muscle Metabolic Profiles in Harp (Pagophilus groenlandicus) and Hooded (Cystophora cristata) Seals

In adult marine mammals, muscles can sustain aerobic metabolism during dives in part because they contain large oxygen (O2) stores and metabolic rates are low. However, young pups have significantly lower tissue O2 stores and much higher mass-specific metabolic rates. To investigate how these differences may influence muscle function during dives, we measured the activities of enzymes involved in aerobic and anaerobic metabolic pathways (citrate synthase [CS], β-hydroxyacyl-coenzyme A dehydrogenase [HOAD], lactate dehydrogenase [LDH]) and the LDH isoform profile in six muscles from 41 harp (Pagophilus groenlandicus) and 30 hooded (Cystophora cristata) seals ranging in age from fetal to adult. All neonatal muscles had significantly higher absolute but lower metabolically scaled CS and HOAD activities than adults (∼ 70% and ∼ 85% lower, respectively). Developmental increases in LDH activity lagged that of aerobic enzymes and were not accompanied by changes in isozyme profile, suggesting that changes in enzyme concentration rather than structure determine activity levels. Biochemical maturation proceeded faster in the major locomotory muscles. In combination, findings suggest that pup muscles are unable to support strenuous aerobic exercise or rely heavily on anaerobic metabolism during early diving activities and that pups' high mass-specific metabolic rates may play a key role in limiting the ability of their muscles to support underwater foraging.

Rapid maturation of the muscle biochemistry that supports diving in pacific walruses (Odobenus rosmarus divergens)

Physiological constraints dictate animals' abilities to exploit habitats. For marine mammals, it is important to quantify physiological limits that influence diving and their ability to alter foraging behaviors. We characterized age-specific dive limits of walruses by measuring anaerobic (acid buffering capacity) and aerobic (myoglobin content) capacities of the muscles that power hind (longissimus dorsi) and fore (supraspinatus) flipper propulsion. Mean buffering capacities were similar across muscles and age classes (a fetus, 5 neonatal calves, a 3-month old, and 20 adults), ranging from 41.31 – 54.14 slykes and 42.00 – 46.93 slykes in the longissimus and supraspinatus, respectively. Mean myoglobin in the fetus and neonatal calves fell within a narrow range (longissimus: 0.92 – 1.68 g 100 g wet muscle mass−1; supraspinatus: 0.88 – 1.64 g wet muscle mass−1). By 3 months postpartum, myoglobin in the longissimus increased by 79%, but levels in the supraspinatus remained unaltered. From 3-months postpartum to adulthood, myoglobin increased by an additional 26% in the longissimus and increased by 126% in the supraspinatus; myoglobin remained greater in the longissimus compared to the supraspinatus. Walruses are unique among marine mammals because they are born with mature muscle acid buffering capacity and attain mature myoglobin content early in life. Despite rapid physiological development, small body size limits the diving capacity of immature walruses and extreme sexual dimorphism reduces the diving capacity of adult females compared to adult males. Thus, free-ranging immature walruses likely exhibit the shortest foraging dives while adult males are capable of the longest foraging dives.

miRNA-directed regulation of VEGF in tilapia under hypoxia condition

The Nile tilapia represents an excellent model for hypoxia tolerance. Vascular endothelial growth factor (VEGF) plays a key role in physiological blood vessel formation and pathological angiogenesis under hypoxia conditions. Tight regulation of VEGF level is necessary for hypoxia adaptation in tilapia. MicroRNAs (miRNAs) function as important regulators of gene expression at the post-transcriptional level, which are usually involved in stress responses. We reasoned that VEGF level could be regulated by miRNAs. Through bioinformatics analysis, we identified a putative miR-204 binding site in the VEGF mRNA. We found that hypoxia leads to a marked up-regulation in VEGF level, but a decrease in miR-204 level. miR-204 directly regulates VEGF expression by targeting its 3'-UTR, and inhibition of miR-204 substantially increases VEGF level in vivo. Moreover, we found that miR-204 loss of function could affect blood O2-carrying capacity, anaerobic metabolism, and antioxidant enzyme activity. Taken together, miR-204 is an endogenous regulator of VEGF expression, which participates in a regulatory circuit that allows rapid gene program transitions upon hypoxia stress.

Changing the paradigm for myoglobin: a novel link between lipids and myoglobin

Myoglobin (Mb) is an oxygen-binding muscular hemeprotein regulated via Ca2+-signaling pathways involving calcineurin (CN), with Mb increases attributed to hypoxia, exercise, and nitric oxide. Here, we show a link between lipid supplementation and increased Mb in skeletal muscle. C2C12 cells were cultured in normoxia or hypoxia with glucose or 5% lipid. Mb assays revealed that lipid cohorts had higher Mb than control cohorts in both normoxia and hypoxia, whereas Mb Western blots showed lipid cohorts having higher Mb than control cohorts exclusively under hypoxia. Normoxic cells were compared with soleus tissue from normoxic rats fed high-fat diets; whereas tissue sample cohorts showed no difference in CO-binding Mb, fat-fed rats showed increases in total Mb protein (similar to hypoxic cells), suggesting increases in modified Mb. Moreover, Mb increases did not parallel CN increases but did, however, parallel oxidative stress marker augmentation. Addition of antioxidant prevented Mb increases in lipid-supplemented normoxic cells and mitigated Mb increases in lipid-supplemented hypoxic cells, suggesting a pathway for Mb regulation through redox signaling independent of CN.

Rapid postnatal development of myoglobin from large liver iron stores in hooded seals

Hooded seals (Cystophora cristata) rely on large stores of oxygen, either bound to hemoglobin or myoglobin (Mb), to support prolonged diving activity. Pups are born with fully developed hemoglobin stores, but their Mb levels are only 25-30% of adult levels. We measured changes in muscle [Mb] from birth to 1 year of age in two groups of captive hooded seal pups, one being maintained in a seawater pool and one on land during the first 2 months. All pups fasted during the first month, but were fed from then on. The [Mb] of the swimming muscle musculus longissimus dorsi (LD) doubled during the month of fasting in the pool group. These animals had significantly higher levels and a more rapid rise in LD [Mb] than those kept on land. The [Mb] of the shoulder muscle, m. supraspinatus, which is less active in both swimming and hauled-out animals, was consistently lower than in the LD and did not differ between groups. This suggests that a major part of the postnatal rise in LD [Mb] is triggered by (swimming) activity, and this coincides with the previously reported rapid early development of diving capacity in wild hooded seal pups. Liver iron concentration, as determined from another 25 hooded seals of various ages, was almost 10 times higher in young pups (1-34 days) than in yearling animals and adults, and liver iron content of pups dropped during the first month, implying that liver iron stores support the rapid initial rise in [Mb].

In the face of hypoxia: myoglobin increases in response to hypoxic conditions and lipid supplementation in cultured Weddell seal skeletal muscle cells

A key cellular adaptation to diving in Weddell seals is enhanced myoglobin concentrations in their skeletal muscles, which serve to store oxygen to sustain a lipid-based aerobic metabolism. The aim of this study was to determine whether seal muscle cells are inherently adapted to possess the unique skeletal muscle adaptations to diving seen in the whole animal. We hypothesized that the seal skeletal muscle cells would have enhanced concentrations of myoglobin de novo that would be greater than those from a C(2)C(12) skeletal muscle cell line and reflect the concentrations of myoglobin observed in previous studies. In addition we hypothesized that the seal cells would respond to environmental hypoxia similarly to the C(2)C(12) cells in that citrate synthase activity and myoglobin would remain the same or decrease under hypoxia and lactate dehydrogenase activity would increase under hypoxia as previously reported. We further hypothesized that β-hydroxyacyl CoA dehydrogenase activity would increase in response to the increasing amounts of lipid supplemented to the culture medium. Our results show that myoglobin significantly increases in response to environmental hypoxia and lipids in the Weddell seal cells, while appearing similar metabolically to the C(2)C(12) cells. The results of this study suggest the regulation of myoglobin expression is fundamentally different in Weddell seal skeletal muscle cells when compared with a terrestrial mammalian cell line in that hypoxia and lipids initially prime the skeletal muscles for enhanced myoglobin expression. However, the cells need a secondary stimulus to further increase myoglobin to levels seen in the whole animal.

Apnea stimulates the adaptive response to oxidative stress in elephant seal pups

Extended breath-hold (apnea) bouts are routine during diving and sleeping in seals. These apneas result in oxygen store depletion and blood flow redistribution towards obligatory oxygen-dependent tissues, exposing seals to critical levels of ischemia and hypoxemia. The subsequent reperfusion/reoxygenation has the potential to increase oxidant production and thus oxidative stress. The contributions of extended apnea to oxidative stress in adapted mammals are not well defined. To address the hypothesis that apnea in seals is not associated with increased oxidative damage, blood samples were collected from northern elephant seal pups (N=6) during eupnea, rest- and voluntary submersion-associated apneas, and post-apnea (recovery). Plasma 4-hydroxynonenal (HNE), 8-isoprostanes (8-isoPGF(2α)), nitrotyrosine (NT), protein carbonyls, xanthine and hypoxanthine (HX) levels, along with xanthine oxidase (XO) activity, were measured. Protein content of XO, superoxide dismutase 1 (Cu,ZnSOD), catalase and myoglobin (Mb), as well as the nuclear content of hypoxia inducible factor 1α (HIF-1α) and NF-E2-related factor 2 (Nrf2), were measured in muscle biopsies collected before and after the breath-hold trials. HNE, 8-iso PGF(2α), NT and protein carbonyl levels did not change among eupnea, apnea or recovery. XO activity and HX and xanthine concentrations were increased at the end of the apneas and during recovery. Muscle protein content of XO, CuZnSOD, catalase, Mb, HIF-1α and Nrf2 increased 25-70% after apnea. Results suggest that rather than inducing the damaging effects of hypoxemia and ischemia/reperfusion that have been reported in non-diving mammals, apnea in seals stimulates the oxidative stress and hypoxic hormetic responses, allowing these mammals to cope with the potentially detrimental effects associated with this condition.

Muscle energy stores and stroke rates of emperor penguins: implications for muscle metabolism and dive performance

In diving birds and mammals, bradycardia and peripheral vasoconstriction potentially isolate muscle from the circulation. During complete ischemia, ATP production is dependent on the size of the myoglobin oxygen (O(2)) store and the concentrations of phosphocreatine (PCr) and glycogen (Gly). Therefore, we measured PCr and Gly concentrations in the primary underwater locomotory muscle of emperor penguin and modeled the depletion of muscle O(2) and those energy stores under conditions of complete ischemia and a previously determined muscle metabolic rate. We also analyzed stroke rate to assess muscle workload variation during dives and evaluate potential limitations on the model. Measured PCr and Gly concentrations, 20.8 and 54.6 mmol kg(-1), respectively, were similar to published values for nondiving animals. The model demonstrated that PCr and Gly provide a large anaerobic energy store, even for dives longer than 20 min. Stroke rate varied throughout the dive profile, indicating muscle workload was not constant during dives as was assumed in the model. The stroke rate during the first 30 s of dives increased with increased dive depth. In extremely long dives, lower overall stroke rates were observed. Although O(2) consumption and energy store depletion may vary during dives, the model demonstrated that PCr and Gly, even at concentrations typical of terrestrial birds and mammals, are a significant anaerobic energy store and can play an important role in the emperor penguin's ability to perform long dives.

Changes in muscle composition during the development of diving ability in the Australian fur seal

During development the Australian fur seal transitions from a terrestrial, maternally dependent pup to an adult marine predator. Adult seals have adaptations that allow them to voluntarily dive at depth for long periods, including increased bradycardic control, increased myoglobin levels and haematocrit. To establish whether the profile of skeletal muscle also changes in line with the development of diving ability, biopsy samples were collected from the trapezius muscle of pups, juveniles and adults. The proportions of different fibre types and their oxidative capacity were determined. Only oxidative fibre types (Type I and IIa) were identified, with a significant change in proportions from pup to adult. There was no change in oxidative capacity of Type I and IIa fibres between pups and juveniles but there was a two-fold increase between juveniles and adults. Myoglobin expression increased between pups and juveniles, suggesting improved oxygen delivery, but with no increase in oxidative capacity, oxygen utilisation within the muscle may still be limited. Adult muscle had the highest oxidative capacity, suggesting that fibres are able to effectively utilise available oxygen during prolonged dives. Elevated levels of total creatine in the muscles of juveniles may act as an energy buffer when fibres are transitioning from a fast to slow fibre type.

Antioxidant capacity develops with maturation in the deep-diving hooded seal

Maturation in hooded seals is characterized by the rapid development of their physiological diving capacity and is accompanied by increases in oxidant production but not oxidative damage. To test the hypothesis that the antioxidant system of hooded seals develops as they transition from a terrestrial to an aquatic environment, we obtained the complete cDNA sequence that encodes the NF-E2-related factor 2 (Nrf2), a central regulator of the antioxidant response, and compared Nrf2 mRNA and protein expression levels in muscle samples from neonate, weaned pups and adult hooded seals, along with glutathione (GSH) levels and the activity/protein content of the antioxidant enzymes catalase, glutathione peroxidase (GPx), peroxyredoxin VI (PrxVI), thioredoxin 1 (Trx1), thioredoxin reductase (TrxR), glutaredoxin 1 (Glrx1), glutathione disulphide reductase, glutathione S-transferase and glutamate-cysteine ligase. The Nrf2 of the hooded seal is 1822 bp long and encodes a protein of 606 amino acids with a leucine zipper domain and Keap1-mediated proteosomal degradation residues, which are key for Nrf2 function and regulation. Although neither Nrf2 mRNA nor Nrf2 nuclear protein content are higher in adults than in pups, GSH levels along with GPx, PrxVI, Trx1, TrxR and Glrx1 activity/protein content increase with maturation, suggesting that the potential for peroxide removal increases with development in hooded seals, and that these enzymes contribute to the regulation of the intracellular redox state and the prevention of oxidative damage in these deep-diving mammals.

Development of the aerobic dive limit and muscular efficiency in northern fur seals (Callorhinus ursinus)

Northern fur seal (Callorhinus ursinus; NFS) populations have been declining, perhaps due to limited foraging ability of pups. Because a marine mammal's proficiency at exploiting underwater prey resources is based on the ability to store large amounts of oxygen (O(2)) and to utilize these reserves efficiently, this study was designed to determine if NFS pups had lower blood, muscle, and total body O(2) stores than adults. Pups (<1-month old) had a calculated aerobic dive limit only ~40% of adult females due to lower blood and, to a much greater extent, muscle O(2) stores. Development of the Pectoralis (Pec) and Longissimus dorsi (LD) skeletal muscles was further examined by determining their myosin heavy chain (MHC) composition and enzyme activities. In all animals, the slow MHC I and fast-twitch IIA proteins typical of oxidative fiber types were dominant, but adult muscles contained more (Pec ~50%; LD ~250% higher) fast-twitch MHC IID/X protein characteristic of glycolytic muscle fibers, than pup muscles. This suggests that adults have greater ability to generate muscle power rapidly and/or under anaerobic conditions. Pup muscles also had lower aerobic and anaerobic ATP production potential, as indicated by lower metabolically scaled citrate synthase, β-hydroxyacyl CoA dehydrogenase, and lactate dehydrogenase activities (all P values ≤0.001). In combination, these findings indicate that pups are biochemically and physiologically limited in their diving capabilities relative to adults. This may contribute to lower NFS first year survival.

In pursuit of Irving and Scholander: a review of oxygen store management in seals and penguins

Since the introduction of the aerobic dive limit (ADL) 30 years ago, the concept that most dives of marine mammals and sea birds are aerobic in nature has dominated the interpretation of their diving behavior and foraging ecology. Although there have been many measurements of body oxygen stores, there have been few investigations of the actual depletion of those stores during dives. Yet, it is the pattern, rate and magnitude of depletion of O2 stores that underlie the ADL. Therefore, in order to assess strategies of O2 store management, we review (a) the magnitude of O2 stores, (b) past studies of O2 store depletion and (c) our recent investigations of O2 store utilization during sleep apnea and dives of elephant seals (Mirounga angustirostris) and during dives of emperor penguins (Aptenodytes forsteri). We conclude with the implications of these findings for (a) the physiological responses underlying O2 store utilization, (b) the physiological basis of the ADL and (c) the value of extreme hypoxemic tolerance and the significance of the avoidance of re-perfusion injury in these animals.

Prolonged fasting increases glutathione biosynthesis in postweaned northern elephant seals

Northern elephant seals experience prolonged periods of absolute food and water deprivation (fasting) while breeding, molting or weaning. The postweaning fast in elephant seals is characterized by increases in the renin-angiotensin system, expression of the oxidant-producing protein Nox4, and NADPH oxidase activity; however, these increases are not correlated with increased oxidative damage or inflammation. Glutathione (GSH) is a potent reductant and a cofactor for glutathione peroxidases (GPx), glutathione-S transferases (GST) and 1-cys peroxiredoxin (PrxVI) and thus contributes to the removal of hydroperoxides, preventing oxidative damage. The effects of prolonged food deprivation on the GSH system are not well described in mammals. To test our hypothesis that GSH biosynthesis increases with fasting in postweaned elephant seals, we measured circulating and muscle GSH content at the early and late phases of the postweaning fast in elephant seals along with the activity/protein content of glutamate-cysteine ligase [GCL; catalytic (GCLc) and modulatory (GCLm) subunits], γ-glutamyl transpeptidase (GGT), glutathione disulphide reductase (GR), glucose-6-phosphate dehydrogenase (G6PDH), GST and PrxVI, as well as plasma changes in γ-glutamyl amino acids, glutamate and glutamine. GSH increased two- to four-fold with fasting along with a 40-50% increase in the content of GCLm and GCLc, a 75% increase in GGT activity, a two- to 2.5-fold increase in GR, G6PDH and GST activities and a 30% increase in PrxVI content. Plasma γ-glutamyl glutamine, γ-glutamyl isoleucine and γ-glutamyl methionine also increased with fasting whereas glutamate and glutamine decreased. Results indicate that GSH biosynthesis increases with fasting and that GSH contributes to counteracting hydroperoxide production, preventing oxidative damage in fasting seals.

Maturation increases superoxide radical production without increasing oxidative damage in the skeletal muscle of hooded seals (Cystophora cristata)

Diving vertebrates represent unique models for the study of the physiological responses to reactive oxygen species (ROS) production and oxidative stress because of their adaptability to cope with dive-derived ROS production. We hypothesized that in the skeletal muscle of a diving mammal, the hooded seal ( Cystophora cristata (Erxleben, 1777)), ROS production increases with maturation but the accumulation of oxidative damage does not. To test this, we analyzed the tissue capacity to produce ROS, the accumulation of oxidative damage, and the activity and protein content of the cooper, zinc, and manganese dependent superoxide dismutases (Cu,ZnSOD, MnSOD) in skeletal muscle from neonates, weaned pups, and adult hooded seals. Our results showed higher tissue capacity to produce ROS, higher Cu,ZnSOD and MnSOD activities, and higher MnSOD protein content in adult seals than in pups. No differences in oxidative damage to lipids, proteins, or DNA were detected among groups. Results suggest that increased SOD activity likely counters the oxidative damage commonly associated with increased ROS production. These findings highlight the unusual tolerance of skeletal muscle of seals to increased ROS production.

Postnatal development of muscle biochemistry in nursing harbor seal (Phoca vitulina) pups: limitations to diving behavior?

Adult marine mammal muscles rely upon a suite of adaptations for sustained aerobic metabolism in the absence of freely available oxygen (O(2)). Although the importance of these adaptations for supporting aerobic diving patterns of adults is well understood, little is known about postnatal muscle development in young marine mammals. However, the typical pattern of vertebrate muscle development, and reduced tissue O(2) stores and diving ability of young marine mammals suggest that the physiological properties of harbor seal (Phoca vitulina) pup muscle will differ from those of adults. We examined myoglobin (Mb) concentration, and the activities of citrate synthase (CS), beta-hydroxyacyl coA dehydrogenase (HOAD), and lactate dehydrogenase (LDH) in muscle biopsies from harbor seal pups throughout the nursing period, and compared these biochemical parameters to those of adults. Pups had reduced O(2) carrying capacity ([Mb] 28-41% lower than adults) and reduced metabolically scaled catabolic enzyme activities (LDH/RMR 20-58% and CS/RMR 29-89% lower than adults), indicating that harbor seal pup muscles are biochemically immature at birth and weaning. This suggests that pup muscles do not have the ability to support either the aerobic or anaerobic performance of adult seals. This immaturity may contribute to the lower diving capacity and behavior in younger pups. In addition, the trends in myoglobin concentration and enzyme activity seen in this study appear to be developmental and/or exercise-driven responses that together work to produce the hypoxic endurance phenotype seen in adults, rather than allometric effects due to body size.