Comparative Respiratory Physiology in Cetaceans

NRBP1 and TSC22D proteins impact distal convoluted tubule physiology through modulation of the WNK pathway

The With No lysine (WNK) kinases regulate processes such as cell volume and epithelial ion transport through the modulation of Cation Chloride Cotransporters such as the NaCl cotransporter, NCC, present in the distal convoluted tubule (DCT) of the kidney. Recently, the interaction of WNKs with Nuclear Receptor Binding Protein 1 (NRBP1) and Transforming Growth Factor β-Stimulated Clone 22 Domain (TSC22D) proteins was reported. Here we explored the effect of NRBP1 and TSC22Ds on WNK signaling in vitro and in the DCT. TSC22D1.1, TSC22D2, and NRBP1 are localized in DCT WNK bodies, which are cytoplasmic biomolecular condensates associated with WNK activation. In HEK293 cells, long TSC22D isoforms and NRBP1 increase WNK4 activity. DCT-specific NRBP1 knockout mice have reduced NCC phosphorylation and activate a compensatory response. Thus, NRBP1 and long TSC22D proteins are positive modulators of WNK signaling and modulate Na+ reabsorption in the kidney. NRBP1 and TSC22Ds likely influence WNK signaling in other tissues, impacting various physiological processes.

Shotgun metabolomic analysis of killer whale (Orcinus orca) exhaled breath condensate

The ocean is facing many anthropogenic stressors caused from both pollution and climate change. These stressors are significantly impacting and changing the ocean's ecosystem, and as such, methods must continually be developed that can improve our ability to monitor the health of marine life. For cetaceans, the current practice for health assessments of individuals requires live capture and release, which is expensive, usually stressful, and for larger species impractical. In this study, we investigated the potential of exhaled breath condensate (EBC) samples to provide unique metabolomic profiles from healthy killer whales (Orcinus orca) of varying known age and sex. EBC collection is a non-invasive procedure that has potential for remote collection using unmanned aerial vehicles, thus improving our ability to understand physiologic parameters within wild populations while minimizing stress from collection procedures. However, descriptions of the available metabolome within EBC and its clinical significance within animals of known health and age must be described before this technique can be considered diagnostically useful. We describe normal variations of the metabolome across age and sex and provide evidence for the potential of this breath analysis method to become a valuable adjunctive tool for assessing the health of managed-care and free-ranging killer whales.

Volumetric imaging and computation to explore contractile function in zebrafish hearts

Despite advancements in cardiovascular engineering, heart diseases remain a leading cause of mortality. The limited understanding of the underlying mechanisms of cardiac dysfunction at the cellular level restricts the development of effective screening and therapeutic methods. To address this, we have developed a framework that incorporates light field detection and individual cell tracking to capture real-time volumetric data in zebrafish hearts, which share structural and electrical similarities with the human heart and generate 120 to 180 beats per minute. Our results indicate that the in-house system achieves an acquisition speed of 200 volumes per second, with resolutions of up to 5.02 ± 0.54 µm laterally and 9.02 ± 1.11 µm axially across the entire depth, using the estimated-maximized-smoothed deconvolution method. The subsequent deep learning-based cell trackers enable further investigation of contractile dynamics, including cellular displacement and velocity, followed by volumetric tracking of specific cells of interest from end-systole to end-diastole in an interactive environment. Collectively, our strategy facilitates real-time volumetric imaging and assessment of contractile dynamics across the entire ventricle at the cellular resolution over multiple cycles, providing significant potential for exploring intercellular interactions in both health and disease.

Cardiorespiratory adaptations in small cetaceans and marine mammals

The dive response, or the 'master switch of life', is probably the most studied physiological trait in marine mammals and is thought to conserve the available O2 for the heart and brain. Although generally thought to be an autonomic reflex, several studies indicate that the cardiovascular changes during diving are anticipatory and can be conditioned. The respiratory adaptations, where the aquatic breathing pattern resembles intermittent breathing in land mammals, with expiratory flow exceeding 160 litres s-1 has been measured in cetaceans, and where exposure to extreme pressures results in alveolar collapse (atelectasis) and recruitment upon ascent. Cardiorespiratory coupling, where breathing results in changes in heart rate, has been proposed to improve gas exchange. Cardiorespiratory coupling has also been reported in marine mammals, and in the bottlenose dolphin, where it alters both heart rate and stroke volume. When accounting for this respiratory dependence on cardiac function, several studies have reported an absence of a diving-related bradycardia except during dives that exceed the duration that is fuelled by aerobic metabolism. This review summarizes what is known about the respiratory physiology in marine mammals, with a special focus on cetaceans. The cardiorespiratory coupling is reviewed, and the selective gas exchange hypothesis is summarized, which provides a testable mechanism for how breath-hold diving vertebrates may actively prevent uptake of N2 during routine dives, and how stress results in failure of this mechanism, which results in diving-related gas emboli.

Evolutionary novelties underlie sound production in baleen whales

Baleen whales (mysticetes) use vocalizations to mediate their complex social and reproductive behaviours in vast, opaque marine environments1. Adapting to an obligate aquatic lifestyle demanded fundamental physiological changes to efficiently produce sound, including laryngeal specializations2-4. Whereas toothed whales (odontocetes) evolved a nasal vocal organ5, mysticetes have been thought to use the larynx for sound production1,6-8. However, there has been no direct demonstration that the mysticete larynx can phonate, or if it does, how it produces the great diversity of mysticete sounds9. Here we combine experiments on the excised larynx of three mysticete species with detailed anatomy and computational models to show that mysticetes evolved unique laryngeal structures for sound production. These structures allow some of the largest animals that ever lived to efficiently produce frequency-modulated, low-frequency calls. Furthermore, we show that this phonation mechanism is likely to be ancestral to all mysticetes and shares its fundamental physical basis with most terrestrial mammals, including humans10, birds11, and their closest relatives, odontocetes5. However, these laryngeal structures set insurmountable physiological limits to the frequency range and depth of their vocalizations, preventing them from escaping anthropogenic vessel noise12,13 and communicating at great depths14, thereby greatly reducing their active communication range.

Bronchial Tree System Analysis of Live Beluga Whale (Delphinapterus leucas) Using Bronchoscopy

Cetaceans, including beluga whales (Delphinapterus leucas), have high morbidity and mortality rates due to bacterial or fungal lower respiratory infections. Bronchoalveolar lavage fluid (BALF) collection by bronchoscopy is beneficial for detecting pathogenic microorganisms in the lower respiratory tract. Efficient and safe bronchoscopy requires characterizing the bronchial tree systems of beluga whales, as no reports exist on bronchial length and bifurcation. In this study, bronchoscopy was performed on five captive beluga whales (9-44 years old) to detect bronchial length and bifurcation. The lengths from the blowhole to the scope impassable points due to the minimized bronchi diameters of the left principal bronchus (LPB), right principal bronchus (RPB), and tracheal bronchus (TB) were 110-155, 110-150, and 80-110 cm, respectively, and were correlated with the body length. Bronchoscopy identified more than 10, 10, and 6 bifurcated bronchi from the LPB, RPB, and TB, respectively. This is the first report to clarify the differences in bronchial tree systems between beluga whales and other cetaceans, as well as the differences for each individual beluga whale. These results could be useful for obtaining BALF via bronchoscopy to detect pathogenic microorganisms causing infections in the lower respiratory tract of beluga whales.

Lymphatic muscle cells are unique cells that undergo aging induced changes

Lymphatic muscle cells (LMCs) within the wall of collecting lymphatic vessels exhibit tonic and autonomous phasic contractions, which drive active lymph transport to maintain tissue-fluid homeostasis and support immune surveillance. Damage to LMCs disrupts lymphatic function and is related to various diseases. Despite their importance, knowledge of the transcriptional signatures in LMCs and how they relate to lymphatic function in normal and disease contexts is largely missing. We have generated a comprehensive transcriptional single-cell atlas-including LMCs-of collecting lymphatic vessels in mouse dermis at various ages. We identified genes that distinguish LMCs from other types of muscle cells, characterized the phenotypical and transcriptomic changes in LMCs in aged vessels, and uncovered a pro-inflammatory microenvironment that suppresses the contractile apparatus in advanced-aged LMCs. Our findings provide a valuable resource to accelerate future research for the identification of potential drug targets on LMCs to preserve lymphatic vessel function as well as supporting studies to identify genetic causes of primary lymphedema currently with unknown molecular explanation.

Cardiorespiratory coupling in the bottlenose dolphin (Tursiops truncatus)

Introduction: The bottlenose dolphin (Tursiops truncatus) is an intermittent breather, where the breath begins with an exhalation followed by inhalation and an extended inter-breath interval ranging from 10 to 40 s. Breathing has been shown to alter both the instantaneous heart rate (if H) and stroke volume (iSV) in the bottlenose dolphin, with a transitory ventilatory tachycardia following the breath, and an exponential decrease to a stable if H around 40 beats • min-1 during the inter-breath period. As the total breath duration in the dolphin is around 1 s, it is not possible to assess the contribution of exhalation and inhalation to these changes in cardiac function during normal breathing. Methods: In the current study, we evaluated the if H response by separating expiration and inspiration of a breath, which allowed us to distinguish their respective contribution to the changes in if H. We studied 3 individual male bottlenose dolphins trained to hold their breath between the different respiratory phases (expiration and inhalation). Results: Our data show that inspiration causes an increase in if H, while expiration appears to result in a decrease in if H. Discussion: These data provide improved understanding of the cardiorespiratory coupling in dolphins, and show how both exhalation and inhalation alters if H.

Allometric scaling of metabolic rate and cardiorespiratory variables in aquatic and terrestrial mammals

While basal metabolic rate (BMR) scales proportionally with body mass (Mb ), it remains unclear whether the relationship differs between mammals from aquatic and terrestrial habitats. We hypothesized that differences in BMR allometry would be reflected in similar differences in scaling of O2 delivery pathways through the cardiorespiratory system. We performed a comparative analysis of BMR across 63 mammalian species (20 aquatic, 43 terrestrial) with a Mb range from 10 kg to 5318 kg. Our results revealed elevated BMRs in small (>10 kg and <100 kg) aquatic mammals compared to small terrestrial mammals. The results demonstrated that minute ventilation, that is, tidal volume (VT )·breathing frequency (fR ), as well as cardiac output, that is, stroke volume·heart rate, do not differ between the two habitats. We found that the "aquatic breathing strategy", characterized by higher VT and lower fR resulting in a more effective gas exchange, and by elevated blood hemoglobin concentrations resulting in a higher volume of O2 for the same volume of blood, supported elevated metabolic requirements in aquatic mammals. The results from this study provide a possible explanation of how differences in gas exchange may serve energy demands in aquatic versus terrestrial mammals.

Role of CD14+ monocyte-derived oxidised mitochondrial DNA in the inflammatory interferon type 1 signature in juvenile dermatomyositis

OBJECTIVES

To define the host mechanisms contributing to the pathological interferon (IFN) type 1 signature in Juvenile dermatomyositis (JDM).

METHODS

RNA-sequencing was performed on CD4+, CD8+, CD14+ and CD19+ cells sorted from pretreatment and on-treatment JDM (pretreatment n=10, on-treatment n=11) and age/sex-matched child healthy-control (CHC n=4) peripheral blood mononuclear cell (PBMC). Mitochondrial morphology and superoxide were assessed by fluorescence microscopy, cellular metabolism by 13C glucose uptake assays, and oxidised mitochondrial DNA (oxmtDNA) content by dot-blot. Healthy-control PBMC and JDM pretreatment PBMC were cultured with IFN-α, oxmtDNA, cGAS-inhibitor, TLR-9 antagonist and/or n-acetyl cysteine (NAC). IFN-stimulated gene (ISGs) expression was measured by qPCR. Total numbers of patient and controls for functional experiments, JDM n=82, total CHC n=35.

RESULTS

Dysregulated mitochondrial-associated gene expression correlated with increased ISG expression in JDM CD14+ monocytes. Altered mitochondrial-associated gene expression was paralleled by altered mitochondrial biology, including 'megamitochondria', cellular metabolism and a decrease in gene expression of superoxide dismutase (SOD)1. This was associated with enhanced production of oxidised mitochondrial (oxmt)DNA. OxmtDNA induced ISG expression in healthy PBMC, which was blocked by targeting oxidative stress and intracellular nucleic acid sensing pathways. Complementary experiments showed that, under in vitro experimental conditions, targeting these pathways via the antioxidant drug NAC, TLR9 antagonist and to a lesser extent cGAS-inhibitor, suppressed ISG expression in pretreatment JDM PBMC.

CONCLUSIONS

These results describe a novel pathway where altered mitochondrial biology in JDM CD14+ monocytes lead to oxmtDNA production and stimulates ISG expression. Targeting this pathway has therapeutical potential in JDM and other IFN type 1-driven autoimmune diseases.

An evolutionary medicine perspective on the cetacean pulmonary immune system - The first identification of SP-D and LBP in the bottlenose dolphin (Tursiops truncatus)

Evolutionary medicine expresses the present status of biomolecules affected by past evolutionary events. To clarify the whole picture of cetacean pneumonia, which is a major threat to cetaceans, their pulmonary immune system should be studied from the perspective of evolutionary medicine. In this in silico study, we focused on cetacean surfactant protein D (SP-D) and lipopolysaccharide-binding protein (LBP) as two representative molecules of the cetacean pulmonary immune system. Sequencing and analyzing SP-D and LBP in the bottlenose dolphin (Tursiops truncatus) lung and liver tissue collected post-mortem elucidated not only basic physicochemical properties but also their evolutionary background. This is the first study to report the sequences and expression of SP-D and LBP in the bottlenose dolphin. Besides, our findings also suggest the direction of an evolutionary arms race in the cetacean pulmonary immune system. These results have important positive implications for cetacean clinical medicine.

Mechanistic insights into robust cardiac I Ks potassium channel activation by aromatic polyunsaturated fatty acid analogues

Voltage-gated potassium (K _V ) channels are important regulators of cellular excitability and control action potential repolarization in the heart and brain. K _V channel mutations lead to disordered cellular excitability. Loss-of-function mutations, for example, result in membrane hyperexcitability, a characteristic of epilepsy and cardiac arrhythmias. Interventions intended to restore K _V channel function have strong therapeutic potential in such disorders. Polyunsaturated fatty acids (PUFAs) and PUFA analogues comprise a class of K _V channel activators with potential applications in the treatment of arrhythmogenic disorders such as Long QT Syndrome (LQTS). LQTS is caused by a loss-of-function of the cardiac I _Ks channel - a tetrameric potassium channel complex formed by K _V 7.1 and associated KCNE1 protein subunits. We have discovered a set of aromatic PUFA analogues that produce robust activation of the cardiac I _Ks channel and a unique feature of these PUFA analogues is an aromatic, tyrosine head group. We determine the mechanisms through which tyrosine PUFA analogues exert strong activating effects on the I _Ks channel by generating modified aromatic head groups designed to probe cation-pi interactions, hydrogen bonding, and ionic interactions. We found that tyrosine PUFA analogues do not activate the I _Ks channel through cation-pi interactions, but instead do so through a combination of hydrogen bonding and ionic interactions.

Virtual clinical trials of anti-PD-1 and anti-CTLA-4 immunotherapy in advanced hepatocellular carcinoma using a quantitative systems pharmacology model

BACKGROUND

Hepatocellular carcinoma (HCC) is the most common form of primary liver cancer and is the third-leading cause of cancer-related death worldwide. Most patients with HCC are diagnosed at an advanced stage, and the median survival for patients with advanced HCC treated with modern systemic therapy is less than 2 years. This leaves the advanced stage patients with limited treatment options. Immune checkpoint inhibitors (ICIs) targeting programmed cell death protein 1 (PD-1) or its ligand, are widely used in the treatment of HCC and are associated with durable responses in a subset of patients. ICIs targeting cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) also have clinical activity in HCC. Combination therapy of nivolumab (anti-PD-1) and ipilimumab (anti-CTLA-4) is the first treatment option for HCC to be approved by Food and Drug Administration that targets more than one immune checkpoints.

METHODS

In this study, we used the framework of quantitative systems pharmacology (QSP) to perform a virtual clinical trial for nivolumab and ipilimumab in HCC patients. Our model incorporates detailed biological mechanisms of interactions of immune cells and cancer cells leading to antitumor response. To conduct virtual clinical trial, we generate virtual patient from a cohort of 5,000 proposed patients by extending recent algorithms from literature. The model was calibrated using the data of the clinical trial CheckMate 040 (ClinicalTrials.gov number, NCT01658878).

RESULTS

Retrospective analyses were performed for different immune checkpoint therapies as performed in CheckMate 040. Using machine learning approach, we predict the importance of potential biomarkers for immune blockade therapies.

CONCLUSIONS

This is the first QSP model for HCC with ICIs and the predictions are consistent with clinically observed outcomes. This study demonstrates that using a mechanistic understanding of the underlying pathophysiology, QSP models can facilitate patient selection and design clinical trials with improved success.

Circus-specific extension of the International Olympic Committee 2020 consensus statement: methods for recording and reporting of epidemiological data on injury and illness in sport

Indepth knowledge of injury and illness epidemiology in circus arts is lacking. Comparing results across studies is difficult due to inconsistent methods and definitions. In 2020, the International Olympic Committee (IOC) consensus group proposed a standard method for recording and reporting epidemiological data on injuries and illnesses in sports and stated that sport-specific extension statements are needed to capture the context of each sport. This is the circus-specific extension to be used with the IOC consensus statement. International circus arts researchers in injury and illness epidemiology and performing arts medicine formed a consensus working group. Consensus statement development included a review of literature, creation of an initial draft by the working group, feedback from external reviewers, integration of feedback into the second draft and a consensus on the final document. This consensus statement contains circus-specific information on (1) injury definitions and characteristics; (2) measures of severity and exposure, with recommendations for calculating the incidence and prevalence; (3) a healthcare practitioner report form; (4) a self-report form capturing health complaints with training and performance exposure; and (5) a demographic, health history and circus experience intake questionnaire. This guideline facilitates comparing results across studies and enables combining data sets on injuries in circus arts. This guideline informs circus-specific injury prevention, rehabilitation, and risk management to improve the performance and health of circus artists.

α2,6-Sialylation is Upregulated in Severe COVID-19 Implicating the Complement Cascade

Better understanding of the mechanisms of COVID-19 severity is desperately needed in current times. Although hyper-inflammation drives severe COVID-19, precise mechanisms triggering this cascade and what role glycosylation might play therein is unknown. Here we report the first high-throughput glycomic analysis of COVID-19 plasma samples and autopsy tissues. We find α2,6-sialylation is upregulated in plasma of patients with severe COVID-19 and in the lung. This glycan motif is enriched on members of the complement cascade, which show higher levels of sialylation in severe COVID-19. In the lung tissue, we observe increased complement deposition, associated with elevated α2,6-sialylation levels, corresponding to elevated markers of poor prognosis (IL-6) and fibrotic response. We also observe upregulation of the α2,6-sialylation enzyme ST6GAL1 in patients who succumbed to COVID-19. Our work identifies a heretofore undescribed relationship between sialylation and complement in severe COVID-19, potentially informing future therapeutic development.

Video-based smartphone app ('VIDEA bewegt') for physical activity support in German adults: a single-armed observational study

OBJECTIVES

The primary objective of this study was to investigate the effect of the video-based smartphone app 'VIDEA bewegt' over eight programme weeks on physical activity in German adults.

DESIGN

The study used a single-arm observational design, assessing the app's effectiveness under real-life conditions. Data were collected from July 2019 to July 2020.

SETTING

The app is enabling users to access video-based educational content via their smartphone. A clinical visit or in-person contact was not required.

PARTICIPANTS

All individuals registered in the freely available app were invited to take part in the study.

INTERVENTIONS

The app aims to increase physical activity in everyday life. It combines educative videos on lifestyle-related benefits and instructional videos of strength and endurance exercises to do at home with motivational components like goal setting, documentation of progress and personalised messages.

PRIMARY AND SECONDARY OUTCOME MEASURES

Primary outcomes were physical activity based one MET minutes per week (metabolic equivalent) and step numbers.Secondary outcomes included physical self-efficacy (motivational, maintenance, recovery self-efficacy), health-related quality of life: Mental Health Component Summary score and Physical Health Component Summary score.

RESULTS

Of 97 people included in the data analysis, 55 successfully completed the programme and all questionnaires. Significant increases over eight programme weeks (between T0 and T2) were observed in physical activity based on MET minutes per week, health-related quality of life, and recovery self-efficacy. Time spent sitting and body mass index significantly decreased for those completing the programme.

CONCLUSIONS

Although significant benefits of physical activity were observed following a complete-case analysis, results should be dealt with caution. Studies with a larger and less heterogeneous sample and robust study designs able to measure causal effects would be desirable.

TRIAL REGISTRATION NUMBER

DRKS00017392.

Diagnosis of Shashi-Pena Syndrome Caused by Chromosomal Rearrangement Using Nanopore Sequencing

Background and Objectives

The aim of this study was to uncover the genetic cause of delayed psychomotor development and variable intellectual disability in a proband whose previous genetic analyses, including chromosome microarray and whole exome sequencing, had been negative.

Methods

Long-read sequencing Oxford Nanopore Technology and RNA-seq analysis were performed on peripheral blood mononuclear cells. Genes with a fold change ≥ 1.5 and p ≤ 0.05 were identified as differentially expressed.

Results

Clinical examinations showed that the proband's features were similar to a rare autosomal-dominant neurodevelopmental syndrome, Shashi-Pena syndrome (MIM #617190). Karyotyping showed that a chromosomal balanced translocation t(2; 11) (p23; q23) was detected in the proband, her father, and her grandmother. Meanwhile, long-read sequencing identified 102 balanced translocations and 145 inversions affecting ASXL2 at an average of 15×. Combined with the family's RNA-seq results, the average mRNA expression of ASXL2 decreased in the patients.

Discussion

We identified a complex chromosomal rearrangement affecting ASXL2 as a pathogenic mechanism of Shashi-Pena syndrome in a Chinese family. This case study suggests that nanopore sequencing is suitable for pathogenic analysis of complex rearrangements, providing new avenues for the diagnosis of genetic diseases.

Scaling of heart rate with breathing frequency and body mass in cetaceans

Plasticity in the cardiac function of a marine mammal facilitates rapid adjustments to the contrasting metabolic demands of breathing at the surface and diving during an extended apnea. By matching their heart rate (fH) to their immediate physiological needs, a marine mammal can improve its metabolic efficiency and maximize the proportion of time spent underwater. Respiratory sinus arrhythmia (RSA) is a known modulation of fH that is driven by respiration and has been suggested to increase cardiorespiratory efficiency. To investigate the presence of RSA in cetaceans and the relationship between fH, breathing rate (fR) and body mass (Mb), we measured simultaneous fH and fR in five cetacean species in human care. We found that a higher fR was associated with a higher mean instantaneous fH (ifH) and minimum ifH of the RSA. By contrast, fH scaled inversely with Mb such that larger animals had lower mean and minimum ifHs of the RSA. There was a significant allometric relationship between maximum ifH of the RSA and Mb, but not fR, which may indicate that this parameter is set by physical laws and not adjusted dynamically with physiological needs. RSA was significantly affected by fR and was greatly reduced with small increases in fR. Ultimately, these data show that surface fHs of cetaceans are complex and the fH patterns we observed are controlled by several factors. We suggest the importance of considering RSA when interpreting fH measurements and particularly how fR may drive fH changes that are important for efficient gas exchange. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.

RESPIRATORY CHANGES IN STRANDED BOTTLENOSE DOLPHINS (TURSIOPS TRUNCATUS)

Lung function (breath duration, respiratory flow [V̇], and tidal volume [V_T]), and end-expiratory O₂ were measured in 19 adult bottlenose dolphins (Tursiops spp.) while at rest in water or beached for up to 10 min. The results show that inspiratory V_T, expiratory V_T, or inspiratory V̇ did not differ on land or in water. The average expiratory V̇ for all dolphins on land decreased by 16%, and the expiratory and total breath durations increased by 5% and 4%, respectively, compared with in water. There were temporal changes observed during beaching, where expired and inspired V_T and inspired V̇ decreased by 13%, 16%, and 9%, respectively, after 10 min on land. These data suggest that dolphins compensate for the effect of gravity by adjusting respiration to maintain alveolar ventilation and gas exchange, but during extended durations, the increased work of breathing may impede ventilation and gas exchange. Continuous monitoring of lung function and gas exchange may help prevent long-term damage during out-of-water medical procedures, optimize animal transport conditions, and improve survival during stranding events.

Lung function assessment in the Pacific walrus (Odobenus rosmarus divergens) while resting on land and submerged in water

In the present study, we examined lung function in healthy resting adult (born in 2003) Pacific walruses (Odobenus rosmarus divergens) by measuring respiratory flow ([Formula: see text]) using a custom-made pneumotachometer. Three female walruses (670-1025 kg) voluntarily participated in spirometry trials while spontaneously breathing on land (sitting and lying down in sternal recumbency) and floating in water. While sitting, two walruses performed active respiratory efforts, and one animal participated in lung compliance measurements. For spontaneous breaths, [Formula: see text] was lower when walruses were lying down (e.g. expiration: 7.1±1.2 l s-1) as compared with in water (9.9±1.4 l s-1), while tidal volume (VT, 11.5±4.6 l), breath duration (4.6±1.4 s) and respiratory frequency (7.6±2.2 breaths min-1) remained the same. The measured VT and specific dynamic lung compliance (0.32±0.07 cmH2O-1) for spontaneous breaths were higher than those estimated for similarly sized terrestrial mammals. VT increased with body mass (allometric mass-exponent=1.29) and ranged from 3% to 43% of the estimated total lung capacity (TLCest) for spontaneous breaths. When normalized for TLCest, the maximal expiratory [Formula: see text] ([Formula: see text]exp) was higher than that estimated in phocids, but lower than that reported in cetaceans and the California sea lion. [Formula: see text]exp was maintained over all lung volumes during spontaneous and active respiratory manoeuvres. We conclude that location (water or land) affects lung function in the walrus and should be considered when studying respiratory physiology in semi-aquatic marine mammals.

Conditioned Variation in Heart Rate During Static Breath-Holds in the Bottlenose Dolphin (Tursiops truncatus)

Previous reports suggested the existence of direct somatic motor control over heart rate (f H) responses during diving in some marine mammals, as the result of a cognitive and/or learning process rather than being a reflexive response. This would be beneficial for O2 storage management, but would also allow ventilation-perfusion matching for selective gas exchange, where O2 and CO2 can be exchanged with minimal exchange of N2. Such a mechanism explains how air breathing marine vertebrates avoid diving related gas bubble formation during repeated dives, and how stress could interrupt this mechanism and cause excessive N2 exchange. To investigate the conditioned response, we measured the f H-response before and during static breath-holds in three bottlenose dolphins (Tursiops truncatus) when shown a visual symbol to perform either a long (LONG) or short (SHORT) breath-hold, or during a spontaneous breath-hold without a symbol (NS). The average f H (if Hstart), and the rate of change in f H (dif H/dt) during the first 20 s of the breath-hold differed between breath-hold types. In addition, the minimum instantaneous f H (if Hmin), and the average instantaneous f H during the last 10 s (if Hend) also differed between breath-hold types. The dif H/dt was greater, and the if Hstart, if Hmin, and if Hend were lower during a LONG as compared with either a SHORT, or an NS breath-hold (P < 0.05). Even though the NS breath-hold dives were longer in duration as compared with SHORT breath-hold dives, the dif H/dt was greater and the if Hstart, if Hmin, and if Hend were lower during the latter (P < 0.05). In addition, when the dolphin determined the breath-hold duration (NS), the f H was more variable within and between individuals and trials, suggesting a conditioned capacity to adjust the f H-response. These results suggest that dolphins have the capacity to selectively alter the f H-response during diving and provide evidence for significant cardiovascular plasticity in dolphins.

Improving estimates of diving lung volume in air-breathing marine vertebrates

The air volume in the respiratory system of marine tetrapods provides a store of O₂ to fuel aerobic metabolism during dives; however, it can also be a liability, as the associated N₂ can increase the risk of decompression sickness. In order to more fully understand the physiological limitations of different air-breathing marine vertebrates, it is therefore important to be able to accurately estimate the air volume in the respiratory system during diving. One method that has been used to do so is to calculate the air volume from glide phases - periods of movement during which no thrust is produced by the animal - which many species conduct during ascent periods, when gases are expanding owing to decreasing hydrostatic pressure. This method assumes that there is conservation of mass in the respiratory system, with volume changes only driven by pressure. In this Commentary, we use previously published data to argue that both the respiratory quotient and differences in tissue and blood gas solubility potentially alter the mass balance in the respiratory system throughout a dive. Therefore, near the end of a dive, the measured volume of gas at a given pressure may be 12-50% less than from the start of the dive; the actual difference will depend on the length of the dive, the cardiac output, the pulmonary shunt and the metabolic rate. Novel methods and improved understanding of diving physiology will be required to verify the size of the effects described here and to more accurately estimate the volume of gas inhaled at the start of a dive.