Peripheral chemoreceptors: function and plasticity of the carotid body

SubSol-HIe is an AMPK-dependent hypoxia-responsive subnucleus of the nucleus tractus solitarius that coordinates the hypoxic ventilatory response and protects against apnoea in mice

Functional magnetic resonance imaging (fMRI) suggests that the hypoxic ventilatory response is facilitated by the AMP-activated protein kinase (AMPK), not at the carotid bodies, but within a subnucleus (Bregma -7.5 to -7.1 mm) of the nucleus tractus solitarius that exhibits right-sided bilateral asymmetry. Here, we map this subnucleus using cFos expression as a surrogate for neuronal activation and mice in which the genes encoding the AMPK-α1 (Prkaa1) and AMPK-α2 (Prkaa2) catalytic subunits were deleted in catecholaminergic cells by Cre expression via the tyrosine hydroxylase promoter. Comparative analysis of brainstem sections, relative to controls, revealed that AMPK-α1/α2 deletion inhibited, with right-sided bilateral asymmetry, cFos expression in and thus activation of a neuronal cluster that partially spanned three interconnected anatomical nuclei adjacent to the area postrema: SolDL (Bregma -7.44 mm to -7.48 mm), SolDM (Bregma -7.44 mm to -7.48 mm) and SubP (Bregma -7.48 mm to -7.56 mm). This approximates the volume identified by fMRI. Moreover, these nuclei are known to be in receipt of carotid body afferent inputs, and catecholaminergic neurons of SubP and SolDL innervate aspects of the ventrolateral medulla responsible for respiratory rhythmogenesis. Accordingly, AMPK-α1/α2 deletion attenuated hypoxia-evoked increases in minute ventilation (normalised to metabolism), reductions in expiration time, and increases sigh frequency, but increased apnoea frequency during hypoxia. The metabolic response to hypoxia in AMPK-α1/α2 knockout mice and the brainstem and spinal cord catecholamine levels were equivalent to controls. We conclude that within the brainstem an AMPK-dependent, hypoxia-responsive subnucleus partially spans SubP, SolDM and SolDL, namely SubSol-HIe, and is critical to coordination of active expiration, the hypoxic ventilatory response and defence against apnoea.

Pioglitazone does not enhance exogenous glucose oxidation or metabolic clearance rate during aerobic exercise in men under acute high-altitude exposure

Insulin insensitivity decreases exogenous glucose oxidation and metabolic clearance rate (MCR) during aerobic exercise in unacclimatized lowlanders at high altitude (HA). Whether use of an oral insulin sensitizer before acute HA exposure enhances exogenous glucose oxidation is unclear. This study investigated the impact of pioglitazone (PIO) on exogenous glucose oxidation and glucose turnover compared with placebo (PLA) during aerobic exercise at HA. With the use of a randomized crossover design, native lowlanders (n = 7 males, means ± SD, age: 23 ± 6 yr, body mass: 84 ± 11 kg) consumed 145 g (1.8 g/min) of glucose while performing 80 min of steady-state (1.43 ± 0.16 V̇o2 L/min) treadmill exercise at HA (460 mmHg; [Formula: see text] 96.6 mmHg) following short-term (5 days) use of PIO (15 mg oral dose per day) or PLA (microcrystalline cellulose pill). Substrate oxidation and glucose turnover were determined using indirect calorimetry and stable isotopes ([13C]glucose and 6,6-[2H2]glucose). Exogenous glucose oxidation was not different between PIO (0.31 ± 0.03 g/min) and PLA (0.32 ± 0.09 g/min). Total carbohydrate oxidation (PIO: 1.65 ± 0.22 g/min, PLA: 1.68 ± 0.32 g/min) or fat oxidation (PIO: 0.10 ± 0.0.08 g/min, PLA: 0.09 ± 0.07 g/min) was not different between treatments. There was no treatment effect on glucose rate of appearance (PIO: 2.46 ± 0.27, PLA: 2.43 ± 0.27 mg/kg/min), disappearance (PIO: 2.19 ± 0.17, PLA: 2.20 ± 0.22 mg/kg/min), or MCR (PIO: 1.63 ± 0.37, PLA: 1.73 ± 0.40 mL/kg/min). Results from this study indicate that PIO is not an effective intervention to enhance exogenous glucose oxidation or MCR during acute HA exposure. Lack of effect with PIO suggests that the etiology of glucose metabolism dysregulation during acute HA exposure may not result from insulin resistance in peripheral tissues.NEW & NOTEWORTHY Short-term (5 days) use of the oral insulin sensitizer pioglitazone does not alter circulating glucose or insulin responses to enhance exogenous glucose oxidation during steady-state aerobic exercise in young healthy men under simulated acute (8 h) high-altitude (460 mmHg) conditions. These results indicate that dysregulations in glucose metabolism in native lowlanders sojourning at high altitude may not be due to insulin resistance at peripheral tissue.

Microglia regulate motor neuron plasticity via reciprocal fractalkine/adenosine signaling

Microglia are innate CNS immune cells that play key roles in supporting key CNS functions including brain plasticity. We now report a previously unknown role for microglia in regulating neuroplasticity within spinal phrenic motor neurons, the neurons driving diaphragm contractions and breathing. We demonstrate that microglia regulate phrenic long-term facilitation (pLTF), a form of respiratory memory lasting hours after repetitive exposures to brief periods of low oxygen (acute intermittent hypoxia; AIH) via neuronal/microglial fractalkine signaling. AIH-induced pLTF is regulated by the balance between competing intracellular signaling cascades initiated by serotonin vs adenosine, respectively. Although brainstem raphe neurons release the relevant serotonin, the cellular source of adenosine is unknown. We tested a model in which hypoxia initiates fractalkine signaling between phrenic motor neurons and nearby microglia that triggers extracellular adenosine accumulation. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF; in contrast, severe AIH drives pLTF by a unique, adenosine-dominant mechanism. Phrenic motor neuron fractalkine knockdown, cervical spinal fractalkine receptor inhibition on nearby microglia, and microglial depletion enhance serotonin-dominant pLTF with moderate AIH but suppress adenosine-dominant pLTF with severe AIH. Thus, microglia play novel functions in the healthy spinal cord, regulating hypoxia-induced neuroplasticity within the motor neurons responsible for breathing.

Neuroanatomical frameworks for volitional control of breathing and orofacial behaviors

Breathing is the only vital function that can be volitionally controlled. However, a detailed understanding how volitional (cortical) motor commands can transform vital breathing activity into adaptive breathing patterns that accommodate orofacial behaviors such as swallowing, vocalization or sniffing remains to be developed. Recent neuroanatomical tract tracing studies have identified patterns and origins of descending forebrain projections that target brain nuclei involved in laryngeal adductor function which is critically involved in orofacial behavior. These nuclei include the midbrain periaqueductal gray and nuclei of the respiratory rhythm and pattern generating network in the brainstem, specifically including the pontine Kölliker-Fuse nucleus and the pre-Bötzinger complex in the medulla oblongata. This review discusses the functional implications of the forebrain-brainstem anatomical connectivity that could underlie the volitional control and coordination of orofacial behaviors with breathing.

Bioelectronic modulation of carotid sinus nerve to treat type 2 diabetes: current knowledge and future perspectives

Bioelectronic medicine are an emerging class of treatments aiming to modulate body nervous activity to correct pathological conditions and restore health. Recently, it was shown that the high frequency electrical neuromodulation of the carotid sinus nerve (CSN), a small branch of the glossopharyngeal nerve that connects the carotid body (CB) to the brain, restores metabolic function in type 2 diabetes (T2D) animal models highlighting its potential as a new therapeutic modality to treat metabolic diseases in humans. In this manuscript, we review the current knowledge supporting the use of neuromodulation of the CSN to treat T2D and discuss the future perspectives for its clinical application. Firstly, we review in a concise manner the role of CB chemoreceptors and of CSN in the pathogenesis of metabolic diseases. Secondly, we describe the findings supporting the potential therapeutic use of the neuromodulation of CSN to treat T2D, as well as the feasibility and reversibility of this approach. A third section is devoted to point up the advances in the neural decoding of CSN activity, in particular in metabolic disease states, that will allow the development of closed-loop approaches to deliver personalized and adjustable treatments with minimal side effects. And finally, we discuss the findings supporting the assessment of CB activity in metabolic disease patients to screen the individuals that will benefit therapeutically from this bioelectronic approach in the future.

Transcriptomic Analysis of Postnatal Rat Carotid Body Development

The carotid body (CB), located bilaterally at the carotid artery bifurcations, is the primary sensory organ for monitoring arterial blood O2 levels. Carotid bodies are immature at birth, exhibiting low sensitivity to hypoxia, and become more sensitive with maturation during the first few weeks of neonatal life. To understand the molecular basis for the postnatal developmental hypoxic responses of CB, we isolated CBs from 5-day and 21-day-old Sprague-Dawley rats and performed RNA sequencing, which allows comprehensive analysis of gene expression. Differentially expressed genes (DEGs) were generated using Edge R, while functional enrichment analysis was performed using gene-set enrichment analysis (GSEA). Analysis of RNA-Seq data showed 2604 DEGs of the total 12,696 genes shared between neonates and adults. Of the 2604 DEGs, 924 genes were upregulated, and 1680 genes were downregulated. Further analysis showed that genes related to oxidative phosphorylation (Ox/phos) and hypoxia-signaling pathways were significantly upregulated in neonatal CBs compared to adult CBs, suggesting a possible link to differential developmental hypoxic responses seen in CB. Genes related to cytokine signaling (INFγ and TNFα) and transcription factors (CREB and NFΚB) mediated pathways were enriched in adult CBs, suggesting that expression of these pathways may be linked to developmental regulation. The RNA-Seq results were verified by analyzing mRNA changes in selected genes by qRT-PCR. Our results of enrichment analysis of biological pathways offer valuable insight into CB hypoxic sensing responses related to the development process.

A role for dopamine in control of the hypoxic ventilatory response via D2 receptors in the zebrafish gill

Dopamine is a neurotransmitter involved in oxygen sensing and control of reflex hyperventilation. In aquatic vertebrates, oxygen sensing occurs in the gills via chemoreceptive neuroepithelial cells (NECs), but a mechanism for dopamine in autonomic control of ventilation has not been defined. We used immunohistochemistry and confocal microscopy to map the distribution of tyrosine hydroxylase (TH), an enzyme necessary for dopamine synthesis, in the gills of zebrafish. TH was found in nerve fibers of the gill filaments and respiratory lamellae. We further identified dopamine active transporter (dat) and vesicular monoamine transporter (vmat2) expression in neurons of the gill filaments using transgenic lines. Moreover, TH- and dat-positive nerve fibers innervated NECs. In chemical screening assays, domperidone, a D2 receptor antagonist, increased ventilation frequency in zebrafish larvae in a dose-dependent manner. When larvae were confronted with acute hypoxia, the D2 agonist, quinpirole, abolished the hyperventilatory response. Quantitative polymerase chain reaction confirmed expression of drd2a and drd2b (genes encoding D2 receptors) in the gills, and their relative abundance decreased following acclimation to hypoxia for 48 h. We localized D2 receptor immunoreactivity to NECs in the efferent gill filament epithelium, and a novel cell type in the afferent filament epithelium. We provide evidence for the synthesis and storage of dopamine by sensory nerve terminals that innervate NECs. We further suggest that D2 receptors on presynaptic NECs provide a feedback mechanism that attenuates the chemoreceptor response to hypoxia. Our studies suggest that a fundamental, modulatory role for dopamine in oxygen sensing arose early in vertebrate evolution.

Blood oxygen regulation via P2Y12R expressed in the carotid body

BACKGROUND

Peripheral blood oxygen monitoring via chemoreceptors in the carotid body (CB) is an integral function of the autonomic cardiorespiratory regulation. The presence of the purinergic P2Y12 receptor (P2Y12R) has been implicated in CB; however, the exact role of the receptor in O2 sensing and signal transduction is unknown.

METHODS

The presence of P2Y12R was established by immunoblotting, RT qPCR and immunohistochemistry. Primary glomus cells were used to assess P2Y12R function during hypoxia and hypercapnia, where monoamines were measured by HPLC; calcium signal was recorded utilizing OGB-1 and N-STORM Super-Resolution System. Ingravescent hypoxia model was tested in anaesthetized mice of mixed gender and cardiorespiratory parameters were recorded in control and receptor-deficient or drug-treated experimental animals.

RESULTS

Initially, the expression of P2Y12R in adult murine CB was confirmed. Hypoxia induced a P2Y12R-dependent release of monoamine transmitters from isolated CB cells. Receptor activation with the endogenous ligand ADP promoted release of neurotransmitters under normoxic conditions, while blockade disrupted the amplitude and duration of the intracellular calcium concentration. In anaesthetised mice, blockade of P2Y12R expressed in the CB abrogated the initiation of compensatory cardiorespiratory changes in hypoxic environment, while centrally inhibited receptors (i.e. microglial receptors) or receptor-deficiency induced by platelet depletion had limited influence on the physiological adjustment to hypoxia.

CONCLUSIONS

Peripheral P2Y12R inhibition interfere with the complex mechanisms of acute oxygen sensing by influencing the calcium signalling and the release of neurotransmitter molecules to evoke compensatory response to hypoxia. Prospectively, the irreversible blockade of glomic receptors by anti-platelet drugs targeting P2Y12Rs, propose a potential, formerly unrecognized side-effect to anti-platelet medications in patients with pulmonary morbidities.

Hypoxic peripheral chemoreflex stimulation-dependent cardiorespiratory coupling is decreased in swimmer athletes

Swimmer athletes showed a decreased ventilatory response and reduced sympathetic activation during peripheral hypoxic chemoreflex stimulation. Based on these observations, we hypothesized that swimmers develop a diminished cardiorespiratory coupling due to their decreased hypoxic peripheral response. To resolve this hypothesis, we conducted a study using coherence time-varying analysis to assess the cardiorespiratory coupling in swimmer athletes. We recruited 12 trained swimmers and 12 control subjects for our research. We employed wavelet time-varying spectral coherence analysis to examine the relationship between the respiratory frequency (Rf ) and the heart rate (HR) time series during normoxia and acute chemoreflex activation induced by five consecutive inhalations of 100% N2 . Comparing swimmers to control subjects, we observed a significant reduction in the hypoxic ventilatory responses to N2 in swimmers (0.012 ± 0.001 vs. 0.015 ± 0.001 ΔVE /ΔVO2 , and 0.365 ± 0.266 vs. 1.430 ± 0.961 ΔVE /ΔVCO2 /ΔSpO2 , both p < 0.001, swimmers vs. control, respectively). Furthermore, the coherence at the LF cutoff during hypoxia was significantly lower in swimmers compared to control subjects (20.118 ± 3.502 vs. 24.935 ± 3.832 area under curve [AUC], p < 0.012, respectively). Our findings strongly indicate that due to their diminished chemoreflex control, swimmers exhibited a substantial decrease in cardiorespiratory coupling during hypoxic stimulation.

Excessive Daytime Sleepiness in Patients With Hypertension: A Systematic Review

We must remember that there are many subclinical cases of obstructive sleep apnea (OSA), even if the patient or family members typically become aware of it through symptoms such as snoring and excessive daytime sleepiness (EDS). EDS is a common symptom among hypertensive patients. This interesting and remarkable systematic review aims to comprehensively survey the current literature on the prevalence and association of EDS among hypertensive patients. PubMed, SCOPUS, Web of Science, and Science Direct were systematically searched for relevant literature. Rayyan QRCI (Rayyan Systems Inc., Cambridge, MA) was employed throughout this comprehensive process. This review included nine studies with a total of 58,517 patients, and 34,398 (58.8%) were males. EDS prevalence among hypertensive patients ranged from 12.1% to 88.3%. This review stated that individuals with hypertension (HTN) had EDS that was worse than that of patients with normotension. In one of the studies included in this analysis, sympathovagal imbalance was noted alongside EDS and HTN. Older age, primary education, being overweight, working, obesity, depression, and having had the condition for longer are all associated with EDS in HTN patients.

Carotid body hypersensitivity in intermittent hypoxia and obtructive sleep apnoea

Carotid bodies are the principal sensory organs for detecting changes in arterial blood oxygen concentration, and the carotid body chemoreflex is a major regulator of the sympathetic tone, blood pressure and breathing. Intermittent hypoxia is a hallmark manifestation of obstructive sleep apnoea (OSA), which is a widespread respiratory disorder. In the first part of this review, we discuss the role of carotid bodies in heightened sympathetic tone and hypertension in rodents treated with intermittent hypoxia, and the underlying cellular, molecular and epigenetic mechanisms. We also present evidence for hitherto-uncharacterized role of carotid body afferents in triggering cellular and molecular changes induced by intermittent hypoxia. In the second part of the review, we present evidence for a contribution of a hypersensitive carotid body to OSA and potential therapeutic intervention to mitigate OSA in a murine model.

Carotid body contribution to the physio-pathological consequences of intermittent hypoxia: role of nitro-oxidative stress and inflammation

Obstructive sleep apnoea (OSA), characterized by chronic intermittent hypoxia (CIH), is considered to be an independent risk for hypertension. The pathological cardiorespiratory consequences of OSA have been attributed to systemic oxidative stress, inflammation and sympathetic overflow induced by CIH, but an emerging body of evidence indicates that a nitro-oxidative and pro-inflammatory milieu within the carotid body (CB) is involved in the potentiation of CB chemosensory responses to hypoxia, which contribute to enhance the sympathetic activity. Accordingly, autonomic and cardiovascular alterations induced by CIH are critically dependent on an abnormally heightened CB chemosensory input to the nucleus of tractus solitarius (NTS), where second-order neurons project onto the rostral ventrolateral medulla (RVLM), activating pre-sympathetic neurons that control pre-ganglionic sympathetic neurons. CIH produces oxidative stress and neuroinflammation in the NTS and RVLM, which may contribute to the long-term irreversibility of the CIH-induced alterations. This brief review is mainly focused on the contribution of nitro-oxidative stress and pro-inflammatory molecules on the hyperactivation of the hypoxic chemoreflex pathway including the CB and the brainstem centres, and whether the persistence of autonomic and cardiorespiratory alterations may depend on the glial-related neuroinflammation induced by the enhanced CB chemosensory afferent input.

Expression of the cellular prion protein by mast cells in the human carotid body

Prion diseases are fatal neurologic disorders that can be transmitted by blood transfusion. The route for neuroinvasion following exposure to infected blood is not known. Carotid bodies (CBs) are specialized chemosensitive structures that detect the concentration of blood gasses and provide feedback for the neural control of respiration. Sensory cells of the CB are highly perfused and densely innervated by nerves that are synaptically connected to the brainstem and thoracic spinal cord, known to be areas of early prion deposition following oral infection. Given their direct exposure to blood and neural connections to central nervous system (CNS) areas involved in prion neuroinvasion, we sought to determine if there were cells in the human CB that express the cellular prion protein (PrPC), a characteristic that would support CBs serving as a route for prion neuroinvasion. We collected CBs from cadaver donor bodies and determined that mast cells located in the carotid bodies express PrPC and that these cells are in close proximity to blood vessels, nerves, and nerve terminals that are synaptically connected to the brainstem and spinal cord.

Does fetal growth restriction induce neuropathology within the developing brainstem?

Fetal growth restriction (FGR) is a complex obstetric issue describing a fetus that does not reach its genetic growth potential. The primary cause of FGR is placental dysfunction resulting in chronic fetal hypoxaemia, which in turn causes altered neurological, cardiovascular and respiratory development, some of which may be pathophysiological, particularly for neonatal life. The brainstem is the critical site of cardiovascular, respiratory and autonomic control, but there is little information describing how chronic hypoxaemia and the resulting FGR may affect brainstem neurodevelopment. This review provides an overview of the brainstem-specific consequences of acute and chronic hypoxia, and what is known in FGR. In addition, we discuss how brainstem structural alterations may impair functional control of the cardiovascular and respiratory systems. Finally, we highlight the clinical and translational findings of the potential roles of the brainstem in maintaining cardiorespiratory adaptation in the transition from fetal to neonatal life under normal conditions and in response to the pathological environment that arises during development in growth-restricted infants. This review emphasises the crucial role that the brainstem plays in mediating cardiovascular and respiratory responses during fetal and neonatal life. We assess whether chronic fetal hypoxaemia might alter structure and function of the brainstem, but this also serves to highlight knowledge gaps regarding FGR and brainstem development.

The glutamatergic drive to breathe is reduced in severe but not moderate hypoxia in Damaraland mole-rats

Damaraland mole-rats (Fukomys damarensis) are a hypoxia-tolerant fossorial species that exhibit a robust hypoxic metabolic response (HMR) and blunted hypoxic ventilatory response (HVR). Whereas the HVR of most adult mammals is mediated by increased excitatory glutamatergic signalling, naked mole-rats, which are closely related to Damaraland mole-rats, do not utilize this pathway. Given their phylogenetic relationship and similar lifestyles, we hypothesized that the signalling mechanisms underlying physiological responses to acute hypoxia in Damaraland mole-rats are like those of naked mole-rats. To test this, we used pharmacological antagonists of glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and N-methyl-d-aspartate receptors (NMDARs), combined with plethysmography, respirometry and thermal RFID chips, to non-invasively evaluate the role of excitatory AMPAR and NMDAR signalling in mediating ventilatory, metabolic and thermoregulatory responses, respectively, to 1 h of 5 or 7% O2. We found that AMPAR or NMDAR antagonism have minimal impacts on the HMR or hypoxia-mediated changes in thermoregulation. Conversely, the 'blunted' HVR of Damaraland mole-rats is reduced by either AMPAR or NMDAR antagonism such that the onset of the HVR occurs in less severe hypoxia. In more severe hypoxia, antagonists have no impact, suggesting that these receptors are already inhibited. Together, these findings indicate that the glutamatergic drive to breathe decreases in Damaraland mole-rats exposed to severe hypoxia. These findings differ from other adult mammals, in which the glutamatergic drive to breathe increases with hypoxia.

Progressive tauopathy disrupts breathing stability and chemoreflexes during presumptive sleep in mice

Rationale: Although sleep apnea occurs in over 50% of individuals with Alzheimer's Disease (AD) or related tauopathies, little is known concerning the potential role of tauopathy in the pathogenesis of sleep apnea. Here, we tested the hypotheses that, during presumptive sleep, a murine model of tauopathy (rTg4510) exhibits: 1) increased breathing instability; 2) impaired chemoreflex function; and 3) exacerbation of these effects with tauopathy progression. Methods: rTg4510 mice initially develop robust tauopathy in the hippocampus and cortex, and eventually progresses to the brainstem. Type I and II post-sigh apnea, Type III (spontaneous) apnea, sigh, and hypopnea incidence were measured in young adult (5-6 months; n = 10-14/group) and aged (13-15 months; n = 22-24/group) non-transgenic (nTg), monogenic control tetracycline transactivator, and bigenic rTg4510 mice using whole-body plethysmography during presumptive sleep (i.e., eyes closed, curled/laying posture, stable breathing for >200 breaths) while breathing room air (21% O2). Peripheral and central chemoreceptor sensitivity were assessed with transient exposures (5 min) to hyperoxia (100% O2) or hypercapnia (3% and 5% CO2 in 21% O2), respectively. Results: We report significant increases in Type I, II, and III apneas (all p < 0.001), sighs (p = 0.002) and hypopneas (p < 0.001) in aged rTg4510 mice, but only Type III apneas in young adult rTg4510 mice (p < 0.001) versus age-matched nTg controls. Aged rTg4510 mice exhibited profound chemoreflex impairment versus age matched nTg and tTA mice. In rTg4510 mice, breathing frequency, tidal volume and minute ventilation were not affected by hyperoxic or hypercapnic challenges, in striking contrast to controls. Histological examination revealed hyperphosphorylated tau in brainstem regions involved in the control of breathing (e.g., pons, medullary respiratory column, retrotrapezoid nucleus) in aged rTg4510 mice. Neither breathing instability nor hyperphosphorylated tau in brainstem tissues were observed in young adult rTg4510 mice. Conclusion: Older rTg4510 mice exhibit profound impairment in the neural control of breathing, with greater breathing instability and near absence of oxygen and carbon-dioxide chemoreflexes. Breathing impairments paralleled tauopathy progression into brainstem regions that control breathing. These findings are consistent with the idea that tauopathy per se undermines chemoreflexes and promotes breathing instability during sleep.

Neurochemical plasticity of the carotid body in hypertension

The carotid body (CB), a main peripheral arterial chemoreceptor, has lately been implicated in the pathophysiology of various cardiovascular disorders. Emerging experimental evidence supports a causal relationship between CB dysfunction and augmented sympathetic outflow which is the common hallmark of human sympathetic-related diseases, including essential hypertension. To gain insight into the neurotransmitter profile of chemosensory cells in the hypertensive CB, we examined the expression and cellular localization of some classical neurotransmitters, neuropeptides, and gaseous signaling molecules as well as neurotrophic factors and their receptors in the CB of spontaneously hypertensive rats, a common animal model of hypertension. Our immunohistochemical experiments revealed an elevated catecholamine and serotonin content in the hypertensive CB compared to normotensive controls. GABA immunostaining was seen in some peripherally located glomus cells in the CB of SHR and it was significantly lower than in control animals. The density of substance P and vasoactive intestinal peptide-immunoreactive fibers was diminished whereas that of neuropeptide Y-immunostained nerve fibers was increased and that of calcitonin gene-related peptide-containing fibers remained almost unchanged in the hypertensive CB. We have further demonstrated that in the hypertensive state the production of nitric oxide is impaired and that the components of the neurotrophin signaling system display an abnormal enhanced expression. Our results provide immunohistochemical evidence that the altered transmitter phenotype of CB chemoreceptor cells and the elevated production of neurotrophic factors modulate the chemosensory processing in hypertensive animals which contributes to autonomic dysfunction and elicits sympathetic hyperactivity, consequently leading to elevated blood pressure.

Magnitude and Mechanism of Phrenic Long-term Facilitation Shift Between Daily Rest Versus Active Phase

Plasticity is a fundamental property of the neural system controlling breathing. One key example of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a persistent increase in phrenic nerve activity elicited by acute intermittent hypoxia (AIH). pLTF can arise from distinct cell signaling cascades initiated by serotonin versus adenosine receptor activation, respectively, and interact via powerful cross-talk inhibition. Here, we demonstrate that the daily rest/active phase and the duration of hypoxic episodes within an AIH protocol have profound impact on the magnitude and mechanism of pLTF due to shifts in serotonin/adenosine balance. Using the historical "standard" AIH protocol (3, 5-min moderate hypoxic episodes), we demonstrate that pLTF magnitude is unaffected by exposure in the midactive versus midrest phase, yet the mechanism driving pLTF shifts from serotonin-dominant (midrest) to adenosine-dominant (midactive). This mechanistic "flip" results from combined influences of hypoxia-evoked adenosine release and daily fluctuations in basal spinal adenosine. Since AIH evokes less adenosine with shorter (15, 1-min) hypoxic episodes, midrest pLTF is amplified due to diminished adenosine constraint on serotonin-driven plasticity; in contrast, elevated background adenosine during the midactive phase suppresses serotonin-dominant pLTF. These findings demonstrate the importance of the serotonin/adenosine balance in regulating the amplitude and mechanism of AIH-induced pLTF. Since AIH is emerging as a promising therapeutic modality to restore respiratory and nonrespiratory movements in people with spinal cord injury or ALS, knowledge of how time-of-day and hypoxic episode duration impact the serotonin/adenosine balance and the magnitude and mechanism of pLTF has profound biological, experimental, and translational implications.

Autonomic control of ventricular function in health and disease: current state of the art

PURPOSE

Cardiac autonomic dysfunction is one of the main pillars of cardiovascular pathophysiology. The purpose of this review is to provide an overview of the current state of the art on the pathological remodeling that occurs within the autonomic nervous system with cardiac injury and available neuromodulatory therapies for autonomic dysfunction in heart failure.

METHODS

Data from peer-reviewed publications on autonomic function in health and after cardiac injury are reviewed. The role of and evidence behind various neuromodulatory therapies both in preclinical investigation and in-use in clinical practice are summarized.

RESULTS

A harmonic interplay between the heart and the autonomic nervous system exists at multiple levels of the neuraxis. This interplay becomes disrupted in the setting of cardiovascular disease, resulting in pathological changes at multiple levels, from subcellular cardiac signaling of neurotransmitters to extra-cardiac, extra-thoracic remodeling. The subsequent detrimental cycle of sympathovagal imbalance, characterized by sympathoexcitation and parasympathetic withdrawal, predisposes to ventricular arrhythmias, progression of heart failure, and cardiac mortality. Knowledge on the etiology and pathophysiology of this condition has increased exponentially over the past few decades, resulting in a number of different neuromodulatory approaches. However, significant knowledge gaps in both sympathetic and parasympathetic interactions and causal factors that mediate progressive sympathoexcitation and parasympathetic dysfunction remain.

CONCLUSIONS

Although our understanding of autonomic imbalance in cardiovascular diseases has significantly increased, specific, pivotal mediators of this imbalance and the recognition and implementation of available autonomic parameters and neuromodulatory therapies are still lagging.

The effects of P2Y12 adenosine receptors' inhibitors on central and peripheral chemoreflexes

Introduction: The most common side effect of ticagrelor is dyspnea, which leads to premature withdrawal of this life-saving medication in 6.5% of patients. Increased chemoreceptors' sensitivity was suggested as a possible pathophysiological explanation of this phenomenon; however, the link between oversensitization of peripheral and/or central chemosensory areas and ticagrelor intake has not been conclusively proved. Methods: We measured peripheral chemoreceptors' sensitivity using hypoxic ventilatory response (HVR), central chemoreceptors' sensitivity using hypercapnic hyperoxic ventilatory response (HCVR), and dyspnea severity before and 4 ± 1 weeks following ticagrelor initiation in 11 subjects with chronic coronary syndrome undergoing percutaneous coronary intervention (PCI). The same tests were performed in 11 age-, sex-, and BMI-matched patients treated with clopidogrel. The study is registered at ClinicalTrials.com at NCT05080478. Results: Ticagrelor significantly increased both HVR (0.52 ± 0.46 vs. 0.84 ± 0.69 L min-1 %-1; p < 0.01) and HCVR (1.05 ± 0.64 vs. 1.75 ± 1.04 L min-1 mmHg-1; p < 0.01). The absolute change in HVR correlated with the change in HCVR. Clopidogrel administration did not significantly influence HVR (0.63 ± 0.32 vs. 0.58 ± 0.33 L min-1%-1; p = 0.53) and HCVR (1.22 ± 0.67 vs. 1.2 ± 0.64 L min-1 mmHg-1; p = 0.79). Drug-related dyspnea was reported by three subjects in the ticagrelor group and by none in the clopidogrel group. These patients were characterized by either high baseline HVR and HCVR or excessive increase in HVR following ticagrelor initiation. Discussion: Ticagrelor, contrary to clopidogrel, sensitizes both peripheral and central facets of chemodetection. Two potential mechanisms of ticagrelor-induced dyspnea have been identified: 1) high baseline HVR and HCVR or 2) excessive increase in HVR or HVR and HCVR. Whether other patterns of changes in chemosensitivities play a role in the pathogenesis of this phenomenon needs to be further investigated.

Hypoxia sensing requires H2S-dependent persulfidation of olfactory receptor 78

Oxygen (O2) sensing by the carotid body is critical for maintaining cardiorespiratory homeostasis during hypoxia. Hydrogen sulfide (H2S) signaling is implicated in carotid body activation by low O2. Here, we show that persulfidation of olfactory receptor 78 (Olfr78) by H2S is an integral component of carotid body activation by hypoxia. Hypoxia and H2S increased persulfidation in carotid body glomus cells and persulfidated cysteine240 in Olfr78 protein in heterologous system. Olfr78 mutants manifest impaired carotid body sensory nerve, glomus cell, and breathing responses to H2S and hypoxia. Glomus cells are positive for GOlf, adenylate cyclase 3 (Adcy3) and cyclic nucleotide-gated channel alpha 2 (Cnga2), key molecules of odorant receptor signaling. Adcy3 or Cnga2 mutants exhibited impaired carotid body and glomus cell responses to H2S and breathing responses to hypoxia. These results suggest that H2S through redox modification of Olfr78 participates in carotid body activation by hypoxia to regulate breathing.

APOE4, Age, and Sex Regulate Respiratory Plasticity Elicited by Acute Intermittent Hypercapnic-Hypoxia

Rationale

Acute intermittent hypoxia (AIH) shows promise for enhancing motor recovery in chronic spinal cord injuries and neurodegenerative diseases. However, human trials of AIH have reported significant variability in individual responses.

Objectives

Identify individual factors (eg, genetics, age, and sex) that determine response magnitude of healthy adults to an optimized AIH protocol, acute intermittent hypercapnic-hypoxia (AIHH).

Methods

In 17 healthy individuals (age = 27 ± 5 yr), associations between individual factors and changes in the magnitude of AIHH (15, 1-min O2 = 9.5%, CO2 = 5% episodes) induced changes in diaphragm motor-evoked potential (MEP) amplitude and inspiratory mouth occlusion pressures (P0.1) were evaluated. Single nucleotide polymorphisms (SNPs) in genes linked with mechanisms of AIH induced phrenic motor plasticity (BDNF, HTR2A, TPH2, MAOA, NTRK2) and neuronal plasticity (apolipoprotein E, APOE) were tested. Variations in AIHH induced plasticity with age and sex were also analyzed. Additional experiments in humanized (h)ApoE knock-in rats were performed to test causality.

Results

AIHH-induced changes in diaphragm MEP amplitudes were lower in individuals heterozygous for APOE4 (i.e., APOE3/4) compared to individuals with other APOE genotypes (P = 0.048) and the other tested SNPs. Males exhibited a greater diaphragm MEP enhancement versus females, regardless of age (P = 0.004). Additionally, age was inversely related with change in P0.1 (P = 0.007). In hApoE4 knock-in rats, AIHH-induced phrenic motor plasticity was significantly lower than hApoE3 controls (P < 0.05).

Conclusions

APOE4 genotype, sex, and age are important biological determinants of AIHH-induced respiratory motor plasticity in healthy adults.

Addition to Knowledge Base

AIH is a novel rehabilitation strategy to induce functional recovery of respiratory and non-respiratory motor systems in people with chronic spinal cord injury and/or neurodegenerative disease. Figure 5 Since most AIH trials report considerable inter-individual variability in AIH outcomes, we investigated factors that potentially undermine the response to an optimized AIH protocol, AIHH, in healthy humans. We demonstrate that genetics (particularly the lipid transporter, APOE), age and sex are important biological determinants of AIHH-induced respiratory motor plasticity.

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

Hypercapnic signaling influences hypoxic signaling in the control of breathing in C57BL6 mice

Interactions between hypoxic and hypercapnic signaling pathways, expressed as ventilatory changes occurring during and following a simultaneous hypoxic-hypercapnic gas challenge (HH-C) have not been determined systematically in mice. This study in unanesthetized male C57BL6 mice addressed the hypothesis that hypoxic (HX) and hypercapnic (HC) signaling events display an array of interactions indicative of coordination by peripheral and central respiratory mechanisms. We evaluated the ventilatory responses elicited by hypoxic (HX-C, 10%, O2, 90% N2), hypercapnic (HC-C, 5% CO2, 21%, O2, 90% N2), and HH-C (10% O2, 5%, CO2, 85% N2) challenges to determine whether ventilatory responses elicited by HH-C were simply additive of responses elicited by HX-C and HC-C, or whether other patterns of interactions existed. Responses elicited by HH-C were additive for tidal volume, minute ventilation and expiratory time, among others. Responses elicited by HH-C were hypoadditive of the HX-C and HC-C responses (i.e., HH-C responses were less than expected by simple addition of HX-C and HC-C responses) for frequency of breathing, inspiratory time and relaxation time, among others. In addition, end-expiratory pause increased during HX-C, but decreased during HC-C and HH-C, therefore showing that HC-C responses influenced the HX-C responses when given simultaneously. Return to room-air responses was additive for tidal volume and minute ventilation, among others, whereas they were hypoadditive for frequency of breathing, inspiratory time, peak inspiratory flow, apneic pause, inspiratory and expiratory drives, and rejection index. These data show that HX-C and HH-C signaling pathways interact with one another in additive and often hypoadditive processes.NEW & NOTEWORTHY We present data showing that the ventilatory responses elicited by a hypoxic gas challenge in male C57BL6 mice are markedly altered by coexposure to hypercapnic gas challenge with hypercapnic responses often dominating the hypoxic responses. These data suggest that hypercapnic signaling processes activated within brainstem regions, such as the retrotrapezoid nuclei, may directly modulate the signaling processes within the nuclei tractus solitarius resulting from hypoxic-induced increase in carotid body chemoreceptor input to these nuclei.

Carotid Artery Stenosis: A Look Into the Diagnostic and Management Strategies, and Related Complications

Carotid stenosis (CS) is a buildup of atherosclerotic plaque within the artery leading to a wide range of symptoms, from mild symptoms, including blurred vision and confusion, to much more life-threatening presentations, including paralysis due to stroke. The presentation is insidious, with symptoms exhibiting predominantly at severe stenosis; hence the emphasis is placed on the importance of early diagnosis, treatment, and lifestyle modifications. CS is seen undergoing almost the same pathogenesis of any atherosclerotic plaque formation, from endothelial damage of the artery lumen to the formation of a fibrous cap with a foam cell, lipid-filled core. The findings of our review article were consistent with the recent literature, depicting that comorbid hypertension, diabetes, and chronic kidney disease (CKD), and lifestyle aspects, including smoking and diet, played the most salient role in plaque development. Among several imaging modalities, duplex ultrasound (DUS) imaging is the widely preferred method in clinical practice. Carotid endarterectomy (CEA) and carotid stenting are the primarily advocated procedures for symptomatic severe stenosis, with similar long-term outcomes. Although, earlier clinical trials showed promising results in mitigating the risk of stroke among asymptomatic severe CS with surgical intervention. However, recent advancements have shifted the focus to medical management alone due to comparable results among the asymptomatic population. Both surgical and medical regimens are beneficial in treating patients, but it is still an ongoing debate as to which is predominantly superior. The currently advancing trials and research will help elucidate definitive guidelines. However, the massive impact of lifestyle modifications advocates some degree of individualized multidisciplinary management strategies.

Autonomic Cardiovascular Control in Health and Disease

Autonomic neural control of the cardiovascular system is formed of complex and dynamic processes able to adjust rapidly to mitigate perturbations in hemodynamics and maintain homeostasis. Alterations in autonomic control feature in the development or progression of a multitude of diseases with wide-ranging physiological implications given the neural system's responsibility for controlling inotropy, chronotropy, lusitropy, and dromotropy. Imbalances in sympathetic and parasympathetic neural control are also implicated in the development of arrhythmia in several cardiovascular conditions sparking interest in autonomic modulation as a form of treatment. A number of measures of autonomic function have shown prognostic significance in health and in pathological states and have undergone varying degrees of refinement, yet adoption into clinical practice remains extremely limited. The focus of this contemporary narrative review is to summarize the anatomy, physiology, and pathophysiology of the cardiovascular autonomic nervous system and describe the merits and shortfalls of testing modalities available. © 2023 American Physiological Society. Compr Physiol 13:4493-4511, 2023.

Assessment of the Carotid Bodies in Magnetic Resonance-A Head-to-Head Comparison with Computed Tomography

OBJECTIVES

To evaluate carotid body visibility in contrast-enhanced magnetic resonance (MR) studies and to compare the results to contrast-enhanced computed tomography (CT).

METHODS

Two observers separately evaluated MR and CT examinations of 58 patients. MR scans were acquired with contrast-enhanced isometric T1-weighted water-only Dixon sequence. CT examinations were performed 90 s after contrast agent administration. Carotid bodies' dimensions were noted and their volumes calculated. To quantify the agreement between both methods, Bland-Altman plots were computed. Receiver operating characteristic (ROC) and its localization-oriented variant (LROC) curves were plotted.

RESULTS

Of the 116 expected carotid bodies, 105 were found on CT and 103 on MR at least by a single observer. Significantly more findings were concordant in CT (92.2%) than in MR (83.6%). The mean carotid body volume was smaller in CT (19.4 mm³) than in MR (20.8 mm³). The inter-observer agreement on volumes was moderately good (ICC (2,k) 0.42, p < 0.001), but with significant systematic error. The diagnostic performance of the MR method added up to 88.4% of the ROC's area under the curve and 78.0% in the LROC algorithm.

CONCLUSIONS

Carotid bodies can be visualized on contrast-enhanced MR with good accuracy and inter-observer agreement. Carotid bodies assessed on MR had similar morphology as described in anatomical studies.

TASK inhibition by mild acidosis increases Ca2+ oscillations to mediate pH sensing in rat carotid body chemoreceptor cells

Severe levels of acidosis (pH < 6.8) have been shown to cause a sustained rise in cytosolic Ca2+ concentration in carotid body Type 1 (glomus) cells. To understand how physiologically relevant levels of acidosis regulate Ca2+ signaling in glomus cells, we studied the effects of small changes in extracellular pH (pHo) on the kinetics of Ca2+ oscillations. A decrease in pHo from 7.4 to 7.3 (designated mild) and 7.2 (designated moderate) acidosis produced significant increases in the frequency and amplitude of Ca2+ oscillations. These effects of acidosis on Ca2+ oscillations were not blocked by NS383 and amiloride [acid-sensing ion channel (ASIC) inhibitors]. Mild and moderate levels of acidosis, however, caused a small but significant inhibition of two-pore domain acid-sensing K+ channels (TASK) (TASK-1- and TASK-3-like channels) and depolarized the cell by 6-13 mV. Acidosis-induced increase in Ca2+ oscillations was inhibited by nifedipine (1 µM; L-type Cav inhibitor) and by TTA-P2 (20 µM; T-type Cav inhibitor). Mild inhibition of TASK activity by N-[(2,4-difluorophenyl)methyl]-2'-[[[2-(4methoxyphenyl)acetyl]amino]methyl][1,1'-biphenyl]-2-carboxamide (A1899) (0.3 µM) and 1-[1-[6-[[1,1'-biphenyl]-4-ylcarbonyl)-5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine-4-yl]-4-piperidinyl]-1-butanon (PK-THPP) (0.1 µM) increased Ca2+ oscillation frequency to levels similar to those observed with mild-moderate acidosis. Mild acidosis (pHo 7.3) and mild hypoxia (∼5%O2) produced similar levels of changes in the kinetics of Ca2+ oscillations. Block of tetraethylammonium (TEA)-sensitive Kv channels did not affect acid-induced increase in Ca2+ oscillations. Our study shows that mild and moderate levels of acidosis increase the frequency and amplitude of Ca2+ oscillations primarily by inhibition of TASK without involving ASICs, and suggests a major role of TASK for signal transduction in response to a physiological change in pHo.

Reactive Oxygen Species and NRF2 Signaling, Friends or Foes in Cancer?

The imbalance between reactive oxygen species (ROS) production and clearance causes oxidative stress and ROS, which play a central role in regulating cell and tissue physiology and pathology. Contingent upon concentration, ROS influence cancer development in contradictory ways, either stimulating cancer survival and growth or causing cell death. Cells developed evolutionarily conserved programs to sense and adapt redox the fluctuations to regulate ROS as either signaling molecules or toxic insults. The transcription factor nuclear factor erythroid 2-related factor 2 (NRF2)-KEAP1 system is the master regulator of cellular redox and metabolic homeostasis. NRF2 has Janus-like roles in carcinogenesis and cancer development. Short-term NRF2 activation suppresses tissue injury, inflammation, and cancer initiation. However, cancer cells often exhibit constitutive NRF2 activation due to genetic mutations or oncogenic signaling, conferring advantages for cancer cells' survival and growth. Emerging evidence suggests that NRF2 hyperactivation, as an adaptive cancer phenotype under stressful tumor environments, regulates all hallmarks of cancer. In this review, we summarized the source of ROS, regulation of ROS signaling, and cellular sensors for ROS and oxygen (O₂), we reviewed recent progress on the regulation of ROS generation and NRF2 signaling with a focus on the new functions of NRF2 in cancer development that reach beyond what we originally envisioned, including regulation of cancer metabolism, autophagy, macropinocytosis, unfolded protein response, proteostasis, and circadian rhythm, which, together with anti-oxidant and drug detoxification enzymes, contributes to cancer development, metastasis, and anticancer therapy resistance.

Acute effects of insulin and insulin-induced hypoglycaemia on carotid body chemoreceptor activity and cardiorespiratory responses in dogs

NEW FINDINGS

What is the central question of this study? What are the effects of insulin and insulin-induced hypoglycaemia on carotid body chemoreceptor activity in vivo and how do carotid body chemoreceptor stimulation-mediated cardiorespiratory responses in beagle dogs compare during euglycaemia and insulin-induced hypoglycaemia? What is the main finding and its importance? Intracarotid insulin administration leads to sustained increase in carotid body chemoreceptor activity and respiratory response with significant cardiovascular effects. Insulin-induced hypoglycaemia exacerbated NaCN-mediated carotid body chemoreceptor activity and respiratory response with enhanced cardiovascular reflex response. These findings suggest that insulin-induced hypoglycaemia augments the carotid body chemoreceptors to initiate the adaptive counter-regulatory responses to restore the normoglycaemic condition.

ABSTRACT

The carotid body chemoreceptors (CBC) play an important role in the adaptive counter-regulatory response to hypoglycaemia by evoking the CBC-mediated sympathetic neuronal system to restore normoglycaemia. Ex vivo studies have shown varied responses of insulin-induced hypoglycaemia on CBC function, and several in vivo studies have indirectly established the role of CBCs in restoring normoglycaemia in both animals and humans. However, a direct effect of insulin and/or insulin-induced hypoglycaemia on CBC activity is not established in animal models. Therefore, the aim of this study was to evaluate in vivo effects of insulin and insulin-induced hypoglycaemia on CBC activity and cardiorespiration in a preclinical large animal model. The carotid sinus nerve (CSN) activity and cardiorespiratory responses to sodium cyanide (NaCN; 25 µg/kg) were compared before (euglycaemic) and after (hypoglycaemic) intracarotid administration of insulin (12.5-100 µU/dogs) in beagle dogs. Insulin administration increased CSN activity and minute ventilation (V ̇ $\dot V$ E ) with significant (P < 0.0001) effects on heart rate and blood pressure. Insulin-mediated effects on CSN and cardiorespiration were sustained and the change inV ̇ $\dot V$ E was driven by tidal volume only. Insulin significantly (P < 0.0001) lowered blood glucose level. NaCN-mediated CSN activity andV ̇ $\dot V$ E were significantly (P < 0.0001) augmented during insulin-induced hypoglycaemia. The augmentedV ̇ $\dot V$ E was primarily driven by respiratory frequency and partially by tidal volume. The cardiovascular reflex response mediated through CBC stimulation was significantly (P < 0.0001) exacerbated during insulin-induced hypoglycaemia. Collectively, these results demonstrate direct effects of insulin and insulin-induced hypoglycaemia on CBC chemosensitivity to potentiate CBC-mediated neuroregulatory pathways to initiate adaptive neuroendocrine and cardiorespiratory counter-regulatory responses to restore normoglycaemia.

Pulmonary ventilation and gas exchange during prolonged exercise in humans: Influence of dehydration, hyperthermia and sympathoadrenal activity

NEW FINDINGS

What is the central question of the study? Ventilation increases during prolonged intense exercise, but the impact of dehydration and hyperthermia, with associated blunting of pulmonary circulation, and independent influences of dehydration, hyperthermia and sympathoadrenal discharge on ventilatory and pulmonary gas exchange responses remain unclear. What is the main finding and its importance? Dehydration and hyperthermia led to hyperventilation and compensatory adjustments in pulmonary CO2 and O2 exchange, such that CO2 output increased and O2 uptake remained unchanged despite the blunted circulation. Isolated hyperthermia and adrenaline infusion, but not isolated dehydration, increased ventilation to levels similar to combined dehydration and hyperthermia. Hyperthermia is the main stimulus increasing ventilation during prolonged intense exercise, partly via sympathoadrenal activation.

ABSTRACT

The mechanisms driving hyperthermic hyperventilation during exercise are unclear. In a series of retrospective analyses, we evaluated the impact of combined versus isolated dehydration and hyperthermia and the effects of sympathoadrenal discharge on ventilation and pulmonary gas exchange during prolonged intense exercise. In the first study, endurance-trained males performed two submaximal cycling exercise trials in the heat. On day 1, participants cycled until volitional exhaustion (135 ± 11 min) while experiencing progressive dehydration and hyperthermia. On day 2, participants maintained euhydration and core temperature (Tc ) during a time-matched exercise (control). At rest and during the first 20 min of exercise, pulmonary ventilation (V ̇ E ${\skew2\dot V_{\rm{E}}}$ ), arterial blood gases, CO2 output and O2 uptake were similar in both trials. At 135 ± 11 min, however,V ̇ E ${\skew2\dot V_{\rm{E}}}$ was elevated with dehydration and hyperthermia, and this was accompanied by lower arterial partial pressure of CO2 , higher breathing frequency, arterial partial pressure of O2 , arteriovenous CO2 and O2 differences, and elevated CO2 output and unchanged O2 uptake despite a reduced pulmonary circulation. The increasedV ̇ E ${\skew2\dot V_{\rm{E}}}$ was closely related to the rise in Tc and circulating catecholamines (R2  ≥ 0.818, P ≤ 0.034). In three additional studies in different participants, hyperthermia independently increasedV ̇ E ${\skew2\dot V_{\rm{E}}}$ to an extent similar to combined dehydration and hyperthermia, whereas prevention of hyperthermia in dehydrated individuals restoredV ̇ E ${\skew2\dot V_{\rm{E}}}$ to control levels. Furthermore, adrenaline infusion during exercise elevated both Tc andV ̇ E ${\skew2\dot V_{\rm{E}}}$ . These findings indicate that: (1) adjustments in pulmonary gas exchange limit homeostatic disturbances in the face of a blunted pulmonary circulation; (2) hyperthermia is the main stimulus increasing ventilation during prolonged intense exercise; and (3) sympathoadrenal activation might partly mediate the hyperthermic hyperventilation.

Mechanisms of Chemosensory Transduction in the Carotid Body

The mammalian carotid body (CB) is a polymodal chemoreceptor, which is activated by blood-borne stimuli, most notably hypoxia, hypercapnia and acidosis, thus ensuring an appropriate cellular response to changes in physical and chemical parameters of the blood. The glomus cells are considered the CB chemosensory cells and the initial site of chemoreceptor transduction. However, the molecular mechanisms by which they detect changes in blood chemical levels and how these changes lead to transmitter release are not yet well understood. Chemotransduction mechanisms are by far best described for oxygen and acid/carbon dioxide sensing. A few testable hypotheses have been postulated including a direct interaction of oxygen with ion channels in the glomus cells (membrane hypothesis), an indirect interface by a reversible ligand like a heme (metabolic hypothesis), or even a functional interaction between putative oxygen sensors (chemosome hypothesis) or the interaction of lactate with a highly expressed in the CB atypical olfactory receptor, Olfr78, (endocrine model). It is also suggested that sensory transduction in the CB is uniquely dependent on the actions and interactions of gaseous transmitters. Apparently, oxygen sensing does not utilize a single mechanism, and later observations have given strong support to a unified membrane model of chemotransduction.

Neurochemical Plasticity of the Carotid Body

A striking feature of the carotid body (CB) is its remarkable degree of plasticity in a variety of neurotransmitter/modulator systems in response to environmental stimuli, particularly following hypoxic exposure of animals and during ascent to high altitude. Current evidence suggests that acetylcholine and adenosine triphosphate are two major excitatory neurotransmitter candidates in the hypoxic CB, and they may also be involved as co-transmitters in hypoxic signaling. Conversely, dopamine, histamine and nitric oxide have recently been considered inhibitory transmitters/modulators of hypoxic chemosensitivity. It has also been revealed that interactions between excitatory and inhibitory messenger molecules occur during hypoxia. On the other hand, alterations in purinergic neurotransmitter mechanisms have been implicated in ventilatory acclimatization to hypoxia. Chronic hypoxia also induces profound changes in other neurochemical systems within the CB such as the catecholaminergic, peptidergic and nitrergic, which in turn may contribute to increased ventilatory and chemoreceptor responsiveness to hypoxia at high altitude. Taken together, current data suggest that complex interactions among transmitters markedly influence hypoxia-induced transmitter release from the CB. In addition, the expression of a wide variety of growth factors, proinflammatory cytokines and their receptors have been identified in CB parenchymal cells in response to hypoxia and their upregulated expression could mediate the local inflammation and functional alteration of the CB under hypoxic conditions.

General Morphology of the Mammalian Carotid Body

The carotid body (CB) is the main peripheral arterial chemoreceptor that registers the levels of pO2, pCO2 and pH in the blood and responds to their changes by regulating breathing. It is strategically located in the bifurcation of each common carotid artery. The organ consists of "glomera" composed of two cell types, glomus and sustentacular cells, interspersed by blood vessels and nerve bundles and separated by connective tissue. The neuron-like glomus or type I cells are considered as the chemosensory cells of the CB. They contain numerous cytoplasmic organelles and dense-cored vesicles that store and release neurotransmitters. They also form both conventional chemical and electrical synapses between each other and are contacted by peripheral nerve endings of petrosal ganglion neurons. The glomus cells are dually innervated by both sensory nerve fibers through the carotid sinus nerve and autonomic fibers of sympathetic origin via the ganglioglomerular nerve. The parasympathetic efferent innervation is relayed by vasomotor fibers of ganglion cells located around or inside the CB. The glial-like sustentacular or type II cells are regarded to be supporting cells although they sustain physiologic neurogenesis in the adult CB and are thus supposed to be progenitor cells as well. The CB is a highly vascularized organ and its intraorgan hemodynamics possibly plays a role in the process of chemoreception.

Carotid Body Dysfunction and Mechanisms of Disease

Emerging evidence shows that the carotid body (CB) dysfunction is implicated in various physiological and pathophysiological conditions. It has been revealed that the CB structure and neurochemical profile alter in certain human sympathetic-related and cardiometabolic diseases. Specifically, a tiny CB with a decrease of glomus cells and their dense-cored vesicles has been seen in subjects with sleep disordered breathing such as sudden infant death syndrome and obstructive sleep apnea patients and people with congenital central hypoventilation syndrome. Moreover, the CB degranulation is accompanied by significantly elevated levels of catecholamines and proinflammatory cytokines in such patients. The intermittent hypoxia stimulates the CB, eliciting augmented chemoreflex drive and enhanced cardiorespiratory and sympathetic responses. High CB excitability due to blood flow restrictions, oxidative stress, alterations in neurotransmitter gases and disruptions of local mediators is also observed in congestive heart failure conditions. On the other hand, the morpho-chemical changes in hypertension include an increase in the CB volume due to vasodilation, altered transmitter phenotype of chemoreceptor cells and elevated production of neurotrophic factors. Accordingly, in both humans and animal models CB denervation prevents the breathing instability and lowers blood pressure. Knowledge of the morphofunctional aspects of the CB, a better understanding of its role in disease and recent advances in human CB translational research would contribute to the development of new therapeutic strategies.

History and Recent Progress in Carotid Body Studies

This chapter describes the history of the carotid body (CB) and the subsequent research on its structure and function. The chronological development of ideas about its anatomical structure as a ganglion, the first descriptions of its glandular nature as a ball of highly vascular tissue (glomus), the discovery of its neural crest origin and relevant embryological views as a true paraganglion toward a more conclusive understanding of its sensory nature as a chemoreceptor for chemical changes in blood have been consistently demonstrated. The knowledge of the CB neurochemistry, physiology and pathophysiology has progressed immensely in the past century and a large and compelling body of evidence for the presence of a neurogenic niche in the CB has accumulated over the last two decades, thus underlying its function and possibility for the development of cell replacement therapies.

Of Mice and Men and Plethysmography Systems: Does LKB1 Determine the Set Point of Carotid Body Chemosensitivity and the Hypoxic Ventilatory Response?

Our recent studies suggest that the level of liver kinase B1 (LKB1) expression in some way determines carotid body afferent discharge during hypoxia and to a lesser extent during hypercapnia. In short, phosphorylation by LKB1 of an as yet unidentified target(s) determines a set point for carotid body chemosensitivity. LKB1 is the principal kinase that activates the AMP-activated protein kinase (AMPK) during metabolic stresses, but conditional deletion of AMPK in catecholaminergic cells, including therein carotid body type I cells, has little or no effect on carotid body responses to hypoxia or hypercapnia. With AMPK excluded, the most likely target of LKB1 is one or other of the 12 AMPK-related kinases, which are constitutively phosphorylated by LKB1 and, in general, regulate gene expression. By contrast, the hypoxic ventilatory response is attenuated by either LKB1 or AMPK deletion in catecholaminergic cells, precipitating hypoventilation and apnea during hypoxia rather than hyperventilation. Moreover, LKB1, but not AMPK, deficiency causes Cheyne-Stokes-like breathing. This chapter will explore further the possible mechanisms that determine these outcomes.

The Carotid Body "Tripartite Synapse": Role of Gliotransmission

In mammals, cardiorespiratory reflexes originating in the carotid body (CB) help maintain homeostasis by matching oxygen supply to oxygen demand. CB output to the brainstem is shaped by synaptic interactions at a "tripartite synapse" consisting of chemosensory (type I) cells, abutting glial-like (type II) cells, and sensory (petrosal) nerve terminals. Type I cells are stimulated by several blood-borne metabolic stimuli, including the novel chemoexcitant lactate. During chemotransduction, type I cells depolarize and release a multitude of excitatory and inhibitory neurotransmitters/neuromodulators including ATP, dopamine (DA), histamine, and angiotensin II (ANG II). However, there is a growing appreciation that the type II cells may not be silent partners. Thus, similar to astrocytes at "tripartite synapses" in the CNS, type II cells may contribute to the afferent output by releasing "gliotransmitters" such as ATP. Here, we first consider whether type II cells can also sense lactate. Next, we review and update the evidence supporting the roles of ATP, DA, histamine, and ANG II in cross talk among the three main CB cellular elements. Importantly, we consider how conventional excitatory and inhibitory pathways, together with gliotransmission, help to coordinate activity within this network and thereby modulate afferent firing frequency during chemotransduction.

Carotid Body: The Primary Peripheral Arterial Chemoreceptor

The carotid body (CB) is a polymodal chemosensory organ that plays an essential role in initiating respiratory and cardiovascular adjustments to maintain blood gas homeostasis. Much of the available evidence suggests that chronic hypoxia induces marked morphological and neurochemical changes within the CB, but the detailed molecular mechanisms by which these affect the hypoxic chemosensitivity still remain to be elucidated. Dysregulation of the CB function and altered oxygen saturation are implicated in various physiological and pathophysiological conditions. Knowledge of the morphological and functional aspects of the CB would improve our current understanding of respiratory and cardiovascular homeostasis in health and disease.

Structural Plasticity of the Carotid Body

The mammalian carotid body (CB) exhibits considerable plasticity of its structure during development and aging and as a consequence of environmental, metabolic and inflammatory stimuli. The structural changes during maturation include an enlargement of the total and vascular volume of the CB. Conversely, aging results in a reduction in the number and volume of glomus cells with progressive cellular degeneration and an apparent increase in the surrounding connective tissue. Age-related structural alterations are similar to those during chronic hypoxia. Long-term hypoxic exposure and sodium nitrate treatment enlarge several-fold the size of the rat CB causing glomus cell hypertrophy and hyperplasia, and evoke changes in its vascular structure, inducing marked vasodilation and neovascularization. In humans, such structural CB adaptation responses to prolonged hypoxia occur during acclimatization to high altitudes. On the other hand, the hyperoxic CB is significantly smaller than those of age-matched normoxic controls. Morphological alterations in the CB in both hypertensive animals and humans are characterized by a slightly enlarged parenchyma without apparent vascular expansion and/or dilation. The CB structural plasticity depends on the existence of a population of multipotent neural crest-derived stem cells, which are activated during hypoxia to proliferate and differentiate into new both neuronal (glomus) and vascular cell types.

Analyzing Angiotensin II Receptor Type 1 Clustering in PC12 Cells in Response to Hypoxia Using Direct Stochastic Optical Reconstruction Microscopy (dSTORM)

Angiotensin II (Ang II) is a hormone that plays a major role in maintaining homeostasis. The Ang II receptor type 1 (AT₁R) is expressed in acute O₂ sensitive cells, including carotid body (CB) type I cells and pheochromocytoma 12 (PC12) cells, and Ang II increases cell activity. While a functional role for Ang II and AT₁Rs in increasing the activity of O₂ sensitive cells has been established, the nanoscale distribution of AT₁Rs has not. Furthermore, it is not known how exposure to hypoxia may alter the single-molecule arrangement and clustering of AT₁Rs. In this study, the AT₁R nanoscale distribution under control normoxic conditions in PC12 cells was determined using direct stochastic optical reconstruction microscopy (dSTORM). AT₁Rs were arranged in distinct clusters with measurable parameters. Across the entire cell surface there averaged approximately 3 AT₁R clusters/μm² of cell membrane. Cluster area varied in size ranging from 1.1 × 10^-4 to 3.9 × 10^-2 μm². Twenty-four hours of exposure to hypoxia (1% O₂) altered clustering of AT₁Rs, with notable increases in the maximum cluster area, suggestive of an increase in supercluster formation. These observations could aid in understanding mechanisms underlying augmented Ang II sensitivity in O₂ sensitive cells in response to sustained hypoxia.

Sleep loss effects on physiological and cognitive responses to systemic environmental hypoxia

In the course of their missions or training, alpinists, but also mountain combat forces and mountain security services, professional miners, aircrew, aircraft and glider pilots and helicopter crews are regularly exposed to altitude without oxygen supplementation. At altitude, humans are exposed to systemic environmental hypoxia induced by the decrease in barometric pressure (<1,013 hPa) which decreases the inspired partial pressure of oxygen (PIO₂), while the oxygen fraction is constant (equal to approximately 20.9%). Effects of altitude on humans occur gradually and depend on the duration of exposure and the altitude level. From 1,500 m altitude (response threshold), several adaptive responses offset the effects of hypoxia, involving the respiratory and the cardiovascular systems, and the oxygen transport capacity of the blood. Fatigue and cognitive and sensory disorders are usually observed from 2,500 m (threshold of prolonged hypoxia). Above 3,500 m (the threshold for disorders), the effects are not completely compensated and maladaptive responses occur and individuals develop altitude headache or acute altitude illness [Acute Mountain Sickness (AMS)]. The magnitude of effects varies considerably between different physiological systems and exhibits significant inter-individual variability. In addition to comorbidities, the factors of vulnerability are still little known. They can be constitutive (genetic) or circumstantial (sleep deprivation, fatigue, speed of ascent.). In particular, sleep loss, a condition that is often encountered in real-life settings, could have an impact on the physiological and cognitive responses to hypoxia. In this review, we report the current state of knowledge on the impact of sleep loss on responses to environmental hypoxia in humans, with the aim of identifying possible consequences for AMS risk and cognition, as well as the value of behavioral and non-pharmacological countermeasures.

Heart rate: control mechanisms, pathophysiology and assessment of the neurocardiac system in health and disease

The monitoring of physiological function and dysfunction is an important principle in modern medicine. Heart rate is a basic example of this type of observation, particularly assessing the neurocardiac system, which entails the autonomic nervous system and intracardiac processes. The neurocardiac axis is an underappreciated and often overlooked system which, if measured appropriately in the clinical setting, may allow identification of patients at risk of disease progression and even mortality. While heart rate itself is a simplistic tool, more information may be gathered through assessing heart rate variability and heart rate recovery time. Studies have demonstrated an association of slow heart rate recovery and lower heart rate variability as markers of elevated sympathetic and lower parasympathetic tone. These parameters have additionally been shown to relate to development of arrhythmia, heart failure, systemic inflammatory processes, ischaemic heart disease and an increased rate of mortality. The aim of this review is to detail how heart rate is homeostatically controlled by the autonomic nervous system, how heart rate can impact on pathophysiological processes, and how heart rate variability and heart rate recovery time may be used in the clinical setting to allow the neurocardiac system to be assessed.

Perioperative blood pressure and heart rate alterations after carotid body tumor excision: a retrospective study of 108 cases

BACKGROUND

Arising from chemoreceptor cells, carotid body tumors (CBTs) are rare neoplasms associated with hemodynamics. Perioperative changes in blood pressure (BP) and heart rate (HR) are not completely understood.

METHODS

This retrospective, observational, controlled study included all CBT patients from 2013 to 2018 in Peking Union Medical College Hospital. Perioperative changes in BP/HR within or between unilateral/bilateral/control groups were investigated. Perioperative details across Shamblin types were also assessed.

RESULTS

This study included 108 patients (116 excised CBTs). The postoperative systolic BP and HR increased in both unilateral (mean difference of systolic BP = 5.9mmHg, 95% CI 3.1 ~ 8.6; mean difference of HR = 3.7 bpm, 95% CI 2.6 ~ 4.9) and bilateral (mean difference of systolic BP = 10.3mmHg, 95% CI 0.6 ~ 19.9; mean difference of HR = 8.4 bpm, 95% CI 0.5 ~ 16.2) CBT patients compared with the preoperative measures. Compared with control group, the postoperative systolic BP increased (difference in the alteration = 6.3mmHg, 95% CI 3.5 ~ 9.0) in unilateral CBT patients; both systolic BP (difference in the alteration = 9.2mmHg, 95% CI 1.1 ~ 17.3) and HR (difference in the alteration = 5.3 bpm, 95% CI 1.0 ~ 9.6) increased in bilateral CBT patients. More CBT patients required extra antihypertensive therapy after surgery than controls (OR = 2.5, 95% CI 1.14 ~ 5.5). Maximum tumor diameter, intraoperative vascular injury, continuous vasoactive agent requirement, total fluid volume, transfusion, estimated blood loss, operation duration, postoperative pathology, overall complications, and intensive care unit/hospital lengths of stay significantly varied among Shamblin types.

CONCLUSION

CBT excision may be associated with subtle perioperative hemodynamic changes. Perioperative management of CBT patients necessitates careful assessment, full preparation and close postoperative monitoring.

Lactate sensing by neuroepithelial cells isolated from the gills of killifish (Fundulus heteroclitus)

Lactate is produced in most vertebrate cells as a by-product of anaerobic metabolism. In addition to its role as a fuel for many tissues, circulating lactate can act as a signalling molecule and stimulates ventilation in air- and water-breathing vertebrates. Recent evidence suggests lactate acts on O2- and CO2/H+-sensitive chemoreceptors located in the mammalian carotid body. While analogous receptors (neuroepithelial cells or NECs) in fish gills are presumed to also function as lactate sensors, direct evidence is lacking. Here, using ratiometric Fura-2 Ca2+ imaging, we show that chemosensitive NECs isolated from killifish gills respond to lactate (5-10 mmol l-1; pHe ∼7.8) with intracellular Ca2+ elevations. These responses were inhibited by an L-type Ca2+ channel blocker (nifedipine; 0.5 µmol l-1), a monocarboxylic acid transporter (MCT) blocker (α-cyano-4-hydroxycinnamate; 300 µmol l-1) or a competitive MCT substrate (pyruvate; 5 mmol l-1). These data provide the first direct evidence that gill NECs act as lactate sensors.

A Methodological Perspective on the Function and Assessment of Peripheral Chemoreceptors in Heart Failure: A Review of Data from Clinical Trials

Augmented peripheral chemoreceptor sensitivity (PChS) is a common feature of many sympathetically mediated diseases, among others, and it is an important mechanism of the pathophysiology of heart failure (HF). It is related not only to the greater severity of symptoms, especially to dyspnea and lower exercise tolerance but also to a greater prevalence of complications and poor prognosis. The causes, mechanisms, and impact of the enhanced activity of peripheral chemoreceptors (PChR) in the HF population are subject to intense research. Several methodologies have been established and utilized to assess the PChR function. Each of them presents certain advantages and limitations. Furthermore, numerous factors could influence and modulate the response from PChR in studied subjects. Nevertheless, even with the impressive number of studies conducted in this field, there are still some gaps in knowledge that require further research. We performed a review of all clinical trials in HF human patients, in which the function of PChR was evaluated. This review provides an extensive synthesis of studies evaluating PChR function in the HF human population, including methods used, factors potentially influencing the results, and predictors of increased PChS.

The HMOX2 polymorphism contributes to the carotid body chemoreflex in European sea-level residents by regulating hypoxic ventilatory responses

This study investigates whether a functional single nucleotide polymorphism of HMOX2 (heme oxygenase-2) (rs4786504 T>C) is involved in individual chemosensitivity to acute hypoxia, as assessed by ventilatory responses, in European individuals. These responses were obtained at rest and during submaximal exercise, using a standardized and validated protocol for exposure to acute normobaric hypoxia. Carriers of the ancestral T allele (n = 44) have significantly lower resting and exercise hypoxic ventilatory responses than C/C homozygous carriers (n = 40). In the literature, a hypoxic ventilatory response threshold to exercise has been identified as an independent predictor of severe high altitude-illness (SHAI). Our study shows that carriers of the T allele have a higher risk of SHAI than carriers of the mutated C/C genotype. Secondarily, we were also interested in COMT (rs4680 G > A) polymorphism, which may be indirectly involved in the chemoreflex response through modulation of autonomic nervous system activity. Significant differences are present between COMT genotypes for oxygen saturation and ventilatory responses to hypoxia at rest. In conclusion, this study adds information on genetic factors involved in individual vulnerability to acute hypoxia and supports the critical role of the ≪ O2 sensor ≫ - heme oxygenase-2 - in the chemosensitivity of carotid bodies in Humans.

Adaptive Responses to Hypoxia and/or Hyperoxia in Humans

Significance: Oxygen is indispensable for aerobic life, but its utilization exposes cells and tissues to oxidative stress; thus, tight regulation of cellular, tissue, and systemic oxygen concentrations is crucial. Here, we review the current understanding of how the human organism (mal-)adapts to low (hypoxia) and high (hyperoxia) oxygen levels and how these adaptations may be harnessed as therapeutic or performance enhancing strategies at the systemic level. Recent Advances: Hyperbaric oxygen therapy is already a cornerstone of modern medicine, and the application of mild hypoxia, that is, hypoxia conditioning (HC), to strengthen the resilience of organs or the whole body to severe hypoxic insults is an important preparation for high-altitude sojourns or to protect the cardiovascular system from hypoxic/ischemic damage. Many other applications of adaptations to hypo- and/or hyperoxia are only just emerging. HC-sometimes in combination with hyperoxic interventions-is gaining traction for the treatment of chronic diseases, including numerous neurological disorders, and for performance enhancement. Critical Issues: The dose- and intensity-dependent effects of varying oxygen concentrations render hypoxia- and/or hyperoxia-based interventions potentially highly beneficial, yet hazardous, although the risks versus benefits are as yet ill-defined. Future Directions: The field of low and high oxygen conditioning is expanding rapidly, and novel applications are increasingly recognized, for example, the modulation of aging processes, mood disorders, or metabolic diseases. To advance hypoxia/hyperoxia conditioning to clinical applications, more research on the effects of the intensity, duration, and frequency of altered oxygen concentrations, as well as on individual vulnerabilities to such interventions, is paramount. Antioxid. Redox Signal. 37, 887-912.

The carotid body: A novel key player in neuroimmune interactions

The idea that the nervous system communicates with the immune system to regulate physiological and pathological processes is not new. However, there is still much to learn about how these interactions occur under different conditions. The carotid body (CB) is a sensory organ located in the neck, classically known as the primary sensor of the oxygen (O2) levels in the organism of mammals. When the partial pressure of O2 in the arterial blood falls, the CB alerts the brain which coordinates cardiorespiratory responses to ensure adequate O2 supply to all tissues and organs in the body. A growing body of evidence, however, has demonstrated that the CB is much more than an O2 sensor. Actually, the CB is a multimodal sensor with the extraordinary ability to detect a wide diversity of circulating molecules in the arterial blood, including inflammatory mediators. In this review, we introduce the literature supporting the role of the CB as a critical component of neuroimmune interactions. Based on ours and other studies, we propose a novel neuroimmune pathway in which the CB acts as a sensor of circulating inflammatory mediators and, in conditions of systemic inflammation, recruits a sympathetic-mediated counteracting mechanism that appears to be a protective response.