Adequate stimuli of the carotid body: more than an oxygen sensor?

Baroreflex and chemoreflex interaction in high-altitude exposure: possible role on exercise performance

The hypoxic chemoreflex and the arterial baroreflex are implicated in the ventilatory response to exercise. It is well known that long-term exercise training increases parasympathetic and decreases sympathetic tone, both processes influenced by the arterial baroreflex and hypoxic chemoreflex function. Hypobaric hypoxia (i.e., high altitude [HA]) markedly reduces exercise capacity associated with autonomic reflexes. Indeed, a reduced exercise capacity has been found, paralleled by a baroreflex-related parasympathetic withdrawal and a pronounced chemoreflex potentiation. Additionally, it is well known that the baroreflex and chemoreflex interact, and during activation by hypoxia, the chemoreflex is predominant over the baroreflex. Thus, the baroreflex function impairment may likely facilitate the exercise deterioration through the reduction of parasympathetic tone following acute HA exposure, secondary to the chemoreflex activation. Therefore, the main goal of this review is to describe the main physiological mechanisms controlling baro- and chemoreflex function and their role in exercise capacity during HA exposure.

Control of Breathing

Substantial advances have been made recently into the discovery of fundamental mechanisms underlying the neural control of breathing and even some inroads into translating these findings to treating breathing disorders. Here, we review several of these advances, starting with an appreciation of the importance of V̇A:V̇CO2:PaCO2 relationships, then summarizing our current understanding of the mechanisms and neural pathways for central rhythm generation, chemoreception, exercise hyperpnea, plasticity, and sleep-state effects on ventilatory control. We apply these fundamental principles to consider the pathophysiology of ventilatory control attending hypersensitized chemoreception in select cardiorespiratory diseases, the pathogenesis of sleep-disordered breathing, and the exertional hyperventilation and dyspnea associated with aging and chronic diseases. These examples underscore the critical importance that many ventilatory control issues play in disease pathogenesis, diagnosis, and treatment.

Autonomic and respiratory consequences of altered chemoreflex function: clinical and therapeutic implications in cardiovascular diseases

The importance of chemoreflex function for cardiovascular health is increasingly recognized in clinical practice. The physiological function of the chemoreflex is to constantly adjust ventilation and circulatory control to match respiratory gases to metabolism. This is achieved in a highly integrated fashion with the baroreflex and the ergoreflex. The functionality of chemoreceptors is altered in cardiovascular diseases, causing unstable ventilation and apnoeas and promoting sympathovagal imbalance, and it is associated with arrhythmias and fatal cardiorespiratory events. In the last few years, opportunities to desensitize hyperactive chemoreceptors have emerged as potential options for treatment of hypertension and heart failure. This review summarizes up to date evidence of chemoreflex physiology/pathophysiology, highlighting the clinical significance of chemoreflex dysfunction, and lists the latest proof of concept studies based on modulation of the chemoreflex as a novel target in cardiovascular diseases.

Insights into the control and consequences of breathing adjustments in fishes-from larvae to adults

Adjustments of ventilation in fishes to regulate the volume of water flowing over the gills are critically important responses to match branchial gas transfer with metabolic needs and to defend homeostasis during environmental fluctuations in O₂ and/or CO₂ levels. In this focused review, we discuss the control and consequences of ventilatory adjustments in fish, briefly summarizing ventilatory responses to hypoxia and hypercapnia before describing the current state of knowledge of the chemoreceptor cells and molecular mechanisms involved in sensing O₂ and CO₂. We emphasize, where possible, insights gained from studies on early developmental stages. In particular, zebrafish (Danio rerio) larvae have emerged as an important model for investigating the molecular mechanisms of O₂ and CO₂ chemosensing as well as the central integration of chemosensory information. Their value stems, in part, from their amenability to genetic manipulation, which enables the creation of loss-of-function mutants, optogenetic manipulation, and the production of transgenic fish with specific genes linked to fluorescent reporters or biosensors.

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.

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.

Acute Effects of Using Added Respiratory Dead Space Volume in a Cycling Sprint Interval Exercise Protocol: A Cross-Over Study

Background: The aim of the study was to compare acute physiological, biochemical, and perceptual responses during sprint interval exercise (SIE) with breathing through a device increasing added respiratory dead space volume (ARDS_V) and without the device. Methods: The study involved 11 healthy, physically active men (mean maximal oxygen uptake: 52.6 ± 8.2 mL∙kg¹∙min^-1). During four visits to a laboratory with a minimum interval of 72 h, they participated in (1) an incremental test on a cycle ergometer; (2) a familiarization session; (3) and (4) cross-over SIE sessions. SIE consisted of 6 × 10-s all-out bouts with 4-min active recovery. During one of the sessions the participants breathed through a 1200-mL ARDSv (SIE_ARDS). Results: The work performed was significantly higher by 4.4% during SIE_ARDS, with no differences in the fatigue index. The mean respiratory ventilation was significantly higher by 13.2%, and the mean oxygen uptake was higher by 31.3% during SIE_ARDS. Respiratory muscle strength did not change after the two SIE sessions. In SIE_ARDS, the mean pH turned out significantly lower (7.26 vs. 7.29), and the mean HCO₃^- concentration was higher by 7.6%. Average La^- and rating of perceived exertion (RPE) did not differ between the sessions. Conclusions: Using ARDS_V during SIE provokes respiratory acidosis, causes stronger acute physiological responses, and does not increase RPE.

Autonomic innervation of the carotid body as a determinant of its sensitivity: implications for cardiovascular physiology and pathology

The motivation for this review comes from the emerging complexity of the autonomic innervation of the carotid body (CB) and its putative role in regulating chemoreceptor sensitivity. With the carotid bodies as a potential therapeutic target for numerous cardiorespiratory and metabolic diseases, an understanding of the neural control of its circulation is most relevant. Since nerve fibres track blood vessels and receive autonomic innervation, we initiate our review by describing the origins of arterial feed to the CB and its unique vascular architecture and blood flow. Arterial feed(s) vary amongst species and, unequivocally, the arterial blood supply is relatively high to this organ. The vasculature appears to form separate circuits inside the CB with one having arterial venous anastomoses. Both sympathetic and parasympathetic nerves are present with postganglionic neurons located within the CB or close to it in the form of paraganglia. Their role in arterial vascular resistance control is described as is how CB blood flow relates to carotid sinus afferent activity. We discuss non-vascular targets of autonomic nerves, their possible role in controlling glomus cell activity, and how certain transmitters may relate to function. We propose that the autonomic nerves sub-serving the CB provide a rapid mechanism to tune the gain of peripheral chemoreflex sensitivity based on alterations in blood flow and oxygen delivery, and might provide future therapeutic targets. However, there remain a number of unknowns regarding these mechanisms that require further research that is discussed.

Update on Chemoreception: Influence on Cardiorespiratory Regulation and Pathophysiology

We examine recent findings that have revealed interdependence of function within the chemoreceptor pathway regulating breathing and sympathetic vasomotor activity and the hypersensitization of these reflexes in chronic disease states. Recommendations are made as to how these states of hyperreflexia in chemoreceptors and muscle afferents might be modified in treating sleep apnea, drug-resistant hypertension, chronic heart failure-induced sympathoexcitation, and the exertional dyspnea of chronic obstructive pulmonary disease.

Role of the carotid chemoreceptors in insulin-mediated sympathoexcitation in humans

We examined the contribution of the carotid chemoreceptors to insulin-mediated increases in muscle sympathetic nerve activity (MSNA) in healthy humans. We hypothesized that reductions in carotid chemoreceptor activity would attenuate the sympathoexcitatory response to hyperinsulinemia. Young, healthy adults (9 male/9 female, 28 ± 1 yr, 24 ± 1 kg/m²) completed a 30-min euglycemic baseline followed by a 90-min hyperinsulinemic (1 mU·kg fat-free mass^-1·min^-1), euglycemic infusion. MSNA (microneurography of the peroneal nerve) was continuously measured. The role of the carotid chemoreceptors was assessed at baseline and during hyperinsulinemia via 1) acute hyperoxia, 2) low-dose dopamine (1-4 µg·kg^-1·min^-1), and 3) acute hyperoxia + low-dose dopamine. MSNA burst frequency increased from baseline during hyperinsulinemia (P < 0.01). Acute hyperoxia had no effect on MSNA burst frequency at rest (P = 0.74) or during hyperinsulinemia (P = 0.83). The insulin-mediated increase in MSNA burst frequency (P = 0.02) was unaffected by low-dose dopamine (P = 0.60). When combined with low-dose dopamine, acute hyperoxia had no effect on MSNA burst frequency at rest (P = 0.17) or during hyperinsulinemia (P = 0.85). Carotid chemoreceptor desensitization in young, healthy men and women does not attenuate the sympathoexcitatory response to hyperinsulinemia. Our data suggest that the carotid chemoreceptors do not contribute to acute insulin-mediated increases in MSNA in young, healthy adults.

Effects of lactate ions on the cardiorespiratory system in rainbow trout (Oncorhynchus mykiss)

Lactate ions are involved in several physiological processes, including a direct stimulation of the carotid body, causing increased ventilation in mammals. A similar mechanism eliciting ventilatory stimulation in other vertebrate classes has been demonstrated, but it remains to be thoroughly investigated. Here, we investigated the effects of lactate ions on the cardiorespiratory system in swimming rainbow trout by manipulating the blood lactate concentration. Lactate elicited a vigorous, dose-dependent elevation of ventilation and bradycardia at physiologically relevant concentrations at constant pH. After this initial confirmation, we examined the chiral specificity of the response and found that only l-lactate induced these effects. By removal of the afferent inputs from the first gill arch, the response was greatly attenuated, and a comparison of the responses to injections up- and downstream of the gills collectively demonstrated that the lactate response was initiated by branchial cells. Injection of specific receptor antagonists revealed that a blockade of serotonergic receptors, which are involved in the hypoxic ventilatory response, significantly reduced the lactate response. Finally, we identified two putative lactate receptors based on sequence homology and found that both were expressed at substantially higher levels in the gills. We propose that lactate ions modulate ventilation by stimulating branchial oxygen-sensing cells, thus eliciting a cardiorespiratory response through receptors likely to have originated early in vertebrate evolution.

Hypoxic tachycardia is not a result of increased respiratory activity in healthy subjects

NEW FINDINGS

What is the central question of this research? Does increased ventilation contribute to the increase in heart rate during transient exposure to hypoxia in humans? What is the main finding and its importance? Voluntary suppression of the ventilatory response to transient hypoxia does not affect the magnitude of the heart rate response to the stimulus. This indicates that hypoxic tachycardia is not secondary to hyperpnoea in humans. Better understanding of the physiology underlying the cardiovascular response to hypoxia might help in identification of new markers of elevated chemoreceptor activity, which has been proposed as a target in treatment of sympathetically mediated diseases.

ABSTRACT

Animal data suggest that hypoxic tachycardia is secondary to hyperpnoea, and for years this observation has been extrapolated to humans, despite a lack of experimental evidence. We addressed this issue in 17 volunteers aged 29 ± 7 (SD) years. A transient hypoxia test, comprising several nitrogen-breathing episodes, was performed twice in each subject. In the first test, the subject breathed spontaneously (spontaneous breathing). In the second test, the subject was repeatedly asked to adjust his or her depth and rate of breathing according to visual (real-time inspiratory flow) and auditory (metronome sound) cues, respectively (controlled breathing), to maintain respiration at the resting level during nitrogen-breathing episodes. Hypoxic responsiveness, including minute ventilation [Hyp-VI; in liters per minute per percentage of blood oxygen saturation ( S p O 2 )], tidal volume [Hyp-VT; in litres per S p O 2 ], heart rate [Hyp-HR; in beats per minute per S p O 2 ], systolic [Hyp-SBP; in millimetres of mercury per S p O 2 ] and mean blood pressure [Hyp-MAP; in millimetres of mercury per S p O 2 ] and systemic vascular resistance [Hyp-SVR; in dynes seconds (centimetres)^-5 per S p O 2 ] was calculated as the slope of the regression line relating the variable to S p O 2 , including pre- and post-hypoxic values. The Hyp-VI and Hyp-VT were reduced by 69 ± 25 and 75 ± 10%, respectively, in controlled versus spontaneous breathing (Hyp-VI, -0.30 ± 0.15 versus -0.11 ± 0.09; Hyp-VT, -0.030 ± 0.024 versus -0.007 ± 0.004; both P < 0.001). However, the cardiovascular responses did not differ between spontaneous and controlled breathing (Hyp-HR, -0.62 ± 0.24 versus -0.71 ± 0.33; Hyp-MAP, -0.43 ± 0.19 versus -0.47 ± 0.21; Hyp-SVR, 9.15 ± 5.22 versus 9.53 ± 5.57; all P ≥ 0.22), indicating that hypoxic tachycardia is not secondary to hyperpnoea. Hyp-HR was correlated with Hyp-SVR (r = -074 and -0.80 for spontaneous and controlled breathing, respectively; both P < 0.05) and resting barosensitivity assessed with the sequence technique (r = -0.60 for spontaneous breathing; P < 0.05). This might suggest that the baroreflex mechanism is involved.

Combined effects of intermittent hyperoxia and intermittent hypercapnic hypoxia on respiratory control in neonatal rats

Chronic exposure to intermittent hyperoxia causes abnormal carotid body development and attenuates the hypoxic ventilatory response (HVR) in neonatal rats. We hypothesized that concurrent exposure to intermittent hypercapnic hypoxia would influence this plasticity. Newborn rats were exposed to alternating bouts of hypercapnic hypoxia (10% O₂/6% CO₂) and hyperoxia (30-40% O₂) (5 cycles h^-1, 24 h d^-1) through 13-14 days of age; the experiment was run twice, once in a background of 21% O₂ and once in a background of 30% O₂ (i.e., "relative hyperoxia"). Hyperoxia had only small effects on carotid body development when combined with intermittent hypercapnic hypoxia: the carotid chemoafferent response to hypoxia was reduced, but this did not affect the HVR. In contrast, sustained exposure to 30% O₂ reduced carotid chemoafferent activity and carotid body size which resulted in a blunted HVR. When given alone, chronic intermittent hypercapnic hypoxia increased carotid body size and reduced the hypercapnic ventilatory response but did not affect the HVR. Overall, it appears that intermittent hypercapnic hypoxia counteracted the effects of hyperoxia on the carotid body and prevented developmental plasticity of the HVR.

Insights into the evolution of polymodal chemoreceptors

Respiratory chemoreceptors in vertebrates are specialized cells that detect chemical changes in the environment or arterial blood supply and initiate autonomic responses, such as hyperventilation or changes in heart rate, to improve O₂ uptake and delivery to tissues. These chemoreceptors are sensitive to changes in O₂, CO₂ and/or H⁺. In fish and mammals, respiratory chemoreceptors may be additionally sensitive to ammonia, hypoglycemia, and numerous other stimuli. Thus, chemoreceptors that affect respiration respond to different types of stimuli (or modalities) and are considered to be "polymodal". This review discusses the polymodal nature of respiratory chemoreceptors in vertebrates with a particular emphasis on chemoreceptors of the carotid body and pulmonary epithelium in mammals, and on neuroepithelial cells in water- and air-breathing fish. A major goal will be to examine the evidence for putative polymodal chemoreceptors in fish within the context of studies on mammalian models, for which polymodal chemoreceptors are well described, in order to improve our understanding of the evolution of polymodal chemoreceptors in vertebrates, and to aid in future studies that aim to identify putative receptors in air- and water-breathing fish.

Is Carotid Body Physiological O2 Sensitivity Determined by a Unique Mitochondrial Phenotype?

The mammalian carotid body (CB) is the primary arterial chemoreceptor that responds to acute hypoxia, initiating systemic protective reflex responses that act to maintain O2 delivery to the brain and vital organs. The CB is unique in that it is stimulated at O2 levels above those that begin to impact on the metabolism of most other cell types. Whilst a large proportion of the CB chemotransduction cascade is well defined, the identity of the O2 sensor remains highly controversial. This short review evaluates whether the mitochondria can adequately function as acute O2 sensors in the CB. We consider the similarities between mitochondrial poisons and hypoxic stimuli in their ability to activate the CB chemotransduction cascade and initiate rapid cardiorespiratory reflexes. We evaluate whether the mitochondria are required for the CB to respond to hypoxia. We also discuss if the CB mitochondria are different to those located in other non-O2 sensitive cells, and what might cause them to have an unusually low O2 binding affinity. In particular we look at the potential roles of competitive inhibitors of mitochondrial complex IV such as nitric oxide in establishing mitochondrial and CB O2-sensitivity. Finally, we discuss novel signaling mechanisms proposed to take place within and downstream of mitochondria that link mitochondrial metabolism with cellular depolarization.

Glucose, insulin, and the carotid body chemoreceptors in humans

Known primarily for its oxygen-sensing capabilities, the carotid body chemoreceptors have recently been implicated, primarily by work in animal models, in the pathophysiology of a number of metabolic conditions. The research presented in this brief review highlights translational work conducted at the Mayo Clinic between 2010 and 2017 in healthy humans and discusses key areas for future work in disease populations.

Lessons from single-cell transcriptome analysis of oxygen-sensing cells

The advent of single-cell RNA-sequencing (RNA-Seq) technology has enabled transcriptome profiling of individual cells. Comprehensive gene expression analysis at the single-cell level has proven to be effective in characterizing the most fundamental aspects of cellular function and identity. This unbiased approach is revolutionary for small and/or heterogeneous tissues like oxygen-sensing cells in identifying key molecules. Here, we review the major methods of current single-cell RNA-Seq technology. We discuss how this technology has advanced the understanding of oxygen-sensing glomus cells in the carotid body and helped uncover novel oxygen-sensing cells and mechanisms in the mice olfactory system. We conclude by providing our perspective on future single-cell RNA-Seq research directed at oxygen-sensing cells.

Reductions in carotid chemoreceptor activity with low-dose dopamine improves baroreflex control of heart rate during hypoxia in humans

The purpose of the present investigation was to examine the contribution of the carotid body chemoreceptors to changes in baroreflex control of heart rate with exposure to hypoxia. We hypothesized spontaneous cardiac baroreflex sensitivity (scBRS) would be reduced with hypoxia and this effect would be blunted when carotid chemoreceptor activity was reduced with low-dose dopamine. Fifteen healthy adults (11 M/4 F) completed two visits randomized to intravenous dopamine or placebo (saline). On each visit, subjects were exposed to 5-min normoxia (~99% SpO2), followed by 5-min hypoxia (~84% SpO2). Blood pressure (intra-arterial catheter) and heart rate (ECG) were measured continuously and scBRS was assessed by spectrum and sequence methodologies. scBRS was reduced with hypoxia (P < 0.01). Using the spectrum analysis approach, the fall in scBRS with hypoxia was attenuated with infusion of low-dose dopamine (P < 0.01). The decrease in baroreflex sensitivity to rising pressures (scBRS "up-up") was also attenuated with low-dose dopamine (P < 0.05). However, dopamine did not attenuate the decrease in baroreflex sensitivity to falling pressures (scBRS "down-down"; P > 0.05). Present findings are consistent with a reduction in scBRS with systemic hypoxia. Furthermore, we show this effect is partially mediated by the carotid body chemoreceptors, given the fall in scBRS is attenuated when activity of the chemoreceptors is reduced with low-dose dopamine. However, the improvement in scBRS with dopamine appears to be specific to rising blood pressures. These results may have important implications for impairments in baroreflex function common in disease states of acute and/or chronic hypoxemia, as well as the experimental use of dopamine to assess such changes.

Concomitant application of sprint and high-intensity interval training on maximal oxygen uptake and work output in well-trained cyclists

PURPOSE

In this study, we compared the effects of two different training modalities on maximal oxygen uptake and work output.

METHODS

Participants included 26 well-trained mountain bike cyclists were divided into two groups. The first group trained using a conventional endurance protocol at steady-state (moderate) intensity and variable-intensity (high-moderate-low) free of maximal efforts. The second group combined endurance training with a sprint and high-intensity interval training protocol, which, respectively, were based on 30 s maximal repetitions and 4 min high intensity repetitions. Training duration was 8 weeks. A graded exercise test was administered pre- and post-training. Work output, oxygen uptake, minute pulmonary ventilation, heart rate and stroke volume were determined during the test.

RESULTS

While work output significantly increased post-training in both groups (P < 0.05), the interval training group showed a greater magnitude of change (from 284.4 ± 91.9 to 314.2 ± 95.1 kJ) than the endurance training group (from 271.8 ± 73.3 to 283.4 ± 72.3 kJ). Significant increases in maximal oxygen uptake (from 57.9 ± 6.8 to 66.6 ± 5.3 ml kg(-1) min(-1)), maximal pulmonary ventilation and stroke volume were observed only in the interval training group.

CONCLUSIONS

An exercise protocol involving endurance and sprint and high-intensity interval training was found to induce positive effects on maximal oxygen uptake in a group of well-trained cyclists with several years athletic experience.

Pathophysiology of human ventilatory control

We review the substantial recent progress made in understanding the underlying mechanisms controlling breathing and the applicability of these findings to selected human diseases. Emphasis is placed on the sites of central respiratory rhythm and pattern generation as well as newly described functions of the carotid chemoreceptors, the integrative nature of the central chemoreceptors, and the interaction between peripheral and central chemoreception. Recent findings that support critical contributions from cortical central command and muscle afferent feedback to exercise hyperpnoea are also reviewed. These basic principles, and the evidence supporting chemoreceptor and ventilatory control system plasticity during and following constant and intermittent hypoxaemia and stagnant hypoxia, are applied to: 1) the pathogenesis, consequences and treatment of obstructive sleep apnoea; and 2) exercise hyperpnoea and its control and limitations with ageing, chronic obstructive pulmonary disease and congestive heart failure.

The human carotid body releases acetylcholine, ATP and cytokines during hypoxia

Studies on experimental animals established that the carotid bodies are sensory organs for detecting arterial blood O2 levels and that the ensuing chemosensory reflex is a major regulator of cardiorespiratory functions during hypoxia. However, little information is available on the human carotid body responses to hypoxia. The present study was performed on human carotid bodies obtained from surgical patients undergoing elective head and neck cancer surgery. Our results show that exposing carotid body slices to hypoxia for a period as brief as 5 min markedly facilitates the release of ACh and ATP. Furthermore, prolonged hypoxia for 1 h induces an increased release of interleukin (IL)-1β, IL-4, IL-6, IL-8 and IL-10. Immunohistochemical analysis revealed that type 1 cells of the human carotid body express an array of cytokine receptors as well as hypoxia-inducible factor-1α and hypoxia-inducible factor-2α. Taken together, these results demonstrate that ACh and ATP are released from the human carotid body in response to hypoxia, suggesting that these neurotransmitters, as in several experimental animal models, play a role in hypoxic signalling also in the human carotid body. The finding that the human carotid body releases cytokines in response to hypoxia adds to the growing body of information suggesting that the carotid body may play a role in detecting inflammation, providing a link between the immune system and the nervous system.

Synaptic and paracrine mechanisms at carotid body arterial chemoreceptors

Mammalian carotid bodies are the main peripheral arterial chemoreceptors, strategically located at the bifurcation of the common carotid artery. When stimulated these receptors initiate compensatory respiratory and cardiovascular reflexes to maintain homeostasis. Thus, in response to low oxygen (hypoxia) or increased CO2/H(+) (acid hypercapnia), chemoreceptor type I cells depolarize and release excitatory neurotransmitters, such as ATP, which stimulate postsynaptic P2X2/3 receptors on afferent nerve terminals. The afferent discharge is shaped by autocrine and paracrine mechanisms involving both excitatory and inhibitory neuromodulators such as adenosine, serotonin (5-HT), GABA and dopamine. Recent evidence suggests that paracrine activation of P2Y2 receptors on adjacent glia-like type II cells may help boost the ATP signal via the opening of pannexin-1 channels. The presence of an inhibitory efferent innervation, mediated by release of nitric oxide, provides additional control of the afferent discharge. The broad array of neuromodulators and their receptors appears to endow the carotid body with a remarkable plasticity, most apparent during natural and pathophysiological conditions associated with chronic sustained and intermittent hypoxia.

Peripheral neural detection of danger-associated and pathogen-associated molecular patterns

Objective:

Bidirectional links between the nervous and immune systems modulate inflammation. The cellular mechanisms underlying the detection of danger-associated molecular patterns and pathogen-associated molecular patterns by the nervous system are not well understood. We hypothesized that the carotid body, a tissue of neural crest origin, detect pathogen associated molecular patterns and danger associated molecular patterns via an inflammasome-dependent mechanism similar to that described in immune cells.

Design:

Randomized, controlled laboratory investigation.

Setting:

University laboratory.

Subjects:

C57Bl/6J mice; juvenile Sprague-Dawley rats, primary human neutrophils.

Interventions:

Rat carotid body chemosensitive cells, and human neutrophils, were treated with TLR agonists to activate inflammasome-dependent pathways. In mice, systemic inflammation was induced by the pathogen associated molecular pattern zymosan (intraperitoneal injection; 500 mg/kg). Isolated carotid body/carotid sinus nerve preparations were used to assess peripheral chemoafferent activity. Ventilation was measured by whole-body plethysmography.

Measurements and Main Results:

Chemosensitive carotid body glomus cells exhibited toll-like receptor (TLR-2 and TLR-4), NLRP1, and NLRP3 inflammasome immunoreactivities. Zymosan increased NLRP3 inflammasome and interleukin-1β expression in glomus cells (p < 0.01). Human neutrophils demonstrated similar LPS-induced changes in inflammasome expression. Carotid body glomus cells also expressed IL-1 receptor and responded to application of IL-1β with increases in intracellular [Ca²⁺]. Four hours after injection of zymosan carotid sinus nerve chemoafferent discharge assessed in vitro (i.e., in the absence of acidosis/circulating inflammatory mediators) was increased five-fold (p < 0.001). Accordingly, zymosan-induced systemic inflammation was accompanied by enhanced respiratory activity.

Conclusions:

In carotid body chemosensitive glomus cells, activation of toll-like receptors increases NLRP3 inflammasome expression, and enhances IL-1β production, which is capable of acting in an autocrine manner to enhance peripheral chemoreceptor drive.

Expanding role of ATP as a versatile messenger at carotid and aortic body chemoreceptors

In mammals, peripheral arterial chemoreceptors monitor blood chemicals (e.g. O(2), CO(2), H(+), glucose) and maintain homeostasis via initiation of respiratory and cardiovascular reflexes. Whereas chemoreceptors in the carotid bodies (CBs), located bilaterally at the carotid bifurcation, control primarily respiratory functions, those in the more diffusely distributed aortic bodies (ABs) are thought to regulate mainly cardiovascular functions. Functionally, CBs sense partial pressure of O(2) ( ), whereas ABs are considered sensors of O(2) content. How these organs, with essentially a similar complement of chemoreceptor cells, differentially process these two different types of signals remains enigmatic. Here, we review evidence that implicates ATP as a central mediator during information processing in the CB. Recent data allow an integrative view concerning its interactions at purinergic P2X and P2Y receptors within the chemosensory complex that contains elements of a 'quadripartite synapse'. We also discuss recent studies on the cellular physiology of ABs located near the aortic arch, as well as immunohistochemical evidence suggesting the presence of pathways for P2X receptor signalling. Finally, we present a hypothetical 'quadripartite model' to explain how ATP, released from red blood cells during hypoxia, could contribute to the ability of ABs to sense O(2) content.

Signal processing at mammalian carotid body chemoreceptors

Mammalian carotid bodies are richly vascularized chemosensory organs that sense blood levels of O(2), CO(2)/H(+), and glucose and maintain homeostatic regulation of these levels via the reflex control of ventilation. Carotid bodies consist of innervated clusters of type I (or glomus) cells in intimate association with glial-like type II cells. Carotid bodies make afferent connections with fibers from sensory neurons in the petrosal ganglia and receive efferent inhibitory innervation from parasympathetic neurons located in the carotid sinus and glossopharyngeal nerves. There are synapses between type I (chemosensory) cells and petrosal afferent terminals, as well as between neighboring type I cells. There is a broad array of neurotransmitters and neuromodulators and their ionotropic and metabotropic receptors in the carotid body. This allows for complex processing of sensory stimuli (e.g., hypoxia and acid hypercapnia) involving both autocrine and paracrine signaling pathways. This review summarizes and evaluates current knowledge of these pathways and presents an integrated working model on information processing in carotid bodies. Included in this model is a novel hypothesis for a potential role of type II cells as an amplifier for the release of a key excitatory carotid body neurotransmitter, ATP, via P2Y purinoceptors and pannexin-1 channels.

Neuroepithelial cells of the gill and their role in oxygen sensing

A highly sensitive oxygen (O(2)) sensing mechanism is critical for the survival of all vertebrate species. In fish, this requirement is fullfilled by the neuroepithelial cells (NECs) of the gill. NECs are neurotransmitter-containing chemosensory cells that are diffusely distributed within a thin epithelial layer of the filaments and respiratory lamellae of all gill arches, and are innervated by afferent fibers from the central nervous system. In acute cell culture, NECs respond immediately, and in a dose-dependent manner, to acute changes in O(2) tension. Thus, hypoxic stimulation of gill NECs appears to initiate the production of adaptive, cardiorespiratory reflexes that contribute to the maintenance of O(2) uptake in order to meet metabolic demands. This review covers the current evidence for the status of NECs as the primary peripheral O(2) sensors in fish. We have included an overview of the phylogeny of O(2) sensing structures among vertebrate groups, and morphological and physiological evidence for the importance of NECs in O(2) sensing.

Changes in Physiology before, during, and after Yawning

The ultimate function of yawning continues to be debated. Here, we examine physiological measurements taken before, during, and after yawns in humans, in an attempt to identify key proximate mechanisms associated with this behavior. In two separate studies we measured changes in heart rate, lung volume, eye closure, skin conductance, ear pulse, respiratory sinus arrhythmia, and respiratory rate. Data were depicted from 75 s before and after yawns, and analyzed at baseline, during, and immediately following yawns. Increases in heart rate, lung volume, and eye muscle tension were observed during or immediately following yawning. Patterns of physiological changes during yawning were then compared to data from non-yawning deep inhalations. In one study, respiration period increased following the execution of a yawn. Much of the variance in physiology surrounding yawning was specific to the yawning event. This was not the case for deep inhalation. We consider our findings in light of various hypotheses about the function of yawning and conclude that they are most consistent with the brain cooling hypothesis.

Peripheral chemoreceptors in air- versus water- breathers

Among the vertebrates, peripheral chemoreceptors have evolved to play a key role in matching oxygen delivery to the metabolic needs of the body cells and tissues. Specialized neuroepithelial cells (NECs) distributed within the gill filaments and/or lamellae of water-breathers appear to subserve this function by initiating an increase in ventilation in response to lowering of blood or water PO(2) (hypoxia). It is only recently, however, that these cells have become amenable for detailed investigations using electrophysiological tools. By contrast, the well-studied specialized neuroendocrine cells (i.e. glomus or type I cells) located principally in the carotid body of air-breathers initiate a similar reflex ventilatory response to hypoxia so as to maintain blood PO(2) homeostasis. In some species, however, the carotid body is immature and relatively insensitive to hypoxia at birth; it is during this period that their sympathoadrenal counterparts in the adrenal medulla act as key PO(2) receptors, critical for the proper transition to air-breathing life. It is becoming increasingly clear that in general these chemoreceptors act as polymodal receptors, i.e. capable of detecting several sensory modalities including high CO(2)/H(+) or acid hypercapnia. Given the phylogenetic and ontogenetic evidence pointing to homology between the mammalian carotid artery and the first gill arch of teleosts, the question arises whether the mechanisms of chemosensing are conserved among these cell types. This review examines some of the anatomical and functional similarities among these peripheral chemoreceptors, while raising the possibility that the fundamental mechanisms of O(2) and CO(2)/H(+) sensing arose first in water-breathers and are conserved among the vertebrates.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Adenosine A₂a receptors and O₂ sensing in development

Reduced mitochondrial oxidative phosphorylation, via activation of adenylate kinase and the resulting exponential rise in the cellular AMP/ATP ratio, appears to be a critical factor underlying O₂ sensing in many chemoreceptive tissues in mammals. The elevated AMP/ATP ratio, in turn, activates key enzymes that are involved in physiologic adjustments that tend to balance ATP supply and demand. An example is the conversion of AMP to adenosine via 5'-nucleotidase and the resulting activation of adenosine A(₂A) receptors, which are involved in acute oxygen sensing by both carotid bodies and the brain. In fetal sheep, A(₂A) receptors associated with carotid bodies trigger hypoxic cardiovascular chemoreflexes, while central A(₂A) receptors mediate hypoxic inhibition of breathing and rapid eye movements. A(₂A) receptors are also involved in hypoxic regulation of fetal endocrine systems, metabolism, and vascular tone. In developing lambs, A(₂A) receptors play virtually no role in O₂ sensing by the carotid bodies, but brain A(₂A) receptors remain critically involved in the roll-off ventilatory response to hypoxia. In adult mammals, A(₂A) receptors have been implicated in O₂ sensing by carotid glomus cells, while central A(₂A) receptors likely blunt hypoxic hyperventilation. In conclusion, A(₂A) receptors are crucially involved in the transduction mechanisms of O₂ sensing in fetal carotid bodies and brains. Postnatally, central A(₂A) receptors remain key mediators of hypoxic respiratory depression, but they are less critical for O₂ sensing in carotid chemoreceptors, particularly in developing lambs.

Effects of fasting on hypoxic ventilatory responses and the contribution of histamine H1 receptors in mice

We tested the hypothesis that fasting affects hypoxic ventilatory responses through metabolic changes via histamine H1 receptors. Wild-type (WT) and histamine H1 receptor knockout (H1RKO) mice were studied in fed and fasted states. In the fed WT, hypoxic-gas exposure elicited an increase and a subsequent decline in ventilation (hypoxic ventilatory decline or HVD). HVD was influenced by fasting in breathing pattern with metabolic rate. Fasting elicited hypoglycemia, a drop in R, and increases in free fatty acid and ketone bodies in the serum. In H1RKO, HVD was blunted in the fed state, but it appeared in the fasted state. There was a minimal drop in R following fasting and a low triglyceride concentration. Thus, fasting affects HVD through a change in energy mobilization from glucose to lipid metabolism. Histamine H1 receptors are involved in HVD during fed and fasted states, resulting in adaptation to the environmental conditions.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Differences in the control of breathing between Andean highlanders and lowlanders after 10 days acclimatization at 3850 m

We used Duffin's isoxic hyperoxic ( mmHg) and hypoxic ( mmHg) rebreathing tests to compare the control of breathing in eight (7 male) Andean highlanders and six (4 male) acclimatizing Caucasian lowlanders after 10 days at 3850 m. Compared to lowlanders, highlanders had an increased non-chemoreflex drive to breathe, characterized by higher basal ventilation at both hyperoxia (10.5 +/- 0.7 vs. 4.9 +/- 0.5 l min(1), P = 0.002) and hypoxia (13.8 +/- 1.4 vs. 5.7 +/- 0.9 l min(1), P < 0.001). Highlanders had a single ventilatory sensitivity to CO(2) that was lower than that of the lowlanders (P < 0.001), whose response was characterized by two ventilatory sensitivities (VeS1 and VeS2) separated by a patterning threshold. There was no difference in ventilatory recruitment thresholds (VRTs) between populations (P = 0.209). Hypoxia decreased VRT within both populations (highlanders: 36.4 +/- 1.3 to 31.7 +/- 0.7 mmHg, P < 0.001; lowlanders: 35.3 +/- 1.3 to 28.8 +/- 0.9 mmHg, P < 0.001), but it had no effect on basal ventilation (P = 0.12) or on ventilatory sensitivities in either population (P = 0.684). Within lowlanders, VeS2 was substantially greater than VeS1 at both isoxic tensions (hyperoxic: 9.9 +/- 1.7 vs. 2.8 +/- 0.2, P = 0.005; hypoxic: 13.2 +/- 1.9 vs. 2.8 +/- 0.5, P < 0.001), although hypoxia had no effect on either of the sensitivities (P = 0.192). We conclude that the control of breathing in Andean highlanders is different from that in acclimatizing lowlanders, although there are some similarities. Specifically, acclimatizing lowlanders have relatively lower non-chemoreflex drives to breathe, increased ventilatory sensitivities to CO(2), and an altered pattern of ventilatory response to CO(2) with two ventilatory sensitivities separated by a patterning threshold. Similar to highlanders and unlike lowlanders at sea-level, acclimatizing lowlanders respond to hypobaric hypoxia by decreasing their VRT instead of changing their ventilatory sensitivity to CO(2).

Differences in the control of breathing between Himalayan and sea-level residents

We compared the control of breathing of 12 male Himalayan highlanders with that of 21 male sea-level Caucasian lowlanders using isoxic hyperoxic ( = 150 mmHg) and hypoxic ( = 50 mmHg) Duffin's rebreathing tests. Highlanders had lower mean +/- s.e.m. ventilatory sensitivities to CO(2) than lowlanders at both isoxic tensions (hyperoxic: 2.3 +/- 0.3 vs. 4.2 +/- 0.3 l min(1) mmHg(1), P = 0.021; hypoxic: 2.8 +/- 0.3 vs. 7.1 +/- 0.6 l min(1) mmHg(1), P < 0.001), and the usual increase in ventilatory sensitivity to CO(2) induced by hypoxia in lowlanders was absent in highlanders (P = 0.361). Furthermore, the ventilatory recruitment threshold (VRT) CO(2) tensions in highlanders were lower than in lowlanders (hyperoxic: 33.8 +/- 0.9 vs. 48.9 +/- 0.7 mmHg, P < 0.001; hypoxic: 31.2 +/- 1.1 vs. 44.7 +/- 0.7 mmHg, P < 0.001). Both groups had reduced ventilatory recruitment thresholds with hypoxia (P < 0.001) and there were no differences in the sub-threshold ventilations (non-chemoreflex drives to breathe) between lowlanders and highlanders at both isoxic tensions (P = 0.982), with a trend for higher basal ventilation during hypoxia (P = 0.052). We conclude that control of breathing in Himalayan highlanders is distinctly different from that of sea-level lowlanders. Specifically, Himalayan highlanders have decreased central and absent peripheral sensitivities to CO(2). Their response to hypoxia was heterogeneous, with the majority decreasing their VRT indicating either a CO(2)-independent increase in activity of peripheral chemoreceptor or hypoxia-induced increase in [H(+)] at the central chemoreceptor. In some highlanders, the decrease in VRT was accompanied by an increase in sensitivity to CO(2), while in others VRT remained unchanged and their sub-threshold ventilations increased, although these were not statistically significant.

Interleukin-6 increases intracellular Ca2+ concentration and induces catecholamine secretion in rat carotid body glomus cells

Although abundant evidence indicates mutual regulation between the immune and the central nervous systems, how the immune signals are transmitted to the brain is still an unresolved question. In a previous study we found strong expression of proinflammatory cytokine receptors, including interleukin (IL)-1 receptor I and IL-6 receptor alpha in the rat carotid body (CB), a well-known arterial chemoreceptor that senses a variety of chemostimuli in the arterial blood. We demonstrated that IL-1 stimulation increases intracellular calcium ([Ca(2+)](i)) in CB glomus cells, releases ATP, and increases the discharge rate in carotid sinus nerve. To explore the effect of IL-6 on CB, here we examine the effect of IL-6 on [Ca(2+)](i) and catecholamine (CA) secretion in rat CB glomus cells. Calcium imaging showed that extracellular application of IL-6 induced a rise in [Ca(2+)](i) in cultured glomus cells. Amperometry showed that local application of IL-6 evoked CA release from glomus cells. Furthermore, the CA secretory response to IL-6 was blocked by 200 microM Cd(2+), a well-known Ca(2+) channel blocker. Our experiments provide further evidence for the responsiveness of the CB to proinflammatory cytokines and indicate that the CB might play a role in inflammation sensing and transmission of such information to the brain.

AQP1 mediates water transport in the carotid body

In this study, we explored the presence of aquaporins (AQPs), a family of membrane water channel proteins, in carotid body (CB) type I chemoreceptor cells. The CB is a polymodal chemoreceptor whose major function is to detect changes in arterial O2 tension to elicit hyperventilation during hypoxia. The CB has also been proposed to function as a systemic osmoreceptor, thus we hypothesized that the presence of AQPs in type I cell membrane may confer higher sensitivity to osmolarity changes and hence accelerate the activation of chemoreceptor cells. We detected the expression of AQP1, AQP7, and AQP8 in the CB and confirmed the location of AQP1 in type I cells. We have also shown that inhibition of AQP1 expression clearly reduced type I cell swelling after a hyposmotic shock, demonstrating that AQP1 has a major contribution in transmembrane water movement in these chemoreceptor cells. Interestingly, CB AQP1 expression levels change during postnatal development, increasing during the first postnatal weeks as the organ matures. In conclusion, in this study, we report the novel observation that AQPs are expressed in the CB. We also show that AQP1 mediates water transport across the cell membrane of type I cells, supporting the contribution of this protein to the osmoreception function of the CB.

Carotid body: new stimuli and new preparations--invited article

Beginning with the pioneering work of Heymans and collaborators in the 1930's, investigations into the role of the mammalian carotid body (CB) in the control of ventilation have attracted much attention. Progress for many years was restricted to the whole animal and organ level, resulting in characterization of the stimulus-response characteristics of the CB with its afferent nerve supply during exposure to chemostimuli such as low PO(2) (hypoxia), elevated PCO(2) (hypercapnia), and low pH (acidity). Major advances on the cellular and molecular mechanisms of chemotransduction occurred 20 years ago with the use of freshly-dissociated CB preparations and single cell studies using patch clamp and spectrofluorimetric techniques. This review will focus on more recent advances based on novel preparations including co-cultures of isolated CB receptor clusters and dispersed sensory or autonomic neurons, thin CB tissue slice preparations, and transgenic models. These preparations have contributed significantly, not only to our understanding of the transduction and neurotransmitter mechanisms that operate in the CB during sensory processing, but also to the identification and characterization of novel CB stimuli such as hypoglycemia. Though the complexity of this remarkable organ still belies its tiny size, these recent advances are slowly unraveling the intricacies surrounding its ability to act as a polymodal detector of blood-borne chemicals and to alter its sensitivity to patterned stimuli.

Postsynaptic action of GABA in modulating sensory transmission in co-cultures of rat carotid body via GABA(A) receptors

GABA is expressed in carotid body (CB) chemoreceptor type I cells and has previously been reported to modulate sensory transmission via presynaptic GABA(B) receptors. Because low doses of clinically important GABA(A) receptor (GABA(A)R) agonists, e.g. benzodiazepines, have been reported to depress afferent CB responses to hypoxia, we investigated the potential contribution of GABA(A)R in co-cultures of rat type I cells and sensory petrosal neurones (PNs). During gramicidin perforated-patch recordings (to preserve intracellular Cl-), GABA and/or the GABA(A) agonist muscimol (50 microm) induced a bicuculline-sensitive membrane depolarization in isolated PNs. GABA-induced whole-cell currents reversed at approximately -38 mV and had an EC50 of approximately 10 microm (Hill coefficient = approximately 1) at -60 mV. During simultaneous PN and type I cell recordings at functional chemosensory units in co-culture, bicuculline reversibly potentiated the PN, but not type I cell, depolarizing response to hypoxia. Application of the CB excitatory neurotransmitter ATP (1 microm) over the soma of functional PN induced a spike discharge that was markedly suppressed during co-application with GABA (2 microm), even though GABA alone was excitatory. RT-PCR analysis detected expression of GABAergic markers including mRNA for alpha1, alpha2, beta2, gamma2S, gamma2L and gamma3 GABA(A)R subunits in petrosal ganglia extracts. Also, CB extracts contained mRNAs for GABA biosynthetic markers, i.e. glutamate decarboxylase (GAD) isoforms GAD 67A,E, and GABA transporter isoforms GAT 2,3 and BGT-1. In CB sections, sensory nerve endings apposed to type I cells were immunopositive for the GABA(A)R beta subunit. These data suggest that GABA, released from the CB during hypoxia, inhibits sensory discharge postsynaptically via a shunting mechanism involving GABA(A) receptors.

Pituitary adenylate cyclase-activating polypeptide maintains neonatal breathing but not metabolism during mild reductions in ambient temperature

Mild reductions in ambient temperature dramatically increase the mortality of neonatal mice deficient in pituitary adenylate cyclase-activating polypeptide (PACAP), with the majority of animals succumbing in the second postnatal week. During anesthesia-induced hypothermia, PACAP−/− mice at this age are also vulnerable to prolonged apneas and sudden death. From these observations, we hypothesized that before the onset of genotype-specific mortality and in the absence of anesthetic, the breathing of PACAP-deficient mice is more susceptible to mild reductions in ambient temperature than wild-type littermates. To test this hypothesis, we recorded breathing in one group of postnatal day 4 PACAP+/+, +/−, and −/− neonates (using unrestrained, flow-through plethysmography) and metabolic rate in a separate group (using indirect calorimetry), both of which were exposed acutely to ambient temperatures slightly below (29°C), slightly above (36°C), or at thermoneutrality (32°C). At 32°C, the breathing frequency of PACAP−/− neonates was significantly less than PACAP+/+ littermates. Reducing the ambient temperature to 29°C caused a significant suppression of tidal volume and ventilation in both PACAP+/− and −/− animals, while the tidal volume and ventilation of PACAP+/+ animals remained unchanged. Genotype had no effect on the ventilatory responses to ambient warming. At all three ambient temperatures, genotype had no influence on oxygen consumption or body temperature. These results suggest that during mild reductions in ambient temperature, PACAP is vital for the preservation of neonatal tidal volume and ventilation, but not for metabolic rate or body temperature.

Measuring the ventilatory response to hypoxia

After defining the current approach to measuring the hypoxic ventilatory response this paper explains why this method is not appropriate for comparisons between individuals or conditions, and does not adequately measure the parameters of the peripheral chemoreflex. A measurement regime is therefore proposed that incorporates three procedures. The first procedure measures the peripheral chemoreflex responsiveness to both hypoxia and CO(2) in terms of hypoxia's effects on the sensitivity and ventilatory recruitment threshold of the peripheral chemoreflex response to CO(2). The second and third procedures employ current methods for measuring the isocapnic and poikilocapnic ventilatory responses to hypoxia, respectively, over a period of 20 min. The isocapnic measure is used to determine the time course characteristics of hypoxic ventilatory decline and the poikilocapnic measure shows the ventilatory response to a hypoxic environment. A measurement regime incorporating these three procedures will permit a detailed assessment of the peripheral chemoreflex response to hypoxia that allows comparisons to be made between individuals and different physiological and environmental conditions.

Does yawning increase arousal through mechanical stimulation of the carotid body?

Yawning is a stereotyped event that occurs in humans and animals from fish to mammals, but neither its mechanisms nor its functions are entirely known. Its widespread nature suggests that it has important physiological functions. It is associated with stretching of muscles in a large area, but the function of this stretching is understood far from completely. It has been proposed that yawning increases arousal and that it is an arousal defense reflex, whose aim is to reverse brain hypoxia. Whilst yawning has been speculated to have an important role in reversing hypoxia, there is a structure in the neck that is known to be intimately involved in the regulation of oxygen homeostasis, namely the carotid body. It senses acute changes in oxygen levels. In spite of this, a connection has never been proposed either between the carotid body and arousal, or between yawning and the carotid body. We propose that yawning stimulates mechanically the carotid body (and possibly other structures in the neck). We further propose that this stimulation gives rise to increased arousal, alertness and wakefulness and that one important physiological function of yawning is increase of arousal through this stimulation. We also propose that mechanical effects on the shunt system of the carotid body may be involved in this stimulation. Our hypothesis is supported by several facts. For example, yawning causes movements and compressions that may affect the carotid body that is situated strategically at the bifurcation of the common carotid artery. Thus, yawning may stimulate the carotid body. The carotid body is highly vascular and compressions may affect its shunt system and blood flow and for example give rise to release of hormones or other substances. Also several facts related to situations where people yawn or do not yawn support our hypothesis and are discussed. Further support comes from facts related to somnogenic substances, hormones and transmitters, and from facts related to the interconnection of homeostatic mechanisms, sleep, arousal and ventilation.

Sensing hypoxia in the carotid body: from stimulus to response

The carotid body is a peripheral sensory organ that can transduce modest falls in the arterial PO2 (partial pressure of oxygen) into a neural signal that provides the afferent limb of a set of stereotypic cardiorespiratory reflexes that are graded according to the intensity of the stimulus. The stimulus sensed is tissue PO2 and this can be estimated to be around 50 mmHg during arterial normoxia, falling to between 10–40 mmHg during hypoxia. The chemoafferent hypoxia stimulus-response curve is exponential, rising in discharge frequency with falling PO2, and with no absolute threshold apparent in hyperoxia. Although the oxygen sensor has not been definitely identified, it is believed to reside within type I cells of the carotid body, and presently two major hypotheses have been put forward to account for the sensing mechanism. The first relies upon alterations in the cell energy status that is sensed by the cytosolic enzyme AMPK (AMP-activated protein kinase) subsequent to hypoxia-induced increases in the cellular AMP/ATP ratio during hypoxia. AMPK is localized close to the plasma membrane and its activation can inhibit both large conductance, calcium-activated potassium (BK) and background, TASK-like potassium channels, inducing membrane depolarization, voltage-gated calcium entry and neurosecretion of a range of transmitter and modulator substances, including catecholamines, ATP and acetylcholine. The alternative hypothesis considers a role for haemoxygenase-2, which uses oxygen as a substrate and may act to gate an associated BK channel through the action of its products, carbon monoxide and possibly haem. It is likely however, that these and other hypotheses of oxygen transduction are not mutually exclusive and that each plays a role, via its own particular sensitivity, in shaping the full response of this organ between hyperoxia and anoxia.