Oxygen and acid chemoreception in the carotid body chemoreceptors

Controlled Mechanical Ventilation in Critically Ill Patients and the Potential Role of Venous Bagging in Acute Kidney Injury

A very low incidence of acute kidney injury (AKI) has been observed in COVID-19 patients purposefully treated with early pressure support ventilation (PSV) compared to those receiving mainly controlled ventilation. The prevention of subdiaphragmatic venous congestion through limited fluid intake and the lowering of intrathoracic pressure is a possible and attractive explanation for this observed phenomenon. Both venous congestion, or "venous bagging", and a positive fluid balance correlate with the occurrence of AKI. The impact of PSV on venous return, in addition to the effects of limiting intravenous fluids, may, at least in part, explain this even more clearly when there is no primary kidney disease or the presence of nephrotoxins. Optimizing the patient-ventilator interaction in PSV is challenging, in part because of the need for the ongoing titration of sedatives and opioids. The known benefits include improved ventilation/perfusion matching and reduced ventilator time. Furthermore, conservative fluid management positively influences cognitive and psychiatric morbidities in ICU patients and survivors. Here, it is hypothesized that cranial lymphatic congestion in relation to a more positive intrathoracic pressure, i.e., in patients predominantly treated with controlled mechanical ventilation (CMV), is a contributing risk factor for ICU delirium. No studies have addressed the question of how PSV can limit AKI, nor are there studies providing high-level evidence relating controlled mechanical ventilation to AKI. For this perspective article, we discuss studies in the literature demonstrating the effects of venous congestion leading to AKI. We aim to shed light on early PSV as a preventive measure, especially for the development of AKI and ICU delirium and emphasize the need for further research in this domain.

Clinical risk factors for increased respiratory drive in intubated hypoxemic patients

BACKGROUND

There is very limited evidence identifying factors that increase respiratory drive in hypoxemic intubated patients. Most physiological determinants of respiratory drive cannot be directly assessed at the bedside (e.g., neural inputs from chemo- or mechano-receptors), but clinical risk factors commonly measured in intubated patients could be correlated with increased drive. We aimed to identify clinical risk factors independently associated with increased respiratory drive in intubated hypoxemic patients.

METHODS

We analyzed the physiological dataset from a multicenter trial on intubated hypoxemic patients on pressure support (PS). Patients with simultaneous assessment of the inspiratory drop in airway pressure at 0.1-s during an occlusion (P0.1) and risk factors for increased respiratory drive on day 1 were included. We evaluated the independent correlation of the following clinical risk factors for increased drive with P0.1: severity of lung injury (unilateral vs. bilateral pulmonary infiltrates, PaO2/FiO2, ventilatory ratio); arterial blood gases (PaO2, PaCO2 and pHa); sedation (RASS score and drug type); SOFA score; arterial lactate; ventilation settings (PEEP, level of PS, addition of sigh breaths).

RESULTS

Two-hundred seventeen patients were included. Clinical risk factors independently correlated with higher P0.1 were bilateral infiltrates (increase ratio [IR] 1.233, 95%CI 1.047-1.451, p = 0.012); lower PaO2/FiO2 (IR 0.998, 95%CI 0.997-0.999, p = 0.004); higher ventilatory ratio (IR 1.538, 95%CI 1.267-1.867, p < 0.001); lower pHa (IR 0.104, 95%CI 0.024-0.464, p = 0.003). Higher PEEP was correlated with lower P0.1 (IR 0.951, 95%CI 0.921-0.982, p = 0.002), while sedation depth and drugs were not associated with P0.1.

CONCLUSIONS

Independent clinical risk factors for higher respiratory drive in intubated hypoxemic patients include the extent of lung edema and of ventilation-perfusion mismatch, lower pHa, and lower PEEP, while sedation strategy does not affect drive. These data underline the multifactorial nature of increased respiratory drive.

The Role of Pharmacological Treatment in the Chemoreflex Modulation

From a physiological point of view, peripheral chemoreceptors (PCh) are the main sensors of hypoxia in mammals and are responsible for adaptation to hypoxic conditions. Their stimulation causes hyperventilation—to increase oxygen uptake and increases sympathetic output in order to counteract hypoxia-induced vasodilatation and redistribute the oxygenated blood to critical organs. While this reaction promotes survival in acute settings it may be devastating when long-lasting. The permanent overfunctionality of PCh is one of the etiologic factors and is responsible for the progression of sympathetically-mediated diseases. Thus, the deactivation of PCh has been proposed as a treatment method for these disorders. We review here physiological background and current knowledge regarding the influence of widely prescribed medications on PCh acute and tonic activities.

Leptin: Master Regulator of Biological Functions that Affects Breathing

Obesity is a global epidemic in developed countries accounting for many of the metabolic and cardiorespiratory morbidities that occur in adults. These morbidities include type 2 diabetes, sleep-disordered breathing (SDB), obstructive sleep apnea, chronic intermittent hypoxia, and hypertension. Leptin, produced by adipocytes, is a master regulator of metabolism and of many other biological functions including central and peripheral circuits that control breathing. By binding to receptors on cells and neurons in the brainstem, hypothalamus, and carotid body, leptin links energy and metabolism to breathing. In this comprehensive article, we review the central and peripheral locations of leptin's actions that affect cardiorespiratory responses during health and disease, with a particular focus on obesity, SDB, and its effects during early development. Obesity-induced hyperleptinemia is associated with centrally mediated hypoventilation with decrease CO₂ sensitivity. On the other hand, hyperleptinemia augments peripheral chemoreflexes to hypoxia and induces sympathoexcitation. Thus, "leptin resistance" in obesity is relative. We delineate the circuits responsible for these divergent effects, including signaling pathways. We review the unique effects of leptin during development on organogenesis, feeding behavior, and cardiorespiratory responses, and how undernutrition and overnutrition during critical periods of development can lead to cardiorespiratory comorbidities in adulthood. We conclude with suggestions for future directions to improve our understanding of leptin dysregulation and associated clinical diseases and possible therapeutic targets. Lastly, we briefly discuss the yin and the yang, specifically the contribution of relative adiponectin deficiency in adults with hyperleptinemia to the development of metabolic and cardiovascular disease. © 2020 American Physiological Society. Compr Physiol 10:1047-1083, 2020.

Hydrogen Sulfide as an O2 Sensor: A Critical Analysis

There is increasing interest in the physiological actions and therapeutic potential of the gasotransmitter hydrogen sulfide (H₂S). In addition to exerting antihypertensive, anti-inflammatory, antioxidant, and pro-angiogenic effects, H₂S has been suggested to play a central and ubiquitous role in O₂ sensing. According to this concept, because H₂S is metabolized by oxidation, its cellular concentration varies inversely with the ambient pO₂ such that hypoxia causes a rise in intracellular [H₂S]; this then acts to induce appropriate cellular responses. In particular, it has been proposed that H₂S underpins O₂ sensing in the carotid body, which triggers increases in ventilation in response to hypoxemia, and also in pulmonary arteries, which constrict in response to local alveolar hypoxia. This process, termed hypoxic pulmonary vasoconstriction (HPV), acts to divert blood to better-oxygenated regions of the lung, thereby maintaining the ventilation-perfusion ratio and minimizing hypoxia-induced falls in blood O₂ saturation. In this chapter, we present a critical review of the evidence supporting and questioning this model in both HPV and the carotid body.

Acute Effects of Systemic Erythropoietin Injections on Carotid Body Chemosensory Activity Following Hypoxic and Hypercapnic Stimulation

The carotid body (CB) chemoreceptors sense changes in arterial blood gases. Upon stimulation CB chemoreceptors cells release one or more transmitters to excite sensory nerve fibers of the carotid sinus nerve. While several neurotransmitters have been described to contribute to the CB chemosensory process less is known about modulatory molecules. Recent data suggest that erythropoietin (Epo) is involved in the control of ventilation, and it has been shown that Epo receptor is constitutively expressed in the CB chemoreceptors, suggesting a possible role for Epo in regulation of CB function. Therefore, in the present study we aimed to determine whether exogenous applications of Epo modulate the hypoxic and hypercapnic CB chemosensory responses. Carotid sinus nerve discharge was recorded in-situ from anesthetized adult male and female Sprague Dawley rats (350 g, n = 8) before and after systemic administration of Epo (2000 UI/kg). CB-chemosensitivity to hypoxia and hypercapnia was calculated by exposing the rat to F_iO₂ 5-15% and F_iCO₂ 10% gas mixtures, respectively. During baseline recordings at normoxia, we found no effects of Epo on CB activity both in male and female rats. In addition, Epo had no effect on maximal CB response to hypoxia in both male and female rats. Epo injections enhanced the maximum CB chemosensory response to hypercapnia in female rats (before vs. after Epo, 72.5 ± 7.1 Hz vs. 108.3 ± 6.9 Hz, p < 0.05). In contrast, Epo had no effect on maximum CB chemosensory response to hypercapnia in male rats but significantly increased the response recovery times (time required to return to baseline discharge following hypercapnic stimulus) from 2.1 ± 0.1 s to 8.2 ± 2.3 s (p < 0.05). Taken together, our results suggest that Epo has some modulatory effect on the CB chemosensory response to hypercapnia.

The oxygen-conserving potential of the diving response: A kinetic-based analysis

We investigated the oxygen-conserving potential of the human diving response by comparing trained breath-hold divers (BHDs) to non-divers (NDs) during simulated dynamic breath-holding (BH). Changes in haemodynamics [heart rate (HR), stroke volume (SV), cardiac output (CO)] and peripheral muscle oxygenation [oxyhaemoglobin ([HbO₂]), deoxyhaemoglobin ([HHb]), total haemoglobin ([tHb]), tissue saturation index (TSI)] and peripheral oxygen saturation (SpO₂) were continuously recorded during simulated dynamic BH. BHDs showed a breaking point in HR kinetics at mid-BH immediately preceding a more pronounced drop in HR (-0.86 bpm.%^-1) while HR kinetics in NDs steadily decreased throughout BH (-0.47 bpm.%^-1). By contrast, SV remained unchanged during BH in both groups (all P > 0.05). Near-infrared spectroscopy (NIRS) results (mean ± SD) expressed as percentage changes from the initial values showed a lower [HHb] increase for BHDs than for NDs at the cessation of BH (+24.0 ± 10.1 vs. +39.2 ± 9.6%, respectively; P < 0.05). As a result, BHDs showed a [tHb] drop that NDs did not at the end of BH (-7.3 ± 3.2 vs. -3.0 ± 4.7%, respectively; P < 0.05). The most striking finding of the present study was that BHDs presented an increase in oxygen-conserving efficiency due to substantial shifts in both cardiac and peripheral haemodynamics during simulated BH. In addition, the kinetic-based approach we used provides further credence to the concept of an "oxygen-conserving breaking point" in the human diving response.

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception

The cascade of transduction of hypoxia and hypercapnia, the natural stimuli to chemoreceptor cells, is incompletely understood. A particular gap in that knowledge is the role played by second messengers, or in a most ample term, of modulators. A recently described modulator of chemoreceptor cell responses is the gaseous transmitter hydrogen sulfide, which has been proposed as a specific activator of the hypoxic responses in the carotid body, both at the level of the chemoreceptor cell response or at the level of the global output of the organ. Since sulfide behaves in this regard as cAMP, we explored the possibility that sulfide effects were mediated by the more classical messenger. Data indicate that exogenous and endogenous sulfide inhibits adenyl cyclase finding additionally that inhibition of adenylyl cyclase does not modify chemoreceptor cell responses elicited by sulfide. We have also observed that transient receptor potential cation channels A1 (TRPA1) are not regulated by sulfide in chemoreceptor cells.

Hypoxic pulmonary vasoconstriction, carotid body function and erythropoietin production in adult rats perinatally exposed to hyperoxia

KEY POINTS

Adult animals that have been perinatally exposed to oxygen-rich atmospheres (hyperoxia), recalling those used for oxygen therapy in infants, exhibit a loss of hypoxic pulmonary vasoconstriction, whereas vasoconstriction elicited by depolarizing agents is maintained. Loss of pulmonary hypoxic vasoconstriction is not linked to alterations in oxygen-sensitive K(+) currents in pulmonary artery smooth muscle cells. Loss of hypoxic vasoconstriction is associated with early postnatal oxidative damage and corrected by an antioxidant diet. Perinatal hyperoxia damages carotid body chemoreceptor cell function and the antioxidant diet does not reverse it. The hypoxia-elicited increase in erythropoietin plasma levels is not affected by perinatal hyperoxia. The potential clinical significance of the findings in clinical situations such as pneumonia, chronic obstructive pulmonary disease or general anaesthesia is considered.

ABSTRACT

Adult mammalians possess three cell systems that are activated by acute bodily hypoxia: pulmonary artery smooth muscle cells (PASMC), carotid body chemoreceptor cells (CBCC) and erythropoietin (EPO)-producing cells. In rats, chronic perinatal hyperoxia causes permanent carotid body (CB) atrophy and functional alterations of surviving CBCC. There are no studies on PASMC or EPO-producing cells. Our aim is to define possible long-lasting functional changes in PASMC or EPO-producing cells (measured as EPO plasma levels) and, further, to analyse CBCC functional alterations. We used 3- to 4-month-old rats born and reared in a normal atmosphere or exposed to perinatal hyperoxia (55-60% O2 for the last 5-6 days of pregnancy and 4 weeks after birth). Perinatal hyperoxia causes an almost complete loss of hypoxic pulmonary vasoconstriction (HPV), which was correlated with lung oxidative status in early postnatal life and prevented by antioxidant supplementation in the diet. O2 -sensitivity of K(+) currents in the PASMC of hyperoxic animals is normal, indicating that their inhibition is not sufficient to trigger HPV. Perinatal hyperoxia also abrogated responses elicited by hypoxia on catecholamine and cAMP metabolism in the CB. An increase in EPO plasma levels elicited by hypoxia was identical in hyperoxic and control animals, implying a normal functioning of EPO-producing cells. The loss of HPV observed in adult rats and caused by perinatal hyperoxia, comparable to oxygen therapy in premature infants, might represent a previously unrecognized complication of such a medical intervention capable of aggravating medical conditions such as regional pneumonias, atelectases or general anaesthesia in adult life.

Pulmonary neuroepithelial bodies are polymodal airway sensors: Evidence for CO2/H+ sensing

Pulmonary neuroepithelial bodies (NEB) in mammalian lungs are thought to function as airway O2 sensors that release serotonin (5-HT) in response to hypoxia. Direct evidence that NEB cells also respond to airway hypercapnia/acidosis (CO2/H+) is presently lacking. We tested the effects of CO2/H+ alone or in combination with hypoxia on 5-HT release from intact NEB cells in a neonatal hamster lung slice model. For the detection of 5-HT release we used carbon fiber amperometry. Fluorescence Ca2+ imaging method was used to assess CO2/H+-evoked changes in intracellular Ca2+. Exposure to 10 and 20% CO2 or pH 6.8–7.2 evoked significant release of 5-HT with a distinct rise in intracellular Ca2+ in hamster NEBs. This secretory response was dependent on the voltage-gated entry of extracellular Ca2+. Moreover, the combined effects of hypercapnia and hypoxia were additive. Critically, an inhibitor of carbonic anhydrase (CA), acetazolamide, suppressed CO2/H+-mediated 5-HT release. The expression of mRNAs for various CA isotypes, including CAII, was identified in NEB cells from human lung, and protein expression was confirmed by immunohistochemistry using a specific anti-CAII antibody on sections of human and hamster lung. Taken together our findings provide strong evidence for CO2/H+ sensing by NEB cells and support their role as polymodal airway sensors with as yet to be defined functions under normal and disease conditions.

Modeling the diving bradycardia: Toward an "oxygen-conserving breaking point"?

PURPOSE

Although it has been demonstrated that the exponential decay model fits the heart rate (HR) kinetics in short static breath holding (BH), this model might be inaccurate when BH is maintained for several minutes. The aim of this study was to build a new meaningful model to quantify HR kinetics during prolonged static BH.

METHODS

Nonlinear regression analysis was used to build a model able to quantify the beat-to-beat HR reduction kinetics observed in prolonged static BH performed both in air and in immersed condition by 11 trained breath-hold divers. Dynamic changes in cardiac autonomic regulation through heart rate variability indices [root mean square of successive difference of R-R intervals (RMSSD); short-term fractal scaling exponent: (DFAα1)] and peripheral oxygen saturation (SpO2) were also analyzed to strengthen the model.

RESULTS

The tri-phasic model showed a sharp exponential drop in HR immediately followed by a slight linear rise up until a breaking point preceding a linear drop in HR. The breaking points had similar level of SpO2 whether in air or in immersed condition (95.1 ± 2.1 vs. 95.2 ± 3.0 %, respectively; P = 0.49), and the subsequent linear drop in HR was concomitant with a shift in cardiac autonomic regulation in air (RMSSD: +109.0 ± 47.8 %; P < 0.001; DFAα1: -18.0 ± 17.4 %; P < 0.05) and in immersion (RMSSD: +112.6 ± 55.8 %; P < 0.001; DFAα1: -26.0 ± 12 %; P < 0.001).

CONCLUSION

In addition to accurately fitting the HR kinetics, the most striking finding is an "oxygen-conserving breaking point" highlighted by the model, which might be interpreted as unique adaptive feature against hypoxic damages in the human diving bradycardia.

The global peripheral chemoreflex drive in patients with systemic sclerosis: a rebreathing and exercise study

BACKGROUND

Exercise intolerance (EI) in systemic sclerosis (SSc) is difficult to manage by the clinician. The peripheral chemoreflex drive compensates for metabolic acidosis during exercise and may be related to EI.

AIM

To assess the global peripheral chemoreflex drive (GPCD) in patients with SSc at rest and during exercise.

METHODS

Consecutively tested SSc patients (n = 49) were evaluated by pulmonary function tests, carbon dioxide (CO2) rebreathing studies and non-invasive cardiopulmonary exercise testing (CPET). Results of their CO2 rebreathing tests were compared with those of controls (n = 32). Respiratory compensation for metabolic acidosis during CPET was defined by the occurrence of a sharp increase in minute ventilation (VdotE) and the ventilatory equivalent for CO2 (V'E and V'CO2) at the end of the isocapnic buffer phase. Euoxic (eVHR) and hyperoxic (hVHR) ventilatory responses to hypercapnia were measured and its difference (eVHR - hVHR) was considered to reflect the GPCD.

RESULTS

In 45 patients with SSc, CPET results showed respiratory compensation at the occurrence of metabolic acidosis. eVHR - hVHR in patients with diffuse cutaneous SSc (dcSSc) differed significantly from that in patients with limited cutaneous SSc (lcSSc) and from that in controls (0.47 ± 0.38 (dcSSc) vs. 0.90 ± 0.77 (lcSSc) and 0.90 ± 0.49 (controls) l/min/mmHg; P = 0.04 and P = 0.03, respectively).

CONCLUSIONS

Respiratory compensation for metabolic acidosis occurred in all patients. However, the GPCD was diminished in dcSSc patients, suggesting an altered control of breathing. Its assessment may help the clinician to better understand reported EI and exertional dyspnea in dcSSc patients.

Fernando de Castro and the discovery of the arterial chemoreceptors

When de Castro entered the carotid body (CB) field, the organ was considered to be a small autonomic ganglion, a gland, a glomus or glomerulus, or a paraganglion. In his 1928 paper, de Castro concluded: “In sum, the Glomus caroticum is innervated by centripetal fibers, whose trophic centers are located in the sensory ganglia of the glossopharyngeal, and not by centrifugal [efferent] or secretomotor fibers as is the case for glands; these are precisely the facts which lead to suppose that the Glomus caroticum is a sensory organ.” A few pages down, de Castro wrote: “The Glomus represents an organ with multiple receptors furnished with specialized receptor cells like those of other sensory organs [taste buds?]…As a plausible hypothesis we propose that the Glomus caroticum represents a sensory organ, at present the only one in its kind, dedicated to capture certain qualitative variations in the composition of blood, a function that, possibly by a reflex mechanism would have an effect on the functional activity of other organs… Therefore, the sensory fiber would not be directly stimulated by blood, but via the intermediation of the epithelial cells of the organ, which, as their structure suggests, possess a secretory function which would participate in the stimulation of the centripetal fibers.” In our article we will recreate the experiments that allowed Fernando de Castro to reach this first conclusion. Also, we will scrutinize the natural endowments and the scientific knowledge that drove de Castro to make the triple hypotheses: the CB as chemoreceptor (variations in blood composition), as a secondary sensory receptor which functioning involves a chemical synapse, and as a center, origin of systemic reflexes. After a brief account of the systemic reflex effects resulting from the CB stimulation, we will complete our article with a general view of the cellular-molecular mechanisms currently thought to be involved in the functioning of this arterial chemoreceptor.

Increase in cytosolic Ca2+ produced by hypoxia and other depolarizing stimuli activates a non-selective cation channel in chemoreceptor cells of rat carotid body

The current model of O2 sensing by carotid body chemoreceptor (glomus) cells is that hypoxia inhibits the outward K(+) current and causes cell depolarization, Ca(2+) influx via voltage-dependent Ca(2+) channels and a rise in intracellular [Ca(2+)] ([Ca(2+)]i). Here we show that hypoxia (<5% O2), in addition to inhibiting the two-pore domain K(+) channels TASK-1/3 (TASK), indirectly activates an ∼20 pS channel in isolated glomus cells. The 20 pS channel was permeable to K(+), Na(+) and Cs(+) but not to Cl(-) or Ca(2+). The 20 pS channel was not sensitive to voltage. Inhibition of TASK by external acid, depolarization of glomus cells with high external KCl (20 mm) or opening of the Ca(2+) channel with FPL64176 activated the 20 pS channel when 1 mm Ca(2+) was present in the external solution. Ca(2+) (10 μm) applied to the cytosolic side of inside-out patches activated the 20 pS channel. The threshold [Ca(2+)]i for activation of the 20 pS channel in cell-attached patches was ∼200 nm. The reversal potential of the 20 pS channel was estimated to be -28 mV. Our results reveal a sequential mechanism in which hypoxia (<5% O2) first inhibits the K(+) conductance and then activates a Na(+)-permeable, non-selective cation channel via depolarization-induced rise in [Ca(2+)]i. Our results suggest that inhibition of K(+) efflux and stimulation of Na(+) influx both contribute to the depolarization of glomus cells during moderate to severe hypoxia.

Differences in cardiovascular and hypothalamic-pituitary-adrenal axis functions between high-altitude visitors and natives during a trek on the Annapurna circuit

OBJECTIVE

Differences in the cardiovascular and hypothalamic-pituitary-adrenal (HPA) axis functions at high altitudes (HAs) between visitors to and natives of HA were examined.

METHODS

The cardiovascular functions and peripheral oxygen saturation (SPO2) were monitored, and the cortisol awakening response (CAR) and nighttime cortisol concentration (NCC), as indices of the HPA axis function, were determined in 25 trekkers and 21 Sherpas during an Annapurna circuit trek.

RESULTS

SPO2 decreased less in the Sherpas than in the trekkers at HAs (3,540, 3,800, and 4,800 m). Blood pressure and heart rate in the Sherpas changed concurrently during the trek; however, a tachycardic response occurred without changes in blood pressure in the trekkers at HAs. The CAR and NCC at HAs in the trekkers differed from those observed at 1,100 m and those observed at HAs in the Sherpas. The trekkers exhibited an elevated morning cortisol level at 3,540 and 3,800 m, a heightened CAR at 4,800 m, and an elevated NCC at 3,800 m. Alteration of the CAR resulted in an increase in the integrated volume of cortisol released within the first hour after awakening (CARauc) in the trekkers. The changes in SPO2 occurred concurrently with the changes in the CARauc and the heart rate in the trekkers.

CONCLUSIONS

The alterations of CAR occurred at HAs where blood pressure levels reached a peak plateau, which is associated with an increase in heart rate at HAs in the trekkers. The CAR was unaltered in the Sherpas during the trek.

Baroreflex and chemoreflex controls of sympathetic activity following intermittent hypoxia

There is a large amount of evidence linking obstructive sleep apnea (OSA), and the associated intermittent hypoxia that accompanies it, with the development of hypertension. For example, cross-sectional studies demonstrate that the prevalence of hypertension increases with the severity of OSA (Bixler et al., 2000; Grote et al., 2001) and an initial determination of OSA is associated with a three-fold increase for future hypertension (Peppard et al., 2000). Interestingly, bouts of intermittent hypoxia have also been shown to affect sympathetic output associated with the baroreflex and chemoreflex, important mechanisms in the regulation of arterial blood pressure. As such, the possibility exists that changes in the baroreflex and chemoreflex may contribute to the development of chronic hypertension observed in OSA patients. The aim of the current article is to briefly review the response of the baroreflex and chemoreflex to intermittent hypoxic exposure and to evaluate evidence for the hypothesis that modification of these autonomic reflexes may, at least in part, support the comorbidity observed between chronic hypertension and OSA.

The Na(+)/H (+) exchanger NHE5 is sorted to discrete intracellular vesicles in the central and peripheral nervous systems

The pH milieu of the central and peripheral nervous systems is an important determinant of neuronal excitability, function, and survival. In mammals, neural acid-base homeostasis is coordinately regulated by ion transporters belonging to the Na(+)/H(+) exchanger (NHE) and bicarbonate transporter gene families. However, the relative contributions of individual isoforms within the respective families are not fully understood. This report focuses on the NHE family, specifically the plasma membrane-type NHE5 which is preferentially transcribed in brain, but the distribution of the native protein has not been extensively characterized. To this end, we generated a rabbit polyclonal antibody that specifically recognizes NHE5. In both central (cortex, hippocampus) and peripheral (superior cervical ganglia, SCG) nervous tissue of mice, NHE5 immunostaining was punctate and highly concentrated in the somas and to lesser amounts in the dendrites of neurons. Very little signal was detected in axons. Similarly, in primary cultures of differentiated SCG neurons, NHE5 localized predominantly to vesicles in the somatodendritic compartment, though some immunostaining was also evident in punctate vesicles along the axons. NHE5 was also detected predominantly in intracellular vesicles of cultured SCG glial cells. Dual immunolabeling of SCG neurons showed that NHE5 did not colocalize with markers for early endosomes (EEA1) or synaptic vesicles (synaptophysin), but did partially colocalize with the transferrin receptor, a marker of recycling endosomes. Collectively, these data suggest that NHE5 partitions into a unique vesicular pool in neurons that shares some characteristics of recycling endosomes where it may serve as an important regulated store of functional transporters required to maintain cytoplasmic pH homeostasis.

Serotonergic neuroepithelial cells of the skin in developing zebrafish: morphology, innervation and oxygen-sensitive properties

In teleost fish, O(2) chemoreceptors of the gills (neuroepithelial cells or NECs) initiate cardiorespiratory reflexes during hypoxia. In developing zebrafish, hyperventilatory and behavioural responses to hypoxia are observed before development of gill NECs, indicating that extrabranchial chemoreceptors mediate these responses in embryos. We have characterised a population of cells of the skin in developing zebrafish that resemble O(2)-chemoreceptive gill NECs. Skin NECs were identified by serotonin immunolabelling and were distributed over the entire skin surface. These cells contained synaptic vesicles and were associated with nerve fibres. Skin NECs were first evident in embryos 24-26 h post-fertilisation (h.p.f.), and embryos developed a behavioural response to hypoxia between 24 and 48 h.p.f. The total number of NECs declined with age from approximately 300 cells per larva at 3 days post-fertilisation (d.p.f.) to ~120 cells at 7 d.p.f., and were rarely observed in adults. Acclimation to hypoxia (30 mmHg) or hyperoxia (300 mmHg) resulted in delayed or accelerated development, respectively, of peak resting ventilatory frequency and produced changes in the ventilatory response to hypoxia. In hypoxia-acclimated larvae, the temporal pattern of skin NECs was altered such that the number of cells did not decrease with age. By contrast, hyperoxia produced a more rapid decline in NEC number. The neurotoxin 6-hydroxydopamine degraded catecholaminergic nerve terminals that made contact with skin NECs and eliminated the hyperventilatory response to hypoxia. These results indicate that skin NECs are sensitive to changes in O(2) and suggest that they may play a role in initiating responses to hypoxia in developing zebrafish.

Activation of the orexin 1 receptor is a critical component of CO2-mediated anxiety and hypertension but not bradycardia

Acute hypercapnia (elevated arterial CO(2)/H(+)) is a suffocation signal that is life threatening and rapidly mobilizes adaptive changes in breathing and behavioral arousal in order to restore acid-base homeostasis. Severe hypercapnia, seen in respiratory disorders (eg, asthma or bronchitis, chronic obstructive pulmonary disease (COPD)), also results in high anxiety and autonomic activation. Recent evidence has demonstrated that wake-promoting hypothalamic orexin (ORX: also known as hypocretin) neurons are highly sensitive to local changes in CO(2)/H(+), and mice lacking prepro-ORX have blunted respiratory responses to hypercapnia. Furthermore, in a recent clinical study, ORX-A, which crosses blood-brain barrier easily, was dramatically increased in the plasma of patients with COPD and hypercapnic respiratory failure. This is consistent with a rodent model of COPD where chronic exposure to cigarette smoke led to a threefold increase in hypothalamic ORX-A expression. In the present study, we determined the role of ORX in the anxiety-like behavior and cardiorespiratory responses to acute exposure to a threshold panic challenge (ie, 20% CO(2)/normoxic gas). Exposing conscious rats to such hypercapnic, but not atmospheric air, resulted in respiratory, pressor, and bradycardic responses, as well as anxiety-like behavior and increased cellular c-Fos responses in ORX neurons. Systemically, pre-treating rats with a centrally active ORX1 receptor antagonist (30 mg/kg SB334867) attenuated hypercapnic gas-induced pressor and anxiety responses, without altering the robust bradycardia response, and only attenuated breathing responses at offset of the CO(2) challenge. Our results show that the ORX system has an important role in anxiety and sympathetic mobilization during hypercapnia. Furthermore, ORX1 receptor antagonists may be a therapeutic option rapidly treating increased anxiety and sympathetic drive seen during panic attacks and in hypercapnic states such as COPD.

Autocrine and paracrine actions of ATP in rat carotid body

Carotid bodies are peripheral chemoreceptors that detect lowering of arterial blood O(2) level. The carotid body comprises clusters of glomus (type I) cells surrounded by glial-like sustentacular (type II) cells. Hypoxia triggers depolarization and cytosolic [Ca(2+)] ([Ca(2+)](i)) elevation in glomus cells, resulting in the release of multiple transmitters, including ATP. While ATP has been shown to be an important excitatory transmitter in the stimulation of carotid sinus nerve, there is considerable evidence that ATP exerts autocrine and paracrine actions in carotid body. ATP acting via P2Y(1) receptors, causes hyperpolarization in glomus cells and inhibits the hypoxia-mediated [Ca(2+)](i) rise. In contrast, adenosine (an ATP metabolite) triggers depolarization and [Ca(2+)](i) rise in glomus cells via A(2A) receptors. We suggest that during prolonged hypoxia, the negative and positive feedback actions of ATP and adenosine may result in an oscillatory Ca(2+) signal in glomus cells. Such mechanisms may allow cyclic release of transmitters from glomus cells during prolonged hypoxia without causing cellular damage from a persistent [Ca(2+)](i) rise. ATP also stimulates intracellular Ca(2+) release in sustentacular cells via P2Y(2) receptors. The autocine and paracrine actions of ATP suggest that ATP has important roles in coordinating chemosensory transmission in the carotid body.

Effects of low glucose on carotid body chemoreceptor cell activity studied in cultures of intact organs and in dissociated cells

The participation of the carotid body (CB) in glucose homeostasis and evidence obtained in simplified cultured CB slices or dissociated cells have led to the proposal that CB chemoreceptor cells are glucoreceptors. However, data generated in intact, freshly excised organs deny CB chemoreceptor cells' glucosensing properties. The physiological significance of the contention has prompted the present study, performed in a newly developed preparation of the intact CB organ in culture that maintains chemoreceptor cells' microenvironment. Chemoreceptor cells of intact CBs in culture retained their capacity to store, synthesize, and secrete catecholamine in response to hypoxia for at least 6 days. Aglycemia did not elicit neurosecretion in dissociated chemoreceptor cells or in intact CB in culture, but potentiated hypoxia-elicited neurosecretion, exclusively, in 1-day-old intact CB cultures and dissociated chemoreceptor cells cultured for 24 h. In fura 2-loaded cells, aglycemia (but not 1 mM) caused a slow Ca(2+)-dependent and nifedipine-insensitive increase in fluorescence at 340- to 380-nm wavelength emission ratio and augmented the fluorescent signal elicited by hypoxia. Association of nifedipine and KBR7943 (a Na(+)/Ca(2+) exchanger inhibitor) completely abolished the aglycemic Ca(2+) response. We conclude that chemoreceptor cells are not sensitive to hypoglycemia. We hypothesize that cultured chemoreceptor cells become transiently more dependent on glycolysis. Consequently, aglycemia would partially inhibit the Na(+)/K(+) pump, causing an increase in intracellular Na(+) concentration, and a reversal of Na(+)/Ca(2+) exchanger. This would slowly increase intracellular Ca(2+) concentration and cause the potentiation of the hypoxic responses. We discuss the nature of the signals detected by chemoreceptor cells for the CB to achieve its glycemic homeostatic role.

Tetrodotoxin as a tool to elucidate sensory transduction mechanisms: the case for the arterial chemoreceptors of the carotid body

Carotid bodies (CBs) are secondary sensory receptors in which the sensing elements, chemoreceptor cells, are activated by decreases in arterial PO₂ (hypoxic hypoxia). Upon activation, chemoreceptor cells (also known as Type I and glomus cells) increase their rate of release of neurotransmitters that drive the sensory activity in the carotid sinus nerve (CSN) which ends in the brain stem where reflex responses are coordinated. When challenged with hypoxic hypoxia, the physiopathologically most relevant stimulus to the CBs, they are activated and initiate ventilatory and cardiocirculatory reflexes. Reflex increase in minute volume ventilation promotes CO₂ removal from alveoli and a decrease in alveolar PCO₂ ensues. Reduced alveolar PCO₂ makes possible alveolar and arterial PO₂ to increase minimizing the intensity of hypoxia. The ventilatory effect, in conjunction the cardiocirculatory components of the CB chemoreflex, tend to maintain an adequate supply of oxygen to the tissues. The CB has been the focus of attention since the discovery of its nature as a sensory organ by de Castro (1928) and the discovery of its function as the origin of ventilatory reflexes by Heymans group (1930). A great deal of effort has been focused on the study of the mechanisms involved in O₂ detection. This review is devoted to this topic, mechanisms of oxygen sensing. Starting from a summary of the main theories evolving through the years, we will emphasize the nature and significance of the findings obtained with veratridine and tetrodotoxin (TTX) in the genesis of current models of O₂-sensing.

Carotid body function and ventilatory responses in intermittent hypoxia. Evidence for anomalous brainstem integration of arterial chemoreceptor input

Obstructive sleep apnea is a frequent medical condition consisting in repetitive sleep-related episodes of upper airways obstruction and concurrent events of arterial blood hypoxia. There is a frequent association of cardiovascular diseases and other pathologies to this condition conforming the obstructive sleep apnea syndrome (OSAS). Laboratory models of OSAS consist in animals exposed to repetitive episodes of intermittent hypoxia (IH) which also develop cardiovascular pathologies, mostly hypertension. The overall OSAS pathophysiology appears to be linked to the repetitive hypoxia, which would cause a sensitization of carotid body (CB) chemoreflex and chemoreflex-driven hyperreactivity of the sympathetic nervous system. However, this proposal is uncertain because hyperventilation, reflecting the CB sensitization, and increased plasma CA levels, reflecting sympathetic hyperreactivity, are not constant findings in patients with OSAS and IH animals. Aiming to solve these uncertainties we have studied the entire CB chemoreflex arch in a rat model of IH, including activity of chemoreceptor cells and CB generated afferent activity to brainstem. The efferent activity was measured as ventilation in normoxia, hypoxia, and hypercapnia. Norepinephrine turnover in renal artery sympathetic endings was also assessed. Findings indicate a sensitization of the CB function to hypoxia evidenced by exaggerated chemoreceptor cell and CB afferent activity. Yet, IH rats exhibited marked hypoventilation in all studied conditions and increased turnover of norepinephrine in sympathetic endings. We conclude that IH produces a bias in the integration of the input arising from the CB with a diminished drive of ventilation and an exaggerated activation of brainstem sympathetic neurons.

Developmental changes in the magnitude and activation characteristics of Na(+) currents of petrosal neurons projecting to the carotid body

Carotid bodies mediate hypoxia sensing for the respiratory system and increase their sensitivity in the post-natal period. The present study examined the characteristics and developmental change of fast Na(+) currents of chemoreceptor afferent neurons. Rat carotid bodies (P2-P19) were harvested intact with the petrosal ganglia and whole-cell recordings obtained from petrosal somas whose axons projected to the carotid body. The magnitude of Na(+) current increased in the post-natal period in parallel with increased conduction velocity and somal size. Voltage-dependence of activation significantly shifted towards negative potentials but no significant change occurred in the voltage dependence of inactivation or the slope factors for activation or inactivation. The leftward shift in activation increased slowly or non-inactivating currents around resting potential which increases afferent neuron excitability, a result confirmed in current clamp recordings. These results suggest that a developmental shift in Na(+) current activation plays a role in chemoreceptor maturation by enhancing excitability of the afferent neuron.

Spermine attenuates carotid body glomus cell oxygen sensing by inhibiting L-type Ca²(+) channels

An increase in intracellular Ca²(+) is crucial to O₂ sensing by the carotid body. Polyamines have been reported to modulate both the extracellular Ca²(+)-sensing receptor (CaR) and voltage-gated Ca²(+) channels in a number of cell types. Using RT-PCR and immunohistochemistry, the predominant voltage-gated Ca²(+) channels expressed in the adult rat carotid body were L (Ca(V)1.2) and N (Ca(V)2.2)-type. CaR mRNA could not be amplified from carotid bodies, but the protein was expressed in the nerve endings. Spermine inhibited the hypoxia-evoked catecholamine release from isolated carotid bodies and attenuated the depolarization- and hypoxia-evoked Ca²(+) influx into isolated glomus cells. In agreement with data from carotid body, recombinant Ca(V)1.2 was also inhibited by spermine. In contrast, the positive allosteric modulator of CaR, R-568, was without effect on hypoxia-induced catecholamine release from carotid bodies and depolarization-evoked Ca²(+) influx into glomus cells. These data show that spermine exerts a negative influence on carotid body O₂ sensing by inhibiting L-type Ca²(+) channels.

Out of thin air: sensory detection of oxygen and carbon dioxide

Oxygen (O₂) and carbon dioxide (CO₂) levels vary in different environments and locally fluctuate during respiration and photosynthesis. Recent studies in diverse animals have identified sensory neurons that detect these external variations and direct a variety of behaviors. Detection allows animals to stay within a preferred environment as well as identify potential food or dangers. The complexity of sensation is reflected in the fact that neurons compartmentalize detection into increases, decreases, and short-range and long-range cues. Animals also adjust their responses to these prevalent signals in the context of other cues, allowing for flexible behaviors. In general, the molecular mechanisms for detection suggest that sensory neurons adopted ancient strategies for cellular detection and coupled them to brain activity and behavior. This review highlights the multiple strategies that animals use to extract information about their environment from variations in O₂ and CO₂.

Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Cardiorespiratory and neural consequences of rats brought past their aerobic dive limit

The mammalian diving response is a dramatic autonomic adjustment to underwater submersion affecting heart rate, arterial blood pressure, and ventilation. The bradycardia is known to be modulated by the parasympathetic nervous system, arterial blood pressure is modulated via the sympathetic system, and still other circuits modulate the respiratory changes. In the present study, we investigate the submergence of rats brought past their aerobic dive limit, defined as the diving duration beyond which blood lactate concentration increases above resting levels. Hemodynamic measurements were made during underwater submergence with biotelemetric transmitters, and blood was drawn from cannulas previously implanted in the rats' carotid arteries. Such prolonged submersion induces radical changes in blood chemistry; mean arterial PCO(2) rose to 62.4 Torr, while mean arterial PO(2) and pH reached nadirs of 21.8 Torr and 7.18, respectively. Despite these radical changes in blood chemistry, the rats neither attempted to gasp nor breathe while underwater. Immunohistochemistry for Fos protein done on their brains revealed numerous Fos-positive profiles. Especially noteworthy were the large number of immunopositive profiles in loci where presumptive chemoreceptors are found. Despite the activation of these presumptive chemoreceptors, the rats did not attempt to breathe. Injections of biotinylated dextran amine were made into ventral parts of the medullary dorsal horn, where central fibers of the anterior ethmoidal nerve terminate. Labeled fibers coursed caudal, ventral, and medial from the injection to neurons on the ventral surface of the medulla, where numerous Fos-labeled profiles were seen in the rats brought past their aerobic dive limit. We propose that this projection inhibits the homeostatic chemoreceptor reflex, despite the gross activation of chemoreceptors.

A revisit to O2 sensing and transduction in the carotid body chemoreceptors in the context of reactive oxygen species biology

Oxygen-sensing and transduction in purposeful responses in cells and organisms is of great physiological and medical interest. All animals, including humans, encounter in their lifespan many situations in which oxygen availability might be insufficient, whether acutely or chronically, physiologically or pathologically. Therefore to trace at the molecular level the sequence of events or steps connecting the oxygen deficit with the cell responses is of interest in itself as an achievement of science. In addition, it is also of great medical interest as such knowledge might facilitate the therapeutical approach to patients and to design strategies to minimize hypoxic damage. In our article we define the concepts of sensors and transducers, the steps of the hypoxic transduction cascade in the carotid body chemoreceptor cells and also discuss current models of oxygen- sensing (bioenergetic, biosynthetic and conformational) with their supportive and unsupportive data from updated literature. We envision oxygen-sensing in carotid body chemoreceptor cells as a process initiated at the level of plasma membrane and performed by a hemoprotein, which might be NOX4 or a hemoprotein not yet chemically identified. Upon oxygen-desaturation, the sensor would experience conformational changes allosterically transmitted to oxygen regulated K+ channels, the initial effectors in the transduction cascade. A decrease in their opening probability would produce cell depolarization, activation of voltage dependent calcium channels and release of neurotransmitters. Neurotransmitters would activate the nerve endings of the carotid body sensory nerve to convey the information of the hypoxic situation to the central nervous system that would command ventilation to fight hypoxia.

Effect of carotid and aortic baroreceptors on cardiopulmonary reflex: the role of autonomic function

We determined the sympathetic and parasympathetic control of heart rate (HR) and the sensitivity of the cardiopulmonary receptors after selective carotid and aortic denervation. We also investigated the participation of the autonomic nervous system in the Bezold-Jarish reflex after selective removal of aortic and carotid baroreceptors. Male Wistar rats (220-270 g) were divided into three groups: control (CG, N = 8), aortic denervation (AG, N = 5) and carotid denervation (CAG, N = 9). AG animals presented increased arterial pressure (12%) and HR (11%) compared with CG, while CAG animals presented a reduction in arterial pressure (16%) and unchanged HR compared with CG. The sequential blockade of autonomic effects by atropine and propranolol indicated a reduction in vagal function in CAG (a 50 and 62% reduction in vagal effect and tonus, respectively) while AG showed an increase of more than 100% in sympathetic control of HR. The Bezold-Jarish reflex was evaluated using serotonin, which induced increased bradycardia and hypotension in AG and CAG, suggesting that the sensitivity of the cardiopulmonary reflex is augmented after selective denervation. Atropine administration abolished the bradycardic responses induced by serotonin in all groups; however, the hypotensive response was still increased in AG. Although the responses after atropine were lower than the responses before the drug, indicating a reduction in vagal outflow after selective denervation, our data suggest that both denervation procedures are associated with an increase in sympathetic modulation of the vessels, indicating that the sensitivity of the cardiopulmonary receptors was modulated by baroreceptor fibers.

Mechanism of inhibition by hydrogen sulfide of native and recombinant BKCa channels

Recent evidence suggests that H(2)S contributes to activation of the carotid body by hypoxia by inhibiting K(+) channels. Here, we determine both the molecular identity of the K(+) channel target within the carotid body and the biophysical characteristics of the H(2)S-evoked inhibition by analyzing native rat and human recombinant BK(Ca) channel activity in voltage-clamped, inside-out membrane patches. Rat glomus cells express the enzymes necessary for the endogenous generation of H(2)S, cystathionine-beta-synthase and cystathionine-gamma-lyase. H(2)S inhibits native carotid body and human recombinant BK(Ca) channels with IC(50) values of around 275 microM. Inhibition by H(2)S is rapid and reversible, works by a mechanism which is distinct from that suggested for CO gas regulation of this channel and does not involve an interaction with either the "Ca bowl" or residues distal to this Ca(2+)-sensing domain. These data show that BK(Ca) is a K(+) channel target of H(2)S, and suggest a mechanism to explain the H(2)S-dependent component of O(2) sensing in the carotid body.

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.

Endogenous N-acyl-dopamines induce COX-2 expression in brain endothelial cells by stabilizing mRNA through a p38 dependent pathway

Cerebral microvascular endothelial cells play an active role in maintaining cerebral blood flow, microvascular tone and blood brain barrier (BBB) functions. Endogenous N-acyl-dopamines like N-arachidonoyl-dopamine (NADA) and N-oleoyl-dopamine (OLDA) have been recently identified as a new class of brain neurotransmitters sharing endocannabinoid and endovanilloid biological activities. Endocannabinoids are released in response to pathogenic insults and may play an important role in neuroprotection. In this study we demonstrate that NADA differentially regulates the release of PGE(2) and PGD(2) in the microvascular brain endothelial cell line, b.end5. We found that NADA activates a redox-sensitive p38 MAPK pathway that stabilizes COX-2 mRNA resulting in the accumulation of the COX-2 protein, which depends on the dopamine moiety of the molecule and that is independent of CB(1) and TRPV1 activation. In addition, NADA inhibits the expression of mPGES-1 and the release of PGE(2) and upregulates the expression of L-PGD synthase enhancing PGD(2) release. Hence, NADA and other molecules of the same family might be included in the group of lipid mediators that could prevent the BBB injury under inflammatory conditions and our findings provide new mechanistic insights into the anti-inflammatory activities of NADA in the central nervous system and its potential to design novel therapeutic strategies to manage neuroinflammatory diseases.

Zebrafish (Danio rerio) gill neuroepithelial cells are sensitive chemoreceptors for environmental CO2

Adult zebrafish exhibit hyperventilatory responses to absolute environmental CO(2) levels as low as 0.13% ( mmHg), more than an order of magnitude lower than the typical arterial levels (40 mmHg) monitored by the mammalian carotid body. The sensory basis underlying the ability of fish to detect and respond to low ambient CO(2) levels is not clear. Here, we show that the neuroepithelial cells (NECs) of the zebrafish gill, known to sense O(2) levels, also respond to low levels of CO(2). An electrophysiological characterization of this response using both current and voltage clamp protocols revealed that for increasing CO(2) levels, a background K(+) channel was inhibited, resulting in a partial pressure-dependent depolarization of the NEC. To elucidate the signalling pathway underlying K(+) channel inhibition, we used immunocytochemistry to show that these NECs express carbonic anhydrase (CA), an enzyme involved in CO(2) sensing in the mammalian carotid body. Further, the NEC response to CO(2) (magnitude of membrane depolarization and time required to achieve maximal response), under conditions of constant pH, was reduced by 50% by the CA-inhibitor acetazolamide. This suggests that the CO(2) detection mechanism involves an intracellular sensor that is responsive to the rate of acidification associated with the hydration of CO(2) and which does not require a change of extracellular pH. Because some cells that were responsive to increasing also responded to hypoxia with membrane depolarization, the present results demonstrate that a subset of the NECs in the zebrafish gill are bimodal sensors of CO(2) and O(2).

Two-pore domain k(+) channels and their role in chemoreception

A number of tandem P-domain K(+)- channels (K(2)P) generate background K(+)-currents similar to those found in enteroreceptors that sense a diverse range of physiological stimuli including blood pH, carbon dioxide, oxygen, potassium and glucose. This review presents an overview of the properties of both cloned K(2)P tandem-P-domain K-channels and the endogenous chemosensitive background K-currents found in central chemoreceptors, peripheral chemoreceptors, the adrenal gland and the hypothalamus. Although the identity of many of these endogenous channels has yet to be confirmed they show striking similarities to a number of K(2)P channels especially those of the TASK subgroup. Moreover these channels seem often (albeit not exclusively) to be involved in pH and nutrient/metabolic sensing.

EPAC signalling pathways are involved in low PO2 chemoreception in carotid body chemoreceptor cells

Chemoreceptor cells of the carotid bodies (CB) are activated by hypoxia and acidosis, responding with an increase in their rate of neurotransmitter release, which in turn increases the electrical activity in the carotid sinus nerve and evokes a homeostatic hyperventilation. Studies in isolated chemoreceptor cells have shown that moderate hypoxias ( 46 mmHg) produces smaller depolarisations and comparable Ca(2+) transients but a much higher catecholamine (CA) release response in intact CBs than intense acidic/hypercapnic stimuli (20% CO(2), pH 6.6). Similarly, intense hypoxia ( 20 mmHg) produces smaller depolarizations and Ca(2+) transients in isolated chemoreceptor cells but a higher CA release response in intact CBs than a pure depolarizing stimulus (30-35 mm external K(+)). Studying the mechanisms responsible for these differences we have found the following. (1) Acidic hypercapnia inhibited I(Ca) (60%; whole cell) and CA release (45%; intact CB) elicited by ionomycin and high K(+). (2) Adenylate cyclase inhibition (SQ-22536; 80 microm) inhibited the hypoxic release response (>50%) and did not affect acidic/hypercapnic release, evidencing that the high gain of hypoxia to elicit neurotransmitter release is cAMP dependent. (3) The last effect was independent of PKA activation, as three kinase inhibitors (H-89, KT 5720 and Rp-cAMP; 10 x IC(50)) did not alter the hypoxic release response. (4) The Epac (exchange protein activated by cAMP) activator (8-pCPT-2-O-Me-cAMP, 100 microm) reversed the effects of the cyclase inhibitor. (5) The Epac inhibitor brefeldin A (100 microm) inhibited (54%) hypoxic induced release. Our findings show for the first time that an Epac-mediated pathway mediates O(2) sensing/transduction in chemoreceptor cells.

MaxiK potassium channels in the function of chemoreceptor cells of the rat carotid body

Hypoxia activates chemoreceptor cells of the carotid body (CB) promoting an increase in their normoxic release of neurotransmitters. Catecholamine (CA) release rate parallels the intensity of hypoxia. Coupling of hypoxia to CA release requires cell depolarization, produced by inhibition of O(2)-regulated K(+) channels, and Ca(2+) entering the cells via voltage-operated channels. In rat chemoreceptor cells hypoxia inhibits large-conductance, calcium-sensitive K channels (maxiK) and a two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) channel (TASK)-like channel, but the significance of maxiK is controversial. A proposal envisions maxiK contributing to set the membrane potential (E(m)) and the hypoxic response, but the proposal is denied by authors finding that maxiK inhibition does not depolarize chemoreceptor cells or alters intracellular Ca(2+) concentration or CA release in normoxia or hypoxia. We found that maxiK channel blockers (tetraethylammonium and iberiotoxin) did not modify CA release in rat chemoreceptor cells, in either normoxia or hypoxia, and iberiotoxin did not alter the Ca(2+) transients elicited by hypoxia. On the contrary, both maxiK blockers increased the responses elicited by dinitrophenol, a stimulus we demonstrate does not affect maxiK channels in isolated patches of rat chemoreceptor cells. We conclude that in rat chemoreceptor cells maxiK channels do not contribute to the genesis of the E(m), and that their full inhibition by hypoxia, preclude further inhibition by maxiK channel blockers. We suggest that full inhibition of this channel is required to generate the spiking behavior of the cells in acute hypoxia.

General redox environment and carotid body chemoreceptor function

Carotid body (CB) chemoreceptor cells detect physiological levels of hypoxia and generate a hyperventilation, homeostatic in nature, aimed to minimize the deleterious effects of hypoxia. Intimate mechanisms involved in oxygen sensing in chemoreceptor cells remain largely unknown, but reactive oxygen species (ROS) had been proposed as mediators of this process. We have determined glutathione levels and calculated glutathione redox potential (E(GSH); indicator of the general redox environment of cells) in rat diaphragms incubated in the presence of oxidizing agents of two types: nonpermeating and permeating through cell membranes; in the latter group, unspecific oxidants and inhibitors of ROS-disposing enzymes were used. Selected concentrations of oxidizing agents were tested for their ability to modify the normoxic and hypoxic activity of chemoreceptor cells measured in vitro as their rate of release of neurotransmitters. Results evidence variable relationships between E(GSH) and the activity of chemoreceptor cells. The independence of chemoreceptor cell activity from the E(GSH) would imply that the ability of the CB to play its homeostatic role is largely preserved in any pathological or toxicological contingency causing oxidative stress. Consistent with this suggestion, it was also found that CB-mediated hypoxic hyperventilation was not altered by treatment of intact animals with agents that markedly decreased the E(GSH) in all tissues assayed.

Molecular identification and functional role of voltage-gated sodium channels in rat carotid body chemoreceptor cells. Regulation of expression by chronic hypoxia in vivo

We have assessed the expression, molecular identification and functional role of Na+ channels (Na(v)) in carotid bodies (CB) obtained from normoxic and chronically hypoxic adult rats. Veratridine evoked release of catecholamines (CA) from an in vitro preparation of intact CBs obtained from normoxic animals, the response being Ca2+ and Na+-dependent and sensitive to tetrodotoxin (TTX). TTX inhibited by 25-50% the CA release response evoked by graded hypoxia. Immunoblot assays demonstrated the presence of Na(v)alpha-subunit (c. 220 kDa) in crude homogenates from rat CBs, being evident an up-regulation (60%) of this protein in the CBs obtained from chronically hypoxic rats (10% O2; 7 days). This up-regulation was accompanied by an enhanced TTX-sensitive release response to veratridine, and by an enhanced ventilatory response to acute hypoxic stimuli. RT-PCR studies demonstrated the expression of mRNA for Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 isoforms. At least three isoforms, Na(v)1.1, Na(v)1.3 and Na(v)1.6 co-localized with tyrosine hydroxylase in all chemoreceptor cells. RT-PCR and immunocytochemistry indicated that Na(v)1.1 isoform was up-regulated by chronic hypoxia in chemoreceptor cells. We conclude that Na(v) up-regulation represents an adaptive mechanism to increase chemoreceptor sensitivity during acclimatization to sustained hypoxia as evidenced by enhanced ventilatory responses to acute hypoxic tests.

TASK-like potassium channels and oxygen sensing in the carotid body

Chemosensing by type-1 cells of the carotid body involves a series of events which culminate in the calcium-dependent secretion of neurotransmitter substances which then excite afferent nerves. This response is mediated via membrane depolarisation and voltage-gated calcium entry. Studies utilising isolated cells indicates that the membrane depolarisation in response to hypoxia, and acidosis, appears to be primarily mediated via the inhibition of a background K(+)-current. The pharmacological and biophysical characteristics of these channels suggest that they are probably closely related to the TASK subfamily of tandem-P-domain K(+)-channels. Indeed they show greatest similarity to TASK-1 and -3. In addition to being sensitive to hypoxia and acidosis, the background K(+)-channels of the type-1 cell are also remarkably sensitive to inhibition of mitochondrial energy metabolism. Metabolic poisons are known potent stimulants of the carotid body and cause membrane depolarisation of type-1 cells. In the presence of metabolic inhibitors hypoxic sensitivity is lost suggesting that oxygen sensing may itself be mediated via depression of mitochondrial energy production. Thus these TASK-like background channels play a central role in mediating the chemotransduction of several different stimuli within the type-1 cell. The mechanisms by which metabolic/oxygen sensitivity might be conferred upon these channels are briefly discussed.

Dose-dependent effect of dobutamine on chemoreflex activity in healthy volunteers

AIMS

beta-adrenergic agonists increase peripheral chemoreceptor sensitivity in humans. We tested the hypothesis that beta(1)-agonist-related increase in peripheral chemoreflex sensitivity is selective and dose-dependent.

METHODS

Using a double-blind, placebo-controlled, randomized, crossover study, we examined the effects of dobutamine (n = 17 healthy subjects) at perfusion rates of 2.5 microg kg(-1) min(-1) (D2.5) and 7.5 microg kg(-1) min(-1) (D7.5) on ventilation, haemodynamics and sympathetic nerve activity during normoxia, isocapnic hypoxia, posthypoxic maximal voluntary end-expiratory apnoea, hyperoxic hypercapnia and cold pressor test (CPT). We analysed the effect of pretreatment with atenolol on dobutamine-evoked chemosensitivity.

RESULTS

Dobutamine dose-dependently increased ventilation (placebo 6.7 +/- 0.5 vs. D2.5 7.8 +/- 0.4 vs. D7.5 8.7 +/- 0.4 l min(-1), P < 0.005) during normoxia, enhanced the ventilatory (placebo 14.4 +/- 0.6 vs. D2.5 17.3 +/- 0.8 vs. D7.5 22.5 +/- 1.9 l min(-1), P < 0.0001) and sympathetic (placebo + 215 +/- 31 vs. D2.5 + 285 +/- 19 vs. D7.5 + 395 +/- 50% of baseline, P < 0.03) responses at the fifth minute of isocapnic hypoxia and enhanced the sympathetic response to apnoea performed after hypoxia (increase after 5 min of hypoxia: + 290 +/- 43% for placebo vs.+ 360 +/- 21% for D2.5 vs. 537 +/- 69% for D7.5, P < 0.05). No differences were observed between dobutamine and placebo in the responses to hyperoxic hypercapnia and CPT. Atenolol inhibited the dobutamine-related hyperventilation and apnoea shortening during normoxia and hypoxia.

CONCLUSION

Dobutamine enhances peripheral chemosensitivity at low infusion rates selectively and in a dose-dependent manner. There is a beta(1) adrenoceptor component in dobutamine-evoked increase in peripheral chemosensititivity; however, a contribution of additional adrenoceptor subtypes cannot be excluded.

The role of maxiK channels in carotid body chemotransduction

MaxiK channels are a unique class of K(+) channels activated by both voltage and intracellular Ca(2+). Derived from a single gene, their diversity arises from extensive splicing, and their wide distribution has led to their implication in a large variety of cellular functions. In the carotid body, they have been proposed to contribute to the resting membrane potential of type I cells, and also to be O(2) sensitive. Thus, they have been suggested to have an important role in hypoxic chemotransduction. Their O(2) sensitivity is preserved when the channels are expressed in HEK 293 cells, permitting detailed studies of candidate mechanisms underlying hypoxic inhibition of maxiK channels. In this article, we review evidence for and against an important role for maxiK channels in chemotransduction. We also consider different mechanisms proposed to account for hypoxic channel inhibition and suggest that, although our understanding of this important physiological process has advanced significantly in recent years, there remain important, unanswered questions as to the importance of maxiK in carotid body chemoreception.

Function of the rat carotid body chemoreceptors in ageing

Some age-related deficits in the ventilatory responses have been attributed to a decline in the functionality of the carotid body (CB) arterial chemoreceptors, but a systematic study of the CB function in ageing is lacking. In rats aged 3-24 months, we have performed quantitative morphometry on specific chemoreceptor tissue, assessed the function of chemoreceptor cells by measuring the content, synthesis and release of catecholamines (a chemoreceptor cell neurotransmitter) in normoxia and hypoxia, and determined the functional activity of the intact organ by measuring chemosensory activity in the carotid sinus nerve (CSN) in normoxia, hypoxia and hypercapnic acidosis. We found that with age CBs enlarge, but at the same time there is a concomitant decrease in the percentage of chemoreceptor tissue. CB content and turnover time for their catecholamines increase with age. Hypoxic stimulation of chemoreceptor cells elicits a smaller release of catecholamines in rats after 12 months of age, but a non-specific depolarizing stimulus elicits a comparable release at all ages. In parallel, there was a marked decrease in the responsiveness to hypoxia, but not to an acidic-hypercapnic stimulus, assessed as chemosensory activity in the CSN. We conclude that in aged mammals chemoreceptor cells become hypofunctional, leading to a decreased peripheral drive of ventilation.

Caffeine inhibition of rat carotid body chemoreceptors is mediated by A2A and A2B adenosine receptors

Caffeine, an unspecific antagonist of adenosine receptors, is commonly used to treat the apnea of prematurity. We have defined the effects of caffeine on the carotid body (CB) chemoreceptors, the main peripheral controllers of breathing, and identified the adenosine receptors involved. Caffeine inhibited basal (IC50, 210 microm) and low intensity (PO2 approximately 66 mm Hg/30 mm K+) stimulation-induced release of catecholamines from chemoreceptor cells in intact preparations of rat CB in vitro. Opposite to caffeine, 5'-(N-ethylcarboxamido)adenosine (NECA; an A2 agonist) augmented basal and low-intensity hypoxia-induced release. 2-p-(2-Carboxyethyl)phenethyl-amino-5'-N-ethylcaboxamido-adenosine hydrochloride (CGS21680), 2-hexynyl-NECA (HE-NECA) and SCH58621 (A2A receptors agents) neither affected catecholamine release nor altered the caffeine effects. The 8-cycle-1,3-dipropylxanthine (DPCPX; an A1/A2B antagonist) and 8-(4-{[(4-cyanophenyl)carbamoylmethyl]-oxy}phenyl)-1,3-di(n-propyl)xanthine (MRS1754; an A2B antagonist) mimicking of caffeine indicated that caffeine effects are mediated by A2B receptors. Immunocytochemical A2B receptors were located in tyrosine hydroxylase positive chemoreceptor cells. Caffeine reduced by 52% the chemosensory discharges elicited by hypoxia in the carotid sinus nerve. Inhibition had two components with pharmacological analysis indicating that A2A and A2B receptors mediate, respectively, the low (17 x 10(-9) m) and high (160 x 10(-6) m) IC50 effects. It is concluded that endogenous adenosine, via presynaptic A2B and postsynaptic A2A receptors, can exert excitatory effects on the overall output of the rat CB chemoreceptors.