Carotid Body Type Research Articles

The Advantages of Non-Adhesive Gel-like Embolic Materials in the Endovascular Treatment of Benign Hypervascularized Lesions of the Head and Neck.

The use of non-adhesive gel-like embolic materials (NAGLEMs) in the endovascular treatment of hypervascularized formations in the head and neck is gaining in popularity because of a number of important characteristics involved. Their primary benefits are their capacity to penetrate diseased vasculature, effectively distribute, and, most importantly, remain controllable during the process. We reviewed the literature and evaluated the results of using NAGLEMs in comparison to other embolizing substances (namely, coils, glue, and particles) as alternative embolizing agents for patients receiving care at our clinic. The process comprised evaluating the safety, effectiveness, and technological elements of endovascular therapy used to treat two categories of hypervascular pathological abnormalities that were surgically corrected between 2015 and 2023. Arteriovenous malformations (AVMs) located in the head, neck, and paragangliomas with jugular/carotid body localization are combined by intense shunting blood flow and shared requirements for the embolic agent used in endovascular treatment (such as penetration, distribution, delayed polymerization, and controllability). An analysis of the literature was also conducted. Results showed 18 patients diagnosed with neck paragangliomas of the carotid body and jugular type. Five patients with arteriovenous malformation (AVM) of the face and neck were included, consisting of sixteen females and seven males with an average age of 55 ± 13 years. Endovascular procedures were performed using NAGLEMs (ONYX (Medtronic, Irvine, CA, USA), SQUID (Balt, Montmorency, France), and PHIL (Microvention, Tustin, CA, USA)) and dimethyl sulfoxide (DMSO)-compatible balloon catheters. All patients achieved complete or partial embolization of hypervascularized formations using one or more stages of endovascular treatment. Additionally, three AVMs of the face and two paragangliomas of the neck were surgically excised following embolization. In other instances, formations were not deemed necessary to be removed. The patients' condition upon discharge was assessed by the modified Rankin Scale (mRs) and rated between 0 and 2. Currently, NAGLEMs are predominantly used to treat hypervascularized formations in the neck and head due to their fundamental properties. These properties include a lack of adhesion and a delay in predictable polymerization (after 30-40 min). NAGLEMs also exhibit excellent distribution and penetration throughout the vascular bed of the formation. Adequate controllability of the process is largely achieved through the presence of embolism forms of different viscosity, as well as excellent X-ray visualization.

Studies on the Mechanism of Lactate-Induced Increase in Ca2+ Oscillations in Carotid Body Type 1 Cells

Blood lactate concentration under normal conditions is ~1 mM and increases during hypoxia and exercise due to elevated anaerobic metabolism. Recent studies suggest that lactate acts directly on carotid body Type 1 chemoreceptor (CB) cells and elevates cytosolic [Ca2+], which increases transmitter secretion from CB cells, carotid sinus afferent nerve activity and ventilation. The mechanism by which lactate increases [Ca2+] is still uncertain. Chang et al (2015) reported that lactate binds to Gs-coupled receptor (OlfR78 in rat/mice) whose signaling leads to a rise in [Ca2+]. Torres-Torrelo et al (2021) reported that lactate enters the cell via the monocarboxylate transporter and increases the ratio of NADH to NAD+, which opens cation channels to cause cell depolarization and Ca2+ influx. Another potential mechanism is that lactate itself inhibits K+ channels and/or activates non-selective cation channels to depolarize CB cells and increase Ca2+ influx. Here we studied the effects of lactate on ion channels, cell Em and Ca2+ oscillations in CB cells. Ion channel and Em studies: Sodium lactate (10 mM, pH 7.4) produced a mild depolarization (<10 mV) in ~30% of CB cells. In HEK cells expressing TASK-1/3, lactate (10 mM) caused mild inhibition of the whole-cell current and cell depolarization (as detected by Fluovolt fluorescence). Lactate activated a 20-pS Ca2+-sensitive monovalent cation channel (CaMCC) in ~50% of CB cells. [Ca2+] oscillations: Lactate (1-20 mM) increased the frequency of Ca2+ oscillations in CB cells (and thus averaged [Ca2+]), and this effect was blunted by removal of extracellular Ca2+, nifedipine and 2-APB. Forskolin that elevated cytosolic [cAMP] had no effect on Ca2+ oscillations, and H-89 (PKA inhibitor) did not block lactate- and hypoxia-induced increase in Ca2+ oscillations. Summary: Our studies do not support the OlfR78-Gs-cAMP signaling pathway in lactate-induced increase in Ca2+ oscillations and [Ca2+] in CB cells. Our study supports a mechanism in which lactate activates CaMCC, possibly initiated by inhibition of TASK and/or other signals, to cause cell depolarization that increases Ca2+ influx and the frequency of Ca2+ oscillations. NIH and Rosalind Franklin University of Medicine and Science This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

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

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

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

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

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

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

Are Multiple Mitochondrial Related Signalling Pathways Involved in Carotid Body Oxygen Sensing?

It is generally acknowledged that the carotid body (CB) type I cell mitochondria are unique, being inhibited by relatively small falls in PaO2 well above those known to inhibit electron transport in other cell types. This feature is suggested to allow for the CB to function as an acute O2 sensor, being stimulated and activating systemic protective reflexes before the metabolism of other cells becomes compromised. What is less clear is precisely how a fall in mitochondrial activity links to type I cell depolarisation, a process that is required for initiation of the chemotransduction cascade and post-synaptic action potential generation. Multiple mitochondrial/metabolic signalling mechanisms have been proposed including local generation of mitochondrial reactive oxygen species (mitoROS), a change in mitochondrial/cellular redox status, a fall in MgATP and an increase in lactate. Although each mechanism is based on compelling experimental evidence, they are all not without question. The current review aims to explore the importance of each of these signalling pathways in mediating the overall CB response to hypoxia. We suggest that there is unlikely to be a single mechanism, but instead multiple mitochondrial related signalling pathways are recruited at different PaO2s during hypoxia. Furthermore, it still remains to be determined if mitochondrial signalling acts independently or in partnership with extra-mitochondrial O2-sensors.

Acute Oxygen-Sensing via Mitochondria-Generated Temperature Transients in Rat Carotid Body Type I Cells.

The Carotid Bodies (CB) are peripheral chemoreceptors that detect changes in arterial oxygenation and, via afferent inputs to the brainstem, correct the pattern of breathing to restore blood gas homeostasis. Herein, preliminary evidence is presented supporting a novel oxygen-sensing hypothesis which suggests CB Type I cell “hypoxic signaling” may in part be mediated by mitochondria-generated thermal transients in TASK-channel-containing microdomains. Distances were measured between antibody-labeled mitochondria and TASK-potassium channels in primary rat CB Type I cells. Sub-micron distance measurements (TASK-1: 0.33 ± 0.04 µm, n = 47 vs TASK-3: 0.32 ± 0.03 µm, n = 54) provided evidence for CB Type I cell oxygen-sensing microdomains. A temperature-sensitive dye (ERthermAC) indicated that inhibition of mitochondrial activity in isolated cells caused a rapid and reversible inhibition of mitochondrial thermogenesis and thus temperature in these microdomains. Whole-cell perforated-patch current-clamp electrophysiological recordings demonstrated sensitivity of resting membrane potential (Vm) to temperature: lowering bath temperature from 37°C to 24°C induced consistent and reversible depolarizations (Vm at 37°C: -48.4 ± 4.11 mV vs 24°C: -31.0 ± 5.69 mV; n = 5; p < 0.01). These data suggest that hypoxic inhibition of mitochondrial thermogenesis may play an important role in oxygen chemotransduction in the CB. A reduction in temperature within cellular microdomains will inhibit plasma membrane ion channels, influence the balance of cellular phosphorylation–dephosphorylation, and may extend the half-life of reactive oxygen species. The characterization of a thermosensory chemotransduction mechanism, that may also be used by other oxygen-sensitive cell types and may impact multiple other chemotransduction mechanisms is critical if we are to fully understand how the CBs, and potentially other oxygen-sensitive cells, respond to hypoxia.

Ethanol and opioids do not act synergistically to depress excitation in carotid body type I cells.

ObjectiveThe combination of opioids and ethanol can synergistically depress breathing and the acute ventilatory response to hypoxia. Multiple studies have shown that the underlying mechanisms for this may involve calcium channel inhibition in central neurons. But we have previously identified opioid receptors in the carotid bodies and shown that their activation inhibits calcium influx into the chemosensitive cells. Given that the carotid bodies contribute to the drive to breathe and underpin the acute hypoxic ventilatory response, we hypothesized that ethanol and opioids may act synergistically in these peripheral sensory organs to further inhibit calcium influx and therefore inhibit ventilation.MethodsCarotid bodies were removed from 56 Sprague–Dawley rats (1021 days old) and then enzymatically dissociated to allow calcium imaging of isolated chemosensitive type I cells. Cells were stimulated with high K+ in the presence and absence of the µ-opioid agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) (10 µM), a maximal sublethal concentration of ethanol (3 g L-1, 65.1 mM) or a combination of both.ResultsDAMGO alone significantly inhibited Ca2+ influx but this effect was not potentiated by the high concentration of ethanol.ConclusionThese results indicate for the first time that while opioids may suppress breathing via an action at the level of the carotid bodies, ethanol is unlikely to potentiate inhibition via this pathway. Thus, the synergistic effects of ethanol and opioids on ventilatory parameters are likely mediated by central rather than peripheral actions.

\u03b2-Adrenoceptor blockade prevents carotid body hyperactivity and elevated vascular sympathetic nerve density induced by chronic intermittent hypoxia

Carotid body (CB) hyperactivity promotes hypertension in response to chronic intermittent hypoxia (CIH). The plasma concentration of adrenaline is reported to be elevated in CIH and our previous work suggests that adrenaline directly activates the CB. However, a role for chronic adrenergic stimulation in mediating CB hyperactivity is currently unknown. This study evaluated whether beta-blocker treatment with propranolol (Prop) prevented the development of CB hyperactivity, vascular sympathetic nerve growth and hypertension caused by CIH. Adult male Wistar rats were assigned into 1 of 4 groups: Control (N), N + Prop, CIH and CIH + Prop. The CIH paradigm consisted of 8 cycles h−1, 8 h day−1, for 3 weeks. Propranolol was administered via drinking water to achieve a dose of 40 mg kg−1 day−1. Immunohistochemistry revealed the presence of both β1 and β2-adrenoceptor subtypes on the CB type I cell. CIH caused a 2–3-fold elevation in basal CB single-fibre chemoafferent activity and this was prevented by chronic propranolol treatment. Chemoafferent responses to hypoxia and mitochondrial inhibitors were attenuated by propranolol, an effect that was greater in CIH animals. Propranolol decreased respiratory frequency in normoxia and hypoxia in N and CIH. Propranolol also abolished the CIH mediated increase in vascular sympathetic nerve density. Arterial blood pressure was reduced in propranolol groups during hypoxia. Propranolol exaggerated the fall in blood pressure in most (6/7) CIH animals during hypoxia, suggestive of reduced sympathetic tone. These findings therefore identify new roles for β-adrenergic stimulation in evoking CB hyperactivity, sympathetic vascular hyperinnervation and altered blood pressure control in response to CIH.

Lactate and Short‐Chain Fatty Acids on Ca2+ Oscillations in Rat Carotid Body Type 1 Cells

Carotid body (CB) Type 1 (glomus) cells express Olfr78, a Gs‐coupled receptor that binds lactate and short chain fatty acids. Lactate is produced in all cells under anaerobic conditions, and short chain fatty acids are produced by bacterial fermentation in the colon. Lactate‐Olfr78 signaling has been proposed to be involved in hypoxia‐induced excitation of CB, but this idea is currently controversial. Ca oscillations are low‐level Ca signals in glomus cells. Here we compared the effects of lactate/short chain fatty acids and hypoxia on Ca oscillations in isolated glomus cell. Lactate ≈ acetate &gt; propionate (1–20 mM) produced a concentration‐dependent increase in the frequency of Ca oscillations with no overall effects on the background TASK K current and resting cell membrane potential. Verapamil abolished Ca oscillations with or without lactate. Forskolin (2–10 μM) produced no significant effect on Ca oscillations, suggesting that adenylyl cyclase/cAMP may not be involved in lactate‐induced changes in Ca signaling. H‐89 (1 μM) did not block lactate‐induced increase in Ca oscillations, questioning the role of PKA. Comparison of lactate and hypoxia effects on Ca oscillations indicated that 5–10 mM lactate and 2–5%O2 (mild‐moderate) hypoxia produce similar effects on Ca oscillations. Thus, an increase of blood lactate to 5–10 mM may be necessary to mimic the effect of mild‐moderate hypoxia on Ca signaling in CB glomus cells.

Carotid body type I cells engage flavoprotein and Pin1 for oxygen sensing.

Carotid body (CB) type I cells sense the blood Po2 and generate a nervous signal for stimulating ventilation and circulation when blood oxygen levels decline. Three oxygen-sensing enzyme complexes may be used for this purpose: 1) mitochondrial electron transport chain metabolism, 2) heme oxygenase 2 (HO-2)-generating CO, and/or 3) an NAD(P)H oxidase (NOX). We hypothesize that intracellular redox changes are the link between the sensor and nervous signals. To test this hypothesis type I cell autofluorescence of flavoproteins (Fp) and NAD(P)H within the mouse CB ex vivo was recorded as Fp/(Fp+NAD(P)H) redox ratio. CB type I cell redox ratio transiently declined with the onset of hypoxia. Upon reoxygenation, CB type I cells showed a significantly increased redox ratio. As a control organ, the non-oxygen-sensing sympathetic superior cervical ganglion (SCG) showed a continuously reduced redox ratio upon hypoxia. CN-, diphenyleneiodonium, or reactive oxygen species influenced chemoreceptor discharge (CND) with subsequent loss of O2 sensitivity and inhibited hypoxic Fp reduction only in the CB but not in SCG Fp, indicating a specific role of Fp in the oxygen-sensing process. Hypoxia-induced changes in CB type I cell redox ratio affected peptidyl prolyl isomerase Pin1, which is believed to colocalize with the NADPH oxidase subunit p47phox in the cell membrane to trigger the opening of potassium channels. We postulate that hypoxia-induced changes in the Fp-mediated redox ratio of the CB regulate the Pin1/p47phox tandem to alter type I cell potassium channels and therewith CND.

Ca2+ oscillations in rat carotid body type 1 cells in normoxia and hypoxia.

We studied the mechanisms by which carotid body glomus (type 1) cells produce spontaneous Ca2+ oscillations in normoxia and hypoxia. In cells perfused with normoxic solution at 37°C, we observed relatively uniform, low-frequency Ca2+ oscillations in >60% of cells, with each cell showing its own intrinsic frequency and amplitude. The mean frequency and amplitude of Ca2+ oscillations were 0.6 ± 0.1 Hz and 180 ± 42 nM, respectively. The duration of each Ca2+ oscillation ranged from 14 to 26 s (mean of ∼20 s). Inhibition of inositol (1,4,5)-trisphosphate receptor and store-operated Ca2+ entry (SOCE) using 2-APB abolished Ca2+ oscillations. Inhibition of endoplasmic reticulum Ca2+-ATPase (SERCA) using thapsigargin abolished Ca2+ oscillations. ML-9, an inhibitor of STIM1 translocation, also strongly reduced Ca2+ oscillations. Inhibitors of L- and T-type Ca2+ channels (Cav; verapamil>nifedipine>TTA-P2) markedly reduced the frequency of Ca2+ oscillations. Thus, Ca2+ oscillations observed in normoxia were caused by cyclical Ca2+ fluxes at the ER, which was supported by Ca2+ influx via Ca2+ channels. Hypoxia (2-5% O2) increased the frequency and amplitude of Ca2+ oscillations, and Cav inhibitors (verapamil>nifedipine>>TTA-P2) reduced these effects of hypoxia. Our study shows that Ca2+ oscillations represent the basic Ca2+ signaling mechanism in normoxia and hypoxia in CB glomus cells.

H2S Contributes to Carotid Body Response to Hypoxia but Not Anoxia

We previously reported that cystathionine‐γ‐lyase (CSE)‐derived H2S mediates carotid body (CB) response to hypoxia (Hx) (Peng et al., PNAS, 2010). A recent study showed that CB Type I cell responses to anoxia (Ax) were unaffected by CSE inhibitors (Kim et al. Respir Physiology, 2015). We re‐examined the role of CSE‐derived H2S in CB sensory and Type I cell responses to Hx and Ax in adult Sprague‐Dawley rats, wild‐type and CSE knockout (KO) mice. Hx (pO2=37± 3mmHg) produced robust sensory excitation, and increased H2S levels in the CB; whereas Ax (pO2 =5±4 mmHg) produced only a weak sensory excitation and had no effect on H2S levels. Ax evoked a robust [Ca2+]i response of Type I cells as compared to hypoxia. CSE inhibitor DL‐propargylglycine (DL‐PAG, 50μM) inhibited Hx but not Ax‐evoked sensory and Type I responses. Higher concentration (300μM) of DL‐PAG alone, or in combination with aminooxyacetic acid (300μM), non‐selectively blocked [Ca2+]i responses of Type I cells to Ax and decreased cell viability. CB sensory excitation and [Ca2+]i elevation in Type I cells were unaffected in CSE null mice as compared to wild‐type (Wt) mice. Breathing stimulation by Hx was absent in CSE KO mice, whereas depressed breathing by Ax was unaffected. These observations suggest that CSE‐derived H2S contributes to carotid body response to “physiologically” relevant hypoxia but not to anoxia.Support or Funding InformationSupported by National Institutes of Health grants P01‐HL‐90554This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

High Fat Feeding in Rats Alters Respiratory Parameters by a Mechanism That Is Unlikely to Be Mediated by Carotid Body Type I Cells.

The carotid bodies (CB) respond to changes in blood gases with neurotransmitter release, thereby increasing carotid sinus nerve firing frequency and ultimately correcting the pattern of breathing. It has previously been demonstrated that acute application of the adipokine leptin augments the hypoxic sensory response of the intact in-vitro CB (Pye RL, Roy A, Wilson RJ, Wyatt CN. FASEB J 30(1 Supplement):983.1, 2016) and isolated CB type I cell (Pye RL, Dunn EJ, Ricker EM, Jurcsisn JG, Barr BL, Wyatt CN. Arterial chemoreceptors in physiology and pathophysiology. Advances in experimental medicine and biology. Springer, Cham, 2015). This study's aim was to examine, in-vivo, if elevated leptin modulated CB function and breathing.Rats were fed high fat or control chow for 16-weeks. High fat fed (HFF) animals gained significantly more weight compared to control fed (CF) animals and had significantly higher serum leptin levels compared to CF. Utilizing whole-body plethysmography, HFF animals demonstrated significantly depressed breathing compared to CF at rest and during hypoxia. However, amplitudes in the change in breathing from rest to hypoxia were not significantly different between groups. CB type I cells were isolated and intracellular calcium levels recorded. Averaged and peak cellular hypoxic responses were not significantly different.Despite a small but significant rise in leptin, differences in breathing caused by high fat feeding are unlikely caused by an effect of leptin on CB type I cells. However, the possibility remains that leptin may have in-vivo postsynaptic effects on the carotid sinus nerve; this remains to be investigated.

Acute oxygen sensing by the carotid body: a rattlebag of molecular mechanisms.

The molecular underpinnings of the oxygen sensitivity of the carotid body Type I cells are becoming better defined as research begins to identify potential interactions between previously separate theories. Nevertheless, the field of oxygen chemoreception still presents the general observer with a bewildering array of potential signalling pathways by which a fall in oxygen levels might initiate Type I cell activation. The purpose of this brief review is to address five of the current oxygen sensing hypotheses: the lactate-Olfr78 hypothesis of oxygen chemotransduction; the role mitochondrial ATP and metabolism may have in chemotransduction; the AMP-activated protein kinase hypothesis and its current role in oxygen sensing by the carotid body; reactive oxygen species as key transducers in the oxygen sensing cascade; and the mechanisms by which H2 S, reactive oxygen species and haem oxygenase may integrate to provide a rapid oxygen sensing transduction system. Over the previous 15 years several lines of research into acute hypoxic chemotransduction mechanisms have focused on the integration of mitochondrial and membrane signalling. This review places an emphasis on the subplasmalemmal-mitochondrial microenvironment in Type I cells and how theories of acute oxygen sensing are increasingly dependent on functional interaction within this microenvironment.

High Fat Feeding in Rats Alters Respiratory Parameters by a Mechanism that is Unlikely to be Mediated by Carotid Body Type I Cells

The carotid bodies (CB), the primary peripheral chemoreceptors, respond to changes in blood gases with neurotransmitter release, thereby increasing carotid sinus nerve firing frequency and ultimately correcting the pattern of breathing. It has previously been demonstrated that acute application of the adipokine leptin caused perturbations of intracellular calcium and membrane ion movement in isolated CB Type I cells (Pye et al, 2015) and blunted the response of the intact CB to hypoxia (Pye et al, 2016). This study's aim was to examine, in‐vivo, if elevated leptin modulated CB function and breathing.Rats were fed high‐fat chow or control chow for 16‐weeks. High‐fat fed (HFF) animals gained significantly more weight compared to control fed (CF) animals (n=18; p&lt;.001; 512.56 g ± 14.70 g vs. 444.11 g ± 7.09 g). HFF animals also had significantly higher serum leptin levels compared to CF (n=18; p&lt;.0001; 3.05 ng/mL ± 0.24 ng/mL vs. 1.29 ng/mL ± 0.12 ng/mL). Whole‐body plethysmography was used to test the acute hypoxic ventilatory response (HVR) in unrestrained, conscious animals. HFF animals had an attenuated 2nd‐phase of the HVR when compared to CF (n=18; p&lt;.05; 710.1 ± 41.9 mL kg−1 min−1 vs. 855.4 ± 44.05 mL kg−1 min−1). CB Type I cells were isolated and intracellular calcium measured; no significant differences in the cellular hypoxic responses between groups were observed.These data show differences in the 2nd‐phase of the HVR caused by high fat feeding are unlikely to be caused by an action of leptin on the Type I cells. However the possibility remains that leptin may have in‐vivo postsynaptic effects on the carotid sinus nerve; this remains to be investigated.Support or Funding InformationThis work was funded by NIH grant RO1HL091836

Commentary: Oxygen regulation of breathing through an olfactory receptor activated by lactate.

Commentary: Oxygen regulation of breathing through an olfactory receptor activated by lactate.

Adrenaline release evokes hyperpnoea and an increase in ventilatory CO2 sensitivity during hypoglycaemia: a role for the carotid body

Hypoglycaemia is counteracted by release of hormones and an increase in ventilation and CO2 sensitivity to restore blood glucose levels and prevent a fall in blood pH. The full counter-regulatory response and an appropriate increase in ventilation is dependent on carotid body stimulation. We show that the hypoglycaemia-induced increase in ventilation and CO2 sensitivity is abolished by preventing adrenaline release or blocking its receptors. Physiological levels of adrenaline mimicked the effect of hypoglycaemia on ventilation and CO2 sensitivity. These results suggest that adrenaline, rather than low glucose, is an adequate stimulus for the carotid body-mediated changes in ventilation and CO2 sensitivity during hypoglycaemia to prevent a serious acidosis in poorly controlled diabetes. Hypoglycaemia in vivo induces a counter-regulatory response that involves the release of hormones to restore blood glucose levels. Concomitantly, hypoglycaemia evokes a carotid body-mediated hyperpnoea that maintains arterial CO2 levels and prevents respiratory acidosis in the face of increased metabolism. It is unclear whether the carotid body is directly stimulated by low glucose or by a counter-regulatory hormone such as adrenaline. Minute ventilation was recorded during infusion of insulin-induced hypoglycaemia (8-17mIUkg(-1) min(-1) ) in Alfaxan-anaesthetised male Wistar rats. Hypoglycaemia significantly augmented minute ventilation (123±4 to 143±7mlmin(-1) ) and CO2 sensitivity (3.3±0.3 to 4.4±0.4mlmin(-1) mmHg(-1) ). These effects were abolished by either β-adrenoreceptor blockade with propranolol or adrenalectomy. In this hypermetabolic, hypoglycaemic state, propranolol stimulated a rise in P aC O2, suggestive of a ventilation-metabolism mismatch. Infusion of adrenaline (1μgkg(-1) min(-1) ) increased minute ventilation (145±4 to 173±5mlmin(-1) ) without altering P aC O2 or pH and enhanced ventilatory CO2 sensitivity (3.4±0.4 to 5.1±0.8mlmin(-1) mmHg(-1) ). These effects were attenuated by either resection of the carotid sinus nerve or propranolol. Physiological concentrations of adrenaline increased the CO2 sensitivity of freshly dissociated carotid body type I cells in vitro. These findings suggest that adrenaline release can account for the ventilatory hyperpnoea observed during hypoglycaemia by an augmented carotid body and whole body ventilatory CO2 sensitivity.

Moderate inhibition of mitochondrial function augments carotid body hypoxic sensitivity.

A functional role for the mitochondria in acute O2 sensing in the carotid body (CB) remains undetermined. Whilst total inhibition of mitochondrial activity causes intense CB stimulation, it is unclear whether this response can be moderated such that graded impairment of oxidative phosphorylation might be a mechanism that sets and modifies the O2 sensitivity of the whole organ. We assessed NADH autofluorescence and [Ca2+]i in freshly dissociated CB type I cells and sensory chemoafferent discharge frequency in an intact CB preparation, in the presence of varying concentrations of nitrite (NO2 −), a mitochondrial nitric oxide (NO) donor and a competitive inhibitor of mitochondrial complex IV. NO2 − increased CB type I cell NADH in a manner that was dose-dependent and rapidly reversible. Similar concentrations of NO2 − raised type I cell [Ca2+]i via L-type channels in a PO2-dependent manner and increased chemoafferent discharge frequency. Moderate inhibition of the CB mitochondria by NO2 − augmented chemoafferent discharge frequency during graded hypoxia, consistent with a heightened CB O2 sensitivity. Furthermore, NO2 − also exaggerated chemoafferent excitation during hypercapnia signifying an increase in CB CO2 sensitivity. These data show that NO2 − can moderate the hypoxia sensitivity of the CB and thus suggest that O2 sensitivity could be set and modified in this organ by interactions between NO and mitochondrial complex IV.

A mitochondrial redox oxygen sensor in the pulmonary vasculature and ductus arteriosus

The mammalian homeostatic oxygen sensing system (HOSS) initiates changes in vascular tone, respiration, and neurosecretion that optimize oxygen uptake and tissue oxygen delivery within seconds of detecting altered environmental or arterial PO2. The HOSS includes carotid body type 1 cells, adrenomedullary cells, neuroepithelial bodies, and smooth muscle cells (SMCs) in pulmonary arteries (PAs), ductus arteriosus (DA), and fetoplacental arteries. Hypoxic pulmonary vasoconstriction (HPV) optimizes ventilation-perfusion matching. In utero, HPV diverts placentally oxygenated blood from the non-ventilated lung through the DA. At birth, increased alveolar and arterial oxygen tension dilates the pulmonary vasculature and constricts the DA, respectively, thereby transitioning the newborn to an air-breathing organism. Though modulated by endothelial-derived relaxing and constricting factors, O2 sensing is intrinsic to PASMCs and DASMCs. Within the SMC's dynamic mitochondrial network, changes in PO2 alter the reduction-oxidation state of redox couples (NAD(+)/NADH, NADP(+)/NADPH) and the production of reactive oxygen species, ROS (e.g., H2O2), by complexes I and III of the electron transport chain (ETC). ROS and redox couples regulate ion channels, transporters, and enzymes, changing intracellular calcium [Ca(2+)]i and calcium sensitivity and eliciting homeostatic responses to hypoxia. In PASMCs, hypoxia inhibits ROS production and reduces redox couples, thereby inhibiting O2-sensitive voltage-gated potassium (Kv) channels, depolarizing the plasma membrane, activating voltage-gated calcium channels (CaL), increasing [Ca(2+)]i, and causing vasoconstriction. In DASMCs, elevated PO2 causes mitochondrial fission, increasing ETC complex I activity and ROS production. The DASMC's downstream response to elevated PO2 (Kv channel inhibition, CaL activation, increased [Ca(2+)]i, and rho kinase activation) is similar to the PASMC's hypoxic response. Impaired O2 sensing contributes to human diseases, including pulmonary arterial hypertension and patent DA.