Carotid Body Chemoreceptors Research Articles

The carotid body chemoreceptors: hard to silence the know-it-alls.

The carotid body chemoreceptors: hard to silence the know-it-alls.

Reductive stress in mitochondria isolated from the carotid body of type 1 diabetic male Wistar rats.

The carotid body (CB) senses changes in arterial O2 partial pressure (pO2) and glucose levels; therefore, it is key for the detection of hypoxia and hypoglycemia. The CB has been suggested to detect pO2 through an increase in reactive oxygen species (ROS) in the mitochondria. However, the mechanism protecting the chemoreceptor cells and their mitochondria from ROS and hyperglycemia is poorly understood. Here we measured glutathione levels in CB mitochondria of control and in streptozotocin (STZ)-induced type 1 diabetic male Wistar rats. We found a dramatic reduction in total glutathione from 11.45 ± 1.30 μmol/mg protein in control rats to 1.45 ± 0.31 μmol/mg protein in diabetic rats. However, the ratio of reduced to oxidized glutathione, a measure of the redox index, was increased in diabetic rats compared to controls. We conclude that the mitochondria of CB chemoreceptor cells in type 1 diabetic male Wistar rats were likely under glutathione-reducing stress.

Role for the Carotid Chemoreceptors in Glucose Tolerance in Human Type 2 Diabetes

Objectives: The carotid body (CB) chemoreceptors have been implicated in the development and progression of type 2 diabetes (T2D). CB denervation can prevent the development and worsening of T2D in pre-clinical models — although these data have yet to be translated to humans. We examined CB chemosensitivity in T2D, as well as the contribution of the CB to glucose tolerance. We hypothesized: 1) individuals with T2D would exhibit exaggerated CB sensitivity to hypoxia (hypoxic ventilatory response, HVR), 2) attenuation of CB chemoreceptor activity (i.e., hyperoxia) would improve glucose tolerance in T2D, and 3) the magnitude of the effect of hyperoxia on glucose tolerance would be related to the level of CB chemosensitivity. Methods: HVR was assessed in sixteen adults with T2D (6M/10F, 55±11 yrs, HbA1c 7.7±1.4%) and twenty-one healthy controls (9M/12F, 50±13 yrs, HbA1c 5.2±0.4%)(NCT05219994). A subset (n=9 per group) completed two additional visits randomized to normoxia (0.21 FiO2) and hyperoxia (1.0 FiO2). During each visit, blood glucose and plasma insulin were measured every 15 min for 2 hrs following consumption of a 75 g glucose drink and responses were calculated as area under the curve (AUC). Pearson correlations between HVR and the difference in AUC glucose and insulin between visits (ΔAUC = hyperoxia-normoxia) were conducted. Results: HVR was augmented in T2D compared to control (control, -0.47±0.39; T2D, -0.83±0.50 L/min/%; p=0.02). There was no effect of hyperoxia on AUCglucose (control, p=0.77; T2D, p=0.73) or AUCinsulin (control, p=0.70; T2D, p=0.41). Correlations were observed between HVR and ΔAUCglucose (R=0.67, p=0.047) and ΔAUCinsulin (R=-0.55, p=0.16)] in adults with T2D, but not in controls [ΔAUCglucose (R=-0.32, p=0.40), ΔAUCinsulin (R=-0.18, p=0.67)]. Conclusions: Individuals with T2D exhibit exaggerated CB sensitivity to hypoxia. Glucose tolerance was unaffected by attenuation of CB chemoreceptor activity; however, when examined in relation to CB sensitivity, glucose tolerance improved with hyperoxia in T2D adults with the highest HVR. Present data advance our understanding of the pathophysiology of T2D and support a potential role for the CB chemoreceptors in glucose tolerance in human T2D. MizzouForward Undergraduate Research Training Grant, University of Missouri Research Council, University of Missouri Alumni Association Richard Wallace Research Incentive Fund. This is the full abstract presented at the American Physiology Summit 2024 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.

Carotid body-mediated peripheral chemoreflex drive sets exercise tolerance in adult mice

Exercise tolerance (EXT) has been primarily explained by individual body/physical features, partly neglecting the complex cardiorespiratory response needed to cope with exercise. While in healthy conditions most people display good EXT, the presence of exercise intolerance (EXINT) has also been documented. More importantly, EXINT increases in several pathological conditions. Despite the precise mechanism underpinning EXINT is not fully understood. It is well known that exaggerated sympathetic outflow to skeletal muscle impairs proper muscle function and that activation of the carotid body (CB) chemoreceptors elicits a robust increase in sympathetic activity. Therefore, it is plausible to hypothesize that an exaggerated CB-chemoreflex drive may impair EXT. Accordingly, we aimed to determine: i) differences in CB-chemoreflex drive between EXT and EXINT mice, and ii) the role played by the CB-chemoreflex on setting EXINT. Male C57/BL6 mice (n=24) were used to study chemoreflex function using whole-body plethysmography. Exercise tolerance/intolerance was determined by a three-day maximal performance exercise test. CB-chemoreflex drive was assessed by allowing the mice to breathe hypoxic gas (FiO2~5%). We found that the incidence of EXINT among healthy mice was ~30%. Notably, compared to EXT mice, animals with EXINT exhibited a heightened CB-mediated chemoreflex drive as evidenced by larger increases in the hypoxic ventilatory responses (HVR). Indeed, HVR was ~1.4-fold larger in EXINT compared to the values obtained in EXT mice (VE: 17 ± 1.6 vs. 12 ± 3.2, mL/min, EXINT vs. EXT, respectively). Then, we tested whether high chemoreflex drive was associated with EXINT. For this, we reduced the chemosensory drive by using the Dejour´s maneuver (FiO2~35%) while EXINT mice continues to exercise. We found that chemoreflex unloading in EXINT markedly improves exercise tolerance. Indeed, exercise tolerance was almost fully restored by reducing chemoreflex drive in EXINT. Our findings underscore the potential role of heightened CB-chemoreflex on setting exercise tolerance. Supported by Fondecyt 1220950. This is the full abstract presented at the American Physiology Summit 2024 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.

Carotid body-mediated chemoreflex drive predicts the development of HFpEF in mice

Heart failure with preserved ejection fraction (HFpEF) is a prevalent pathophysiological condition affecting millions of people worldwide. Several factors have been pointed out to play a role in the development of HFpEF; however, there are no molecular/physiological variables that can predict the incidence of HFpEF. Interestingly, carotid body (CB) chemoreceptors have been implicated in worse outcomes in reduced ejection fraction HF. Then, we hypothesized that CB chemoreflex sensitivity may offer prognostic value in HFpEF. Accordingly, we aimed to study whether high chemoreflex drive predicts the development of HFpEF in mice. A longitudinal study was performed in adult C57BLJ6 mice (P90) that underwent 16 weeks treatment with a high fat diet (60% high fat diet, HFD) and a nitric oxide synthase blocker (L-NAME 0.6 g/L) to induce HFpEF. Mice were recorded for cardiac/hemodynamic and ventilatory function before the onset of HFD and L-NAME treatment by using 2D-echocardiography, tissue doppler imaging, blood pressure and whole-body plethysmography, respectively. At 16 weeks after HFD+L-NAME all mice were recorded again. CB-chemoreflex drive was assessed by the hypoxic ventilatory response (HVR, FiO2»10%). We found that »50% of total mice (n=24) develop HFpEF after 16 weeks of HFD+L-NAME as evidenced by a significant (p<0.05) increase in the E/A ratio (1.60±0.09 vs. 1.30±0.01 mmHg, HFpEF vs. nonHFpEF), blood pressure (MAP: 98.7±1.9 vs. 88.4±1.4 mmHg, HFpEF vs. nonHFpEF) and body weight (31±1 vs. 29±0 g, HFpEF vs. nonHFpEF). Remarkably, a higher HVR before the onset of HFD+L-NAME was closely associated with the prevalence of HFpEF. Indeed, (4/5) mice that displayed a high HVR chemoreflex (³0.15 ΔVE/FiO2%) develop HFpEF after 16 weeks of HFD+L-NAME treatment. Our results showed for the first time that CB-mediated chemoreflex sensitivity predicts the development of HFpEF and support the notion that peripheral chemoreflex drive may be useful to evaluate the risk of developing HFpEF. Supported by Fondecyt 1220950. This is the full abstract presented at the American Physiology Summit 2024 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.

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

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

The Thirty-Fifth Anniversary of K+ Channels in O2 Sensing: What We Know and What We Don’t Know

On the thirty-fifth anniversary of the first description of O2-sensitive K+ channels in the carotid body chemoreceptors O2 sensing remains a salient issue in the literature. Whereas much has been learned about this subject, important questions such as the identity of the specific K+ channel subtype(s) responsible for O2 sensing by chemoreceptors and the mechanism(s) by which their activities are altered by hypoxia have not yet been definitively answered. O2 sensing is a fundamental biological process necessary for the acute and chronic responses to varying environmental O2 levels which allow organisms to adapt to hypoxia. Whereas chronic responses depend on the modulation of hypoxia-inducible transcription factors which determine the expression of numerous genes encoding enzymes, transporters and growth factors, acute responses rely mainly on the dynamic modulation of ion channels by hypoxia, causing adaptive changes in cell excitability, contractility and secretory activity in specialized tissues. The most widely studied oxygen-sensitive ion channels are potassium channels, but oxygen sensing by members of both the calcium and sodium channel families has also been demonstrated. Given the explosion of information on this topic, in this review, we will focus on the mechanisms of physiological oxygen chemotransduction by PO2-dependent K+ channels, with particular emphasis on their function in carotid body chemoreceptor cells (CBCC) and pulmonary artery smooth muscle cells (PASMC), highlighting areas of consensus and controversy within the field. We will first describe the most well-established concepts, those reproduced in multiple laboratories, and then discuss selected observations or questions that remain unresolved, and that limit our progress in this field.

Contributions of carotid bodies, retrotrapezoid nucleus neurons and preBötzinger complex astrocytes to the CO2 -sensitive drive for breathing.

Current models of respiratory CO2 chemosensitivity are centred around the function of a specific population of neurons residing in the medullary retrotrapezoid nucleus (RTN). However, there is significant evidence suggesting that chemosensitive neurons exist in other brainstem areas, including the rhythm-generating region of the medulla oblongata - the preBötzinger complex (preBötC). There is also evidence that astrocytes, non-neuronal brain cells, contribute to central CO2 chemosensitivity. In this study, we reevaluated the relative contributions of the RTN neurons, the preBötC astrocytes, and the carotid body chemoreceptors in mediating the respiratory responses to CO2 in experimental animals (adult laboratory rats). To block astroglial signalling via exocytotic release of transmitters, preBötC astrocytes were targeted to express the tetanus toxin light chain (TeLC). Bilateral expression of TeLC in preBötC astrocytes was associated with ∼20% and ∼30% reduction of the respiratory response to CO2 in conscious and anaesthetized animals, respectively. Carotid body denervation reduced the CO2 respiratory response by ∼25%. Bilateral inhibition of RTN neurons transduced to express Gi-coupled designer receptors exclusively activated by designer drug (DREADDGi ) by application of clozapine-N-oxide reduced the CO2 response by ∼20% and ∼40% in conscious and anaesthetized rats, respectively. Combined blockade of astroglial signalling in the preBötC, inhibition of RTN neurons and carotid body denervation reduced the CO2 -induced respiratory response by ∼70%. These data further support the hypothesis that the CO2 -sensitive drive to breathe requires inputs from the peripheral chemoreceptors and several central chemoreceptor sites. At the preBötC level, astrocytes modulate the activity of the respiratory network in response to CO2 , either by relaying chemosensory information (i.e. they act as CO2 sensors) or by enhancing the preBötC network excitability to chemosensory inputs. KEY POINTS: This study reevaluated the roles played by the carotid bodies, neurons of the retrotrapezoid nucleus (RTN) and astrocytes of the preBötC in mediating the CO2 -sensitive drive to breathe. The data obtained show that disruption of preBötC astroglial signalling, blockade of inputs from the peripheral chemoreceptors or inhibition of RTN neurons similarly reduce the respiratory response to hypercapnia. These data provide further support for the hypothesis that the CO2 -sensitive drive to breathe is mediated by the inputs from the peripheral chemoreceptors and several central chemoreceptor sites.

TRPA1 channel in the airway underlies protection against airborne threats by modulating respiration and behaviour.

Transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of cation channels, is broadly expressed in sensory neural pathways, including the trigeminal neurons innervating the nasal cavity and vagal neurons innervating the trachea and the lung. TRPA1 acts as a detector of various irritant chemicals as well as hypoxia and hyperoxia. For the past 15years, we have characterised its role in respiratory and behavioural modulation in vivo using Trpa1 knockout (KO) mice and wild-type (WT) littermates. Trpa1 KO mice failed to detect, wake up from sleeping, and escape from formalin vapour and a mild hypoxic (15% O2) environment. Respiratory augmentation induced by mild hypoxia was absent in either Trpa1 KO mice or WT mice treated with a TRPA1 antagonist. Irritant gas introduced into the nasal cavity inhibited respiratory responses in WT mice but not in the KO mice. The effect of TRPA1 on the olfactory system seemed minimal because olfactory bulbectomized WT mice reacted similarly to the intact mice. Immunohistological analyses using a cellar activation marker, the phosphorylated form of extracellular signal-regulated kinase, confirmed activation of trigeminal neurons in WT mice but not in Trpa1 KO mice in response to irritant chemicals and mild hypoxia. These data collectively show that TRPA1 is necessary for multiple chemical-induced protective responses in respiration and behaviour. We propose that TRPA1 channels in the airway may play a sentinel role for environmental threats and prevent incoming damage.

Immunohistochemical distribution of Ca2+/calmodulin-dependent protein kinase II subunits in the rat carotid body

Carotid body (CB) activity stimulated by a lower partial oxygen pressure in rats is enhanced by exposure to chronic intermittent hypoxia. However, the mechanisms that modulate CB activity remain unclear. In the present study, the expression and distribution of one of the candidate molecules to modulate reactivity, Ca2+/calmodulin-dependent protein kinase II (CaMKII) were examined in the rat CB using reverse transcriptional polymerase chain reaction and immunofluorescence with isoform-specific antibodies. CaMKIIγ and CaMKIIδ were distributed in CB chemoreceptor cells, and exhibited intense immunoreactivity in dopamine β-hydroxylase-positive chemoreceptor cells. CaMKIIβ and CaMKIIγ were distributed in sensory nerve endings attached to chemoreceptor cells of the CB. In the petrosal ganglion, immunoreactivities for CaMKIIα, CaMKIIβ, CaMKIIγ, and CaMKIIδ were detected in the perinuclear region of ganglion cells. The present results indicate that CaMKIIγ and CaMKIIδ in chemoreceptor cells and CaMKIIβ and CaMKIIγ in sensory nerve endings enhanced reciprocal synaptic transmission, i.e., noradrenaline and ATP for cells to neurons and glutamate for neurons to cells.

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

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

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.

Role of Peripheral Chemoreceptors on Enhanced Central Chemoreflex Drive in Nonischemic Heart Failure.

Heart failure (HF) is a prevalent disease in elderly population. Potentiation of the ventilatory chemoreflex drive plays a pivotal role in disease progression, at least in part, through their contribution to the generation/maintenance of breathing disorders. Peripheral and central chemoreflexes are mainly regulated by carotid body (CB) and the retrotrapezoid nuclei (RTN), respectively. Recent evidence showed an enhanced central chemoreflex drive in rats with nonischemic HF along with breathing disorders. Importantly, increase activity from RTN chemoreceptors contribute to the potentiation of central chemoreflex response to hypercapnia. The precise mechanism driving RTN potentiation in HF is still elusive. Since interdependency of RTN and CB chemoreceptors has been described, we hypothesized that CB afferent activity is required to increase RTN chemosensitivity in the setting of HF. Accordingly, we studied central/peripheral chemoreflex drive and breathing disorders in HF rats with and without functional CBs (CB denervation). We found that CB afferent activity was required to increase central chemoreflex drive in HF. Indeed, CB denervation restored normal central chemoreflex drive and reduced the incidence of apneas by twofold. Our results support the notion that CB afferent activity plays an important role in central chemoreflex potentiation in rats with HF.

Carotid Body-Mediated Chemoreflex Function in Aging and the Role of Receptor-Interacting Protein Kinase.

Ventilatory impairment during aging has been linked to carotid body (CB) dysfunction. Anatomical/morphological studies evidenced CB degeneration and reductions in the number of CB chemoreceptor cells during aging. The mechanism(s) related to CB degeneration in aging remains elusive. Programmed cell death encompasses both apoptosis and necroptosis. Interestingly, necroptosis can be driven by molecular pathways related to low-grade inflammation, one hallmark of the aging process. Accordingly, we hypothesized that necrotic cell death dependent on receptor-interacting protein kinase-3 (RIPK3) may contribute, at least in part, to impair CB function during aging. Adult (3months) and aged (24months) wild type (WT) and RIPK3-/- mice were used to study chemoreflex function. Aging results in significant reductions in both the hypoxic (HVR) and hypercapnic ventilatory responses (HCVR). Adult RIPK3-/- mice showed normal HVR and HCVR compared to adult WT mice. Remarkable, aged RIPK3-/- mice displayed no reductions in HVR nor in HCVR. Indeed, chemoreflex responses obtained in aged RIPK3-/- KO mice were undistinguishable from the ones obtained in adult WT mice. Lastly, we found high prevalence of breathing disorders during aging and this was absent in aged RIPK3-/- mice. Together our results support a role for RIPK3-mediated necroptosis in CB dysfunction during aging.

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

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

Peripheral chemoreflex modulation of renal hemodynamics and renal tissue PO2 in chronic heart failure with reduced ejection fraction.

Aberrant carotid body chemoreceptor (CBC) function contributes to increased sympathetic nerve activity (SNA) and reduced renal blood flow (RBF) in chronic heart failure (CHF). Intermittent asphyxia (IA) mimicking sleep apnea is associated with additional increases in SNA and may worsen reductions in RBF and renal PO2 (RPO2) in CHF. The combined effects of decreased RBF and RPO2 may contribute to biochemical changes precipitating renal injury. This study sought to determine the role of CBC activity on glomerular filtration rate (GFR), RBF and RPO2 in CHF, and to assess the additive effects of IA. Furthermore, we sought to identify changes in gene expression that might contribute to renal injury. We hypothesized that GFR, RBF, and RPO2 would be reduced in CHF, that decreases in RBF and RPO2 would be worsened by IA, and that these changes would be ameliorated by CBC ablation (CBD). Finally, we hypothesized that CHF would be associated with pro-oxidative pro-fibrotic changes in renal gene expression that would be ameliorated by CBD. CHF was induced in adult male Sprague Dawley rats using coronary artery ligation (CAL). Carotid body denervation was performed by cryogenic ablation. GFR was assessed in conscious animals at the beginning and end of the experimental period. At 8-weeks post-CAL, cardiac function was assessed via echocardiography, and GFR, baseline and IA RBF and RPO2 were measured. Renal gene expression was measured using qRT-PCR. GFR was lower in CHF compared to sham (p < 0.05) but CBD had no salutary effect. RBF and RPO2 were decreased in CHF compared to sham (p < 0.05), and this effect was attenuated by CBD (p < 0.05). RBF and RPO2 were reduced to a greater extent in CHF vs. sham during exposure to IA (p < 0.05), and this effect was attenuated by CBD for RBF (p < 0.05). Downregulation of antioxidant defense and fibrosis-suppressing genes was observed in CHF vs. sham however CBD had no salutary effect. These results suggest that aberrant CBC function in CHF has a clear modulatory effect on RBF during normoxia and during IA simulating central sleep apnea.

Neurochemical plasticity of the carotid body in hypertension.

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

Respiratory responses to hypoxia during rest and exercise in individuals born pre-term: a state-of-the-art review.

The pre-term birth survival rate has increased considerably in recent decades, and research investigating the long-term effects of premature birth is growing. Moreover, altitude sojourns are increasing in popularity and are often accompanied by various levels of physical activity. Individuals born pre-term appear to exhibit altered acute ventilatory responses to hypoxia, potentially predisposing them to high-altitude illness. These impairments are likely due to the use of perinatal hyperoxia stunting the maturation of carotid body chemoreceptors, but may also be attributed to limited lung diffusion capacity and/or gas exchange inefficiency. Aerobic exercise capacity also appears to be reduced in this population. This may relate to the aforementioned respiratory impairments, or could be due to physiological limitations in pulmonary blood flow or at the exercising muscle (e.g. mitochondrial efficiency). However, surprisingly, the debilitative effects of exercise when performed at altitude do not seem to be exacerbated by premature birth. In fact, it is reasonable to speculate that pre-term birth could protect against the consequences of exercise combined with hypoxia. The mechanisms that underlie this assertion might relate to differences in oxidative stress responses or in cardiopulmonary morphology in pre-term individuals, compared to their full-term counterparts. Further research is required to elucidate the independent effects of neonatal treatment, sex differences and chronic lung disease, and to establish causality in some of the proposed mechanisms that could underlie the differences discussed throughout this review. A more in-depth understanding of the acclimatisation responses to chronic altitude exposures would also help to inform appropriate interventions in this clinical population.

The Role of Carotid Body Chemoreceptor Afferents in Mediating Abnormal Ventilation in Acute Lung Injury

Background‐Acute lung injury (ALI) induces inflammation that disrupts the normal alveolar‐capillary endothelial barrier which impairs gas exchange to induce hypoxemia that reflexively increases respiration. Increased respiratory rate (RR) usually develops within a few hours to a few days after ALI. This disorder of acute respiratory failure affects approximately 200,000 new cases each year in the US alone and has high morbidity and mortality. The neural mechanisms underlying the respiratory dysfunction after ALI are not fully understood. The purpose of this study involved investigating the role of carotid body chemoreceptor afferents in mediating abnormal ventilation in ALI.Methods and Results‐ALI was induced in male SD rats (200‐250g) using a single intra‐tracheal injection of bleomycin (Bleo) (day 0) and respiratory variables‐ RR, TV (Tidal Volume), and MV (Minute Ventilation) in response to 10% hypoxia (10% O2, 0% CO2) and 5% hypercapnia (21% O2, 5% CO2) were measured weekly from W1‐W4 using whole‐ body plethysmography (WBP). MV at 21% O2 increased from baseline (W0) to W1 and W2 in bleo treated rats (p≤0.01 compared to saline treated sham rats). Changes in MV were due to a significant increase in RR while TV showed no changes. Chemoreflex was assessed by measuring the absolute difference between 21% O2 and hypoxia (10% O2, 0% CO2) &amp; hypercapnia (21% O2, 5% CO2). Chemoreflex activation is a fundamental protective reflex that maintains oxygen homeostasis. Our data indicated a blunted chemoreflex at W1 and a sensitized chemoreflex in W3‐W4 during hypoxia and hypercapnia. We hypothesized that the blunted chemoreflex in W1 could be due to maximal pre‐activation of chemoreceptors, called the ‘ceiling effect’. To test this possibility, 90% hyperoxia (90% O2, 0% CO2) was given to bleo treated rats to inhibit the chemoreflex and our results showed no changes in RR, suggesting absence of the tonic activity at W1 post‐ALI.Conclusions‐These data suggest that chemoreflex activation in response to hypoxia and hypercapnia is blunted at acute stage of lung injury but became sensitized at the recovery stage.

Regulation of heart rate in vertebrates during hypoxia: A comparative overview.

Acute exposure to low oxygen (hypoxia) places conflicting demands on the heart. Whilst an increase in heart rate (tachycardia) may compensate systemic oxygen delivery as arterial oxygenation falls, the heart itself is an energetically expensive organ that may benefit from slowing (bradycardia) to reduce work when oxygen is limited. Both strategies are apparent in vertebrates, with tetrapods (mammals, birds, reptiles, and amphibians) classically exhibiting hypoxic tachycardia and fishes displaying characteristic hypoxic bradycardia. With a richer understanding of the ontogeny and evolution of the responses, however, we see similarities in the underlying mechanisms between vertebrate groups. For example, in adult mammals, primary bradycardia results from the hypoxic stimulation of carotid body chemoreceptors that are overwhelmed by mechano-sensory feedback from the lung associated with hyperpnoea. Fish-like bradycardia prevails in the mammalian foetus (which, at this stage, is incapable of pulmonary ventilation), and in fish and foetus alike, the bradycardia ensues despite an elevation of circulating catecholamines. In both cases, the reduced heart rate may primarily serve to protect the heart. Thus, the comparative perspective offers fundamental insight into how and why different vertebrates regulate heart rate in different ways during periods of hypoxia.