Carotid Chemoreflex Research Articles

ALTERED CONTROL OF BREATHING IN A RAT MODEL OF ALLERGIC LOWER AIRWAY INFLAMMATION.

Obstructive sleep apnea (OSA) is highly prevalent in patients with asthma. Asthma, dose-dependently to its duration, promotes incident OSA, suggesting that asthma plays a role in OSA pathogenesis. We hypothesized that asthma-related inflammation alters breathing control mechanisms, specifically the carotid chemoreflex. Accordingly, we measured hypoxic ventilatory responses (HRV) in awake, unrestrained, ovalbumin (OVA)-sensitized Brown Norway rats and compared them with responses in sham-sensitized (SALINE) controls. To differentiate the role of allergic inflammation from bronchoconstriction, we repeated HVR after administration of formoterol, a long-acting bronchodilator. Blood and bronchoalveolar lavage (BAL) fluid were collected for quantification of inflammatory cytokines. The rise in ventilatory equivalent for O2 evoked by acute exposure to hypoxia was augmented following sensitization by OVA, whereas it remained stable after SALINE. This augmentation was driven by increased breathing frequency with no change in tidal volume. Tachypneic hyperventilation in normoxia was also observed with OVA. Neither the increased HVR nor excessive normoxic ventilation was affected by formoterol, suggesting that they were not secondary to lung mechanical constraints. Higher levels of inflammatory cytokines were observed in BAL fluid and serum of OVA vs. SALINE. In OVA, serum interleukin-5 correlated with change (baseline to post-sensitization) in ventilatory response to severe hypoxia (FIO2, 0.09). These observations are consistent with inflammation-induced enhancement of carotid chemoreflex function, i.e. increased controller gain, and they suggest a possible role for asthma-related allergic inflammation in the ventilatory instability known to promote upper airway collapse and sleep apnea in humans.

Gefapixant as a P2X3 receptor antagonist treatment for obstructive sleep apnea: a randomized controlled trial.

Obstructive sleep apnea (OSA) is a highly prevalent disorder with serious health consequences but limited therapeutic options. For a subset of those with OSA, a key underlying mechanism is hypersensitive chemoreflex control of breathing. There is no approved therapy that targets this endotypic trait. Here we determine whether the P2X3 receptor antagonist gefapixant, which is predicted to attenuate hypersensitive carotid chemoreflexes, reduces OSA severity in patients with chemoreflex-dependent OSA. In a randomized placebo-controlled cross-over study, 24 patients with moderate-to-severe OSA (aged 39-68 years, non-CPAP users) whose disorder was partially responsive to supplemental oxygen (chemoreflex-dependent OSA) were treated with gefapixant 180 mg (or placebo) administered as tablets taken orally before bedtime for 7 days and assessed via overnight polysomnography. The primary analysis examined whether gefapixant treatment resulted in a greater reduction in the apnea-hypopnea index (AHI) from baseline than placebo. Gefapixant did not lower the AHI significantly more than placebo; the estimated ratio of the AHI on gefapixant versus placebo was 0.92 [90% CI: 0.73, 1.17]. Notably, nocturnal hypoxemia was increased (ratio of total sleep time with SpO2 <90% on gefapixant versus placebo = 2.08 [90% CI: 1.53, 2.82]), consistent with reduced chemoreflex output. Commonly reported adverse events with gefapixant included ageusia, dysgeusia, oral hypoaesthesia, nausea, somnolence, and taste disorders. Gefapixant, while generally well tolerated, did not reduce OSA severity in patients with chemoreflex-dependent OSA. P2X3 receptor antagonism is unlikely to provide an avenue for therapeutic intervention in OSA. Registry: ClinicalTrials.gov; Name: Safety and Tolerability of Gefapixant (MK-7264) in Participants With Obstructive Sleep Apnea (MK-7264-039); URL: https://clinicaltrials.gov/study/NCT03882801; Identifier: NCT03882801.

Acute Intermittent Hypoxia Induces Chemoreflex-Independent Sympathetic Plasticity and Improves Cardiovascular Function in a Rodent Model of Spinal Cord Injury

Background: Cardiovascular complications are one of the leading causes of morbidity and mortality in people with spinal cord injury (SCI). Activation of spinal sympathetic circuitry is critical to offset cardiovascular dysregulation and disease in those with high-level SCI. Acute intermittent hypoxia (AIH) elicits plasticity (i.e., a gradual increase in nerve activity post-AIH exposure) in phrenic and non-phrenic spinal motor circuitry and improves motor function — yet the cardiovascular benefits of AIH after SCI are largely unexplored. Aims: Determine mechanisms (Aim 1) and potential benefits (Aim 2) of AIH-induced sympathetic plasticity post-SCI. Hypothesis: We hypothesized that AIH elicits chemoreflex-dependent sympathetic plasticity and improves cardiovascular function in rats with SCI. Methods: For Aim 1, 40 adult (10-12 weeks) male Wistar rats were subjected to a spinal cord contusion injury (T3, 300 kdyn) and randomly assigned to 5 groups of 8 each: AIH alone, AIH after carotid body denervation (CBX, no peripheral chemoreflex), time control with no intervention (TC), simulated hemorrhage (to mimic AIH-induced hypotension), and AIH plus phenylephrine (to prevent hypotension during hypoxic episodes). Two weeks post-SCI recovery, rats were anesthetized with urethane (~2.1g.kg−1 i.v.) and underwent femoral venous and arterial catheterizations to monitor blood pressure (BP), sample arterial blood gases, and enable intravenous fluid and drug delivery. Additionally, splanchnic sympathetic nerve activity (sSNA) was recorded using a suction electrode at baseline and for up to 90-min following each experimental paradigm. AIH consisted of 10 x 1 min episodes of FiO2 = 0.1 (balanced N2) interspersed with 2 min of FiO2 = 1. Hexamethonium bromide (30 mg.kg−1) was infused to subtract noise levels from each recording, after which rats were euthanized with chloral hydrate. For Aim 2, 19 additional rats were subjected to the same injury model and time post-injury and allocated to 2 groups: AIH ( n = 11) and TC ( n = 8). These rats were instrumented with left-ventricular (LV) and arterial catheters and assessed simultaneously for cardiac and peripheral hemodynamic function at baseline and 90-min post-AIH/TC. Results: Aim 1) When expressed as % change from baseline, sSNA increased at 90-min post-treatment in AIH (93±67%, P = 0.02) and CBX (193±182%, P &lt; 0.001), but not TC rats (50±45%, P = 0.40). Preventing BP reduction during hypoxic episodes with i.v. phenylephrine blocked the increase in sSNA 90-min post-AIH (12±66%, P &gt; 0.99). Mimicking the AIH-induced BP reduction with simulated hemorrhage (10 x 1-min BP reductions of ~20-25 mmHg equal to those during AIH, interspersed by 2-min of euvolemia), increased sSNA 90-min post-intervention (106±86%, P = 0.01). Aim 2) The changes in maximal LV pressure (AIH = 11±8 mmHg vs. TC = 2±5 mmHg, P = 0.05) and mean arterial pressure (AIH = 8±7 mmHg vs. TC = 0±4 mmHg, P = 0.05) were greater at 90-min post-AIH treatment. Conclusions: We conclude that AIH elicits sympathetic plasticity and improves cardiovascular function post-SCI. Since AIH-induced sympathetic plasticity remains in the presence of a bilateral CBX, is replicated by episodic simulated hemorrhage, and is blocked by phenylephrine, AIH-induced sympathetic plasticity is independent of carotid (peripheral) chemoreflex activation, and is closely linked to episodic hypotension. We suggest that AIH-induced sympathetic plasticity arises from episodic reductions in spinal/medullary cardiovascular centers blood flow (and oxygen delivery) post-SCI. International Spinal Research Trust and Natural Sciences, Engineering Research Council of Canada, International Collaboration on Repair Discoveries. 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.

Exercise-induced potentiation of the acute hypoxic ventilatory response: Neural mechanisms and implications for cerebral blood flow.

A given dose of hypoxia causes a greater increase in pulmonary ventilation during physical exercise than during rest, representing an exercise-induced potentiation of the acute hypoxic ventilatory response (HVR). This phenomenon occurs independently from hypoxic blood entering the contracting skeletal muscle circulation or metabolic byproducts leaving skeletal muscles, supporting the contention that neural mechanisms per se can mediate the HVR when humoral mechanisms are not at play. However, multiple neural mechanisms might be interacting intricately. First, we discuss the neural mechanisms involved in the ventilatory response to hypoxic exercise and their potential interactions. Current evidence does not support an interaction between the carotid chemoreflex and central command. In contrast, findings from some studies support synergistic interactions between the carotid chemoreflex and the muscle mechano- and metaboreflexes. Second, we propose hypotheses about potential mechanisms underlying neural interactions, including spatial and temporal summation of afferent signals into the medulla, short-term potentiation and sympathetically induced activation of the carotid chemoreceptors. Lastly, we ponder how exercise-induced potentiation of the HVR results in hyperventilation-induced hypocapnia, which influences cerebral blood flow regulation, with multifaceted potential consequences, including deleterious (increased central fatigue and impaired cognitive performance), inert (unchanged exercise) and beneficial effects (protection against excessive cerebral perfusion).

Carotid body dysregulation contributes to Long COVID symptoms

BackgroundThe symptoms of long COVID, which include fatigue, breathlessness, dysregulated breathing, and exercise intolerance, have unknown mechanisms. These symptoms are also observed in heart failure and are partially driven by increased sensitivity of the carotid chemoreflex. As the carotid body has an abundance of ACE2 (the cell entry mechanism for SARS-CoV-2), we investigated whether carotid chemoreflex sensitivity was elevated in participants with long COVID.MethodsNon-hositalised participants with long-COVID (n = 14) and controls (n = 14) completed hypoxic ventilatory response (HVR; the measure of carotid chemoreflex sensitivity) and cardiopulmonary exercise tests. Parametric and normally distributed data were compared using Student’s unpaired t-tests or ANOVA. Nonparametric equivalents were used where relevant. Peason’s correlation coefficient was used to examine relationships between variables.ResultsDuring cardiopulmonary exercise testing the VE/VCO2 slope (a measure of breathing efficiency) was higher in the long COVID group (37.8 ± 4.4) compared to controls (27.7 ± 4.8, P = 0.0003), indicating excessive hyperventilation. The HVR was increased in long COVID participants (−0.44 ± 0.23 l/min/ SpO2%, R2 = 0.77 ± 0.20) compared to controls (−0.17 ± 0.13 l/min/SpO2%, R2 = 0.54 ± 0.38, P = 0.0007). The HVR correlated with the VE/VCO2 slope (r = −0.53, P = 0.0036), suggesting that excessive hyperventilation may be related to carotid body hypersensitivity.ConclusionsThe carotid chemoreflex is sensitised in long COVID and may explain dysregulated breathing and exercise intolerance in these participants. Tempering carotid body excitability may be a viable treatment option for long COVID patients.

The role of the peripheral chemoreceptor reflex in non-hospitalised patients with post-COVID-19 syndrome

Abstract Background Long COVID (post COVID-19 syndrome) is a common debilitating illness. Little is known regarding the underlying mechanisms driving ongoing symptoms, which include lethargy, breathlessness, dysregulated breathing, and exercise intolerance. The carotid body is important in the control of breathing, is abundant in ACE2 (cell entry mechanism for SARS-CoV-2) and is known to drive similar symptoms in patients with heart failure (HF). We hypothesised that the carotid chemoreflex becomes hyperreactive in patients with long COVID-19 syndrome, which could be important in driving several ongoing symptoms. Purpose We aimed to examine whether carotid chemoreflex sensitivity was elevated in patients with long COVID (with initial mild-moderate symptoms) versus an age-matched control group and whether this could help to explain dysregulated breathing and poor exercise tolerance in long COVID. Methods Long COVID participants (n=14; 12F) were prospectively recruited from a tertiary cardiology long COVID clinic (after being cleared for cardiac conditions) and from advertising publicly. Patients were non-hospitalised and had received a diagnosis of long COVID via their GP or long COVID clinic. Participants had a positive PCR or sero-positive antibody test for their initial infection. Controls were healthy with no diagnosed comorbidities (n=14; 11F). Eight controls had a confirmed previous infection and did not develop ongoing symptoms. Participants completed baseline spirometry, a resting 12-lead electrocardiogram, and an incremental cardiopulmonary exercise test (CPET) to volitional exhaustion on a cycle ergometer. On a separate day, the carotid chemoreflex sensitivity was measured using the hypoxic ventilatory response (HVR) [1]. Averaged group data were compared using statistical software (GraphPad, Prism). Data are mean ± SD unless stated otherwise. Results Age and body mass index were similar between groups (Table 1). Resting spirometry indicated that lung function was similar between groups (Table 1). The long COVID group had a lower maximum volume of inspired oxygen (VO2 peak) and a similar maximum heart rate (as a % of predicted) vs. controls (Table 1). Ventilatory efficiency (VE/VCO2) slope, VE/VCO2 ratio at anaerobic threshold (AT), and VE/VCO2 nadir were higher in the long COVID group, indicating poor breathing efficiency, similar to the level observed in HF (Table 1) [2]. Finally, the HVR was elevated in the long COVID group (-0.45±0.23 l/min/SpO2%) versus the control group (-0.18±0.13 l/min/SpO2%, Figure 1, P=0.0007). The HVR correlated with the VE/VCO2 slope (r=0.54, P=0.0012). Conclusions These data suggest that the carotid chemoreflex is sensitised in people with long COVID and may help to explain dysregulated breathing in these patients. The carotid chemoreflex could be a viable target to help improve symptoms in people suffering with ongoing symptoms following even a mild original SARS-CoV-2 infection, but more research is needed.Table 1Figure 1

Cardiovascular Effects of the Exercise Pressor Reflex and Carotid Chemoreflex Interaction in Patients with Heart Failure with Reduced Ejection Fraction

Background: The interaction of the exercise pressor reflex (EPR) and the carotid chemoreflex (CR) affects circulatory control during physical activity in health. It has been shown that patients with heart failure with reduced ejection fraction (HFrEF) have an abnormal cardiovascular response to EPR or CR activation, which contributes to impaired limb blood flow during exercise. However, the impact of the reflex interaction in HFrEF is unclear. Purpose: To evaluate the interaction between EPR and CR in HFrEF. Method: Five patients with HFrEF and 6 control subjects (Ctrl) completed two experimental sessions. In session 1, participants performed dynamic single-leg knee-extension at 60% of peak power output for 4 min while breathing room air (normoxia; NormC: SaO2~96%, PaO2~81 mmHg, PaCO2~33 mmHg, arterial pH~7.43) and pure O2 (hyperoxia; HyperC: SaO2~100%, PaO2~408 mmHg, PaCO2~35 mmHg, arterial pH~7.40). In session 2, following lumbar intrathecal fentanyl administration to attenuate group III/IV leg muscle afferent feedback, participants repeated the same exercise under normoxic and hyperoxic conditions (NormF and HyperF). Mean arterial pressure (MAP, radial artery catheter), heart rate (HR, 12-lead ECG), stroke volume (SV, finger photoplethysmography), cardiac output (CO), systemic vascular conductance (SVC), femoral blood flow of the working leg (LBF, Doppler ultrasound), and leg vascular conductance (LVC) were continuously quantified during exercise. Results: EPR activation (i.e., ΔHyperC-HyperF) in HFrEF resulted in no changes in MAP and HR, but significantly decreased SV (~10 mL), CO (~1 L min−1), SVC (~9 mL min−1 mmHg−1), LBF (~0.4 L min−1) and LVC (~4 mL min−1 mmHg−1). In contrast, EPR activation in Ctrl significantly increased MAP (~10 mmHg) and decreased LVC (~3 mL min−1 mmHg−1), but did not affect HR, SV, CO, SVC and LBF ( p≥0.20). CR activation in HFrEF (i.e., ΔNormF-HyperF) had no effects on any of the cardiovascular variables ( p≥0.12). CR activation in Ctrl significantly increased HR (~3 beats min−1) and SVC (~3 mL min−1 mmHg−1), but had no effects on MAP, SV, CO, LBF and LVC ( p≥0.10). During EPR:CR co-activation (i.e., ΔNormC-HyperF) in HFrEF, the observed responses were not significantly different from the sum of the responses evoked by each reflex individually. In Ctrl, MAP during EPR:CR co-activation was greater than the summated response (16±7 vs. 10±9 mmHg; p&lt;0.01) and SV and SVC were significantly lower than the summated responses; there were no differences between the observed and summated responses of other parameters. Conclusion: Our findings indicate that during exercise in healthy older adults, EPR and CR interact to synergistically augment MAP by constricting vessels in non-active tissue/organs, which may facilitate blood flow to the exercising limb. The circulatory consequences of the reflex interaction appear to be altered in HFrEF and contribute to the impaired limb blood flow characterizing this population during exercise. National Heart, Lung, and Blood Institute (HL-116579), U.S. Veterans Affairs Rehabilitation Research and Development (E3343-R) 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.

Role of the angiotensin type 1 receptor in modulating the carotid chemoreflex in an ovine model of renovascular hypertension.

The carotid body has been implicated as an important mediator and putative target for hypertension. Previous studies have indicated an important role for angiotensin II in mediating carotid body function via angiotensin type-1 receptors (AT1R); however, their role in modulating carotid body function during hypertension is unclear. Using a large preclinical ovine model of renovascular hypertension, we hypothesized that acute AT1R blockade would lower blood pressure and decrease carotid body-mediated increases in arterial pressure. Adult ewes underwent either unilateral renal artery clipping or sham surgery. Two weeks later, flow probes were placed around the contralateral renal and common carotid arteries. In both hypertensive and sham animals, carotid body stimulation using potassium cyanide caused dose-dependent increases in mean arterial pressure but a reduction in renal vascular conductance. These responses were not different between groups. Infusion of angiotensin II led to an increase in arterial pressure and reduction in renal blood flow. The sensitivity of the renal vasculature to angiotensin II was significantly attenuated in hypertension compared with the sham animals. Systemic inhibition of the AT1R did not alter blood pressure in either group. Interestingly carotid body-evoked arterial pressure responses were attenuated by AT1R blockade in renovascular hypertension but not in shams. Taken together, our findings indicate a decrease in vascular reactivity of the non-clipped kidney to angiotensin II in hypertension. The CB-evoked increase in blood pressure in hypertension is mediated in part, by the AT1R. These findings indicate a differential role of the AT1R in the carotid body versus the renal vasculature.

Investigating the control of exercise hyperpnoea: A synergy of contributions.

Investigating the control of exercise hyperpnoea: A synergy of contributions.

Duration at high altitude influences the onset of arrhythmogenesis during apnea.

Autonomic control of the heart is balanced by sympathetic and parasympathetic inputs. Excitation of both sympathetic and parasympathetic systems occurs concurrently during certain perturbations such as hypoxia, which stimulate carotid chemoreflex to drive ventilation. It is well established that the chemoreflex becomes sensitized throughout hypoxic exposure; however, whether progressive sensitization alters cardiac autonomic activity remains unknown. We sought to determine the duration of hypoxic exposure at high altitude necessary to unmask cardiac arrhythmias during instances of voluntary apnea. Measurements of steady-state chemoreflex drive (SS-CD), continuous electrocardiogram (ECG) and SpO2 (pulse oximetry) were collected in 22 participants on 1day at low altitude (1045m) and over eight consecutive days at high-altitude (3800m). SS-CD was quantified as ventilation (L/min) over stimulus index (PETCO2/SpO2). Bradycardia during apnea was greater at high altitude compared to low altitude for all days (p < 0.001). Cardiac arrhythmias occurred during apnea each day but became most prevalent (> 50%) following Day 5 at high altitude. Changes in saturation during apnea and apnea duration did not affect the magnitude of bradycardia during apnea (ANCOVA; saturation, p = 0.15 and apnea duration, p = 0.988). Interestingly, the magnitude of bradycardia was correlated with the incidence of arrhythmia per day (r = 0.8; p = 0.004). Our findings suggest that persistent hypoxia gradually increases vagal tone with time, indicated by augmented bradycardia during apnea and progressively increased the incidence of arrhythmia at high altitude.

Duration at High Altitude Influences the Onset of Arrhythmogenesis During Apnea

Excitation of both sympathetic and parasympathetic systems occur concurrently at high-altitude (chronic hypoxia), via the carotid chemoreflex. We have demonstrated this autonomic conflict manifests as cardiac arrhythmias during voluntary apnea. We sought to determine the duration of hypoxic exposure at high-altitude necessary to unmask cardiac arrhythmias during instances of voluntary apnea. We hypothesized that the occurrence of cardiac arrhythmias during apnea would be increased following 24 hours of high-altitude exposure. Measurements of oxygen saturation (SpO2; pulse oximetry) and continuous electrocardiogram (ECG; lead II) were collected in 22 participants (8 females) at low altitude (1045m) and over 8 consecutive days at high-altitude (3800m). Following a 2 minute baseline participants performed an end-expiratory apnea to volitional breakpoint. ECG rate and rhythm were evaluated at baseline and during apnea. The nadir heart rate was used to quantify bradycardia and abnormal ECG rhythm was classified based on origin (e.g., sinus node) and type (e.g., sinus arrest). Differences in resting ventilation, SpO2, and the heart rate response to apnea on each day at high-altitude (Day 1-8) were compared to that at low altitude (Day 0) using a repeated-measures ANOVA design, with a Holm-Sidak post-hoc analysis where main effect of time was significant. Baseline SpO2 was lower for all days at high-altitude (average of all days, 89±3) compared to low altitude (98 ± 2) (p<0.01). At high-altitude (all days), baseline heart rate was higher compared to low altitude (p<0.01). A main effect of time (p<0.01) was identified for the delta change in heart rate during apnea. At low altitude, the average apnea induced decrease in heart rate was -9±15bpm, whereas the average decrease in heart rate at high altitude (all days) was -24±16bpm. Bradycardia became more pronounced with acclimatization, with the greatest drop in heart rate (-31±16bpm) occurring on Day 5. At low altitude 14% (3/22) of participants developed arrhythmias during apnea. At high-altitude, cardiac arrhythmias during apnea became most prevalent (>50%) following Day 5. Changes in saturation during apnea and apnea duration were not associated with the magnitude of bradycardia during apnea (saturation, r= -0.007 p=0.92; apnea duration, r=0.005 p=0.94). Interestingly, the magnitude of bradycardia was correlated with the incidence (percentage) of arrhythmia per day (r=0.8; p=0.004), suggesting a similar underlying mechanism. Our data indicate chronic hypoxia is associated with a progressive increase in vagal tone, as indicated by augmented bradycardia during apnea and progressively increased incidence of arrhythmia at high-altitude. Additionally, the increased incidence of arrhythmia with acclimatization suggests that chemoreflex sensitization may play a role in this phenomenon. This study provides insight into cardiac electrophysiology at high-altitude and has implications as a future functional model for exploring the effects of heightened chemoreflex stress on autonomic control of heart.

Measuring Peripheral Chemoreflex Hypersensitivity in Heart Failure.

Heart failure with reduced ejection fraction (HFrEF) induces chronic sympathetic activation. This disturbance is a consequence of both compensatory reflex disinhibition in response to lower cardiac output and patient-specific activation of one or more excitatory stimuli. The result is the net adrenergic output that exceeds homeostatic need, which compromises cardiac, renal, and vascular function and foreshortens lifespan. One such sympatho-excitatory mechanism, evident in ~40–45% of those with HFrEF, is the augmentation of carotid (peripheral) chemoreflex ventilatory and sympathetic responsiveness to reductions in arterial oxygen tension and acidosis. Recognition of the contribution of increased chemoreflex gain to the pathophysiology of HFrEF and to patients’ prognosis has focused attention on targeting the carotid body to attenuate sympathetic drive, alleviate heart failure symptoms, and prolong life. The current challenge is to identify those patients most likely to benefit from such interventions. Two assumptions underlying contemporary test protocols are that the ventilatory response to acute hypoxic exposure quantifies accurately peripheral chemoreflex sensitivity and that the unmeasured sympathetic response mirrors the determined ventilatory response. This Perspective questions both assumptions, illustrates the limitations of conventional transient hypoxic tests for assessing peripheral chemoreflex sensitivity and demonstrates how a modified rebreathing test capable of comprehensively quantifying both the ventilatory and sympathoneural efferent responses to peripheral chemoreflex perturbation, including their sensitivities and recruitment thresholds, can better identify individuals most likely to benefit from carotid body intervention.

Functional glutamate transporters are expressed in the carotid chemoreceptor

BackgroundThe carotid body (CB) plays a critical role in cyclic intermittent hypoxia (CIH)-induced chemosensitivity; however, the underlying mechanism remains uncertain. We have demonstrated the presence of multiple inotropic glutamate receptors (iGluRs) in CB, and that CIH exposure alters the level of some iGluRs in CB. This result implicates glutamatergic signaling in the CB response to hypoxia. The glutamatergic neurotransmission is not only dependent on glutamate and glutamate receptors, but is also dependent on glutamate transporters, including vesicular glutamate transporters (VGluTs) and excitatory amino acid transporters (EAATs). Here, we have further assessed the expression and distribution of VGluTs and EAATs in human and rat CB and the effect of CIH exposure on glutamate transporters expression.MethodsThe mRNA of VGluTs and EAATs in the human CB were detected by RT-PCR. The protein expression of VGluTs and EAATs in the human and rat CB were detected by Western blot. The distribution of VGluT3, EAAT2 and EAAT3 were observed by immunohistochemistry staining and immunofluorescence staining. Male Sprague-Dawley (SD) rats were exposed to CIH (FIO2 10–21%, 3 min/3 min for 8 h per day) for 2 weeks. The unpaired Student's t-test was performed.ResultsHere, we report on the presence of mRNAs for VGluT1–3 and EAAT1–3 in human CB, which is consistent with our previous results in rat CB. The proteins of VGluT1 and 3, EAAT2 and 3, but not VGluT2 and EAAT1, were detected with diverse levels in human and rat CB. Immunostaining showed that VGluT3, the major type of VGluTs in CB, was co-localized with tyrosine hydroxylase (TH) in type I cells. EAAT2 and EAAT3 were distributed not only in type I cells, but also in glial fibrillary acidic protein (GFAP) positive type II cells. Moreover, we found that exposure of SD rats to CIH enhanced the protein level of EAAT3 as well as TH, but attenuated the levels of VGluT3 and EAAT2 in CB.ConclusionsOur study suggests that glutamate transporters are expressed in the CB, and that glutamate transporters may contribute to glutamatergic signaling-dependent carotid chemoreflex to CIH.

The interaction between the carotid chemoreflex and the muscle mechanoreflex: differential cardiovascular consequences in men and women

BackgroundAlthough the carotid chemoreflex (CR) and the muscle mechanoreflex (MR) are recognized as sympatho‐excitatory feedback mechanisms, the consequence of their interaction for circulatory control remains unclear in humans.PurposeTo evaluate the cardiovascular consequence of the interaction between the CR and the MR in healthy men and women.MethodWe administered a hypoxic inspirate and passive leg movement (PLM) to activate the CR and the MR, respectively. Thirteen volunteers (7 males, 6 females) completed 2 experimental sessions. In one session, subjects were seated resting in a normoxic control condition (NormRest; SpO2 ~97%, PETO2 ~84 mmHg, PETCO2 ~33 mmHg) and in isocapnic hypoxia (HypoRest; SpO2 ~85%, PETO2 ~48 mmHg, PETCO2 ~33 mmHg). In the other session, PLM was performed for 1 min in the conditions of normoxia (NormPLM) and isocapnic hypoxia (HypoPLM). Thus, while NormRest was considered a condition with no reflexes activated, HypoRest, NormPLM, and HypoPLM activated the CR alone, the MR alone, and both of the CR and the MR, respectively. All sessions and conditions were conducted in randomized order. Mean arterial blood pressure (MAP), cardiac output (CO), and femoral blood flow (QL) were quantified using finger photoplethysmography and Doppler ultrasound.ResultsThe CR activation alone (i.e., ΔHypoRest‐NormRest) significantly increased CO in both men and women (~0.6 L min−1). The MR activation alone by PLM (i.e., ΔNormPLM‐NormRest) decreased MAP (~−7 mmHg; p &lt; 0.05), but significantly increased CO (~1.1 L min−1), QL (~1.0 L min−1), and leg vascular conductance (LVC, ~11 ml min−1 mmHg−1) in both groups. During co‐activation of the CR and the MR (i.e., ΔHypoPLM‐NormRest) in men, the observed QL and LVC responses were significantly lower when compared with the summated responses evoked by each reflex alone (QL: 1.1 ± 0.1 vs. 1.4 ± 0.2 L min−1; LVC: 11 ± 1 vs. 14 ± 2 ml min−1 mmHg−1); there were no differences between the observed and summated responses of MAP and CO. In contrast, CO, QL, and LVC observed during the reflex co‐activation in women were significantly greater than the summated responses to each reflex alone (CO: 1.5 ± 0.5 vs. 1.0 ± 0.4 L min−1; QL: 0.9 ± 0.1 vs. 0.6 ± 0.1 L min−1; LVC: 9 ± 2 vs. 7 ± 2 ml min−1 mmHg−1), whereas the observed MAP did not differ from the summated response.ConclusionIn men, the interaction of the CR and the MR is additive in terms of MAP and CO, but hypo‐additive in terms of QL and LVC (i.e., impedes peripheral hemodynamics). In women, the interaction of the CR and the MR is additive in terms of MAP, but hyper‐additive in terms of CO, QL, and LVC (i.e., facilitates central and peripheral hemodynamics). The sex‐related discrepancy in the peripheral hemodynamic responses to the reflex interaction is perhaps due to greater β‐adrenoreceptor‐mediated vasodilation in women compared with men. Taken together, despite different cardiovascular consequences, our findings suggest that the interaction between the CR and the MR further augments sympatho‐excitation in healthy men and women.Support or Funding InformationNational Heart, Lung, and Blood Institute grants (HL‐116579)

INFLUENCE OF BODY POSITION ON PULMONARY VENTILATION DURING ARREST AND RELEASE OF LOWER LIMB CIRCULATION AFTER EXERCISE IN HEALTHY HUMANS

Pulmonary ventilation (V̇E) response to exercise is greater when exercise is performed seated versus supine. Maybe this phenomenon is partially attributed to interaction between neural mechanisms involved on the regulation of V̇E. For instance, venous return is reduced during seated exercise, which activates the cardiopulmonary baroreflex. Therefore we sought to investigate if the cardiopulmonary baroreflex interacts with the muscle metaboreflex and the carotid chemoreflex to the regulation of V̇E in healthy humans. Thirteen young healthy subjects participated in the study (7 men). Subjects performed isometric plantar flexion with the dominant limb for 1.5 min in the semi recumbent position. Close to the end of exercise, the circulation of the dominant leg was arrested at the thigh level via cuff inflation. Then, subjects stopped exercising and their position was shifted from semi‐recumbent to either: 1) seated with the non‐dominant limb lowered to inhibit the cardiopulmonary baroreflex; or 2) supine with the non‐dominant limb raised to activate the cardiopulmonary baroreflex. Circulatory arrest took 2 min and aimed to trap metabolites into the exercised muscles to sustain muscle metaboreflex activation. Next, the tight cuff was deflated, releasing metabolites from skeletal muscles to the systemic circulation. Another experiment, under free flow recovery, served as unconcluded control. A subset of the subjects (n = 3) have undergone hyperoxia breathing (100% O2) only during release of circulatory arrest at the seated position to inhibit the carotid chemoreflex. A normoxic experiment served as control. A rebreathing system was used in an attempt to maintain the end‐tidal partial pressure of CO2 constant throughout all experiments. We found that circulatory arrest did not change V̇E versus unconcluded control in the seated (P &gt; 0.05) and supine (P &gt; 0.05) positions throughout the 120‐s period of cuff inflation. V̇E increased above unconcluded control in the seated position after 60 s of circulatory release (mean ± SD: Δ = 20 ± 29%, P = 0.05) and it remained elevated until the end of data recording at 120 s (Δ = 21 ± 31%, P = 0.04). However, release of circulatory arrest did not change V̇E versus unconcluded control in the supine position (P &gt; 0.05). Furthermore, hyperoxia did not change the V̇E increase after circulatory release at the seated position. Collectively, these results suggest that the cardiopulmonary baroreflex did not interact with either the muscle metaboreflex or the carotid chemoreflex. The underling mechanisms of the V̇E increase during release of circulatory arrest at the seated position deserve further investigation.Support or Funding InformationCAPES and CNPq

Carotid chemoreflex and muscle metaboreflex interact to the regulation of ventilation in patients with heart failure with reduced ejection fraction.

Synergism among reflexes probably contributes to exercise hyperventilation in patients with heart failure with reduced ejection fraction (HFrEF). Thus, we investigated whether the carotid chemoreflex and the muscle metaboreflex interact to the regulation of ventilation (V˙E) in HFrEF. Ten patients accomplished 4‐min cycling at 60% peak workload and then recovered for 2 min under either: (a) 21% O2 inhalation (tonic carotid chemoreflex activity) with legs’ circulation free (inactive muscle metaboreflex); (b) 100% O2 inhalation (suppressed carotid chemoreflex activity) with legs’ circulation occluded (muscle metaboreflex activation); (c) 21% O2 inhalation (tonic carotid chemoreflex activity) with legs’ circulation occluded (muscle metaboreflex activation); or (d) 100% O2 inhalation (suppressed carotid chemoreflex activity) with legs’ circulation free (inactive muscle metaboreflex) as control. V˙E, tidal volume (VT) and respiratory frequency (f R) were similar between each separated reflex (protocols a and b) and control (protocol d). Calculated sum of separated reflexes effects was similar to control. Oppositely, V˙E (mean ± SEM: Δ vs. control = 2.46 ± 1.07 L/min, p = .05) and f R (Δ = 2.47 ± 0.77 cycles/min, p = .02) increased versus control when both reflexes were simultaneously active (protocol c). Therefore, the carotid chemoreflex and the muscle metaboreflex interacted to V˙E regulation in a f R‐dependent manner in patients with HFrEF. If this interaction operates during exercise, it can have some contribution to the HFrEF exercise hyperventilation.

Carotid Chemoreflex Regulation of Post‐Exercise Cardiac Autonomic Control in Healthy Humans: Influence of Exercise Intensity

The recovery of cardiac autonomic control from exercise has two phases. The fast phase lasts about 60 s after exercise, and is largely dependent on the cardiac vagal reactivation. The slow phase is 300 s in duration, and is dependent on both the cardiac vagal reactivation and sympathetic withdrawal. Indexes of post‐exercise cardiac autonomic control are widely used in the clinical practice to predict morbidity and mortality. However, the mechanisms that regulate the post‐exercise cardiac autonomic control remain unclear. The carotid chemoreflex regulates cardiac autonomic control at rest, and carotid chemoreceptors are stimulated by humoral and neural signals related to exercise. As the extent of the humoral and neural responses are dependent upon exercise intensity, we hypothesized the carotid chemoreflex could regulate post‐exercise cardiac autonomic control in an exercise‐intensity‐dependent manner.Thirteen healthy humans performed ramp incremental exercise up to either moderate‐intensity (MI; i.e., ventilatory threshold) or high‐intensity exercise (HI; i.e., 90% peak workload). Both MI and HI exercise were followed by 5 min of active recovery (i.e., unloaded cycling) while breathing, in random order, normoxia (21% of O2, control), hyperoxia (100% of O2, carotid chemoreflex inhibition) or hypoxia (12% of O2, carotid chemoreflex stimulation). Gas administration started 10 s before the onset of recovery from exercise. The fast phase of cardiac autonomic control recovery from exercise was assessed by the heart rate (HR) reduction at 60 s post‐exercise (HRR60s). The slow phase was assessed by the time constant of exponential HR decay (HRRτ) over 300 s and the HR reduction at 300 s post‐exercise (HRR300s). Fast and slow phases were assessed by the square root of mean squared differences of successive R‐R intervals of 30‐s data segments (RMSSD30s).As compared to normoxia, hyperoxia did not change HRR60s at either exercise intensities. Hypoxia did not change HRR60s at MI, but decreased HRR60s at HI (normoxia: 29 ± 8 vs hypoxia: 17 ± 9 bpm; P &lt; 0.01). Hyperoxia and hypoxia did not change HRRτ. Hyperoxia did not affect HRR300s, but hypoxia decreased HRR300s similarly at both exercise intensities (MI: normoxia = 32 ± 14 vs hypoxia = 26 ± 17 bpm; HI: normoxia = 55 ± 7 vs hypoxia = 48 ± 11 bpm; post hoc for gas main effect: P = 0.03). Finally, regardless of the time, hyperoxia increased RMSSD30s (MI: normoxia = 15 ± 7 vs hyperoxia = 19 ± 10 ms; HI: normoxia = 5 ± 3 vs hyperoxia = 6 ± 3 ms, post hoc for gas main effect: P &lt; 0.01), whereas hypoxia decreased RMSSD30s (MI: hypoxia = 13 ± 9 ms vs normoxia; HI: hypoxia = 4 ± 2 vs normoxia, post hoc for gas main effect: P &lt; 0.01) at both exercise intensities.In conclusion, the carotid chemoreflex regulates both the fast and slow phases of post‐exercise cardiac autonomic control in healthy humans; however, only the fast phase regulation is dependent on the exercise intensity. These results suggest that sensitization of the carotid chemoreflex with exercise plays an important role for the post‐exercise cardiac vagal reactivation.Support or Funding InformationFAPESP 2015/22198‐2; 2018/03501‐4This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Carotid chemoreflex activity restrains post-exercise cardiac autonomic control in healthy humans and in patients with pulmonary arterial hypertension.

Dysfunction of post-exercise cardiac autonomic control is associated with increased mortality risk in healthy adults and in patients with cardiorespiratory diseases. The afferent mechanisms that regulate the post-exercise cardiac autonomic control remain unclear. We found that afferent signals from carotid chemoreceptors restrain the post-exercise cardiac autonomic control in healthy adults and patients with pulmonary arterial hypertension (PAH). Patients with PAH had higher carotid chemoreflex sensitivity, and the magnitude of carotid chemoreceptor restraint of autonomic control was greater in patients with PAH as compared to healthy adults. The results demonstrate that the carotid chemoreceptors contribute to the regulation of post-exercise cardiac autonomic control, and suggest that the carotid chemoreceptors may be a potential target to treat post-exercise cardiac autonomic dysfunction in patients with PAH. Dysfunction of post-exercise cardiac autonomic control predicts mortality, but its underlying mechanisms remain unclear. We tested whether carotid chemoreflex activity restrains post-exercise cardiac autonomic control in healthy adults (HA), and whether such restraint is greater in patients with pulmonary arterial hypertension (PAH) who may have both altered carotid chemoreflex and altered post-exercise cardiac autonomic control. Twenty non-hypoxaemic patients with PAH and 13 age- and sex-matched HA pedalled until 90% of peak work rate observed in a symptom-limited ramp-incremental exercise test. Recovery consisted of unloaded pedalling for 5min followed by seated rest for 6min. During recovery, subjects randomly inhaled either 100% O2 (hyperoxia) to inhibit the carotid chemoreceptor activity, or 21% O2 (normoxia) as control. Post-exercise cardiac autonomic control was examined via heart rate (HR) recovery (HRR; HR change after 30, 60, 120 and 300s of recovery, using linear and non-linear regressions of HR decay) and HR variability (HRV; time and spectral domain analyses). As expected, the PAH group had higher carotid chemosensitivity and worse post-exercise HRR and HRV than HA. Hyperoxia increased HRR at 30, 60 and 120s and absolute spectral power HRV in both groups. Additionally, hyperoxia resulted in an accelerated linear HR decay and increased time domain HRV during active recovery only in the PAH group. In conclusion, the carotid chemoreceptors restrained recovery of cardiac autonomic control from exercise in HA and in patients with PAH, with the restraint greater for some autonomic indexes in patients with PAH.

Interaction between baroreflex and chemoreflex in the cardiorespiratory responses to stimulation of the carotid sinus/nerve in conscious rats.

Electrical stimulation of the carotid baroreflex has been thoroughly investigated for treating drug-resistant hypertension in humans. However, a previous study from our laboratory, performed in conscious rats, has demonstrated that electrical stimulation of the carotid sinus/nerve (CS) activated both the carotid baroreflex as well as the carotid chemoreflex, resulting in hypotension. Additionally, we also demonstrated that the carotid chemoreceptor deactivation potentiated this hypotensive response. Therefore, to further investigate this carotid baroreflex/chemoreflex interaction, besides the hemodynamic responses, we evaluated the respiratory responses to the electrical stimulation of the CS in both intact (CONT) and carotid chemoreceptors deactivated (CHEMO-X) conscious rats. CONT rats showed increased ventilation in response to electrical stimulation of the CS as measured by the respiratory frequency (fR), tidal volume (VT) and minute ventilation (VE), suggesting a carotid chemoreflex activation. The carotid chemoreceptor deactivation abolished all respiratory responses to the electrical stimulation of the CS. Regarding the hemodynamic responses, the electrical stimulation of the CS caused hypotensive responses in CONT rats, which were potentiated by the carotid chemoreceptors deactivation. Heart rate (HR) responses did not differ between groups. In conclusion, the present study showed that the electrical stimulation of the CS, in conscious rats, activates both the carotid baroreflex and the carotid chemoreflex driving an increase in ventilation and a decrease in AP. These findings further contribute to our understanding of the electrical stimulation of CS.

Turnaround in the history of carotid chemoreflex contribution to cardiorespiratory control in COPD: what are the upcoming chapters?

Turnaround in the history of carotid chemoreflex contribution to cardiorespiratory control in COPD: what are the upcoming chapters?