Carotid Body Chemosensory Responses Research Articles

A comparative study of carotid body chemosensitivity in lean and obese mice: Implication for obesity induced hypertension

Obesity is associated with increased circulating level of the hormone leptin. Previously we have shown in mice that, leptin induces hypertension acting on transient receptor potential melastatin 7 (Trpm7) channels in the carotid body (CB), a multi-modal peripheral chemoreceptor organ that controls breathing and drives sympathetic tone. Further, in mice leptin augments hypoxic ventilatory response which is abolished by CB denervation. This suggests a potential involvement of CB chemosensitivity. However, no comparative study of CB chemosensory responses has been conducted in lean and obese mice to support this notion. Based on these premises, we hypothesize that obese mice should have higher leptin induced CB chemosensory responses during normoxia and hypoxia compared to lean mice, and Trpm7 receptor antagonist should abolish/reduce these responses. To prove this, we measured afferent carotid sinus nerve (CSN) activity, using a novel ex vivo perfused mouse (male) CB preparation from lean (b.w.; 35 ± 0.5 gm) and diet induced obese (DIO) mice (b.w.; 53 ± 1.5gm) with C57BL/6J background. We found that compared to lean mice; (a) CSN response to hypoxia (PO2 =60 Torr) was 50% more in obese mice, (b) in presence of leptin (50nM), CSN responses to normoxia (PO2=100Torr) and hypoxia were augmented 50% and 150% more respectively in obese mice. Finally, leptin induced increase in CSN responses were partially reduced by Trpm7 antagonist FTY720 (3μM) in obese compared to lean mice. In conclusion, CB chemosensitivity is enhanced in obese compared to lean mice and CB Trpm7 channel could be a therapeutic target for obesity related hypertension. Grant support: NIH R01 HL1331000. 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 contribution to the physio-pathological consequences of intermittent hypoxia: role of nitro-oxidative stress and inflammation.

Obstructive sleep apnoea (OSA), characterized by chronic intermittent hypoxia (CIH), is considered to be an independent risk for hypertension. The pathological cardiorespiratory consequences of OSA have been attributed to systemic oxidative stress, inflammation and sympathetic overflow induced by CIH, but an emerging body of evidence indicates that a nitro-oxidative and pro-inflammatory milieu within the carotid body (CB) is involved in the potentiation of CB chemosensory responses to hypoxia, which contribute to enhance the sympathetic activity. Accordingly, autonomic and cardiovascular alterations induced by CIH are critically dependent on an abnormally heightened CB chemosensory input to the nucleus of tractus solitarius (NTS), where second-order neurons project onto the rostral ventrolateral medulla (RVLM), activating pre-sympathetic neurons that control pre-ganglionic sympathetic neurons. CIH produces oxidative stress and neuroinflammation in the NTS and RVLM, which may contribute to the long-term irreversibility of the CIH-induced alterations. This brief review is mainly focused on the contribution of nitro-oxidative stress and pro-inflammatory molecules on the hyperactivation of the hypoxic chemoreflex pathway including the CB and the brainstem centres, and whether the persistence of autonomic and cardiorespiratory alterations may depend on the glial-related neuroinflammation induced by the enhanced CB chemosensory afferent input.

Carbamylated form of human erythropoietin normalizes cardiorespiratory disorders triggered by intermittent hypoxia mimicking sleep apnea syndrome.

Chronic intermittent hypoxia (CIH), one of the main features of obstructive sleep apnea (OSA), enhances carotid body-mediated chemoreflex and induces hypertension and breathing disorders. The carbamylated form of erythropoietin (cEpo) may have beneficial effects as it retains its antioxidant/anti-inflammatory and neuroprotective profile without increasing red blood cells number. However, no studies have evaluated the potential therapeutic effect of cEpo on CIH-related cardiorespiratory disorders. We aimed to determine whether cEpo normalized the CIH-enhanced carotid body ventilatory chemoreflex, the hypertension and ventilatory disorders in rats. Male Sprague-Dawley rats (250 g) were exposed to CIH (5% O2, 12/h, 8 h/day) for 28 days. cEPO (20 μg/kg, i.p) was administrated from day 21 every other day for one more week. Cardiovascular and respiratory function were assessed in freely moving animals. Twenty-one days of CIH increased carotid body-mediated chemoreflex responses as evidenced by a significant increase in the hypoxic ventilatory response (FiO2 10%) and triggered irregular eupneic breathing, active expiration, and produced hypertension. cEpo treatment significantly reduced the carotid body--chemoreflex responses, normalizes breathing patterns and the hypertension in CIH. In addition, cEpo treatment effectively normalized carotid body chemosensory responses evoked by acute hypoxic stimulation in CIH rats. Present results strongly support beneficial cardiorespiratory therapeutic effects of cEpo during CIH exposure.

Global and Transient Effects of Intermittent Hypoxia on Heart Rate Variability Markers: Evaluation Using an Obstructive Sleep Apnea Model

Intermittent hypoxia (IH) produces autonomic dysfunction that promotes the development of arrhythmia and hypertension in patients with obstructive sleep apnea (OSA). This paper investigated different heart rate variability (HRV) indices in the context of IH using a rat model for OSA. Linear and non-linear HRV parameters were assessed from ultra-short (15-s segments) and short-term (5 min) analyses of heartbeat time-series. Transient changes observed from pre-apnea segments to hypoxia episodes were evaluated, besides the relative and global impact of IH, as a function of its severity. Results showed an overall increase in ultra-short HRV markers as immediate response to hypoxia: standard deviation of normal RR intervals, SDNN = 1.2 ms (IQR: 1.1-2.1) vs 1.4 ms (IQR: 1.2-2.2), p = 0.015; root mean square of the successive differences, RMSSD = 1.7 ms (IQR: 1.5-2.2) vs 1.9 ms (IQR: 1.6-2.4), p = 0.031. The power in the very low frequency (VLF) band also showed a significant increase: 0.09 ms <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (IQR: 0.05-0.20) vs 0.16 ms <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (IQR: 0.12-0.23), p = 0.016, probably associated with the potentiation of the carotid body chemo-sensory response to hypoxia. Moreover, a clear link between severity of IH and short-term HRV measures was found in VLF and LF power, besides their progressive increase seen throughout the experiment after each apnea sequence. However, only those markers quantifying fragmentation levels in RR series were significantly affected when the experiment ended, as compared to baseline measures: percentage of inflection points, PIP = 49% (IQR: 45-51) vs 53% (IQR: 47-53), p = 0.031; percentage of short (geq 3 RR intervals) accelerated/decelerated segments, PSS = 75% (IQR: 51-81) vs 87% (IQR: 51-90), p = 0.046. These findings suggest a significant deterioration of cardiac rhythm with a more erratic behavior beyond the normal sinus arrhythmia, that may lead to a future cardiac condition.

Endothelin-1-Mediated Mechanisms in the Carotid Body Modulates Cardiovascular Responses in Rats Exposed to Chronic Intermittent Hypoxia

Chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnea (OSA), is associated with hypertension. The increased of carotid body (CB) sensitivity due to enhanced sympathetic efferent may be mainly responsible for the elevation of blood pressure. Accordingly, we studied this effect of Endothelin-1 (ET-1)-induced CB chemosensory response to CIH, as a vasoactive peptide expressed in CB. The purpose of this study was to investigate the mean arterial blood pressure (MAP) and renal sympathetic nerve activity (RSNA) responses in CIH group by injecting ET-1 to directly stimulate CB chemoreceptor. Furthermore, whether ET receptor-mediated PKC and p38MAPK signaling pathway was involved in CIH-induced CB activation was also studied. Male Sprague–Dawley rats were exposed to CIH (8 h/day for 3 weeks) and the MAP and RSNA were recorded in CIH rats and Sham rats. Our results demonstrated that ET-1-induced MAP and RSNA increase were mainly mediated by ETA receptor activation in CB chemosensory after CIH exposure. Moreover, P38MAPK and PKC signaling pathway might be involved in ET-1-induced increase of MAP and RSNA in CIH group, which provided a potential therapeutic target of OSA.

Ventilatory and Carotid Body Chemosensory Responses In Rats Exposed To Short‐Term Sustained Hypoxia

Previous studies performed in in situ rat preparations reported that the exposure to short‐term (24 h) sustained hypoxia (SH) generates active expiration at rest, increases baseline sympathetic activity during expiration and enhances sympathetic responses to peripheral chemoreceptor stimulation. In the present study, we evaluated basal and hypoxic‐induced changes in ventilatory parameters of unanesthetized juvenile male Holtzman rats (80–90 g) previously exposed to SH (10% O2, 24 hs). In addition, we examined the chemosensory response of carotid body chemoreceptors of decerebrated, artificially‐perfused in situ preparations of juvenile rats (60–80 g) submitted to SH. At resting conditions, SH rats (n=12) exhibited augmented (P&lt;0.05) respiratory frequency (109±2 vs 97±1 cpm), tidal volume (14±1 vs 10±1 ml/kg) and minute ventilation (1550±137 vs 979±114 ml/kg.min) compared to control rats (n=12). SH rats also showed greater tachypneic and hyperventilatory responses (P&lt;0.05) to a new hypoxia episode (7% O2, 20 min) in comparison to control rats. The magnitude of increase in tidal volume was similar between groups. The in situ preparations of SH rats (n=8), but not controls (n=8), exhibited additional bursts in baseline abdominal motor activity, indicating the presence of active expiration at rest. Interestingly, basal afferent activity of carotid sinus nerve (CSN) was similar (0.67±0.18 vs 0.65±0.18 μV) between SH and control rats. Moreover, the CSN excitatory response to stimulation of peripheral chemoreceptors with potassium cyanide was not different between groups (ΔCSN: 1235±372 vs 922±278 %), although the in situ preparations of SH rats showed greater sympathoexcitatory reflex responses (P&lt;0.05). Our findings indicate that augmented basal ventilation and enhanced ventilatory and sympathetic responses to hypoxia in rats exposed to SH are not dependent on increased basal and sensorial activity of carotid body chemoreceptors, but may rely on central mechanisms.Support or Funding InformationFAPESP (2013/17.251‐6) and CNPq

Chronic phenytoin treatment reduces rat carotid body chemosensory responses to acute hypoxia

Ventilation is peripherally controlled by afferent activity arising from the peripheral chemoreceptors. In the rat, chemosensory activity is conveyed to the central nervous system through axons of neurons located in the nodose-petrosal-jugular-complex. These neurons have distinct electrophysiological properties, including a persistent Na+ current. Acute blockade of this current with phenytoin and other anti-epileptic drugs reduces normoxic chemosensory activity and responses to acute hypoxia. However, because anti-epileptic therapy is prolonged and there is no information on the effects of chronic phenytoin treatment on peripheral chemosensory activity, we studied the effects of long-lasting phenytoin treatment (~25 days) on afferent chemosensory activity, on a wide range of oxygen inspiratory fractions. Osmotic pumps containing dissolved phenytoin (166mg/mL) or vehicle (daily flow: 60µL) were implanted subcutaneously in male adult Sprague Dawley rats. At the end of the treatment, the animals were anesthetized and carotid sinus nerve activity was recorded in vivo. Afferent chemosensory activity in normoxia was not significantly different between control (71.2±2.2Hz) and phenytoin treated (95.4±2.1Hz) rats. In contrast, carotid body chemosensory responses to acute hypoxic challenges were markedly reduced in phenytoin treated rats, specifically in the lowest part of the hypoxic range (control 133.5±18.0Hz vs phenytoin treated 50.2±29.4, at 5% FIO2). Chronic phenytoin treatment severely impaired the chemosensory responses to acute hypoxia, suggesting that long-term phenytoin treatment in patients may result in a reduced peripheral respiratory drive together with a reduction in the respiratory responses to hypoxic challenges.

Intermittent Hypoxia-Induced Carotid Body Chemosensory Potentiation and Hypertension Are Critically Dependent on Peroxynitrite Formation.

Oxidative stress is involved in the development of carotid body (CB) chemosensory potentiation and systemic hypertension induced by chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnea. We tested whether peroxynitrite (ONOO−), a highly reactive nitrogen species, is involved in the enhanced CB oxygen chemosensitivity and the hypertension during CIH. Accordingly, we studied effects of Ebselen, an ONOO− scavenger, on 3-nitrotyrosine immunoreactivity (3-NT-ir) in the CB, the CB chemosensory discharge, and arterial blood pressure (BP) in rats exposed to CIH. Male Sprague-Dawley rats were exposed to CIH (5% O2, 12 times/h, 8 h/day) for 7 days. Ebselen (10 mg/kg/day) was administrated using osmotic minipumps and BP measured with radiotelemetry. Compared to the sham animals, CIH-treated rats showed increased 3-NT-ir within the CB, enhanced CB chemosensory responses to hypoxia, increased BP response to acute hypoxia, and hypertension. Rats treated with Ebselen and exposed to CIH displayed a significant reduction in 3-NT-ir levels (60.8 ± 14.9 versus 22.9 ± 4.2 a.u.), reduced CB chemosensory response to 5% O2 (266.5 ± 13.4 versus 168.6 ± 16.8 Hz), and decreased mean BP (116.9 ± 13.2 versus 82.1 ± 5.1 mmHg). Our results suggest that CIH-induced CB chemosensory potentiation and hypertension are critically dependent on ONOO− formation.

Inflammation and oxidative stress during intermittent hypoxia: the impact on chemoreception.

What is the topic of this review? This article describes the contribution of oxidative stress and pro-inflammatory cytokines to the enhanced carotid body chemosensory responsiveness to the hypoxia and systemic hypertension induced by chronic intermittent hypoxia. What advances does it highlight? Chronic intermittent hypoxia enhances the carotid body chemosensory discharge during normoxia and hypoxia, leading to sympathetic overactivity and hypertension. New evidence suggests that chronic intermittent hypoxia increases pro-inflammatory cytokines. Here, we discuss the role of inflammation in the alterations of the carotid chemoreceptor function as well as the cardiorespiratory alterations following chronic intermittent hypoxia. Chronic intermittent hypoxia (CIH), the main characteristic of obstructive sleep apnoea, enhances carotid body (CB) chemosensory discharges during normoxia and hypoxia and elicits hypertension. These alterations are attributed to oxidative stress, because antioxidants prevent the enhanced CB chemosensory discharges and the hypertension. In this report, we discuss new evidence supporting the suggestion that oxidative stress-induced upregulation of pro-inflammatory cytokines (i.e. tumour necrosis factor-α and interleukin-1β) in the CB is involved in the chemosensory potentiation and the hypertension following CIH. Anti-inflammatory treatment with ibuprofen prevents the increased tumour necrosis factor-α and interleukin-1β levels in the CB and the hypertension, but does not reduce the enhanced chemosensory hypoxic response and the local oxidative stress in the CB. In contrast, antioxidant treatment with ascorbic acid prevents the increase in cytokine concentrations and CB oxidative stress, the chemosensory potentiation and the hypertension. Thus, the enhanced CB chemosensory responses to hypoxia depend critically on the oxidative stress, but not on the increased tumour necrosis factor-α and interleukin-1β in the CB. We discuss a possible role for pro-inflammatory cytokines in development of the hypertension produced by CIH, acting on cardiorespiratory centres located in the CNS.

Intermittent hypoxia: endothelin‐1 and hypoxic carotid body chemosensory potentiation

Intermittent hypoxia: endothelin‐1 and hypoxic carotid body chemosensory potentiation

Inhibition of hydrogen sulfide restores normal breathing stability and improves autonomic control during experimental heart failure

Cardiovascular autonomic imbalance and breathing instability are major contributors to the progression of heart failure (CHF). Potentiation of the carotid body (CB) chemoreflex has been shown to contribute to these effects. Hydrogen sulfide (H2S) recently has been proposed to mediate CB hypoxic chemoreception. We hypothesized that H2S synthesis inhibition should decrease CB chemoreflex activation and improve breathing stability and autonomic function in CHF rats. Using the irreversible inhibitor of cystathione γ-lyase dl-propargylglycine (PAG), we tested the effects of H2S inhibition on resting breathing patterns, the hypoxic and hypercapnic ventilatory responses, and the hypoxic sensitivity of CB chemoreceptor afferents in rats with CHF. In addition, heart rate variability (HRV) and systolic blood pressure variability (SBPV) were calculated as an index of autonomic function. CHF rats, compared with sham rats, exhibited increased breath interval variability and number of apneas, enhanced CB afferent discharge and ventilatory responses to hypoxia, decreased HRV, and increased low-frequency SBPV. Remarkably, PAG treatment reduced the apnea index by 90%, reduced breath interval variability by 40-60%, and reversed the enhanced hypoxic CB afferent and chemoreflex responses observed in CHF rats. Furthermore, PAG treatment partially reversed the alterations in HRV and SBPV in CHF rats. Our results show that PAG treatment restores breathing stability and cardiac autonomic function and reduces the enhanced ventilatory and CB chemosensory responses to hypoxia in CHF rats. These results support the idea that PAG treatment could potentially represent a novel pathway to control sympathetic outflow and breathing instability in CHF.

Selective contribution of inflammation and oxidative stress to the cardiorespiratory and carotid body alterations following intermittent hypoxia

Chronic intermittent hypoxia (CIH), a main feature of obstructive sleep apnea, potentiates carotid body (CB) chemosensory and ventilatory hypoxic responses, and induces hypertension. These alterations are attributed to oxidative stress, since antioxidants prevent the enhanced chemosensory and ventilatory hypoxic responses and the hypertension. We tested if oxidative stressinduced upregulation of pro‐inflammatory cytokines TNF‐α and IL‐1β are involved in the CB and cardiorespiratory alterations during CIH. We studied the effects of ibuprofen (40 mg/Kg day) on TNF‐α, IL‐1β and 3‐nitrotyrosine (3‐NT) CB levels, chemosensory and ventilatory hypoxic responses, and blood pressure in male Sprague‐Dawley rats exposed to CIH (5%O2, 12 times/h for 8 h/day) for 21 days. CIH increased CB TNF‐α, IL‐1β and 3‐NT, enhanced chemosensory and ventilatory hypoxic responses, and led to hypertension. Ibuprofen prevented the increased TNF‐α and IL‐1β CB levels and cardioventilatory alterations, but not the enhanced chemosensory hypoxic response and 3‐NT accumulation. Contrarily, ascorbic acid prevented the increased CB cytokines and 3‐NT expression, the chemosensory and ventilatory potentiation, and the hypertension. Thus, present results show that CIH‐induced potentiation of CB chemosensory responses to hypoxia depends critically on the oxidative stress, but not on the increased TNF‐α and IL‐1β in the CB. Thus, we provide new evidence suggesting a plausible role for pro‐inflammatory cytokines in the control of breathing and arterial pressure during CIH acting through a CB independent pathway.Supported by FONDECYT 1100405

VEGF up‐regulation and vascular area enlargement induced by intermittent hypoxia in the rat carotid body is not secondary to oxidative stress

Chronic intermittent hypoxia (CIH), a main feature of obstructive sleep apnea, potentiated the carotid body (CB) chemosensory responses to acute hypoxia, enlarges the vascular area and increased the VEGF expression in the CB. Since oxidative stress plays an essential role in the CIH‐induced CB chemosensory potentiation and by the fact that VEGF may be directly up‐regulated by oxidative stress, we hypothesized if the treatment with an antioxidant may impede the increased expression of VEGF and the enlarged vascular area in the CB of rats exposed to CIH. Thus, we tested the effects of ascorbic acid on the vascular area enlargement and the VEGF immunoreactivity (VEGF‐ir) in the CB of rats exposed to 5%O2, 12 times/hr for 8 hrs or sham condition for 21 days. We found that CIH increased VEGF‐ir and enlarged the vascular area in the CB by increasing the size of the blood vessels, although the number of blood vessels was unchanged. Ascorbic acid, which prevented the CB chemosensory potentiation to hypoxia failed to impede the vascular enlargement and the increased VEGF‐ir in the CB. Thus, our results suggest that CB vascular changes, including VEGF up‐regulation may result of the direct hypoxic stimulation during CIH exposure, and not secondary to the increased oxidative stress, which is involved in the potentiation of the CB chemosensory responses to hypoxia.Supported by FONDECYT 1100405

Differential expression of pro-inflammatory cytokines, endothelin-1 and nitric oxide synthases in the rat carotid body exposed to intermittent hypoxia

The enhanced carotid body (CB) chemosensory response to hypoxia induced by chronic intermittent hypoxia (CIH) has been attributed to oxidative stress, which is expected to increase the expression of chemosensory modulators including chemoexcitatory pro-inflammatory cytokines in the CB. Accordingly, we studied the time-course of the changes in the immunohistological expression of TNF-α, IL-1β, IL-6, ET-1, iNOS, eNOS and 3-nitrotyrosine in the CB, along with the progression of enhanced CB chemosensory responses to acute hypoxia in male Sprague-Dawley rats exposed to CIH (5%O 2, 12 times/h per 8 h) for 7, 14 and 21 days. Exposure to CIH for 7 days resulted in a sustained potentiation of CB chemosensory responses to acute hypoxia, which persisted until 21 days of CIH. The chemosensory potentiation was paralleled by an increased 3-nitrotyrosine expression in the CB. On the contrary, CIH produced a transient 2-fold increase of ET-1 immunoreactivity at 7 days, a decrease in eNOS immunoreactivity, and a delayed but progressive increase of TNF-α, IL-1β and iNOS immunoreactivity, which was not associated with changes in systemic plasma levels or immune cell invasion within the CB. Thus, present results suggest that the local expression of chemosensory modulators and pro-inflammatory cytokines in the CB may have different temporal contribution to the CB chemosensory potentiation induced by CIH.

Carotid body chemosensory responses in mice deficient of TASK channels

Background K+ channels of the TASK family are believed to participate in sensory transduction by chemoreceptor (glomus) cells of the carotid body (CB). However, studies on the systemic CB-mediated ventilatory response to hypoxia and hypercapnia in TASK1- and/or TASK3-deficient mice have yielded conflicting results. We have characterized the glomus cell phenotype of TASK-null mice and studied the responses of individual cells to hypoxia and other chemical stimuli. CB morphology and glomus cell size were normal in wild-type as well as in TASK1−/− or double TASK1/3−/− mice. Patch-clamped TASK1/3-null glomus cells had significantly higher membrane resistance and less hyperpolarized resting potential than their wild-type counterpart. These electrical parameters were practically normal in TASK1−/− cells. Sensitivity of background currents to changes of extracellular pH was drastically diminished in TASK1/3-null cells. In contrast with these observations, responsiveness to hypoxia or hypercapnia of either TASK1−/− or double TASK1/3−/− cells, as estimated by the amperometric measurement of catecholamine release, was apparently normal. TASK1/3 knockout cells showed an enhanced secretory rate in basal (normoxic) conditions compatible with their increased excitability. Responsiveness to hypoxia of TASK1/3-null cells was maintained after pharmacological blockade of maxi-K+ channels. These data in the TASK-null mouse model indicate that TASK3 channels contribute to the background K+ current in glomus cells and to their sensitivity to external pH. They also suggest that, although TASK1 channels might be dispensable for O2/CO2 sensing in mouse CB cells, TASK3 channels (or TASK1/3 heteromers) could mediate hypoxic depolarization of normal glomus cells. The ability of TASK1/3−/− glomus cells to maintain a powerful response to hypoxia even after blockade of maxi-K+ channels, suggests the existence of multiple sensor and/or effector mechanisms, which could confer upon the cells a high adaptability to maintain their chemosensory function.

A Role for TASK-1 (KCNK3) Channels in the Chemosensory Control of Breathing

Acid-sensitive K+ channels of the tandem P-domain K+-channel family (TASK-1 and TASK-3) have been implicated in peripheral and central respiratory chemosensitivity; however, because of the lack of decisive pharmacological agents, the final proof of the role of the TASK channel in the chemosensory control of breathing has been missing. In the mouse, TASK-1 and TASK-3 channels are dispensable for central respiratory chemosensitivity (Mulkey et al., 2007). Here, we have used knock-out animals to determine whether TASK-1 and TASK-3 channels play a role in the carotid body function and chemosensory control of breathing exerted by the carotid body chemoreceptors. Ventilatory responses to hypoxia (10% O2 in inspired air) and moderate normoxic hypercapnia (3-6% CO2 in inspired air) were significantly reduced in TASK-1 knock-out mice. In contrast, TASK-3-deficient mice showed responses to both stimuli that were similar to those developed by their wild-type counterparts. TASK-1 channel deficiency resulted in a marked reduction of the hypoxia (by 49%)- and CO2 (by 68%)-evoked increases in the carotid sinus nerve chemoafferent discharge recorded in the in vitro superfused carotid body/carotid sinus nerve preparations. Deficiency in both TASK-1 and TASK-3 channels increased baseline chemoafferent activity but did not cause a further reduction of the carotid body chemosensory responses. These observations provide direct evidence that TASK-1 channels contribute significantly to the increases in the carotid body chemoafferent discharge in response to a decrease in arterial P(O2) or an increase in P(CO2)/[H+]. TASK-1 channels therefore play a key role in the control of ventilation by peripheral chemoreceptors.

Expression and Immunolocalization of Endothelin Peptides and Its Receptors, ETA and ETB, in the Carotid Body Exposed to Chronic Intermittent Hypoxia

Increased levels of endothelin-1 (ET-1) in the carotid body (CB) contribute to the enhancement of chemosensory responses to acute hypoxia in cats exposed to chronic intermittent hypoxia (CIH). However, it is not known if the ET receptor types A (ETA-R) and B (ETB-R) are upregulated. Thus, we studied the expression and localization of ETA-R and ETB-R using Western blot and immunohistochemistry (IHC) in CBs from cats exposed to cyclic hypoxic episodes, repeated during 8 hr for 4 days. In addition, we determined if ET-1 is expressed in the chemoreceptor cells using double immunofluorescence for ET-1 and tyrosine hydroxylase (TH). We found that ET-1 expression was ubiquitous in the blood vessels and CB parenchyma, although double ET-1 and TH-positive chemoreceptor cells were mostly found in the parenchyma. ETAR was expressed in most chemoreceptor cells and blood vessels of the CB vascular pole. ETB-R was expressed in chemoreceptor cells, parenchymal capillaries, and blood vessels of the vascular pole. CIH upregulated ETB-R expression by approximately 2.1 (Western blot) and 1.6-fold (IHC) but did not change ETA-R expression. Present results suggest that ET-1,ETA-R, and ETB-R are involved in the enhanced CB chemosensory responses to acute hypoxia induced by CIH.

Contribution of endothelin-1 to the enhanced carotid body chemosensory responses induced by chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) enhances carotid body (CB) chemosensory responses to acute hypoxia. We tested the hypothesis that endothelin-1 (ET-1), an excitatory modulator of CB chemoreception may contribute to the enhanced CB chemosensory responses in cats exposed to cyclic hypoxic episodes repeated during 8 h for 4 days. Accordingly, we measured the ET-1 immunoreactivity (ET-ir) in the CB and plasma. Using a perfused CB preparation, we studied the effects of exogenous ET-1 and bosentan, a non-selective endothelin receptor type A and B antagonist, on the frequency of chemosensory discharges ( ƒ x ) during normoxia, mild and severe hypoxia. We found that CIH increased ET-ir in the CB by ∼10-fold leaving ET-1 plasma levels unchanged. Application of ET-1 to control and CIH-treated CBs produced long-lasting dose-dependent increases in ƒ x , although the dose–response curve showed a rightward-shift in the CIH-treated CBs. CIH increased baseline ƒ x and hypoxic chemosensory responses, which were reduced by 50 μM bosentan in CBs from CIH-treated cats. Present results suggest that a local increase of ET-1 in the CB may contribute to the enhanced chemosensory responses induced by CIH predominantly through a vasomotor mechanism.

Doxapram stimulates the carotid body via a different mechanism than hypoxic chemotransduction

To determine if doxapram stimulates the carotid body through the same mechanism as hypoxia, we compared the effects of doxapram and hypoxia on isolated-perfused carotid bodies in rabbits. Doxapram stimulated the carotid body in a dose-dependent manner. In Ca(2+)-free solution, neither doxapram nor hypoxia stimulated the carotid body. Although, doxapram had an additive effect on the carotid body chemosensory response to hypercapnia, a synergistic effect was not observed. Also, we investigated the various K(+) channel activators on the response to doxapram and hypoxia: pinacidil and levcromakalim as ATP-sensitive K(+) channel activators; NS-1619 as a Ca(2+)-sensitive K(+) channel activator; and halothane as a TASK-like background K(+) channel activator. The hypoxic response was partially reduced by halothane only, while pinacidil, levcromakalim and NS-1619 had no effect. Interestingly, the effect of doxapram was partially inhibited by NS-1619. Neither pinacidil nor levcromakalim affected the stimulatory effect of doxapram. We conclude that doxapram stimulates the carotid body via a different mechanism than hypoxic chemotransduction.

Chronic intermittent hypoxia enhances cat chemosensory and ventilatory responses to hypoxia.

The carotid body (CB) chemoreceptors may play an important role in the enhanced hypoxic ventilatory response induced by chronic intermittent hypoxia (CIH). We studied the effects of cyclic hypoxic episodes of short duration on cat cardiorespiratory reflexes, heart rate variability, and CB chemosensory activity. Cats were exposed to cyclic hypoxic episodes (PO2 approximately 75 Torr) repeated during 8 h for 2-4 days. Cats were anaesthetized with sodium pentobarbitone (40 mg kg(-1) i.p., followed by 8-12 mg i.v.), and ventilatory and cardiovascular responses to NaCN (0.1-100 microg kg(-1) i.v.) and several isocapnic levels of oxygen (PO2 approximately 20-740 Torr) were studied. After studying the reflex responses, we recorded the CB chemosensory responses induced by the same stimuli. Results showed that CIH for 4 days selectively enhanced cat CB ventilatory (VT and VI) responses to hypoxia, while responses to NaCN remained largely unchanged. Similarly, basal CB discharges and responses to acute hypoxia (PO2 < 100 Torr) were larger in CIH than in control cats, without modification of the responses to NaCN. Exposure to CIH did not increase basal arterial pressure, heart rate, or their changes induced by acute hypoxia or hyperoxia. However, the spectral analysis of heart rate variability of CIH cats showed a marked increase of the low-/high-frequency ratio and an increase of the power spectral distribution of low frequencies of heart rate variability. Thus, the enhanced CB reactivity to hypoxia may contribute to the augmented ventilatory response to hypoxia, as well as to modified heart rate variability due to early changes in autonomic activity.