Mechanisms of alpha2-adrenoceptor-mediated inhibition in rabbit carotid body

Loss of ganglioglomerular nerve input to the carotid body impacts the hypoxic ventilatory response in freely-moving rats

The carotid bodies are the primary sensors of blood pH, pO2 and pCO2. The ganglioglomerular nerve (GGN) provides post-ganglionic sympathetic nerve input to the carotid bodies, however the physiological relevance of this innervation is still unclear. The main objective of this study was to determine how the absence of the GGN influences the hypoxic ventilatory response in juvenile rats. As such, we determined the ventilatory responses that occur during and following five successive episodes of hypoxic gas challenge (HXC, 10% O2, 90% N2), each separated by 15 min of room-air, in juvenile (P25) sham-operated (SHAM) male Sprague Dawley rats and in those with bilateral transection of the ganglioglomerular nerves (GGNX). The key findings were that 1) resting ventilatory parameters were similar in SHAM and GGNX rats, 2) the initial changes in frequency of breathing, tidal volume, minute ventilation, inspiratory time, peak inspiratory and expiratory flows, and inspiratory and expiratory drives were markedly different in GGNX rats, 3) the initial changes in expiratory time, relaxation time, end inspiratory or expiratory pauses, apneic pause and non-eupneic breathing index (NEBI) were similar in SHAM and GGNX rats, 4) the plateau phases obtained during each HXC were similar in SHAM and GGNX rats, and 5) the ventilatory responses that occurred upon return to room-air were similar in SHAM and GGNX rats. Overall, these changes in ventilation during and following HXC in GGNX rats raises the possibility the loss of GGN input to the carotid bodies effects how primary glomus cells respond to hypoxia and the return to room-air.

Neurochemical Anatomy of the Mammalian Carotid Body

Carotid body (CB) glomus cells in most mammals, including humans, contain a broad diversity of classical neurotransmitters, neuropeptides and gaseous signaling molecules as well as their cognate receptors. Among them, acetylcholine, adenosine triphosphate and dopamine have been proposed to be the main excitatory transmitters in the mammalian CB, although subsequently dopamine has been considered an inhibitory neuromodulator in almost all mammalian species except the rabbit. In addition, co-existence of biogenic amines and neuropeptides has been reported in the glomus cells, thus suggesting that they store and release more than one transmitter in response to natural stimuli. Furthermore, certain metabolic and transmitter-degrading enzymes are involved in the chemotransduction and chemotransmission in various mammals. However, the presence of the corresponding biosynthetic enzyme for some transmitter candidates has not been confirmed, and neuroactive substances like serotonin, gamma-aminobutyric acid and adenosine, neuropeptides including opioids, substance P and endothelin, and gaseous molecules such as nitric oxide have been shown to modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. It is likely that the fine balance between excitatory and inhibitory transmitters and their complex interactions might play a more important than suggested role in CB plasticity.

The superior cervical ganglia modulate ventilatory responses to hypoxia independently of preganglionic drive from the cervical sympathetic chain

Superior cervical ganglia (SCG) postganglionic neurons receive preganglionic drive via the cervical sympathetic chains (CSC). The SCG projects to structures like the carotid bodies (e.g., vasculature, chemosensitive glomus cells), upper airway (e.g., tongue, nasopharynx), and to the parenchyma and cerebral arteries throughout the brain. We previously reported that a hypoxic gas challenge elicited an array of ventilatory responses in sham-operated (SHAM) freely moving adult male C57BL6 mice and that responses were altered in mice with bilateral transection of the cervical sympathetic chain (CSCX). Since the CSC provides preganglionic innervation to the SCG, we presumed that mice with superior cervical ganglionectomy (SCGX) would respond similarly to hypoxic gas challenge as CSCX mice. However, while SCGX mice had altered responses during hypoxic gas challenge that occurred in CSCX mice (e.g., more rapid occurrence of changes in frequency of breathing and minute ventilation), SCGX mice displayed numerous responses to hypoxic gas challenge that CSCX mice did not, including reduced total increases in frequency of breathing, minute ventilation, inspiratory and expiratory drives, peak inspiratory and expiratory flows, and appearance of noneupneic breaths. In conclusion, hypoxic gas challenge may directly activate subpopulations of SCG cells, including subpopulations of postganglionic neurons and small intensely fluorescent (SIF) cells, independently of CSC drive, and that SCG drive to these structures dampens the initial occurrence of the hypoxic ventilatory response, while promoting the overall magnitude of the response. The multiple effects of SCGX may be due to loss of innervation to peripheral and central structures with differential roles in breathing control.NEW & NOTEWORTHY We present data showing that the ventilatory responses elicited by a hypoxic gas challenge in male C57BL6 mice with bilateral superior cervical ganglionectomy are not equivalent to those reported for mice with bilateral transection of the cervical sympathetic chain. These data suggest that hypoxic gas challenge may directly activate subpopulations of superior cervical ganglia (SCG) cells, including small intensely fluorescent (SIF) cells and/or principal SCG neurons, independently of preganglionic cervical sympathetic chain drive.

Loss of Cervical Sympathetic Chain Input to the Superior Cervical Ganglia Affects the Ventilatory Responses to Hypoxic Challenge in Freely-Moving C57BL6 Mice

The cervical sympathetic chain (CSC) innervates post-ganglionic sympathetic neurons within the ipsilateral superior cervical ganglion (SCG) of all mammalian species studied to date. The post-ganglionic neurons within the SCG project to a wide variety of structures, including the brain (parenchyma and cerebral arteries), upper airway (e.g., nasopharynx and tongue) and submandibular glands. The SCG also sends post-ganglionic fibers to the carotid body (e.g., chemosensitive glomus cells and microcirculation), however, the function of these connections are not established in the mouse. In addition, nothing is known about the functional importance of the CSC-SCG complex (including input to the carotid body) in the mouse. The objective of this study was to determine the effects of bilateral transection of the CSC on the ventilatory responses [e.g., increases in frequency of breathing (Freq), tidal volume (TV) and minute ventilation (MV)] that occur during and following exposure to a hypoxic gas challenge (10% O₂ and 90% N₂) in freely-moving sham-operated (SHAM) adult male C57BL6 mice, and in mice in which both CSC were transected (CSCX). Resting ventilatory parameters (19 directly recorded or calculated parameters) were similar in the SHAM and CSCX mice. There were numerous important differences in the responses of CSCX and SHAM mice to the hypoxic challenge. For example, the increases in Freq (and associated decreases in inspiratory and expiratory times, end expiratory pause, and relaxation time), and the increases in MV, expiratory drive, and expiratory flow at 50% exhaled TV (EF₅₀) occurred more quickly in the CSCX mice than in the SHAM mice, although the overall responses were similar in both groups. Moreover, the initial and total increases in peak inspiratory flow were higher in the CSCX mice. Additionally, the overall increases in TV during the latter half of the hypoxic challenge were greater in the CSCX mice. The ventilatory responses that occurred upon return to room-air were essentially similar in the SHAM and CSCX mice. Overall, this novel data suggest that the CSC may normally provide inhibitory input to peripheral (e.g., carotid bodies) and central (e.g., brainstem) structures that are involved in the ventilatory responses to hypoxic gas challenge in C57BL6 mice.

Model for Identifying High Carotid Body Chemosensitivity in Patients with Obstructive Sleep Apnea

OBJECTIVE

The carotid body (CB) is a major peripheral respiratory chemoreceptor. In patients with obstructive sleep apnea (OSA), high CB chemosensitivity (CBC) is associated with refractory hypertension and insulin resistance and known to further aggravate OSA. Thus, the identification of high CB (hCBC) among OSA patients is of clinical significance, but detection methods are still limited. Therefore, this study aimed to explore the association of CBC with OSA severity and to develop a simplified model that can identify patients with hCBC.

METHODS

In this cross-sectional study of subjects who underwent polysomnography (PSG), CBC was measured using the Dejours test. We defined hCBC as a decrease of >12% in respiratory rate (RR) after breathing of pure O₂. The association of CBC with OSA severity was explored by logistic regression, and a model for identifying hCBC was constructed and confirmed using receiver operating characteristic analysis.

RESULTS

Patients with OSA (n=142) and individuals without OSA (n=38) were enrolled. CBC was higher in patients with OSA than in those without OSA (% decrease in RR, 15.2%±13.3% vs 9.1%±7.5%, P<0.05). Apnea-hypopnea index (AHI), fraction of apnea-hypopnea events in rapid-eye-movement sleep (F_{events-in-REM}), and longest time of apnea (LTA) were associated with hCBC independently (odds ratio [OR]=1.048, OR=1.082, and OR=1.024 respectively; all P<0.05). The model for identifying hCBC allocated a score to each criterion according to its OR values, ie, 1 (LTA >48.4 s), 2 (AHI >15.7 events/hour), and 3 (F_{events-in-REM} >12.7%). A score of 3 or greater indicated hCBC with a sensitivity of 79.4% and specificity of 88.2%.

CONCLUSION

High CBC is associated with the severity of OSA. A simplified scoring system based on clinical variables from PSG can be used to identify hCBC.

Autonomic innervation of the carotid body as a determinant of its sensitivity: implications for cardiovascular physiology and pathology

The motivation for this review comes from the emerging complexity of the autonomic innervation of the carotid body (CB) and its putative role in regulating chemoreceptor sensitivity. With the carotid bodies as a potential therapeutic target for numerous cardiorespiratory and metabolic diseases, an understanding of the neural control of its circulation is most relevant. Since nerve fibres track blood vessels and receive autonomic innervation, we initiate our review by describing the origins of arterial feed to the CB and its unique vascular architecture and blood flow. Arterial feed(s) vary amongst species and, unequivocally, the arterial blood supply is relatively high to this organ. The vasculature appears to form separate circuits inside the CB with one having arterial venous anastomoses. Both sympathetic and parasympathetic nerves are present with postganglionic neurons located within the CB or close to it in the form of paraganglia. Their role in arterial vascular resistance control is described as is how CB blood flow relates to carotid sinus afferent activity. We discuss non-vascular targets of autonomic nerves, their possible role in controlling glomus cell activity, and how certain transmitters may relate to function. We propose that the autonomic nerves sub-serving the CB provide a rapid mechanism to tune the gain of peripheral chemoreflex sensitivity based on alterations in blood flow and oxygen delivery, and might provide future therapeutic targets. However, there remain a number of unknowns regarding these mechanisms that require further research that is discussed.

G-Protein-Coupled Receptor (GPCR) Signaling in the Carotid Body: Roles in Hypoxia and Cardiovascular and Respiratory Disease

The carotid body (CB) is an important organ located at the carotid bifurcation that constantly monitors the blood supplying the brain. During hypoxia, the CB immediately triggers an alarm in the form of nerve impulses sent to the brain. This activates protective reflexes including hyperventilation, tachycardia and vasoconstriction, to ensure blood and oxygen delivery to the brain and vital organs. However, in certain conditions, including obstructive sleep apnea, heart failure and essential/spontaneous hypertension, the CB becomes hyperactive, promoting neurogenic hypertension and arrhythmia. G-protein-coupled receptors (GPCRs) are very highly expressed in the CB and have key roles in mediating baseline CB activity and hypoxic sensitivity. Here, we provide a brief overview of the numerous GPCRs that are expressed in the CB, their mechanism of action and downstream effects. Furthermore, we will address how these GPCRs and signaling pathways may contribute to CB hyperactivity and cardiovascular and respiratory disease. GPCRs are a major target for drug discovery development. This information highlights specific GPCRs that could be targeted by novel or existing drugs to enable more personalized treatment of CB-mediated cardiovascular and respiratory disease.

Differences in the expression of catecholamine-synthesizing enzymes between vesicular monoamine transporter 1- and 2-immunoreactive glomus cells in the rat carotid body

Vesicular monoamine transporters (VMAT) 1 and 2 are responsible for monoamine transportation into secretary vesicles and are tissue-specifically expressed in central and peripheral monoaminergic tissues, including the carotid body (CB). The aim of the present study was to examine the expression of catecholamine-synthesizing enzymes in VMAT1- and VMAT2-immunoreactive glomus cells in the rat CB using multiple immunolabeling. The expression of VMAT1 and VMAT2 mRNA in the CB was confirmed by RT-PCR. Immunohistochemistry revealed that VMAT1 immunoreactivity was predominant in glomus cells rather than VMAT2 immunoreactivity. Glomus cells with VMAT1 immunoreactivity exhibited weak/negative VMAT2 immunoreactivity, and vice versa. Immunoreactivities for VMAT1 and tyrosine hydroxylase, the rate-limiting enzyme for catecholamine biosynthesis, were co-localized in the same glomus cells and a positive correlation was confirmed between the two immunoreactivities (Spearman's coefficient = 0.82; p <  0.05). Although some glomus cells showed co-localization of VMAT2 and dopamine β-hydroxylase immunoreactivity, the biosynthetic enzyme for noradrenaline, VMAT2 immunoreactivity appeared to be less associated with both catecholamine-synthesizing enzymes as indicated by a correlation analysis (TH: Spearman's coefficient = 0.38, DBH: Spearman's coefficient = 0.26). These results indicate that heterogeneity on functional role would exist among glomus cells in terms of VMAT isoform and catecholamine-synthesizing enzymes expression.

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 (fR ) 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 fR (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 fR -dependent manner in patients with HFrEF. If this interaction operates during exercise, it can have some contribution to the HFrEF exercise hyperventilation.

Vesicular nucleotide transporter-immunoreactive type I cells associated with P2X3-immunoreactive nerve endings in the rat carotid body

ATP is the major excitatory transmitter from chemoreceptor type I cells to sensory nerve endings in the carotid body, and has been suggested to be released by exocytosis from these cells. We investigated the mRNA expression and immunohistochemical localization of vesicular nucleotide transporter (VNUT) in the rat carotid body. RT-PCR detected mRNA expression of VNUT in extracts of the tissue. Immunoreactivity for VNUT was localized in a part of type I cells immunoreactive for synaptophysin (SYN), but not in glial-like type II cells immunoreactive for S100 and S100B. Among SYN-immunoreactive type I cells, VNUT immunoreactivity was selectively localized in the sub-population of tyrosine hydroxylase (TH)-immunorective type I cells associated with nerve endings immunoreactive for the P2X3 purinoceptor; however, it was not detected in the sub-population of type I cells immunoreactive for dopamine beta-hydroxylase. Multi-immunolabeling for VNUT, P2X3, and Bassoon revealed that Bassoon-immunoreactive products were localized in type I cells with VNUT immunoreactivity, and accumulated on the contact side of P2X3-immunoreactive nerve endings. These results revealed the selective localization of VNUT in the subpopulation of TH-immunoreactive type I cells attached to sensory nerve endings and suggested that these cells release ATP by exocytosis for chemosensory transmission in the carotid body.

Age-related changes in immunoreactivity for dopamine β-hydroxylase in carotid body glomus cells in spontaneously hypertensive rats

The purpose of this study was to investigate immunoreactivity for dopamine β-hydroxylase (DBH) and tyrosine hydroxylase (TH) in carotid body (CB) glomus cells in spontaneously hypertensive rats (SHR/Izm) at 4 (prehypertensive stage), 8 (early stage of developmental hypertension), 12 (later stage of developmental hypertension), and 16weeks of age (established hypertensive stage). Age-matched Wistar Kyoto rats (WKY/Izm) were used as controls. Staining properties for TH were similar between both strains at each age. Regarding DBH immunostaining, although some glomus cells showed intense DBH immunoreactivity at 4weeks of age, these cells were rarely observed at 8, 12, and 16weeks of age in WKY/Izm. In SHR/Izm, intense DBH immunoreactivity was observed in some glomus cells at 4weeks of age, these cells were also observed at 8 and 12weeks of age, and their number increased at 16weeks of age. An image analysis showed that the percentage of DBH-immunopositive glomus cells in WKY/Izm was approximately 30% at 4weeks of age and significantly decreased to approximately 10% at 8, 12, and 16weeks of age (p<0.05). This percentage in SHR/Izm was approximately 40% at each age. The gray scale intensity for DBH immunoreactivity in DBH-immunopositive glomus cells was similar in both strains at 4weeks of age, but became significantly lower in WKY/Izm and higher in SHR/Izm with increase in age (p<0.05). These results suggest that noradrenaline in glomus cells plays an important role in the regulation of neurotransmission between CB and afferent nerves during developmental hypertension.

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

KEY POINTS

Hypoglycaemia is counteracted by release of hormones and an increase in ventilation and CO2 sensitivity to restore blood glucose levels and prevent a fall in blood pH. The full counter-regulatory response and an appropriate increase in ventilation is dependent on carotid body stimulation. We show that the hypoglycaemia-induced increase in ventilation and CO2 sensitivity is abolished by preventing adrenaline release or blocking its receptors. Physiological levels of adrenaline mimicked the effect of hypoglycaemia on ventilation and CO2 sensitivity. These results suggest that adrenaline, rather than low glucose, is an adequate stimulus for the carotid body-mediated changes in ventilation and CO2 sensitivity during hypoglycaemia to prevent a serious acidosis in poorly controlled diabetes.

ABSTRACT

Hypoglycaemia in vivo induces a counter-regulatory response that involves the release of hormones to restore blood glucose levels. Concomitantly, hypoglycaemia evokes a carotid body-mediated hyperpnoea that maintains arterial CO2 levels and prevents respiratory acidosis in the face of increased metabolism. It is unclear whether the carotid body is directly stimulated by low glucose or by a counter-regulatory hormone such as adrenaline. Minute ventilation was recorded during infusion of insulin-induced hypoglycaemia (8-17 mIU kg(-1)  min(-1) ) in Alfaxan-anaesthetised male Wistar rats. Hypoglycaemia significantly augmented minute ventilation (123 ± 4 to 143 ± 7 ml min(-1) ) and CO2 sensitivity (3.3 ± 0.3 to 4.4 ± 0.4 ml min(-1)  mmHg(-1) ). These effects were abolished by either β-adrenoreceptor blockade with propranolol or adrenalectomy. In this hypermetabolic, hypoglycaemic state, propranolol stimulated a rise in P aC O2, suggestive of a ventilation-metabolism mismatch. Infusion of adrenaline (1 μg kg(-1)  min(-1) ) increased minute ventilation (145 ± 4 to 173 ± 5 ml min(-1) ) without altering P aC O2 or pH and enhanced ventilatory CO2 sensitivity (3.4 ± 0.4 to 5.1 ± 0.8 ml min(-1)  mmHg(-1) ). These effects were attenuated by either resection of the carotid sinus nerve or propranolol. Physiological concentrations of adrenaline increased the CO2 sensitivity of freshly dissociated carotid body type I cells in vitro. These findings suggest that adrenaline release can account for the ventilatory hyperpnoea observed during hypoglycaemia by an augmented carotid body and whole body ventilatory CO2 sensitivity.

Effects of dexmedetomidine on cardiorespiratory regulation in spontaneously breathing newborn rats

BACKGROUND

Dexmedetomidine, a selective α2-adrenoceptor agonist, is a new sedative agent.

OBJECTIVE

To examine the dexmedetomidine-associated changes in cardiorespiratory indices in spontaneously breathing newborn rats.

METHODS

An abdominal catheter to administer drugs and subcutaneous electrodes to record electrocardiographic data were inserted into 2- to 4-day-old rats under isoflurane anesthesia; the rats were then placed in individual chambers. After recovery from the anesthesia, the rats received intraperitoneal administrations of normal saline (NS, vehicle), dexmedetomidine (50 μg·kg(-1)), or dexmedetomidine (50 μg·kg(-1)) followed 5 min later with NS or the selective α2-adrenoceptor antagonist atipamezole (1 mg·kg(-1)) (n = 10 in each group). Cardiorespiratory indices were recorded for each animal throughout the experiment.

RESULTS

Dexmedetomidine administration significantly decreased heart rate (HR) and minute ventilation (V'E) (P < 0.05) compared with control, whereas NS administration did not. The decrease in HR and V'E after dexmedetomidine administration was significantly less in rats that received atipamezole (P < 0.05) than in those that received NS after dexmedetomidine administration. The dexmedetomidine-associated V'E depression was attributed to a significant decrease in respiratory frequency (fR) but not tidal volume (VT ). The change in fR was reversed by atipamezole administration, which itself induced no significant changes in HR and fR.

CONCLUSION

In spontaneously breathing immature rats, dexmedetomidine administration significantly reduced HR and V'E. Because atipamezole fully reversed decreases in fR and therefore V'E, dexmedetomidine-related respiratory suppression occurs predominantly through α2-adrenoceptor-related suppression of fR.

Revisiting cAMP signaling in the carotid body

Chronic carotid body (CB) activation is now recognized as being essential in the development of hypertension and promoting insulin resistance; thus, it is imperative to characterize the chemotransduction mechanisms of this organ in order to modulate its activity and improve patient outcomes. For several years, and although controversial, cyclic adenosine monophosphate (cAMP) was considered an important player in initiating the activation of the CB. However, its relevance was partially displaced in the 90s by the emerging role of the mitochondria and molecules such as AMP-activated protein kinase and O₂-sensitive K⁺ channels. Neurotransmitters/neuromodulators binding to metabotropic receptors are essential to chemotransmission in the CB, and cAMP is central to this process. cAMP also contributes to raise intracellular Ca²⁺ levels, and is intimately related to the cellular energetic status (AMP/ATP ratio). Furthermore, cAMP signaling is a target of multiple current pharmacological agents used in clinical practice. This review (1) provides an outline on the classical view of the cAMP-signaling pathway in the CB that originally supported its role in the O₂/CO₂ sensing mechanism, (2) presents recent evidence on CB cAMP neuromodulation and (3) discusses how CB activity is affected by current clinical therapies that modify cAMP-signaling, namely dopaminergic drugs, caffeine (modulation of A_2A/A_2B receptors) and roflumilast (PDE4 inhibitors). cAMP is key to any process that involves metabotropic receptors and the intracellular pathways involved in CB disease states are likely to involve this classical second messenger. Research examining the potential modification of cAMP levels and/or interactions with molecules associated with CB hyperactivity is currently in its beginning and this review will open doors for future explorations.

Short-term hypoxia transiently increases dopamine β-hydroxylase immunoreactivity in glomus cells of the rat carotid body

Under long-term hypoxia, noradrenaline (NA) content in the carotid body (CB) increases, suggesting that NA plays an important role in CB chemotransduction. However, it is unknown whether short-term hypoxia upregulates NA biosynthesis in CB. Therefore, we examined dopamine β-hydroxylase (DBH) expression in the CB of rats exposed to hypoxia (10% O(2)) for 0 to 24 hr with immunoblotting and immunohistochemistry. Using immunoblotting, the signal intensity for DBH appeared to be the most intense in rats exposed to hypoxia for 12 hr. Using immunohistochemistry, DBH immunoreactivity was observed in the cytoplasm of some glomus cells and varicosities in controls and rats exposed to hypoxia for 6 hr. In rats exposed to hypoxia for 12 hr, DBH immunoreactive intensities in DBH-positive glomus cells were significantly higher compared with controls (p<0.05). In the CB of rats exposed to hypoxia for 18 and 24 hr, DBH immunoreactive intensities in DBH-positive glomus cells were significantly lower than that of rats exposed to hypoxia for 12 hr (p<0.05). These results demonstrate that DBH immunoreactivity is transiently increased in glomus cells by short-term hypoxia, suggesting that NA biosynthesis is transiently facilitated in glomus cells at an early stage of hypoxia.

Increased total volume and dopamine β-hydroxylase immunoreactivity of carotid body in spontaneously hypertensive rats

Under hypertension, it has been reported that the carotid body (CB) is enlarged and noradrenaline (NA) content in CB is increased. Therefore, it is hypothesized that morphological and neurochemical changes in CB are induced in hypertensive animal models. In the present study, we examined the morphological features and dopamine β-hydroxylase (DBH) immunoreactivity in CB of spontaneously hypertensive rats (SHR/Izm) and Wistar Kyoto rats (WKY/Izm). The CB of SHR/Izm was elongated in terms of the cross section of center and was enlarged in the reconstructed images compared with that of WKY/Izm, and the total volume of CB in SHR/Izm (0.048 ± 0.004 mm³) was significantly (p<0.05) increased compared with the value in WKY/Izm (0.032 ± 0.006 mm³). By immunohistochemistry, immunoreactivity for tyrosine hydroxylase in CB was mainly observed in glomus cells and the immunostaining properties were similar between WKY/Izm and SHR/Izm. On the other hand, DBH immunoreactivity was mainly observed in nerve fibers around blood vessels and observed in a few glomus cells in CB of WKY/Izm. The number of glomus cells with strong DBH immunoreactivity was increased in SHR/Izm compared with that in WKY/Izm. In conclusion, the present study exhibited the enlargement of CB as three-dimensional image and revealed the enhanced immunoreactivity for DBH of glomus cells in SHR/Izm. These results suggest that the morphology of CB is affected by the effect of sympathetic nerve and that the signal transduction from CB is regulated by NA in glomus cells under hypertensive conditions.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

An antagonistic interaction between A2B adenosine and D2 dopamine receptors modulates the function of rat carotid body chemoreceptor cells

We have previously demonstrated that adenosine controls the release of catecholamines (CA) from carotid body (CB) acting on A2B receptors. Here, we have tested the hypothesis that the control is exerted via an interaction between adenosine A2B and dopamine D2 receptors present in chemoreceptor cells. Experiments were performed in vitro in CB from 3 months rats. The effect of A2B adenosine and D2 dopamine agonists and antagonists applied alone or in combination were studied on basal (20%O2) and hypoxia (10%O2)-evoked release of CA and cAMP content of CB. We have found that adenosine A2 agonists and D2 antagonists dose-dependently increased basal and evoked release CA from the CB while A2 antagonists and D2 agonists had an inhibitory action. The existence of A2B-D2 receptor interaction was established because the inhibitory action of A2 antagonists was abolished by D2 antagonists, and the stimulatory action of A2 agonists was abolished by D2 agonists. Further, A2 agonists increased and D2 agonist decreased cAMP content in the CB; their co-application eliminated the response. The present results provide direct pharmacological evidence that an antagonistic interaction between A2B adenosine and D2 dopamine receptors exist in rat CB and would explain the dopamine-adenosine interactions on ventilation previously observed.

Hypoxia transduction by carotid body chemoreceptors in mice lacking dopamine D(2) receptors

Hypoxia-induced dopamine (DA) release from carotid body (CB) glomus cells and activation of postsynaptic D(2) receptors have been proposed to play an important role in the neurotransmission process between the glomus cells and afferent nerve endings. To better resolve the role of D(2) receptors, we examined afferent nerve activity, catecholamine content and release, and ventilation of genetically engineered mice lacking D(2) receptors (D(2)(-/-) mice). Single-unit afferent nerve activities of D(2)(-/-) mice in vitro were significantly reduced by 45% and 25% compared with wild-type (WT) mice during superfusion with saline equilibrated with mild hypoxia (Po(2) approximately 50 Torr) or severe hypoxia (Po(2) approximately 20 Torr), respectively. Catecholamine release in D(2)(-/-) mice was enhanced by 125% in mild hypoxia and 75% in severe hypoxia compared with WT mice, and the rate of rise was increased in D(2)(-/-) mice. We conclude that CB transduction of hypoxia is still present in D(2)(-/-) mice, but the response magnitude is reduced. However, the ventilatory response to acute hypoxia is maintained, perhaps because of an enhanced processing of chemoreceptor input by brain stem respiratory nuclei.

Clonidine-evoked respiratory effects in anaesthetized rats

The respiratory effects of stimulation of alpha2-adrenergic receptors were studied in spontaneously breathing anaesthetized rats that were neurally intact, or bilaterally vagotomized, or subjected to bilateral combined midcervical vagotomy and section of the carotid sinus nerves. An intravenous clonidine bolus (15 microg kg(-1)) evoked a prolonged slowing of the respiratory rate in all the neural states explored. Vagotomy reduced the early clonidine-evoked decline, but not the augmentation of tidal volume that followed the decline. After section of the carotid sinus nerves, clonidine challenge continued to decrease the respiratory rate, but not the tidal volume. Blockade of alpha2-adrenergic receptors with intravenous doses of SKF 86466 (200 microg kg(-1)) abolished all respiratory effects of the clonidine challenge. In all the neural states studied, clonidine evoked a significant short-lived rise in mean arterial blood pressure followed by a decrease below the respective prechallenge value. The SKF 86466 pretreatment lowered mean arterial blood pressure control values and reduced the magnitude of postclonidine changes. These results indicate that: (i) clonidine-evoked activation of alpha2-adrenergic receptors affects the two components of the breathing pattern differently, and this occurs beyond the lung vagi; and (ii) changes in tidal volume result from excitation of the carotid bodies and are coupled with centrally mediated slowing of the respiratory rhythm.

Differential respiratory effects of [Dmt1]DALDA and morphine in mice

H-Dmt-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA, dDAL), a highly selective mu-opioid peptide, produces potent analgesia without respiratory depression after intrathecal administration. Despite carrying 3+ net charge, dDAL is also a potent analgesic after systemic administration. We compared the respiratory effects of dDAL and morphine after subcutaneous administration in mice using whole body plethysmography. Analgesic doses of 3 and 10 times ED(50) were examined. Both drugs dose-dependently decreased respiratory frequency and minute volume in room air. Tidal volume was increased by the lower dose of morphine, while it was decreased by the higher dose of dDAL. The decrease in minute volume by dDAL and morphine was completely reversed by naloxone. No difference in ventilatory response to CO(2) was observed between dDAL and morphine at three times ED(50). Ventilatory response to hypoxia was significantly diminished by dDAL compared to morphine and saline, and this effect of dDAL was naloxone-irreversible. Thus dDAL likely reduces the sensitivity of the peripheral chemoreflex loop through a non-opioid action.

Insulin-like growth factor II plays a central role in atherosclerosis in a mouse model

Insulin-like growth factor II is a fetal promoter of cell proliferation that is involved in some forms of cancer and overgrowth syndromes in humans. Here, we provide two sources of genetic evidence for a novel, pivotal role of locally produced insulin-like growth factor II in the development of atherosclerosis. First, we show that homozygosity for a disrupted insulin-like growth factor II allele in mice lacking apolipoprotein E, a widely used animal model of atherosclerosis, results in aortic lesions that are approximately 80% smaller and contain approximately 50% less proliferating cells compared with mice lacking only apolipoprotein E. Second, targeted expression of an insulin-like growth factor II transgene in smooth muscle cells, but not the mere elevation of circulating levels of the peptide, causes per se aortic focal intimal thickenings. The insulin-like growth factor II transgenics presented here are the first viable mutant mice spontaneously developing intimal masses. These observations provide the first direct evidence for an atherogenic activity of insulin-like growth factor II in vivo.

Cellular mechanisms of oxygen sensing at the carotid body: heme proteins and ion channels

The purpose of this article is to highlight some recent concepts on oxygen sensing mechanisms at the carotid body chemoreceptors. Most available evidence suggests that glomus (type I) cells are the initial site of transduction and they release transmitters in response to hypoxia, which in turn depolarize the nearby afferent nerve ending, leading to an increase in sensory discharge. Two main hypotheses have been advanced to explain the initiation of the transduction process that triggers transmitter release. One hypothesis assumes that a biochemical event associated with a heme protein triggers the transduction cascade. Supporting this idea it has been shown that hypoxia affects mitochondrial cytochromes. In addition, there is a body of evidence implicating non-mitochondrial enzymes such as NADPH oxidases, NO synthases and heme oxygenases located in glomus cells. These proteins could contribute to transduction via generation of reactive oxygen species, nitric oxide and/or carbon monoxide. The other hypothesis suggests that a K(+) channel protein is the oxygen sensor and inhibition of this channel and the ensuing depolarization is the initial event in transduction. Several oxygen sensitive K(+) channels have been identified. However, their roles in initiation of the transduction cascade and/or cell excitability are unclear. In addition, recent studies indicate that molecular oxygen and a variety of neurotransmitters may also modulate Ca(2+) channels. Most importantly, it is possible that the carotid body response to oxygen requires multiple sensors, and they work together to shape the overall sensory response of the carotid body over a wide range of arterial oxygen tensions.

Intracellular Ca(2+) stores in chemoreceptor cells of the rabbit carotid body: significance for chemoreception

The notion that intracellular Ca(2+) (Ca(i)(2+)) stores play a significant role in the chemoreception process in chemoreceptor cells of the carotid body (CB) appears in the literature in a recurrent manner. However, the structural identity of the Ca(2+) stores and their real significance in the function of chemoreceptor cells are unknown. To assess the functional significance of Ca(i)(2+) stores in chemoreceptor cells, we have monitored 1) the release of catecholamines (CA) from the cells using an in vitro preparation of intact rabbit CB and 2) the intracellular Ca(2+) concentration ([Ca(2+)](i)) using isolated chemoreceptor cells; both parameters were measured in the absence or the presence of agents interfering with the storage of Ca(2+). We found that threshold [Ca(2+)](i) for high extracellular K(+) (K(e)(+)) to elicit a release response is approximately 250 nM. Caffeine (10-40 mM), ryanodine (0.5 microM), thapsigargin (0.05-1 microM), and cyclopiazonic acid (10 microM) did not alter the basal or the stimulus (hypoxia, high K(e)(+))-induced release of CA. The same agents produced Ca(i)(2+) transients of amplitude below secretory threshold; ryanodine (0.5 microM), thapsigargin (1 microM), and cyclopiazonic acid (10 microM) did not alter the magnitude or time course of the Ca(i)(2+) responses elicited by high K(e)(+). Several potential activators of the phospholipase C system (bethanechol, ATP, and bradykinin), and thereby of inositol 1,4,5-trisphosphate receptors, produced minimal or no changes in [Ca(2+)](i) and did not affect the basal release of CA. It is concluded that, in the rabbit CB chemoreceptor cells, Ca(i)(2+) stores do not play a significant role in the instant-to-instant chemoreception process.

Characterization of the synthesis and release of catecholamine in the rat carotid body in vitro

The aim of this work was to determine contents and turnover rates for dopamine (DA) and norepinephrine (NE) and to identify the catecholamine (CA) released during stimulation of the rat carotid body (CB). Turnover rates and the release of CA were measured in an in vitro preparation using a combination of HPLC and radioisotopic methods. Mean rat CB levels of DA and NE were 209 and 45 pmol/mg tissue, respectively. With [(3)H]tyrosine as precursor, rat CB synthesized [(3)H]CA in a time- and concentration-dependent manner; calculated turnover times for DA and NE were 5.77 and 11.4 h, respectively. Hypoxia and dibutyryl adenosine 3',5'-cyclic monophosphate significantly increased [(3)H]CA synthesis. In normoxia, rat CB released [(3)H]DA and [(3)H]NE in a ratio of 5:1, comparable to that of the endogenous tissue CA. Hypoxia and high K(+) preferentially released [(3)H]DA, nicotine preferentially released [(3)H]NE, and acidic stimuli released both amines in proportion to tissue content. Release of [(3)H]CA induced by hypoxia and high K(+) was nearly fully dependent on extracellular Ca(2+), whereas basal normoxic release was not altered by removal of Ca(2+) from the incubating solution. We conclude that the rat CB is an organ with higher levels of DA than NE that preferentially releases DA or NE in a stimulus-specific manner.

Nitric oxide inhibits L-type Ca2+ current in glomus cells of the rabbit carotid body via a cGMP-independent mechanism

Previous studies have shown that nitric oxide (NO) inhibits carotid body sensory activity. To begin to understand the cellular mechanisms associated with the actions of NO in the carotid body, we monitored the effects of NO donors on the macroscopic Ca2+ current in glomus cells isolated from rabbit carotid bodies. Experiments were performed on freshly dissociated glomus cells from adult rabbit carotid bodies using the whole cell configuration of the patch-clamp technique. The NO donors sodium nitroprusside (SNP; 600 microM, n = 7) and spermine nitric oxide (SNO; 100 microM, n = 7) inhibited the Ca2+ current in glomus cells in a voltage-independent manner. These effects of NO donors were rapid in onset and peaked within 1 or 2 min. In contrast, the outward K+ current was unaffected by SNP (600 microM, n = 6), indicating that the inhibition by SNP was not a nonspecific membrane effect. 2-(4-carboxyphenyl)-4,4,5, 5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO; 500 microM), an NO scavenger, prevented inhibition of the Ca2+ current by SNP (n = 7), whereas neither superoxide dismutase (SOD; 2,000 U/ml, n = 4), a superoxide scavenger, nor sodium hydrosulfite (SHS; 1 mM, n = 7), a reducing agent, prevented inhibition of the Ca2+ current by SNP. However, SNP inhibition of the Ca2+ current was reversible in the presence of either SOD or SHS. These results suggest that NO itself inhibits Ca2+ current in a reversible manner and that subsequent formation of peroxynitrites results in irreversible inhibition. SNP inhibition of the Ca2+ current was not affected by 30 microM LY 83, 583 (n = 7) nor was it mimicked by 600 microM 8-bromoguanosine 3':5'-cyclic monophosphate (8-Br-cGMP; n = 6), suggesting that the effects of NO on the Ca2+ current are mediated, in part, via a cGMP-independent mechanism. N-ethylmaleimide (NEM; 2.5 mM, n = 6) prevented the inhibition of the Ca2+ current by SNP, indicating that SNP is acting via a modification of sulfhydryl groups on Ca2+ channel proteins. Norepinephrine (NE; 10 microM) further inhibited the Ca2+ current in the presence of NEM (n = 7), implying that NEM did not nonspecifically eliminate Ca2+ current modulation. Nisoldipine, an L-type Ca2+ channel blocker (2 microM, n = 6), prevented the inhibition of Ca2+ current by SNP, whereas omega-conotoxin GVIA, an N-type Ca2+ channel blocker (1 microM, n = 9), did not prevent the inhibition of Ca2+ current by SNP. These results demonstrate that NO inhibits L-type Ca2+ channels in adult rabbit glomus cells, in part, due to a modification of calcium channel proteins. The inhibition might provide one plausible mechanism for efferent inhibition of carotid body activity by NO.