Carotid body chemoreceptor function: hypothesis based on a new circuit model

Hypoxia releases S-nitrosocysteine from carotid body glomus cells-relevance to expression of the hypoxic ventilatory response

We have provided indirect pharmacological evidence that hypoxia may trigger release of the S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), from primary carotid body glomus cells (PGCs) of rats that then activates chemosensory afferents of the carotid sinus nerve to elicit the hypoxic ventilatory response (HVR). The objective of this study was to provide direct evidence, using our capacitive S-nitrosothiol sensor, that L-CSNO is stored and released from PGCs extracted from male Sprague Dawley rat carotid bodies, and thus further pharmacological evidence for the role of S-nitrosothiols in mediating the HVR. Key findings of this study were that 1) lysates of PGCs contained an S-nitrosothiol with physico-chemical properties similar to L-CSNO rather than S-nitroso-L-glutathione (L-GSNO), 2) exposure of PGCs to a hypoxic challenge caused a significant increase in S-nitrosothiol concentrations in the perfusate to levels approaching 100 fM via mechanisms that required extracellular Ca2+, 3) the dose-dependent increases in minute ventilation elicited by arterial injections of L-CSNO and L-GSNO were likely due to activation of small diameter unmyelinated C-fiber carotid body chemoafferents, 4) L-CSNO, but not L-GSNO, responses were markedly reduced in rats receiving continuous infusion (10 μmol/kg/min, IV) of both S-methyl-L-cysteine (L-SMC) and S-ethyl-L-cysteine (L-SEC), 5) ventilatory responses to hypoxic gas challenge (10% O2, 90% N2) were also due to the activation of small diameter unmyelinated C-fiber carotid body chemoafferents, and 6) the HVR was markedly diminished in rats receiving L-SMC plus L-SEC. This data provides evidence that rat PGCs synthesize an S-nitrosothiol with similar properties to L-CSNO that is released in an extracellular Ca2+-dependent manner by hypoxia.

Expression of group II and III mGluRs in the carotid body and its role in the carotid chemoreceptor response to acute hypoxia

The carotid body (CB) contributes significantly to oxygen sensing. It is unclear, however, whether glutamatergic signaling is involved in the CB response to hypoxia. Previously, we reported that ionotropic glutamate receptors (iGluRs) and multiple glutamate transporters are present in the rat CB. Except for iGluRs, glutamate receptors also include metabotropic glutamate receptors (mGluRs), which are divided into the following groups: Group I (mGluR1/5); group II (mGluR2/3); group III (mGluR4/6/7/8). We have studied the expression of group I mGluRs in the rat CB and its physiological function response to acute hypoxia. To further elucidate the states of mGluRs in the CB, this study’s aim was to investigate the expression of group II and III mGluRs and the response of rat CB to acute hypoxia. We used reverse transcription-polymerase chain reaction (RT-PCR) to observed mRNA expression of GRM2/3/4/6/7/8 subunits by using immunostaining to show the distribution of mGluR2 and mGluR8. The results revealed that the GRM2/3/4/6/7/8 mRNAs were expressed in both rat and human CB. Immunostaining showed that mGluR2 was localized in the type I cells and mGluR8 was localized in type I and type II cells in the rat CB. Moreover, the response of CB to acute hypoxia in rats was recorded by in vitro carotid sinus nerve (CSN) discharge. Perfusion of group II mGluRs agonist or group III mGluRs agonist (LY379268 or L-SOP) was applied to examine the effect of group II and III mGluRs on rat CB response to acute hypoxia. We found that LY379268 and L-SOP inhibited hypoxia-induced enhancement of CSN activity. Based on the above findings, group II and III mGluRs appear to play an inhibitory role in the carotid chemoreceptor response to acute hypoxia.

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.

A comparative analysis of putative oxygen-sensing cells in the fish gill

We investigated the distribution of serotonin (5-HT)-containing neuroepithelial cells (NECs), the putative O(2) sensing cells, in the gills of four species of fish: trout (Oncorhynchus mykiss), goldfish (Carassius auratus), trairão (Hoplias lacerdae) and traira (Hoplias malabaricus) using immunohistochemical markers for 5-HT, synaptic vesicles and neural innervation. We found that all fish had a cluster of innervated, serotonergic NECs at the filament tips, but there were species-specific distributions of serotonin-containing NECs within the primary gill filaments. Trout gill filaments had a greater number of serotonin-containing NECs than both trairão and traira, whereas goldfish primary filaments had none. Serotonin-containing NECs in the secondary lamellae were most numerous in goldfish, present in trairão and traira, but absent in trout. Those found in the primary filament were generally associated with the efferent filamental artery. Innervated, serotonin-containing cells (NECs or Merkel-like cells) were also found in the gill rakers of trout and goldfish although vesicular serotonin was only found in the gill rakers of goldfish. These differences in serotonergic NEC distribution appear to reflect paracrine versus chemoreceptive roles related to hypoxia tolerance in the different fish species.

The molecular basis of O2-sensing and hypoxia tolerance in pheochromocytoma cells

Hypoxia is a common environmental stimulus. However, very little is known about the mechanisms by which cells sense and respond to changes in oxygen. Our laboratory has utilized the PC12 cell line in order to study the biophysical and molecular response to hypoxia. The current review summarizes our results. We demonstrate that the O2-sensitive K(+) channel, Kv1.2, is present in PC12 cells and plays a critical role in the hypoxia-induced depolarization of PC12 cells. Previous studies have shown that PC12 cells secrete a variety of autocrine/paracrine factors, including dopamine, norepinephrine, and adenosine during hypoxia. We investigated the mechanisms by which adenosine modulates cell function and the effect of chronic hypoxia on this modulation. Finally, we present results identifying the mitogen- and stress-activated protein kinases (MAPKs and SAPKs) as hypoxia-regulated protein kinases. Specifically, we show that p38 and an isoform, p38gamma, are activated by hypoxia. In addition, our results demonstrate that the p42/p44 MAPK protein kinases are activated by hypoxia. We further show that p42/p44 MAPK is critical for the hypoxia-induced transactivation of endothelial PAS-domain protein 1 (EPAS1), a hypoxia-inducible transcription factor. Together, these results provide greater insight into the mechanisms by which cells sense and adapt to hypoxia.

EPAS1 trans-activation during hypoxia requires p42/p44 MAPK

Hypoxia is a common environmental stress that regulates gene expression and cell function. A number of hypoxia-regulated transcription factors have been identified and have been shown to play critical roles in mediating cellular responses to hypoxia. One of these is the endothelial PAS-domain protein 1 (EPAS1/HIF2-alpha/HLF/HRF). This protein is 48% homologous to hypoxia-inducible factor 1-alpha (HIF1-alpha). To date, virtually nothing is known about the signaling pathways that lead to either EPAS1 or HIF1-alpha activation. Here we show that EPAS1 is phosphorylated when PC12 cells are exposed to hypoxia and that p42/p44 MAPK is a critical mediator of EPAS1 activation. Pretreatment of PC12 cells with the MEK inhibitor, PD98059, completely blocked hypoxia-induced trans-activation of a hypoxia response element (HRE) reporter gene by transfected EPAS1. Likewise, expression of a constitutively active MEK1 mimicked the effects of hypoxia on HRE reporter gene expression. However, pretreatment with PD98059 had no effect on EPAS1 phosphorylation during hypoxia, suggesting that MAPK targets other proteins that are critical for the trans-activation of EPAS1. We further show that hypoxia-induced trans-activation of EPAS1 is independent of Ras. Finally, pretreatment with calmodulin antagonists nearly completely blocked both the hypoxia-induced phosphorylation of MAPK and the EPAS1 trans-activation of HRE-Luc. These results demonstrate that the MAPK pathway is a critical mediator of EPAS1 activation and that activation of MAPK and EPAS1 occurs through a calmodulin-sensitive pathway and not through the GTPase, Ras. These results are the first to identify a specific signaling pathway involved in EPAS1 activation.

Norepinephrine inhibits a toxin resistant Ca2+ current in carotid body glomus cells: evidence for a direct G protein mechanism

Previous studies have demonstrated that endogenous norepinephrine (NE) inhibits carotid body (CB) sensory discharge, and the cellular actions of NE have been associated with inhibition of Ca2+ current in glomus cells. The purpose of the present study was to elucidate the characteristics and mechanism of NE inhibition of whole cell Ca2+ current isolated from rabbit CB glomus cells and to determine the type(s) of Ca2+ channel involved. NE (10 microM) inhibited 24 +/- 2% (SE) of the macroscopic Ca2+ current measured at the end of a 25 ms pulse to 0 mV and slowed activation of the current. The alpha2 adrenergic receptor antagonist, SK&F 86466, attenuated these effects. Inhibition by NE was fast and voltage-dependent i.e., maximal at -10 mV and then diminished with stronger depolarizations. This is characteristic of G protein betagamma subunit interaction with the alpha1 subunit of certain Ca2+ channels, which can be relieved by depolarizing steps. A depolarizing step (30 ms to +80 mV) significantly increased (14 +/- 1%) current in the presence of NE, whereas it had no effect before application of NE (1 +/- 1%). To further test for the involvement of G proteins, NE was applied to cells where intracellular GTP was replaced by GDP-betaS. NE had little or no effect on Ca2+ current in cells dialyzed with GDP-betaS. To determine whether NE was inhibiting N- and/or P/Q-type channels, we applied NE in the presence of omega-conotoxin MVIIC (MVIIC). In the presence of 2.5 microM MVIIC, NE was equally potent at inhibiting the Ca2+ current (23 +/- 4% vs. 23 +/- 4% in control), suggesting that NE was not exclusively inhibiting N- or P/Q-type channels. NE was also equally potent (30 +/- 2% vs. 26 +/- 4% in control) at inhibiting the Ca2+ current in the presence of 2 microM nisoldipine, suggesting that NE was not inhibiting L-type channels. Further, NE inhibited a significantly larger proportion (47 +/- 6%) of the resistant Ca2+ current remaining in the presence of NISO and MVIIC. These results suggest that NE inhibition of Ca2+ current in rabbit CB glomus cells is mediated in most part by effects on the resistant, non L-, N-, or P/Q-type channel and involves a direct G protein betagamma interaction with this channel.

Endogenous carbon monoxide in control of respiration

It is being increasingly appreciated that gas molecules such as nitric oxide (NO) function as chemical messengers in the nervous system. Recent studies suggest that carbon monoxide (CO) is another gas molecule that has similar biological actions as NO. The purpose of this article is to highlight the current information on the significance of endogenously generated CO in control of breathing. In mammalian cells, CO is generated during oxidative cleavage of heme by heme oxygenases (HO) and molecular oxygen is essential for this reaction. Two forms of HO have been identified including an inducible HO-1, that resembles stress-inducible protein HSP-32, and a constitutively expressed HO-2. HO-2 is expressed in many respiratory related neural structures including airway ganglion, carotid body, petrosal and nodose ganglia., nucleus of the tractus solitarius (nTS), and neurons of the rostral ventrolateral medulla (RVLM). Basal expression of HO-1 is either very low or even absent, but can be elevated during oxidative stress and hypoxia. Physiological studies have shown that CO might be of importance in vagally mediated contractions of airways. Several lines of evidence indicate that endogenously generated CO is a physiological modulator of the ventilatory response to hypoxia via its actions on carotid bodies and perhaps at brainstem neurons. In addition, CO might play a role in ventilatory adaptation to hypoxia, as low oxygen is a potent inducer of HO-1. Many of the neuronal structures that express HO also contain NOS, the enzyme that generates NO. Much remains to be studied on regulatory interactions between CO and NO and their impact on breathing.

Ultrastructure of the carotid body of the goat

The carotid body of the goat was found to be a small oval or rounded parenchymatous organ. It was characterized by its profound vascularity. Delicate septa divided the parenchyma into small feebly defined lobules. Electron microscopy revealed that the parenchyma comprised type I cells, type II cells, nerve endings, axons and fenestrated dilated capillaries. Type I cells were characterized with electron dense-cored vesicles. They showed variations in size and concentration of the dense-cored vesicles and number of mitochondria. The possibility that these variations are reflections of different stages of activity is discussed. Type II cells were less numerous than type I cells, relatively small and devoid of dense-cored vesicles. They usually surrounded small groups of type I cells and associated nerve endings and axons. Presumptive afferent nerve endings characterized with many clear vesicles, occasional large granular vesicles and varying numbers of slender mitochondria, lay apposed to type I cells. Nerve endings of this kind showed afferent and efferent synaptic junctions with type I cells. Presumptive sympathetic efferent endings were occasionally seen within the lobules but never lay apposed to type I cells or afferent nerve ending.

Dopamine beta-hydroxylase-like immunoreactivity in the rat and cat carotid body: a light and electron microscopic study

Immunocytochemical localization of dopamine beta-hydroxylase (DBH) was used to study the synthesis and storage sites of norepinephrine (noradrenaline) in the rat and cat carotid bodies. In the rat carotid body some parenchymal cells exhibited strong DBH-like immunoreactivity (DBH-I), while others displayed only faint DBH-I. In a typical parenchymal cell cluster, most cells with strong DBH-I were irregular in shape and appeared to partially surround those with weak DBH-I which usually were rounded in contour. In the cat carotid body most parenchymal cells showed a strong to moderate DBH-I. In both the rat and cat carotid bodies varicose nerve fibres with DBH-I were associated primarily with blood vessels. All autonomic ganglion cells examined, which were associated with the rat carotid body, showed DBH-I. Electron microscopy revealed that most DBH-I in the strongly positive cells of the rat carotid body was associated with dense granules (possibly corresponding to dense-cored vesicles of various sizes), although some was found in other sites. In oval cells with less DBH-I, reactivity resided in some of the large granules. In the cat carotid body the glomus cells contained more granules of various sizes and shapes than did those of the rat carotid body. Most of the cat glomus cell granules exhibited DBH-I activity. Our results indicate that some of glomus cells in the rat and most of the glomus cells in the cat contain DBH and therefore may be sites of norepinephrine synthesis.

Influence of age on the carotid bodies of spontaneously hypertensive (SHR) and normotensive rats. II. Alterations of the vascular wall

The carotid bodies of spontaneously hypertensive rats (SHR) of the Okamoto-Aoki-strain and of age-matched normotensive Wistar rats (NWR) were studied with respect to their size and the histological appearance of their arterial vessels. The animals were aged 3-6 d, and 5-6, 15-20, 30-40 and 50-70 weeks. The development of hypertension in the SHR started at an age of 5-6 weeks and was fully established at 15-20 weeks (mean systemic arterial blood pressure at about 160 mm Hg). When compared with the age-matched normotensive control rats (NWR) the SHR in the established phase of hypertension showed enlarged carotid bodies, an increase of the thickness of the media of the carotid body artery and circumscript pad-like thickening of the vascular wall within the carotid bodies. Repeatedly in particular in the old SHR, also a hyalinosis of the small branches of the glomic artery was found. These pathological changes regularly narrowed the lumen of the vessels seized; sometimes to a considerable extent. Such vascular alterations were never found in the newborn (3-6 d old) SHR and were also not demonstrable in the NWR. Thus these vascular alterations in the carotid body vessels of the adult SHR are supposed to be the result of the high systemic arterial blood pressure. The data indicate that long-lasting high systemic arterial blood pressure leads to changes of the wall of the arterial vessels of the carotid and presumably also the aortic bodies thus inducing an ischemic hypoxia of the specific chemoreceptive tissue and a chronic stimulation of the arterial chemoreceptors.

Two types of glomus cell in the rat carotid body as revealed by alpha-bungarotoxin binding

Horseradish peroxidase (HRP)-conjugated alpha-bungarotoxin (alpha Bgt) was used to localize alpha Bgt-acetylcholine receptor sites in the rat carotid body. Two types of glomus cell were differentiated on the basis of the staining of their plasma membranes by the conjugate: type A, devoid of staining or only partly stained; and type B, exhibiting staining over the entire cell surface. The parts of type A glomus and supporting cells stained were always in direct apposition to type B glomus cells. It is concluded that type B glomus cells are possibly the only cell types exhibiting specific binding sites of alpha Bgt. Other morphological characteristics and quantitative studies indicated that the type A and type B glomus cells presented in this study were equivalent to those described in the rat carotid body by other investigators (McDonald & Mitchell, 1975). alpha Bgt-HRP staining facilitated the observation of the distribution pattern of glomus cells in the parenchyma: type A glomus cells were arranged in groups and often showed polarity toward neural elements and sinusoidal capillaries; and clusters of type B glomus cells were frequently situated in a demilune -like fashion over groups of type A glomus cells. Because of differences in morphology, synaptology, alpha Bgt-binding affinity, and polarity toward the blood vessels, we propose that type A and type B glomus cells in the rat carotid body represent functionally distinct cell types.

Cytochemical evidence for the existence of norepinephrine-containing glomus cells in the rat carotid body

Some investigators have postulated that glomus cells of the rat carotid body contain only dopamine (DA), and that the norepinephrine (NE) measured in the carotid body resides only in sympathetic nerve endings and ganglion cells. To investigate this hypothesis, we employed a pharmacologic drug sequence which depleted all carotid body catecholamines and then selectively restored DA levels while keeping NE levels significantly lowered. Analysis of carotid body catecholamines by high performance liquid chromatography (HPLC) validated this drug regimen. Employing this drug treatment, we examined glomus cells after potassium dichromate cytochemical staining in an effort to distinguish those glomus cell vesicles which still contained appreciable amounts of catecholamine, presumably DA. Glomus cells from rats receiving vehicle or L-dopa (100 mg kg-1 ip) alone had 83 and 97% of their cells stained, respectively. However, glomus cells from reserpinized (5 mg kg-1 ip) animals were largely unstained (89%). Carotid bodies from animals treated with reserpine and then, 24 h later, with L-dopa 90 min prior to sacrifice had about 46% of their glomus cells stained while 54% of the cells were unstained. The results of this last group, coupled with our biochemical data which demonstrated that DA levels were comparable to control values but that NE was 80% depleted, suggest that a significant number of glomus cells did not contain enough catecholamine to react with the dichromate. We believe that these unstained cells may normally contain NE and that glomus cells may be of several types, some containing predominantly DA and others NE.

Localization of enkephalin-like immunoreactivity in the cat carotid and aortic body chemoreceptors

The purpose of this study was to determine if enkephalin-like immunoreactivity was present in the glomus cells of the carotid and aortic body peripheral arterial chemoreceptors. Cat carotid and aortic bodies were reacted with antisera to met- and leu-enkephalin using the indirect peroxidase-antiperoxidase immunocytochemical method of Sternberger (1979). Both the carotid and aortic bodies demonstrated clusters of immunoreactive cells for both met- and leu-enkephalin. Additionally, met-enkephalin-like immunoreactivity was observed in many of the dense-core vesicles of the glomus cells of the carotid body. The glomus cells of these chemoreceptors are known to contain catecholamines which may modulate chemoreceptor activity. The presence of opioid peptide-like substances co-existing with the glomus cell catecholamines, perhaps in the same vesicles, may have important implications for a trophic influence of these peptides on glomus cell chemoreceptor modulation.

Autoradiographic localization of alpha-bungarotoxin-binding sites in the carotid body of the rat

Radioiodinated alpha-bungarotoxin (alpha-Bgt) was used to localize alpha-Bgt-acetylcholine receptors in the carotid body of the rat. The gamma spectrometer analyses indicated a high uptake of [125I] alpha-Bgt in carotid bodies incubated in vitro (1.51 fmole per organ). Incorporation of the isotope was effectively blocked by pretreatment of carotid bodies with d-tubocurarine and unlabeled alpha-Bgt, but not by atropine. Light microscopic autoradiography showed a heavy labeling of some parenchymal cells. Electron-microscopic autoradiography revealed that labeling was localized along the interface between parenchymal cells, especially where their cytoplasmic processes engage in complex interdigitations. The silver grain counts on electron-microscopic autoradiographs suggest that labelings are preferentially associated with the plasma membrane of certain Type I cells. It is suggested that these Type I cells in the rat's carotid body probably are provided with nicotinic acetylcholine receptors on their plasma membranes.

Electron-dense precipitates in glomus cells of rat carotid body after fixation in glutaraldehyde and pyroantimonate-osmium tetroxide mixture as possible indicators of calcium localization

An attempt was made to study the subcellular localization of calcium in carotid body glomus cells of adult rats using fixation with glutaraldehyde followed by treatment with a mixture of pyroantimonate and osmium tetroxide. Precipitates were seen as electron-dense particles (EDP) in the glomus cells, mostly within membrane-bound organelles, such as dense-cored vesicles, mitochondria, small clear vesicles, multivesicular bodies, and especially in lysosomes. However, EDP were also seen in the nuclei and in the free cytoplasm of the glomus cells and even outside them. Preincubation of carotid bodies in media containing calcium and either high potassium or calcium-ionophore A 23187 resulted in a marked increase in the general precipitation pattern, there being an increased amount of EDP both in the glomus cell nuclei and in the cytoplasm. Dense-cored vesicles more often showed precipitates than those in the controls. Some dense-cored vesicles contained multiple precipitates, typically located in the electron-lucent area between core and vesicle membrane. Extensive diffusion of ions probably occurred during fixation before precipitation, making the localization of calcium and other precipitating cations unreliable. However, it is possible that precipitates, which were regularly seen in the dense-cored vesicles, may reflect the content of bound calcium. The possible significance of calcium in glomus cell function is discussed, and the need for more adequate methods is emphasized.