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

The Adult Carotid Body: A Germinal Niche at the Service of Physiology

The carotid body is the most relevant oxygen sensor in mammalian organisms. This organ helps to detect acute changes in PO₂, but it is also crucial for the organismal adaptation to a maintained hypoxemia. Profound angiogenic and neurogenic processes take place in the carotid body to facilitate this adaptation process. We have described a plethora of multipotent stem cells and restricted progenitors, from both vascular and neuronal lineages, existing in the quiescent normoxic carotid body, ready to contribute to organ growth and adaptation upon the arrival of the hypoxic stimulus. Our deep understanding of the functioning of this stunning germinal niche will very likely facilitate the management and treatment of an important group of diseases that course with carotid body over-activation and malfunction.

Neurotransmitter Modulation of Carotid Body Germinal Niche

The carotid body (CB), a neural-crest-derived organ and the main arterial chemoreceptor in mammals, is composed of clusters of cells called glomeruli. Each glomerulus contains neuron-like, O₂-sensing glomus cells, which are innervated by sensory fibers of the petrosal ganglion and are located in close contact with a dense network of fenestrated capillaries. In response to hypoxia, glomus cells release transmitters to activate afferent fibers impinging on the respiratory and autonomic centers to induce hyperventilation and sympathetic activation. Glomus cells are embraced by interdigitating processes of sustentacular, glia-like, type II cells. The CB has an extraordinary structural plasticity, unusual for a neural tissue, as it can grow several folds its size in subjects exposed to sustained hypoxia (as for example in high altitude dwellers or in patients with cardiopulmonary diseases). CB growth in hypoxia is mainly due to the generation of new glomeruli and blood vessels. In recent years it has been shown that the adult CB contains a collection of quiescent multipotent stem cells, as well as immature progenitors committed to the neurogenic or the angiogenic lineages. Herein, we review the main properties of the different cell types in the CB germinal niche. We also summarize experimental data suggesting that O₂-sensitive glomus cells are the master regulators of CB plasticity. Upon exposure to hypoxia, neurotransmitters and neuromodulators released by glomus cells act as paracrine signals that induce proliferation and differentiation of multipotent stem cells and progenitors, thus causing CB hypertrophy and an increased sensory output. Pharmacological modulation of glomus cell activity might constitute a useful clinical tool to fight pathologies associated with exaggerated sympathetic outflow due to CB overactivation.

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.

The Logic of Carotid Body Connectivity to the Brain

The carotid body has emerged as a therapeutic target for cardio-respiratory-metabolic diseases. With the expansive functions of the chemoreflex, we sought mechanisms to explain differential control of individual responses. We purport a remarkable correlation between phenotype of a chemosensory unit (glomus cell-sensory afferent) with a distinct component of the reflex response. This logic could permit differential modulation of distinct chemoreflex responses, a strategy ideal for therapeutic exploitation.

Progenitor Cell Heterogeneity in the Adult Carotid Body Germinal Niche

Somatic stem cells confer plasticity to adult tissues, permitting their maintenance, repair and adaptation to a changing environment. Adult germinal niches supporting somatic stem cells have been thoroughly characterized throughout the organism, including in central and peripheral nervous systems. Stem cells do not reside alone within their niches, but they are rather accompanied by multiple progenitor cells that not only contribute to the progression of stem cell lineage but also regulate their behavior. Understanding the mechanisms underlying these interactions within the niche is crucial to comprehend associated pathologies and to use stem cells in cell therapy. We have described a stunning germinal niche in the adult peripheral nervous system: the carotid body. This is a chemoreceptor organ with a crucial function during physiological adaptation to hypoxia. We have shown the presence of multipotent stem cells within this niche, escorted by multiple restricted progenitor cell types that contribute to niche physiology and hence organismal adaptation to the lack of oxygen. Herein, we discuss new and existing data about the nature of all these stem and progenitor cell types present in the carotid body germinal niche, discussing their role in physiology and their clinical relevance for the treatment of diverse pathologies.

Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword

The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca²⁺/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca²⁺]_i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.

Advances in cellular and integrative control of oxygen homeostasis within the central nervous system

Mammals must continuously regulate the levels of O2 and CO2 , which is particularly important for the brain. Failure to maintain adequate O2 /CO2 homeostasis has been associated with numerous disorders including sleep apnoea, Rett syndrome and sudden infant death syndrome. But, O2 /CO2 homeostasis poses major regulatory challenges, even in the healthy brain. Neuronal activities change in a differentiated, spatially and temporally complex manner, which is reflected in equally complex changes in O2 demand. This raises important questions: is oxygen sensing an emergent property, locally generated within all active neuronal networks, and/or the property of specialized O2 -sensitive CNS regions? Increasing evidence suggests that the regulation of the brain's redox state involves properties that are intrinsic to many networks, but that specialized regions in the brainstem orchestrate the integrated control of respiratory and cardiovascular functions. Although the levels of O2 in arterial blood and the CNS are very different, neuro-glial interactions and purinergic signalling are critical for both peripheral and CNS chemosensation. Indeed, the specificity of neuroglial interactions seems to determine the differential responses to O2 , CO2 and the changes in pH.

Fast neurogenesis from carotid body quiescent neuroblasts accelerates adaptation to hypoxia

Unlike other neural peripheral organs, the adult carotid body (CB) has a remarkable structural plasticity, as it grows during acclimatization to hypoxia. The CB contains neural stem cells that can differentiate into oxygen-sensitive glomus cells. However, an extended view is that, unlike other catecholaminergic cells of the same lineage (sympathetic neurons or chromaffin cells), glomus cells can divide and thus contribute to CB hypertrophy. Here, we show that O₂-sensitive mature glomus cells are post-mitotic. However, we describe an unexpected population of pre-differentiated, immature neuroblasts that express catecholaminergic markers and contain voltage-dependent ion channels, but are unresponsive to hypoxia. Neuroblasts are quiescent in normoxic conditions, but rapidly proliferate and differentiate into mature glomus cells during hypoxia. This unprecedented "fast neurogenesis" is stimulated by ATP and acetylcholine released from mature glomus cells. CB neuroblasts, which may have evolved to facilitate acclimatization to hypoxia, could contribute to the CB oversensitivity observed in highly prevalent human diseases.

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.

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.

Short-term hypoxia increases tyrosine hydroxylase immunoreactivity in rat carotid body

Neurochemical and morphological changes in the carotid body are induced by chronic hypoxia, leading to regulation of ventilation. In this study, we examined the time courses of changes in immunohistochemical intensity for tyrosine hydroxylase (TH) and cellular volume of glomus cells in rats exposed to hypoxia (10% O(2)) for up to 24 hr. Grayscale intensity for TH immunofluorescence was significantly increased in rats exposed to hypoxia for 12, 18, and 24 hr compared with control rats (p<0.05). The transectional area of glomus cells was not significantly different between experimental groups. The TH fluorescence intensity of the glomus cells exhibited a strong negative correlation with the transectional area in control rats (Spearman's rho = -0.70). This correlation coefficient decreased with exposure time, and it was lowest for the rats exposed to hypoxia for 18 hr (rho = -0.18). The histogram of TH fluorescence intensity showed a single peak in control rats. The peaks were gradually shifted to the right and became less pronounced in hypoxia-exposed rats, suggesting that a hypoxia-induced increase in TH immunoreactivity occurred uniformly in glomus cells. In conclusion, this study demonstrates that short-term hypoxia induces an increase in TH protein expression in rat carotid body glomus cells.

Oxygen and carotid body chemotransduction: the cholinergic hypothesis - a brief history and new evaluation

Oxygen can be said to be the most fundamentally necessary substrate for life. In those organisms having a cardiopulmonary system for delivering it in blood to the tissues the carotid body functions as the principal detector of decreases in arterial oxygen. Such a decrease stimulates an increase in neural output from the carotid body to the nucleus tractus solitarii, and this can precipitate a wide array of systemic reflex responses. The neural mechanisms involved in the genesis of increased signal from the carotid body remain unclear. But a current model of carotid body chemotransduction postulates that transmitter-laden glomus cells initiate the neural activity by being depolarized by hypoxemia and releasing an excitatory transmitter which binds to postsynaptic receptors of the adjacent sensory afferent fibers as well as to presynaptic glomus cell autoreceptors. This Frontiers Review evaluates anew the data supporting the hypothesis that acetylcholine (ACh) is an (the) essential excitatory transmitter in this process by examining ACh's fulfillment of criteria required to establish a substance as a synaptic transmitter. All eight criteria are fulfilled in the case of ACh. Indeed, additional data further support the Cholinergic Hypothesis.

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.

Presence of nicotinic acetylcholine receptors in cat carotid body afferent system

With immunocytochemical techniques using a monoclonal antibody for alpha7 subunits of neuronal nicotinic acetylcholine receptors, we have found these subunits to be exclusively expressed in nerve fibers in the carotid body. Double-immunostaining showed that alpha7 subunit-positive nerve endings enveloped tyrosine hydroxylase-positive glomus cells. Some carotid sinus nerve fibers and tyrosine hydroxylase-positive petrosal ganglion neurons also expressed alpha7 subunits. These data support a role for acetylcholine in carotid body neurotransmission.

Stimulus-specific mobilization of dopamine and norepinephrine stores in cat carotid body

The catecholamines (CAs), dopamine (DA) and norepinephrine (NE), are synthesized and stored in carotid body chemosensory type I cells. Previous studies in our laboratory demonstrated that low concentrations of nicotine preferentially evoke the release of NE from rabbit type I cells, whereas hypoxia mobilizes DA and NE in proportion to their stores in the tissue. The primary objective of the present study was to examine whether hypoxia, nicotine and elevated concentrations (30 mM) of K+ evoke the preferential release of DA vs. NE from cat carotid bodies superfused in vitro. In this species, where tissue stores of DA and NE are nearly equal, hypoxia evoked the preferential release of DA from normal carotid bodies. This pattern of release evoked by low O2 was also present following chronic removal of the superior cervical ganglion, which eliminated NE contained in the sympathetic innervation to the carotid body. In contrast, nicotine and high-K+ preferentially mobilized NE in these sympathectomized animals. Sympathectomy also reduced the percent of DA (but not NE) content released from type I cells in response to any of the three stimuli. Our findings suggest that chemosensory type I cells possess stimulus-specific mechanisms for CA mobilization and that the sympathetic innervation modulates the metabolism and release of CAs in the cat carotid body.

Muscarinic and nicotinic receptors raise intracellular Ca2+ levels in rat carotid body type I cells

1. The effects of cholinergic agonists upon intracellular free Ca2+ levels ([Ca2+]i) have been studied in enzymically isolated rat carotid body single type I cells, using indo-1. 2. Acetylcholine (ACh) dose-dependently increased [Ca2+]i in 55% of cells studied (EC50 = 13 microM). These [Ca2+]i rises were partially inhibited by atropine or mecamylamine. 3. Specific nicotinic and muscarinic agonists also elevated [Ca2+]i in a dose-dependent manner (nicotine, EC50 = 15 microM; methacholine, EC50 = 20 microM). 4. While the majority of the ACh-sensitive cells responded to both classes of cholinergic agonist, 29% responded exclusively to nicotinic stimulation and 9% responded exclusively to muscarinic stimulation. 5. In the presence of nicotinic agonists, Ca2+i responses were transient. In the presence of muscarinic agonists, Ca2+i responses consisted of an initial rise, which then declined to a lower plateau level. 6. Nicotinic responses were rapidly abolished in Ca(2+)-free medium, suggesting that they are dependent on Ca2+ influx. 7. The plateau component of the muscarinic-activated response was also abolished in Ca(2+)-free conditions. The rapid initial [Ca2+]i rise, however, could still be evoked after several minutes in Ca(2+)-free medium. Muscarine also increased Mn2+ quenching of intracellular fura-2 fluorescence. These data suggest that the full muscarinic response depends on both Ca2+ release from intracellular stores and Ca2+o influx. 8. The results indicate that, in rat carotid body type I cells, both nicotinic and muscarinic acetylcholine receptors increase [Ca2+]i, but achieve this via different mechanisms. ACh may therefore play a role in carotid body function by modulating Ca2+i in the chemosensory type I cells.

Nicotinic acetylcholine receptors in isolated type I cells of the neonatal rat carotid body

Electrophysiological responses of enzymatically isolated type I cells from the neonatal rat carotid body to cholinergic agonists were examined using the whole-cell patch-clamp technique. Inward currents were evoked in cells clamped at -70 mV in response to bath-applied carbachol and two selective nicotinic agonists, nicotine and dimethylphenylpiperazinium. Muscarine failed to produce any change in membrane current. Responses to nicotine were concentration-dependent and also voltage-dependent, showing strong rectification positive to -40 mV. Currents evoked by nicotine were reduced or abolished in the presence of mecamylamine and also by high concentrations of atropine (10 or 100 microM). Under "current-clamp", nicotine was shown to depolarize type I cells, an effect which was only slowly reversible, but which could be rapidly attenuated by introduction of mecamylamine to the perfusate. In voltage-clamped cells, nicotine could evoke inward currents when extracellular Na+ was replaced by Ca2+. Our results demonstrate the presence of functional nicotinic acetylcholine receptors on type I cells of the neonatal rat carotid body. Activation of these receptors could lead to excitation of the intact carotid body by either of two possible mechanisms: depolarization of type I cells sufficient to open voltage-gated Ca2+ channels, or Ca2+ influx through the receptor pore itself. Either (or both) mechanisms could trigger catecholamine release from type I cells, which is a fundamental step in chemotransmission.

Characterization of cultured chemoreceptor cells dissociated from adult rabbit carotid body

Short-term cell cultures were obtained from enzymatically dissociated carotid bodies from adult rabbits, and morphological and functional characterization of the cultured chemoreceptor cells were carried out. Under phase contrast, freshly isolated type I cells are round, bright, and 10-14 microns in diameter and exhibit strong fluorescence when stained with the glyoxylic acid technique. The content of endogenous dopamine in the cultures increased from 80 pmol/10(5) cells 2 h after plating the cells to 200 pmol/10(5) cells on the 3rd day, and the rate of synthesis and storage of [3H]dopamine from the precursor [3H]tyrosine increased from 1.7 pmol.10(5) cells-1.h-1 in 1-day cultures to 4 pmol.10(5) cells-1.h-1 on the 3rd day; the later values represent 80-85% of the expected values for the intact carotid body. After labeling with [3H]tyrosine, cultured chemoreceptor cells release [3H]dopamine when challenged by hypoxia, high external K+, or the protonophore dinitrophenol, the pattern of response being similar to that of the intact carotid body. When studied by whole cell clamp recording, individual chemoreceptor cells exhibit a marked variability in the properties of some ionic currents; the data, however, do not support the existence of distinct subpopulations of type I cells.

The co-existence of biogenic amines and neuropeptides in the type I cells of the cat carotid body

The mammalian carotid body consists of preneural type I (glomus) cells synaptically coupled to afferent axon terminals and enveloped by type II (sustentacular) cells. Recent studies indicate the presence of multiple putative neurotransmitters in this arterial chemoreceptor organ. A double-labeling immunocytochemical technique was utilized which allows simultaneous visualization of two neurochemicals in a single cell. The issue of transmitter co-occurrence in type I cells of the cat carotid body was addressed using specific antibodies for seven neurochemical agents: tyrosine hydroxylase, dopamine-beta-hydroxylase, choline acetyltransferase, serotonin, substance P, met-enkephalin and chromogranin. A high degree (greater than 70%) of co-localization was observed for most pairs of markers, indicating the co-existence of multiple neuroactive agents in type I cells of the cat carotid body. The intensity of staining varied greatly among cells but formed a pattern. Thus, for tyrosine hydroxylase and dopamine-beta-hydroxylase, the majority of double-labeled type I cells exhibited equivalently low or high levels of both, while for the neuropeptides unequal levels of the two markers predominated. Neuropeptides also co-existed in type I cells with catecholamine-synthesizing enzymes and with serotonin. The functional significance of such patterns of multiple co-existence involving biogenic amines and neuropeptides is discussed. Our results indicate a high degree of co-occurrence of reaction product for amine-synthesizing enzymes (tyrosine hydroxylase, dopamine-beta-hydroxylase and choline acetyltransferase), the indoleamine serotonin, and the neuropeptides substance P and met-enkephalin.

Effects of relative hypoxia and hypercapnia on intracellular pH and membrane potential of cultured carotid body glomus cells

Clusters of glomus cells, isolated from rat carotid bodies, were cultured for up to 2 weeks. Afterwards, we simultaneously measured the intracellular pH (pHi) and membrane potentials (Em) of single cells with pH-sensitive and KCl-filled microelectrodes. In 89 control cells (bathed in saline equilibrated with 50% O2 in N2) pHi was 6.87 +/- 0.014 (SE) and Em -36.3 +/- 0.45 mV. In 42 cells, switching to air (about 20% O2) lowered pHi in 60% of them by as much as 0.14 unit (mean decrease, 0.05). In the remaining cells, pHi increased by as much as 0.18 unit (mean increase, 0.06). Application of 2.5% CO2 in 50% O2 (balance N2) reduced the pHi in 90% of 47 cells by as much as 0.44 unit (mean decrease, 0.14). pHi increased to a maximum of 0.05 unit (mean increase, 0.04) in the others. Either stimulus depolarized or hyperpolarized glomus cells (-5 to 8 mV) in approximately equal proportions. There was a significant and positive correlation between delta Em and delta pHi. This observation confirms the idea that the Em of glomus cell is H(+)-dependent. Results do not agree with the acidic hypothesis for chemoreception.

Co-existence of tyrosine hydroxylase and dopamine beta-hydroxylase immunoreactivity in glomus cells of the cat carotid body

Catecholamines are thought to play an important role in sensory transduction in the arterial chemoreceptors of the mammalian carotid body, and classical cytochemical techniques have demonstrated their presence in the type I (glomus) cells of this organ. However, it remains controversial whether dopamine (DA) and norepinephrine (NE) occur in the same or in different subtypes of glomus cells. In the present study, we have addressed this issue using immunocytochemistry to compare the localization of tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (D beta H) in the cat carotid body. Both pre- and post-embedding double-labelling immunohistochemical techniques were employed. TH and D beta H were found to co-exist in over 90% of the glomus cells, and they were co-localized at equivalent levels in almost 80% of the cells; less than 5% contained only TH. The results suggest that DA and NE are synthesized and stored in a common cell population in the cat carotid body.

Differential stimulus coupling to dopamine and norepinephrine stores in rabbit carotid body type I cells

Recent studies suggest that preneural type I (glomus) cells in the arterial chemoreceptor tissue of the carotid body act as primary transducer elements which respond to natural stimuli (low O2, pH or increased CO2) by releasing chemical transmitter agents capable of exciting the closely apposed afferent nerve terminals. These type I cells contain multiple putative transmitters, but the identity of the natural excitatory agents remains an unresolved problem in carotid body physiology. Characterization of putative transmitter involvement in the response to natural and pharmacological stimuli has therefore become fundamental to further understanding of chemotransmission in this organ. The present study demonstrates that a natural stimulus (hypoxia) evokes the release of dopamine (DA) and norepinephrine (NE) in approximate proportion to their unequal stores in rabbit carotid body (DA release/NE release = 8.2). In contrast, nicotine (100 microM), a cholinomimetic agent thought to act on the nicotinic receptors present on the type I cells, evokes the preferential release of NE (DA release/NE release = 0.17). These findings suggest that distinct mechanisms are involved in a differential mobilization of these two catecholamines from the rabbit carotid body.

Immunocytochemical localization of choline acetyltransferase in the carotid body of the cat and rabbit

The carotid body consists of afferent axon terminals in synaptic association with preneural type I cells and enveloping type II cells. The presence of acetylcholine (ACh) in this organ and its pharmacological actions are well established; however, its precise localization remains uncertain. In the present study, choline acetyltransferase was immunocytochemically localized to type I cells of the cat and rabbit. These data, combined with previous demonstrations of cholinergic receptor action, suggest that ACh may be involved in neurotransmitter coupling in the carotid body.

Localization of acetylcholinesterase in dissociated cell cultures of the carotid body of the rat

The localization of acetylcholinesterase (AChE) was investigated at the cellular and subcellular levels in dissociated cell cultures of the carotid body of the neonatal rat, prepared by the methods of Fishman and Schaffner (1984). In the presence of iso-OMPA, which blocks nonspecific cholinesterase, staining was confined almost exclusively to glomus-cell clusters and occasional isolated cells. These clusters grow as discrete islands scattered throughout the culture and display typical catecholamine (CA) fluorescence as in vivo. AChE staining was abolished or reduced by the cholinesterase inhibitors eserine (30-100 microM), or (the poorly lipid soluble) echothiophate (8 microM). Processing of the same culture sequentially for the demonstration of both AChE and CA revealed that glomus-cell clusters and individual glomus cells were consistently positive for both. In electron micrographs AChE reaction product was associated intracellularly with the nuclear envelope and cytoplasm of glomus cells (identified by their characteristic dense cored granules), as well as extracellularly with the boundaries of contiguous glomus cells. Significantly, reaction product occurred in some glomus cell profiles that had both dense-cored and clear (cholinergic-like) vesicles. These findings are discussed in the context of a possible dual (adrenergic/cholinergic) function status of glomus cells in the rat's carotid body.

Cat carotid body chemoreceptor responses before and after nicotine receptor blockade with alpha-bungarotoxin

The nature of nicotine receptors in the carotid body was studied in anesthetized, paralyzed and artificially ventilated cats. Chemoreceptor discharge in single or few-fiber preparations of the carotid sinus nerve was measured during isocapnic hypoxia, hyperoxic hypercapnia and in response to nicotine injections before and after administration of alpha-bungarotoxin (10 cats) and after alpha-bungarotoxin plus mecamylamine (7 cats) which binds to neuromuscular-type nicotine cholinergic receptors. alpha-Bungarotoxin caused a slight enhancement of the chemoreceptor response to hypoxia without affecting the chemoreceptor stimulation by nicotine. Mecamylamine (1-5 mg, i.v.), a ganglionic-type nicotinic receptor blocker, had no further effect on the response to hypoxia while it completely abolished the chemoreceptor stimulation by nicotine. Thus the nicotinic receptors in the cat carotid body which elicit excitation of chemosensory fibers appear to be of the ganglionic-type. Blockade of neuromuscular and ganglionic types of nicotinic receptors in the carotid body by alpha-bungarotoxin and mecamylamine does not attenuate the chemosensory responses to either hypoxia or hypercapnia. These nicotinic receptors therefore, do not appear to play an essential role in hypoxic or hypercapnic chemoreception in the cat carotid body.

Synthesis and release of catecholamines by the cat carotid body in vitro: effects of hypoxic stimulation

The role of catecholamines (CAs) in cat carotid body chemoreception has been controversial. On the basis of pharmacological experiments, it would appear that endogenous dopamine (DA) may act either as an inhibitory or excitatory transmitter. Neurochemical studies on the effects of natural stimulation on the release of carotid body CAs in the cat have also been inconclusive. In the present study, we have characterized the synthesis and release of CAs in the in vitro cat carotid body preparation in response to different levels of hypoxic stimulation and have correlated these measures with the chemosensory activity of the carotid sinus nerve. The synthesis of [3H]DA and [3H]norepinephrine was linear for at least 4 h in carotid bodies incubated with their natural precursor [3H]tyrosine. Synthesis of both [3H]CAs plateaued when the [3H]tyrosine concentration in the media reached 40 microM, which is a concentration similar to that found in cat plasma. Exposure of the animals to an atmosphere of 10% O2 in N2 for 3 h prior to removal and incubation of the carotid bodies with [3H]tyrosine resulted in an approximately 100% increase in the rate of [3H]DA synthesis but no change in [3H]norepinephrine synthesis. This selective increase in [3H]DA synthesis was not detected when [3H]dihydroxyphenylalanine was used as precursor. Carotid bodies first incubated with [3H]tyrosine and later superfused with solutions equilibrated with different gas mixtures (0-100% O2 in N2) exhibited an increase in [3H]DA release and carotid sinus nerve discharge which were inversely related to the oxygen concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium influx in cultured carotid body cells is stimulated by acetylcholine and hypoxia

Calcium influx was measured in cultured carotid body and glioma cells. In carotid body cells stimulation with acetylcholine (ACh, 10(-5) mol/l) increased the calcium influx to 135% of control values after 1 min and to 163% after 30 min. With a reduction of the pO2 to nearly zero calcium influx increased to 170% of control values. In glioma cells there was only a slight or no increase. This sensitivity of carotid body cells is discussed in relation to their function in chemoreception.

Localization and function of cat carotid body nicotinic receptors

Acetylcholine and nicotinic agents excite cat carotid body chemoreceptors and modify their response to natural stimuli. The present experiments utilized [125I]alpha-bungarotoxin [( 125I]alpha-BGT) to localize within the chemosensory tissue the possible sites of action of exogenous and endogenous nicotinic cholinergic substances. In vitro equilibrium binding studies of intact carotid bodies determined a Kd of 5.57 nM and a Bmax of 9.21 pmol/g of tissue. Chronic section (12-15 days) of the carotid sinus nerve (CSN) did not change the amount of displaceable toxin binding. In contrast, the specific binding was reduced by 46% following removal of the superior cervical ganglion. Light microscope autoradiography of normal, CSN-denervated and sympathectomized carotid bodies revealed displaceable binding sites concentrated in lobules of type I and type II cells. Treatment of carotid bodies with 50 nM alpha-BGT in vitro reduced by 50% the release of [3H]dopamine (synthesized from [3H]tyrosine) caused by hypoxia or nicotine, and also significantly reduced the stimulus-evoked discharges recorded from the CSN. The data suggest an absence of alpha-BGT binding sites on the afferent terminals of the CSN and that nicotinic receptors located with parenchymal cell lobules may modulate the release of catecholamines from these cells.

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.