Autoradiographic localization of muscarinic receptors in rabbit carotid body

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.

Responses induced by acetylcholine and ATP in the rabbit petrosal ganglion

Acetylcholine and ATP appear to mediate excitatory transmission between receptor (glomus) cells and the petrosal ganglion (PG) neuron terminals in the carotid body. In most species these putative transmitters are excitatory, while inhibitory effects had been reported in the rabbit. We studied the effects of the application of acetylcholine and ATP to the PG on the carotid nerve activity in vitro. Acetylcholine and ATP applied to the PG increased the carotid nerve activity in a dose-dependent manner. Acetylcholine-induced responses were mimicked by nicotine, antagonized by hexamethonium, and enhanced by atropine. Bethanechol had no effect on basal activity, but reduced acetylcholine-induced responses. Suramin antagonized ATP-induced responses, and AMP had little effect on the carotid nerve activity. Our results suggest that rabbit PG neurons projecting through the carotid nerve are endowed with nicotinic acetylcholine and purinergic P2 receptors that increase the carotid nerve activity, while simultaneous activation of muscarinic cholinergic receptors reduce the maximal response evoked by nicotinic cholinergic receptor activation.

Role of acetylcholine in neurotransmission of the carotid body

Acetylcholine (ACh) has been considered an important excitatory neurotransmitter in the carotid body (CB). Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca(2+) from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K(+) and Ca(2+) channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. We discuss these issues in light of current knowledge of neuroscience.

Oxygen sensing in the body

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

Identification of M1 and M2 muscarinic acetylcholine receptors in the cat carotid body chemosensory system

The carotid body is a major arterial chemoreceptor that senses low O2 tension, high CO2 tension and low pH in the arterial blood. It is generally believed that neurotransmitters, including acetylcholine (ACh), participate in the genesis of afferent neural output from the carotid body and modulate the function of chemoreceptor cells (glomus cells). Previous pharmacological studies suggest that M1 and M2 muscarinic ACh receptors (mAChRs) are involved in these processes. This study was designed to demonstrate the presence and localization of M1 and M2 mAChRs in the carotid body and in the petrosal ganglion of the cat. Since DNA sequences of the cat M1 and M2 mAChRs were not known, we first determined partial DNA sequences. These sequences and deduced amino acid sequences highly resembled those of human and the rat. Subsequent reverse transcription-polymerase chain reaction (RT-PCR)analysis has demonstrated that mRNAs for M1 and M2 mAChRs are present in the carotid body and the petrosal ganglion of the cat. Immunohistochemistry has indicated that the localization of these receptors appears different. Immunoreactivity for M1 mAChR was strong in nerves in the carotid body. Nerve endings positively stained for M1 mAChR appear to innervate glomus cells. Weak staining for M1 mAChRs was seen in glomus cells. On the other hand, M2 receptor protein seems to be present in glomus cells but not on nerve endings. One third of the neurons in the petrosal ganglion showed immunoreactivity for M1 mAChR. Many neurons and nerve fibers in the petrosal ganglion expressed M2 mAChR immunoreactivity. The results were consistent with previous pharmacological studies. Thus, activation of M1 mAChRs on afferent nerve endings may be linked to the increase in neural output during hypoxia. Further, M1 and M2 mAChRs on glomus cells modulate the release of neurotransmitters.

Expression of functional nicotinic acetylcholine receptors in neuroepithelial bodies of neonatal hamster lung

Pulmonary neuroepithelial bodies (NEB) are presumed airway chemoreceptors involved in respiratory control, especially in the neonate. Nicotine is known to affect both lung development and control of breathing. We report expression of functional nicotinic acetylcholine receptors (nAChR) in NEB cells of neonatal hamster lung using a combination of morphological and electrophysiological techniques. Nonisotopic in situ hybridization method was used to localize mRNA for the beta 2-subunit of nAChR in NEB cells. Double-label immunofluorescence confirmed expression of alpha 4-, alpha 7-, and beta 2-subunits of nAChR in NEB cells. The electrophysiological characteristics of nAChR in NEB cells were studied using the whole cell patch-clamp technique on fresh lung slices. Application of nicotine ( approximately 0.1-100 microM) evoked inward currents that were concentration dependent (EC50 = 3.8 microM; Hill coefficient = 1.1). ACh (100 microM) and nicotine (50 microM) produced two types of currents. In most NEB cells, nicotine-induced currents had a single desensitizing component that was blocked by mecamylamine (50 microM) and dihydro-beta-erythroidine (50 microM). In some NEB cells, nicotine-induced current had two components, with fast- and slow-desensitizing kinetics. The fast component was selectively blocked by methyllcaconitine (MLA, 10 nM), whereas both components were inhibited by mecamylamine. Choline (0.5 mM) also induced an inward current that was abolished by 10 nM MLA. These studies suggest that NEB cells in neonatal hamster lung express functional heteromeric alpha 3 beta 2, alpha 4 beta 2, and alpha 7 nAChR and that cholinergic mechanisms could modulate NEB chemoreceptor function under normal and pathological conditions.

Feedback Inhibition of Ca2+ Currents by Dopamine in Glomus Cells of the Carotid Body

The effect of dopamine on the voltage-dependent ionic channels of enzymatically dispersed glomus cells from rabbit carotid bodies was studied. Whole-cell currents were recorded on isolation with patch electrodes and dopamine applied to the bath solution. Dopamine at nanomolar concentrations produced a reversible attenuation of the calcium current whereas sodium and potassium currents remained unaltered. Dopamine inhibition of Ca2+ current was observed in all cells tested (n=48) and at a saturating concentration (1 microM) the average reduction was of 40 +/- 6.5% (n=8). The effect of dopamine was probably caused by a decrease in the number of channels activatable on depolarization since it did not modify the voltage-dependent parameters of the current. These results indicate that dopamine, which is the major transmitter secreted by glomus cells, regulates further transmitter release by feedback inhibition of Ca2+ channels.

Muscarinic modulation of hypoxia-induced release of catecholamines from the cat carotid body

Chemotransduction of arterial hypoxemia by the cat carotid body is generally thought to begin with a hypoxia-induced depolarization of the glomus cells (GCs) of the carotid body (CB). This depolarization activates voltage-gated calcium channels with the subsequent entry of calcium, movement of transmitter-containing vesicles to the synaptic-like juncture between the GC and apposed sensory afferent neuron. The vesicles exocytotically release their transmitters which then proceed to the receptors on both the postsynaptic neuron and on the GCs themselves (autoreceptors). Action potentials and their modulation in the sensory fibers are the result, along with the modulation of further neurotransmitter release from the GCs. The purpose of the present study was to: (1) determine the parameters of an incubated cat CB preparation capable of releasing measurable amounts of catecholamines (CAs) in response to hypoxia; (2) determine the impact of muscarinic activities on CA release during the hypoxic challenge; (3) determine if the muscarinic activity preferentially modified the release of one CA more than another; (4) determine if there were any differences in the pattern of hypoxia-induced release of dopamine (DA) vs. norepinephrine (NE). CBs were harvested from deeply anesthetized cats. Cleaned of fat and connective tissue, they were incubated in Krebs Ringer bicarbonate solution at 37 degrees C, and bubbled with a hyperoxic mixture of gases (95% O(2)-5% CO(2)) for 30 min. The first series of experiments to address the CB's hypoxia-induced release of CAs explored the effects of incubating CBs for 2 h with hyperoxia vs. normoxia (21% O(2)-6% CO(2)) followed by a 30 min hypoxic challenge, with or without L-dihydroxyphenylalanine (L-DOPA). In the second series of experiments the CBs, after the first 30 min of hyperoxia, were next challenged with hypoxia (4% O(2)-5% CO(2)) for intervals of 3-20 min with intervening recovery periods of hyperoxia to determine the effect of the duration of the hypoxic exposure on CA release. In the third series of experiments the CBs, after the first 30 min of hyperoxia, were challenged with hypoxia for intervals of 10-40 min in the presence or absence of an M1 or M2 muscarinic receptor antagonist. CAs released into the incubation medium were analyzed by means of high performance liquid chromatography-electrochemical detection using standard procedures. Incubated cat CBs challenged for 2 h with hyperoxia followed by 30 min of hypoxia, released much more measurable amounts of CAs in the presence of 40 microM L-DOPA than without it. Moving from hyperoxia to hypoxia produced a better yield than moving from normoxia to hypoxia, and at least 10-20 min exposures were needed for measurable amounts of CAs. The M1 muscarinic receptor antagonist, pirenzepine, reduced the hypoxia-induced release of CAs during each exposure. Further, the reduction appeared to be dose-related. The M2 muscarinic receptor antagonist, methoctramine, enhanced the hypoxia-induced release of CAs during each exposure. These data support a role for acetylcholine (ACh) in the hypoxia-induced release of CAs, and suggest a significant, if modest, muscarinic dimension to it. And although hypoxia induced a greater release of DA than of NE, the muscarinic modulation of the release (both decreasing it and increasing it) may have had a greater impact on NE release than on DA release. Finally, the patterns of hypoxia-induced release of DA and NE from incubated cat carotid bodies are significantly different.

Acetylcholine is released from in vitro cat carotid bodies during hypoxic stimulation

Previous pharmacological, immunocytochemical, electrophysiological, and microfluorometric studies have suggested that acetylcholine (ACh) is a critically important excitatory transmitter in the chemotransduction of hypoxia by the cat carotid body (CB). With the use of HPLC this study shows that the in vitro cat CB releases ACh under normoxic conditions; this release is increased when the CB is challenged with hypoxia. The preliminary observation that greater amounts of ACh are liberated in the presence of gallamine and AFDX116 suggests the presence of functioning M2 muscarinic receptors on the glomus cells of the CB.

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.

Acetylcholine release from cat carotid bodies

Hypoxia, hypercapnia and acidosis stimulate the carotid body (CB) sending increased neural activity via a branch of the glossopharyngeal nerve to nucleus tractus solitarius; this precipitates an impressive array of cardiopulmonary, endocrine and renal reflex responses. However, the cellular mechanisms by which these stimuli generate the increased CB neural output are only poorly understood. Central to the understanding of these mechanisms is the determination of which agents are released within the CB in response to hypoxia, and serve as the stimulating transmitter(s) for chemosensory nerve endings. Acetylcholine (ACh) has been proposed as such an agent from the outset, but this proposal has been, and remains, controversial. The present study tests two hypotheses: (1) The CB releases ACh under normoxic/normocapnic conditions; and (2) The amount released increases during hypoxia and other conditions known to increase neural output from the CB. These hypotheses were tested in 12 experiments in which both CBs were removed from the anesthetized cat and incubated at 37 degrees C in a physiological salt solution while the solution was bubbled with four different concentrations of oxygen and carbon dioxide. The incubation medium was exchanged at 10 min intervals for 30 min (three periods of incubation). The medium was analyzed with high performance liquid chromatography-electrochemical detection for ACh content. Normoxic/normocapnic conditions (21% O2/6% CO2) produced a total of 0.639 +/- 0.106 pmol/150 microl (mean +/- S.E.M.; n = 12). All stimulating conditions produced larger total outputs: 4% O2/2% CO2 produced 1.773 +/- 0.46 pmol/150 microl; 0% O2/5% CO2, 0.868 +/- 0.13 pmol/150 microl; 4% O2/10% CO2, 1.077 +/- 0.21 pmol/150 microl. These three amounts were significantly greater than the normoxic/normocapnic condition, but indistinguishable among themselves. Further, the amount of ACh released did not diminish over the 30 min of stimulation. These data support the concept that during hypoxia ACh functions as a stimulating transmitter in the CB, and are consistent with the earlier reports of cholinergic enzymes and receptors found in the CB.

Further cholinergic aspects of carotid body chemotransduction of hypoxia in cats

From the 1930s into the 1970s, the role of acetylcholine (ACh) in the carotid body's chemotransduction of hypoxia was debated. Since the late 1970s, the issue has been pursued only intermittently or not at all. The purpose of this study was to test again with a new preparation the hypothesis that ACh is an excitatory neurotransmitter in the cat carotid body's chemotransduction of hypoxia. We tested the effect of the specific nicotinic blocker mecamylamine and the muscarinic blocker of all five muscarinic receptors, atropine. We further tested the effects of M1 and M2 muscarinic-receptor blockers. The carotid body region was selectively perfused with hypoxic Krebs-Ringer bicarbonate (KRB) solutions that were blocker free or contained varying doses of the blockers. Both mecamylamine and atropine reduced the response to hypoxic KRB in a dose-related manner. The M2 muscarinic-receptor blockers gallamine and AFDX 116 increased the response to hypoxic KRB, whereas the M1 muscarinic-receptor blocker pirenzepine reduced the response to hypoxic KRB. These data are consistent with an excitatory role for ACh in the carotid body chemotransduction of hypoxia in the cat.

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.

Effects of M1-selective antimuscarinics on respiratory chemosensitivity in humans

We examined effects of selective M1 antagonists on hypercapnic and hypoxic ventilatory responses in 17 healthy human volunteers. Subjects were intravenously treated with placebo, pirenzepine (10 mg) and biperiden lactate (4 mg) on three separate days in a randomized double-blind design. Ventilatory responses to hyperoxic progressive hypercapnia and isocapnic progressive hypoxia were studied after the drug administration. There were no statistically significant differences in the mean delta VE/delta PET CO2 or delta VE/delta SaO2 among the three treatments. However, the delta VE/delta PET CO2 with placebo negatively correlated with the difference in delta VE/delta PET CO2 between the biperiden and placebo studies (r=-0.65, P < 0.01), but not with that between the pirenzepine and placebo studies. On the other hand, the delta VE/delta SaO2 with placebo negatively correlated with the difference in delta VE/delta SaO2 between the pirenzepine and placebo studies (r = -0.79, P < 0.001), but not with that between the biperiden and placebo studies. These data suggest the possible involvement of M1 cholinergic receptors in the central CO2 and peripheral O2 sensing mechanisms in humans, although the degree of its involvement is not consistent among subjects. These findings may explain the interindividual variation in the control of breathing in humans.

Immunocytochemical localization of cAMP and cGMP in cells of the rat carotid body following natural and pharmacological stimulation

Although the chemoreceptive function of the carotid body has been known for many decades, the cellular mechanisms of sensory transduction in this organ remain obscure. Common elements in the transductive processes of many cells are the cyclic nucleotide second messengers, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Studies from our laboratory have revealed stimulus-induced changes in cyclic nucleotide levels in the carotid body as measured by RIA, but such changes in second messenger levels have not been localized to specific cellular elements in the organ. The present immunocytochemical study utilized the avidin-biotin-peroxidase method to investigate the distribution of cAMP and cGMP in the rat carotid body and to assess changes in the intensity of immunostaining following in vitro stimulation by hypoxia, forskolin, sodium nitroprusside, high potassium, and atrial natriuretic peptide. Both cAMP and cGMP immunoreactivity were localized to type I cells of organs maintained in vivo and fixed by perfusion. Organs exposed to 100% O2-equilibrated media in vitro produced low but visible levels of cAMP immunoreactivity in a majority of type I cells; hypoxia (5% O2-equilibrated media) for 10 min moderately increased the level of immunoreactivity; forskolin (10(-5) M), or forskolin combined with hypoxia, dramatically increased cAMP levels in virtually all cells. Moderate levels of cGMP immunoreactivity in control carotid bodies in vitro were strikingly reduced by hypoxia; a significant increase in cGMP levels occurred following incubation in high potassium (100 mM), and under these conditions, the decrease in cGMP immunoreactivity with hypoxia was much more pronounced.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic receptor localization and function in rabbit carotid body

Acetylcholine and muscarinic agonists inhibit chemosensory activity in the rabbit carotid sinus nerve (CSN). Because the mechanism of this inhibition is poorly understood, we have investigated the kinetics and distribution of muscarinic receptors in the rabbit carotid body with the specific muscarinic antagonist [3H]quinuclidinylbenzilate ([3H]QNB). Equilibrium binding experiments identified displaceable binding sites (1 microM atropine) with a Kd = 71.46 pM and a Bmax = 9.23 pmol/g tissue. These binding parameters and the pharmacology of the displaceable [3H]QNB binding sites are similar to specific muscarinic receptors identified in numerous other nervous, muscular and glandular tissues. Comparisons of specific binding in normal and chronic CSN-denervated carotid bodies suggest that muscarinic receptors are absent on afferent terminals in the carotid body; however, nearly 50% of the specific [3H]QNB binding is lost following chronic sympathectomy, suggesting the presence of presynaptic muscarinic receptors on the sympathetic innervation supplying the carotid body vasculature. Autoradiographic studies have localized the remainder of [3H]QNB binding sites to lobules of type I and type II parenchymal cells. In separate experiments, the muscarinic agonists, oxotremorine (100 microM) stimulation of the in vitro carotid body. Our data suggest that muscarinic inhibition in the rabbit carotid body is mediated by receptors located on type I cells which are able to modulate the excitatory actions of acetylcholine at nicotinic sites.

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.

Effects of Putative Neurotransmitters of the Carotid Body on its Own Glomus Cells

Carotid body glomus cells produce and release acetylcholine (ACh), catecholamines, and neuropeptides, and there is biochemical evidence that these cells possess receptors for these substances. Thus, we studied the effects of cholinergics [ACh, nicotine (Nic), bethanechol (BN)] and peptides [met-enkephalin (ME), substance P (SP)] on the membrane potential (Em), voltage noise (Erms), and input resistance (Ro) of glomus cells. Sliced carotid bodies (for cell visualization) of cats, rabbits, and mice were used. The mean Em and Ro of rabbit glomus cells were lower than those of cat and mouse. Ro of mouse cells was the largest, whereas Erms was similar in all species. The various agents had qualitatively similar effects on the cells of the three species although some quantitative differences were sometimes observed. But, for simplicity, results were pooled. ACh depolarized most cells (effect depressed by zero [Ca2+]o and Mn2+), reduced their resistance, and induced variable changes in Erms. Different ACh doses produced non-linear effects on DeltaEm. Nic and BN also depolarized most cells, reducing Ro and Erms. Atropine depressed the cell responses to BN; alpha-bungarotoxin the depolarizing response to Nic. ME and SP depolarized most cells, but only ME significantly reduced Ro. Neither peptide significantly changed voltage noise. Comparing the effects of all drugs showed that BN was the most effective depolarizing agent, producing the largest reductions in Ro. There were negative correlations between DeltaEm and DeltaRo with the cholinergics and SP; correlations between DeltaErms and DeltaRo were significant and positive only with the cholinergics. These results confirm the presence of nicotinic, muscarinic, and peptidergic receptors in glomus cells. The similar effects of cholinergics and peptides and those of flow interruption and anoxia suggest that the latter may partly act via autoreceptors for the released transmitters.

Effects of hypoxia on cyclic nucleotide formation in rabbit carotid body in vitro

The present experiments measured cAMP and cGMP in the arterial chemosensory tissue of the rabbit carotid body exposed for 10 min in vitro to normoxic or hypoxic conditions, or to specific activators of adenylate cyclase (forskolin) and guanylate cyclase (sodium nitroprusside). The enzyme activators elevated the basal levels of cAMP (48 x) and cGMP (3.7 x), respectively. Hypoxic media increased cAMP in the carotid body by 3.6-fold, but the levels of cGMP were reduced by 33% in media equilibrated with low O2. The data are consistent with the notion that cyclic nucleotides are involved in the transduction of natural stimuli and/or the neurotransmitter feedback modulation of chemosensory type I cells.

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.