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Oxygen (O2) and Carbon Dioxide (CO2) are the two gases to be detected and controlled. Of interest might be a query of the evolutionary origin of each. From the cooling of the Big Bang (~13.8 Billion Years Ago [BYA]) came a quark-gluon plasma from which protons and neutrons emerged, producing H, He, Li. As H and He collapsed into the first stars at ~13.3 BYA carbon and monatomic oxygen were generated. Some 3 billion years ago greater amounts of diatomic oxygen (O2) were provided by earth’s photosynthesizing bacteria until earth’s atmosphere had sufficient amounts to sustain the life processes of multicellular animals, and finally higher vertebrates. Origin of CO2 is somewhat unclear, though it probably came from the erupting early volcanoes. Photosynthesis produced sugars with O2 a waste product. Animal life took sugars and O2 needed for life. Clearly, animal detection and control of each was critical. Many chapters involving great heroes describe phases involved in detecting each, both in the CNS and in peripheral detectors. The carotid body (CB) has played a crucial role in the detection of each. What reflex responses the stimulated CB generates, and the mechanisms as to how it does so have been a fascinating story over the last 1.5 centuries, but principally over the last 50 years. Explorations to detect these gases have proceeded from the organismal/system/ organ levels down to the sub-cell and genetic levels.

The carotid body (CB) is a tiny structure, bilaterally located at the bifurcation of the common carotid artery into its internal and external branches, is innervated by a branch of cranial nerve IX, and sends its neural output to the nucleus tractus solitarius in the medulla. Arterial flow through the CB is the highest of any organ ever measured. The CB is the unique sensor of decreases in PaO2, but is also stimulated by low glucose as well as increases in PaCO2, [H+]a, temperature, and osmolarity. Hypoxia-generated increases in neural activity result from K+ channels in the CB's neurotransmitter-containing glomus (Type I) cells being blocked; this depolarizes the cells allowing calcium to enter, which promotes the movement of the glomus cells' transmitter-containing vesicles to exocytose ACh and ATP into the gap between cell and an abutting afferent fiber, where these agents bind to cholinergic and purinergic postsynaptic receptors. Stimulation of the CB's provokes reflex responses in the respiratory, cardiovascular, endocrine and renal systems. Recent research has identified the CB's malfunctioning in chronic heart failure (CHF). The decreased blood flow in this pathology reduces shear stress on the endothelial cells of the CB's vasculature, reducing the level of critically important molecules, and making the CB hypersensitive. The increased neural output from the CB's promotes an increase in output of the sympathetic nervous system which has a deleterious impact on breathing, cardiovascular and renal function. Techniques have been found in animal models that reduce the CB's impact during CHF. The very large role of this tiny structure in both health and disease underline its organismal importance.

Do cat carotid bodies (CBs) increase their release of acetylcholine and ATP in response to H2S? Two CBs, incubated in a Krebs Ringer bicarbonate solution at 37 °C, exhibited a normal response to hypoxia—increased release of acetylcholine (ACh) and ATP. They were challenged with several concentrations of Na2S, an H2S donor. H2S, a new gasotransmitter, is reported to open KATP channels. Under normoxic conditions the CBs reduced their release of ACh and ATP below control values. They responded identically to pinacidil, a well-known KATP channel opener. CB glomus cells exhibited a positive immunohistochemical signal for cystathione-β-synthetase, a H2S synthesizing enzyme, and for a subunit of the KATP channel. The data suggest that Na2S may have opened the glomus cells’ KATP channels, hyperpolarizing the cells, thus reducing their tonic release of ACh and ATP. Since during hypoxia H2S levels rise, the glomus cells responding very actively to hypoxia may be protected from over-exertion by the H2S opening of the KATP channels.

The carotid body (CB) is a polymodal chemosensor of arterial blood located next to the internal carotid artery. The basic chemosensing unit is composed of the neurotransmitter (NT)-containing glomus cells (GCs) and the sensory afferent fibers synapsing onto the GCs. Nicotinic and muscarinic receptors have been found on both the sensory afferent fibers and on the GCs. Neural output from the CB (CBNO) increases when arterial blood perfusing it is hypoxic, hypoglycemic, hypercapnic, or acidic. The increased CBNO due to GC release of excitatory NTs must be preceded by an entrance of calcium into the GCs. With repeated release of ACh from the GCs, cholinergic receptors could become desensitized, particularly nicotinic receptors which function as calcium channels. The purpose of the present study was to see if adenosine (ADO), known to alter receptor sensitivities, could attenuate or eliminate any desensitization of the nicotinic receptors occurring during the repeated application of ACh. Cat CBs were harvested with techniques approved by the University's Animal Care/Use Committee. The GCs were cultured and prepared for detecting [Ca(++)](i) with standard techniques. Repeated application of ACh produced a progressively decreasing increase in [Ca(++)](i). With the use of ADO or an A2(A) ADO receptor agonist the decrease was avoided. Though ADO also increased GC [Ca(++)](i), the sum of ADO increase and ACh increase, when superfused separately, was less than the increase when they were both included in the same superfusion. This suggested the possible involvement of a new path in the action. Potential mechanisms to explain the phenomena are discussed.

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.

The carotid body (CB) is the primary hypoxic chemosensory organ. Its hypoxic response appears to be genetically controlled. We have hypothesized that: 1) genes related to CB function are expressed less in the A/J mice (low responder to hypoxia) compared with DBA/2J mice (high responder to hypoxia); and 2) gene expression levels of morphogenic and trophic factors of the CB are significantly lower in the A/J mice than DBA/2J mice. This study utilizes microarray analysis to test these hypotheses. Three sets of CBs were harvested from both strains. RNA was isolated and used for global gene expression profiling (Affymetrix Mouse 430 v2.0 array). Statistically significant gene expression was determined as a minimum six counts of nine pairwise comparisons, a minimum 1.5-fold change, and P ≤ 0.05. Our results demonstrated that 793 genes were expressed less and that 568 genes were expressed more in the A/J strain vs. the DBA/2J strain. Analysis of individual genes indicates that genes encoding ion channels are differentially expressed between the two strains. Genes related to neurotransmitter metabolism, synaptic vesicles, and the development of neural crest-derived cells are expressed less in the A/J CB vs. the DBA/2J CB. Through pathway analysis, we have constructed a model that shows gene interactions and offers a roadmap to investigate CB development and hypoxic chemosensing/chemotransduction processes. Particularly, Gdnf, Bmp2, Kcnmb2, Tph1, Hif1a, and Arnt2 may contribute to the functional differences in the CB between the two strains. Bmp2, Phox2b, Dlx2, and Msx2 may be important for the morphological differences.

As a counterpoint to the volumes of beautiful work exploring how the carotid bodies (CBs) sense and transduce stimuli into neural traffic, this study explored one organismal reflex response to such stimulation. We challenged the anesthetized, paralyzed, artificially ventilated cat with two forms of acute hypoxemia: 10 % O(2)/balance N(2) (hypoxic hypoxia [HH] and carbon monoxide hypoxia [COH]). HH stimulates both CBs and aortic bodies (ABs), whereas COH stimulates only the ABs. Our design was to stimulate both with HH (HHint), then to stimulate only the ABs with COH (COHint); then, after aortic depressor nerve transaction, only the CBs with HH (HHabr), and finally neither with COH (COHabr). We recorded whole animal responses from Group 1 cats (e.g., cardiac output, arterial blood pressure, pulmonary arterial pressure/and vascular resistance) before and after sectioning the aortic depressor nerves. From Group 2 cats (intact) and Group 3 cats (aortic body resected) we recorded the vascular resistance in several organs (e.g., brain, heart, spleen, stomach, pancreas, adrenal glands, eyes). The HHint challenge was the most effective at keeping perfusion pressures adequate to maintain homeostasis in the face of a systemic wide hypoxemia with locally mediated vasodilation. The spleen and pancreas, however, showed a vasoconstrictive response. The adrenals and eyes showed a CB-mediated vasodilation. The ABs appeared to have a significant impact on the pulmonary vasculature as well as the stomach. Chemoreceptors via the sympathetic nervous system play the major role in this organism's response to hypoxemia.

Respiratory and Metabolic Acidosis can result from a variety of diseases and be sensed by receptors in the respiratory control system. But acid/base imbalance also directly affects the performance ...

The carotid body (CB) is a sensor of oxygen, carbon dioxide, hydrogen ion, and glucose in the arterial blood. Many studies of the CB's responses to low oxygen (hypoxia) have been reported. Recently attention has been increasingly focused on its responses to elevated CO2 (hypercapnia). An increase in ventilation or carotid body neural output (CBNO) can result from stimulating the CB with blood or perfusion fluids having an elevated CO2 or H+. The increase in ventilation seen with a hypoxic stimulus is accompanied with an increase in CBNO and an increased release of both acetylcholine (ACh) and ATP from the CB. The present in vitro study using both CBs harvested from six cats was undertaken to determine if hypercapnia also provoked an increased release of ACh from the incubated CBs. The anesthetizing, handling, and euthanizing of the animals were according to the guidelines of the Johns Hopkins Animal Care and Use Committee which are totally consonant with those of the NIH. CBs, once harvested and prepared for the experimental protocol, were subjected to the following steps each lasting 10 min: (1) control; (2) stress; (3) recovery. The stresses were respiratory acidosis (RAC; acidic hypercapnia), compensated respiratory acidosis (CRAC; isohydric hypercapnia), and metabolic acidosis (MtAC). The first and last forms of acidosis generated small but significant increases in the release of ACh from the CBs; the second generated a very small and insignificant increase in ACh release. Since it is generally accepted that ACh is a key excitatory neurotransmitter in the CB along with ATP, these data are consistent with other studies measuring the increase in ventilation in response to a small increase in CO2 and those studies recording CBNO in response to hypercapnia. In five of the six animals the responses to RAC and MtAC were compared to the responses to hypoxia. The latter were statistically indistinguishable from the former two.

NO is known to reduce the hypoxia-induced increase in carotid body neural activity (CBNA). Acetylcholine (ACh), a known excitatory transmitter in the cat carotid body (CB), is released during hypoxia. This study addressed the impact of an NO precursor on ACh release during hypoxia. Both CBs from nine cats were prepared for incubation, then inserted into a medium and bubbled with three consecutive gas mixtures, hyperoxic, hypoxic, and a final hyperoxic mixture. This series of exposures was performed in the absence of l -arginine, followed by the three exposures in a 1 mM l -arginine medium, and followed, thirdly, in a 10 mM l -arginine medium. l -Arginine significantly attenuated the hypoxia-induced release of ACh. Two post-arginine procedures suggested strongly that the reduction in the ACh release was not due to a gradual exhaustion of carotid body ACh stores over the course of the experiment. The data are consistent with those reports showing that NO donors and precursors reduce the hypoxia-induced increase in CBNA, and further support a role for ACh in the hypoxia-induced increase in CBNA.

In a previous study, DBA/2J and A/J inbred mice showed extremely different hypoxic ventilatory responses, suggesting variations in their carotid bodies. We have assessed the morphological and functional differences of the carotid bodies in these mice. Histological examination revealed a clearly delineated carotid body only in the DBA/2J mice. Many typical glomus cells and glomeruli appeared in the DBA/2J but not in the A/J mice. The size of the carotid body in the DBA/2J and A/J mice was 6.3 ± 0.5 × 106and 1.5 ± 0.3 × 106μm3, respectively. The area immunostained for tyrosine hydroxylase, an estimation of the glomus cell quantity, was four times larger in the DBA/2J mice than in the A/J mice. The individual data points in the DBA/2J mice segregated from those in the A/J mice. ACh increased intracellular Ca2+in most clusters (81%) of cultured carotid body cells from the DBA/2J mice, but only in 18% of clusters in the A/J mice. These data suggest that genetic determinants account for the strain differences in the structure and function of the carotid body.

The purpose of this study was to determine if H2S relaxed human airway smooth muscle cells. If so, we explored the mechanisms responsible for the phenomenon. Human airway smooth muscle cells (HASM) were prepared from human bronchi obtained from lungs unsuitable for transplantation. Dynamic changes in stiffness were measured as an indicator of contraction and relaxation of isolated HASM, using Magnetic Twisting Cytometry (MTC). Acute exposure of the cells to two agents capable of generating H2S, Na2S and GYY4137, decreased cell stiffness in a dose-dependent fashion, with maximal relaxation at 10mM and 5mM, respectively. In addition, cells exposed to varying doses of GYY4137 for 24 hours exhibited dose-dependent decreases in cell stiffness that were substantially greater than acute responses (21% relaxation at 300s vs 50% relaxation at 24h). We further determined that, like vascular smooth muscle, HASM exhibited the presence of ATP-sensitive K channels and the H2S synthesizing enzyme, cystathionine-g-lyase....

Here we explored the impact of hydrogen sulfide (H2S) on biophysical properties of the primary human airway smooth muscle (ASM)–the end effector of acute airway narrowing in asthma. Using Magnetic Twisting Cytometry (MTC), we measured dynamic changes in the stiffness of isolated ASM, at the single-cell level, in response to varying doses of GYY4137 (1–10 mM). GYY4137 slowly released appreciable levels of H2S in the range of 10–275 µM, and H2S released was long lived. In isolated human ASM cells, GYY4137 acutely decreased stiffness (i.e. an indicator of the single-cell relaxation) in a dose-dependent fashion, and stiffness decreases were sustained in culture for 24h. Human ASM cells showed protein expressions of cystathionine-γ-lyase (CSE; a H2S synthesizing enzyme) and ATP-sensitive potassium (KATP) channels. The KATP channel opener pinacidil effectively relaxed isolated ASM cells. In addition, pinacidil-induced ASM relaxation was completely inhibited by the treatment of cells with the KATP channel blocker glibenclamide. Glibenclamide also markedly attenuated GYY4137-mediated relaxation of isolated human ASM cells. Taken together, our findings demonstrate that H2S causes the relaxation of human ASM and implicate as well the role for sarcolemmal KATP channels. Finally, given that ASM cells express intrinsic enzymatic machinery of generating H2S, we suggest thereby this class of gasotransmitter can be further exploited for potential therapy against obstructive lung disease.

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.

Shirahata, Machiko, Robert S. Fitzgerald, and James S. K. Sham. Acetylcholine increases intracellular calcium of arterial chemoreceptor cells of adult cats. J. Neurophysiol. 78: 2388–2395, 1997. Several neurotransmitters have been reported to play important roles in the chemoreception of the carotid body. Among them acetylcholine (ACh) appears to be involved in excitatory processes in the cat carotid body. As one of the steps to elucidate possible roles of ACh in carotid body chemoreception in the cat, we examined the effect of ACh on intracellular calcium concentration ([Ca2+]i) of cultured carotid body cells. The carotid body from adult cats was dissociated and cultured for up to 2 wk. [Ca2+]iwas measured from clusters of cells with a microfluorometric technique using Indo-1 AM. Experiments were performed at 37°C, and cells were continuously superfused with modified Krebs solutions equilibrated with 5% CO2-16% O2-79% N2. ACh (100 μM) caused a marked increase in [Ca2+]iin ∼70% of clusters, and the responses to 1–300 μM of ACh were concentration dependent. The magnitude and kinetics of the ACh response were mimicked by the application of nicotine, whereas muscarinic agonists, pilocarpine, and muscarine failed to evoke a similar response. ACh-induced increase in [Ca2+]iwas dependent on extracellular Ca2+: it was greatly reduced or completely abolished by a transient removal of extracellular Ca2+. The response was consistently but only partially reduced by caffeine (5 mM) or nifedipine (10 μM). The effect of mecamylamine (100 μM) was inhibitory but small. Moreover, the increase in [Ca2+]iin response to ACh was also observed in some clusters that did not respond to high K (100 mM) Krebs. These results suggest that ACh increases [Ca2+]iof cultured carotid body cells by activating neuronal nicotinic ACh receptors, leading to Ca2+influx via nicotinic channels. In addition, other pathways such as Ca2+influx through L-type calcium channels, perhaps secondary to membrane depolarization, and Ca2+release from intracellular stores may participate in increasing [Ca2+]iin response to ACh. Muscarinic receptors appear to play only a small role, if any.

Fitzgerald, Robert S., Machiko Shirahata, and Tohru Ide.Further cholinergic aspects of carotid body chemotransduction of hypoxia in cats. J. Appl. Physiol.82(3): 819–827, 1997.—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.

Tankersley, Clarke G., Robert S. Fitzgerald, Roy C. Levitt, Wayne A. Mitzner, Susan L. Ewart, and Steven R. Kleeberger.Genetic control of differential baseline breathing pattern. J. Appl. Physiol. 82(3): 874–881, 1997.—The purpose of the present study was to determine the genetic control of baseline breathing pattern by examining the mode of inheritance between two inbred murine strains with differential breathing characteristics. Specifically, the rapid, shallow phenotype of the C57BL/6J (B6) strain is consistently distinct from the slow, deep phenotype of the C3H/HeJ (C3) strain. The response distributions of segregant and nonsegregant progeny were compared with the two progenitor strains to determine the mode of inheritance for each ventilatory characteristic. The BXH recombinant inbred (RI) strains derived from the B6 and C3 progenitors were examined to establish strain distribution patterns for each ventilatory trait. To establish the mode of inheritance, baseline breathing frequency (f), tidal volume, and inspiratory time (Ti) were measured five times in each of 178 mature male animals from the two progenitor strains and their progeny by using whole body plethysmography. With respect to f and Ti, the two progenitor strains were consistently distinct, and segregation analyses of the inheritance pattern suggest that the most parsimonious genetic model for response distributions of f and Ti is a two-loci model. In similar experiments conducted on 82 mature male animals from 12 BXH RI strains, each parental phenotype was represented by one or more of the RI strains. Intermediate phenotypes emerged to confirm the likelihood that parental strain differences in f and Ti were determined by more than one locus. Taken together, these studies suggest that the phenotypic difference in baseline respiratory timing between male B6 and C3 mice is best explained by a genetic model that considers at least two loci as major determinants.

Among the several topics included in respiratory studies investigators have focused on the control of breathing for a relatively few number of years, perhaps only the last 75 to 80. For a very long time, the phenomenon of respiration presented a great mystery. The Chinese had suggestions for proper breathing, and later the Egyptians sought to understand its purpose. But in the western world, the early Greeks made the more significant observations. Centuries passed before the anatomical structures pertinent to respiration were properly visualized and located. There followed efforts to understand if lung movement was necessary for life and what happened in the lung. The rise of chemistry in the 18th century eventually clarified the roles of the gases significant in respiratory behavior. More time was needed to understand what gases provoked increases in breathing and where those gases worked. At this point, control of breathing became a significant focus of respiratory investigators. Studies included identifying the structures and functions of central and peripheral chemoreceptors, and airway receptors, sources of respiratory rhythm and pattern generation, the impact of the organism's status on its breathing including environment and disease/trauma. At this same time, mid- to late-20th century, efforts to mathematicize the variables in the control of breathing appeared. So though wonderment about the mysterious phenomenon of respiration began over two millennia ago, serious physiological investigation into its control is by comparison very young.

This study aimed to dissect the roles played by the autonomic interoreceptors, the carotid bodies (cbs) and the aortic bodies (abs) on the vascular resistances of several organs in anesthetized, paralyzed, artificially ventilated cats challenged by systemic hypoxemia. Two 15 min challenges stimulated each of 5 animals in two different groups: (1) in the intact group hypoxic hypoxia (10% O2 in N2; HH) stimulated both abs and cbs, increasing neural output to the nucleus tractus solitarius (NTS); (2) in this group carbon monoxide hypoxia (30% O2 in N2 with the addition of CO; COH) stimulated only the abs, increasing neural output to the NTS. (3) In the second group in which their bilateral aortic depressor nerves had been transected only the cbs increased neural output to the NTS during the HH challenge; (4) in this aortic body resected group during COH neither abs nor cbs increased neural traffic to the NTS. CO and 10% O2 reduced Hb saturation to the same level. With the use of radiolabeled microspheres blood flow was measured in a variety of organs. Organ vascular resistance was calculated by dividing the aortic pressure by that organ's blood flow. The spleen and pancreas revealed a vasoconstriction in the face of systemic hypoxemia, thought to be sympathetic nervous system (SNS)-mediated. The adrenals and the eyes vasodilated only when cbs were stimulated. Vasodilation in the heart and diaphragm showed no effect of chemoreceptor stimulated increase in SNS output. Different chemoreceptor involvement had different effects on the organs.

This study aimed to determine the roles played by the autonomic interoreceptors, the carotid bodies (cbs) and the aortic bodies (abs) in anesthetized, paralyzed, artificially ventilated cats' response to systemic hypoxemia. Four 15min challenges stimulated each of 15 animals: (1) hypoxic hypoxia (10%O₂ in N₂; HH) in the intact (int) cat where both abs and cbs sent neural traffic to the nucleus tractus solitarius (NTS); (2) carbon monoxide hypoxia (30%O₂ in N₂ with the addition of CO; COH) in the intact cat where only the abs sent neural traffic to the NTS; (3) HH in the cat after transection of both aortic depressor nerves, resecting the aortic bodies (HHabr), where only the cbs sent neural traffic to the NTS; (4) COH to the abr cat where neither abs nor cbs sent neural traffic to the NTS. Cardiac output (C.O.), contractility (dP/dt(MAX)), systolic/diastolic pressures, aortic blood pressure, total peripheral resistance, pulmonary arterial pressure, and pulmonary vascular resistance (PVR) were measured. When both cbs and abs were active the maximum increases were observed except for PVR which decreased. Some variables showed the cbs to have a greater effect than the abs. The abs proved to be important during some challenges for maintaining blood pressure. The data support the critically important role for the chemoreceptor-sympathetic nervous system connection during hypoxemia for maintaining viable homeostasis, with some differences between the cbs and the abs.

H2S, the most recently explored gasotransmitter, has been found to have actions in at least three types of tissues.

The purpose of this study was to test the hypothesis that acetylcholine (ACh) is an excitatory neurotransmitter during the hypoxic stimulation of the carotid body. Cats were anesthetized, paralyzed, and artificially ventilated. The common carotid artery was fitted with a loop containing a stopcock for selectively perfusing the carotid body. Neural activity was recorded from the whole carotid sinus nerve. After the cats had been ventilated on 10% O2 for 3 min with the carotid body being normally perfused with its own hypoxic arterial blood, the stopcock was turned, and either equally hypoxic Krebs-Ringer bicarbonate solution (KRB) containing alpha-bungarotoxin, mecamylamine, and atropine or hypoxic blocker-free KRB perfused the carotid body for 2 min. The stopcock was returned to its original position, allowing blocker-free hypoxic blood to perfuse the carotid body once again. With this protocol we found 1) the cholinergic blockers reduced the carotid body response to hypoxic KRB in a dose-dependent manner; 2) carotid baroreceptor activity was not reduced by the blockers, suggesting that the action of the blockers was not nonspecific (whereas lidocaine rapidly reduced both chemoreceptor and baroreceptor activity); 3) inclusion of the blockers in perfused hypoxic blood also reduced neural output from the carotid body; and 4) the blockers reduced the carotid body's neural response to hypoxic KRB containing substance P (20 micrograms/100 ml), suggesting that substance P may be linked to ACh in the carotid body. We conclude that these data provide good evidence supportive of an excitatory role for ACh in carotid body hypoxic excitation.

We tested the hypothesis that altering the pattern and/or magnitude of discharge of pulmonary stretch receptors (PSRs) would alter baroreceptor reflexes in anesthetized aortic-denervated cats. Carotid baroreceptor control of mean arterial pressure (MAP), heart rate (HR), and hindlimb perfusion pressure (PPhl) was examined by changing carotid sinus pressure (CSP) from 50 to 225 mmHg. The pattern of PSR discharge was changed by switching conventional mechanical ventilation (CMV) to high-frequency ventilation (HFV). Magnitude of PSR discharge was altered by changing positive end-expiratory pressure (PEEP). Altering the discharge pattern of PSR had no effect on CSP-MAP or CSP-PPhl relationships; small changes in HR were observed. Increasing PSR activity by increasing PEEP during CMV (from 3 to 9 cmH2O) depressed CSP-MAP relationship, set point, and threshold pressure. However, the depression in CSP-MAP relationship and set point during PEEP was unrelated to PSR activation, because these changes were not abolished after bilateral vagotomy. CSP-PPhl relationship was significantly elevated during PEEP before and after vagotomy, suggesting activation of a nonvagally mediated vasoconstrictory mechanism instead of PSR-mediated depressor reflex. CSP-HR relationship during PEEP showed a slight elevation, which was abolished after vagotomy. We conclude that despite minor increases in HR, altering the pattern of magnitude of PSR activity with HFV and PEEP has no significant effect on carotid baroreceptor regulation of systemic circulation. Hemodynamic changes observed during PEEP were likely due to its mechanical effect on cardiac output and activation of other cardiopulmonary receptors rather than to the increase in PSR activity.

Do cat carotid bodies (CBs) increase their release of acetylcholine and ATP in response to H(2)S? Two CBs, incubated in a Krebs Ringer bicarbonate solution at 37 ° C, exhibited a normal response to hypoxia-increased release of acetylcholine (ACh) and ATP. They were challenged with several concentrations of Na(2)S, an H(2)S donor. H(2)S, a new gasotransmitter, is reported to open K(ATP) channels. Under normoxic conditions the CBs reduced their release of ACh and ATP below control values. They responded identically to pinacidil, a well-known K(ATP) channel opener. CB glomus cells exhibited a positive immunohistochemical signal for cystathione-β-synthetase, a H(2)S synthesizing enzyme, and for a subunit of the K(ATP) channel. The data suggest that Na(2)S may have opened the glomus cells' K(ATP) channels, hyperpolarizing the cells, thus reducing their tonic release of ACh and ATP. Since during hypoxia H(2)S levels rise, the glomus cells responding very actively to hypoxia may be protected from over-exertion by the H(2)S opening of the K(ATP) channels.

The role of the carotid body (CB) as a primary chemosensor is well established in large mammals. Due to our ability to better modify the genome the mouse has been used for hypoxic chemosensing rese...

The ventilatory response to hypoxia depends on the carotid body function and sleep-wake states. Therefore, the response must be measured in a consistent sleep-wake state. In mice, EMG with behavioral indices (coordinated movements, CMs; myoclonic twitches, MTs) has been used to assess sleep-wake states. However, in neonatal mice EMG instrumentation could induce stress, altering their behavior and ventilation. Accordingly, we examined: (1) if EMG can be eliminated for assessing sleep-wake states; and (2) behavioral characteristics and carotid body-mediated respiratory control during sleep with EMG (EMG+) or without EMG (EMG-). Seven-day-old DBA/2J and A/J mice were divided into EMG+ and EMG- groups. In both strains, CMs occurred when EMG was high; MTs were present during silent/low EMG activity. The durations of high EMG activity and of CMs were statistically indifferent. Thus, CMs can be used to indicate wake state without EMG. The stress caused by EMG instrumentation may be distinctively manifested based on genetic background. Prolonged agitation was observed in some EMG+ DBA/2J (5 of 13), but not in A/J mice. The sleep time and MT counts were indifferent between the groups in DBA/2J mice. The EMG+ A/J group showed longer sleep time and less MT counts than the EMG- A/J group. Mean respiratory variables (baseline, hyperoxic/hypoxic responses) were not severely influenced by EMG+ in either strain. Individual values were more variable in EMG+ mice. Carotid body-mediated respiratory responses (decreased ventilation upon hyperoxia and increased ventilation upon mild hypoxia) during sleep were clearly observed in these neonatal mice with or without EMG instrumentation.

The carotid body (CB) is a polymodal sensor which increases its neural output to the nucleus tractus solitarii with a subsequent activation of several reflex cardiopulmonary responses. Current reports identify acetylcholine (ACh) and adenosine triphosphate (ATP) as two essential excitatory neurotransmitters in the cat and rat CBs. This study explored the impact of hypoxia, low glucose, and the two together on the release of both ACh and ATP from two incubated cat CBs. The CBs were prepared with standard procedures in accordance with the policies and regulations of the Institutional Animal Care and Use Committee. When normalized to their controls, a significant increase of ACh in the incubation medium was measured in response to hypoxia, low glucose, and the combined stimuli. When normalized to their controls, a significant increase in ATP in the incubation medium was measured in response to hypoxia and to the combined stimuli. Low glucose generated an increase in ATP which was not statistically significant (P>0.05). Second, normalizing the initial 3-4 or 2-3 min Time Segment of the challenge Stage to the final 3-4 or 2-3 min Time Segment of the control Stage for both ACh and ATP generated significant increases in response to hypoxia, low glucose (ACh only), and the combined stimuli. The data suggested the possibility that in the cat the increased CB neural output in response to low glucose might be due primarily to ACh.

This brief review first touches on the origins of the earth's oxygen. It then identifies and locates the principal oxygen sensor in vertebrates, the carotid body (CB). The CB is unique in that in human subjects, it is the only sensor of lower than normal levels in the partial pressure of oxygen (hypoxia, HH). Another oxygen sensor, the aortic bodies, are mostly vestigial in higher vertebrates. At least they play a much smaller role than the CB. In such an important role, the many reflexes in response to CB stimulation by HH are presented. After briefly reviewing what CB stimulation does, the next topic is to describe how the CB chemotransduces HH into neural signals to the brain. Several mechanisms are known, but critical steps in the mechanisms of chemosensation and chemotransduction are still under investigation. Finally, a brief glance at the operation of the CB in chronic heart failure patients is presented. Specifically, the role of nitric oxide, NO, is discussed.

The Carotid Body (CB) senses hypoxia, hypercapnia, and acidosis in the arterial blood. The resulting increase in CB neural output (CBNO) to the nucleus tractus solitarius in the medulla promotes reflex responses in the respiratory, circulatory, renal, and endocrine systems. Increases in CBNO are commonly thought to be due to the release of neurotransmitters from glomus cells in the CB. Additional to the action of these released transmitters on the postsynaptic afferent neurons which abut on the glomus cells the transmitters act presynaptically on glomus cell autoreceptors. Among the several transmitters contained in the glomus cells there now exists considerable evidence supporting excitatory roles for both acetylcholine (ACh) and ATP and an inhibitory role for dopamine (DA) and norepinephrine (NE) (Fitzgerald, 2000). The release of ACh (Fitzgerald et al., 1999; Kim et al., 2004) and catecholamines (Wang and Fitzgerald, 2002) appears to be influenced by modulators. The present study investigated the action of adenosine (ADO) on the release of ACh, DA, and NE since it has been reported that ADO influences CBNO (McQueen and Ribeiro, 1981) and CB-mediated increases in ventilation (Monteiro and Ribeiro, 1987). The study further investigated the action of nitric oxide (NO) on the release of ACh since NO has been reported to reduce the hypoxia-induced increase in CBNO (Wang, et al., 1994).

The role of the carotid body (CB) in response to hypoxia is very well defined (Fitzgerald and Shirahata, 1997). The hypoxic ventilatory response (HVR) is characterized by an increase in ventilation, but this response remains variable among individuals (Eisele et al., 1992; Vizek et al., 1987; Weil 1970). Genetics may play a critical role in explaining this variability. Indeed, longitudinal and twin studies do demonstrate the role of genetics in the HVR (Collins et al., 1978; Kawakami et al., 1982). Studies utilizing inbred strains of mice have also demonstrated the effect of genetics on the response to hypoxia (Tankersley et al., 1994 & 2000). Two strains of mice in these studies were identified as having extreme responses to hypoxia. The DBA/2J strain demonstrated the highest HVR, whereas the A/J strain demonstrated the lowest HVR (Tankersley et al., 1994). In another study analyzing the role of genetics in a mouse model of sleepinduced hypoxia, the DBA/2J strain demonstrated an increased sensitivity to hypoxia during sleep, compared to that of the A/J strain (Rubin et al., 2004). In order to elucidate a potential explanation that may contribute to this difference in hypoxic sensitivity, we examined CB morphology and volume in adult DBA/2J and A/J strains (Yamaguchi et al., 2003). Results demonstrated a significantly larger volume as well as an increased glomus cell quantity in the CB of DBA/2J strain compared to that of the A/J strain. A question arises whether these differences exist from early neonatal ages. Or, developmental plasticity may contribute to these strain differences. In this study, we analyzed CB volume, glomus cell quantity, and ventilation during development in the DBA/2J and A/J strains of mice. We also introduced a glimpse into the genetic influence on chemoreceptor development with emphasis on the role of glial-cell-line-derived neurotrophic factor (GDNF).

The carotid body is a major chemosensory organ for hypoxia, hypercapnia and acidosis in the arterial blood (Fitzgerald and Shirahata 1997; Gonzalez et al., 1994). During hypoxia, the neural output from the carotid body increases and reflexly modifies several variables in the respiratory system. A prominent response is an increase in ventilation, but the hypoxic ventilatory response (HVR) among individuals varies widely (Eisele et al., 1992; Vizek et al., 1987). Studies in humans (Collins et al., 1978; Kawakami et al., 1982; Nishimura et al., 1991; Thomas et al., 1993) suggest that genetic factors significantly contribute to those differences. Similarly, studies in mice (Tankersley et al., 1994) and rats (Weil et al., 1998) clearly indicate that genetic determinants robustly influenced HVR. Among several inbred strains of mice the DBA/2J mice demonstrated the highest HVR and the A/J mice the lowest HVR (Tankersley et al., 1994). The differences in HVR between these two strains of mice may be closely related to the structural differences of the carotid body (Yamaguchi et al., 2003). The size of the carotid body and the quantity of glomus cells in the DBA/2J mice are significantly larger than those in the A/J mice. Those differences were clearly segregated between the strains, suggesting that genetic factors strongly influence the observed phenotypic differences between the DBA/2J and A/J mice. The purpose of the current study was to confirm that the morphological characteristic differences in the carotid body between the DBA/2J and A/J mice are controlled by genetic factors. Thus, we generated the first-filial progeny (F1) by a crossing the DBA/2J (female) and A/J (male) strains of mice, and examined the morphological differences of the carotid body in the DBA/2J, A/J and their F1 mice.