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PubMed1976

Interactions between carotid sinus mechanoreceptor and chemoreceptor reflex loops.

Our experiments were designed to evaluate the combined influence of the mechanoreceptor and chemoreceptor control loops on the cardiovascular and respiratory systems. Both carotid sinus areas were isolated by a complete blind sac preparation in chloralose anesthetized dogs in which a flow probe had been implanted previously on the ascending aorta. The following variables were continuously monitored: central aortic pressure, heart rate, cardiac output, vascular resistance of a hind limb, tidal volume, respiratory rate, and oxygen consumption. While one sinus was submitted to step changes of pressure, the opposite sinus was kept at constant mean pressure. In addition, one sinus was perfused with either oxygenated of deoxygenated blood at constant flow rate. These studies show a definite interaction between the control loops initiated by the carotid sinus mechanoreceptors and those initiated by the chemoreceptors. The influence of the chemoreceptors is most marked in the peripheral beds, where it shifts the whole response curve of the mechanoreceptor control loop upward. This response accounts for most of the upward shift of the pressure response curve. On the other hand, the mechanoreceptor loop affects the ventilation, but this effect is range dependent, being most marked for the low pressure perfusion of the carotid sinus.

Animals↗
PubMed1991

Studies of the cyclic adenosine monophosphate chemoreceptor of Paramecium.

A doublet of proteins (approximately 48,000 Mr) from the Paramecium cell body membrane fits several criteria for the external cAMP chemoreceptor. These criteria include: (i) selective elution from a cAMP affinity column, matching a specificity that could be predicted from the behavioral response and whole-cell binding; (ii) binding to wheat germ agglutinin indicating the presence of carbohydrate moieties indicating surface exposure; and (iii) selective inhibition of the intact cells' chemoresponse to cAMP by antibodies against the doublet. Additional evidence for the existence of a receptor, in general, comes from selective elimination of the cAMP chemoresponse by photoaffinity labeling of while cells with 8-N3-cAMP. The doublet proteins are not identical to the regulatory subunit of a cAMP-dependent protein kinase from Paramecium, the Dictyostelium cAMP chemoreceptor, or the 42-45 kDa range proteins related to the large surface glycoprotein in Paramecium. The doublet proteins are not readily separable and, as in Dictyostelium, may represent two different covalent modification states of the same protein. Amino acid analysis indicates that the proteins are similar, but does not distinguish between the possibilities of proteolysis and covalent modification. Once cloned, this doublet may prove to be only the fifth external, eukaryotic chemoreceptor to be identified.

Amino Acids↗
PubMed1980

Relationship between tissue po2 and chemoreceptor activity of the carotid body in vitro.

Tissue pO2 (pgO2) and sinus nerve activity were recorded in the carotid body in vitro under hypoxic conditions produced either by interrupting the superfusion flow or by lowering the pO2 of the medium (pmO2). The pgO2 gradient is the steeper the higher pmO2 is. These findings point to a pO2-dependent oxygen consumption. Under hypoxia produced by interrupting the superfusion flow, pgO2 declines slowly down to final values and, concomitantly, the chemoreceptor discharge increases. Under hypoxia produced by lowering pmO2, pgO2 decreases rapidly down to values of about 3 torr, whereas the chemoreceptor discharge at first increases and then decreases, in spite of a maintained low pgO2. The pO2 threshold where the chemoreceptor starts firing under hypoxia, varies between 9 and 90 torr.

Animals↗
PubMed1994

A midline area in the nucleus commissuralis of NTS mediates the phrenic nerve responses to carotid chemoreceptor stimulation.

The carotid body chemoreceptor afferents have been reported to project to a discrete area located in the nucleus commissuralis of nucleus tractus solitarius [A. Vardhan et al., Am. J. Physiol., 264 (1993) R41-R50]. The afore-mentioned study was done in spontaneously breathing rats and the afferents and efferents located in the chest wall and the respiratory tract of these animals were intact. In order to exclude the role, if any, of these afferents and efferents, in the present experiments respiratory changes were monitored by recording the phrenic nerve activity instead of tracheal airflow. Experiments were carried out in pentobarbital-anesthetized, bilaterally vagotomized, paralyzed and artificially ventilated rats with a pneumothorax. The carotid body chemoreceptors were stimulated with tracheal administration of nitrogen for 7-10 s. The chemoreceptor stimulation induced an increase in the frequency and amplitude of phrenic nerve bursts. A decrease in the duration of inspiratory (T1), expiratory (TE) and total cycles (TTOT) was observed in the phrenic nerve activity. Inhibition of neuronal cell bodies by microinjections of muscimol (140 pmol/20 nl) into a discrete area in the commissural subnucleus of the nucleus tractus solitarius (coordinates in mm: 0.3 rostral to 0.5 caudal, 0 to 0.5 lateral and 0.3 to 0.5 deep with respect to the calamus scriptorius), attenuated the phrenic nerve responses to the carotid body stimulation. On the other hand, control injections of saline (0.9%) into this site did not alter the phrenic nerve response to the carotid body stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1975

Stimulus interaction in the responses of carotid body chemoreceptor single afferent fibers.

The characteristics of steady-state responses of single afferent fibers of carotid chemoreceptors to independent changes in arterial Po2, and Pco2 were investigated in cats. The arterial blood pressure was maintained within the normal limits (115-130 torr). Single chemoreceptor afferent fibers responded to changes both in arterial Po2 and Pco2. The relationship between the activity of chemoreceptors and changes in arterial Pco2 was linear at a constant arterial Po2. The two stimuli showed multiplicative interaction. The activity approached zero (threshold) as arterial Pco2 was decreased at a constant arterial Po2; a decrease in arterial Po2 decreased the arterial Pco2 threshold. These response characteristics of a single fiber suggest that the sensory receptor may be activated through a single mechanism by the two stimuli. The data fit into an idea that the mechanism may involve a conformational change in the membrane-bound polymeric chromophore group which reacts with O2 reversibly and shows a Bohr-shift.

Animals↗
PubMed1975

Medullary relay neurons in the carotid-body chemoreceptor pathway of cats.

Central terminations of the carotid body chemoreceptor afferents were localized by recording field potentials and unit activity evoked by electrical stimulation of the carotid sinus nerve, and unit activity during chemical stimulation of the chemoreceptors. Areas where evoked responses with short latency could be recorded and which contained neurons that responded to carotid body excitation were located in two regions of the medulla, about the level of the obex: a dorsal region which included the nucleus tractus solitarius and the reticular formation just below this nucleus; and a ventrolateral region which included the nucleus ambiguus and the ventrolateral reticular formation around that nucleus. The evoked field responses in the two regions were similar. In these two regions, the only neurons which increased their firing both to NaCN and a decreased PIO2 had a respiratory, bursting activity which was phase locked with phrenic nerve firing. The authors' findings suggest that there is a direct synaptic input from carotid body chemoreceptor afferents onto medullary respiratory neurons.

Action Potentials↗
PubMed1991

Hypoxia-induced bronchial responsiveness in awake sheep: role of carotid chemoreceptors.

Hypoxia enhances bronchial responsiveness in sheep and dogs and we recently reported a similar effect in asthmatic subjects. Activation of a reflex arc due to stimulation of peripheral chemoreceptors has been suggested to explain this finding. We evaluated the contribution of carotid body chemoreceptors to hypoxia-induced bronchial responsiveness in conscious sheep. Incremental concentrations of aerosolized methacholine were given to each of seven sheep ventilated successively (in a random order) with a normoxic and a hypoxic gas mixture (15% O2 in N2), both before and after peripheral surgical chemodenervation. Hypoxia alone did not affect lung mechanics in intact or chemodenervated animals. Bronchial responsiveness to methacholine was significantly increased by hypoxia in intact sheep, as compared to chemodenervated sheep. We conclude that hypoxia has a stimulating effect on bronchial responsiveness in awake sheep similar to that seen in human patients with asthma. This increased responsiveness may be mediated by hypoxic stimulation of carotid body chemoreceptors, which in turn may influence the nervous control of airway calibre.

Airway Resistance↗
PubMed1995

Effects of potassium and lactic acid on chemoreceptor discharge in anaesthetized cats.

Both increasing [K+]a and falling pHa stimulate ventilation through an action on the peripheral chemoreceptors. We have examined the effect on afferent carotid chemoreceptor discharge, of intravenous infusion of lactic acid alone, KCl alone, and both combined at constant PETCO2 in anaesthetized, artificially ventilated cats. Infusions of lactic acid alone and KCl alone caused similar increases in both the mean and amplitude of oscillation of chemoreceptor discharge. In the case of the lactic acid alone infusion the increase in the amplitude of oscillation could be accounted for by the resultant increase in carbon dioxide production. Simultaneous infusion of KCl and lactic acid caused an increase in the mean and amplitude of the discharge which was greater than either given alone, although the combined effect was less than additive. The alterations in mean and amplitude of oscillation of discharge during infusion of both agents together may be completely accounted for by a combined effect of increased carbon dioxide production and elevated [K+]a.

Anesthesia↗
PubMed1979

Conduction velocity in spinal descending pathways of baro- and chemoreceptor reflex.

Activity in the white rami T3 and L2 or 3 has been recorded and averaged with respect to excitation of the carotid sinus baroreceptor afferents produced by the pulsatile blood pressure (baroreceptor reflex) and with respect to brief trains of electrical stimuli exciting low threshold chemoreceptor afferents in the left carotid sinus nerve (chemoreceptor reflex). Experiments were performed on chloralose anaesthetized cats with both vago-depressor nerves cut. From the latency difference between the onset of the responses at the thoracic and their arrival at the lumbar level the spinal conduction velocity for the pathway of each reflex has been calculated. The baroreceptor reflex pathway has slower spinal conduction velocity 3.3 +/- 0.7 m/sec than the chemoreceptor pathway 5.5 +/- 0.9 m/sec. These results indicate that there are separate descending spinal pathways for the two types of reflexes.

Animals↗
PubMed1993

N-methyl-D-aspartate receptors but not non-N-methyl-D-aspartate receptors mediate hypertension induced by carotid body chemoreceptor stimulation in the rostral ventrolateral medulla of the rat.

In urethane-anesthetized rats, excitatory amino acid antagonists were microinjected into the rostral ventrolateral medulla (RVLM) and their effects on the pressor response and tachycardia evoked by carotid chemoreceptor stimulation were examined. Microinjections of the N-methyl-D-aspartate (NMDA) receptor antagonists 2-amino-5-phosphonovalerate (AP5) and MK-801 into the RVLM inhibited these chemoreceptor reflex responses whereas these responses were not affected by injection of the non-NMDA receptor antagonist CNQX. AP5 and MK-801 but not CNQX abolished the pressor response evoked by NMDA whereas CNQX but not AP5 and MK-801 abolished that evoked by AMPA or kainate. These results provide evidence that NMDA receptors in the RVLM of the rat are involved in the carotid chemoreceptor reflex.

Animals↗
PubMed2004

Neuroanatomical and electrophysiological studies of identified contact chemoreceptors on the ventral ovipositor valve of 3rd instar larvae of lubber grasshoppers (Taeniopoda eques).

A large number of contact chemoreceptors are located on the ovipositor valves of adult female grasshoppers. These receptors play an important role in many aspects of grasshopper life such as detecting the chemical composition of the soil before and during oviposition. It is surprising, however, to find these types of receptors on the ovipositor valves of instar larvae which are not able to oviposit. Thus, these receptors may serve functions other than to search for a suitable site for egg laying. Observation under the scanning electron microscope revealed the presence of uniporous basiconic contact chemoreceptors in addition to different types of trichoid mechanoreceptors on the ovipositor valve of lubber grasshopper 3rd instar larvae. Neuroanatomical studies have shown that these sensilla are multiply innervated, containing one mechanosensory neuron and four chemosensory neurons that project locally and intersegmentally. The tip recording technique from single basiconic sensilla demonstrated mechanosensory responses to deflections of the sensillum as well as gustatory activity when in contact with different chemical solutions. The electrophysiological studies have shown that these sensilla serve as contact chemoreceptors and not as olfactory receptors.

Journal Article↗
PubMed1999

Sensitivity of the mosquito Aedes aegypti (Culicidae) labral apical chemoreceptors to blood plasma components.

The phagostimulants from the cellular fraction of blood induce gorging of Aedes aegypti (L.), and this process is enhanced by some plasma components. This project examines the responses of the labral apical chemoreceptors to plasma components enhancing phagostimulation. From the electrophysiological responses of the labral apical chemoreceptors four cells were identified by the waveform of their action potentials. Three of the cells (Cell 2, Cell 3 and Cell 4) responded in a dose dependent manner to NaCl. The responses of Cell 2 and Cell 3 to NaCl concentrations from 1 to 500 mmol/l can be described by a logarithmic equation. The response of Cell 2 to 150 mmol/l NaCl is modulated when a buffer is added. The magnitude of the modulation of the response is determined by the nature of the buffer: NaHCO(3) inhibits while Na(2)HPO(4) enhances the response. High osmotic pressure inhibits the response of Cell 4, regardless of how it is achieved. Cell 4 responds with a high frequency to the presence of L-alanine, the C-terminal amino acid of albumin, but shows a reduced response to the same concentration of albumin. From these results it can be concluded that labral apical chemoreceptors of A. aegypti are capable of detecting the plasma components involved in blood recognition.

Journal Article↗
PubMed2001

The highly conserved domain of the Caulobacter McpA chemoreceptor is required for its polar localization.

We have fused GFP to the C-terminus of McpA to study chemoreceptor polar localization in Caulobacter crescentus. The full-length McpA-GFP fusion is polarly localized and methylated. The methylation is dependent on the chemoreceptor methyltransferase (cheR) and chemoreceptor methylesterase (cheB) genes present in the mcpA operon. C-terminal and internal deletions of McpA were constructed and fused to the N-terminus of GFP to identify the domains required for polar localization. When the R1 methylation domain was deleted, the McpA-GFP fusion was still polarly localized, suggesting that this domain is dispensable for polar localization. However, when the highly conserved domain (HCD), which is involved in interacting with CheW, was deleted either by an internal deletion or C-terminal deletion, the resulting McpA-GFP fusions were completely delocalized. When the mcpA operon, which contains the cheW and cheA homologues, was deleted, the full-length McpA-GFP fusion was delocalized. Although additional chemotaxis genes are required for the polar localization of McpA-GFP, the presence of the single polar flagellum is not required. However, in filamentous cells, which are frequently found in C. crescentus fliF mutants, the McpA-GFP fusion was observed at mid-cell positions.

Amino Acid Sequence↗
PubMed2002

Chemoreceptor activity is normal in mice lacking the NK1 receptor.

Substance P has been proposed to be an important neurotransmitter in the carotid body with the neurokinin 1 (NK1) receptor, mediating excitation between the glomus cells and afferent nerve endings. In order to better understand the role of substance P, this study examined chemoreceptor afferent activity, in vitro, and tissue catecholamine levels and release in adult, wild-type mice and mice lacking the gene for the NK1 receptor (NK1-KO). Groups did not differ significantly in body weight, carotid body dopamine content or carotid body norepinephrine content. In wild-type mice, single unit activity increased from 0.59 +/- 0.14 Hz to 19.78 +/- 2.27 Hz during superfusion with strong hypoxia (PO2 approximately 25 Torr). Chemoreceptor activity in NK1-KO mice, increased from 0.71 +/- 0.23 to 21.50 +/- 3.62 Hz, and neither baseline or peak frequencies were significantly different from the wild-type group. Less severe hypoxia (PO2 approximately 45 torr), evoked peak activities of 12.50 +/- 1.88 and 10.64 +/- 3.72 Hz in wild-type and NK1-KO mice, which were also not significantly different. In response to severe hypoxia, free-tissue catecholamine increased to 4.92 +/- 0.85 microm in wild-type mice and 4.26 +/- 0.63 microm in NK1-KO mice, which were also not significantly different. It may therefore be concluded that loss of NK1 receptors has little effect on chemoreceptor function in the mouse, and thus they play, at best, a minor role in the hypoxic chemoreception process.

Animals↗
PubMed1993

Cloning and characterization of the Salmonella typhimurium-specific chemoreceptor Tcp for taxis to citrate and from phenol.

Salmonella typhimurium shows an attractant response to citrate and a repellent response to phenol, and a chemoreceptor mediating these responses has been identified and named Tcp (taxis to citrate and away from phenol). Tcp is one of the methyl-accepting chemotaxis proteins that have a molecular mass of approximately 60 kDa estimated by SDS/PAGE, and its methylation level is increased by citrate and decreased by phenol. Tcp also mediates an attractant response to metal-citrate complexes. The complete nucleotide sequence of the tcp coding region has been determined. The deduced amino acid sequence of Tcp, consisting of 547-amino acid residues, is homologous with that of the aspartate chemoreceptor of S. typhimurium. Thus, Tcp is another member of the bacterial transmembrane chemoreceptor family. Because citrate is a good carbon source for S. typhimurium but is not a carbon source for the closely related species Escherichia coli and because citrate utilization is used as a key diagnostic character to distinguish these species, it is reasonable to assume that Tcp is specific to S. typhimurium.

Amino Acid Sequence↗
PubMed1996

Detecting the conformational change of transmembrane signaling in a bacterial chemoreceptor by measuring effects on disulfide cross-linking in vivo.

Transmembrane signaling by bacterial chemoreceptors is thought to involve relative movement among the four transmembrane helices of the homodimer. We assayed that movement by measuring effects of ligand occupancy on rates of oxidative cross-linking between cysteines introduced into neighboring helices of the transmembrane domain of chemoreceptor Trg from Escherichia coli. Measurements were done on chemoreceptors in their native environment, intact cells that were motile and chemotactically responsive. Receptor occupancy did not appear to cause drastic rearrangement of the four-helix structure since, among 67 cysteine pairs tested, the same 19 exhibited oxidative cross-linking in the presence or absence of saturating chemoattractant. However, occupancy did cause subtle changes that were detected as effects on rates of cross-linking. Among the seven disulfides appropriate for measurements of initial rates of formation, ligand occupancy had significant and different effects on all three cross-links that connected the two helices within a subunit but had minimal effects on the four that spanned the packing interface between subunits. This constitutes direct evidence that the conformational change of transmembrane signaling involves significant movement within a subunit and minimal movement between subunits, a pattern deduced from several previous studies and now documented directly. Among possible modes of movement between the two helices of a subunit, axial sliding of one helix relative to the other was the conformational change that best accounted for the observed effects on cross-linking.

Bacterial Proteins↗
PubMed1998

Two large families of chemoreceptor genes in the nematodes Caenorhabditis elegans and Caenorhabditis briggsae reveal extensive gene duplication, diversification, movement, and intron loss.

The str family of genes encoding seven-transmembrane G-protein-coupled or serpentine receptors related to the ODR-10 diacetyl chemoreceptor is very large, with at least 197 members in the Caenorhabditis elegans genome. The closely related stl family has 43 genes, and both families are distantly related to the srd family with 55 genes. Analysis of the structures of these genes indicates that a third of them are clearly or likely pseudogenes. Preliminary surveys of other candidate chemoreceptor families indicates that as many as 800 genes and pseudogenes or 6% of the genome might encode 550 functional chemoreceptors constituting 4% of the C. elegans protein complement. Phylogenetic analyses of the str and stl families, and comparisons with a few orthologs in Caenorhabditis briggsae, reveal ongoing processes of gene duplication, diversification, and movement. The reconstructed ancestral gene structures for these two families have eight introns each, four of which are homologous. Mapping of intron distributions on the phylogenetic tree reveals that each intron has been lost many times independently. Most of these introns were lost individually, which might best be explained by precise in-frame deletions involving nonhomologous recombination between short direct repeats at their termini. [Alignment of the putatively functional proteins in the str and stl families is available from Pfam (http://genome. wustl.edu/Pfam); alignments of all translations are available at http://cshl.org/gr; alignments of the genes are available from the author at hughrobe@uiuc.edu]

Amino Acid Sequence↗
PubMed2005

Clustering requires modified methyl-accepting sites in low-abundance but not high-abundance chemoreceptors of Escherichia coli.

Chemotaxis signalling complexes of Escherichia coli, composed of chemoreceptors, CheA and CheW, form clusters located predominantly at cell poles. As the only kind of receptor in a cell, high-abundance receptors are polar and clustered whereas low-abundance chemoreceptors are polar but largely unclustered. We found that clustering was a function of the cytoplasmic, carboxyl-terminal domain and that effective clustering was conferred on low-abundance receptors by addition of the approximately 20-residue sequence from the carboxyl terminus of either high-abundance receptor. These sequences are different but share a carboxyl-terminal pentapeptide that enhances adaptational covalent modification and allows a physiological balance between modified and unmodified methyl-accepting sites, implying that receptor modification might influence clustering. Thus we investigated directly effects of modification state on chemoreceptor clustering. As the sole receptor type in a cell, low-abundance receptors were clustered only if modified, but high-abundance receptors were clustered independent of extent of modification. This difference could mean that the two receptor types are fundamentally different or that they are poised at different positions in the same conformational equilibrium. Notably, no receptor perturbation we tested altered a predominant location at cell poles, emphasizing a distinction between determinants of clustering and polar localization.

Amino Acid Sequence↗