Inhibitory and excitatory effects of dopamine on carotid chemoreceptors

The effect of intravenous dopamine on carotid body chemoreceptor activity was investigated in 6 anesthetized cats which were paralyzed and artificially ventilated. Studies were performed at 4 steady-state p_aO₂ levels at a constant p_aCO₂ and at 4 levels of p_aCO₂ during hyperoxia. Dopamine inhibited carotid chemoreceptors before and excited them after haloperidol. Moderate stimulation of the receptors by hypoxia and hypercapnia augmented dopamine's effects. These results indicate that both inhibitory and excitatory dopamine receptors are present in the carotid body.     

Immunosensory signalling by carotid body chemoreceptors

Injections of lipopolysaccharide (LPS) have been used to produce the signs of sepsis and study their underlying mechanisms. Intravenous (IV) injections of LPS in anesthetized cats induce tachypnea, tachycardia and hypotension, but ventilatory changes are suppressed after sectioning carotid and aortic nerves. Otherwise, LPS increases the basal frequency of carotid chemosensory discharges, but reduces ventilatory and chemosensory responses to hypoxia and nicotine injections. Increases in cytokines (IL-1β, IL-6 and TNF-α) are observed in plasma and tissues after injecting LPS. In carotid bodies perfused in vitro, TNF-α reduces chemosensory discharges induced by hypoxia. The rat carotid body and its sensory ganglion constitutively express LPS canonical receptor, TLR4, as well as TNF-α and its receptors (TNF-R1 and TNF-R2). Increases of TNF-α and TNF-R2 expression occur after LPS administration. The activation of peripheral and central autonomic pathways induced by LPS or IL's is partly dependent on intact vagus nerves. Thus, the carotid and vagus nerves provide routes between the immune system and CNS structures involved in systemic inflammatory responses.     

The effect of ephedrine on carotid chemoreceptors

Summary The carotid sinus was isolated by the method of Moiseev-Heymans-Anichkov, respiration serving as a test. Perfusion of 150.000 solution of ephedrine caused respiration to be lowered by acetylcholine and enhanced by the sodium cyanide. Thus, it has been shown that ephedrine accelerates sensitivity of the carotid body to the cyanides.Cyanides are anoxic poisons, their effect being like that of oxygen insufficiency. Hence, ephedrine enhances the sensitivity of the carotid body to oxygen insufficiency accelerating its participation in the control of blood oxygen.Submitted by Active Member AMS USSR Professor S. V. Anichkov     

Pharmacology of pH effects on carotid body chemoreceptors in vitro

1. The carotid body and the carotid nerve were removed from anaesthetized cats and placed in a small Perspex channel through which Locke solution (at various pH values and usually equilibrated with 50% O(2) in N(2)) was allowed to flow. The glomus was immersed in the flowing solution while the nerve was lifted into oil covering the saline. Sensory discharges were recorded from the nerve and their frequency was used as an index of receptor activity. At times, a small segment of carotid artery, containing pressoreceptor endings, was removed together with the glomus. In this case, pressoreceptor discharges were recorded from the nerve.2. The amplitude of either chemo- or pressoreceptor discharges was not changed by strong acid solutions. Acid decreased the frequency of the baroreceptor discharges only when pH fell to less than 4.0. Solutions at low pH increased the chemosensory discharge, but acid depressed the increased chemoreceptor discharge elicited by KCl. These experiments indicated that H(+) ions probably acted as membrane ;stabilizers' without depolarizing either the nerve fibres or endings.3. Acid solutions increased the action of acetylcholine chloride (AChCl) (100-200 mug) on chemoreceptors. This effect probably was due either to inactivation of tissue cholinesterase or to enhanced sensitivity of the sensory endings to ACh.4. Choline chloride (10(-3)M), which favours ACh synthesis, protected the preparation against decay during prolonged experimentation. Hemicholinium-3 (HC-3), which blocks ACh synthesis in low concentrations (10(-5)M), depressed the chemosensory response to acid and to hypoxia when such stimuli were applied repeatedly. This concentration of HC-3 did not change effects of applied ACh.5. Substances which affect ACh release markedly changed the chemoreceptor discharge increase induced by acidity and other forms of stimulation. In the absence of Ca(2+), acid, anoxia, and interruption of flow provoked receptor depression while receptor excitation induced by ACh and KCl persisted. All stimuli excited and showed increased effectiveness as the Ca(2+) concentration was raised, but their effects declined as Ca(2+) was increased above normal values. Mg(2+) ions depressed the chemoreceptor effects induced by all these stimuli. The action of Mg(2+) was not due entirely to nerve ending block. Morphine sulphate (which decreases ACh release in other structures) also depressed the receptor response to acid and flow interruption.6. Cholinergic blocking agents such as mecamylamine, hexamethonium, atropine, dihydro-beta-erithroidine (DHE), HC-3 (10(-4)M), choline and acetylcholine (in combination with choline) depressed the effects of acid and ACh on the chemoreceptors. The effect induced by interruption of flow was depressed only by mecamylamine and DHE.7. Agents which affect the fate of released ACh, such as acetylcholinesterase and eserine salicylate, did not affect clearly the response of chemoreceptors to acid.8. The results suggest that acid stimulates chemoreceptor fibres through an indirect mechanism, viz. through increased release and/or decreased destruction of a presynaptic transmitter from the glomus cell. This transmitter is probably ACh (see following paper, Eyzaguirre & Zapata, 1968).     

Stimulus-specific signaling pathways in rabbit carotid body chemoreceptors

The carotid body is an arterial chemosensory organ which responds to multiple natural and pharmacological stimuli, including hypoxia and nicotine. Numerous studies have investigated the initial molecular events which activate chemosensory type I cells in the carotid body, but less attention has been focused on later steps in the transduction cascade, which mediate neurotransmitter release from type I cells and excitation of chemoreceptor afferent fibers in the carotid sinus nerve. In the present study, we examined the effects of a highly specific inhibitor of calcium/calmodulin-dependent kinase II, KN-62, and a calmodulin inhibitor, trifluoperazine, on carotid sinus nerve activity and catecholamine release evoked from rabbit carotid bodies superfused in vitro. KN-62 did not alter sinus nerve activity and catecholamine release evoked by hypoxia, but this agent significantly reduced nerve activity and neurotransmitter release evoked by 100 microM nicotine. Trifluoperazine (10 microM), likewise inhibited activity evoked by nicotine, as well as hypoxia. Basal levels of nerve activity and catecholamine release (established in superfusate equilibrated with 100% O2) were unaffected by all drug treatments. Separate biochemical experiments showed that Ca2+/calmodulin-dependent incorporation of 32P into carotid body particulate proteins is significantly reduced following incubation of intact carotid bodies in nicotine, but not following exposure to hypoxia. Our observations suggest that excitation of the carotid body by diverse stimuli may involve the activation of distinct, stimulus-specific transduction pathways. Furthermore, these data correlate with our previous findings which showed that hypoxia, on the one hand, and nicotine on the other, evoke the preferential release of either dopamine or norepinephrine, respectively, from carotid bodies incubated in vitro.     

Stimulus-specific signaling pathways in rabbit carotid body chemoreceptors

The carotid body is an arterial chemosensory organ which responds to multiple natural and pharmacological stimuli, including hypoxia and nicotine. Numerous studies have investigated the initial molecular events which activate chemosensory type I cells in the carotid body, but less attention has been focused on later steps in the transduction cascade, which mediate neurotransmitter release from type I cells and excitation of chemoreceptor afferent fibers in the carotid sinus nerve. In the present study, we examined the effects of a highly specific inhibitor of calcium/calmodulin-dependent kinase II, KN-62, and a calmodulin inhibitor, trifluoperazine, on carotid sinus nerve activity and catecholamine release evoked from rabbit carotid bodies superfused in vitro. KN-62 did not alter sinus nerve activity and catecholamine release evoked by hypoxia, but this agent significantly reduced nerve activity and neurotransmitter release evoked by 100 μM nicotine. Trifluoperazine (10 μM), likewise inhibited activity evoked by nicotine, as well as hypoxia. Basal levels of nerve activity and catecholamine release (established in superfusate equilibrated with 100% O₂) were unaffected by all drug treatments. Separate biochemical experiments showed that Ca²⁺/calmodulin-dependent incorporation of ³²P into carotid body particulate proteins is significantly reduced following incubation of intact carotid bodies in nicotine, but not following exposure to hypoxia.Our observations suggest that excitation of the carotid body by diverse stimuli may involve the activation of distinct, stimulus-specific transduction pathways. Furthermore, these data correlate with our previous findings which showed that hypoxia, on the one hand, and nicotine on the other, evoke the preferential release of either dopamine or norepinephrine, respectively, from carotid bodies incubated in vitro.     

Effects of dopamine superfusion on the activity of rabbit carotid chemoreceptors in vitro

The effect of different concentrations (0.001, 0.01, 0.1 and 1 mM) of dopamine on chemo-afferent activity was studied in the rabbit carotid body superfused in vitro. Excitation was the sole effect observed: it was always present for dopamine tests at 0.1 and 1 mM but was found in only 4 out of 9 tests at 0.01 mM and in 1 out of 5 tests at 0.001 mM. By comparison with a natural stimulus like hypoxia, dopamine excitation was delayed and had a much slower time course. Dopamine antagonists, (+)-butaclamol and haloperidol did not affect the responses to dopamine and to hypoxia. The results were not significantly altered when CO2 was added to the superfusing medium. It is concluded that dopamine is not a likely excitatory transmitter for chemoreception in the rabbit carotid body.     

The role of NADPH oxidase in carotid body arterial chemoreceptors

O(2)-sensing in the carotid body occurs in neuroectoderm-derived type I glomus cells where hypoxia elicits a complex chemotransduction cascade involving membrane depolarization, Ca(2+) entry and the release of excitatory neurotransmitters. Efforts to understand the exquisite O(2)-sensitivity of these cells currently focus on the coupling between local P(O2) and the open-closed state of K(+)-channels. Amongst multiple competing hypotheses is the notion that K(+)-channel activity is mediated by a phagocytic-like multisubunit enzyme, NADPH oxidase, which produces reactive oxygen species (ROS) in proportion to the prevailing P(O2). In O(2)-sensitive cells of lung neuroepithelial bodies (NEB), multiple studies confirm that ROS levels decrease in hypoxia, and that E(M) and K(+)-channel activity are indeed controlled by ROS produced by NADPH oxidase. However, recent studies in our laboratories suggest that ROS generated by a non-phagocyte isoform of the oxidase are important contributors to chemotransduction, but that their role in type I cells differs fundamentally from the mechanism utilized by NEB chemoreceptors. Data indicate that in response to hypoxia, NADPH oxidase activity is increased in type I cells, and further, that increased ROS levels generated in response to low-O(2) facilitate cell repolarization via specific subsets of K(+)-channels.     

Peripheral arterial chemoreceptors and the evolution of the carotid body

There has been a reduction in the distribution of peripheral respiratory O(2) chemoreceptors from multiple, dispersed sites in fish and amphibia to a single dominant receptor site in birds and mammals. In the process, the cells in the fish gill associated with O(2) chemosensing (5-HT containing neuroepithelial cells often found in association with ACh/catecholamine (CA) containing cells) are replaced by the glomus cells of the mammalian carotid body (which contain multiple putative neurotransmitter substances, including 5-HT, CA and ACh, all within the same cells), although this difference may be more superficial than first appears. While still highly speculative, these trends would appear to be correlated with the transition from aquatic respiration and bimodal breathing, and from animals with intra-cardiac shunts (two situations where the ability to sense O(2) at multiple sites would be an advantage), to strictly air breathing in animals with no intra-cardiac shunts. It is also tempting to speculate that while the basic O(2)-sensing mechanism is the same for all receptor cells, the receptor groups in fish have evolved in such a way to make the responses of some more sensitive to changes in O(2) delivery than others. The net result is that those receptors associated with the first gill arch of fish (the third branchial arch) become the carotid body in higher vertebrates associated with the regulation of ventilation and ensuring oxygen supply to the gas exchange surface. Those receptors associated with the second gill arch (fourth branchial arch) become the aortic bodies capable of sensing changes in oxygen content of the blood and primarily involved in regulating oxygen transport capacity through erythropoiesis and changes in blood volume.     

The effects of blood osmolality changes on cat carotid body chemoreceptors in vivo

The possibility that carotid chemoreceptors respond to changes in plasma osmolality was investigated in the cat, perfusing the carotid artery with blood made hyper- or hypo-osmotic and recording chemoreceptor activity from carotid nerve fibers. Blood made hyperosmotic with sucrose or NaCl reduced the chemoreceptor discharge, while hypoosmotic blood increased chemoreceptor activity. The minimal osmolality variation necessary to obtain a detectable frequency change was 3–8% of the control. Frequency changes of 30% of the control were obtained with a 20% variation in osmolality. The frequency variations produced by the osmotic changes lasted as long as the infusion was maintained (up to 15 min). In some instances a rebound was observed when iso-osmotic saline was perfused again. A transient change in frequency and a clear rebound were obtained when blood made hyperosmotic with glycerol was perfused. These effects probably reflect a rapid change in intracellular osmolality due to the free passage of glycerol across cellular membranes.The modifications in chemoreceptor activity consecutive to osmolality variations are the opposite of those observed in isolated and superfused carotid bodies. As it is known that osmolality values affect the smooth muscle of the blood vessels, we conclude that our results are mainly produced by changes in carotid body blood flow due to a direct effect of hyper- and hypo-osmotic solutions on vascular muscle tone. Chemoreceptor excitation during a decrease in blood osmolality may contribute reflexly to the increased vascular resistance observed during acute osmolality reductions in man.     

Effects of combined cholinergic-purinergic block upon cat carotid body chemoreceptors in vitro

Since acetylcholine (ACh) and ATP have been proposed as excitatory co-transmitters at synapses between glomus cells and sensory nerve endings of the carotid body (CB), we tested such hypothesis by studying the effects of combined cholinergic-purinergic block on the chemosensory activity recorded from cat's carotid bodies perfused and/or superfused in vitro. The preparations were bathed with Tyrode's solution, either normoxic (PO2=98.5+/-13.5 Torr) or hypoxic (PO2=31.8+/-5.2 Torr), and the frequency of chemosensory impulses (fchi) was recorded from the carotid (sinus) nerve. Dose-response curves for fchi increases evoked by intra-stream boluses of acetylcholine, nicotine and ATP were studied. A combination of mecamylamine 2 microM and suramin 50 microM, applied through the perfusate or superfusate, suppressed nicotine- and ATP-induced increases in fchi, but the basal chemosensory activity in normoxia and the chemosensory excitation elicited by hypoxic superfusion were preserved, although variably reduced in most preparations. Thus, in spite of the excitatory effects provoked by applying ACh and ATP to the perfused/superfused CB in vitro, a co-release of these substances cannot account entirely for the chemosensory excitation induced by hypoxic stimulation of the CB.     

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

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

Vagal baro‐ and chemoreceptors in middle internal carotid artery and carotid body in rat

The glossopharyngeal nerve, via the carotid sinus nerve (CSN), presents baroreceptors from the internal carotid artery (ICA) and chemoreceptors from the carotid body. Although neurons in the nodose ganglion were labelled after injecting tracer into the carotid body, the vagal pathway to these baro‐ and chemoreceptors has not been identified. Neither has the glossopharyngeal intracranial afferent/sensory pathway that connects to the brainstem been defined. We investigated both of these issues in male Sprague–Dawley rats (n = 40) by injecting neural tracer wheat germ agglutinin‐horseradish peroxidase into: (i) the peripheral glossopharyngeal or vagal nerve trunk with or without the intracranial glossopharyngeal rootlet being rhizotomized; or (ii) the nucleus of the solitary tract right after dorsal and ventral intracranial glossopharyngeal rootlets were dissected. By examining whole‐mount tissues and brainstem sections, we verified that only the most rostral rootlet connects to the glossopharyngeal nerve and usually four caudal rootlets connect to the vagus nerve. Furthermore, vagal branches may: (i) join the CSN originating from the pharyngeal nerve base, caudal nodose ganglion, and rostral or caudal superior laryngeal nerve; or (ii) connect directly to nerve endings in the middle segment of the ICA or to chemoreceptors in the carotid body. The aortic depressor nerve always presents and bifurcates from either the rostral or the caudal part of the superior laryngeal nerve. The vagus nerve seemingly provides redundant carotid baro‐ and chemoreceptors to work with the glossopharyngeal nerve. These innervations confer more extensive roles on the vagus nerve in regulating body energy that is supplied by the cardiovascular, pulmonary and digestive systems.