In vitro activation of cyclo-oxygenase in the rabbit carotid body: effect of its blockade on [3H]catecholamine release

Cyclooxygenase and nitric oxide synthase pathways mediate the respiratory effects of TNF-α in rats

TNF-α is the key inflammatory cytokine. TNF-α receptors are expressed in brain stem regions involved in respiratory control and also in the carotid bodies, which are the sensory organs monitoring arterial blood O₂. We hypothesised that the circulating tumour necrosis factor (TNF)-α may affect the lung ventilation and modulate the hypoxic ventilatory response via activation of cyclooxygenase (COX) and nitric oxide synthase (NOS) pathways. The aim of the current study was to compare the respiratory effects of TNF-α before and after pretreatment with diclofenac or L-NG-nitro arginine methyl ester (L-NAME) nonspecific inhibitors of COX and NOS, respectively. The hypoxic ventilatory response was measured in anaesthetised rats using rebreathing techniques. We found that TNF-α increased the lung ventilation in normoxia but decreased the ventilatory response to hypoxia. Pretreatment with each of these inhibitors reduced respiratory effects of TNF-α. We believe that activation of COX and NOS-related pathways and also "cross-talk" between them mediates the TNF-α respiratory effects and underlies the impact of inflammation on the respiratory function.

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception

The cascade of transduction of hypoxia and hypercapnia, the natural stimuli to chemoreceptor cells, is incompletely understood. A particular gap in that knowledge is the role played by second messengers, or in a most ample term, of modulators. A recently described modulator of chemoreceptor cell responses is the gaseous transmitter hydrogen sulfide, which has been proposed as a specific activator of the hypoxic responses in the carotid body, both at the level of the chemoreceptor cell response or at the level of the global output of the organ. Since sulfide behaves in this regard as cAMP, we explored the possibility that sulfide effects were mediated by the more classical messenger. Data indicate that exogenous and endogenous sulfide inhibits adenyl cyclase finding additionally that inhibition of adenylyl cyclase does not modify chemoreceptor cell responses elicited by sulfide. We have also observed that transient receptor potential cation channels A1 (TRPA1) are not regulated by sulfide in chemoreceptor cells.

Inflammatory and immunomodulatory mechanisms in the carotid body

Evidence is available about the role of inflammatory/immunological factors in the physiology and plasticity of the carotid body, with potential clinical implications in obstructive sleep apnea syndrome and sudden infant death syndrome. In humans, lymphomonocytic aggregations (chronic carotid glomitis) have been reported in aging and opiate addiction. Glomus cells produce prostaglandin E2 and the cytokines interleukin 1β, interleukin 6 and TNF-α, with corresponding receptors. These factors modulate glomus cell excitability, catecholamine release and/or chemoreceptor discharge. The above cytokines are up-regulated in chronic sustained or intermittent hypoxia, and prevention of these changes, with ibuprofen or dexamethasone, may modulate hypoxia-induced changes in carotid body chemosensitivity. The main transcription factors considered to be involved are NF-kB and HIFs. Circulating immunogens (lipopolysaccharide) and cytokines may also affect peripheral arterial chemoreception, with the carotid body exerting an immunosensing function.

Influence of indomethacin on ventilatory and cerebrovascular responsiveness to CO2 and breathing stability: the influence of PCO2 gradients

Indomethacin (INDO), a reversible cyclooxygenase inhibitor, is a useful tool for assessing the role of cerebrovascular reactivity on ventilatory control. Despite this, the effect of INDO on breathing stability during wakefulness has yet to be examined. Although the effect of reductions in cerebrovascular CO(2) reactivity on ventilatory CO(2) sensitivity is likely dependent upon the method used, no studies have compared the effect of INDO on steady-state and modified rebreathing estimates of ventilatory CO(2) sensitivity. The latter method includes the influence of PCO(2) gradients and cerebral perfusion, whereas the former does not. We examined the hypothesis that INDO-induced reduction in cerebrovascular CO(2) reactivity would 1) cause unstable breathing in conscious humans and 2) increase ventilatory CO(2) sensitivity during the steady-state method but not during rebreathing methods. We measured arterial blood gases, ventilation (VE), and middle cerebral artery velocity (MCAv) before and 90 min following INDO ingestion (100 mg) or placebo in 12 healthy participants. There were no changes in resting arterial blood gases or Ve following either intervention. INDO increased the magnitude of Ve variability (index of breathing stability) during spontaneous air breathing (+4.3 +/- 5.2 Deltal/min, P = 0.01) and reduced MCAv (-25 +/- 19%, P < 0.01) and MCAv-CO(2) reactivity during steady-state (-47 +/- 27%, P < 0.01) and rebreathing (-32 +/- 25%, P < 0.01). The Ve-CO(2) sensitivity during the steady-state method was increased with INDO (+0.5 +/- 0.5 l x min(-1) x mmHg(-1), P < 0.01), while no changes were observed during rebreathing (P > 0.05). These data indicate that the net effect of INDO on ventilatory control is an enhanced ventilatory loop gain resulting in increased breathing instability. Our findings also highlight important methodological and physiological considerations when assessing the effect of INDO on ventilatory CO(2) sensitivity, whereby the effect of INDO-induced reduction of cerebrovascular CO(2) reactivity on ventilatory CO(2) sensitivity is unmasked with the rebreathing method.

Influence of cerebral blood flow on breathing stability

Our previous work showed a diminished cerebral blood flow (CBF) response to changes in Pa(CO(2)) in congestive heart failure patients with central sleep apnea compared with those without apnea. Since the regulation of CBF serves to minimize oscillations in H(+) and Pco(2) at the site of the central chemoreceptors, it may play an important role in maintaining breathing stability. We hypothesized that an attenuated cerebrovascular reactivity to changes in Pa(CO(2)) would narrow the difference between the eupneic Pa(CO(2)) and the apneic threshold Pa(CO(2)) (DeltaPa(CO(2))), known as the CO(2) reserve, thereby making the subjects more susceptible to apnea. Accordingly, in seven normal subjects, we used indomethacin (Indo; 100 mg by mouth) sufficient to reduce the CBF response to CO(2) by approximately 25% below control. The CO(2) reserve was estimated during non-rapid eye movement (NREM) sleep. The apnea threshold was determined, both with and without Indo, in NREM sleep, in a random order using a ventilator in pressure support mode to gradually reduce Pa(CO(2)) until apnea occurred. results: Indo significantly reduced the CO(2) reserve required to produce apnea from 6.3 +/- 0.5 to 4.4 +/- 0.7 mmHg (P = 0.01) and increased the slope of the ventilation decrease in response to hypocapnic inhibition below eupnea (control vs. Indo: 1.06 +/- 0.10 vs. 1.61 +/- 0.27 l x min(-1) x mmHg(-1), P < 0.05). We conclude that reductions in the normal cerebral vascular response to hypocapnia will increase the susceptibility to apneas and breathing instability during sleep.

Adaptation to chronic hypoxia involves immune cell invasion and increased expression of inflammatory cytokines in rat carotid body

Exposure to chronic hypoxia (CH; 3-28 days at 380 Torr) induces adaptation in mammalian carotid body such that following CH an acute hypoxic challenge elicits an abnormally large increase in carotid sinus nerve impulse activity. The current study examines the hypothesis that CH initiates an immune response in the carotid body and that chemoreceptor hyperexcitability is dependent on the expression and action of inflammatory cytokines. CH resulted in a robust invasion of ED1(+) macrophages, which peaked on day 3 of exposure. Gene expression of proinflammatory cytokines, IL-1beta, TNFalpha, and the chemokine, monocyte chemoattractant protein-1, was increased >2-fold after 1 day of hypoxia followed by a >2-fold increase in IL-6 on day 3. After 28 days of CH, IL-6 remained elevated >5-fold, whereas expression of other cytokines recovered to normal levels. Cytokine expression was not restricted to immune cells. Studies of cultured type I cells harvested following 1 day of in vivo hypoxia showed elevated transcript levels of inflammatory cytokines. In situ hybridization studies confirmed expression of IL-6 in type I cells and also showed that CH induces IL-6 expression in supporting type II cells. Concurrent treatment of CH rats with anti-inflammatory drugs (ibuprofen or dexamethasone) blocked immune cell invasion and severely reduced CH-induced cytokine expression in carotid body. Drug treatment also blocked the development of chemoreceptor hypersensitivity in CH animals. Our findings indicate that chemoreceptor adaptation involves novel neuroimmune mechanisms, which may alter the functional phenotypes of type I cells and chemoafferent neurons.

α2-Adrenoreceptor mediated sympathoinhibition of heart rate during acute hypoxia is diminished in conscious prostacyclin synthase deficient mice

Acute hypoxia increases ventilatory drive in conscious animals, resulting in tachycardia. Sustained hypoxia changes the initial chemoreflex ventilatory increase to secondary ventilatory depression, which then evokes a gradual secondary heart rate (HR) reduction. Prostacyclin (PGI(2)) release is known to potentiate alpha(2)-adrenoreceptor (alpha(2)-AR) mediated inhibition of sympathoactivation during ischaemia and hypoxia. We examined whether alpha(2)-AR mediated sympathoinhibition was responsible for limiting hypoxic heart rate increases during initial sympathoactivation, and subsequent secondary HR depression, and if PGI(2) is required for sympathoinhibition of HR. The responses of unrestrained PGI(2) synthase deficient (PGID) and wild type (WT) mice to acute hypoxia (10% O(2) for 30 min) were investigated by simultaneous telemetry, whole body plethysmography and open-flow respirometry. PGID mice exhibited potentiated .V(E) (p < 0.007) after intraperitoneal vehicle injection (n = 8), but not so HR responses compared to WT mice during sustained hypoxia. Idazoxan (alpha(2)-AR antagonist, i.p. bolus 3 mg/kg) pretreatment did not change hypoxic ventilatory response in either group, but significantly elevated hypoxic HR in WT mice only (p < 0.013). Sodium meclofenamate (cyclooxygenase inhibition, i.p. bolus 25 mg/kg) pretreatment eliminated the potentiated .V(E) of PGID and caused significant basal hypotension that led to a transient hypertensive response to hypoxia. From these results, we suggest that alpha(2)-AR activation is required for coupling HR to central inspiratory drive during acute hypoxia, and that PGI(2) is required to enhance the inhibition of sympathoactivation.

Carotid body I1-imidazoline receptors: binding, visualization and modulatory function

The carotid body is influenced by many neurotransmitter receptors. A novel receptor specific for imidazolines has been implicated in cardiorespiratory regulation in the brain. To test for both I1-imidazoline and alpha2-adrenergic receptors, which also recognize imidazolines, specific [125I]p-iodoclonidine binding to carotid body membranes was characterized. The specific alpha2-agents epinephrine (100 microM) or SK&F 86466 (10 microM) inhibited only a portion of specific [125I]p-iodoclonidine binding in both cat and rabbit carotid bodies, indicating the presence of I1-imidazoline as well as alpha2-adrenergic sites. The distribution of [125I]p-iodoclonidine binding sites was visualized autoradiographically. The cat carotid body was intensely labeled by [125I]p-iodoclonidine, with both I1-imidazoline and alpha2-adrenergic sites expressed. The relevance of I1-imidazoline receptors in modulation of chemosensory discharge was determined in seven cats after alpha2-adrenergic blockade. Clonidine (100 microg/kg) facilitated chemosensory activity particularly under hypoxia. We conclude that I1-imidazoline receptors are expressed within the carotid body and may potentiate chemosensory discharge, in contrast to the inhibitory action of alpha2-adrenergic receptors.

Arachidonic acid inhibits both K+ and Ca2+ currents in isolated type I cells of the rat carotid body

Whole-cell patch-clamp recordings were used to investigate the effects of arachidonic acid (AA) on K+ and Ca2+ channels in isolated rat type I carotid body cells. AA (2-20 microM) produced a concentration-dependent inhibition of both K+ currents and Ca2+ channel currents. The effects of AA on K+ currents were unaffected by indomethacin (5 microM), phenidone (5 microM) or 1-aminobenzotriazole (3 mM), suggesting that AA did not exert its effects via cyclo-oxygenase, lipoxygenase or cytochrome P-450 (cP-450) metabolism. Our results suggest that AA directly and non-selectively inhibits ionic currents in rat type I carotid body cells.

Fatty acid acylation of dopamine in the carotid body

In this article, we put forward a hypothesis concerning the assembling and storage of dopamine molecules in the dense-core vesicles of the carotid body chemoreceptor cell. We posit that dopamine molecules are packed and sustained in the vesicular form due to the formation of N-acyldopamine, a condensation product of fatty acid acyl chain and dopamine at the amino group of the latter. N-acyldopamine would then be stored in a micelle-like supramolecular structure formed due to self-association through the hydrophilic dopamine headgroups. This hypothesis may help explain the perennial problem of the role of dopamine in chemoception. It also draws attention to the possibility of the existence of neurotransmitters in the N-acylated form. This could lead to the design of acylated compounds that would play a role of prodrugs slow-releasing active substances by hydrolysis into the desired environment.

The effects of a nonsteroidal antiinflammatory drug (ketoprofen) on morphine respiratory depression: a double-blind, randomized study in volunteers

Nonsteroidal antiinflammatory drugs (NSAIDs) decrease the postoperative requirements for opioid analgesic medication. To determine whether NSAIDs potentiate the respiratory effects of opioids, we studied the effects of ketoprofen (K), an NSAID, on respiratory depression induced by morphine (M) in volunteers. After ethics committee approval, 12 healthy male volunteers received infusions of K (1.5 mg/kg), M (0.1 mg/ kg), and KM (1.5 mg/kg + 0.1 mg/kg) in a double-blind, randomized, three-treatment, three-period cross-over trial. During the three sessions, CO2 rebreathing challenges for ventilatory and occlusion pressure responses to CO2 were performed immediately before and 10, 70, 130, 190, and 250 min after drug infusion over 10 min. Venous blood samples for plasma drug concentrations were withdrawn at the same times. Comparisons were made on slopes of ventilatory and occlusion pressure responses to CO2. Venous blood samples confirmed that morphine plasma concentrations were similar when subjects had received morphine alone and when they had received the combination of drugs. Morphine alone induced a respiratory depression with a decrease in both ventilatory and occlusion pressure responses to CO2. Ketoprofen alone did not produce any respiratory effects. The combination of drugs induced a decrease in ventilatory responses to CO2, but intergroup comparisons showed that this was significantly less marked than the decrease induced by morphine alone. In conclusion, for similar morphine plasma concentrations, respiratory depression was less marked with the combination of drugs than with morphine alone. Therefore, ketoprofen may reduce the respiratory depression induced by morphine.

Endogenous cyclo-oxygenase substrates mediate the neuroactivity of evening primrose oil in rats

The role of cyclo-oxygenase and or its substrate(s) on the neuroactivity of evening primrose oil was investigated on the basis that a blockade of cyclo-oxygenase activity using aspirin would inhibit neuroactivity of primrose oil if this effect was mediated by prostanoids. Streptozotocin diabetic rats and controls were all given large doses of aspirin, but only subgroups of them received primrose oil. Saphenous sensory A- and C-fibre, and sciatic motor conduction velocities were measured to assess neuroactivity of primrose oil. Aspirin enhanced the neuroactivity of primrose oil thus indicating that prostanoids are unlikely to mediate this neuroactivity, but also suggesting that substrates of cyclo-oxygenase are neuroactive. It is likely that cyclo-oxygenase antagonises neuroactivity of primrose oil by competing with the process for substrates. Thickly myelinated sensory A-fibres were most affected by primrose oil suggesting that the predominant sensory symptoms in diabetic neuropathy could be due to the sensitivity of sensory nerves to the metabolic aberration in diabetes. Normal nerve function is probably preserved by cyclo-oxygenase during an influx of neuroactive fatty acids from the gut, since inhibition of the enzyme rendered non-diabetic nerves vulnerable to dietary primrose oil.

Inhibition of [3H]catecholamine release and Ca2+ currents by prostaglandin E2 in rabbit carotid body chemoreceptor cells

Basal release of [3H]catecholamine ([3H]CA) from rabbit carotid bodies (CBs), previously incubated in the presence of [3H]tyrosine, was not significantly modified by prostaglandin E2 (PGE2). On the contrary, PGE2 (3-300 nM) produced a dose-dependent inhibition of the low PO2-evoked release of [3H]CA. The inhibition was greatest (55%) at a low intensity of hypoxic stimulation (incubating solution PO2 approximately 66 mmHg) and decreased with increasing intensities of hypoxia. Chronic denervation of the CB did not modify the response to PGE2. The release of [3H]CA induced by incubating the CBs in a hypercapnic-acidic solution (PCO2 approximately 132 mmHg; pH = 6.60) and by dinitrophenol (100 microM) was not significantly modified by 300 nM PGE2. PGE2 (300 nM) inhibited the release of [3H]CA elicited by incubating the CBs in a high K+ (35 mM)-containing solution. The release response elicited by high K+ (25 mM) was strongly augmented by a dihydropyridine agonist of Ca2+ channels, Bay K 8644, at a concentration of 1 microM. The Bay K 8644 effect was partly inhibited by PGE2 (300 nM). Using whole-cell recordings in freshly dispersed or short-term cultured chemoreceptor cells from adult rabbits it was found that Ca2+ currents (ICa) were reversibly inhibited by bath application of PGE2. A good parallelism exits between the dose-response curves for PGE2 inhibition of ICa in isolated chemoreceptor cells and high extracellular [K+]- or hypoxia-evoked release of [3H]CA from the whole CB.(ABSTRACT TRUNCATED AT 250 WORDS)