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

Premature birth, homeostatic plasticity and respiratory consequences of inflammation

Infants who are born premature can have persistent apnea beyond term gestation, reemergence of apnea associated with inflammation during infancy, increased risk of sudden unexplained death, and sleep disorder breathing during infancy and childhood. The autonomic nervous system, particularly the central neural networks that control breathing and peripheral and central chemoreceptors and mechanoreceptors that modulate the activity of the central respiratory network, are rapidly developing during the last trimester (22-37 weeks gestation) of fetal life. With advances in neonatology, in well-resourced, developed countries, infants born as young as 23 weeks gestation can survive. Thus, a substantial part of maturation of central and peripheral systems that control breathing occurs ex-utero in infants born at the limit of viability. The balance of excitatory and inhibitory influences dictates the ultimate output from the central respiratory network. We propose in this review that simply being born early in the last trimester can trigger homeostatic plasticity within the respiratory network tipping the balance toward inhibition that persists in infancy. We discuss the intersection of premature birth, homeostatic plasticity and biological mechanisms leading to respiratory depression during inflammation in former premature infants.

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.

The action of prostaglandins on ion channels

Prostaglandins, in particular PGE(2) and prostacyclin PGI(2) have diverse biological effects. Most importantly, they are involved in inflammation and pain. Prostaglandins in nano- and micromolar concentrations sensitize nerve cells, i.e. make them more sensitive to electrical or chemical stimuli. Sensitization arises from the effect of prostaglandins on ion channels and occurs both at the peripheral terminal of nociceptors at the site of tissue injury (peripheral sensitization) and at the synapses in the spinal cord (central sensitization). The first step is the binding of prostaglandins to receptors in the cell membrane, mainly EP and IP receptors. The receptors couple via G proteins to enzymes such as adenylate cyclase and phospholipase C (PLC). Activation of adenylate cyclase leads to increase of cAMP and subsequent activation of protein kinase A (PKA) or PKA-independent effects of cAMP, e.g. mediated by Epac (=exchange protein activated by cAMP). Activation of PLC causes increase of inositol phosphates and increase of cytosolic calcium. This article summarizes the effects of PGE(2), PGE(1), PGI2 and its stable analogues on non-selective cation channels and sodium, potassium, calcium and chloride channels. It describes the mechanism responsible for the facilitatory or inhibitory prostaglandin effects on ion channels. Understanding these mechanisms is essential for the development of useful new analgesics.

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.

Effects of reducing agents on glutathione metabolism and the function of carotid body chemoreceptor cells

Two current hypotheses of O2 sensing in the carotid body (CB) chemoreceptors suggest participation of oxygen reactive (ROS) species, but they are mechanistically opposed. One postulates that hypoxia decreases ROS levels; the other that hypoxia increases them. Yet, both propose that the ensuing alteration in the cellular redox environment is the key signal triggering hypoxic chemoreception. Since the glutathione redox pair is the main cellular buffer for ROS and the main determinant of the general redox environment of the cells, a way to test whether ROS participate in chemoreception is to determine glutathione levels and to correlate them with the activity of CB chemoreceptor cells. We found that hypoxia does not alter the glutathione reduction potential but that it activates chemoreceptor cell neurosecretion. Incubation of tissues with reduced glutathione increases the glutathione-reducing potential but does not activate chemoreceptor cells in normoxia nor does it modify hypoxic activation. Like reduced glutathione, N-acetylcysteine promoted a general reducing environment in the cells without alteration of chemoreceptor cell activity. N-(mercaptopropionyl)-glycine, like the two previous agents, increases the reduction potential of glutathione. In contrast, the compound activated chemoreceptor cells in normoxia, promoting a dose- and Ca(2+)-dependent neurosecretion and a potentiation of the hypoxic responses. The existence of multiple relationships between glutathione reduction potential in the cells and their activity indicates that the general cellular redox environment is not a factor determining chemoreceptor cell activation. It cannot be excluded that the local redox environments of restricted microdomain(s) in the cells with specific regulating mechanisms are important signals for chemoreceptor cell activity.

CO(2) and pH independently modulate L-type Ca(2+) current in rabbit carotid body glomus cells

The carotid bodies respond to changes in arterial O(2), CO(2), and pH, and Ca(2+) influx via voltage-gated Ca(2+) channels is an important step in the chemoreception process. The objectives of the present study were as follows: 1) to determine whether hypercapnia modulates Ca(2+) current in glomus cells, and if so, to determine if this modulation is secondary to changes in pH; 2) to examine the mechanism of CO(2) modulation of the Ca(2+) current; and 3) to determine whether the effects of hypercapnia and hypoxia on Ca(2+) channel activity in glomus cells are synergistic. The effects of CO(2) on Ca(2+) current were monitored in glomus cells isolated from rabbit carotid bodies using both perforated and conventional patch-clamp techniques. Raising CO(2) in the extracellular solution from 5 to 10% (hypercapnia) reversibly augmented the whole-cell Ca(2+) current. This augmentation was rapid and increased the whole-cell Ca(2+) current similarly in both the perforated and the conventional patch configurations by 16 +/- 2% (n = 5) and 15 +/- 1% (n = 32), respectively. The following observations suggest that the effects of CO(2) are not secondary to changes in pH: 1) isohydric hypercapnia (pH maintained at 7.4) augmented the Ca(2+) current by 24 +/- 2% (n = 6); 2) decreasing the pH of the extra- or intracellular solutions decreased the Ca(2+) current by 43 +/- 4% (n = 8) and 13 +/- 1% (n = 5), respectively; and 3) hypercapnia did not shift the half-maximal activation voltage (V(1/2)), whereas intracellular and extracellular acidosis alone caused shifts in V(1/2). Furthermore, 100 nM of a membrane-permeable protein kinase A inhibitor prevented the augmentation by CO(2), and 500 microM 8-Br-cAMP mimicked the effect of CO(2) by augmenting the Ca(2+) current by 10 +/- 2% (n = 6). Also, cyclic AMP levels in carotid bodies increased from 1.98 +/- 0.6 to 9.0 +/- 2 pmol/microg protein in response to hypercapnia. In contrast, decreasing pH in the nominal absence of CO(2) did not affect cAMP levels in rabbit carotid bodies. Further, nisoldipine, but not omega-conotoxin MVIIC, prevented augmentation of the Ca(2+) current by CO(2). In addition, when combined, hypercapnia and hypoxia augmented the Ca(2+) current by 26 +/- 4% (n = 7), which is greater than either stimulus alone, suggesting the effects are additive. Taken together, these results indicate that L-type Ca(2+) current is augmented by hypercapnia. The effect of CO(2) is not secondary to changes in pH and seems to be mediated by a protein kinase A-dependent mechanism. Furthermore, hypercapnia and hypoxia act additively in stimulating Ca(2+) current in glomus cells.

Augmentation of L-type calcium current by hypoxia in rabbit carotid body glomus cells: evidence for a PKC-sensitive pathway

Previous studies have suggested that voltage-gated Ca(2+) influx in glomus cells plays a critical role in sensory transduction at the carotid body chemoreceptors. The purpose of the present study was to determine the effects of hypoxia on the Ca(2+) current in glomus cells and to elucidate the underlying mechanism(s). Experiments were performed on freshly dissociated glomus cells from rabbit carotid bodies. Ca(2+) current was monitored using the whole cell configuration of the patch-clamp technique, with Ba(2+) as the charge carrier. Hypoxia (pO(2) = 40 mmHg) augmented the Ca(2+) current by 24 +/- 3% (n = 42, at 0 mV) in a voltage-independent manner. This effect was seen in a CO(2)/HCO(3)(-)-, but not in a HEPES-buffered extracellular solution at pH 7.4 (n = 6). When the pH of a HEPES-buffered extracellular solution was lowered from 7.4 to 7. 0, hypoxia augmented the Ca(2+) current by 20 +/- 5% (n = 4, at 0 mV). Nisoldipine, an L-type Ca(2+) channel blocker (2 microM, n = 6), prevented, whereas, omega-conotoxin MVIIC (2 microM, n = 6), an inhibitor of N and P/Q type Ca(2+) channels, did not prevent augmentation of the Ca(2+) current by hypoxia, implying that low oxygen affects L-type Ca(2+) channels in glomus cells. Protein kinase C (PKC) inhibitors, staurosporine (100 nM, n = 6) and bisindolylmaleimide (2 microM, n = 8, at 0 mV), prevented, whereas, a protein kinase A inhibitor (4 nM PKAi, n = 10) did not prevent the hypoxia-induced increase of the Ca(2+) current. Phorbol 12-myristate 13-acetate (PMA, 100 nM), a PKC activator, augmented the Ca(2+) current by 20 +/- 3% (n = 8, at 0 mV). In glomus cells treated with PMA overnight (100 nM), hypoxia did not augment the Ca(2+) current (-3 + 4%, n = 5, at 0 mV). Immunocytochemical analysis revealed PKCdelta-like immunoreactivity in the cytosol of the glomus cells. Following hypoxia (6% O(2) for 5 min), PKCdelta-like immunoreactivity translocated to the plasma membrane in 87 +/- 3% of the cells, indicating PKC activation. These results demonstrate that hypoxia augments Ca(2+) current through L-type Ca(2+) channels via a PKC-sensitive mechanism.

Ca2+ current in rabbit carotid body glomus cells is conducted by multiple types of high-voltage-activated Ca2+ channels

Ca2+ current in rabbit carotid body glomus cells is conducted by multiple types of high-voltage-activated Ca2+ channels. J. Neurophysiol. 78: 2467-2474, 1997. Carotid bodies are sensory organs that detect changes in arterial oxygen. Glomus cells are presumed to be the initial sites for sensory transduction, and Ca2+-dependent neurotransmitter release from glomus cells is believed to be an obligatory step in this response. Some information exists on the Ca2+ channels in rat glomus cells. However, relatively little is known about the types of Ca2+ channels present in rabbit glomus cells, the species in which most of the neurotransmitter release studies have been performed. Therefore we tested the effect of specific Ca2+ channel blockers on current recorded from freshly dissociated, adult rabbit carotid body glomus cells using the whole cell configuration of the patch-clamp technique. Macroscopic Ba2+ current elicited from a holding potential of -80 mV activated at a Vm of approximately -30 mV, peaked between 0 and +10 mV and did not inactivate during 25-ms steps to positive test potentials. Prolonged ( approximately 2 min) depolarized holding potentials inactivated the current with a V1/2 of -47 mV. There was no evidence for T-type channels. On steps to 0 mV, 6 mM Co2+ decreased peak inward current by 97 +/- 1% (mean +/- SE). Nisoldipine (2 mu M), 1 mu M omega-conotoxin GVIA, and 100 nM omega-agatoxin IVa each blocked a portion of the macroscopic Ca2+ current (30 +/- 5, 33 +/- 5, and 19 +/- 3% after rundown correction, respectively). Simultaneous application of these blockers revealed a resistant current that was not affected by 1 mu M omega-conotoxin MVIIC. This resistant current constituted 27 +/- 5% of the total macroscopic Ca2+ current. Each blocker had an effect in every cell so tested. However, the relative proportion of current blocked varied from cell to cell. These results suggest that L, N, P, and resistant channel types each conduct a significant proportion of the macroscopic Ca2+ current in rabbit glomus cells. Hypoxia-induced neurotransmitter release from glomus cells may involve one or more of these channels.

Cholera and pertussis toxins reveal multiple regulation of cAMP levels in the rabbit carotid body

It is known that hypoxia (PO2 approximately equal to 66-18 mm Hg), acting via unknown receptors, increases carotid body cAMP levels in Ca(2+)-free solutions, indicating that low PO2 activates adenylate cyclases independently of the action of the released neurotransmitters. The aim of the present work was to investigate the involvement of G proteins in the genesis of the basal level of cAMP and on the increase in cAMP induced by low PO2. In carotid body homogenates, cholera toxin- and pertussis toxin-induced [32P]ADP-ribosylation of two protein bands of approximately equal to 42 and 45 kDa, and approximately equal to 39 and 40 kDa respectively; in both cases, prior incubation of the carotid bodies with the toxins reduced [32P]ADP-ribosylation by > 90%. In intact carotid bodies, cholera toxin treatment increased cAMP levels more in normoxic than in hypoxic organs, indicating that hypoxia releases neurotransmitters acting on receptors negatively coupled to adenylate cyclases. Cholera toxin-treated carotid bodies incubated in Ca(2+)-free solution had identical cAMP levels in normoxia and in hypoxia. In pertussis toxin-treated normoxic carotid bodies the cAMP level was close to control, but in pertussis toxin-treated hypoxic carotid bodies cAMP rose to a level similar to those seen in normoxic cholera toxin-treated organs, indicating that low PO2 releases neurotransmitters acting on receptors positively coupled to adenylate cyclases. Pertussis toxin-treated carotid bodies incubated in Ca(2+)-free solution lost their capacity to increase cAMP in response to hypoxia, indicating that a G protein sensitive to pertussis toxin is needed for this response. This implies that the carotid bodies express a pertussis toxin-sensitive G protein positively coupled to adenylate cyclases, or that a Gs protein requiring the cooperative action of Go/Gi donated beta gamma subunits mediates the increase in cAMP level produced by hypoxia.

L- and N-type Ca2+ channels in adult rat carotid body chemoreceptor type I cells

1. Whole-cell voltage-dependent Ca2+ currents recorded from chemoreceptor type I cells of the adult rat carotid body had maximum amplitudes of -94 pA in 10 mM Ca2+ and were half-inactivated at a holding potential of -38 mV. Somatostatin and dopamine inhibited whole-cell Ca2+ current in type I cells. 2. The dihydropyridine agonist (+)202-791 increased the Ca2+ current amplitude by 106% at a step potential of -18 mV. The dihydropyridine antagonist nimodipine decreased the Ca2+ current amplitude by 40% from a holding potential of -80 mV, and by 74% from a holding potential of -60 mV. The nimodipine-sensitive current had a maximum amplitude at a membrane potential of -12 mV. omega-Conotoxin GVIA (omega-CgTX GVIA) blocked the whole-cell Ca2+ current by 40%. The omega-CgTX GVIA-sensitive current had a maximum amplitude at a membrane potential of +2 mV. 3. In summary, type I cells of the adult rat carotid body have dihydropyridine-sensitive L-type and omega-conotoxin GVIA-sensitive N-type voltage-dependent Ca2+ channels. These channels may play a role in the voltage-gated entry of Ca2+ necessary for stimulus-secretion coupling in response to changes in arterial PO2, PCO2 and pH. Inhibition of the Ca2+ currents by somatostatin and dopamine may alter the chemotransduction signal in type I cells.

Cellular mechanisms of oxygen chemoreception in the carotid body

The carotid bodies (CB) are arterial chemoreceptors that by sensing changes of arterial PO2, PCO2 and pH can initiate and modify ventilatory and cardiovascular reflexes in order to maintain PO2, PCO2 and pH within physiological levels. It is now generally accepted that the glomus or type I cells of the CB are the transducers of hypoxic stimuli, and relay chemosensory information to the brainstem via neurotransmitter release at synaptic contacts with afferent terminals of the carotid sinus nerve. This article reviews the mechanisms of the O2-sensing process at the cellular level. We consider first the transduction of the hypoxic stimulus, in which most of the experimental evidence currently favors a mechanism involving modulation of the electrical properties of type I cells. The last part of the article deals with the transmission of the stimulus between type I cells and afferent nerve terminals, and we present an overview on the issue of neurotransmission in the CB, summarizing the actions of the main neurotransmitters present in the organ.

Effects of fluoride and cholera and pertussis toxins on sensory transduction in the carotid body

The regulation of the chemoreceptor cell function by G proteins has been studied by measuring the release of 3H-labeled catecholamines ([3H]CA) in carotid bodies (CBs) treated with fluoride, cholera toxin (CTX), and pertussis toxin (PTX). Fluoride augmented the basal release of [3H]CA in a dose- (5-20 mM) and Ca(2+)-dependent manner. Nisoldipine (1 microM) and ethylisopropyl amiloride (EIPA; 10 microM) inhibited this effect by approximately 60%, and both drugs combined inhibited it in full. BAY K 8644 (1 microM) doubled the effect of fluoride. The effects of fluoride on the stimulus-evoked release of [3H]CA varied with the type of stimulus and the duration of the treatment. Simultaneous application of fluoride with the stimulus increased by five times the release evoked by hypoxia and by two times that by K+ and dinitrophenol (DNP). Preincubation with fluoride for 1 h caused an inhibition (approximately 70%) of the release evoked by high K+ and veratridine, whereas that evoked by DNP and low PO2 was still augmented (approximately 2 times). Preincubation (4 h) of the CBs with CTX (3 micrograms/ml) reduced by 54% the release of [3H]CA evoked by 35 mM K+ but did not affect that evoked by low PO2 or DNP. A similar treatment with PTX (1 microgram/ml) affected only the release of [3H]CA evoked by DNP, reducing it by 65%. The data show that fluoride, CTX, and PTX have different effects on the release of [3H]CA evoked by high external K+, DNP, and low PO2, indicating that the stimulus-secretion coupling process for each stimulus is differently regulated by G proteins.