Calpain activation by ROS mediates human ether-a-go-go-related gene protein degradation by intermittent hypoxia.

Human ether-a-go-go-related gene (hERG) channels conduct delayed rectifier K(+) current. However, little information is available on physiological situations affecting hERG channel protein and function. In the present study we examined the effects of intermittent hypoxia (IH), which is a hallmark manifestation of sleep apnea, on hERG channel protein and function. Experiments were performed on SH-SY5Y neuroblastoma cells, which express hERG protein. Cells were exposed to IH consisting of alternating cycles of 30 s of hypoxia (1.5% O2) and 5 min of 20% O2. IH decreased hERG protein expression in a stimulus-dependent manner. A similar reduction in hERG protein was also seen in adrenal medullary chromaffin cells from IH-exposed neonatal rats. The decreased hERG protein was associated with attenuated hERG K(+) current. IH-evoked hERG protein degradation was not due to reduced transcription or increased proteosome/lysomal degradation. Rather it was mediated by calcium-activated calpain proteases. Both COOH- and NH2-terminal sequences of the hERG protein were the targets of calpain-dependent degradation. IH increased reactive oxygen species (ROS) levels, intracellular Ca(2+) concentration ([Ca(2+)]i), calpain enzyme activity, and hERG protein degradation, and all these effects were prevented by manganese-(111)-tetrakis-(1-methyl-4-pyridyl)-porphyrin pentachloride, a membrane-permeable ROS scavenger. These results demonstrate that activation of calpains by ROS-dependent elevation of [Ca(2+)]i mediates hERG protein degradation by IH.

Long-term regulation of carotid body function: acclimatization and adaptation--invited article.

Physiological responses to hypoxia either continuous (CH) or intermittent (IH) depend on the O(2)-sensing ability of the peripheral arterial chemoreceptors, especially the carotid bodies, and the ensuing reflexes play important roles in maintaining homeostasis. The purpose of this article is to summarize the effects of CH and IH on carotid body function and the underlying mechanisms. CH increases baseline carotid body activity and sensitizes the response to acute hypoxia. These effects are associated with hyperplasia of glomus cells and neovascularization. Enhanced hypoxic sensitivity is due to alterations in ion current densities as well as changes in neurotransmitter dynamics and recruitment of additional neuromodulators (endothelin-1, ET-1) in glomus cells. Morphological alterations are in part due to up-regulation of growth factors (e.g. VEGF). Hypoxia-inducible factor-1 (HIF-1), a transcriptional activator might underlie the remodeling of carotid body structure and function by CH. Chronic IH, on the other hand, is associated with recurrent apneas in adults and premature infants. Two major effects of chronic IH on the adult carotid body are sensitization of the hypoxic sensory response and long-lasting increase in baseline activity i.e., sensory long-term facilitation (LTF) which involve reactive oxygen species (ROS) and HIF-1. In neonates, chronic IH leads to sensitization of the hypoxic response but does not induce sensory LTF. Chronic IH-induced sensitization of the carotid body response to hypoxia increases the likelihood of unstable breathing perpetuating in more number of apneas, whereas sensory LTF may contribute to increased sympathetic tone and systemic hypertension associated with recurrent apneas.

NADPH oxidase is required for the sensory plasticity of the carotid body by chronic intermittent hypoxia.

Respiratory motoneuron response to hypoxia is reflex in nature and carotid body sensory receptor constitutes the afferent limb of this reflex. Recent studies showed that repetitive exposures to hypoxia evokes long term facilitation of sensory nerve discharge (sLTF) of the carotid body in rodents exposed to chronic intermittent hypoxia (CIH). Although studies with anti-oxidants suggested the involvement of reactive oxygen species (ROS)-mediated signaling in eliciting sLTF, the source of and the mechanisms associated with ROS generation have not yet been investigated. We tested the hypothesis that ROS generated by NADPH oxidase (NOX) mediate CIH-evoked sLTF. Experiments were performed on ex vivo carotid bodies from rats and mice exposed either to 10 d of CIH or normoxia. Acute repetitive hypoxia evoked a approximately 12-fold increase in NOX activity in CIH but not in control carotid bodies, and this effect was associated with upregulation of NOX2 mRNA and protein, which was primarily localized to glomus cells of the carotid body. sLTF was prevented by NOX inhibitors and was absent in mice deficient in NOX2. NOX activation by CIH required 5-HT release and activation of 5-HT(2) receptors coupled to PKC signaling. Studies with ROS scavengers revealed that H(2)O(2) generated from O(2).(-) contributes to sLTF. Priming with H(2)O(2) elicited sLTF of carotid bodies from normoxic control rats and mice, similar to that seen in CIH-treated animals. These observations reveal a novel role for NOX-induced ROS signaling in mediating sensory plasticity of the carotid body.

Significance of pulmonary vagal afferents for respiratory muscle activity in the cat.

The influence of vagal stretch receptor afferents on respiratory motor-output and respiratory changes in esophageal pressure (DeltaP(es)) was studied in anaesthetized cats. Tracheal occlusions and lung inflations were performed during hyperoxic normocapnia, during electrical stimulation of one carotid sinus nerve (CSN) or the intracranial medullary chemosensitivity (MCS), during hypercapnia or the combination of CSN and hypercapnia. Tracheal occlusions during inspiration led to increased and prolonged inspiratory muscle (IM) activity. Moderate hyperinflation in inspiration decreased and shortened inspiratory motor output. Changes in esophageal pressure and in amplitude and discharge duration of IM are largely proportional (0.84>r<0.98) to lung volume above normal endexpiratory volume (FRC). The effects are described as the Hering-Breuer inspiration inhibitory reflex (HB-IIR). Tracheal occlusion or hyperinflation in end-inspiratory position not only prolonged expiration but also activated expiratory muscles (EM). The effects linearly (0.86>r<0.98) increased with elevation of lung volume. We refer to these effects as the Hering-Breuer expiration facilitatory reflex (HB-EFR). Severe hyperinflation or rapid inflation of the lungs during inspiration, however, led to an inspiratory facilitation with increased IM activity. During concomitant chemoreflex activation, CSN or MCS stimulation, respiratory hypercapnia, or the combination of both, the extent of the above described responses of IM and EM activity were significantly (0.05>p<0.0002) enlarged. The changes in the discharge period of IM and EM following lung inflation were smaller in the presence of the increased chemical respiratory drive (0.01>p<0.005). The relative changes in EM responses to lung inflations during increased respiratory drive were greater than those of IM. Bilateral vagotomy abolished the respiratory responses to tracheal occlusion and hyperinflation of the lungs. The results of the present investigation show that aside from the well-known inhibition of inspiration, vagal slowly adapting lung stretch receptors facilitate expiration. The sensitivity of the lung reflexes is enhanced with increasing respiratory drive. The HB-inspiration inhibitory reflex limits the depth of lung inflation, whereas the HB-expiration facilitatory reflex promotes an effective lung deflation. Both reflex mechanisms, the inspiratory and expiratory one, are present in eupnoeic breathing, but play an important role during increased chemoreflex drive and obstruction of expiration, e.g., with increased external airway resistance.

Acute lung injury augments hypoxic ventilatory response in the absence of systemic hypoxemia.

The objective of the present study was to examine the impact of early stages of lung injury on ventilatory control by hypoxia and hypercapnia. Lung injury was induced with intratracheal instillation of bleomycin (BM; 1 unit) in adult, male Sprague-Dawley rats. Control animals underwent sham surgery with saline instillation. Five days after the injections, lung injury was present in BM-treated animals as evidenced by increased neutrophils and protein levels in bronchoalveolar lavage fluid, as well as by changes in lung histology and computed tomography images. There was no evidence of pulmonary fibrosis, as indicated by lung collagen content. Basal core body temperature, arterial Po(2), and arterial Pco(2) were comparable between both groups of animals. Ventilatory responses to hypoxia (12% O(2)) and hypercapnia (7% CO(2)) were measured by whole body plethysmography in unanesthetized animals. Baseline respiratory rate and the hypoxic ventilatory response were significantly higher in BM-injected compared with control animals (P = 0.003), whereas hypercapnic ventilatory response was not statistically different. In anesthetized, spontaneously breathing animals, response to brief hyperoxia (Dejours' test, an index of peripheral chemoreceptor sensitivity) and neural hypoxic ventilatory response were augmented in BM-exposed relative to control animals, as measured by diaphragmatic electromyelograms. The enhanced hypoxic sensitivity persisted following bilateral vagotomy, but was abolished by bilateral carotid sinus nerve transection. These data demonstrate that afferent sensory input from the carotid body contributes to a selective enhancement of hypoxic ventilatory drive in early lung injury in the absence of pulmonary fibrosis and arterial hypoxemia.

Oxygen sensing in the body.

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

Modulation of the hypoxic sensory response of the carotid body by 5-hydroxytryptamine: role of the 5-HT2 receptor.

Previous studies have shown that glomus cells of the carotid body express 5-hydroxytryptamine (5-HT). The aim of this study was to elucidate the role of 5-HT on the hypoxic sensory response (HSR) of the carotid body. Sensory activity was recorded from multi-fiber (n=16) and single-fiber (n=8) preparations of ex vivo carotid bodies harvested from anesthetized, adult rats. 5-HT (3 microM) had no significant effect on the magnitude or on the onset of the HSR. However, 5-HT consistently prolonged the time necessary for the sensory activity to return to baseline following the termination of the hypoxic challenge. Ketanserin (40 microM), a 5-HT2 receptor antagonist completely prevented 5-HT-induced prolongation of the HSR, whereas had no effect on the control HSR (onset, magnitude, and time for decay without 5-HT). Carotid bodies expressed 5-HT, but hypoxia did not facilitate 5-HT release. These observations suggest that 5-HT is not critical for the HSR of the rat carotid body, but it modulates the dynamics of the HSR via its action on 5-HT2 receptors.

Intermittent hypoxia: cell to system.

This symposium was organized to present research dealing with the effects of intermittent hypoxia on cardiorespiratory systems and cellular mechanisms. The pattern of neural impulse activity has been shown to be critical in the induction of genes in neuronal cells and involves distinct signaling pathways. Mechanisms associated with different patterns of intermittent hypoxia might share similar mechanisms. Chronic intermittent hypoxia selectively augments carotid body sensitivity to hypoxia and causes long-lasting activation of sensory discharge. Intermittent hypoxia also activates hypoxia-inducible factor-1. Reactive oxygen species are critical in altering carotid body function and hypoxia-inducible factor-1 activation caused by intermittent hypoxia. Blockade of serotonin function in the spinal cord prevents long-term facilitation in respiratory motor output elicited by episodic hypoxia and requires de novo protein synthesis. Chronic intermittent hypoxia leads to sustained elevation in arterial blood pressure and is associated with upregulation of catecholaminergic and renin-angiotensin systems and downregulation of nitric oxide synthases.

Selected Contribution: Improved anoxic tolerance in rat diaphragm following intermittent hypoxia.

Intermittent hypoxia (IH), associated with obstructive sleep apnea, initiates adaptive physiological responses in a variety of organs. Little is known about its influence on diaphragm. IH was simulated by exposing rats to alternating 15-s cycles of 5% O2 and 21% O2 for 5 min, 9 sets/h, 8 h/day, for 10 days. Controls did not experience IH. Diaphragms were excised 20-36 h after IH. Diaphragm bundles were studied in vitro or analyzed for myosin heavy chain isoform composition. No differences in maximum tetanic stress were observed between groups. However, peak twitch stress (P < 0.005), twitch half-relaxation time (P < 0.02), and tetanic stress at 20 or 30 Hz (P < 0.05) were elevated in IH. No differences in expression of myosin heavy chain isoforms or susceptibility to fatigue were seen. Contractile function after 30 min of anoxia (95% N2-5% CO2) was markedly preserved at all stimulation frequencies during IH and at low frequencies after 15 min of reoxygenation. Anoxia-induced increases in passive muscle force were eliminated in the IH animals (P < 0.01). These results demonstrate that IH induces adaptive responses in the diaphragm that preserve its function in anoxia.

Oxygen sensing during intermittent hypoxia: cellular and molecular mechanisms.

To the majority of the population, recurrent episodes of hypoxia are more likely encountered in life than sustained hypoxia. Until recently, much of the information on the long-term effects of intermittent hypoxia has come from studies on human subjects experiencing chronic recurrent apneas. Recent development of animal models of intermittent hypoxia and techniques for exposing cell cultures to alternating cycles of hypoxia have led to new information on the effects of episodic hypoxia on oxygen-sensing mechanisms in the carotid body chemoreceptors and regulation of gene expression. The purpose of this review is to highlight some recent studies on the effects of intermittent hypoxia on oxygen sensing at the carotid bodies and regulation of gene expression. In a rodent model, chronic intermittent hypoxia selectively enhances hypoxic sensitivity of the carotid body chemoreceptors. More interestingly, chronic intermittent hypoxia also induces a novel form of plasticity in the carotid body, leading to long-term facilitation in the sensory discharge. Studies on cell cultures reveal that intermittent hypoxia is more potent in activating activator protein-1 and hypoxia-inducible factor-1 transcription factors than sustained hypoxia. Moreover, some evidence suggests that intermittent hypoxia utilizes intracellular signaling pathways distinct from sustained hypoxia. Reactive oxygen species generated during the reoxygenation phase of intermittent hypoxia might play a key role in the effects of intermittent hypoxia on carotid body function and gene expression. Global gene profile analysis in cell cultures suggests that certain genes are selectively affected by intermittent hypoxia, some upregulated and some downregulated. It is suggested that, in intact animals, coordinated gene regulation of gene expression might be critical for eliciting phenotypic changes in the cardiorespiratory systems in response to intermittent hypoxia. It is hoped that future studies will unravel new mechanisms that are unique to intermittent hypoxia that may lead to a better understanding of the changes in the cardiorespiratory systems and new therapies for diseases associated with chronic recurrent episodes of hypoxia.

Release of substance P by low oxygen in the rabbit carotid body: evidence for the involvement of calcium channels.

Carotid bodies from diverse species contain substance P (SP), an 11-residue peptide that belongs to the tachykinin peptide family. Previous studies indicated that SP is excitatory to the carotid body and is associated with sensory response to hypoxia. However, release of SP from the carotid body during hypoxia has not been documented. In the present study, we determined whether hypoxia releases SP from the carotid body and further characterized the mechanism(s) associated with SP release by low oxygen. The release of SP from superfused rabbit carotid body was determined by an enzyme immunoassay (EIA). SP-like immunoreactivity was localized to many glomus cells and nerve fibers and the concentration of SP in the rabbit carotid body was 1.5+/-0.1 ng/mg protein. For release studies, carotid bodies (n=56) were superfused with a modified Tyrode medium containing Hepes buffer, pH 7.4, saturated with either room air (normoxia) or hypoxic gas mixtures. The basal release of SP during normoxia was 51.0+/-1.5 fmol/min per mg protein. Hypoxia increased SP release from the carotid body and the magnitude of release is dependent on the severity of hypoxic stimulus. Moderate hypoxia (pO2, 79+/-4 mmHg) stimulated SP release by approximately 50%, whereas SP release during severe hypoxia (pO2, 11+/-6 mmHg) was 2-fold higher than the normoxic control. A similar pattern of SP release was also observed when superfusion medium containing CO2-HCO3 buffer, pH 7.4, was used for release studies. To examine the mechanism(s) associated with hypoxia-induced SP release from the carotid body, moderate level of hypoxia (12% O2+N2) was used. Omission of calcium in the superfusion medium markedly attenuated hypoxia-induced SP release (>95%), whereas the basal release of SP was unaffected. Cd2+ (100 microM), a voltage-dependent Ca2+ channel blocker, abolished hypoxia-induced SP release. About 85% of SP release by hypoxia was inhibited by omega-conotoxin GVIA (1 microM), an N-type Ca2+ channel blocker, whereas nitrendipine (1.5 microM), an inhibitor of L-type Ca2+ channel partially attenuated ( approximately 65%) hypoxia-induced SP release. By contrast, omega-agatoxin TK (50 nM), a P/Q-type Ca2+ channel inhibitor, had no significant effect (P>0.05, n=6). These results suggest that SP is released from the rabbit carotid body by hypoxia that depends on the severity of the hypoxic stimulus. Further, SP release by hypoxia is a calcium-dependent process and is primarily mediated by N- and L-type Ca2+ channels.

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.

Cellular mechanisms of oxygen sensing at the carotid body: heme proteins and ion channels.

The purpose of this article is to highlight some recent concepts on oxygen sensing mechanisms at the carotid body chemoreceptors. Most available evidence suggests that glomus (type I) cells are the initial site of transduction and they release transmitters in response to hypoxia, which in turn depolarize the nearby afferent nerve ending, leading to an increase in sensory discharge. Two main hypotheses have been advanced to explain the initiation of the transduction process that triggers transmitter release. One hypothesis assumes that a biochemical event associated with a heme protein triggers the transduction cascade. Supporting this idea it has been shown that hypoxia affects mitochondrial cytochromes. In addition, there is a body of evidence implicating non-mitochondrial enzymes such as NADPH oxidases, NO synthases and heme oxygenases located in glomus cells. These proteins could contribute to transduction via generation of reactive oxygen species, nitric oxide and/or carbon monoxide. The other hypothesis suggests that a K(+) channel protein is the oxygen sensor and inhibition of this channel and the ensuing depolarization is the initial event in transduction. Several oxygen sensitive K(+) channels have been identified. However, their roles in initiation of the transduction cascade and/or cell excitability are unclear. In addition, recent studies indicate that molecular oxygen and a variety of neurotransmitters may also modulate Ca(2+) channels. Most importantly, it is possible that the carotid body response to oxygen requires multiple sensors, and they work together to shape the overall sensory response of the carotid body over a wide range of arterial oxygen tensions.

Intracellular pathways linking hypoxia to activation of c-fos and AP-1.

Organisms respond to hypoxia through detection of blood oxygen levels by sensors at peripheral chemoreceptors and by receptors in certain key cells of the body. The pathways over which peripheral chemoreceptor signals are transmitted to respiratory muscles are well established. However, the intracellular pathways that transmit hypoxic stimulus to gene activation are just being identified. Using anti-sense c-fos strategy, we have shown that c-fos is essential for the activation of activator protein-1 transcription factor complex (AP-1) and subsequent stimulation of downstream genes such as tyrosine hydroxylase (TH; Mishra et al. 1998). The purpose of the present study was to identify intracellular pathways that link hypoxia to activation of c-fos. The results of the present study show that hypoxia causes Ca2+ influx through L-type voltage gated Ca2+ channels and that hypoxia-induced c-fos gene expression is Ca2+/calmodulin dependent. We also demonstrate that hypoxia activates the extracellular-regulated kinase (ERK) and p38, but not JNK. Further, phosphorylation of ERK is essential for c-fos activation via SRE cis-element. Further characterization of nuclear signalling pathways provides evidence for the involvement of Src, a non receptor protein tyrosine kinase, and Ras, a small G protein, in the hypoxia-induced c-fos gene expression. These results suggest a possible role for non-receptor protein tyrosine kinases in propagating signals from G-protein coupled receptors to the activation of immediate early genes such as c-fos during hypoxia.