Medullary and Hypothalamic Functional Magnetic Imaging During Acute Hypoxia in Tracing Human Peripheral Chemoreflex Responses.

[Figure: see text].

Carotid chemoreceptors have a limited role in mediating the hyperthermia-induced hyperventilation in exercising humans.

Hyperthermia causes hyperventilation at rest and during exercise. We previously reported that carotid chemoreceptors partly contribute to the hyperthermia-induced hyperventilation at rest. However, given that a hyperthermia-induced hyperventilation markedly differs between rest and exercise, the results obtained at rest may not be representative of the response in exercise. Therefore, we evaluated whether carotid chemoreceptors contribute to hyperthermia-induced hyperventilation in exercising humans. Eleven healthy young men (23 ± 2 yr) cycled in the heat (37°C) at a fixed submaximal workload equal to ~55% of the individual's predetermined peak oxygen uptake (moderate intensity). To suppress carotid chemoreceptor activity, 30-s hyperoxia breathing (100% O2) was performed at rest (before exercise) and during exercise at increasing levels of hyperthermia as defined by an increase in esophageal temperature of 0.5°C (low), 1.0°C (moderate), 1.5°C (high), and 2.0°C (severe) above resting levels. Ventilation during exercise gradually increased as esophageal temperature increased (all P ≤ 0.05), indicating that hyperthermia-induced hyperventilation occurred. Hyperoxia breathing suppressed ventilation in a greater manner during exercise (-9 to -13 l/min) than at rest (-2 ± 1 l/min); however, the magnitude of reduction during exercise did not differ at low (0.5°C) to severe (2.0°C) increases in esophageal temperature (all P > 0.05). Similarly, hyperoxia-induced changes in ventilation during exercise as assessed by percent change from prehyperoxic levels were not different at all levels of hyperthermia (~15-20%, all P > 0.05). We show that in young men carotid chemoreceptor contribution to hyperthermia-induced hyperventilation is relatively small at low-to-severe increases in body core temperature induced by moderate-intensity exercise in the heat. NEW & NOTEWORTHY Exercise-induced increases in hyperthermia cause a progressive increase in ventilation in humans. However, the mechanisms underpinning this response remain unresolved. We showed that in young men hyperventilation associated with exercise-induced hyperthermia is not predominantly mediated by carotid chemoreceptors. This study provides important new insights into the mechanism(s) underpinning the regulation of hyperthermia-induced hyperventilation in humans and suggests that factor(s) other than carotid chemoreceptors play a more important role in mediating this response.

Preventing acute asthmatic symptoms by targeting a neuronal mechanism involving carotid body lysophosphatidic acid receptors.

Asthma accounts for 380,000 deaths a year. Carotid body denervation has been shown to have a profound effect on airway hyper-responsiveness in animal models but a mechanistic explanation is lacking. Here we demonstrate, using a rat model of asthma (OVA-sensitized), that carotid body activation during airborne allergic provocation is caused by systemic release of lysophosphatidic acid (LPA). Carotid body activation by LPA involves TRPV1 and LPA-specific receptors, and induces parasympathetic (vagal) activity. We demonstrate that this activation is sufficient to cause acute bronchoconstriction. Moreover, we show that prophylactic administration of TRPV1 (AMG9810) and LPA (BrP-LPA) receptor antagonists prevents bradykinin-induced asthmatic bronchoconstriction and, if administered following allergen exposure, reduces the associated respiratory distress. Our discovery provides mechanistic insight into the critical roles of carotid body LPA receptors in allergen-induced respiratory distress and suggests alternate treatment options for asthma.

Research Report: Intermittent hypobaric hypoxia and hyperbaric oxygen on GAP-43 in the rat carotid body.

Adaptive changes in the carotid body (CB) including the expression of the growth-associated protein-43 (GAP-43) have been studied in response to low, but not high, oxygen exposure. Expression of GAP-43 in the CB of rats under different atmospheric pressures and oxygen partial pressure (PO2) conditions was investigated. Mature male Sprague-Dawley rats were exposed to intermittent hypobaric hypoxia (IHH, 0, 1, 2 and 3 weeks), intermittent hyperbaric oxygen (IHBO2, 0, 1, 5 and 10 days, sacrificed six hours or 24 hours after the last HBO2 exposure), and intermittent hyperbaric normoxia (IHN, same treatment pattern as IHBO2). GAP-43 was highly expressed (mainly in type I cells) in the CB of normal rats. IHH u-regulated GAP-43 expression in the CB with significant differences (immunohistochemical staining [IHC]: F(3,15)=40.64, P < 0.01; western blot [WB]: F(3,16) = 53.52, P < 0.01) across the subgroups. GAP-43 expression in the CB was inhibited by IHBO2 (controls vs. IHBO2 groups, IHC: F(6,30) = 15.85, P < 0.01; WB: F(6,29) = 15.95, P < 0.01). No detectable changes in GAP-43 expression were found for IHN. These findings indicated that different PO2 conditions, but not air pressures, played an important role in the plasticity of the CB, and that GAP-43 might be a viable factor for the plasticity of the CB.

Nitric oxide in the hypothalamus-pituitary axis mediates increases in brain glucose retention induced by carotid chemoreceptor stimulation with cyanide in rats.

Neuronal nitric oxide synthase (nNOS), which catalyzes the generation of nitric oxide (NO), is expressed by neuron subpopulations in the CNS. Nitric oxide is involved in neurotransmission and central glucose homeostasis. Our prior studies have shown that carotid body receptors participate in brain glucose regulation in vivo, and suggest the presence of a NO tonic mechanism in the solitary tract nucleus (STn). However, the role of NO within STn in glucose control remains unknown. In this study, we explored the potential regulatory role of NO on brain glucose retention induced by carotid body chemoreceptor anoxic stimulation with sodium cyanide (NaCN) which inhibits oxidative metabolism. Intracisternal infusions of nitroxidergic drugs before carotid chemoreceptor stimulation in anesthetized rats, elicited changes in nitrite concentration in plasma and hypothalamus-pituitary (H-P) tissue, as well as in gene expression of neuronal and inducible isoforms (nNOS and iNOS) in H-P tissue. The changes observed in above variables modified brain glucose retention in an opposite direction. When the NO donor, sodium nitroprusside (SNP), was given before carotid stimulation, nitrite concentration in plasma and H-P tissue, and gene expression of nNOS and iNOS in H-P tissue increased, whereas brain glucose retention decreased. In contrast, when the NOS inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) was infused immediately before carotid chemoreceptor stimulation, nitrite levels and nNOS expression decreased in plasma and H-P tissue, whereas brain glucose retention increased. Anoxic stimulation by itself induced an increase in the expression of both genes studied. All these results indicate that de novo expression of the nNOS gene in H-P tissue may be critically involved in central glucose changes observed after anoxic carotid chemoreceptor stimulation in conjunction with NO.

Structural alterations in adult rat carotid bodies exposed to hyperbaric oxygenation.

UNLABELLED: Inhibition of carotid body (CB) function is the main mechanism involved in the attenuation of respiratory drive observed during hyperoxia. However, only a few studies at 5.0 atmospheres absolutes (ATA) have analyzed carotid body structure or function in hyperbaric oxygenation (HBO2) situations. We hypothesized that rats will present CB structural alterations when exposed to different lower hyperbaric oxygen doses enough to alter their chemosensory response to hypoxia. METHODS: Twenty-one adult male Wistar rats, divided into three groups, were maintained in room air or exposed to O2 at 2.4 or 3.0 ATA for six hours. Histological, ultrastructural and immunohistochemical analyses for neuronal nitric oxide synthase (nNOS) and F2-isoprostane were performed in the excised CBs. RESULTS: Histological analyses revealed signs of intracellular edema in animals exposed to both conditions, but this was more marked in the 3.0 ATA group, which showed ultrastructural alterations at the mitochondrial level. There was a significant increase in the volume density of intraglomic-congested capillaries in the 3.0 ATA group associated with an arteriolar vasoconstriction. In the 2.4 ATA group, there was a relative increase of glomic light cells and a decrease of glomic progenitor cells. Additionally, there was a stronger immunoreactivity for F2-isoprostane in the 3.0 ATA O2-exposed carotid bodies. The glomic cells stained positive for nNOS, but no difference was observed between the groups. Our results show that high O2 exposures may induce structural alterations in glomic cells with signs of lipid peroxidation. We further suggest that deviation of blood flow toward intraglomic capillaries occurs in hyperbaric hyperoxia.

RT-PCR and pharmacological analysis of L-and T-type calcium channels in rat carotid body.

Mechanisms involved in carotid body (CB) chemoreceptor cells O(2)-sensing and responses are not fully understood. So far, it is known that hypoxia depolarizes chemoreceptor cells via O(2)-sensitive K(+)-channel inhibition; calcium influx via voltage-gated channels and neurotransmitter secretion follow. Presence of high voltage activated (HVA) calcium channels in rat CB chemoreceptor cells is well documented, but the presence of low voltage activated (LVH) or T-type calcium channels has not been reported to date. The fact that O(2)-sensitive PC12 cells express T-type channels and that they are inducible by chronic hypoxia (CH) lead us to hypothesize they could be present and play a role in the genesis of the hypoxic response in rat CB chemoreceptor cells. We have analyzed the expression of the three isoforms of T-type calcium channels (alpha1G, alpha1H and alpha1I) and the isoforms alpha1C and alpha1D of L-type calcium channels in rat CB by RT-PCR. We found that rat CB expresses alpha1G and alpha1C subunits. After chronic hypoxic treatment of adult rats (10 degrees O(2), 8 days), expression of alpha1G seems to be down-regulated whereas alpha1C expression is up-regulated. Functionally, it was found that the release of catecholamine induced by hypoxia and high external K({+}) from CB chemoreceptor cells was fully sensitive to L-type channel inhibition (nisoldipine, 2 microM), while specific inhibition of T-channels (mibefradil, 2 microM) inhibited exclusively hypoxia-induced release (50 degrees ). As a whole, present findings demonstrate the presence of T-type as well as L-type calcium channels in rat CB and suggest a selective participation of the T-type channels in the hypoxic activation of chemoreceptor cells.

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).

Hyperbaric oxygenation alters carotid body ultrastructure and function.

We previously demonstrated that chronic normobaric hyperoxia (NH) for 60-67 h attenuated the carotid chemosensory response to hypoxia, probably initiated by the generation of reactive oxygen species (ROS). Since biological systems are affected by oxygen in a dose-dependent manner, we hypothesized that hyperbaric oxygenation (HBO) would affect the cellular mechanisms of oxygen chemoreception in a shorter time. To test the hypothesis, we studied the effects of oxygen at 5 atmospheres absolute (ATA) on cats (n = 7) carotid body ultrastructure and chemosensory responses to hypoxia, hypercapnia, and to bolus injections of cyanide, nicotine and dopamine. Four control cats breathed room air at 1 ATA. At the termination of the experiments, carotid bodies from 4 cats in each group were fixed and prepared for electron microscopy and morphometry. On the average, HBO diminished the chemosensory responsiveness to hypoxia (P < 0.01, unpaired t-test) within about 2 h, supporting the hypothesis. The responses to hypercapnia or bolus injections of cyanide, nicotine and dopamine were normal. HBO did not diminish the distribution of the dense-cored vesicles but significantly increased the mean volume-density of mitochondria and decreased the cristated area per mitochondrion in the glomus cells. The latter suggests a link between oxidative metabolism and chemosensing, and the former excludes availability of neurotransmitters being the cause of the blunted chemosensory response to hypoxia.