Effect of carbon dioxide on neonatal mouse lung: a genomic approach

Mild and moderate chronic hypercapnia elicit distinct transcriptomic responses of immune function in cardiorespiratory nuclei

Chronic hypercapnia (CH) is a hallmark of respiratory-related diseases, and the level of hypercapnia can acutely or progressively become more severe. Previously, we have shown time-dependent adaptations in steady-state physiology during mild (arterial Pco2 ∼55 mmHg) and moderate (∼60 mmHg) CH in adult goats, including transient (mild CH) or sustained (moderate CH) suppression of acute chemosensitivity suggesting limitations in adaptive respiratory control mechanisms as the level of CH increases. Changes in specific markers of glutamate receptor plasticity, interleukin-1ß, and serotonergic modulation within key nodes of cardiorespiratory control do not fully account for the physiological adaptations to CH. Here, we used an unbiased approach (bulk tissue RNA sequencing) to test the hypothesis that mild or moderate CH elicits distinct gene expression profiles in important brain stem regions of cardiorespiratory control, which may explain the contrasting responses to CH. Gene expression profiles from the brain regions validated the accuracy of tissue biopsy methodology. Differential gene expression analyses revealed greater effects of CH on brain stem sites compared with the medial prefrontal cortex. Mild CH elicited an upregulation of predominantly immune-related genes and predicted activation of immune-related pathways and functions. In contrast, moderate CH broadly led to downregulation of genes and predicted inactivation of cellular pathways related to the immune response and vascular function. These data suggest that mild CH leads to a steady-state activation of neuroinflammatory pathways within the brain stem, whereas moderate CH drives the opposite response. Transcriptional shifts in immune-related functions may underlie the cardiorespiratory network's capability to respond to acute, more severe hypercapnia when in a state of progressively increased CH.NEW & NOTEWORTHY Mild chronic hypercapnia (CH) broadly upregulated immune-related genes and a predicted activation of biological pathways related to immune cell activity and the overall immune response. In contrast, moderate CH primarily downregulated genes related to major histocompatibility complex signaling and vasculature function that led to a predicted inactivation of pathways involving the immune response and vascular endothelial function. The severity-dependent effect on immune responses suggests that neuroinflammation has an important role in CH and may be important in the maintenance of proper ventilatory responses to acute and chronic hypercapnia.

Carbon dioxide-dependent signal transduction in mammalian systems

Carbon dioxide (CO2) is a fundamental physiological gas known to profoundly influence the behaviour and health of millions of species within the plant and animal kingdoms in particular. A recent Royal Society meeting on the topic of 'Carbon dioxide detection in biological systems' was extremely revealing in terms of the multitude of roles that different levels of CO2 play in influencing plants and animals alike. While outstanding research has been performed by leading researchers in the area of plant biology, neuronal sensing, cell signalling, gas transport, inflammation, lung function and clinical medicine, there is still much to be learned about CO2-dependent sensing and signalling. Notably, while several key signal transduction pathways and nodes of activity have been identified in plants and animals respectively, the precise wiring and sensitivity of these pathways to CO2 remains to be fully elucidated. In this article, we will give an overview of the literature relating to CO2-dependent signal transduction in mammalian systems. We will highlight the main signal transduction hubs through which CO2-dependent signalling is elicited with a view to better understanding the complex physiological response to CO2 in mammalian systems. The main topics of discussion in this article relate to how changes in CO2 influence cellular function through modulation of signal transduction networks influenced by pH, mitochondrial function, adenylate cyclase, calcium, transcriptional regulators, the adenosine monophosphate-activated protein kinase pathway and direct CO2-dependent protein modifications. While each of these topics will be discussed independently, there is evidence of significant cross-talk between these signal transduction pathways as they respond to changes in CO2. In considering these core hubs of CO2-dependent signal transduction, we hope to delineate common elements and identify areas in which future research could be best directed.

Hypercapnia Regulates Gene Expression and Tissue Function

Carbon dioxide (CO₂) is produced in eukaryotic cells primarily during aerobic respiration, resulting in higher CO₂ levels in mammalian tissues than those in the atmosphere. CO₂ like other gaseous molecules such as oxygen and nitric oxide, is sensed by cells and contributes to cellular and organismal physiology. In humans, elevation of CO₂ levels in tissues and the bloodstream (hypercapnia) occurs during impaired alveolar gas exchange in patients with severe acute and chronic lung diseases. Advances in understanding of the biology of high CO₂ effects reveal that the changes in CO₂ levels are sensed in cells resulting in specific tissue responses. There is accumulating evidence on the transcriptional response to elevated CO₂ levels that alters gene expression and activates signaling pathways with consequences for cellular and tissue functions. The nature of hypercapnia-responsive transcriptional regulation is an emerging area of research, as the responses to hypercapnia in different cell types, tissues, and species are not fully understood. Here, we review the current understanding of hypercapnia effects on gene transcription and consequent cellular and tissue functions.

Hypercapnia: An Aggravating Factor in Asthma

Asthma is a common chronic respiratory disorder with relatively good outcomes in the majority of patients with appropriate maintenance therapy. However, in a small minority, patients can experience severe asthma with respiratory failure and hypercapnia, necessitating intensive care unit admission. Hypercapnia occurs due to alveolar hypoventilation and insufficient removal of carbon dioxide (CO₂) from the blood. Although mild hypercapnia is generally well tolerated in patients with asthma, there is accumulating evidence that elevated levels of CO₂ can act as a gaso-signaling molecule, triggering deleterious effects in various organs such as the lung, skeletal muscles and the innate immune system. Here, we review recent advances on pathophysiological response to hypercapnia and discuss potential detrimental effects of hypercapnia in patients with asthma.

Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in Mammals

Molecular oxygen (O₂) and carbon dioxide (CO₂) are the primary gaseous substrate and product of oxidative phosphorylation in respiring organisms, respectively. Variance in the levels of either of these gasses outside of the physiological range presents a serious threat to cell, tissue, and organism survival. Therefore, it is essential that endogenous levels are monitored and kept at appropriate concentrations to maintain a state of homeostasis. Higher organisms such as mammals have evolved mechanisms to sense O₂ and CO₂ both in the circulation and in individual cells and elicit appropriate corrective responses to promote adaptation to commonly encountered conditions such as hypoxia and hypercapnia. These can be acute and transient nontranscriptional responses, which typically occur at the level of whole animal physiology or more sustained transcriptional responses, which promote chronic adaptation. In this review, we discuss the mechanisms by which mammals sense changes in O₂ and CO₂ and elicit adaptive responses to maintain homeostasis. We also discuss crosstalk between these pathways and how they may represent targets for therapeutic intervention in a range of pathological states.

Hypercapnia Alters Expression of Immune Response, Nucleosome Assembly and Lipid Metabolism Genes in Differentiated Human Bronchial Epithelial Cells

Hypercapnia, the elevation of CO2 in blood and tissues, commonly occurs in severe acute and chronic respiratory diseases, and is associated with increased risk of mortality. Recent studies have shown that hypercapnia adversely affects innate immunity, host defense, lung edema clearance and cell proliferation. Airway epithelial dysfunction is a feature of advanced lung disease, but the effect of hypercapnia on airway epithelium is unknown. Thus, in the current study we examined the effect of normoxic hypercapnia (20% CO2 for 24 h) vs normocapnia (5% CO2), on global gene expression in differentiated normal human airway epithelial cells. Gene expression was assessed on Affymetrix microarrays, and subjected to gene ontology analysis for biological process and cluster-network representation. We found that hypercapnia downregulated the expression of 183 genes and upregulated 126. Among these, major gene clusters linked to immune responses and nucleosome assembly were largely downregulated, while lipid metabolism genes were largely upregulated. The overwhelming majority of these genes were not previously known to be regulated by CO2. These changes in gene expression indicate the potential for hypercapnia to impact bronchial epithelial cell function in ways that may contribute to poor clinical outcomes in patients with severe acute or advanced chronic lung diseases.

Carbon dioxide-dependent regulation of NF-κB family members RelB and p100 gives molecular insight into CO2-dependent immune regulation

CO₂ is a physiological gas normally produced in the body during aerobic respiration. Hypercapnia (elevated blood pCO₂ >≈50 mm Hg) is a feature of several lung pathologies, e.g. chronic obstructive pulmonary disease. Hypercapnia is associated with increased susceptibility to bacterial infections and suppression of inflammatory signaling. The NF-κB pathway has been implicated in these effects; however, the molecular mechanisms underpinning cellular sensitivity of the NF-κB pathway to CO₂ are not fully elucidated. Here, we identify several novel CO₂-dependent changes in the NF-κB pathway. NF-κB family members p100 and RelB translocate to the nucleus in response to CO₂ A cohort of RelB protein-protein interactions (e.g. with Raf-1 and IκBα) are altered by CO₂ exposure, although others are maintained (e.g. with p100). RelB is processed by CO₂ in a manner dependent on a key C-terminal domain located in its transactivation domain. Loss of the RelB transactivation domain alters NF-κB-dependent transcriptional activity, and loss of p100 alters sensitivity of RelB to CO₂ Thus, we provide molecular insight into the CO₂ sensitivity of the NF-κB pathway and implicate altered RelB/p100-dependent signaling in the CO₂-dependent regulation of inflammatory signaling.

α-Tocopherol transfer protein mediates protective hypercapnia in murine ventilator-induced lung injury

RATIONALE

Hypercapnia is common in mechanically ventilated patients. Experimentally, 'therapeutic hypercapnia' can protect, but it can also cause harm, depending on the mechanism of injury. Hypercapnia suppresses multiple signalling pathways. Previous investigations have examined mechanisms that were known a priori, but only a limited number of pathways, each suppressed by CO₂, have been reported.

OBJECTIVE

Because of the complexity and interdependence of processes in acute lung injury, this study sought to fill in knowledge gaps using an unbiased screen, aiming to identify a specifically upregulated pathway.

METHODS AND RESULTS

Using genome-wide gene expression analysis in a mouse model of ventilator-induced lung injury, we discovered a previously unsuspected mechanism by which CO₂ can protect against injury: induction of the transporter protein for α-tocopherol, α-tocopherol transfer protein (αTTP). Pulmonary αTTP was induced by inspired CO₂ in two in vivo murine models of ventilator-induced lung injury; the level of αTTP expression correlated with degree of lung protection; and, absence of the αTTP gene significantly reduced the protective effects of CO₂. α-Tocopherol is a potent antioxidant and hypercapnia increased lung α-tocopherol in wild-type mice, but this did not alter superoxide generation or expression of NRF2-dependent antioxidant response genes in wild-type or in αTTP^-/- mice. In concordance with a regulatory role for α-tocopherol in lipid mediator synthesis, hypercapnia attenuated 5-lipoxygenase activity and this was dependent on the presence of αTTP.

CONCLUSIONS

Inspired CO₂ upregulates αTTP which increases lung α-tocopherol levels and inhibits synthesis of a pathogenic chemoattractant.

Therapeutic hypercapnia prevents bleomycin-induced pulmonary hypertension in neonatal rats by limiting macrophage-derived tumor necrosis factor-α

Bleomycin-induced lung injury is characterized in the neonatal rat by inflammation, arrested lung growth, and pulmonary hypertension (PHT), as observed in human infants with severe bronchopulmonary dysplasia. Inhalation of CO2 (therapeutic hypercapnia) has been described to limit cytokine production and to have anti-inflammatory effects on the injured lung; we therefore hypothesized that therapeutic hypercapnia would prevent bleomycin-induced lung injury. Spontaneously breathing rat pups were treated with bleomycin (1 mg/kg/d ip) or saline vehicle from postnatal days 1–14 while being continuously exposed to 5% CO2 (PaCO2 elevated by 15–20 mmHg), 7% CO2 (PaCO2 elevated by 35 mmHg), or normocapnia. Bleomycin-treated animals exposed to 7%, but not 5%, CO2, had significantly attenuated lung tissue macrophage influx and PHT, as evidenced by normalized pulmonary vascular resistance and right ventricular systolic function, decreased right ventricular hypertrophy, and attenuated remodeling of pulmonary resistance arteries. The level of CO2 neither prevented increased tissue neutrophil influx nor led to improvements in decreased lung weight, septal thinning, impaired alveolarization, or decreased numbers of peripheral arteries. Bleomycin led to increased expression and content of lung tumor necrosis factor (TNF)-α, which was found to colocalize with tissue macrophages and to be attenuated by exposure to 7% CO2. Inhibition of TNF-α signaling with the soluble TNF-2 receptor etanercept (0.4 mg/kg ip from days 1–14 on alternate days) prevented bleomycin-induced PHT without decreasing tissue macrophages and, similar to CO2, had no effect on arrested alveolar development. Our findings are consistent with a preventive effect of therapeutic hypercapnia with 7% CO2 on bleomycin-induced PHT via attenuation of macrophage-derived TNF-α. Neither tissue macrophages nor TNF-α appeared to contribute to arrested lung development induced by bleomycin. That 7% CO2 normalized pulmonary vascular resistance and right ventricular function without improving inhibited airway and vascular development suggests that vascular hypoplasia does not contribute significantly to functional changes of PHT in this model.

Blood protein concentrations in the first two postnatal weeks associated with early postnatal blood gas derangements among infants born before the 28th week of gestation. The ELGAN Study

AIM

To explore the relationships between blood gas derangements and blood concentrations of inflammation-related proteins shortly after preterm birth.

DESIGN

Observational cohort.

SETTING

Fourteen neonatal intensive care units.

SUBJECTS

Seven hundred and forty five infants born before the 28th week of gestation who were classified by their blood gas derangements during the first three postnatal days and by the concentrations of 25 proteins in their blood on days 1, 7, and 14. We classified these newborns by whether or not they had a highest or lowest PaO2, PCO2, and lowest pH in the most extreme quartile, and by whether or not they had a protein concentration in the highest quartile.

RESULTS

Blood gas derangements on two days were much more likely to be accompanied or followed by sustained or recurrent systemic inflammation than a derangement on only one day. This was most evident for acidemia, and slightly less so for hypercapnia.

CONCLUSIONS

Our finding that protein concentration patterns indicative of systemic inflammation are associated with several blood gas derangements raises the possibility that organ damage attributed to these derangements might be accompanied by or involve an inflammatory response.

Regulation of gene expression by carbon dioxide

Carbon dioxide (CO(2)) is a physiological gas found at low levels in the atmosphere and produced in cells during the process of aerobic respiration. Consequently, the levels of CO(2) within tissues are usually significantly higher than those found externally. Shifts in tissue levels of CO(2) (leading to either hypercapnia or hypocapnia) are associated with a number of pathophysiological conditions in humans and can occur naturally in niche habitats such as those of burrowing animals. Clinical studies have indicated that such altered CO(2) levels can impact upon disease progression. Recent advances in our understanding of the biology of CO(2) has shown that like other physiological gases such as molecular oxygen (O(2)) and nitric oxide (NO), CO(2) levels can be sensed by cells resulting in the initiation of physiological and pathophysiological responses. Acute CO(2) sensing in neurons and peripheral and central chemoreceptors is important in rapidly activated responses including olfactory signalling, taste sensation and cardiorespiratory control. Furthermore, a role for CO(2) in the regulation of gene transcription has recently been identified with exposure of cells and model organisms to high CO(2) leading to suppression of genes involved in the regulation of innate immunity and inflammation. This latter, transcriptional regulatory role for CO(2), has been largely attributed to altered activity of the NF-B family of transcription factors. Here, we review our evolving understanding of how CO(2) impacts upon gene transcription.

Morphometry of the epidermis of an invasive megascoelecid earthworm (Amynthas gracilis, Kinberg 1867) inhabiting actively volcanic soils in the Azores archipelago

For the first time, the structure, dimensions, and composition of the epidermis of an invasive earthworm species that has successfully colonized hostile conditions in actively volcanic soil on São Miguel (Azores) have been measured. Metal concentrations in actively volcanic (Furnas) and volcanically inactive (Fajã) soils were similar; however, Furnas soil was characterised by elevated temperature (10°C differential), relative hypoxia, extremely high CO(2) tension, and accompanying acidity. The epidermis of earthworm's resident at Fajã was approximately twice the thickness of the epidermis of conspecifics resident in Furnas soil. Reference worms transferred to Furnas soil for 14 days experienced an epidermal thinning of approximately 51%. In comparison, when Furnas earthworms were transferred to mesocosms at the relatively benign Fajã site, their epidermal thickness increased by approximately 21% over 14 days. Earthworms resident in Furnas soil had higher goblet cell counts than the residents of volcanically inactive soil on a neighbouring island (S. Maria). Transferring worms from S. Maria to mesocosms at Furnas induced a significant increase in goblet cell counts. Clearly, the active volcanic environment at Furnas poses a multifactorial stress challenge to the epigeic A. gracilis colonizer.

Chronic hypercapnia alters lung matrix composition in mouse pups

RATIONALE

permissive hypercapnia, a stretch-limiting ventilation strategy, often results in high Pa(CO(2)). This strategy is associated with reduced morbidity and mortality in premature infants and its benefits have been attributed to diminished barotrauma. However, little is known about the independent effect of high CO(2) levels during the lung development.

METHODS

mice were exposed to 8% CO(2) or room air for 2 wk either from postnatal day 2 through 17 or as adults (approximately 2 mo of age). Lungs were excised and processed for protein, RNA, histology, and total lung volumes.

RESULTS

histologic analysis demonstrated that alveolar walls of CO(2)-exposed mouse pups were thinner than those of controls and had twice the total lung volume. Molecular analysis revealed that several matrix proteins in the lung were downregulated in mouse pups exposed to hypercapnia. Interstitial collagen type I alpha1, type III alpha1, elastin and fibronectin protein, and mRNA levels were less than half of controls while collagen IV alpha 5 was unaffected. This decrease in interstitial collagen could thus account for the thinning of the interstitial matrix and the altered lung biomechanics. Matrix metalloproteinase (MMP)-8, a collagenase that has specificity for collagen types I and III, increased in hypercapnic mouse pups, suggesting increased collagen degradation. Moreover, tissue inhibitor of MMP (TIMP)-1, a potent inhibitor of MMP-8, was significantly decreased. However, unlike pups, adult mice exposed to hypercapnia demonstrated only a mild increase in total lung volumes and did not exhibit similar molecular or histologic changes.

CONCLUSIONS

although permissive hypercapnia may prevent lung injury from barotrauma, our study revealed that exposure to hypercapnia may be an important factor in lung remodeling and function, especially in early life.

Therapeutic effects of hypercapnia on chronic lung injury and vascular remodeling in neonatal rats

Permissive hypercapnia, achieved using low tidal volume ventilation, has been an effective protective strategy in patients with acute respiratory distress syndrome. To date, no such protective effect has been demonstrated for the chronic neonatal lung injury, bronchopulmonary dysplasia. The objective of our study was to determine whether evolving chronic neonatal lung injury, using a rat model, is resistant to the beneficial effects of hypercapnia or simply requires a less conservative approach to hypercapnia than that applied clinically to date. Neonatal rats inhaled air or 60% O2 for 14 days with or without 5.5% CO2. Lung parenchymal neutrophil and macrophage numbers were significantly increased by hyperoxia alone, which was associated with interstitial thickening and reduced secondary crest formation. The phagocyte influx, interstitial thickening, and impaired alveolar formation were significantly attenuated by concurrent hypercapnia. Hyperoxic pups that received 5.5% CO2 had a significant increase in alveolar number relative to air-exposed pups. Increased tyrosine nitration, a footprint for peroxynitrite-mediated reactions, arteriolar medial wall thickening, and both reduced small peripheral pulmonary vessel number and VEGF and angiopoietin-1 (Ang-1) expression, which were observed with hyperoxia, was attenuated by concurrent hypercapnia. We conclude that evolving chronic neonatal lung injury in a rat model is responsive to the beneficial effects of hypercapnia. Inhaled 5.5% CO2 provided a significant degree of protection against parenchymal and vascular injury in an animal model of chronic neonatal lung injury likely due, at least in part, to its inhibition of a phagocyte influx.

Elevated CO2 suppresses specific Drosophila innate immune responses and resistance to bacterial infection

Elevated CO(2) levels (hypercapnia) frequently occur in patients with obstructive pulmonary diseases and are associated with increased mortality. However, the effects of hypercapnia on non-neuronal tissues and the mechanisms that mediate these effects are largely unknown. Here, we develop Drosophila as a genetically tractable model for defining non-neuronal CO(2) responses and response pathways. We show that hypercapnia significantly impairs embryonic morphogenesis, egg laying, and egg hatching even in mutants lacking the Gr63a neuronal CO(2) sensor. Consistent with previous reports that hypercapnic acidosis can suppress mammalian NF-kappaB-regulated innate immune genes, we find that in adult flies and the phagocytic immune-responsive S2* cell line, hypercapnia suppresses induction of specific antimicrobial peptides that are regulated by Relish, a conserved Rel/NF-kappaB family member. Correspondingly, modest hypercapnia (7-13%) increases mortality of flies inoculated with E. faecalis, A. tumefaciens, or S. aureus. During E. faecalis and A. tumefaciens infection, increased bacterial loads were observed, indicating that hypercapnia can decrease host resistance. Hypercapnic immune suppression is not mediated by acidosis, the olfactory CO(2) receptor Gr63a, or by nitric oxide signaling. Further, hypercapnia does not induce responses characteristic of hypoxia, oxidative stress, or heat shock. Finally, proteolysis of the Relish IkappaB-like domain is unaffected by hypercapnia, indicating that immunosuppression acts downstream of, or in parallel to, Relish proteolytic activation. Our results suggest that hypercapnic immune suppression is mediated by a conserved response pathway, and illustrate a mechanism by which hypercapnia could contribute to worse outcomes of patients with advanced lung disease, who frequently suffer from both hypercapnia and respiratory infections.

Effects of chronic hypercapnia in the neonatal mouse lung and brain

BACKGROUND

Permissive hypercapnia is increasingly utilized in the care of premature infants to prevent bronchopulmonary dysplasia. In a previous investigation, we described gene expression changes in the neonatal mouse lung exposed to chronic hypercapnia that might contribute to lung protection and accelerated maturation. However, it is unknown whether chronic hypercapnia increases alveolar formation, nor if it has detrimental effects in other developing organs such as the brain.

OBJECTIVE

To determine whether chronic hypercapnia accelerates early alveolar formation and increases neuronal cell injury in the developing mouse lung and brain, respectively.

DESIGN

Mouse pups were exposed to 8% CO(2) + 21% O(2) starting at postnatal day (P) 2 until P7. Control animals were maintained in room air. Animals were sacrificed at P4 or P7, and lungs and brains were excised and analyzed.

RESULTS

Exposure to 8% CO(2) resulted in an increased expression of alpha-smooth muscle actin (alpha-sma) which localized to the tips of developing alveolar buds, and also an increased number of alveolar buds at P7. Importantly, hypercapnic animals also demonstrated evidence of increased TUNEL-positive cells in the brain.

CONCLUSIONS

Exposure to chronic hypercapnia may lead to early initiation of alveolar budding in the neonatal mouse, but may also lead to increased TUNEL-positive cells in the developing brain.

Hypercapnia and the neonate

'Permissive hypercapnia' is a familiar term in neonatal intensive care, given the widespread adoption of low-tidal-volume ventilation strategies applied with the goal of decreasing respiratory morbidity. Recent evidence suggesting that hypercapnic acidosis may itself have protective effects on the lung and other organs has led to the coining of a new phrase, 'therapeutic hypercapnia', which also encompasses the use of supplemental inspired CO(2).

CONCLUSION

Experimental evidence suggests that mild-moderate hypercapnia can improve tissue oxygenation and perfusion, which may ameliorate injury to the immature lung and brain. However, hypercapnia may also be associated with adverse outcomes, and the range of PaCO(2) levels that are both safe and effective for specific subsets of neonates has yet to be determined.

Modulatory effects of hypercapnia on in vitro and in vivo pulmonary endothelial-neutrophil adhesive responses during inflammation

Reducing tidal volume as a part of a protective ventilation strategy may result in hypercapnia. In this study, we focused on the influence of hypercapnia on endothelial-neutrophil responses in models of inflammatory-stimulated human pulmonary microvascular endothelial cells (HMVEC) and in an animal model of lipopolysaccharide (LPS)-induced acute lung injury. Neutrophil adhesion and adhesion molecules expression and nuclear factor-kappaB (NF-kappaB) were analyzed in TNF-alpha and LPS-treated HMVEC exposed to either eucapnia or hypercapnia. In the in vivo limb, bronchoalveolar lavage fluid cell counts and differentials, adhesion molecule and chemokine expression were assessed in LPS-treated rabbits ventilated with either low tidal volume ventilation and eucapnia or hypercapnia. In both the in vitro and in vivo models, hypercapnia significantly increased neutrophil adhesion and adhesion molecule expression compared to eucapnia. Activity of NF-kappaB was significantly enhanced by hypercapnia in the in vitro experiments. IL-8 expression was greatest both in vitro and in vivo under conditions of hypercapnia and concomitant inflammation. CD11a expression was greatest in isolated human neutrophils exposed to hypercapnia+LPS. Our results demonstrate that endothelial-neutrophil responses per measurement of fundamental molecules of adhesion are significantly increased during hypercapnia and that hypercapnia mimics conditions of eucapnia+inflammation.

Effect of chronic elevated carbon dioxide on the expression of acid-base transporters in the neonatal and adult mouse

Several pulmonary and neurological conditions, both in the newborn and adult, result in hypercapnia. This leads to disturbances in normal pH homeostasis. Most mammalian cells maintain tight control of intracellular pH (pHi) using a group of transmembrane proteins that specialize in acid-base transport. These acid-base transporters are important in adjusting pHiduring acidosis arising from hypoventilation. We hypothesized that exposure to chronic hypercapnia induces changes in the expression of acid-base transporters. Neonatal and adult CD-1 mice were exposed to either 8% or 12% CO2for 2 wk. We used Western blot analysis of membrane protein fractions from heart, kidney, and various brain regions to study the response of specific acid-base transporters to CO2. Chronic CO2increased the expression of the sodium hydrogen exchanger 1 (NHE1) and electroneutral sodium bicarbonate cotransporter (NBCn1) in the cerebral cortex, heart, and kidney of neonatal but not adult mice. CO2increased the expression of electrogenic NBC (NBCe1) in the neonatal but not the adult mouse heart and kidney. Hypercapnia decreased the expression of anion exchanger 3 (AE3) in both the neonatal and adult brain but increased AE3 expression in the neonatal heart. We conclude that: 1) chronic hypercapnia increases the expression of the acid extruders NHE1, NBCe1 and NBCn1 and decreases the expression of the acid loader AE3, possibly improving the capacity of the cell to maintain pHiin the face of acidosis; and 2) the heterogeneous response of tissues to hypercapnia depends on the level of CO2and development.

Effects of overinflation on procollagen type III expression in experimental acute lung injury

Introduction

In acute lung injury (ALI), elevation of procollagen type III (PC III) occurs early and has an adverse impact on outcome. We examined whether different high-inflation strategies of mechanical ventilation (MV) in oleic acid (OA) ALI alter regional expression of PC III.

Methods

We designed an experimental, randomized, and controlled protocol in which rats were allocated to two control groups (no injury, recruited [alveolar recruitment maneuver after tracheotomy without MV; n = 4 rats] and control [n = 5 rats]) or four injured groups (one exposed to OA only [n = 10 rats] and three OA-injured and ventilated). The three OA-injured groups were ventilated for 1 hour according to the following strategies: LVHP-S (low volume-high positive end-expiratory pressure [PEEP], supine; n = 10 rats, tidal volume [V_T] = 8 ml/kg, PEEP = 12 cm H₂O), HVLP-S (high volume-low PEEP, supine; n = 10 rats, V_T = 20 ml/kg, PEEP = 5 cm H₂O), and HVLP-P (high volume-low PEEP, prone; n = 10 rats). Northern blot analysis for PC III and interleukin-1-beta (IL-1β) and polymorphonuclear infiltration index (PMI) counting were performed in nondependent and dependent regions. Regional differences between groups were assessed by two-way analysis of variance after logarithmic transformation and post hoc tests.

Results

A significant interaction for group and region effects was observed for PC III (p = 0.012) with higher expression in the nondependent region for HVLP-S and LVHP-S, intermediate for OA and HVLP-P, and lower for control (group effect, p < 0.00001, partial η² = 0.767; region effect, p = 0.0007, partial η² = 0.091). We found high expression of IL-1β (group effect, p < 0.00001, partial η² = 0.944) in the OA, HVLP-S, and HVLP-P groups without regional differences (p = 0.16). PMI behaved similarly (group effect, p < 0.00001, partial η² = 0.832).

Conclusion

PC III expression is higher in nondependent regions and in ventilatory strategies that caused overdistension. This response was partially attenuated by prone positioning.

Chronic High-Inspired CO2 Decreases Excitability of Mouse Hippocampal Neurons

To examine the effect of chronically elevated CO2 on excitability and function of neurons, we exposed mice to 8 and 12% CO2 for 4 wk (starting at 2 days of age), and examined the properties of freshly dissociated hippocampal neurons obtained from slices. Chronic CO2-treated neurons (CC) had a similar input resistance ( Rm) and resting membrane potential ( Vm) as control (CON). Although treatment with 8% CO2 did not change the rheobase (64 ± 11 pA, n = 9 vs. 47 ± 12 pA, n = 8 for CC 8% vs. CON; means ± SE), 12% CO2 treatment increased it significantly (73 ± 8 pA, n = 9, P = 0.05). Furthermore, the 12% CO2 but not the 8% CO2 treatment decreased the Na+ channel current density (244 ± 36 pA/pF, n = 17, vs. 436 ± 56 pA/pF, n = 18, for CC vs. CON, P = 0.005). Recovery from inactivation was also lowered by 12% but not 8% CO2. Other gating properties of Na+ current, such as voltage-conductance curve, steady-state inactivation, and time constant for deactivation, were not modified by either treatment. Western blot analysis showed that the expression of Na+ channel types I–III was not changed by 8% CO2 treatment, but their expression was significantly decreased by 20–30% ( P = 0.03) by the 12% treatment. We conclude from these data and others that neuronal excitability and Na+ channel expression depend on the duration and level of CO2 exposure and maturational changes occur in early life regarding neuronal responsiveness to CO2.