Therapeutic hypercapnia prevents chronic hypoxia-induced pulmonary hypertension in the newborn rat

Obstructive Sleep Apnea, Obesity Hypoventilation Syndrome, and Pulmonary Hypertension: A State-of-the-Art Review

The pathophysiological interplay between sleep-disordered breathing (SDB) and pulmonary hypertension (PH) is complex and can involve a variety of mechanisms by which SDB can worsen PH. These mechanistic pathways include wide swings in intrathoracic pressure while breathing against an occluded upper airway, intermittent and/or sustained hypoxemia, acute and/or chronic hypercapnia, and obesity. In this review, we discuss how the downstream consequences of SDB can adversely impact PH, the challenges in accurately diagnosing and classifying PH in the severely obese, and review the limited literature assessing the effect of treating obesity, obstructive sleep apnea, and obesity hypoventilation syndrome on PH.

Hypoxia-Induced Kidney Injury in Newborn Rats

Exposure to hypoxia during the early postnatal period can have adverse effects on vital organs. Neonatal Sprague-Dawley rats housed in a hypoxic chamber were compared to those in a normoxic chamber from postnatal days 0 to 7. Arterial blood was collected to evaluate renal function and hypoxia. Kidney morphology and fibrosis were evaluated using staining methods and immunoblotting. In the kidneys of the hypoxic group, protein expressions of hypoxia-inducible factor-1 were higher than those in the normoxic group. Hypoxic rats had higher levels of hematocrit, serum creatinine, and lactate than normoxic rats. Body weight was reduced, and protein loss of kidney tissue was observed in hypoxic rats compared to normoxic rats. Histologically, hypoxic rats showed glomerular atrophy and tubular injury. Renal fibrosis with collagen fiber deposition was observed in the hypoxic group. The expression of nicotinamide adenine dinucleotide phosphate oxidases was enhanced in the kidneys of hypoxic rats. Proteins involved in apoptosis were upregulated in the kidneys of hypoxic rats. An increase in the expression of pro-inflammatory cytokines was also observed in the kidneys of hypoxic rats. Hypoxic kidney injury in neonatal rats was associated with oxidative stress, inflammation, apoptosis, and fibrosis.

Pulmonary hypertension associated with cardiopulmonary bypass and cardiac surgery

BACKGROUND AND AIM

Pulmonary hypertension (PH) is frequently associated with cardiovascular surgery and is a common complication that has been observed after surgery utilizing cardiopulmonary bypass (CPB). The purpose of this review is to explain the characteristics of PH, the mechanisms of PH induced by cardiac surgery and CPB, treatments for postoperative PH, and future directions in treating PH induced by cardiac surgery and CPB using up-to-date findings.

METHODS

The PubMed database was utilized to find published articles.

RESULTS

There are many mechanisms that contribute to PH after cardiac surgery and CPB which involve pulmonary vasomotor dysfunction, cyclooxygenase, the thromboxane A2 and prostacyclin pathway, the nitric oxide pathway, inflammation, and oxidative stress. Furthermore, there are several effective treatments for postoperative PH within different types of cardiac surgery.

CONCLUSIONS

By possessing a deep understanding of the mechanisms that contribute to PH after cardiac surgery and CPB, researchers can develop treatments for clinicians to use which target the mechanisms of PH and ultimately reduce and/or eliminate postoperative PH. Additionally, learning about the most up-to-date studies regarding treatments can allow clinicians to choose the best treatments for patients who are undergoing cardiac surgery and CPB.

Effects of Intermittent Normoxia on Chronic Hypoxic Pulmonary Hypertension and Right Ventricular Hypertrophy in Rats

Liu, Chunlei, Xu Chen, Ge Guo, Xiang Xu, Xin Li, Qingxia Wei, Yanying Shen, Hanlu Li, Jianxiu Hao, Ya Ping Tian, and Kunlun He. Effects of intermittent normoxia on chronic hypoxic pulmonary hypertension and right ventricular hypertrophy in rats. High Alt Med Biol. 22: 184-192, 2021. Background: Individuals with chronically low arterial oxygen tension owing to high altitude develop elevated rates of pulmonary hypertension (PH) and right ventricular (RV) hypertrophy. However, the effects of the frequency and duration of normoxic exposure on PH and RV hypertrophy have not been adequately assessed; thus, we aimed to analyze the same. Materials and Methods: PH and RV hypertrophy were induced in 60 rats using a hypobaric chamber. Of these 60 rats, every 10 were exposed to normoxic conditions for 30 minutes once (1T/D), three times (3T/D), or five times daily (5T/D), or for one 150-minute recovery daily (1LT/D). Furthermore, 10 rats were housed in a normoxic environment, and another 10 were subjected to continuous hypoxia. After 4 weeks, hemodynamic measurements were recorded, and the hearts were harvested for pathomorphological observations. Results: Average pulmonary arterial pressures (PAP) of control rats and those exposed to hypobaric hypoxia were 14.1 and 32.3 mmHg, respectively. After 30 minutes of exposure to normoxia 3T/D, 5T/D, or 1LT/D, PAP values were reduced to 27.1, 27.9, or 26.8 mmHg, respectively. Four weeks of hypoxic exposure elevated the RV/heart weight (HW) ratios, while exposure to normoxia 3T/D, 5T/D, and 1LT/D significantly reduced RV/HW. In addition, exposure to normoxia 3T/D, 5T/D, 1LT/D reduced the percentage wall thickness of the pulmonary artery as well as the hypertrophy indices of atrial natriuretic peptide, brain natriuretic peptide, and myosin heavy chain 7 (MYH-7). Conclusions: Thirty-minute exposure to normoxic conditions of 3T/D, 5T/D, or 1LT/D effectively ameliorates PH and RV thickening.

Influence of Intermittent Hypoxia/Hypercapnia on Atherosclerosis, Gut Microbiome, and Metabolome

Obstructive sleep apnea (OSA), a common sleep disorder characterized by intermittent hypoxia and hypercapnia (IHC), increases atherosclerosis risk. However, the contribution of intermittent hypoxia (IH) or intermittent hypercapnia (IC) in promoting atherosclerosis remains unclear. Since gut microbiota and metabolites have been implicated in atherosclerosis, we examined whether IH or IC alters the microbiome and metabolome to induce a pro-atherosclerotic state. Apolipoprotein E deficient mice (ApoE-/- ), treated with IH or IC on a high-fat diet (HFD) for 10 weeks, were compared to Air controls. Atherosclerotic lesions were examined, gut microbiome was profiled using 16S rRNA gene amplicon sequencing and metabolome was assessed by untargeted mass spectrometry. In the aorta, IC-induced atherosclerosis was significantly greater than IH and Air controls (aorta, IC 11.1 ± 0.7% vs. IH 7.6 ± 0.4%, p < 0.05 vs. Air 8.1 ± 0.8%, p < 0.05). In the pulmonary artery (PA), however, IH, IC, and Air were significantly different from each other in atherosclerotic formation with the largest lesion observed under IH (PA, IH 40.9 ± 2.0% vs. IC 20.1 ± 2.6% vs. Air 12.2 ± 1.5%, p < 0.05). The most differentially abundant microbial families (p < 0.001) were Peptostreptococcaceae, Ruminococcaceae, and Erysipelotrichaceae. The most differentially abundant metabolites (p < 0.001) were tauro-β-muricholic acid, ursodeoxycholic acid, and lysophosphoethanolamine (18:0). We conclude that IH and IC (a) modulate atherosclerosis progression differently in distinct vascular beds with IC, unlike IH, facilitating atherosclerosis in both aorta and PA and (b) promote an atherosclerotic luminal gut environment that is more evident in IH than IC. We speculate that the resulting changes in the gut metabolome and microbiome interact differently with distinct vascular beds.

A Complex Life Habitable Zone Based On Lipid Solubility Theory

To find potentially habitable exoplanets, space missions employ the habitable zone (HZ), which is the region around a star (or multiple stars) where standing bodies of water could exist on the surface of a rocky planet. Follow-up atmospheric characterization could yield biosignatures signifying life. Although most iterations of the HZ are agnostic regarding the nature of such life, a recent study argues that a complex life HZ would be considerably smaller than that used in classical definitions. Here, I use an advanced energy balance model to show that such an HZ would be considerably wider than originally predicted given revised CO₂ limits and (for the first time) N₂ respiration limits for complex life. The width of this complex life HZ (CLHZ) increases by ~35% from ~0.95–1.2 AU to 0.95–1.31 AU in our solar system. Similar extensions are shown for stars with stellar effective temperatures between 2,600–9,000 K. I define this CLHZ using lipid solubility theory, diving data, and results from animal laboratory experiments. I also discuss implications for biosignatures and technosignatures. Finally, I discuss the applicability of  the CLHZ and other HZ variants to the search for both simple and complex life.

Mitochondrial Fission-Mediated Lung Development in Newborn Rats With Hyperoxia-Induced Bronchopulmonary Dysplasia With Pulmonary Hypertension

Background: Bronchopulmonary dysplasia (BPD) is the most common chronic respiratory disease in premature infants. Oxygen inhalation and mechanical ventilation are common treatments, which can cause hyperoxia-induced lung injury, but the underlying mechanism is not yet understood. Mitochondrial fission is essential for mitochondrial homeostasis. The objective of this study was to determine whether mitochondrial fission (dynamin-related protein 1, Drp1) is an important mediator of hyperoxia lung injury in rats. Methods: The animal model of BPD was induced with high oxygen (80-85% O₂). Pulmonary histological changes were observed by hematoxylin-eosin (HE) staining. Pulmonary microvessels were observed by immunofluorescence staining of von Willebrand Factor (vWF). Protein expression levels of Drp1 and p-Drp1 (Ser616) were observed using Western Blot. We used echocardiography to measure pulmonary artery acceleration time (PAT), pulmonary vascular resistance index (PVRi), peak flow velocity of the pulmonary artery (PFVP), pulmonary arteriovenous diameter, and pulmonary vein peak velocity. Mitochondrial division inhibitor-1 (Mdivi-1) was used as an inhibitor of Drp1, and administered through intraperitoneal injection (25 mg/kg). Results: Pulmonary artery resistance of the hyperoxide-induced neonatal rat model of BPD increased after it entered normoxic convalescence. During the critical stage of alveolar development in neonatal rats exposed to high oxygen levels for an extended period, the expression and phosphorylation of Drp1 increased in lung tissues. When Drp1 expression was inhibited, small pulmonary vessel development improved and PH was relieved. Conclusion: Our study shows that excessive mitochondrial fission is an important mediator of hyperoxia-induced pulmonary vascular injury, and inhibition of mitochondrial fission may be a useful treatment for hyperoxia-induced related pulmonary diseases.

Effects of chronic hypercapnia on ammonium transport in the mouse kidney

Hypercapnia and subsequent respiratory acidosis are serious complications in many patients with respiratory disorders. The acute response to hypercapnia is buffering of H+ by hemoglobin and cellular proteins but this effect is limited. The chronic response is renal compensation that increases HCO3- reabsorption, and stimulates urinary excretion of titratable acids (TA) and NH4+ . However, the main effective pathway is the excretion of NH4+ in the collecting duct. Our hypothesis is that, the renal NH3 /NH4+ transporters, Rhbg and Rhcg, in the collecting duct mediate this response. The effect of hypercapnia on these transporters is unknown. We conducted in vivo experiments on mice subjected to chronic hypercapnia. One group breathed 8% CO2 and the other breathed normal air as control (0.04% CO2 ). After 3 days, the mice were euthanized and kidneys, blood, and urine samples were collected. We used immunohistochemistry and Western blot analysis to determine the effects of high CO2 on localization and expression of the Rh proteins, carbonic anhydrase IV, and pendrin. In hypercapnic animals, there was a significant increase in urinary NH4+ excretion but no change in TA. Western blot analysis showed a significant increase in cortical expression of Rhbg (43%) but not of Rhcg. Expression of CA-IV was increased but pendrin was reduced. These data suggest that hypercapnia leads to compensatory upregulation of Rhbg that contributes to excretion of NH3 /NH4+ in the kidney. These studies are the first to show a link among hypercapnia, NH4+ excretion, and Rh expression.

Effect of hypercapnia on respiratory and peripheral skeletal muscle loss during critical illness - A pilot study

INTRODUCTION

Critical illness has profound effects on muscle strength and long-term physical morbidity. However, there remains a paucity of evidence for the aetiology of critical illness related weakness. Recent animal model research identified that hypercapnia may reduce the rate of muscle loss. The aim of this study was to determine the effect of hypercapnia on respiratory and peripheral skeletal muscle in patients with critical illness.

METHODS

A pilot observational study of mechanically ventilated critically ill patients at a tertiary critical care unit who were retrospectively categorised as: 1) Respiratory failure with normocapnia; 2) Respiratory failure with hypercapnia; and 3) brain injury. Diaphragm thickness and quadriceps rectus femoris cross-sectional area (RFCSA) were measured using ultrasound imaging at baseline and at days 3, 5, 7 and 10 of mechanical ventilation.

RESULTS

Significant reductions in RFCSA muscle loss were observed for all time-points when compared to baseline [day 10: -14.9%±8.2 p< 0.001], and in diaphragm thickness between baseline and day 7 [day 7: -5.8%±9.5 p=0.029). No correlation was identified between the rate of muscle mass loss in the diaphragm and RFCSA.

CONCLUSION

In this pilot study, peripheral skeletal muscle weakness occurred early and rapidly within the critical care population, irrespective of carbon dioxide levels.

PLCγ1-PKCε-IP3R1 signaling plays an important role in hypoxia-induced calcium response in pulmonary artery smooth muscle cells

Hypoxia-induced pulmonary vasoconstriction (HPV) is attributed to an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in pulmonary artery smooth muscle cells (PASMCs). We have reported that phospholipase C-γ1 (PLCγ1) plays a significant role in the hypoxia-induced increase in [Ca²⁺]_i in PASMCs and attendant HPV. In this study, we intended to determine molecular mechanisms for hypoxic Ca²⁺ and contractile responses in PASMCs. Our data reveal that hypoxic vasoconstriction occurs in pulmonary arteries, but not in mesenteric arteries. Hypoxia caused a large increase in [Ca²⁺]_i in PASMCs, which is diminished by the PLC inhibitor U73122 and not by its inactive analog U73433 . Hypoxia augments PLCγ1-dependent inositol 1,4,5-trisphosphate (IP₃) generation. Exogenous ROS, hydrogen peroxide (H₂O₂), increases PLCγ1 phosphorylation at tyrosine-783 and IP₃ production. IP₃ receptor-1 (IP₃R1) knock-down remarkably diminishes hypoxia- or H₂O₂-induced increase in [Ca²⁺]_i. Hypoxia or H₂O₂ increases the activity of IP₃Rs, which is significantly reduced in protein kinase C-ε (PKCε) knockout PASMCs. A higher PLCγ1 expression, activity, and basal [Ca²⁺]_i are found in PASMCs, but not in mesenteric artery smooth muscle cells from mice exposed to chronic hypoxia (CH) for 21 days. CH enhances H₂O₂- and ATP-induced increase in [Ca²⁺]_i in PASMCs and PLC-dependent, norepinephrine-evoked pulmonary vasoconstriction. In conclusion, acute hypoxia uniquely causes ROS-dependent PLCγ1 activation, IP₃ production, PKCε activation, IP₃R1 opening, Ca²⁺ release, and contraction in mouse PASMCs; CH enhances PASM PLCγ1 expression, activity, and function, playing an essential role in pulmonary hypertension in mice.

Intermittent Hypoxia and Hypercapnia Accelerate Atherosclerosis, Partially via Trimethylamine-Oxide

Obstructive sleep apnea (OSA) is a common disorder characterized by intermittent hypoxia and hypercapnia (IHC) during sleep. OSA has been shown to be a risk factor for atherosclerosis, but the relation of IHC to the induction or progression of atherosclerosis is not well understood. To dissect the mechanisms involved, we compared atherosclerotic lesion formation in two mouse models, i.e., apolipoprotein E (ApoE) and low density lipoprotein receptor (Ldlr)-deficient mice, with or without IHC exposure. Ten-week-old ApoE-/- or Ldlr-/- mice were fed a high-fat diet for 4 or 8 weeks while being exposed to IHC for 10 hours/day or room air (RA) for 24 hours/day. En face lesions of the aorta, aortic arch, and pulmonary artery (PA) were examined. Moreover, 3,3-dimethyl-1-butanol (DMB), an inhibitor of microbial trimethylamine (TMA) production, was used to determine the contribution of TMA-oxide (TMAO) to IHC-induced atherosclerosis. Eight weeks of IHC exposure expedited the formation of atherosclerosis in both the PA and aortic arch of ApoE-/- mice, but only in the PA of Ldlr-/- mice (ApoE-/- PA 8 wk, IHC 35.4 ± 1.9% versus RA 8.0 ± 2.8%, P < 0.01). The atherosclerotic lesions evolved faster and to a more severe extent in ApoE-/- mice as compared with Ldlr-/- mice (PA IHC 8 wk, ApoE-/- 35.4 ± 1.9% versus Ldlr-/- 8.2 ± 1.5%, P < 0.01). DMB significantly attenuated but did not totally eliminate IHC-induced PA atherosclerosis. Our findings suggest that IHC, a hallmark of OSA, accelerates the progression of atherosclerosis in the aorta and especially in the PA. This process is partly inhibited by DMB, demonstrating that microbial metabolites may serve as therapeutic targets for OSA-induced atherosclerosis.

mTOR-Notch3 signaling mediates pulmonary hypertension in hypoxia-exposed neonatal rats independent of changes in autophagy

BACKGROUND/AIM

Mammalian target of rapamycin (mTOR) is a pivotal regulator of cell proliferation, survival, and autophagy. Autophagy is increased in adult experimental chronic pulmonary hypertension (PHT), but its contributory role to pulmonary vascular disease remains uncertain and has yet to be explored in the neonatal animal. Notch is a major pro-proliferative pathway activated by mTOR. A direct relationship between autophagy and Notch signaling has not been previously explored. Our aim was to examine changes in mTOR-, Notch-, and autophagy-related pathways and the therapeutic effects of autophagy modulators in experimental chronic neonatal PHT secondary to chronic hypoxia.

METHODS

Rat pups were exposed to normoxia or hypoxia (13% O₂ ) from postnatal days 1-21, while receiving treatment with temsirolimus (mTOR inhibitor), DAPT (Notch inhibitor), or chloroquine (inhibitor of autophagic flux).

RESULTS

Exposure to hypoxia up-regulated autophagy and Notch3 signaling markers in lung, pulmonary artery (PA), and PA-derived smooth muscle cells (SMCs). Temsirolimus prevented chronic PHT and attenuated PA and SMC signaling secondary to hypoxia. These effects were replicated by DAPT. mTOR or Notch inhibition also down-regulated smooth muscle content of platelet-derived growth factor β-receptor, a known contributor to vascular remodeling. In contrast, chloroquine had no modifying effects on markers of chronic PHT. Knockdown of Beclin-1 in SMCs had no effect on hypoxia-stimulated Notch3 signaling.

CONCLUSIONS

mTOR-Notch3 signaling plays a critical role in experimental chronic neonatal PHT. Inhibition of autophagy did not suppress Notch signaling and had no effect on markers of chronic PHT.

Enhanced NO-dependent pulmonary vasodilation limits increased vasoconstrictor sensitivity in neonatal chronic hypoxia

Augmented vasoconstrictor reactivity is thought to play an important role in the development of chronic hypoxia (CH)-induced neonatal pulmonary hypertension. However, whether this response to CH results from pulmonary endothelial dysfunction and reduced nitric oxide (NO)-mediated vasodilation is not well understood. We hypothesized that neonatal CH enhances basal tone and pulmonary vasoconstrictor sensitivity by limiting NO-dependent pulmonary vasodilation. To test this hypothesis, we assessed the effects of the NO synthase (NOS) inhibitor N^ω-nitro-l-arginine (l-NNA) on baseline pulmonary vascular resistance (PVR) and vasoconstrictor sensitivity to the thromboxane mimetic U-46619 in saline-perfused lungs (in situ) from 2-wk-old control and CH (12-day exposure, 0.5 atm) Sprague-Dawley rats. Basal tone was defined as that reversed by exogenous NO (spermine NONOate). CH neonates displayed elevated right ventricular systolic pressure (in vivo) and right ventricular hypertrophy, indicative of pulmonary hypertension. Perfused lungs from CH rats demonstrated greater baseline PVR, basal tone, and U-46619-mediated vasoconstriction compared with control rats in the absence of l-NNA. l-NNA markedly increased baseline PVR and reactivity to U-46619 in lungs from CH neonates, further augmenting vasoconstrictor sensitivity compared with control lungs. Exposure to CH also enhanced NO-dependent vasodilation to arginine vasopressin, pulmonary expression of NOS III [endothelial NOS (eNOS)], and eNOS phosphorylation at activation residue Ser¹¹⁷⁷ However, CH did not alter lung nitrotyrosine levels, a posttranslational modification reflecting [Formula: see text] scavenging of NO. We conclude that, in contrast to our hypothesis, enhanced basal tone and agonist-induced vasoconstriction after neonatal CH is limited by increased NO-dependent pulmonary vasodilation resulting from greater eNOS expression and phosphorylation at activation residue Ser¹¹⁷⁷NEW & NOTEWORTHY This research is the first to demonstrate enhanced nitric oxide-dependent vasodilation that limits increased vasoconstrictor reactivity in neonatal pulmonary hypertension. These results suggest that augmented vasoconstriction in this setting reflects changes in smooth muscle reactivity rather than a reduction in nitric oxide-dependent pulmonary vasodilation.

The Interaction between Trolox and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic Acid on Hypoxic Pulmonary Vasoconstriction in the Isolated Rabbit Lung

BACKGROUND

The mechanism of hypoxic pulmonary vasoconstriction (HPV) is still debatable. It has been proposed that reactive oxygen species (ROS) might be involved in HPV. However, there is no special transporter for superoxide anion in the cell membrane and it may release from the cells via anion exchanger. Therefore, the aim of this study was to investigate the interaction of ROS and anion exchanger in acute HPV.

METHODS

The present study was performed in the isolated rabbit lung. After preparation, the lungs were divided into four hypoxic groups of control, Trolox (antioxidant)-treated, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, anion exchanger inhibitor)-treated, and Trolox+DIDS-treated. Pulmonary artery pressure, left atrial pressure, and lung weight were continuously registered and PVR was then calculated. PO₂, PCO₂, HCO₃^-, pH, and NO metabolites of the perfusate were measured during steady-state and at the end of experiments (30 minutes). All data were compared with ANOVA and t-test and significance was considered when P<0.05.

RESULTS

Ventilation of the lungs with hypoxic gas induced HPV in the control group. DIDS did not have a further effect on HPV compared with the control group. The combination of Trolox and DIDS decreased HPV rather than Trolox per se at 5 minutes. Furthermore, HPV was abolished in both the Trolox and Trolox+DIDS groups at 30 minutes. Concentrations of NO metabolites in the Trolox+DIDS group were more than other groups.

CONCLUSION

The present study indicates a possible interaction between ROS and anion exchanger in acute HPV. It also suggests the modulatory effect of NO at above condition.

Intermittent hypoxia during recovery from neonatal hyperoxic lung injury causes long-term impairment of alveolar development: A new rat model of BPD

Bronchopulmonary dysplasia (BPD) is a chronic lung injury characterized by impaired alveologenesis that may persist into adulthood. Rat models of BPD using varying degrees of hyperoxia to produce injury either cause early mortality or spontaneously recover following removal of the inciting stimulus, thus limiting clinical relevance. We sought to refine an established rat model induced by exposure to 60% O₂ from birth by following hyperoxia with intermittent hypoxia (IH). Rats exposed from birth to air or 60% O₂ until day 14 were recovered in air with or without IH (FI_O2 = 0.10 for 10 min every 6 h) until day 28 Animals exposed to 60% O₂ and recovered in air had no evidence of abnormal lung morphology on day 28 or at 10-12 wk. In contrast, 60% O₂-exposed animals recovered in IH had persistently increased mean chord length, more dysmorphic septal crests, and fewer peripheral arteries. Recovery in IH also increased pulmonary vascular resistance, Fulton index, and arterial wall thickness. IH-mediated abnormalities in lung structure (but not pulmonary hypertension) persisted when reexamined at 10-12 wk, accompanied by increased pulmonary vascular reactivity and decreased exercise tolerance. Increased mean chord length secondary to IH was prevented by treatment with a peroxynitrite decomposition catalyst [5,10,15,20-Tetrakis(4-sulfonatophenyl)-21H,23H-porphyrin iron (III) chloride, 30 mg/kg/day, days 14-28], an effect accompanied by fewer inflammatory cells. We conclude that IH during recovery from hyperoxia-induced injury prevents recovery of alveologenesis and leads to changes in lung and pulmonary vascular function lasting into adulthood, thus more closely mimicking contemporary BPD.

Leukotriene B4 mediates macrophage influx and pulmonary hypertension in bleomycin-induced chronic neonatal lung injury

Systemically-administered bleomycin causes inflammation, arrested lung growth, and pulmonary hypertension (PHT) in the neonatal rat, similar to human infants with severe bronchopulmonary dysplasia (BPD). Leukotrienes (LTs) are inflammatory lipid mediators produced by multiple cell types in the lung. The major LTs, LTB4 and cysteinyl LTs, are suggested to contribute to BPD, but their specific roles remain largely unexplored in experimental models. We hypothesized that LTs are increased in bleomycin-induced BPD-like injury, and that inhibition of LT production would prevent inflammatory cell influx and thereby ameliorate lung injury. Rat pups were exposed to bleomycin (1 mg·kg(-1)·day(-1) ip) or vehicle (control) from postnatal days 1-14 and were treated with either zileuton (5-lipoxygenase inhibitor), montelukast (cysteinyl LT1 receptor antagonist), or SC57461A (LTA4 hydrolase inhibitor) 10 mg·kg(-1)·day(-1) ip. Bleomycin led to increased lung content of LTB4, but not cysteinyl LTs. Bleomycin-induced increases in tissue neutrophils and macrophages and lung contents of LTB4 and tumor necrosis factor-α were all prevented by treatment with zileuton. Treatment with zileuton or SC57461A also prevented the hemodynamic and structural markers of chronic PHT, including raised pulmonary vascular resistance, increased Fulton index, and arterial wall remodeling. However, neither treatment prevented impaired alveolarization or vascular hypoplasia secondary to bleomycin. Treatment with montelukast had no effect on macrophage influx, PHT, or on abnormal lung structure. We conclude that LTB4 plays a crucial role in lung inflammation and PHT in experimental BPD. Agents targeting LTB4 or LTB4-mediated signaling may have utility in infants at risk of developing BPD-associated PHT.

The impact of chronic intermittent hypoxia on hematopoiesis and the bone marrow microenvironment

Obstructive sleep apnea (OSA) is a highly prevalent sleep-related breathing disorder which is associated with patient morbidity and an elevated risk of developing hypertension and cardiovascular diseases. There is ample evidence for the involvement of bone marrow (BM) cells in the pathophysiology of cardiovascular diseases but a connection between OSA and modulation of the BM microenvironment had not been established. Here, we studied how chronic intermittent hypoxia (CIH) affected hematopoiesis and the BM microenvironment, in a rat model of OSA. We show that CIH followed by normoxia increases the bone marrow hypoxic area, increases the number of multipotent hematopoietic progenitors (CFU assay), promotes erythropoiesis, and increases monocyte counts. In the BM microenvironment of CIH-subjected animals, the number of VE-cadherin-expressing blood vessels, particularly sinusoids, increased, accompanied by increased smooth muscle cell coverage, while vWF-positive vessels decreased. Molecularly, we investigated the expression of endothelial cell-derived genes (angiocrine factors) that could explain the cellular phenotypes. Accordingly, we observed an increase in colony-stimulating factor 1, vascular endothelium growth factor, delta-like 4, and angiopoietin-1 expression. Our data shows that CIH induces vascular remodeling in the BM microenvironment, which modulates hematopoiesis, increasing erythropoiesis, and circulating monocytes. Our study reveals for the first time the effect of CIH in hematopoiesis and suggests that hematopoietic changes may occur in OSA patients.

The Effects of Lung Protective Ventilation or Hypercapnic Acidosis on Gas Exchange and Lung Injury in Surfactant Deficient Rabbits

Background

Permissive hypercapnia has been shown to reduce lung injury in subjects with surfactant deficiency. Experimental studies suggest that hypercapnic acidosis by itself rather than decreased tidal volume may be a key protective factor.

Objectives

To study the differential effects of a lung protective ventilatory strategy or hypercapnic acidosis on gas exchange, hemodynamics and lung injury in an animal model of surfactant deficiency.

Methods

30 anesthetized, surfactant-depleted rabbits were mechanically ventilated (FiO₂ = 0.8, PEEP = 7cmH₂O) and randomized into three groups: Normoventilation-Normocapnia (NN)-group: tidal volume (Vt) = 7.5 ml/kg, target PaCO₂ = 40 mmHg; Normoventilation-Hypercapnia (NH)-group: Vt = 7.5 ml/kg, target PaCO₂ = 80 mmHg by increasing FiCO₂; and a Hypoventilation-Hypercapnia (HH)-group: Vt = 4.5 ml/kg, target PaCO₂ = 80 mmHg. Plasma lactate and interleukin (IL)-8 were measured every 2 h. Animals were sacrificed after 6 h to perform bronchoalveolar lavage (BAL), to measure lung wet-to-dry weight, lung tissue IL-8, and to obtain lung histology.

Results

PaO₂ was significantly higher in the HH-group compared to the NN-group (p<0.05), with values of the NH-group between the HH- and NN-groups. Other markers of lung injury (wet-dry-weight, BAL-Protein, histology-score, plasma-IL-8 and lung tissue IL-8) resulted in significantly lower values for the HH-group compared to the NN-group and trends for the NH-group towards lower values compared to the NN-group. Lactate was significantly lower in both hypercapnia groups compared to the NN-group.

Conclusion

Whereas hypercapnic acidosis may have some beneficial effects, a significant effect on lung injury and systemic inflammatory response is dependent upon a lower tidal volume rather than resultant arterial CO₂ tensions and pH alone.

Hypercapnia: clinical relevance and mechanisms of action

PURPOSE OF REVIEW

Multiple clinical and laboratory studies have been conducted to illustrate the effects of hypercapnia in a range of injuries, and to understand the mechanisms underlying these effects. The aim of this review is to highlight and interpret information obtained from these recent reports and discuss how they may inform the clinical context.

RECENT FINDINGS

In the last decade, several important articles have addressed key elements of how carbon dioxide interacts in critical illness states. Among them the most important insights relate to how hypercapnia affects critical illness and include the effects and mechanisms of carbon dioxide in pulmonary hypertension, infection, inflammation, diaphragm dysfunction, and cerebral ischemia. In addition, we discuss molecular insights that apply to multiple aspects of critical illness.

SUMMARY

Experiments involving hypercapnia have covered a wide range of illness models with varying degrees of success. It is becoming evident that deliberate hypercapnia in the clinical setting should seldom be used, except wherever necessitated to avoid ventilator-associated lung injury. A more complete understanding of the molecular mechanisms must be established.

Arginase inhibition prevents bleomycin-induced pulmonary hypertension, vascular remodeling, and collagen deposition in neonatal rat lungs

Arginase is an enzyme that limits substrate L-arginine bioavailability for the production of nitric oxide by the nitric oxide synthases and produces L-ornithine, which is a precursor for collagen formation and tissue remodeling. We studied the pulmonary vascular effects of arginase inhibition in an established model of repeated systemic bleomycin sulfate administration in neonatal rats that results in pulmonary hypertension and lung injury mimicking the characteristics typical of bronchopulmonary dysplasia. We report that arginase expression is increased in the lungs of bleomycin-exposed neonatal rats and that treatment with the arginase inhibitor amino-2-borono-6-hexanoic acid prevented the bleomycin-induced development of pulmonary hypertension and deposition of collagen. Arginase inhibition resulted in increased L-arginine and L-arginine bioavailability and increased pulmonary nitric oxide production. Arginase inhibition also normalized the expression of inducible nitric oxide synthase, and reduced bleomycin-induced nitrative stress while having no effect on bleomycin-induced inflammation. Our data suggest that arginase is a promising target for therapeutic interventions in neonates aimed at preventing lung vascular remodeling and pulmonary hypertension.

The efficacy of antihypertensive drugs in chronic intermittent hypoxia conditions

Sleep apnea/hypopnea disorders include centrally originated diseases and obstructive sleep apnea (OSA). This last condition is renowned as a frequent secondary cause of hypertension (HT). The mechanisms involved in the pathogenesis of HT can be summarized in relation to two main pathways: sympathetic nervous system stimulation mediated mainly by activation of carotid body (CB) chemoreflexes and/or asphyxia, and, by no means the least important, the systemic effects of chronic intermittent hypoxia (CIH). The use of animal models has revealed that CIH is the critical stimulus underlying sympathetic activity and hypertension, and that this effect requires the presence of functional arterial chemoreceptors, which are hyperactive in CIH. These models of CIH mimic the HT observed in humans and allow the study of CIH independently without the mechanical obstruction component. The effect of continuous positive airway pressure (CPAP), the gold standard treatment for OSA patients, to reduce blood pressure seems to be modest and concomitant antihypertensive therapy is still required. We focus this review on the efficacy of pharmacological interventions to revert HT associated with CIH conditions in both animal models and humans. First, we explore the experimental animal models, developed to mimic HT related to CIH, which have been used to investigate the effect of antihypertensive drugs (AHDs). Second, we review what is known about drug efficacy to reverse HT induced by CIH in animals. Moreover, findings in humans with OSA are cited to demonstrate the lack of strong evidence for the establishment of a first-line antihypertensive regimen for these patients. Indeed, specific therapeutic guidelines for the pharmacological treatment of HT in these patients are still lacking. Finally, we discuss the future perspectives concerning the non-pharmacological and pharmacological management of this particular type of HT.

Vasopressin improves survival compared with epinephrine in a neonatal piglet model of asphyxial cardiac arrest

BACKGROUND

Epinephrine is a component of all resuscitation algorithms. Vasopressin is a pulmonary vasodilator and systemic vasopressor. We investigated the effect of epinephrine vs. vasopressin on survival and hemodynamics after neonatal porcine cardiac arrest (CA).

METHODS

A 4-min asphyxial CA was induced, after which cardiopulmonary resuscitation (CPR) was commenced. Animals were randomized to low- (LDE: 0.01 mg/kg) or high-dose epinephrine (HDE: 0.03 mg/kg), low- (LDV: 0.2 U/kg) or high-dose vasopressin (HDV: 0.4 U/kg), or control (saline). Clinical and echocardiography indexes were monitored.

RESULTS

Sixty-nine animals were randomized. Survival was greater in HDV (n = 8 (89%); P < 0.05 ANOVA) vs. control (n = 7 (43%)) and LDE (n = 5 (36%)) but not vs. HDE (n = 7 (64%)) or LDV (n = 6 (75%)). Animals resuscitated with LDE required more shocks (2.5 (interquartile range: 2-6); P < 0.05) and higher doses of energy (15 J (interquartile range: 10-20); P < 0.05). Left ventricular output was comparable between groups, but a greater increase in superior vena caval flow was seen after HDV (P < 0.001 vs. control, LDE, and HDE). Plasma troponin was greatest in the HDE group (P < 0.05 vs. control and HDV).

CONCLUSION

Vasopressin results in improved survival, lower postresuscitation troponin, and less hemodynamic compromise after CA in newborn piglets. Vasopressin may be a candidate for testing in human neonates.

Acidosis in the critically ill - balancing risks and benefits to optimize outcome

Acidosis is associated with poor outcome in critical illness. However, acidosis - both hypercapnic and metabolic - has direct effects that can limit tissue injury induced by many causes. There is also a clear potential for off-target harm with acute exposure (for example, raised intracranial pressure, pulmonary hypertension), and with exposure for prolonged periods (for example, increased risk of infection) or at high doses. Ongoing comprehensive determination of molecular, cellular and physiologic impact across a range of representative pathologies will allow us to understand better the risks and benefits of hypercapnia and acidosis during critical illness.

Therapeutic hypercapnia prevents inhaled nitric oxide-induced right-ventricular systolic dysfunction in juvenile rats

Chronic pulmonary hypertension in the neonate and infant frequently presents with right-ventricular (RV) failure. Current clinical management may include protracted treatment with inhaled nitric oxide (iNO), with the goal of reducing RV afterload. We have previously reported that prolonged exposure to iNO causes RV systolic dysfunction in the chronic hypoxia-exposed juvenile rat, which was prevented by a peroxynitrite decomposition catalyst. Given that inhalation of CO2 (therapeutic hypercapnia) may limit oxidative stress and upregulated cytokine expression in the lung and other organs, we hypothesized that therapeutic hypercapnia would attenuate cytokine-mediated nitric oxide synthase (NOS) upregulation, thus limiting peroxynitrite generation. Sprague-Dawley rat pups were exposed to chronic hypoxia (13% O2) from postnatal day 1 to 21, while receiving iNO (20 ppm) from day 14 to 21, with or without therapeutic hypercapnia (10% CO2). Therapeutic hypercapnia completely normalized RV systolic function, RV hypertrophy, and remodeling of pulmonary resistance arteries in animals exposed to iNO. Inhaled nitric oxide-mediated increases in RV peroxynitrite, apoptosis, and contents of tumor necrosis factor (TNF)-α, interleukin (IL)-1α, and NOS-2 were all attenuated by therapeutic hypercapnia. Inhibition of NOS-2 activity with 1400 W (1 mg/kg/day) prevented iNO-mediated upregulation of peroxynitrite and led to improved RV systolic function. Blockade of IL-1 receptor signaling with anakinra (500 mg/kg/day) decreased NOS-2 content and had similar effects compared to NOS-2 inhibition on iNO-mediated effects, whereas blockade of TNF-α signaling with etanercept (0.4 mg/kg on alternate days) had no effects on these parameters. We conclude that therapeutic hypercapnia prevents the adverse effects of sustained exposure to iNO on RV systolic function by limiting IL-1-mediated NOS-2 upregulation and consequent nitration. Therapeutic hypercapnia also acts synergistically with iNO in normalizing RV hypertrophy, vascular remodeling, and raised pulmonary vascular resistance secondary to chronic hypoxia.

A comprehensive review: the evolution of animal models in pulmonary hypertension research; are we there yet?

Pulmonary hypertension (PH) is a disorder that develops as a result of remodeling of the pulmonary vasculature and is characterized by narrowing/obliteration of small pulmonary arteries, leading to increased mean pulmonary artery pressure and pulmonary vascular resistance. Subsequently, PH increases the right ventricular afterload, which leads to right ventricular hypertrophy and eventually right ventricular failure. The pathophysiology of PH is not fully elucidated, and current treatments have only a modest impact on patient survival and quality of life. Thus, there is an urgent need for improved treatments or a cure. The use of animal models has contributed extensively to the current understanding of PH pathophysiology and the investigation of experimental treatments. However, PH in current animal models may not fully represent current clinical observations. For example, PH in animal models appears to be curable with many therapeutic interventions, and the severity of PH in animal models is also believed to correlate poorly with that observed in humans. In this review, we discuss a variety of animal models in PH research, some of their contributions to the field, their shortcomings, and how these have been addressed. We highlight the fact that the constant development and evolution of animal models will help us to more closely model the severity and heterogeneity of PH observed in humans.

Newborn rat response to single vs. combined cGMP-dependent pulmonary vasodilators

Inhaled nitric oxide (NO) and other cGMP- or cAMP-dependent pulmonary vasodilators are often used in combination for the treatment of the persistent pulmonary hypertension of the newborn syndrome. There is in vitro evidence to indicate that NO downregulate the pulmonary vascular response to cGMP-dependent agonists raising concern as to whether a synergistic effect is observed when employing a combined strategy in newborns. Hypothesizing that a synergistic effect is absent, we evaluated newborn and juvenile rat pulmonary arteries to determine the individual and combined vasodilatory effect of cGMP- and cAMP-dependent agonists. In precontracted near-resistance pulmonary arteries, the addition of sildenafil reduced vasorelaxation response to NO donor S-nitroso-N-acetyl penicillamine (SNAP). A similar decrease in SNAP-induced vasodilation was observed in arteries pretreated with BAY 41-2272 (10(-9) M), a soluble guanylate cyclase stimulator cGMP, and its downstream protein kinase activator. cGMP also reduced the vasorelaxant response to the cAMP-dependent forskolin. Inhibition of endogenous vascular NO generation enhanced SNAP-induced relaxation. The present data suggest that the mechanism involved in the cGMP desensitization to other relaxant agonists involves downregulation of the small heat shock protein HSP20 and is evident in rat pulmonary and systemic vascular smooth muscle cells. In newborn rats with chronic hypoxia-induced pulmonary hypertension, the combination of sildenafil and inhaled NO resulted in a lesser reduction in pulmonary vascular resistance compared with their individual effect. These data suggest that clinical exposure to one cGMP-dependent pulmonary vasodilator may affect the response to other cGMP- or cAMP-mediated agonists.

Hypercapnia: is it protective in lung injury?

Hypercapnic acidosis has been regarded as a tolerated side effect of protective lung ventilation strategies. Various in vivo and ex vivo animal studies have shown beneficial effects in acute lung injury setting, but some recent work raised concerns about its anti-inflammatory properties. This mini-review article aims to expand the potential clinical spectrum of hypercapnic acidosis in critically ill patients with lung injury. Despite the proven benefits of hypercapnic acidosis, further safety studies including dose-effect, level-and-onset of anti-inflammatory effect, and safe applicability period need to be performed in various models of lung injury in animals and humans to further elucidate its protective role.

Intermittent hypoxia and hypercapnia induce pulmonary artery atherosclerosis and ventricular dysfunction in low density lipoprotein receptor deficient mice

Patients with obstructive sleep apnea, who experience episodic hypoxia and hypercapnia during sleep, often demonstrate increased inflammation, oxidative stress, and dyslipidemia. We hypothesized that sleep apnea patients would be predisposed to the development of atherosclerosis. To dissect the mechanisms involved, we developed an animal model in mice whereby we expose mice to intermittent hypoxia/hypercapnia (IHH) in normobaric environments. Two- to three-month-old low-density lipoprotein receptor deficient (Ldlr(-/-)) mice were fed a high-fat diet for 8 or 16 wk while being exposed to IHH for either 10 h/day or 24 h/day. Plasma lipid levels, pulmonary artery and aortic atherosclerotic lesions, and cardiac function were then assayed. Surprisingly, atherosclerosis in the aorta of IHH mice was similar compared with controls. However, in IHH mice, atherosclerosis was markedly increased in the trunk and proximal branches of the pulmonary artery of exposed mice; even though plasma cholesterol and triglycerides were lower than in controls. Hemodynamic analysis revealed that right ventricular maximum pressure and isovolumic relaxation constant were significantly increased in IHH exposed mice and left ventricular % fractional shortening was reduced. In conclusion, 1) Intermittent hypoxia/hypercapnia remarkably accelerated atherosclerotic lesions in the pulmonary artery of Ldlr(-/-) mice and 2) increased lesion formation in the pulmonary artery was associated with right and left ventricular dysfunction. These findings raise the possibility that patients with obstructive sleep apnea may be susceptible to atherosclerotic disease in the pulmonary vasculature, an observation that has not been previously recognized.

Hypercapnia attenuates the hypoxia-induced blunting of the reactivity in chronically hypoxic rats

Chronic hypoxia causes oxidative injury of pulmonary vessels and attenuates their reactivity to different stimuli. When combined with hypercapnia, biochemical markers of this injury are reduced but the effect of concomitant hypoxia and hypercapnia on vascular reactivity is not fully understood. This study was therefore designed to test whether hypercapnia can prevent also the hypoxia-induced loss of reactivity of pulmonary vessels. The reactivity of vessels from rats exposed either to hypoxia or hypoxia combined with hypercapnia was tested using a small vessel myograph (M 500A, Linton, Norfolk, GB). The second and third intrapulmonary branches of pulmonary arteries were isolated under a dissecting microscope from lungs of 8 control rats (group N), 6 rats exposed to hypoxia for 5 days (isobaric, 10 % O(2), group H) and 7 rats exposed to hypoxia combined with hypercapnia for 5 days (10 % O(2), 5 % CO(2), group H+CO(2)). The transmural pressure was set by automatic normalization to 30 mm Hg. The vessel size did not vary among the groups. After stabilization we challenged the vessels twice with KCl (80 mM) and once with PGF(2alpha) (0.1 mM). There were no significant differences in KCl induced contractions among the groups. The responses to PGF(2alpha) were expressed as a ratio to the maximal tension obtained by the exposure to 80 mM KCl. Contractions induced by PGF(2alpha) were markedly reduced in group H (0.07+/-0.02) and in group H+CO(2) (0.26+/-0.03) in comparison with group N (0.83+/-0.07). The vessels of group H responded to PGF(2alpha) less than those of group H+CO(2). However we observed the attenuated reactivity also in group H+CO(2) in comparison with N. Hypercapnia therefore partially blunted the hypoxia-induced loss of reactivity in pulmonary arteries. This finding supports the hypothesis that hypercapnia significantly alters the nature of lung injury induced by chronic hypoxia.

Moderate and prolonged hypercapnic acidosis may protect against ventilator-induced diaphragmatic dysfunction in healthy piglet: an in vivo study

Introduction

Protective ventilation by using limited airway pressures and ventilation may result in moderate and prolonged hypercapnic acidosis, as often observed in critically ill patients. Because allowing moderate and prolonged hypercapnia may be considered protective measure for the lungs, we hypothesized that moderate and prolonged hypercapnic acidosis may protect the diaphragm against ventilator-induced diaphragmatic dysfunction (VIDD). The aim of our study was to evaluate the effects of moderate and prolonged (72 hours of mechanical ventilation) hypercapnic acidosis on in vivo diaphragmatic function.

Methods

Two groups of anesthetized piglets were ventilated during a 72-hour period. Piglets were assigned to the Normocapnia group (n = 6), ventilated in normocapnia, or to the Hypercapnia group (n = 6), ventilated with moderate hypercapnic acidosis (PaCO₂ from 55 to 70 mm Hg) during the 72-hour period of the study. Every 12 hours, we measured transdiaphragmatic pressure (Pdi) after bilateral, supramaximal transjugular stimulation of the two phrenic nerves to assess in vivo diaphragmatic contractile force. Pressure/frequency curves were drawn after stimulation from 20 to 120 Hz of the phrenic nerves. The protocol was approved by our institutional animal-care committee.

Results

Moderate and prolonged hypercapnic acidosis was well tolerated during the study period. The baseline pressure/frequency curves of the two groups were not significantly different (Pdi at 20 Hz, 32.7 ± 8.7 cm H₂O, versus 34.4 ± 8.4 cm H₂O; and at 120 Hz, 56.8 ± 8.7 cm H₂O versus 60.8 ± 5.7 cm H₂O, for Normocapnia and Hypercapnia groups, respectively). After 72 hours of ventilation, Pdi decreased by 25% of its baseline value in the Normocapnia group, whereas Pdi did not decrease in the Hypercapnia group.

Conclusions

Moderate and prolonged hypercapnic acidosis limited the occurrence of VIDD during controlled mechanical ventilation in a healthy piglet model. Consequences of moderate and prolonged hypercapnic acidosis should be better explored with further studies before being tested on patients.

Effect of chronic perinatal hypoxia on the role of rho-kinase in pulmonary artery contraction in newborn lambs

Exposure to chronic hypoxia during gestation predisposes infants to neonatal pulmonary hypertension, but the underlying mechanisms remain unclear. Here, we test the hypothesis that moderate continuous hypoxia during gestation causes changes in the rho-kinase pathway that persist in the newborn period, altering vessel tone and responsiveness. Lambs kept at 3,801 m above sea level during gestation and the first 2 wk of life were compared with those with gestation at low altitude. In vitro studies of isolated pulmonary arterial rings found a more forceful contraction in response to KCl and 5-HT in high-altitude compared with low-altitude lambs. There was no difference between the effects of blockers of various pathways of extracellular Ca(2+) entry in low- and high-altitude arteries. In contrast, inhibition of rho-kinase resulted in significantly greater attenuation of 5-HT constriction in high-altitude compared with low-altitude arteries. High-altitude lambs had higher baseline pulmonary artery pressures and greater elevations in pulmonary artery pressure during 15 min of acute hypoxia compared with low-altitude lambs. Despite evidence for an increased role for rho-kinase in high-altitude arteries, in vivo studies found no significant difference between the effects of rho-kinase inhibition on hypoxic pulmonary vasoconstriction in intact high-altitude and low-altitude lambs. We conclude that chronic hypoxia in utero results in increased vasopressor response to both acute hypoxia and serotonin, but that rho-kinase is involved only in the increased response to serotonin.

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.

Sustained therapeutic hypercapnia attenuates pulmonary arterial Rho-kinase activity and ameliorates chronic hypoxic pulmonary hypertension in juvenile rats

Sustained therapeutic hypercapnia prevents pulmonary hypertension in experimental animals, but its rescue effects on established disease have not been studied. Therapies that inhibit Rho-kinase (ROCK) and/or augment nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) signaling can reverse or prevent progression of chronic pulmonary hypertension. Our objective in the present study was to determine whether sustained rescue treatment with inhaled CO2 (therapeutic hypercapnia) would improve structural and functional changes of chronic hypoxic pulmonary hypertension. Spontaneously breathing pups were exposed to normoxia (21% O2) or hypoxia (13% O2) from postnatal days 1–21 with or without 7% CO2 (PaCO2 elevated by ∼25 mmHg) or 10% CO2 (PaCO2 elevated by ∼40 mmHg) from days 14 to 21. Compared with hypoxia alone, animals exposed to hypoxia and 10% CO2 had significantly ( P < 0.05) decreased pulmonary vascular resistance, right-ventricular systolic pressure, right-ventricular hypertrophy, and medial wall thickness of pulmonary resistance arteries as well as decreased lung phosphodiesterase (PDE) V, RhoA, and ROCK activity. Rescue treatment with 10% CO2, or treatment with a ROCK inhibitor (15 mg/kg ip Y-27632 twice daily from days 14 to 21), also increased pulmonary arterial endothelial nitric oxide synthase and lung NO content. In contrast, cGMP content and cGMP-dependent protein kinase (PKG) activity were increased by exposure to 10% CO2, but not by ROCK inhibition with Y-27632. In vitro exposure of pulmonary artery smooth muscle cells to hypercapnia suppressed serum-induced ROCK activity, which was prevented by inhibition of PKG with Rp-8-Br-PET-cGMPS. We conclude that sustained hypercapnia dose-dependently inhibited ROCK activity, augmented NO-cGMP-PKG signaling, and led to partial improvements in the hemodynamic and structural abnormalities of chronic hypoxic PHT in juvenile rats. Increased PKG content and activity appears to play a major upstream role in CO2-induced suppression of ROCK activity in pulmonary arterial smooth muscle.

Impaired iNOS-sGC-cGMP signalling contributes to chronic hypoxic and hypercapnic pulmonary hypertension in rat

Nitric oxide (NO) is an important vascular modulator in the development of pulmonary hypertension. NO exerts its regulatory effect mainly by activating soluble guanylate cyclase (sGC) to synthesize cyclic guanosine monophosphate (cGMP). Exposure to hypoxia causes pulmonary hypertension. But in lung disease, hypoxia is commonly accompanied by hypercapnia. The aim of this study was to examine the changes of sGC enzyme activity and cGMP content in lung tissue, as well as the expression of inducible nitric oxide synthase (iNOS) and sGC in rat pulmonary artery after exposure to hypoxia and hypercapnia, and assess the role of iNOS-sGC-cGMP signal pathway in the development of hypoxic and hypercapnic pulmonary hypertension. Male Sprague-Dawley rats were exposed to hypoxia and hypercapnia for 4 weeks to establish model of chronic pulmonary hypertension. Weight-matched rats exposed to normoxia served as control. After exposure to hypoxia and hypercapnia, mean pulmonary artery pressure, the ratio of right ventricle/left ventricle+septum, and the ratio of right ventricle/body weight were significantly increased. iNOS mRNA and protein levels were significantly increased, but sGC α(1) mRNA and protein levels were significantly decreased in small pulmonary arteries of hypoxic and hypercapnic exposed rat. In addition, basal and stimulated sGC enzyme activity and cGMP content in lung tissue were significantly lower after exposure to hypoxia and hypercapnia. These results demonstrate that hypoxia and hypercapnia lead to the upregulation of iNOS expression, downregulation of sGC expression and activity, which then contribute to the development of pulmonary hypertension.

Improved pulmonary vascular reactivity and decreased hypertrophic remodeling during nonhypercapnic acidosis in experimental pulmonary hypertension

Pulmonary hypertension (PH) is characterized by pulmonary arteriolar remodeling with excessive pulmonary vascular smooth muscle cell (VSMC) proliferation. This results in decreased responsiveness of pulmonary circulation to vasodilator therapies. We have shown that extracellular acidosis inhibits VSMC proliferation and migration in vitro. Here we tested whether induction of nonhypercapnic acidosis in vivo ameliorates PH and the underlying pulmonary vascular remodeling and dysfunction. Adult male Sprague-Dawley rats were exposed to hypoxia (8.5% O(2)) for 2 wk, or injected subcutaneously with monocrotaline (MCT, 60 mg/kg) to develop PH. Acidosis was induced with NH(4)Cl (1.5%) in the drinking water 5 days prior to and during the 2 wk of hypoxic exposure (prevention protocol), or after MCT injection from day 21 to 28 (reversal protocol). Right ventricular systolic pressure (RVSP) and Fulton's index were measured, and pulmonary arteriolar remodeling was analyzed. Pulmonary and mesenteric artery contraction to phenylephrine (Phe) and high KCl, and relaxation to acetylcholine (ACh) and sodium nitroprusside (SNP) were examined ex vivo. Hypoxic and MCT-treated rats demonstrated increased RVSP, Fulton's index, and pulmonary arteriolar thickening. In pulmonary arteries of hypoxic and MCT rats there was reduced contraction to Phe and KCl and reduced vasodilation to ACh and SNP. Acidosis prevented hypoxia-induced PH, reversed MCT-induced PH, and resulted in reduction in all indexes of PH including RVSP, Fulton's index, and pulmonary arteriolar remodeling. Pulmonary artery contraction to Phe and KCl was preserved or improved, and relaxation to ACh and SNP was enhanced in NH(4)Cl-treated PH animals. Acidosis alone did not affect the hemodynamics or pulmonary vascular function. Phe and KCl contraction and ACh and SNP relaxation were not different in mesenteric arteries of all groups. Thus nonhypercapnic acidosis ameliorates experimental PH, attenuates pulmonary arteriolar thickening, and enhances pulmonary vascular responsiveness to vasoconstrictor and vasodilator stimuli. Together with our finding that acidosis decreases VSMC proliferation, the results are consistent with the possibility that nonhypercapnic acidosis promotes differentiation of pulmonary VSMCs to a more contractile phenotype, which may enhance the effectiveness of vasodilator therapies in PH.

Intermittent hypoxia augments pulmonary vascular smooth muscle reactivity to NO: regulation by reactive oxygen species

Intermittent hypoxia (IH) resulting from sleep apnea can lead to pulmonary hypertension. IH causes oxidative stress that may limit bioavailability of the endothelium-derived vasodilator nitric oxide (NO) and thus contribute to this hypertensive response. We therefore hypothesized that increased vascular superoxide anion (O(2)(-)) generation reduces NO-dependent pulmonary vasodilation following IH. To test this hypothesis, we examined effects of the O(2)(-) scavenger tiron on vasodilatory responses to the endothelium-dependent vasodilator ionomycin and the NO donor S-nitroso-N-acetylpenicillamine in isolated lungs from hypocapnic-IH (H-IH; 3 min cycles of 5% O(2)/air flush, 7 h/day, 4 wk), eucapnic-IH (E-IH; cycles of 5% O(2), 5% CO(2)/air flush), and sham-treated (air/air cycled) rats. Next, we assessed effects of endogenous O(2)(-) on NO- and cGMP-dependent vasoreactivity and measured O(2)(-) levels using the fluorescent indicator dihydroethidium (DHE) in isolated, endothelium-disrupted small pulmonary arteries from each group. Both E-IH and H-IH augmented NO-dependent vasodilation; however, enhanced vascular smooth muscle (VSM) reactivity to NO following H-IH was masked by an effect of endogenous O(2)(-). Furthermore, H-IH and E-IH similarly increased VSM sensitivity to cGMP, but this response was independent of either O(2)(-) generation or altered arterial protein kinase G expression. Finally, both H-IH and E-IH increased arterial O(2)(-) levels, although this response was more pronounced following H-IH, and H-IH exposure resulted in greater protein tyrosine nitration indicative of increased NO scavenging by O(2)(-). We conclude that IH increases pulmonary VSM sensitivity to NO and cGMP. Furthermore, endogenous O(2)(-) limits NO-dependent vasodilation following H-IH through an apparent reduction in bioavailable NO.

Bench-to-bedside review: hypercapnic acidosis in lung injury--from 'permissive' to 'therapeutic'

Modern ventilation strategies for patients with acute lung injury and acute respiratory distress syndrome frequently result in hypercapnic acidosis (HCA), which is regarded as an acceptable side effect ('permissive hypercapnia'). Multiple experimental studies have demonstrated advantageous effects of HCA in several lung injury models. To date, however, human trials studying the effect of carbon dioxide per se on outcome in patients with lung injury have not been performed. While significant concerns regarding HCA remain, in particular the possible unfavorable effects on bacterial killing and the inhibition of pulmonary epithelial wound repair, the potential for HCA in attenuating lung injury is promising. The underlying mechanisms by which HCA exerts its protective effects are complex, but dampening of the inflammatory response seems to play a pivotal role. After briefly summarizing the physiological effects of HCA, a critical analysis of the available evidence on the potential beneficial effects of therapeutic HCA from in vitro, ex vivo and in vivo lung injury models and from human studies will be reviewed. In addition, the potential concerns in the clinical setting will be outlined.

Rescue treatment with a Rho-kinase inhibitor normalizes right ventricular function and reverses remodeling in juvenile rats with chronic pulmonary hypertension

Chronic pulmonary hypertension in infancy and childhood is characterized by a fixed and progressive increase in pulmonary arterial pressure and resistance, pulmonary arterial remodeling, and right ventricular hypertrophy and systolic dysfunction. These abnormalities are replicated in neonatal rats chronically exposed to hypoxia from birth in which increased activity of Rho-kinase (ROCK) is critical to injury, as evidenced by preventive effects of ROCK inhibitors. Our objective in the present study was to examine the reversing effects of a late or rescue approach to treatment with a ROCK inhibitor on the pulmonary and cardiac manifestations of established chronic hypoxic pulmonary hypertension. Rat pups were exposed to air or hypoxia (13% O(2)) from postnatal day 1 and were treated with Y-27632 (15 mg/kg) or saline vehicle by twice daily subcutaneous injection commencing on day 14, for up to 7 days. Treatment with Y-27632 significantly attenuated right ventricular hypertrophy, reversed arterial wall remodeling, and completely normalized right ventricular systolic function in hypoxia-exposed animals. Reversal of arterial wall remodeling was accompanied by increased apoptosis and attenuated content of endothelin (ET)-1 and ET(A) receptors. Treatment of primary cultured juvenile rat pulmonary artery smooth muscle cells with Y-27632 attenuated serum-stimulated ROCK activity and proliferation and increased apoptosis. Smooth muscle apoptosis was also induced by short interfering RNA-mediated knockdown of ROCK-II, but not of ROCK-I. We conclude that sustained rescue treatment with a ROCK inhibitor reversed both the hemodynamic and structural abnormalities of chronic hypoxic pulmonary hypertension in juvenile rats and normalized right ventricular systolic function. Attenuated expression and activity of ET-1 and its A-type receptor on pulmonary arterial smooth muscle was a likely contributor to the stimulatory effects of ROCK inhibition on apoptosis. In addition, our data suggest that ROCK-II may be dominant in enhancing survival of pulmonary arterial smooth muscle.

Bench-to-bedside review: carbon dioxide

Carbon dioxide is a waste product of aerobic cellular respiration in all aerobic life forms. PaCO2 represents the balance between the carbon dioxide produced and that eliminated. Hypocapnia remains a common - and generally underappreciated - component of many disease states, including early asthma, high-altitude pulmonary edema, and acute lung injury. Induction of hypocapnia remains a common, if controversial, practice in both adults and children with acute brain injury. In contrast, hypercapnia has traditionally been avoided in order to keep parameters normal. More recently, advances in our understanding of the role of excessive tidal volume has prompted clinicians to use ventilation strategies that result in hypercapnia. Consequently, hypercapnia has become increasingly prevalent in the critically ill patient. Hypercapnia may play a beneficial role in the pathogenesis of inflammation and tissue injury, but may hinder the host response to sepsis and reduce repair. In contrast, hypocapnia may be a pathogenic entity in the setting of critical illness. The present paper reviews the current clinical status of low and high PaCO2 in the critically ill patient, discusses the insights gained to date from studies of carbon dioxide, identifies key concerns regarding hypocapnia and hypercapnia, and considers the potential clinical implications for the management of patients with acute lung injury.

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.

Effects of rho-kinase inhibition on pulmonary hypertension, lung growth, and structure in neonatal rats chronically exposed to hypoxia

Rho-kinase (ROCK) inhibitors prevent pulmonary hypertension (PHT) in adult rodents, but little is known about their effects on the neonatal lung. Our objective was to examine the effects of ROCK inhibition on chronic hypoxia (CH)-induced PHT and abnormal lung structure in the neonatal rat. Pups were exposed to air or CH from postnatal d 1-14 while receiving Y-27632 (5 or 10 mg x kg(-1) x d(-1)), fasudil (20 mg x kg(-1) x d(-1)), or saline intraperitoneally. Relative to air, CH-exposed pups had increased pulmonary vascular resistance, right ventricular hypertrophy, arterial medial wall thickening, and abnormal distal airway morphology characterized by septal thinning and decreased secondary septation. Treatment with 10 mg/kg Y-27632 or fasudil attenuated the structural and hemodynamic changes of PHT while having no effect on septal thinning or inhibited secondary septation. In addition, Y-27632 (10 mg/kg) and fasudil augmented CH-induced somatic growth restriction. Pulmonary arteries of CH-exposed pups had increased ROCK activity, up-regulated expression of PDGF-BB and increased smooth muscle DNA synthesis, all of which were attenuated by treatment with 10 mg/kg Y-27632. Systemically administered ROCK inhibitors prevented PHT in the CH-exposed neonatal rat but at the cost of inhibited somatic growth. Limiting effects on vascular remodeling likely resulted, in major part, from attenuated vascular PDGF-BB/beta-receptor signaling.

Does hypercapnia ameliorate hyperoxia-induced lung injury in neonatal rats?

Therapeutic hypercapnia (TH), an intentional inhalation of CO(2), has been shown to improve pulmonary function in certain models of lung injury. We tested the null hypothesis that TH does not improve hyperoxic lung injury in neonatal rats. The prospective, randomized study was set at Research laboratory in Children's Hospital. Forty-five newborn rats were randomly assigned to three groups (n = 15/group), and exposed to 96 h of normoxia (FiO(2) = 0.21), hyperoxia (FiO(2) > 0.98), and TH (FiO(2) = 0.95, FiCO(2) = 0.05). Lung histology, wet-weight to dry-weight ratio, and concentrations of pro- and anti-inflammatory cytokines (IL-1beta, IL-6, TNF-alpha, and IL-10) were used to evaluate pulmonary damage. Using a scale of 0-4, the total scores for lungs hypercellularity, inflammation, and hemorrhage was significantly increased from a median value of 1.5 in normoxia to 2.5 in hyperoxia (P < 0.05) and 3.0 with TH (P < 0.001, nonparametric ANOVA). The interstitial space relative to the alveolar space, as a measure of hypercellularity, was increased by 18% during hyperoxia and by 44% with TH compared with normoxia. TH significantly increased the size of the interstitial space by 22% compared with hyperoxia (P < 0.001). The lung wet-weight to dry-weight ratio was increased by 10% in both hyperoxic groups (P < 0.001). Both hyperoxic groups showed significant reductions in the concentration of IL-1beta compared with normoxia (P < 0.001), whereas the ratio of IL-1beta to IL-10 was significantly decreased, indicating an anti-inflammatory trend. TH does not prevent histological manifestations of hyperoxic lung injury in spontaneously breathing neonatal rats and may worsen the outcome.

Mechanisms of intermittent hypoxia induced hypertension

Exposing rodents to brief episodes of hypoxia mimics the hypoxemia and the cardiovascular and metabolic effects observed in patients with obstructive sleep apnoea (OSA), a condition that affects between 5% and 20% of the population. Apart from daytime sleepiness, OSA is associated with a high incidence of systemic and pulmonary hypertension, peripheral vascular disease, stroke and sudden cardiac death. The development of animal models to study sleep apnoea has provided convincing evidence that recurrent exposure to intermittent hypoxia (IH) has significant vascular and haemodynamic impact that explain much of the cardiovascular morbidity and mortality observed in patients with sleep apnoea. However, the molecular and cellular mechanisms of how IH causes these changes is unclear and under investigation. This review focuses on the most recent findings addressing these mechanisms. It includes a discussion of the contribution of the nervous system, circulating and vascular factors, inflammatory mediators and transcription factors to IH-induced cardiovascular disease. It also highlights the importance of reactive oxygen species as a primary mediator of the systemic and pulmonary hypertension that develops in response to exposure to IH.