No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Benzamil, a blocker of epithelial Na+ channel-induced upregulation of artery oxygen pressure level in acute lung injury rabbit ventilated with high frequency oscillation
Abstract
The epithelial Na(+) transport via an epithelial Na(+) channel (ENaC) expressed in the lung epithelium would play a key role in recovery from lung edema at acute lung injury by removing the fluid in lung luminal space. The lung edema causes dysfunction of gas exchange, decreasing oxygen pressure level of artery [P(aO(2))]. To study if ENaC plays a key role in recovering P(aO(2)) from a decreased level to a normal one in acute lung injury, we applied benzamil (20microM, a specific blocker of ENaC) to the lung luminal space in acute lung injury treated with high frequency oscillation ventilation (HFOV) that is a lung-protective ventilation with a lower tidal volume and a smaller pressure swing than conventional mechanical ventilation (CMV). Benzamil facilitated the recovery of P(aO(2)) in acutely injured lung with HFOV but not CMV. The observation suggests that in acutely injured lung treated with HFOV an ENaC blocker, benzamil, can be applied as a therapeutic drug for acute lung injury combing with HFOV.
... Sodium is absorbed through the basolateral membrane of the epithelial sodium channel (ENaC) and sodium pump (Na + /K + -ATPase)-type cells. The associated chloride transport mechanisms have not yet been elucidated [18]. (iii) Passive water transport is accomplished mainly by aquaporins on type I cells. ...
Acute lung injury (ALI) is a common and serious disease. Numerous treatment options are available but they do not improve quality of life or reduce mortality for ALI patients. Here, we review the treatments for ALI to provide basic data for ALI drug therapy research and development. Chinese Materia Medica (CMM) has long been the traditional clinical approach in China for the treatment of ALI and it has proven efficacy. The continued study of CMM has disclosed new potential therapeutic ingredients for ALI. However, few reviews summarize the currently available CMM-based anti-ALI drugs. Therefore, the systematic analysis of research progress in anti-ALI CMM is of great academic and clinical value. The aim of the present review is to describe CMM-based research progress in ALI treatment. Data were compiled by electronic retrieval (CNKI, SciFinder, PubMeds, Google Scholar, Web of Science) and from articles, patents and ethnopharmacological literature in university libraries were systematically studied. This review introduces progress in research on the etiology and mechanisms of ALI, the anti-ALI theory and modes of action in traditional Chinese medicine (TCM), anti-ALI active constituents of CMM, research progress in experimental methods of CMM anti-ALI, the anti-ALI molecular mechanisms of CMM, the anti-ALI efficacy of CMM formulae, and the potential toxicity of CMM and the antidotes for it. Scholars have investigated the anti-ALI molecular mechanism of CMM from various direction and have made substantial progress. This research explored the above aspects, enriched the anti-ALI theory of CMM and established the clinical significance and developmental prospects of ALI treatment by CMM. Because of the high frequency of drugs such as glucocorticoids or antibiotics, Western medicine lacks the advantages of CMM in terms of overall anti-ALI efficacy. In the future, the development of CMM-based anti-ALI therapies will become a major trend in the field of ALI drug development. Successful clinical safety and efficacy validations will promote and encourage the use of CMM. It provides fundamental theoretical support for the discovery and use of CMM resources through the comprehensive analysis of various anti-ALI CMM report databases.
... Отек легких нарушает транспорт газов, снижает парциальное давление кислорода в артериальной крови. Применение се лективного блокатора ENaC при ОПЛ, вызванном высокочастотной вентиляцией (ВЧВ), способст вует ликвидации отека и может быть предложено в качестве терапевтического средства при ОПЛ в условиях проведения ВЧВ [18]. ...
Morphological studies were used to examine the lungs from 92 persons who had died from brain and concomitant injuries and massive blood loss and from 117 experimental animals (rats) in systemic circulatory disorders and vascular thrombosis of the pulmonary microcirculatory bed. The purpose of the study was to assess the nature, intensity, and developmental periods of the morphological signs characterizing acute pulmonary lesion. Disorders of circulation (including microcirculation), damages to bronchial and bronchiolar mucosae, the development of atelectases, dyst-electases, and focal emphysema were found to be early structural changes in injury and massive blood loss. Pulmonary edema and the signs of a systemic inflammatory reactions form within the first hours following injury and massive blood loss. Experimental studies confirm that microcirculatory disorders and individual traits play an important role in the development of acute pulmonary lesion.
... Na + flux through ENaC is crucial for clearance of alveolar edema (Factor, Mutlu et al. 2007) yet, apparently paradoxically, activity of channel is reduced under hypoxic conditions (Jain and Sznajder 2005) . However, inhibition of ENaC under hypoxic conditions, leading to an increase in airway surface liquid may be beneficial for clearance of mucus (Taguchi, Niisato et al. 2005;Mall 2008 Minimum concentration of haem necessary to induce inhibition of NPo was ≈40 nM. ...
... To the best of our knowledge, Ben is the first chemical compound that is capable of enhancing UPS activity in a concentrationdependent manner. The fact that Ben also shows beneficial effects in lung (54) and cardiac (55) disease models extends its potential use as a drug in human. Ami, on the other hand, has already been prescribed as an adjunctive treatment with thiazide diuretics or other kaliuretic agents in congestive heart failure or hypertension (56) in human patients with known pharmacokinetics. ...
Huntington's disease (HD) is a fatal neurodegenerative disorder. Despite a tremendous effort to develop therapeutic tools
in several HD models, there is no effective cure at present. Acidosis has been observed previously in cellular and in in vivo models as well as in the brains of HD patients. Here we challenged HD models with amiloride (Ami) derivative benzamil (Ben),
a chemical agent used to rescue acid-sensing ion channel (ASIC)-dependent acidotoxicity, to examine whether chronic acidosis
is an important part of the HD pathomechanism and whether these drugs could be used as novel therapeutic agents. Ben markedly
reduced the huntingtin-polyglutamine (htt-polyQ) aggregation in an inducible cellular system, and the therapeutic value of
Ben was successfully recapitulated in the R6/2 animal model of HD. To reveal the mechanism of action, Ben was found to be
able to alleviate the inhibition of the ubiquitin-proteasome system (UPS) activity, resulting in enhanced degradation of soluble
htt-polyQ specifically in its pathological range. More importantly, we were able to demonstrate that blocking the expression
of a specific isoform of ASIC (asic1a), one of the many molecular targets of Ben, led to an enhancement of UPS activity and this blockade also decreased htt-polyQ
aggregation in the striatum of R6/2 mice. In conclusion, we believe that chemical compounds that target ASIC1a or pharmacological
alleviation of UPS inhibition would be an effective and promising approach to combat HD and other polyQ-related disorders.
... Na ϩ flux through ENaC is crucial for clearance of alveolar edema (41), yet apparently paradoxically, channel activity is reduced under hypoxic conditions (8). However, inhibition of ENaC under hypoxic conditions, leading to an increase in airway surface liquid, may be beneficial for clearance of mucus (42,43). Thus, activity of ENaC, a channel pivotal to lung function, may be ''regulated'' by PO 2 and hemeoxygenase , a process that may be crucial to survival of lung injury or infection. ...
Epithelial sodium channels (ENaCs) are of immense importance, controlling Na(+) transport across epithelia and thus playing a central role in all aspects of fluid clearance as well as numerous other functions. Regulation of these channels is critical. Here, we show that haem, a regulator of Na(+) transport, directly influences ENaC activity, decreasing channel-open probability (but not unitary conductance) in inside-out patches (but not outside-out). Conversely, exposure to the protein in the presence of NADPH and at normoxic O(2) tension (requirements for activity of hemeoxygenase) increases channel activity. CO, a product of hemeoxygenase activity, activated ENaC in a manner similar to that of haem plus NADPH. However, under hypoxic conditions, inhibition of ENaC by haem occurred even in the presence of NADPH. These data demonstrate a potent, O(2)-sensitive mechanism for regulation of ENaC, in which hemeoxygenase acts as the O(2) sensor, its substrate and product inhibiting and stimulating (respectively) the activity of ENaC.
... In addition, excessive Na ϩ absorption occurs when the apical membrane CFTR density is reduced by genetic mutations that cause cystic fibrosis, a process that appears to be linked to disease pathogenesis (19,111). The effects manifested in genetic diseases point to potentially significant roles for Na ϩ channel dysregulation that may result in conditions such as pulmonary edema (116,138), chronic obstructive pulmonary disease (111), acute respiratory distress syndrome (182) and nephrosis (92), as well as hypertension (9,178,181). The salt-sensitive hypertension of Liddle's syndrome is now well understood as being caused by an ENaC mutation which alters its trafficking and cell surface expression, the mechanism of which was elucidated through basic studies of ENaC function (1,57,68,69,105,152,163). ...
The epithelial Na(+) channel (ENaC) is a major regulator of salt and water reabsorption in a number of epithelial tissues. Abnormalities in ENaC function have been directly linked to several human disease states including Liddle's syndrome, psuedohypoaldosteronism, and cystic fibrosis and may be implicated in states as diverse as salt-sensitive hypertension, nephrosis, and pulmonary edema. ENaC activity in epithelial cells is highly regulated both by open probability and number of channels. Open probability is regulated by a number of factors, including proteolytic processing, while ENaC number is regulated by cellular trafficking. This review discusses current understanding of apical membrane delivery, cell surface stability, endocytosis, retrieval, and recycling of ENaC and the molecular partners that have so far been shown to participate in these processes. We review known sites and mechanisms of hormonal regulation of trafficking by aldosterone, vasopressin, and insulin. While many details of the regulation of ENaC trafficking remain to be elucidated, knowledge of these mechanisms may provide further insights into ENaC activity in normal and disease states.
The amiloride-sensitive epithelial sodium channel, ENaC, is a heteromultimeric protein made up of three homologous subunits (alpha, beta and gamma) (1,2). In vitro, assembly and expression of functional active sodium channels in the Xenopus oocyte is strictly dependent on alpha-ENaC--the beta and gamma subunits by themselves are unable to induce an amiloride-sensitive sodium current in this heterologous expression system (2). In vivo, ENaC constitutes the limiting step for sodium absorption in epithelial cells that line the distal renal tubule, distal colon and the duct of several exocrine glands. The adult lung expresses alpha, beta and gamma ENaC (3,4), and an amiloride-sensitive electrogenic sodium reabsorption has been documented in upper and lower airways (3-7), but it is not established whether this sodium transport is mediated by ENaC in vivo. We inactivated the mouse alpha-ENaC gene by gene targeting. Amiloride-sensitive electrogenic Na+ transport was abolished in airway epithelia from alpha-ENaC(-/-) mice. Alpha-ENaC(-/-) neonates developed respiratory distress and died within 40 h of birth from failure to clear their lungs of liquid. This study shows that ENaC plays a critical role in the adaptation of the newborn lung to air breathing.
The capacity of the alveolar epithelial barrier to remove excess alveolar fluid from the airspaces of the lung was studied in an experimental model of moderate hyperoxic lung injury. Rats were exposed to 100% oxygen for 40 h in an exposure chamber and compared with control animals exposed to room air. Extravascular lung water was calculated gravimetrically. Alveolar and lung liquid clearance were studied over 1 h by instillation of a 5% albumin solution with 1.5 microCi of 125I-labeled albumin (6 mL/kg into both lungs). The concentration of both the unlabeled and labeled albumin was used to calculate alveolar liquid clearance. Hyperoxic rats developed pulmonary edema, with a 33% increase in extravascular lung water to 5.3 +/- 0.1 g of water per gram of dry lung, compared with 4.0 +/- 0.2 g of water per gram of dry lung in control rats (p < 0.05). This degree of edema was associated with a significant increase in the alveolar-arterial oxygen difference (241 +/- 61 vs 124 +/- 14 mm Hg in control animals exposed to room air, p < 0.05). Despite this moderate degree of lung injury, alveolar fluid clearance was normal (30 +/- 3%) compared with control rats (33 +/- 6%). Furthermore, the hyperoxic injured rats responded normally to an exogenous beta-adrenergic agonist (terbutaline, 10(-4) mol/L) with a 67% increase in the rate of alveolar liquid clearance (50 +/- 5%). Thus, in the setting of moderate hyperoxic lung injury, the alveolar epithelial barrier is still capable of removing fluid at a normal rate and responding to beta-adrenergic agonist treatment. These experimental results have potential clinical implications for patients with acute lung injury.
Reexpansion of a collapsed lung induces increased microvascular permeability leading to reexpansion pulmonary edema (REPE). This study was designed to prove the hypothesis that local overproduction of interleukin-8 (IL-8) induces inflammatory cell accumulation which leads to the induction of REPE. Initially, we examined the detailed characteristics of a rabbit model of REPE in association with IL-8 production and its mRNA expression. The lung tissue to plasma ratio of radiolabeled albumin (T/P ratio), the lung wet to dry ratio, and bronchoalveolar lavage (BAL) neutrophil counts were significantly increased in the reexpanded lung. IL-8 concentrations and mRNA expression were significantly increased in the reexpanded lung homogenate. Immunohistochemically, alveolar macrophages (AMs) and epithelial cells in the reexpanded lung and AMs in the collapsed lung were positive for IL-8. Second, we examined the effect of pretreatment with a specific monoclonal anti-IL-8 antibody (Ab) or control IgG on the development of REPE. The T/P ratio and BAL neutrophil counts were conspicuously decreased by pretreatment with anti-IL-8 Ab, but not with control IgG. On a histopathological study, lung injury and leukocyte infiltration were attenuated by the pretreatment with anti-IL-8 Ab. In conclusion, IL-8 production is enhanced in the reexpanded lung, and contributes to the development of REPE. The pretreatment with anti-IL-8 antibody may be useful as a novel protective therapy for this disease.
Interleukin (IL)-13 has been associated with asthma, allergic rhinitis, and chronic sinusitis, all conditions where an imbalance in epithelial fluid secretion and absorption could impact upon the disease. We have investigated the effects of IL-13 on the ion transport characteristics of human bronchial epithelial cells cultured at an apical-air interface. Ussing chamber studies indicated that 48 h pretreatment with IL-13 or IL-4 significantly reduced the basal short-circuit current (I(sc)) and inhibited the amiloride-sensitive current by >98%. Furthermore, the I(sc) responses were increased by more than six- and twofold over control values when stimulated with UTP or forskolin, respectively, after cytokine treatment. The IL-13-enhanced response to UTP/ionomycin was sensitive to bumetanide and DIDS and was reduced in a low-chloride, bicarbonate-free solution. Membrane permeablization studies indicated that IL-13 induced the functional expression of an apical Ca(2+)-activated anion conductance and that changes in apical or basolateral K(+) conductances could not account for the increased I(sc) responses to UTP or ionomycin. The results indicate that IL-13 converts the human bronchial epithelium from an absorptive to a secretory phenotype that is the result of loss of amiloride-sensitive current and an increase in a DIDS-sensitive apical anion conductance.
Asthma is an inflammatory disease, and the epithelial mesenchymal unit appears to be of importance in regulating the disease mechanisms. Cell-cell adhesion plays an important role in tissue morphogenesis and homeostasis and is commonly mediated by cadherins, a family of Ca(2+)-dependent transmembrane adhesion receptors. The cadherin family is involved in control of the cellular architecture. Proinflammatory cytokines such as tumor necrosis factor (TNF)-alpha are involved in asthma and may interfere with epithelial integrity. In the present study, we investigated the role of TNF-alpha and dexamethasone on the expression of E-cadherin, beta-catenin, and gamma-catenin. We used two bronchial epithelial cell models: primary small airway epithelial cell cultures and primary culture obtained from human bronchial tubes. After 48 h of TNF-alpha stimulation with or without dexamethasone expression of E-cadherin, beta-catenin and gamma-catenin were analyzed using Western blot analysis and immunofluorescence. This study showed a decrease in the expression of adhesion molecules in both epithelial cell cultures after stimulation. Dexamethasone and anti-TNF-alpha inhibited this effect. In unstimulated cells, E-cadherin and beta- and gamma-catenin expression was membranous, expressed only on the lateral cell wall with minimal cytoplasmic expression. Immunoreactivity was cytoplasmic in stimulated cells. We demonstrated, using Western blot analysis and immunofluorescence, that proinflammatory cytokines could be responsible for structural damage to the epithelium and that this process was potentially reversed by steroids.
Severe acute chest syndrome afflicts patients with sickle cell disease and can cause hypoxemia refractory to conventional treatments. Obstructive mucus plugging and the development of acute respiratory distress syndrome may underlie the pathophysiology of refractory hypoxemia in acute chest syndrome. Although high-frequency oscillatory ventilation (HFOV) is well established in the treatment of pediatric acute respiratory distress syndrome, there is no support in the literature for its role in managing hypoxemia in acute chest syndrome. In disease processes with high airways resistance and obstructive mucus plugging, HFOV may predispose to air-trapping and increased morbidity secondary to air leak syndromes. We report the first successful HFOV management of pediatric patients suffering from severe acute chest syndrome and hypoxic respiratory failure. These cases suggest that HFOV should be strongly considered for patients with severe acute chest syndrome that is refractory to conventional mechanical ventilation.
The epithelial sodium channel (ENaC) plays a key role in the regulation of fluid absorption in the kidney, lung, colon and exocrine glands, and in the regulation of blood pressure. Abnormal functioning of ENaC is associated with several human diseases, including pseudohypoaldosteronism type I, Liddle's syndrome, pulmonary edema, and cystic fibrosis. ENaC is regulated by several hormones, ions and accessory proteins. This review focuses on the regulation of ENaC by recently described accessory proteins, mainly Nedd4, syntaxin 1A, CFTR, sgk, K-Ras2A and Cap-1.
Lung epithelial ion transport promotes salt and water movement across the fetal and neonatal lung epithelium. The mechanism is dependent on basolateral membrane Na-K-ATPase and the apical membrane Cl– and Na+ channels. During fetal life active secretion of Cl– and parallel movement of Na+ across the epithelium into the developing lung lumen induce accumulation of liquid into the future airspaces. Postnatally, however, absorption of fluid from the airspaces must start. Present evidence suggests that activation of Na+ transport from the lumen into the basolateral direction drives fluid absorption and results in an essentially dry air-filled alveolus. In laboratory animals amiloride, a Na+ channel blocker, induces respiratory distress and impedes lung fluid clearance. One of the epithelial amiloride-sensitive Na+ channels, ENaC, is composed of three homologous subunits that differentially respond to glucocorticoid hormone. In newborn infants an increase in pulmonary fluid and a defective Na+ transport associate with respiratory distress. The ontogeny, subunit composition and function of ENaC along the respiratory tract are currently under investigation. It will be interesting to find out whether the subunit composition and function of lung ENaC respond to the therapy of the critically ill newborn infant.
The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. We have reviewed the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.
To determine whether epithelial ion transport is physiologically important for lung water clearance after birth, the sodium transport inhibitor amiloride or its vehicle saline was given intratracheally to newborn full-term guinea pigs before the first breath. Guinea pigs given saline intratracheally breathed normally and had arterial O2 saturations (SaO2) greater than 94%. In contrast, guinea pigs that had an estimated 10(-4) M intra-alveolar concentration of amiloride had chest wall retractions and 88 +/- 3.6% (SD) SaO2 (P less than 0.01). Extravascular lung water (EVLW) per gram of dry lung weight 4 h after birth was significantly greater in newborns that received amiloride (8.3 +/- 1.1, n = 5) than in those that received saline (5.6 +/- 0.9, n = 7, P less than 0.01). The degree of perivascular fluid cuffing at 25 cmH2O inflation was quantitatively similar in amiloride- and saline-treated animals. The effect of amiloride was dose dependent. Intratracheal amiloride did not affect EVLW in 9-day-old guinea pigs. This study demonstrates that intratracheal amiloride before the first breath results in respiratory distress, hypoxemia, and an abnormally high EVLW. Epithelial sodium transport contributes normal lung liquid clearance after birth.
1. The purpose of the investigation was to characterize the luminal membrane and the paracellular pathway of rat lung alveolar epithelium. Experiments were performed on lungs in situ instilled with isotonic, buffered Ringer solution and perfused with blood from a donor rat using cross-circulation technique. 2. The rate of active Na+ transport was 4.4 pmol/(cm2s). The fluid absorption was 156 nl/s, and was unaffected by the presence of protein in the instillate (166 nl/s). In the absence of Na+, fluid absorption was zero. Amiloride (10(-3) M) reduced fluid absorption by 60%. Amiloride, combined with absence of D-glucose, arrested fluid absorption completely. Phloridzin at the luminal side reduced fluid absorption whilst phloretin had no effect. Amiloride together with phloridzin (10(-3) M) also arrested absorption. Thus, there are two entry systems for Na+ in the luminal membrane: Na+ channels and a Na+-D-glucose symport. These results show that alveolar fluid absorption is due to cellular activity. 3. Substitution of Cl- with gluconate not only stopped fluid absorption, but led to slight reversal of net fluid movement. 4. Passive unidirectional flux of Na+, determined with 22Na+, was 9.9 pmol/(cm2s) and that of Cl-, determined with 36Cl-, was 12.4 pmol/(cm2s). These fluxes were based on an assumed alveolar surface area of 5000 cm2. Transference numbers calculated from these figures are close to those in free solution, suggesting a neutral or weakly charged intercellular junctional pathway. The D-mannitol permeability in the paracellular pathway was 1.7 X 10(-8) cm/s. 5. It is a consequence of the proposed mechanism for fluid absorption that it becomes inoperative if the normally high reflexion coefficients for Na+ and Cl- are lowered in pathological states. In such conditions pulmonary oedema may develop depending on the net balance of passive mechanical and colloid-osmotic forces. 6. An explanation of the reversal of fluid transport at the time of birth is presented.
Three new concepts concerning lung liquid and protein exchange are considered. The first is that the microvascular surface area is as important as the microvascular hydrostatic pressure in assessing filtration in the lung. One of the problems in differentiating hemodynamic from increased permeability edema is the inability to determine whether the microvascular surface area has changed. Several agents, as well as exercise, affect liquid filtration. A new, dynamic procedure that is more sensitive for the detection of increased permeability than static measurements of lung water content is described, along with its limitations. The second concept is that water and electrolytes are cleared from the alveoli by a separate mechanism from protein. Water clearance is fast and occurs mainly by an active process, which can be inhibited by amiloride or phloridzin and accelerated by beta-agonists. The mechanism appears to depend on metabolically regulated sodium transport across the alveolar epithelium. Protein clearance is very slow and is relatively independent of alveolar concentration. The protein clearance mechanism is unknown but may involve transcytosis. The third concept is that during edema formation there are two pathways for liquid clearance in addition to the lymphatic system: into the pleural space and along the bronchovascular connective tissue into the mediastinum. During recovery from edema, reabsorption into blood is important if the edema liquid has a low protein osmotic pressure. Clearance into the mediastinum may be the major pathway for liquid sequestered in the loose, binding connective tissue.
Although the mechanisms responsible for alveolar liquid clearance have been studied in several species, there has not been any information regarding the effect of ion transport agonists or antagonists on alveolar liquid clearance in the human lung. Therefore, we studied alveolar liquid clearance in the recently resected human lung from patients who underwent surgery for lung cancer. A test solution of 40 ml of isosmolar albumin solution was instilled into one segment of a resected lobe within 10 min of resection. Because protein leaves the air spaces very slowly, the concentration of alveolar protein over 4 h was used to quantify alveolar liquid clearance. Basal alveolar liquid clearance was 12 +/- 2% over 4 h. Amiloride (10(-5) M), an inhibitor of apical Na+ uptake, and ouabain (10(-3) M), an inhibitor of Na,K-ATPase activity, reduced alveolar liquid clearance by 40 and 49%, respectively (p < 0.005). Terbutaline (10(-3) or 10(-4) M) doubled alveolar liquid clearance to 28 +/- 9% over 4 h (p < 0.05). Propranolol (10(-4) M) and amiloride (10(-5) M) inhibited the terbutaline-induced increase in alveolar liquid clearance. In conclusion, (1) alveolar liquid clearance in the human lung can be markedly reduced by inhibition of apical sodium channel uptake or Na,K-ATPase activity, and (2) beta-adrenergic stimulation markedly increases the rate of alveolar liquid clearance in the resected human lung without pulmonary perfusion.
The amiloride-sensitive epithelial sodium channel constitutes the rate-limiting step for sodium reabsorption in epithelial cells that line the distal part of the renal tubule, the distal colon, the duct of several exocrine glands, and the lung. The activity of this channel is upregulated by vasopressin and aldosterone, hormones involved in the maintenance of sodium balance, blood volume and blood pressure. We have identified the primary structure of the alpha-subunit of the rat epithelial sodium channel by expression cloning in Xenopus laevis oocytes. An identical subunit has recently been reported. Here we identify two other subunits (beta and gamma) by functional complementation of the alpha-subunit of the rat epithelial Na+ channel. The ion-selective permeability, the gating properties and the pharmacological profile of the channel formed by coexpressing the three subunits in oocytes are similar to that of the native channel.
The epithelial amiloride-sensitive sodium channel constitutes the rate limiting step for sodium reabsorbtion by the epithelial lining the distal part of the kidney tubule, the urinary bladder and the distal colon. Reabsorbtion of sodium through this channel, which is regulated by hormones such as aldosterone and vasopressin, is one of the essential mechanisms involved in the regulation of sodium balance, blood volume and blood pressure. Here we isolate a DNA from epithelial cells of rat distal colon and identify it by functional expression of an amiloride-sensitive sodium current in Xenopus oocyte. The deduced polypeptide (698 amino acids) has at least two putative transmembrane segments. Expression of this protein in Xenopus oocytes reconstitutes the functional properties of the highly selective amiloride-sensitive, epithelial sodium channel. The gene encoding this rat sodium channel subunit shares significant sequence similarity with mec-4 and deg-1, members of a family of Caenorhabditis elegans genes involved in sensory touch transduction and, when mutated, neuronal degeneration. We propose that the gene products of these three genes are members of a gene family coding for cation channels.
The recent immunopurification and cloning of various lung Na+ channel proteins has provided the necessary tools to study Na+ transport at a fundamental level across a number of epithelial tissues. Various macroscopic measurements of Na+ transport have shown that Na+ ions enter the cytoplasm of alveolar cells mainly through amiloride-inhibitable Na+ channels. Molecular biology studies have shown the existence of three Na+ channel subunit mRNAs (alpha-, beta-, and gamma-rENaC) in mature fetal (FDLE) and adult alveolar type II (ATII) cells. Patch-clamp studies have demonstrated the existence of various types of amiloride-inhibitable Na+ channels, located in the apical membranes of FDLE and ATII cells. beta-Agonists and agents that enhance intracellular adenosine 3',5'-cyclic monophosphate levels increase the open probability of these channels, leading to increased Na+ transport across the alveolar epithelium in vivo. Immunopurification of a putative channel protein from adult ATII cells showed that it contains an amiloride-binding subunit with a molecular mass of 150 kDa. When this protein was reconstituted in planar lipid bilayers, it exhibited single channels with a conductance of 25 pS, which were moderately selective for Na+ over K+. The open probability of these channels was increased by the addition of protein kinase A (PKA) and ATP, and was decreased to the same extent by addition of [N-ethyl-N-isopropyl]-2'-4'-amiloride (EIPA) and amiloride (1 microM each) in the apical side of the bilayer, in agreement with the results of patch-clamp studies in ATII cells. Exposure of rats to sublethal hyperoxia increased alpha-rENaC mRNA and the functional expression of Na+ channels in alveolar epithelial cells and limited alveolar edema. These findings indicate that alveolar epithelial channels contain at least one family of amiloride-sensitive Na+ channel proteins, which displays a number of unique properties, including sensitivity to EIPA.
1. In cell-attached patches formed on the apical membrane of fetal alveolar epithelium, terbutaline (a specific beta2-adrenergic agonist) increased the open probability (Po) of an amiloride-sensitive Na+-permeable non-selective cation (NSC) channel (control, 0.03 +/- 0.04; terbutaline, 0.62 +/- 0.18; n = 8, P < 0. 00001) by increasing the mean open time 100-fold without any significant change in the mean closed time and without any change in the single channel conductance (control, 27.8 +/- 2.3 pS; terbutaline, 28.2 +/- 2.1 pS; n = 8). 2. The Po of the unstimulated channel increased when the apical membrane was depolarized due to a decrease in the closing rate and an increase in the opening rate, while the Po of the terbutaline-stimulated channel did not depend on the membrane potential. 3. Increased cytosolic [Ca2+] also increased the Po of the channel in a manner consistent with one Ca2+-binding site on the cytosolic surface of the channel. Terbutaline increased the sensitivity of the channel to cytosolic Ca2+ by shifting the concentration of cytosolic Ca2+ ([Ca2+]c) required for half-maximal activation to a lower [Ca2+]c value, leading to an increase in Po. 4. An increase in the cytosolic Cl- concentration ([Cl-]c) decreased the Po of the channel consistent with two Cl--binding sites by increasing the closing rate without any significant change in the opening rate. Terbutaline increased Po by reducing the effect of cytosolic Cl- to promote channel closing. 5. Taken together, these observations indicate that terbutaline activates a Ca2+-activated, Cl--inhibitable, amiloride-sensitive, Na+-permeable NSC channel in fetal rat alveolar epithelium in two ways: first, through an increase in Ca2+ sensitivity, and second, through a reduction in the effect of cytosolic Cl- to promote channel closing.
At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na(+)-selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 microM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of < 1 microM). Both FDLE and ATII cells contain the alpha-, beta-, and gamma-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na(+)-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'-amiloride. A variety of agents, including cAMP, oxygen, glucocorticoids, and in some cases Ca2+, increased the activity and/or rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na(+)-absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn alpha-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.
The highly amiloride-sensitive epithelial sodium channel (ENaC) is an apical membrane constituent of cells of many salt-absorbing epithelia. In the kidney, the functional relevance of ENaC expression has been well established. ENaC mediates the aldosterone-dependent sodium reabsorption in the distal nephron and is involved in the regulation of blood pressure. Mutations in genes encoding ENaC subunits are causative for two human inherited diseases: Liddle's syndrome, a severe form of hypertension associated with ENaC hyperfunction, and pseudohypoaldosteronism (PHA-1), a salt-wasting syndrome caused by decreased ENaC function. Transgenic mouse technologies provide a useful tool to study the role of ENaC in vivo. Different mouse lines have been established in which each of the ENaC subunits was affected. The phenotypes observed in these mice demonstrated that each subunit is essential for survival and for regulation of sodium transport in kidney and colon. Moreover, the alpha subunit plays a specific role in the control of fluid absorption in the airways at birth. Such mice can now be used to study the role of ENaC in various organs and can serve as models to understand the pathophysiology of these human diseases.
Lung epithelial ion transport promotes salt and water movement across the fetal and neonatal lung epithelium. The mechanism is dependent on basolateral membrane Na-K-ATPase and the apical membrane Cl(-) and Na(+) channels. During fetal life active secretion of Cl(-) and parallel movement of Na(+) across the epithelium into the developing lung lumen induce accumulation of liquid into the future airspaces. Postnatally, however, absorption of fluid from the airspaces must start. Present evidence suggests that activation of Na(+) transport from the lumen into the basolateral direction drives fluid absorption and results in an essentially dry air-filled alveolus. In laboratory animals amiloride, a Na(+) channel blocker, induces respiratory distress and impedes lung fluid clearance. One of the epithelial amiloride-sensitive Na(+) channels, ENaC, is composed of three homologous subunits that differentially respond to glucocorticoid hormone. In newborn infants an increase in pulmonary fluid and a defective Na(+) transport associate with respiratory distress. The ontogeny, subunit composition and function of ENaC along the respiratory tract are currently under investigation. It will be interesting to find out whether the subunit composition and function of lung ENaC respond to the therapy of the critically ill newborn infant.
WEQAS is one of the largest and the most rapidly expanding national EQA Schemes in the UK with over 500 participants. Since April 1995, the Scheme has distributed a tonometered protein-based aqueous material for blood gas analytes (H(+), pCO2, pO2), electrolytes, glucose and lactate. Current performance of the new generation of blood gas analysers show marked bias between analysers which a number of manufacturers have attributed to a "matrix" effect. This study was undertaken to investigate the "true" analytical performance of these analysers utilising a tonometered haemolysate blood product with oxygen saturation kinetics identical to that of fresh blood. One hundred laboratories that represented the major analyzers used in the UK were selected to participate in the study. Participants were asked to analyse the samples within 1 week of tonometry of the material. Performance data, expressed as interlaboratory variation, was compared with concentration matched data from the same group using the aqueous material. Significant improvement in instrument Bias was observed for the AVL Omni and iStat analysers for pO2). Surprisingly, there was also an improvement in bias for the electrolyte parameters on the IL synthesis. However, a significant positive bias was observed for the AVL Opti for Potassium and for the Nova analyzer for glucose using this material. Similar performance was observed for the co-oximetry, parametres for fresh and lyophilised material for the majority of analyzers.
Active transepithelial transport of sodium from the airspaces to the lung interstitium is a primary mechanism driving alveolar fluid clearance. This mechanism depends on sodium uptake by amiloride-sensitive sodium channels on the apical membrane of alveolar type II cells followed by extrusion of sodium on the basolateral surface by the Na-K-ATPase. Injury to the alveolar epithelium can disrupt the integrity of the alveolar barrier or downregulate ion transport pathways thus reducing net alveolar fluid reabsorption, and enhancing the extent of alveolar edema. Endogenous catecholamines upregulate alveolar fluid clearance in several experimental models of acute lung injury, but this upregulation is short-term and often not sufficient to counterbalance alveolar flooding. There is new evidence, however, that pharmacological treatment with beta-adrenergic agonists and/or epithelial growth factors may induce a more sustained stimulation of alveolar fluid reabsorption and in turn facilitate recovery from experimental pulmonary edema. Similar results have been achieved experimentally by gene transfer enhancing the abundance of sodium transporters in the alveolar epithelium. Clinical studies show that impaired alveolar fluid transport mechanisms contribute to the development, severity and outcome of pulmonary edema in humans. Very recent data suggest that mechanisms that augment transepithelial sodium transport and enhance the clearance of alveolar edema may lead to more effective prevention or treatment for pulmonary edema and acute lung injury.
The epithelial sodium channel (ENaC) in the apical membrane of polarized epithelial cells is the rate-limiting step for Na entry into the cell; in series with the basolateral Na pump, it allows the vectorial transepithelial transport of Na ions. ENaC is expressed in different epithelia like the distal nephron or colon, and the airways epithelium. In the lung ENaC controls the composition and the amount of pulmonary fluid, whereas in the distal nephron ENaC under the control of aldosterone and vasopressin, is essential to adapt the amount of Na+ reabsorbed with the daily sodium intake. Activating mutations of ENaC cause severe disturbances of Na+ homeostasis leading to hypertension in human and in mouse models. Functional expression of ENaC in different cell systems allowed the identification of structural domains of the protein that are essential for channel function and/or modulation of channel activity. Site-directed mutations in specific domains of the channel protein lead to channel hyperactivity or channel loss of function. Knowledge about ENaC structure-function relationships opens new opportunities for development of pharmacological tools for controlling ENaC activity, such as channel activators of potential benefit in the treatment of pulmonary edema, or highly potent ENaC blockers with natriuretic effects.
A beta-adrenergic agonist (beta-agonist), terbutaline, stimulated amiloride-sensitive Na(+) absorption in fetal rat alveolar type II epithelium, contributing to the clearance of lung fluid. Cytosolic Ca(2+) plays an important role in terbutaline-stimulated Na(+) absorption, since Ca(2+)-activated, amiloride-sensitive Na(+)-permeable channels are involved in transcellular Na(+) absorption and terbutaline stably elevates the cytosolic Ca(2+) concentration by stimulating Ca(2+) influx. Therefore, we studied whether Ca(2+) channel blockers (Ni(2+), verapamil, and nifedipine) affect terbutaline-stimulated transcellular Na(+) absorption. Ni(2+) partially blocked the channel responsible for the terbutaline-stimulated Na(+) absorption at the Na(+) entry pathway across the apical membrane of the epithelium, but did not diminish the terbutaline-stimulated transcellular Na(+) absorption. By measuring the capacity of the Na(+),K(+)-pump activity, we determined that the rate-limiting step of the terbutaline-stimulated transcellular Na(+) absorption was the extrusion step across the basolateral membrane by the Na(+),K(+)-pump. The other Ca(2+) channel blockers, verapamil and nifedipine, had effects identical to those of Ni(2+). Based upon these observations, we conclude that, in the beta-agonist-stimulated fetal rat alveolar type II epithelium, Ca(2+) channel blockers diminish amiloride-sensitive channels, but do not affect transcellular Na(+) absorption, since under the beta-agonist-stimulated condition the Na(+),K(+)-pump is the rate-limiting step in Na(+) transport.
The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.
In patients with asthma, eosinophils are primed and massively infiltrate lung tissues and migrate across epithelia into airways. Using blocking monoclonal antibodies, we found that eosinophil transmigration across a lung epithelial cell monolayer depended on the functions of alphaMbeta2 integrin CD11b/CD18. To study the role of Ca2+ in eosinophil priming and transepithelial migration, we treated eosinophils with eotaxin or thapsigargin (TG), reagents that increase cytoplasmic free Ca2+ concentrations by receptor- or nonreceptor-mediated mechanisms, respectively. Pretreatment of eosinophils with TG enhanced CD11b/CD18-dependent transmigration across lung epithelium. Within minutes, TG time- and dose-dependently upregulated the expression of CD11b/CD18 but did not upregulate the expression of alphaL (CD11a) or beta1 (CD29) integrin. The upregulation of CD11b/CD18 expression by eotaxin or TG was prevented when Ca2+ entry was blocked. The priming of eosinophil transmigration by TG was also abrogated by the blockade of Ca2+ entry. Our results indicate that induction of Ca2+ entry by the depletion of Ca2+ from intracellular stores upregulates CD11b/CD18 expression on eosinophils and primes eosinophil transmigration across lung epithelium. Both responses are therefore elicited by extracellular Ca2+. We suggest that, as an important priming signal for human eosinophil functional responses, store-operated Ca2+ entry may be one of the underlying mechanisms of eosinophilic inflammation in asthma.
To evaluate the survival rate and factors affecting the outcome of pediatric patients treated with high-frequency oscillatory ventilation (HFOV) for diffuse alveolar disease (DAD) compatible with acute respiratory distress syndrome (ARDS).
A cohort study was conducted at the pediatric intensive care unit of Queen Siritkit National Institute of Child Health from 1st January 1999 to 31st December 2001. Children who suffered from DAD compatible with ARDS were enrolled. Inclusion criteria were PaO2/FiO2 < 200 and oxygenation index (OI) > 10. High-frequency oscillatory ventilator (3100A Sensor Medics Corp, Yorba Linda, Calif) was used applying high volume strategy of treatment. Patients were weaned to conventional ventilation (CV) once clinical improvement occurred. Demographic data, duration of CV mode before changing to HFOV, duration of HFOV, ventilator parameters and gas exchange variables from beginning and during the course of HFOV were recorded, so patient data could be compared between surviving and non-surviving groups.
A total of 21 children were enrolled during the 3 year period. There were 4 patients with simultaneous air leak syndrome and a total of 10 male patients. The average age was 3.58 +/- 3.9 years. There were 11 surviving patients (52.4%). Data of ventilator parameters and gas exchange variables after changing to HFOV for 4-6 hours for the two groups, FiO2 was higher (0.99 +/- 0.32 vs 0.84 +/- 0.18; p = 0.02) and alveolar arterial oxygen gradient [P(A-a)O2] was lower (448.5 +/- 140.8 vs 562.7 +/- 99.9 mmHg; p = 0.047) in the surviving group than in the non-surviving group. Concerning mean airway pressure (Paw), oxygenation index (OI), P(A-a)O2 and PaO2/FiO2 at initiation and during the course of HFOV with comparison of the surviving and non-surviving groups: Paw and OI decreased in the surviving group and was significantly different at 36 and 24 hours respectively. P(A-a)O2 was statistically significantly lower at 6 hours after HFOV initiation in the surviving group. PaO2/FiO2 was statistically significantly increased at 24 hours in the surviving group.
Implement of HFOV is useful in patients with DAD, ARDS and air leak syndrome from the initial phase of illness which fulfill criteria for decreasing ventilator induced lung injury and thus decrease the mortality rate from ARDS. Predisposing survival factor showing statistically significant differences was lower Paw during CV before changing to HFOV, lower Paw at 36 hours, lower OI at 24 hours, lower P(A-a)O2 at 6 hours and higher PaO2/FiO2 at 24 hours. These parameters are good indicators for the prognosis of ARDS for patients responding or not responding to HFOV.
Cystic fibrosis (CF) is a lethal genetic disease caused by a mutation in a membrane protein, the cystic fibrosis transmembrane conductance regulator (CFTR), which mainly (but not exclusively) functions as a chloride channel. The main clinical symptoms are chronic obstructive lung disease, which is responsible for most of the morbidity and mortality associated with CF, and pancreatic insufficiency. About 1000 mutations of the gene coding for CFTR are currently known; the most common of these, present in the great majority of the patients (Delta508) results in the deletion of a phenylalanine at position 508. In this mutation, the aberrant CFTR is not transported to the membrane but degraded in the ubiquitin-proteasome pathway. The aim of this review is to give an overview of the pharmacologic strategies currently used in attempts to overcome the ion transport defect in CF. One strategy to develop pharmacologic treatment for CF is to inhibit the breakdown of DeltaF508-CFTR by interfering with the chaperones involved in the folding of CFTR. At least in in vitro systems, this can be accomplished by sodium phenylbutyrate, or S-nitrosoglutathione (GSNO), and also by genistein or benzo[c]quinolizinium compounds. It is also possible to stimulate CFTR or its mutated forms, when present in the plasma membrane, using xanthines, genistein, and various other compounds, such as benzamidizoles and benzoxazoles, benzo[c]quinolizinium compounds or phenantrolines. Experimental results are not always unambiguous, and adverse effects have been incompletely tested. Some clinical tests have been done on sodium phenyl butyrate, GSNO and genistein, mostly in respect to other diseases, and the results demonstrate that these drugs are reasonably well tolerated. Their efficiency in the treatment of CF has not yet been demonstrated, however. An alternative strategy is to compensate for the defective chloride transport by CFTR by stimulation of other chloride channels. This can be done via purinergic receptors. A phase I study using a stable uridine triphosphate analog has recently been completed. A second alternative strategy is to attempt to maintain hydration of the airway mucus by inhibiting Na(+) uptake by the epithelial Na(+) channel using amiloride or stable analogs of amiloride. Clinical tests so far have been inconclusive. A number of other suggestions are currently being explored. The minority of patients with CF who have a stop mutation may benefit from treatment with gentamicin. The difficulties in finding a pharmacologic treatment for CF may be due to the fact that CFTR has additional functions besides chloride transport, and interfering with CFTR biosynthesis or activation implies interference with central cellular processes, which may have undesirable adverse effects.
In a recent review of the data for fluid strategies and ARDS, fluid restriction or diuretic use was graded as "reasonably justifiable by available scientific evidence" and as "strongly supported by expert critical care opinion". Until the ARDS Network trial is published, only general guidelines regarding fluid management with or without specific vascular filling pressures from a pulmonary artery catheter can be made. Ultimately, the rationale for restricting fluid is to reduce hydrostatic pressures as much as possible. It seems most reasonable to maintain the lowest PAOP in ARDS patients that still maintains adequate circulating blood volume, mean arterial perfusion pressures, and cardiac output to provide sufficient oxygen delivery. Other clinical variables such as central venous pressure, urinary output, acid-base status, and lactate, serum urea nitrogen, and serum creatinine levels may help in judging the adequacy of a patient's intravascular volume, especially if central vascular pressure measurements are not available. Measures to reduce total body water, including flood restriction and diuretic use, seem to be of some benefit. Vasopressor use is especially important when systemic perfusion pressures are inadequate to maintain organ blood flow but should not be used to create supranormal levels of oxygen delivery.
In the experimental setting, repeated derecruitments of the lungs of ARDS models accentuate lung injury during mechanical ventilation, whereas open lung concept strategies can attenuate the injury. In the clinical setting, recruitment manuevers that use a continuous positive airway pressure of 40 cmH2O for 40 secs improve oxygenation in patients with early ARDS who do not have an impairment in the chest wall. High intermittent positive end-expiratory pressure (PEEP), intermitent sighs, or high-pressure controlled ventilation improves short-term oxygenation in ARDS patients. Both conventional and electrical impedance thoracictomography studies at the clinical setting indicate that high PEEP associated with low levels of pressure control ventilation recruit the collapsed portions of the ARDS lungs and that adequate PEEP levels are necessary to keep the ARDS lungs opened allowing a more homogenous ventilation. High PEEP/low tidal volume ventilation was seen to reduce inflammatory mediators in both bronchoalveolar lavage and plasma, compared to low PEEP/high tidal volume ventilation, after 36 hours of mechanical ventilation in ARDS patients. Recruitment maneuvers that used continuous positive airway pressure levels of 35-40 cmH2O for 40 secs, with PEEP set at 2 cmH2O above the lower inflection point of the pressure-volume curve, and tidal volume < 6 mL/kg were associated with a 28-day intensive care unit survival rate of 62%. This contrasted with a survival rate of only 29% with conventional ventilation (defined as the lowest PEEP for acceptable oxygenation without hemodynamic impairment with a tidal volume of 12 mL/kg), without recruitment manuevers (number needed to treat = 3; p < 0.001). In the near future, thoracic computed tomography associated with high-performance monitoring of regional ventilation may be used at the bedside to determine the optimal mechanical ventilation of the ARDS keeping an opened lung with a homogenous ventilation.
Amiloride-sensitive sodium channels in the lung play an important role in lung fluid balance. Particularly in the alveoli, sodium transport is closely regulated to maintain an appropriate fluid layer on the surface of the alveoli. Alveolar type II cells appear to play an important role in this sodium transport, with the role of alveolar type I cells being less clear. In alveolar type II cells, there are a variety of different amiloride-sensitive, sodium-permeable channels. This significant diversity appears to play a role in both normal lung physiology and in pathological states. In many epithelial tissues, amiloride-sensitive epithelial sodium channels (ENaC) are formed from three subunit proteins, designated alpha-, beta-, and gamma-ENaC. At least part of the diversity of sodium-permeable channels in lung arises from the assembling of different combinations of these subunits to form channels with different biophysical properties and different mechanisms for regulation. This leads to epithelial tissue in the lung, which has enormous flexibility to alter the magnitude and regulation of salt and water transport. In this review, we discuss the biophysical properties and occurrence of these various channels and some of the mechanisms for their regulation.
Numerous studies suggest setting positive end-expiratory pressure during conventional ventilation according to the static pressure-volume (P-V) curve, whereas data on how to adjust mean airway pressure (P(aw)) during high-frequency oscillatory ventilation (HFOV) are still scarce. The aims of the current study were to (1) examine the respiratory and hemodynamic effects of setting P(aw) during HFOV according to the static P-V curve, (2) assess the effect of increasing and decreasing P(aw) on slice volumes and aeration patterns at the lung apex and base using computed tomography, and (3) study the suitability of the P-V curve to set P(aw) by comparing computed tomography findings during HFOV with those obtained during recording of the static P-V curve at comparable pressures.
Saline lung lavage was performed in seven adult pigs. P-V curves were obtained with computed tomography scanning at each volume step at the lung apex and base. The lower inflection point (Pflex) was determined, and HFOV was started with P(aw) set at Pflex. The pigs were provided five 1-h cycles of HFOV. P(aw), first set at Pflex, was increased to 1.5 times Pflex (termed 1.5 Pflex(inc)) and 2 Pflex and decreased thereafter to 1.5 times Pflex and Pflex (termed 1.5 Pflex(dec) and Pflex(dec)). Hourly measurements of respiratory and hemodynamic variables as well as computed tomography scans at the apex and base were made.
High-frequency oscillatory ventilation at a P(aw) of 1.5 Pflex(inc) reestablished preinjury arterial oxygen tension values. Further increase in P(aw) did not change oxygenation, but it decreased oxygen delivery as a result of decreased cardiac output. No differences in respiratory or hemodynamic variables were observed when comparing HFOV at corresponding P(aw) during increasing and decreasing P(aw). Variation in total slice lung volume (TLVs) was far less than expected from the static P-V curve. Overdistended lung volume was constant and less than 3% of TLVs. TLVs values during HFOV at Pflex, 1.5 Pflex(inc), and 2 Pflex were significantly greater than TLVs values at corresponding tracheal pressures on the inflation limb of the static P-V curve and located near the deflation limb. In contrast, TLVs values during HFOV at decreasing P(aw) (i.e., 1.5 Pflex(dec) and Pflex(dec)) were not significantly greater than corresponding TLV on the deflation limb of the static P-V curves. The marked hysteresis observed during static P-V curve recordings was absent during HFOV.
High-frequency oscillatory ventilation using P(aw) set according to a static P-V curve results in effective lung recruitment, and slice lung volumes during HFOV are equal to those from the deflation limb of the static P-V curve at equivalent pressures.
Acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) affect hundreds of thousands of people each year worldwide, resulting in a significant healthcare burden. Over the past four decades, much has been discovered regarding the pathophysiology of lung injury, yet little progress has been made in advancing effective treatment strategies. In this article, we discuss the current knowledge as to fluid balance in the pathophysiology of ALI/ARDS and the recent innovations that have been described related to manipulations of hydrostatic or oncotic pressure in this condition.
Hypoproteinemia is a clear marker for ALI/ARDS and may play a pathophysiologic role given its independent prognostic value. Fluid balance and oncotic pressure alterations induced by diuretic and colloid therapy improve respiratory physiology and likely alter net flux of fluid across the injured capillary-alveolar membrane. Chest radiographs serve as a useful adjunctive tool in monitoring longitudinal fluid balance manipulations in ALI/ARDS.
Manipulation of Starling forces in established ALI/ARDS produces significant physiologic benefit and may influence outcome. Future research should focus on determining a mortality benefit with this readily available intervention.
Acute respiratory failure requiring mechanical ventilation continues to contribute to mortality and affect long-term functional outcomes in patients admitted to the pediatric intensive care unit (ICU). Studies in adults with acute respiratory distress syndrome (ARDS) far outnumber those conducted in the pediatric age group, and pediatric intensivists are left with the task of carefully selecting and critically appraising relevant adult data and extrapolating results to their domain of practice.
The recent ARDSNet study reinforces the use of low tidal volumes. Administration of surfactant is safe, but once again its beneficial effect was not sustained in a randomized trial. Surfactant proteins A and D have been shown to be of prognostic value in cases of acute lung injury. The effect of inhaled nitric oxide (NO) in patients with ARDS can be enhanced by aggressive lung recruitment strategies such as can be achieved using high-frequency oscillatory ventilation (HFOV). A recent adult trial shows good response rates but no significant long-term outcome benefit from prone positioning in patients with ARDS. Routine scheduled assessments of readiness for weaning and extubation may be more important than specific weaning modes and weaning criteria for children. A recent meta-analysis suggests that prophylactic dexamethasone use may decrease postextubation stridor and possibly reduce the need for reintubation in selected patients. Outcome data in children requiring mechanical support is encouraging, especially for high-risk groups such as bone marrow transplant (BMT) recipients, and may guide ethically challenging decision-making for these patients.
Mechanical ventilation strategies aiming for optimal alveolar recruitment with the judicious use of positive end-expiratory pressure (PEEP) and low tidal volumes will remain the mainstay for managing respiratory failure in children. Dexamethasone may prevent postextubation stridor. Prone positioning, surfactant therapy, HFOV, and inhaled NO are used sporadically and need to be evaluated for their effect on mortality and duration of ventilation.
Acute lung injury remains a major cause of morbidity and mortality in paediatric intensive care units. Research over the past decade has altered our understanding of the pathophysiology of acute lung injury and the effects of mechanical ventilation on the lung. As a result, approaches to conventional mechanical ventilation of the injured lung are now largely centred around preservation of adequate gas exchange while protecting the lung from further ventilator-induced lung injury. Current techniques for accomplishing these goals include adjusting the ventilator based on the measurement and interpretation of pressure-volume curves, limitation of inspiratory tidal volumes, use of elevated levels of positive end-expiratory pressure, recruiting manoeuvres and prone positioning. The currently available data regarding the efficacy and appropriate use of these techniques are reviewed.
Biophysical properties of sodium channels in lung alveolar epithelial cells
- Matalon