Hypercapnic Acidosis Impairs Plasma Membrane Wound Resealing in Ventilator-injured Lungs

Abstract

The objective of this study was to assess the effects of hypercapnic acidosis on lung cell injury and repair by confocal microscopy in a model of ventilator-induced lung injury. Three groups of normocapnic, hypocapnic, and hypercapnic rat lungs were perfused ex vivo, either during or after injurious ventilation, with a solution containing the membrane-impermeant label propidium iodide. In lungs labeled during injurious ventilation, propidium iodide fluorescence identifies all cells with plasma membrane wounds, both permanent and transient, whereas in lungs labeled after injurious ventilation propidium iodide fluorescence identifies only cells with permanent plasma membrane wounds. Hypercapnia minimized the adverse effects of high-volume ventilation on vascular barrier function, whereas hypocapnia had the opposite effect. Despite CO 2 -dependent differences in lung mechanics and edema the number of injured subpleural cells per alveolus was similar in the three groups (0.48 Ϯ 0.34 versus 0.51 Ϯ 0.19 versus 0.43 Ϯ 0.20 for hypocapnia, normocapnia, and hypercapnia, respectively). However, compared with normocapnia the probability of wound repair was significantly reduced in hypercapnic lungs (63 versus 38%; p Ͻ 0.02). This finding was subsequently confirmed in alveolar epithelial cell scratch models. The potential relevance of these observations for lung inflammation and remodeling after mechanical injury is discussed.Keywords: permissive hypercapnia; plasma membrane wounding and repair; ventilator-induced lung injury So-called lung protective mechanical ventilation strategies emphasize lung recruitment and the avoidance of large tidal volumes. Such strategies are often associated with hypercapnic acidosis. Many experts view "permissive hypercapnia" as a necessary evil of a low tidal ventilation strategy (1). Concern about detrimental effects of acidemia on renal and cardiovascular function have motivated attempts to enhance CO 2 removal by tracheal gas insufflation (2) and have led to unsubstantiated recommendations about the use of bicarbonate buffers in hypercapnic patients (3). More recent data suggest that hypercapnia may actually protect the lung from certain manifestations of ischemiareperfusion, endotoxin, and mechanical ventilation-related injury (4-9). Hypocapnia, in contrast, and correction of acidemia may be harmful (10-12). The specific mechanisms through which hypercapnia influences lung injury and vascular barrier function remain uncertain (13). CO 2 generates H ϩ ions, which react with titratable groups in certain amino acids and/or interacts directly (Received in original form September 3, 2003; accepted in final form January 21, 2005) Supported by grants from the National Institutes of Health (HL-63178), GlaxoSmithKline, and the Brewer Foundation. with free amine groups in proteins to form carbamate residues (14-16). CO 2 -dependent effects on the generation of reactive oxygen species and reactive nitrogen species as well as effects on ion channel conductivity have all been considered (17, 18).V...

Impact of buffering hypercapnic acidosis on cell wounding in ventilator-injured rat lungs

“…With the differential timing of administration of the fluorescent label PI, we showed that, under normocapnic conditions, most plasma membrane wounds inflicted by injurious ventilation repair themselves (7). Subsequent experiments by Doerr et al (5) demonstrated that this repair process is impaired under conditions of hypercapnic acidosis. We have replicated those findings with the current set of experiments, showing similar rates of subpleural cellular injury but little repair as determined from the PI/Alv ratios during (group A) and after (group B) fluorescent dye labeling under unbuffered conditions (Fig.…”

“…The protocol was approved by the Mayo Clinic Institutional Review Board (IRB) and the Institutional Animal Care and Use Committee (IACUC). The isolated perfused rat lung VILI model is detailed in previous publications (5,7). In brief, isolated perfused rat lungs were mechanically ventilated ex vivo under hypercapnic conditions with or without pH buffering.…”

Elevated CO 2 levels affect development, motility, and fertility and extend life span in Caenorhabditis elegans

“…Some investigators have proposed that high CO 2 levels had beneficial effects in models of acute lung injury and proposed the term ''permissive hypercapnia'' and even ''therapeutic hypercapnia'' (1,2). However, more recent studies have suggested that high pCO 2 can cause oxidative stress in the lung, and injury (3). More recently it has been reported that in rat lungs and human epithelial cells, high pCO 2 decreased alveolar fluid clearance independently of pH and ROS (4,5).…”

The physiological and molecular effects of elevated CO2levels

“…22 The advantages of permissive hypercapnia have been questioned by several studies due to the deleterious side effects observed as a result of the elevated CO 2 levels. [23][24][25][26][27] Hypercapnia under conditions of normal pH had detrimental effects on monolayers in coordinating the initial response of the animal to hypercapnia and include, among others, many nuclear hormone receptors, several 7-transmembrane domain (7-TM), soluble guanylyl cyclase and ubiquitin ligase genes. After a 6-hour exposure to 19% CO 2 , 374 genes are upregulated and 283 genes are downregulated at least two fold.…”