Hubmayr, Rolf D.

Data from the current study support the hypothesis that the pressure-generating capacity of the diaphragm at high lung volume is compromised primarily due to a mechanism that involves a combined mechanical effect of hyperinflation and substantial muscle contraction rather than a significant loss of its muscle fiber curvature.

Abstract The lung parenchyma comprises a large number of thin‐walled alveoli, forming an enormous surface area, which serves to maintain proper gas exchange. The alveoli are held open by the transpulmonary pressure, or prestress, which is balanced by tissues forces and alveolar surface film forces. Gas exchange efficiency is thus inextricably linked to three fundamental features of the lung: parenchymal architecture, prestress, and the mechanical properties of the parenchyma. The prestress is a key determinant of lung deformability that influences many phenomena including local ventilation, regional blood flow, tissue stiffness, smooth muscle contractility, and alveolar stability. The main pathway for stress transmission is through the extracellular matrix. Thus, the mechanical properties of the matrix play a key role both in lung function and biology. These mechanical properties in turn are determined by the constituents of the tissue, including elastin, collagen, and proteoglycans. In addition, the macroscopic mechanical properties are also influenced by the surface tension and, to some extent, the contractile state of the adherent cells. This chapter focuses on the biomechanical properties of the main constituents of the parenchyma in the presence of prestress and how these properties define normal function or change in disease. An integrated view of lung mechanics is presented and the utility of parenchymal mechanics at the bedside as well as its possible future role in lung physiology and medicine are discussed. © 2011 American Physiological Society. Compr Physiol 1:1317‐1351, 2011.

Stress failure of alveolar epithelial cells is a common feature of ventilator‐injured lungs. While some injured cells undergo necrosis, others repair their plasma membrane (PM) wounds through non‐secretory exocytosis of intracellular vesicles (e.g. lysosomes) to and cytoskeleton remodeling at and around the wound. We measured the timing of lysosomal trafficking and F‐actin reorganization following micropuncture injury in primary type I alveolar epithelial cells (ATI). ATI were isolated from female rats and loaded with Calcein AM, N‐rhodamine phosphatidylethanolamine (lysosome membrane label) or transfected with LifeAct (GFP tagged actin label). Changes in fluorescence intensity were indicators of time to wound repair (cessation of calcein fluorescence decay following injury), lysosomal trafficking to the wound site, and actin remodeling. ATI repaired wounds within 27.9±3.9s. Recruitment of lysosomes to the wound site was detected at 2.9±1.5s following injury and preceded actin polymerization by 23.7±5.4s. Failure to repair PM wounds was associated with a weak or absent actin polymerization response. We conclude that local actin polymerization is a critical step in PM wound repair and postulate that adhesive interactions between PM lipids and the remodeled actin cytoskeleton serve as a driving force for wound closure by promoting lateral PM lipid bilayer flow. Support: HL63178

Cell wounding and repair are important drivers of the innate immune response of ventilator‐injured lungs. Cells repair plasma membrane (PM) wounds by several mechanisms including non‐secretory exocytosis and it is therefore not surprising that extra‐cellular ATP, a secretagogue, enhances PM repair. Neither the responsible purinergic receptor sub‐type nor the downstream signaling pathways have been identified. Expression of P2Y2R in A549 cells was validated by immunocytochemistry and immunoblot experiments using anti‐P2Y2R (Chemicon). Confluent A549 cells were incubated with the P2Y2 antagonist Suramin (400μM), ATP (50ìM) or ATP hydrolase Apyrase (20U/ml) and stretched by 30% at 8 cycles/min for 5 min. 2 min later, the cells were labeled with Propidium Iodide (PI; 4μl/ml), trypsinized and subjected to flow cytometry. PI + cells were considered wounded and necrotic. Pretreatment of A549 cells with Suramin showed a significantly higher fraction of PI + cells compared to controls (47 ± 9 % vs 22 ± 5%, p<0.05), while the addition of extracellular ATP appeared to promote PM wound repair (6± 1%, p<0.01). In contrast, pretreatment with apyrase did not significantly alter the number of necrotic cells (26 ± 6%) Our results suggest that ATP promotes PM wound repair by a P2Y2R dependent mechanism. The Apyrase data raises the possibility that ATP metabolites may also modulate PM repair. Support HL63178

Abstract Studies showed that TRIM72 is essential for repair of alveolar cell membrane disruptions, and exogenous recombinant human TRIM72 protein (rhT72) demonstrated tissue-mending properties in animal models of tissue injury. Here we examine the mechanisms of rhT72-mediated lung cell protection in vitro and test the efficacy of inhaled rhT72 in reducing tissue pathology in a mouse model of ventilator-induced lung injury. In vitro lung cell injury was induced by glass beads and stretching. Ventilator-induced lung injury was modeled by injurious ventilation at 30 ml/kg tidal volume. Affinity-purified rhT72 or control proteins were added into culture medium or applied through nebulization. Cellular uptake and in vivo distribution of rhT72 were detected by imaging and immunostaining. Exogenous rhT72 maintains membrane integrity of alveolar epithelial cells subjected to glass bead injury in a dose-dependent manner. Inhaled rhT72 decreases the number of fatally injured alveolar cells, and ameliorates tissue-damaging indicators and cell injury markers after injurious ventilation. Using in vitro stretching assays, we reveal that rhT72 improves both cellular resilience to membrane wounding and membrane repair after injury. Image analysis detected rhT72 uptake by rat alveolar epithelial cells, which can be inhibited by a cholesterol-disrupting agent. In addition, inhaled rhT72 distributes to the distal lungs, where it colocalizes with phosphatidylserine detection on nonpermeabilized lung slices to label wounded cells. In conclusion, our study showed that inhaled rhT72 accumulates in injured lungs and protects lung tissue from ventilator injury, the mechanisms of which include improving cell resilience to membrane wounding, localizing to injured membrane, and augmenting membrane repair.

This review describes the current understanding of the lungs' response to deforming stress under conditions of both normal physiology and acute lung injury. Several limiting assumptions are needed to infer lung parenchymal stress and strain from airway pressure, volume, and flow data from mechanically ventilated patients with injured lungs. These assumptions include the effects of the chest wall on lung-surface pressure, its topographical distribution, and the effects of non-uniform tissue properties on local parenchymal stresses. In addition, there is a spectrum of biophysical lung injury mechanisms that involves normal as well as tangential alveolar wall stresses. To these are added important secondary effects on pulmonary vascular resistance and right heart function. Understanding both the assumptions of lung mechanics and the scope of injury mechanisms operating during ARDS is necessary to interpret the results of clinical trials that inform prevailing ventilator-management guidelines. The implications issuing from these 3 topics inform a safer approach to setting and adjusting the ventilator to minimize the risk of ventilator-induced lung injury. This is enumerated in a 5-step approach that can be used to guide ventilator management of unstable patients with severe lung injury.

Our objectives were to characterize colonic viscoelastic properties of the human descending colon by assessing pressure-volume (P-V) relationships during barostatic balloon distension. In 16 healthy subjects, a balloon was inflated to 44 mmHg and then deflated to 0 mmHg in 4-mmHg steps at 10, 30, and 60 ml/min, allowing volume fluctuations to stabilize at each pressure increment. Thereafter, these “quasi-static” P-V curves were compared with “dynamic” distensions to 300 ml, at 1 and 10 ml/s, before and after intravenous atropine in another five subjects. During quasi-static curves, balloon volume stabilized at each pressure increment. Quasi-static P-V curves were reproducible within individuals and approximated to a power exponential function and revealed hysteresis, indicative of viscoelasticity. Body mass index influenced quasi-static P-V curves during inflation but not during deflation. The colon was less compliant during dynamic distensions at 10 ml/s than during quasi-static distensions. Atropine increased quasi-static compliance and attenuated differences between quasi-static and rapid distensions. We conclude that colonic viscoelastic properties can be assessed by quasi-static P-V curves. Rapid colonic distension activated neural reflexes, thereby reducing colonic compliance compared with quasi-static distensions.