Barrier effects of hyperosmolar signaling in microvascular endothelium of rat lung

Microvascular fluid flow in ex vivo and engineered lungs

In recent years, it has become common to experiment with ex vivo perfused lungs for organ transplantation and to attempt regenerative pulmonary engineering using decellularized lung matrices. However, our understanding of the physiology of ex vivo organ perfusion is imperfect; it is not currently well understood how decreasing microvascular barrier affects the perfusion of pulmonary parenchyma. In addition, protocols for lung perfusion and organ culture fluid-handling are far from standardized, with widespread variation on both basic methods and on ideally controlled parameters. To address both of these deficits, a robust, noninvasive, and mechanistic model is needed which is able to predict microvascular resistance and permeability in perfused lungs while providing insight into capillary recruitment. Although validated mathematical models exist for fluid flow in native pulmonary tissue, previous models generally assume minimal intravascular leak from artery to vein and do not assess capillary bed recruitment. Such models are difficult to apply to both ex vivo lung perfusions, in which edema can develop over time and microvessels can become blocked, and to decellularized ex vivo organomimetic cultures, in which microvascular recruitment is variable and arterially perfused fluid enters into the alveolar space. Here, we develop a mathematical model of pulmonary microvascular fluid flow which is applicable in both instances, and we apply our model to data from native, decellularized, and regenerating lungs under ex vivo perfusion. The results provide substantial insight into microvascular pressure-flow mechanics, while producing previously unknown output values for tissue-specific capillary-alveolar hydraulic conductivity, microvascular recruitment, and total organ barrier resistance.NEW & NOTEWORTHY We present a validated model of pulmonary microvascular fluid mechanics and apply this model to study the effects of increased capillary permeability in decellularized and regenerating lungs. We find that decellularization alters microvascular steady-state mechanics and that re-endothelialization partially rescues key biologic parameters. The described model provides powerful insight into intraorgan microvascular dynamics and may be used to guide regenerative engineering experiments. We include all data and derivations necessary to replicate this work.

Hypertonic stress modulates eNOS function through O-GlcNAc modification at Thr-866

O-GlcNAcylation, an energy-sensitive posttranslational modification, can regulate the activity of endothelial nitric oxide synthase (eNOS). Previous studies found that Thr866 is the key site for low-glucose-mediated regulation of eNOS O-GlcNAc. However, it is not known whether this activity functions through the Thr866 site concomitant with other physical and chemical factors. Therefore, we first explored the effects of physical and chemical factors on eNOS O-GlcNAc and its Thr866 site. In this study, hypertonic stress, hyperthermia and hydrogen peroxide all increased the expression levels of eNOS O-GlcNAc, whereas hypoxia and high levels of alcohol had no effect. on the expression levels of eNOS O-GlcNAc; by contrast, low pH led to a decrease in eNOS O-GlcNAc levels. Notably, eNOS O-GlcNAc protein levels were unchanged after Thr866 site mutation only under hypertonic conditions, suggesting that hypertonic stress may act through the Thr866 site. Upon exploring the mechanism of hypertonic stress on eNOS O-GlcNAc activity and function, we found that hypertonic stress can upregulate the expression of O-linked N-acetylglucosamine (GlcNAc) transferase (OGT), which is dependent on AMPK. When AMPK was knocked out, the upregulation of OGT expression and increased O-GlcNAc modifications induced by hypertonic stress were reversed.

Functional vascularized lung grafts for lung bioengineering

End-stage lung disease is the third leading cause of death worldwide, accounting for 400,000 deaths per year in the United States alone. To reduce the morbidity and mortality associated with lung disease, new therapeutic strategies aimed at promoting lung repair and increasing the number of donor lungs available for transplantation are being explored. Because of the extreme complexity of this organ, previous attempts at bioengineering functional lungs from fully decellularized or synthetic scaffolds lacking functional vasculature have been largely unsuccessful. An intact vascular network is critical not only for maintaining the blood-gas barrier and allowing for proper graft function but also for supporting the regenerative cells. We therefore developed an airway-specific approach to removing the pulmonary epithelium, while maintaining the viability and function of the vascular endothelium, using a rat model. The resulting vascularized lung grafts supported the attachment and growth of human adult pulmonary cells and stem cell-derived lung-specified epithelial cells. We propose that de-epithelialization of the lung with preservation of intact vasculature could facilitate cell therapy of pulmonary epithelium and enable bioengineering of functional lungs for transplantation.

Role played by paxillin and paxillin tyrosine phosphorylation in hepatocyte growth factor/sphingosine-1-phosphate-mediated reactive oxygen species generation, lamellipodia formation, and endothelial barrier function

Paxillin is a multifunctional and multidomain focal adhesion adaptor protein. It serves as an important scaffolding protein at focal adhesions by recruiting and binding to structural and signaling molecules. Paxillin tyrosine phosphorylation at Y31 and Y118 is important for paxillin redistribution to focal adhesions and angiogenesis. Hepatocyte growth factor (HGF) and sphingosine-1-phosphate (S1P) are potent stimulators of lamellipodia formation, a prerequisite for endothelial cell migration. The role played by paxillin and its tyrosine phosphorylated forms in HGF- or S1P-induced lamellipodia formation and barrier function is unclear. HGF or S1P stimulated lamellipodia formation, tyrosine phosphorylation of paxillin at Y31 and Y118, and c-Abl in human lung microvascular endothelial cells (HLMVECs). Knockdown of paxillin with small interfering RNA (siRNA) or transfection with paxillin mutants (Y31F or Y118F) mitigated HGF- or S1P-induced lamellipodia formation, translocation of p47 (phox) to lamellipodia, and reactive oxygen species (ROS) generation in HLMVECs. Furthermore, exposure of HLMVECs to HGF or S1P stimulated c-Abl-mediated tyrosine phosphorylation of paxillin at Y31 and Y118 in a time-dependent fashion, and down-regulation of c-Abl with siRNA attenuated HGF- or S1P-mediated lamellipodia formation, translocation of p47 (phox) to lamellipodia, and endothelial barrier enhancement. In vivo, knockdown of paxillin with siRNA in mouse lungs attenuated ventilator-induced lung injury. Together, these results suggest that c-Abl-mediated tyrosine phosphorylation of paxillin at Y31 and Y118 regulates HGF- or S1P-mediated lamellipodia formation, ROS generation in lamellipodia, and endothelial permeability.

Acid contact in the rodent pulmonary alveolus causes proinflammatory signaling by membrane pore formation

Although gastric acid aspiration causes rapid lung inflammation and acute lung injury, the initiating mechanisms are not known. To determine alveolar epithelial responses to acid, we viewed live alveoli of the isolated lung by fluorescence microscopy, then we microinjected the alveoli with HCl at pH of 1.5. The microinjection caused an immediate but transient formation of molecule-scale pores in the apical alveolar membrane, resulting in loss of cytosolic dye. However, the membrane rapidly resealed. There was no cell damage and no further dye loss despite continuous HCl injection. Concomitantly, reactive oxygen species (ROS) increased in the adjacent perialveolar microvascular endothelium in a Ca(2+)-dependent manner. By contrast, ROS did not increase in wild-type mice in which we gave intra-alveolar injections of polyethylene glycol (PEG)-catalase, in mice overexpressing alveolar catalase, or in mice lacking functional NADPH oxidase (Nox2). Together, our findings indicate the presence of an unusual proinflammatory mechanism in which alveolar contact with acid caused membrane pore formation. The effect, although transient, was nevertheless sufficient to induce Ca(2+) entry and Nox2-dependent H(2)O(2) release from the alveolar epithelium. These responses identify alveolar H(2)O(2) release as the signaling mechanism responsible for lung inflammation induced by acid and suggest that intra-alveolar PEG-catalase might be therapeutic in acid-induced lung injury.

Protective effects of high-molecular weight polyethylene glycol (PEG) in human lung endothelial cell barrier regulation: role of actin cytoskeletal rearrangement

Acute lung injury represents the result of multiple pathways initiated by local or systemic insults and is characterized by profound vascular permeability, pulmonary edema, and life-threatening respiratory failure. Permeability-reducing therapies are of potential clinical utility but are currently unavailable. We hypothesized that polyethylene glycol (PEG) compounds, inert and non-toxic polymers that serve as a surrogate mucin lining in intestinal epithelium, may attenuate agonist-mediated lung endothelial cell (EC) barrier dysfunction. High molecular weight PEG (PEG15-20) produced rapid, dose-dependent increases in transendothelial electrical resistance (TER) in human lung endothelium cultured on gold microelectrodes, reflecting increased paracellular integrity. The maximal effective concentration of 8% PEG induced a sustained 125% increase in TER (40 h), results similar to barrier-enhancing agonists such as sphingosine 1-phosphate (40% increase in TER). Maximal PEG barrier enhancement was achieved at 45-60 min and PEG effectively reversed both thrombin- and LPS-induced EC barrier dysfunction. Consistent with the increase in TER, immunofluorescent studies demonstrated that PEG produced significant cytoskeletal rearrangement with formation of well-defined cortical actin rings and lamellipodia containing the actin-binding proteins, cortactin and MLCK, known participants in cell-matrix and cell-cell junctional adhesion. Finally, PEG challenge induced rapid alterations in levels of MAP kinase and MLC phosphorylation. In summary, PEG joins a number of EC barrier-regulatory agents which rapidly activate barrier-enhancing signal transduction pathways which target the cytoskeleton and provides a potential therapeutic strategy in inflammatory lung injury.

Concentration-dependent inhibition of angiogenesis by mesenchymal stem cells

Mesenchymal stem cells (MSCs), which potentially transdifferentiate into multiple cell types, are increasingly reported to be beneficial in models of organ system injury. However, the molecular mechanisms underlying interactions between MSCs and host cells, in particular endothelial cells (ECs), remain unclear. We show here in a matrigel angiogenesis assay that MSCs are capable of inhibiting capillary growth. After addition of MSCs to EC-derived capillaries in matrigel at EC:MSC ratio of 1:1, MSCs migrated toward the capillaries, intercalated between ECs, established Cx43-based intercellular gap junctional communication (GJC) with ECs, and increased production of reactive oxygen species (ROS). These events led to EC apoptosis and capillary degeneration. In an in vivo tumor model, direct MSC inoculation into subcutaneous melanomas induced apoptosis and abrogated tumor growth. Thus, our findings show for the first time that at high numbers, MSCs are potentially cytotoxic and that when injected locally in tumor tissue they might be effective antiangiogenesis agents suitable for cancer therapy.

Effect of hypertonic saline pre-treatment on ischemia-reperfusion lung injury in pig

BACKGROUND

Hypertonic saline may be administered in the setting of lung transplantation but may affect the development of ischemia-reperfusion lung injury. This study investigated the effects of the pre-treatment by intravenous hypertonic saline in a pig model of single lung ischemia-reperfusion.

METHODS

Forty-three pigs (34 +/- 4 kg) under mechanical ventilation were randomly assigned to a left lung ischemia-reperfusion alone or preceded by 4-ml/kg 7.5% hypertonic saline, 33-ml/kg normal saline, or by the infusion of the vasodilator nicardipine. Animals without ischemia served as controls. After euthanasia, the left lung was sampled for histologic analysis and measurement of lung water and alveolar-capillary permeability.

RESULTS

Ischemia-reperfusion resulted in high-permeability pulmonary edema, hypoxemia, and increased interleukin-6 serum level. Hypertonic saline pre-treatment worsened pulmonary edema of the left lung (6.6 +/- 0.7 vs 4.8 +/- 0.8 ml/kg of body weight, p < 0.05) and resulted in a higher ratio of the protein level in the alveolar fluid to the serum protein level (0.41 +/- 0.04 vs 0.21 +/- 0.09, p < 0.05) and in a higher histologic damage score (11 [range, 9-11.75] vs 6.5 [range, 4.5-7.5], p < 0.05) without promoting pulmonary or systemic inflammation. Lung injury was affected neither by normal saline nor by nicardipine pre-treatment. Nicardipine did not influence the deleterious effect of hypertonic saline.

CONCLUSIONS

Pre-treatment by intravenous hypertonic saline worsened ischemia-reperfusion lung injury independently of its effects on the cardiac index or pulmonary circulation but probably through a direct effect of hyperosmolarity on endothelial permeability.

Chloride-dependent secretion of alveolar wall liquid determined by optical-sectioning microscopy

The liquid layer lining the pulmonary alveolar wall critically determines the lung's immune defense against inhaled pathogens, because it provides a liquid milieu in the air-filled alveolus for dispersal of immune cells and defensive surfactant proteins. However, mechanisms underlying formation of the liquid are unknown. We achieved visualization of the alveolar wall liquid (AWL) in situ in mouse lungs by means of optical-sectioning microscopy. Continuous liquid secretion was present in alveoli of wild-type (WT) mice under baseline conditions. This secretion was blocked by inhibitors of the cystic fibrosis transmembrane regulator (CFTR). The secretion was absent in Cftr(-/-) mice, and it was blocked when chloride was depleted from the perfusate of WT mice, providing the first evidence that CFTR-dependent chloride secretion causes AWL formation. Injected microparticles demonstrated flow of the AWL. The flow was blocked by CFTR inhibition and was absent in Cftr(-/-) mice. We conclude that CFTR-dependent liquid secretion is present in alveoli of the adult mouse. Defective alveolar secretion might impair alveolar immune defense and promote alveolar disease.

Evaluation of two-way protein fluxes across the alveolo-capillary membrane by scintigraphy in rats: effect of lung inflation

Pulmonary microvascular and alveolar epithelial permeability were evaluated in vivo by scintigraphic imaging during lung distension. A zone of alveolar flooding was made by instilling a solution containing99mTc-albumin in a bronchus. Alveolar epithelial permeability was estimated from the rate at which this tracer left the lungs. Microvascular permeability was simultaneously estimated measuring the accumulation of111In-transferrin in lungs. Four levels of lung distension (corresponding to 15, 20, 25, and 30 cmH2O end-inspiratory airway pressure) were studied during mechanical ventilation. Computed tomography scans showed that the zone of alveolar flooding underwent the same distension as the contralateral lung during inflation with gas. Increasing lung tissue stretch by ventilation at high airway pressure immediately increased microvascular, but also alveolar epithelial, permeability to proteins. The same end-inspiratory pressure threshold (between 20 and 25 cmH2O) was observed for epithelial and endothelial permeability changes, which corresponded to a tidal volume between 13.7 ± 4.69 and 22.2 ± 2.12 ml/kg body wt. Whereas protein flux from plasma to alveolar space (111In-transferrin lung-to-heart ratio slope) was constant over 120 min, the rate at which99mTc-albumin left air spaces decreased with time. This pattern can be explained by changes in alveolar permeability with time or by a compartment model including an intermediate interstitial space.

Hyperosmotic mannitol induces Src kinase-dependent phosphorylation of beta-catenin in cerebral endothelial cells

Mannitol, which is a cell-impermeable and nontoxic polyalcohol, has been shown to be a useful tool for reversible opening of the blood-brain barrier (BBB). Despite successful clinical trials, the molecular mechanism of the mannitol-induced changes in cerebral endothelial cells (CECs) are poorly understood. For our experiments, we used CECs in culture, which were treated with different, clinically relevant concentrations of mannitol. We found that mannitol induced a rapid, concentration-dependent, and reversible tyrosine phosphorylation of a broad range of proteins between 50 and 190 kDa. One of the targets of tyrosine phosphorylation turned out to be the adherens junction protein beta-catenin. Phosphorylation of beta-catenin on tyrosine residues caused its subcellular redistribution and its dissociation from cadherin and alpha-catenin as shown by coimmunoprecipitation studies. All these effects could be inhibited by the Src kinase inhibitor PP-1 but not by the Erk inhibitor U0126, the Rho kinase inhibitor Y27632, or the calcium channel blocker verapamil. Because beta-catenin is a key component of the junctional complex, its Src-mediated phpsphorylation may play an important role in the mannitol induced reversible opening of the BBB.

Mitogen-activated protein kinase pathways and cardiac surgery

Mitogen-activated protein kinases are serine-threonine protein kinases that are involved in several processes important to cardiac surgery such as vascular permeability, cytokine production, vasomotor function, and reperfusion injury. Mitogen-activated protein kinases are expressed in multiple cell types including cardiomyocytes, vascular endothelial cells, and vascular smooth muscle cells. Mitogen-activated protein kinases function in cellular signal transduction cascades and are activated by a diverse range of stimuli including ischemia, shear stress, and vasoactive agents. Three major mitogen-activated protein kinase families were identified as the extracellular signal-regulated kinases, c-Jun NH(2)-terminal protein kinases, and p38 kinases. Extensive investigation has established roles for extracellular signal-regulated kinases, c-Jun NH(2)-terminal protein kinases, and p38 kinases in cardiovascular signal transduction pathways. Activity of these signal cascades may contribute to the increased pulmonary vascular permeability and myocardial reperfusion injury observed after cardiac surgery with cardioplegia and cardiopulmonary bypass. Recent findings from our laboratory suggest that alterations in the activity of myocardial extracellular signal-regulated kinase pathways occur as a result of cardioplegia-cardiopulmonary bypass in humans. In addition, these differences in extracellular signal-regulated kinase activity were shown to mediate coronary microcirculatory dysfunction associated with cardioplegia-cardiopulmonary bypass. The resulting deficit in coronary microcirculatory regulation may potentially lead to detrimental effects on organ perfusion and function. As mitogen-activated protein kinase pathways are further characterized, our potential to develop methods to prevent morbidity associated with cardiac surgery and cardiopulmonary bypass may be greatly improved.

Hyperosmolarity enhances the lung capillary barrier

Although capillary barrier deterioration underlies major inflammatory lung pathology, barrier-enhancing strategies are not available. To consider hyperosmolar therapy as a possible strategy, we gave 15-minute infusions of hyperosmolar sucrose in lung venular capillaries imaged in real time. Surprisingly, this treatment enhanced the capillary barrier, as indicated by quantification of the capillary hydraulic conductivity. The barrier enhancement was sufficient to block the injurious effects of thrombin, TNF-alpha, and H2O2 in single capillaries, and of intratracheal acid instillation in the whole lung. Capillary immunofluorescence indicated that the hyperosmolar infusion markedly augmented actin filament formation and E-cadherin expression at the endothelial cell periphery. The actin-depolymerizing agent latrunculin B abrogated the hyperosmolar barrier enhancement as well as the actin filament formation, suggesting a role for actin in the barrier response. Furthermore, hyperosmolar infusion blocked TNF-alpha-induced P-selectin expression in an actin-dependent manner. Our results provide the first evidence to our knowledge that in lung capillaries, hyperosmolarity remodels the endothelial barrier and the actin cytoskeleton to enhance barrier properties and block proinflammatory secretory processes. Hyperosmolar therapy may be beneficial in lung inflammatory disease.

Activation of pulmonary mitogen-activated protein kinases during cardiopulmonary bypass

PURPOSE

Cardiopulmonary bypass (CPB) produces an inflammatory response associated with pulmonary dysfunction. Mitogen-activated protein kinases (MAPK) have been shown to mediate pulmonary injury. We hypothesized that MAPK are activated during CPB and potentially contribute to lung injury.

METHODS

Pigs were placed on CPB (n = 6) for 90 min, which included 80 min of cardioplegic arrest, followed by 180 min of post-CPB reperfusion. Control animals (n = 6) underwent sternotomy and heparinization only. Lung samples were collected at baseline, during CPB, and during post-CPB reperfusion. Activated forms of extracellular signal-regulated kinases 1/2 (ERK1/2) and p38 were measured by Western blot. Immunohistochemistry was used for tissue localization of activated MAPK. Pulmonary inflammation was determined by histology. Pulmonary edema was estimated by tissue water percentage.

RESULTS

Activated ERK1/2 and p38 were increased after 90 min of CPB compared with controls (3.94 +/- 0.61- and 2.49 +/- 0.15-fold increase, respectively; both P < 0.01). At 180 min of post-CPB reperfusion, ERK1/2 activity was increased by nearly 5-fold compared with controls (P < 0.01), whereas p38 activity returned to baseline levels. By immunohistochemistry, activated ERK1/2 and p38 in the CPB group were localized to alveolar epithelial cells, vascular endothelial cells, and bronchial smooth muscle. Histologic signs of lung injury included leukocyte infiltration in the CPB group. Tissue water percentage was increased with CPB (89.9 +/- 1.5% versus 82.5 +/- 1.0%, CPB versus control, P < 0.05).

CONCLUSIONS

The results of our study demonstrate that CPB increases pulmonary p38 activity and causes sustained activation of ERK1/2. MAPK activation thus may in part mediate the pulmonary inflammatory response and provide a potential site of intervention to prevent pulmonary dysfunction due to CPB.

Endothelial barrier strengthening by activation of focal adhesion kinase

Endothelial cell barrier (EC) properties regulate blood tissue fluid flux. To determine the role of endothelial-matrix interactions in barrier regulation, we induced cell shrinkage by exposing confluent endothelial monolayers to hyperosmolarity. The dominant effect of a 15-min hyperosmolar exposure was an increase in the trans-endothelial electrical resistance, indicating the induction of barrier strengthening. Hyperosmolar exposure also increased activity of focal adhesion kinase and E-cadherin accumulation at the cell periphery. Concomitantly, the density of actin filaments increased markedly. In EC monolayers stably expressing constitutively active or dominant negative isoforms of Rac1, the actin response to hyperosmolar exposure was enhanced or blocked, respectively, although the response in trans-endothelial resistance was unaffected, indicating that the endothelial barrier enhancement occurred independently of actin. However, in monolayers expressing a kinase-deficient mutant of focal adhesion kinase, the hyperosmolarity-induced increases in activity of focal adhesion and peripheral E-cadherin enhancement were blocked and the induced increase of electrical resistance was markedly blunted. These findings indicate that in EC exposed to hyperosmolar challenge, the involvement of focal adhesion kinase was critical in establishing barrier strengthening.

Hyperosmotically induced volume change and calcium signaling in intervertebral disk cells: the role of the actin cytoskeleton

Loading of the spine alters the osmotic environment in the intervertebral disk (IVD) as interstitial water is expressed from the tissue. Cells from the three zones of the IVD, the anulus fibrosus (AF), transition zone (TZ), and nucleus pulposus (NP), respond to osmotic stress with altered biosynthesis through a pathway that may involve calcium (Ca(2+)) as a second messenger. We examined the hypothesis that IVD cells respond to hyperosmotic stress by increasing the concentration of intracellular calcium ([Ca(2+)](i)) through a mechanism involving F-actin. In response to hyperosmotic stress, control cells from all zones decreased in volume and cells from the AF and TZ exhibited [Ca(2+)](i) transients, while cells from the NP did not. Extracellular Ca(2+) was necessary to initiate [Ca(2+)](i) transients. Stabilization of F-actin with phalloidin prevented the Ca(2+) response in AF and TZ cells and decreased the rate of volume change in cells from all zones, coupled with an increase in the elastic moduli and apparent viscosity. Conversely, actin breakdown with cytochalasin D facilitated Ca(2+) signaling while decreasing the elastic moduli and apparent viscosity for NP cells. These results suggest that hyperosmotic stress induces volume change in IVD cells and may initiate [Ca(2+)](i) transients through an actin-dependent mechanism.

Reactive oxygen intermediates are involved in IL-8 production induced by hyperosmotic stress in human bronchial epithelial cells

Changes in the osmolarity of the airway surface fluid have been described to be involved in the pathogenesis of exercise induced asthma, and are suggested as the major cause of the lung disease in cystic fibrosis. In this study, we examined the signaling pathway of hyperosmotic challenge to interleukin-8 (IL-8). Hyperosmolarity (NaCl) caused a time- and concentration-dependent increase in IL-8 expression and secretion in bronchial epithelial cells. These effects could be blocked by antioxidants, such as DMSO, DMTU, DTT, and beta-mercaptoethanol, suggesting an involvement of reactive oxygen intermediates (ROI) in the signal transduction of hyperosmolarity-induced IL-8 synthesis. Since IL-8 is regulated by MAP kinases, we examined the influence of MAP kinase inhibitors on hyperosmolarity-induced IL-8 expression. The results show that this induction is regulated by p38 MAPK and not by ERK1/2. Furthermore, antioxidants blocked the activation of p38 MAPK induced by hyperosmolarity. These results suggest that ROIs are critical for p38 MAPK mediated IL-8 expression by hyperosmolarity.

Amino acids are compatible osmolytes for volume recovery after hypertonic shrinkage in vascular endothelial cells

The response to chronic hypertonic stress has been studied in human endothelial cells derived from saphenous veins. In complete growth medium the full recovery of cell volume requires several hours and is neither associated with an increase in cell K+ nor hindered by bumetanide but depends on an increased intracellular pool of amino acids. The highest increase is exhibited by neutral amino acid substrates of transport system A, such as glutamine and proline, and by the anionic amino acid glutamate. Transport system A is markedly stimulated on hypertonic stress, with an increase in activity roughly proportional to the extent and the duration of the osmotic shrinkage. Cycloheximide prevents the increase in transport activity of system A and the recovery of cell volume. It is concluded that human endothelial cells counteract hypertonic stress through the stimulation of transport system A and the consequent expansion of the intracellular amino acid pool.