Thromboxane receptor stimulation/inhibition and perfusion redistribution after acute lung injury

Effects of COX-2 inhibitor on ventilator-induced lung injury in rats

BACKGROUND

Mechanical ventilation especially with large tidal volume has been demonstrated to activate inflammatory response inducing lung injury, which could be attenuated by cyclooxygenase (COX)-2 inhibitors. As the main small integral membrane proteins that selectively conduct water molecules' transportation, aquaporin (AQP)-1 downregulation significantly related to lung edema and inflammation. This study aims to investigate the role of AQP1 in ventilator-induced lung injury in rats and evaluates the effects of COX-2 inhibition.

METHODS

Forty rats were allocated into four groups, where rats in Groups LD (low volume+DMSO) and LN (low volume+NS-398) were given intravenously 2ml DMSO and 8mg/kg NS-398 (a specific COX-2 inhibitor, dissolved in 2ml DMSO) before 4-hour lower tidal volume ventilation (8ml/kg), respectively, while DMSO and NS-398 were administrated in the same manner before 4-hour injurious ventilation (40ml/kg) in Groups HD (high volume+DMSO) and HN (high volume+NS-398). The arachidonic acid metabolites (6-keto prostaglandin F1α, thromboxane B2), inflammatory cytokines (tumor necrosis factor-α, interleukin-1β, 6, 8) and total protein levels in bronchoalveolar lavage (BAL) fluid and COX-2 mRNA and AQP1 protein expression in lung tissue were detected; water content and lung morphology were also evaluated.

RESULTS

Compared to Groups LD and LN, the rats in Groups HD and HN suffered obvious lung morphological changes with higher wet-to-dry weight ratio and lung injury score, and the levels of arachidonic acid metabolites, inflammatory cytokines and total protein in BAL fluid were increased, the expression of COX-2 mRNA was significantly upregulated and AQP1 protein was downregulated in lung tissue (p<0.05). The changes in BAL fluid and the severity of lung injury were attenuated, and AQP1 expression was upregulated in Group HN as compared to HD (p<0.05).

CONCLUSIONS

Ventilation with large tidal volume causes inflammatory mediator production and AQP1 downregulation, which could be attenuated by COX-2 inhibition.

Physiologic, biochemical, and imaging characterization of acute lung injury in mice

RATIONALE

Most models of acute lung injury in mice have yet to be fully characterized.

OBJECTIVES

To directly compare and contrast endotoxin and oleic acid models of acute lung injury in mice in terms of their physiologic, biochemical, histopathologic, and imaging manifestations.

METHODS

Survival studies, lung weights, x-ray computed tomographic scanning, light and electron microscopy, bronchoalveolar lavage, lung uptake of ((18)F)fluorodeoxyglucose, tissue myeloperoxidase, arterial blood gases, mean arterial pressure, and lung tissue prostanoids were measured in separate groups of C57Bl/6 mice (normal animals, endotoxin only [20 microg/g], oleic acid only [0.15 microl/g], or endotoxin + oleic acid).

RESULTS

Endotoxin alone caused only mild pulmonary neutrophilic inflammation with little functional or structural damage to the alveolar architecture. In contrast, oleic acid caused severe alveolar damage with the development of alveolar edema of the increased-permeability type with associated abnormalities in gas exchange. When given together, endotoxin and oleic acid acted synergistically to increase pulmonary edema and to worsen gas exchange and hemodynamics, thereby increasing mortality. This synergism was significantly attenuated by the prior administration of the endotoxin antagonist E5564 (eritoran).

CONCLUSIONS

Under the conditions of these studies, only mice exposed to oleic acid showed both structural and functional characteristics of acute lung injury. Nevertheless, endotoxin had potent synergistic physiologic effects that increased mortality. Overall, these models, which can be translated to genetically altered mice, are amenable to study with state-of-the-art imaging techniques, and with experimental interventions that can probe the underlying mechanisms of injury.

Neutrophils and neutrophil products do not mediate pulmonary hemodynamic effects of endotoxin on oleic acid-induced lung injury

Small-dose endotoxin (Etx) prevents pulmonary perfusion redistribution away from edematous dorsal lung regions after oleic acid (OA)-induced injury in dogs, causing a significant deterioration in oxygenation. We hypothesized that small-dose Etx might mediate this effect via polymorphonuclear neutrophil (PMN) priming with release of inflammatory mediators such as platelet activating factor (PAF) or secretory phospholipase A(2) (sPLA(2)). To test this hypothesis, we administered specific inhibitors directed against each mediator and used two strategies to generate neutropenia. PAF and sPLA(2) inhibitors were administered before OA injury, followed 2 h later by small-dose Etx (n = 4 each group). PMN depletion was achieved by hydroxyurea administration for 5 days before the study to achieve absolute neutrophil counts <1000/mm(3) (n = 4). Inhibition of PMN adherence to lung endothelium was achieved by the administration of an anti-CD18 monoclonal antibody immediately before lung injury (n = 5). Positron emission tomography was used to evaluate pulmonary perfusion distribution and lung water content. We observed no effect of these interventions on the perfusion pattern after Etx + OA. Thus, neither neutrophils nor PAF or sPLA(2) mediate the effects of Etx on the pattern of perfusion in this model of lung injury.

IMPLICATIONS

Acute respiratory failure is characterized by severe decreases in blood oxygen. The pattern of blood flow within the lungs can contribute to this problem. This study investigated the potential role of white blood cells and their products in mediating abnormal pulmonary blood flow patterns in an experimental animal model of respiratory failure.

Components of diesel exhaust particles differentially affect lung expression of cyclooxygenase-2 related to bacterial endotoxin

We have reported previously that components of diesel exhaust particles (DEP) differently affect acute lung injury related to lipopolysaccharide (LPS) in mice. This study examined the effects of components of DEP on the lung expression of cyclooxygenase (COX)-1 and -2 in the presence or absence of LPS. ICR mice were divided into six experimental groups that received vehicle, LPS (2.5 mg kg(-1)), organic chemicals in DEP (DEP-OC) extracted with dichloromethane (4 mg kg(-1)), residual carbonaceous nuclei after the extraction (washed DEP: 4 mg kg(-1)), DEP-OC (4 mg kg(-1)) + LPS (2.5 mg kg(-1)) or washed DEP (4 mg kg(-1)) + LPS (2.5 mg kg(-1)) intratracheally. The expression of mRNA for both COXs in the lung was evaluated 4 h after the intratracheal administration. The magnitude of COX-1 mRNA expression was not altered in each group. The LPS treatment enhanced the COX-2 gene expression compared with vehicle treatment. Washed DEP combined with LPS further increased its expression compared with LPS alone. In contrast, combined treatment of DEP-OC with LPS decreased COX-2 gene expression compared with LPS alone. These results suggest that the residual carbonaceous nuclei of DEP predominantly enhance lung expression of COX-2 rather than the extracted organic chemicals from DEP in the presence of LPS, which is concomitant with the magnitude of acute lung injury in our previous study.

Effect of position, nitric oxide, and almitrine on lung perfusion in a porcine model of acute lung injury

In a porcine model of oleic acid-induced lung injury, the effects of inhaled nitric oxide (iNO) and intravenous almitrine bismesylate (ivALM), which enhances the hypoxic pulmonary vasoconstriction on the distribution of regional pulmonary blood flow (PBF), were assessed. After injection of 0.12 ml/kg oleic acid, 20 anesthetized and mechanically ventilated piglets [weight of 25 +/- 2.6 (SD) kg] were randomly divided into four groups: supine position, prone position, and 10 ppm iNO for 40 min followed by 4 microg x kg(-1) x min(-1) ivALM for 40 min in supine position and in prone position. PBF was measured with positron emission tomography and H(2)15O. The redistribution of PBF was studied on a pixel-by-pixel basis. Positron emission tomography scans were performed before and then 120, 160, and 200 min after injury. With prone position alone, although PBF remained prevalent in the dorsal regions it was significantly redistributed toward the ventral regions (P < 0.001). A ventral redistribution of PBF was also obtained with iNO regardless of the position (P = 0.043). Adjunction of ivALM had no further effect on PBF redistribution. PP and iNO have an additive effect on ventral redistribution of PBF.

Importance of hypoxic vasoconstriction in maintaining oxygenation during acute lung injury

OBJECTIVE

To investigate the role of hypoxic pulmonary vasoconstriction in the intrapulmonary blood flow redistribution and gas exchange protection during oleic acid acute lung injury.

DESIGN

Prospective, controlled animal study.

SETTING

Research laboratory of an academic institution.

SUBJECTS

Three groups of five mongrel dogs.

INTERVENTIONS

Induction of acute lung injury by 0.08 mL/kg oleic acid intravenously. Hypoxic pulmonary vasoconstriction inhibition by Escherichia coli endotoxin microdose (15 microg/kg) pretreatment or by metabolic alkalosis (pH 7.60).

MEASUREMENTS AND MAIN RESULTS

Pulmonary arterial and venous resistances were determined by flow-pressure curves and by capillary pressure estimation. Regional lung water and pulmonary blood flow were assessed by positron emission tomography. Oleic acid alone increased the arterial and venous resistances, redistributed blood flow away from edematous areas, and decreased the Pao2 from 507 +/- 16 to 373 +/- 60 torr. on Fio2 1.0 and positive end-expiratory pressure 5 cm H2O. Endotoxin pretreatment inhibited the increase in arterial resistance, suppressed the redistribution, and decreased the Pao2 to 105 +/- 22 torr. Alkalosis inhibited the increase in arterial and venous resistances, suppressed the redistribution, and decreased the Pao2 to 63 +/- 12 torr. Reversal of the alkalosis increased the arterial and venous resistances, restored the perfusion redistribution, and improved the Pao2 to 372 +/- 63 torr. Changes in blood gases conformed to predictions of a computer lung model in which hypoxic pulmonary vasoconstriction was suppressed by endotoxin and alkalosis.

CONCLUSIONS

We conclude that in oleic acid-induced lung injury, a) pulmonary hypertension results from increases in both arterial and venous resistances; b) the increase in arterial resistance is the primary mechanism responsible for the perfusion redistribution and the gas exchange protection; and c) the increase in arterial resistance is most consistent with hypoxic pulmonary vasoconstriction.

Effect of endotoxin on oleic acid lung injury does not depend on priming

Recent studies have demonstrated significant synergistic physiological and biochemical effects between low-dose endotoxin (Etx) administration and oleic acid (OA)-induced canine lung injury. To evaluate whether this interaction depends on Etx priming of some key cell population, we compared the effects of giving low-dose Etx both after as well as before inducing lung injury with OA. In addition to hemodynamic and blood-gas measurements, positron emission tomographic imaging was used to measure edema accumulation and intrapulmonary blood flow distribution. Biochemical measurements of the stable metabolites of prostacyclin and thromboxane were obtained as well as measurements of isoprostanes and reactive sulfhydryls as evidence for possible concomitant oxidant production. We found that the physiological and biochemical effects of low-dose Etx developed 30-45 min after its administration, regardless of whether Etx was administered before or after OA. No increase in either isoprostane or reactive sulfhydryl production after Etx and/or OA was detected. These data suggest that the synergistic effect of low-dose Etx and OA-induced lung injury is not due to a priming effect of Etx.

Endogenous adenosine and secondary injury after chest trauma

BACKGROUND

No previous studies have examined actions of adenosine or related compounds after blunt chest trauma, but we have shown that the prototype adenosine-regulating agent, acadesine (aminoimidazole carboxamide ribonucleotide [AICAR]), has multiple favorable anti-inflammatory actions after other forms of trauma, ischemia, hemorrhage, and sepsis; and that a progressive inflammatory response in the contralateral (uninjured) lung after unilateral blunt chest trauma is caused (in part) by activation and sequestration of circulating leukocytes (white blood cells [WBCs]). Thus, we hypothesized that AICAR would ameliorate WBC-dependent, secondary pathophysiologic changes after blunt chest trauma.

METHODS

Mongrel pigs (28+/-1 kg, n = 21) were anesthetized, mechanically ventilated, and injured on the right chest (pulmonary contusion) with a captive bolt gun. Either AICAR (1 mg/kg + 0.2 mg/kg/min) or its saline vehicle were administered for a 12-hour period, beginning 15 minutes before injury.

RESULTS

Injury caused a three- to fourfold increase in bronchoalveolar lavage (BAL) WBC counts, 10- to 20-fold increases in BAL protein, and 200% increases in lung edema as measured by wet-dry ratio (all p < 0.05), in both the injured (right) and the noninjured (left) lungs. With AICAR versus saline, BAL WBC counts, lung myeloperoxidase levels, and systemic hemodynamics were similar. However, the increases in BAL protein were attenuated by 30% to 50% (p < 0.14, NS) and edema was reduced (p < 0.05) in both lungs. Furthermore, oxygenation, hypercapnia, acidosis (all p < 0.05), and survival were improved (9 of 10 vs. 4 of 11, p < 0.04).

CONCLUSION

Pretreatment with AICAR before experimental pulmonary contusion ameliorates the trauma-induced destruction of the alveolar capillary membrane, and attenuates the delayed secondary injury in the contralateral uninjured lung, by a mechanism that may be independent of leukocytes. Endogenous adenosine could have a role in the pathophysiologic response after blunt chest injury, with potential sites of action including the endothelium and alveolar macrophage. Adenosine-regulating agents may have therapeutic potential after blunt chest injury, but further studies are needed in clinically relevant models, with administration begun at the time of resuscitation.

Effect of partial liquid ventilation on pulmonary vascular permeability and edema after experimental acute lung injury

We evaluated the effects of partial liquid ventilation (PLV) with two different dosages of the perfluorocarbon LiquiVent (perflubron) on pulmonary vascular permeability and edema formation after oleic acid (OA)-induced acute lung injury in dogs. We used imaging with positron emission tomography to measure fractional pulmonary blood flow, lung water concentration (LWC), and the pulmonary transcapillary escape rate (PTCER) of (68)Ga-labeled transferrin at 5 and 21 h after lung injury in five dogs undergoing conventional mechanical ventilation (CMV), five dogs undergoing low-dose PLV (perflubron at 10 ml/kg), and four dogs undergoing high dose PLV (perflubron at 30 ml/kg). A positive end-expiratory pressure of 7.5 cm H(2)O was used in all dogs. After OA (0.08 ml/kg)- induced lung injury, there were no significant differences or trends for PTCER or LWC at any time when the PLV groups were compared with the CMV group. However, lung tissue myeloperoxidase activity was significantly lower in the combined PLV group than in the CMV group (p = 0.016). We conclude that after OA-induced lung injury, the addition of PLV to CMV does not directly attenuate pulmonary vascular leak or lung water accumulation. Rather, the benefits of such treatment may be due to modifications of the inflammatory response.

Regional pulmonary blood flow during partial liquid ventilation in normal and acute oleic acid-induced lung-injured piglets

OBJECTIVE

To determine the spatial distribution of pulmonary blood flow in three groups of piglets: partial liquid ventilation in normal piglets, partial liquid ventilation during acute lung injury, and conventional gas ventilation during acute lung injury.

DESIGN

Prospective randomized study.

SETTING

A university medical school laboratory approved for animal research.

SUBJECTS

Neonatal piglets.

INTERVENTIONS

Regional pulmonary blood flow was studied in 21 piglets in the supine position randomized to three different groups: a normal group that received partial liquid ventilation (Normal-PLV) and two acute lung injury groups that received an oleic acid-induced lung injury: partial liquid ventilation during acute lung injury (OA-PLV) and conventional gas ventilation during acute lung injury (OA-Control). Acute lung injury was induced by infusing oleic acid (0.15 mL/kg iv) over 30 mins. Partial liquid ventilation was instituted with perflubron (LiquiVent, 30 mL/kg) after 30 mins in the Normal-PLV and OA-PLV groups.

MEASUREMENTS AND MAIN RESULTS

Arterial and venous blood gases, hemodynamics, and pulmonary mechanics were measured every 15 mins throughout the hour-long study. Pulmonary blood flow was assessed by fluorescent microsphere technique at baseline and after 30, 45, and 60 mins. In the Normal-PLV piglets, pulmonary blood flow decreased from baseline (before injury or partial liquid ventilation) in the most dependent areas of the lung (F ratio = 3.227; p < .001). In the OA-PLV piglets, pulmonary blood flow was preserved over time throughout the lung (F ratio = 1.079; p = .38). In the OA-Control piglets, pulmonary blood flow decreased in the most dependent areas of the lung and increased from baseline in less dependent slices over time (F ratio = 2.48; p = .003).

CONCLUSIONS

The spatial distribution of regional pulmonary blood flow is preserved during partial liquid ventilation compared with gas ventilation in oleic acid-induced lung injury.

Role of cyclooxygenase-2 in oleic acid-induced acute lung injury

Eicosanoid production appears to be important to both edemagenesis and the pattern of pulmonary perfusion in experimental acute lung injury (ALI). We hypothesized that these effects could be mediated by the inducible form of cyclooxygenase (COX-2). We used positron emission tomography to evaluate the pulmonary perfusion pattern in dogs given oleic acid (OA) only (n = 6), the novel COX-2 inhibitor SC-236 50 min before OA (n = 3), and SC-236 given 20 min before endotoxin (Etx), followed by OA given 30 min after Etx (n = 5). Thromboxane B(2) (TXB(2)) and prostacyclin (6-keto prostaglandin F(1alpha); 6-keto PGF(1alpha)) metabolite concentrations in plasma and lung tissue were measured in these groups and in another group given Etx + OA (n = 4). Inhibition of COX-2 before administration of OA alone or before administration of Etx and OA did not have any significant effect on plasma or lung tissue concentrations of TXB(2). However, inhibition of COX-2 prior to Etx and OA significantly reduced the plasma and lung tissue concentrations of 6-keto PGF(1alpha) as compared with those in the group given only Etx + OA. Moreover, SC-236 prevented the expected loss of perfusion redistribution associated with Etx + OA only. The effect of endotoxin on pulmonary perfusion in ALI is therefore the result of a COX-2-mediated increase in prostacyclin production in lung tissue.

Prostanoids: early mediators in the secondary injury that develops after unilateral pulmonary contusion

BACKGROUND

We have previously shown a sequence of events after unilateral pulmonary contusion that suggests the release of blood-borne prostanoid mediators and that culminates in refractory bilateral pulmonary failure.

PURPOSE

To determine the role of platelet-derived thromboxane and endothelial-derived prostacyclin in the primary and secondary injury after unilateral blunt chest trauma, and to determine whether pretreatment with the cyclooxygenase inhibitor indomethacin alters the progression of secondary injury.

METHODS

Anesthetized, ventilated (FIO2 = 0.50) pigs received a unilateral, blunt injury to the right thorax (n = 20) or sham injury (n = 5) and were monitored for 24 hours. Either indomethacin (5 mg/kg i.v.; n = 10) or its saline vehicle (n = 10) were administered 15 minutes before injury. Serial bronchoalveolar lavages of each lung were analyzed for protein and neutrophil (polymorphonuclear neutrophil (PMN)) content.

RESULTS

Contusion caused profound hypoxemia; PaO2 partially recovered within 1 hour of injury to 50% of baseline. Thereafter, worsening hypoxemia required positive end-expiratory pressure. With indomethacin compared with vehicle, PaO2 was higher at any given level of positive end-expiratory pressure (p < 0.05). There was an early increase in serial bronchoalveolar lavage protein on the injured side (peak at 2 hours), with a delayed pulmonary capillary leak on the contralateral side (peak at 6 hours), which correlated with increasing PMN infiltration; this was reduced by 40 to 60% with indomethacin (p < 0.05). Thromboxane peaked within 1 hour after contusion at 800% baseline, then fell off rapidly. This peak preceded the maximal increase in permeability and was completely blocked by indomethacin. Prostacyclin slowly rose to 300% baseline by 3 hours and remained elevated; this change was blocked by indomethacin for 18 hours.

CONCLUSIONS

Contusion of the right thorax induced a delayed pulmonary capillary leak in the left lung, which reflects a progressive secondary inflammatory response. Elevations in thromboxane and prostacyclin preceded progressive bilateral PMN infiltration. Indomethacin blocked thromboxane and prostacyclin and attenuated, but did not prevent, the progression to pulmonary failure. Overall, these data suggest that prostanoids are released soon after unilateral contusion and initiate an inflammatory response in both lungs that is sustained by PMN infiltration.

Response to inhaled nitric oxide in acute lung injury depends on distribution of pulmonary blood flow prior to its administration

Responses to inhaled nitric oxide (iNO) in acute lung injury (ALI), as evidenced by improvements in oxygenation, are variable. We hypothesized that the effect of iNO may be related to the pre-iNO distribution of pulmonary blood flow (PBF). In the present study we evaluated the effect of iNO on PBF in normal healthy dogs and in a canine model of ALI induced by oleic acid (OA). In Group "OA only" (n = 5), ALI was induced by central venous injection of 0.08 ml/kg OA. In Group "E+OA" (n = 5), hypoxic pulmonary vasoconstriction after ALI was blocked with low-dose endotoxin (15 microg/kg of Escherichia coli endotoxin) administered 30 min before giving the same dose of OA. Measurements of regional PBF and lung water concentration (LWC) using positron emission tomography (PET) and H215O were performed before and after OA or placebo, and then again at concentrations of 10, 40, and 0 ppm iNO. One hundred twenty minutes after OA injury, PaO2/FIO2 fell significantly in Group OA only, from 567 +/- 32 to 437 +/- 67 mm Hg. In these animals, PBF redistributed from the dorsal edematous regions of the lungs to the nondependent zones, thus partially preserving normal ventilation/ perfusion relationships. As in the normal animals, in Group OA only, iNO did not significantly change either PBF or oxygenation. In Group E+OA, the administration of low-dose endotoxin eliminated perfusion redistribution from the dorsal edematous lung regions. As a result, PaO2/FIO2 fell from 558 +/- 70 to 119 +/- 53 mm Hg, a decrease that was significantly greater than that in Group OA only. In Group E+OA, administration of iNO restored perfusion redistribution to a similar level as in Group OA only, which was associated with a significant improvement in PaO2/FIO2, from 119 +/- 53 to 251 +/- 159 (10 ppm iNO), and 259 +/- 165 mm Hg (40 ppm iNO). We conclude that the effect of iNO on oxygenation after ALI depends on the pre-iNO perfusion pattern, which may help explain the variable response to iNO often observed in patients with acute respiratory distress syndrome.

The evaluation of lung function with PET

In many ways, the lung is an ideal organ for study with positron emission tomography (PET). First, structure-function relations are homogeneous over larger areas than in other organs (reducing problems associated with otherwise relatively poor spatial resolution and partial-volume averaging). Second, many physiologic and metabolic processes can be studied, including pulmonary blood flow, ventilation, vascular permeability, endothelial receptor and enzyme function, among others. A variety of radiotracers have been used to evaluate pulmonary blood flow with PET, including 68Ga- or 11C-albumin microspheres administered intravenously, H2 15O administered by i.v. infusion, and 13N-N2 administered by inhalation. Pulmonary ventilation has been evaluated with both 13N-N2 and 19Ne gas, also administered by inhalation. In general, the relative advantage of one approach over another depends on site-specific cyclotron capacity and experience, and on the nature and timing of concomitant studies with other positron-emitting radiopharmaceuticals. The various blood flow methods have been used primarily in studies of pulmonary gas exchange, in both experimental animals and in humans. Acute lung injury is usually defined by both an increase in extravascular water (pulmonary edema) and an increase in the permeability of the pulmonary endothelium to protein. Both processes can easily be evaluated with PET. Extravascular water is measured by a combination of scans with i.v. H2 15O and C15O. The latter is administered by inhalation to label the blood pool (to calculate intravascular water concentrations). Pulmonary vascular permeability has been evaluated with dynamic sequential imaging after either 68Ga-transferrin or 11C-methylalbumin infusions. The rate of uptake of either tracer into the pulmonary extravascular space is an index of "leakiness" of the pulmonary endothelium, and is quantified as the pulmonary transcapillary escape rate, or PTCER. PTCER appears to be a highly sensitive index of acute lung injury. Two receptor/ enzyme systems that have been evaluated include the beta-adrenergic receptor system (using 11CGP-12177 as the ligand) and angiotensin converting enzyme (using 18F-fluorocaptopril). In each case, the object is to measure Bmax, or the maximum binding-capacity for the ligand in question. Changes in Bmax can be used to infer changes in protein expression of the receptor or enzyme, or can be used to quantify adequacy of therapy with inhibitor drugs. Given the highly active nature of the pulmonary endothelium, it is likely that many other pulmonary receptor or enzyme systems can be studied in a similar fashion.

Synergistic hemodynamic effects of low-dose endotoxin and acute lung injury

We evaluated the effects of low-dose endotoxin (15 microg/kg) on the pulmonary and systemic responses to oleic acid (OA)-induced acute lung injury in dogs. Animals given endotoxin alone (n = 5) showed a modest decrease in arterial blood pressure, but no effects on pulmonary hemodynamics, blood gases, cardiac output, or lung water accumulation. Animals (n = 6) given only OA (0.08 ml/kg) showed the expected development of mild-moderate pulmonary hypertension, a comparable reduction in arterial blood pressure, hypoxemia, increased lung water concentration, and an altered intrapulmonary perfusion pattern, as assessed by positron emission tomography. Animals (n = 7) given the same dose of endotoxin, followed 30 min later by the same dose of OA, developed a similar increase in lung water concentration as the group given OA alone, but failed to develop pulmonary hypertension or to redistribute pulmonary blood flow away from the edematous lung regions. In addition, arterial blood pressure fell significantly more than in the other groups. These responses were associated with a 30-fold increase in circulating prostacyclin (assayed as 6-keto prostaglandin F1 alpha [PGF1alpha]). The effects on systemic blood pressure, intrapulmonary blood flow redistribution, and eicosanoid production were eliminated by pretreating (n = 5) animals with meclofenamate (2 mg/kg). The results are consistent with a "priming" effect of low-dose endotoxin on the pulmonary endothelium, with exaggerated prostacyclin production in response to a subsequent lung injury. This interaction leads to altered intrapulmonary hemodynamics that exacerbate the development of hypoxemia, and to significant decreases in systemic blood pressure. To the extent that the lung is the most likely source of the increased prostacyclin production, the synergistic effects of low-dose endotoxin and lung injury may produce a kind of "lung shock."

Postinjury thromboxane receptor blockade ameliorates acute lung injury

BACKGROUND

Acute lung injury is associated with pulmonary hypertension, intrapulmonary shunting, and increased microvascular permeability, leading to altered oxygenation capacity. Thromboxane A2 has been found to be a central mediator in the development of septic and oleic acid (OA)-induced acute lung injury. Our previous study demonstrated a beneficial effect of preinjury thromboxane A2 receptor blockade. The current study examines the efficacy of postinjury receptor blockade on oxygenation capacity and pulmonary hemodynamics in an isolated lung model of OA-induced acute lung injury.

METHODS

Four groups of rabbit heart-lung preparations were studied for 60 minutes in an ex vivo perfusion-ventilation system. Saline control lungs received saline solution during the first 20 minutes of study. Injury control lungs received an OA-ethanol solution during the first 20 minutes. Two treatment groups were used: T10, in which the thromboxane receptor antagonist, SQ30741, was infused 10 minutes after the initiation of OA infusion; and T30, in which the thromboxane receptor antagonist was infused 30 minutes after OA infusion.

RESULTS

Significant differences were found in oxygenation (oxygen tension in T10 = 62.6 +/- 11.7 mm Hg, T30 = 68.2 +/- 21.2 mm Hg; injury control = 40.2 +/- 9.0 mm Hg, saline control = 123.5 +/- 16.01 mm Hg; p < 0.001) and percentile change in pulmonary artery pressure (T10 = 1.1% +/- 19.4% increase, T30 = 11.2% +/- 7.3% increase; injury control = 47.6% +/- 20.5%, saline control = 4.2% +/- 6.81%; p < 0.001).

CONCLUSIONS

This study demonstrates that blockade of the thromboxane A2 receptor, even after the initiation of acute lung injury, eliminates pulmonary hypertension and improves oxygenation.

Thromboxane receptor blockade improves oxygenation in an experimental model of acute lung injury

BACKGROUND

Adult respiratory distress syndrome remains a major cause of morbidity and mortality. We investigated the role of thromboxane receptor antagonism in an experimental model of acute lung injury that mimics adult respiratory distress syndrome.

METHODS

Three groups of rabbit heart-lung preparations were studied for 30 minutes in an ex vivo blood perfusion/ventilation system. Saline control (SC) lungs received saline solution during the first 20 minutes of study. Injury control (IC) lungs received an oleic acid-ethanol solution during the first 20 minutes. Thromboxane receptor blockade (TRB) lungs received the same injury as IC lungs, but a thromboxane receptor antagonist (SQ30741) was added to the blood perfusate just prior to study. Blood gases were obtained at 10-minute intervals, and tidal volume, pulmonary artery pressure, and lung weight were continuously recorded. Oxygenation was assessed by measuring the percent change in oxygen tension over the 30-minute study period. Tissue samples were collected from all lungs for histologic evaluation.

RESULTS

Significant differences were found between SC and IC lungs as well as TRB and IC lungs when comparing pulmonary artery pressure (SC = 33.1 +/- 2.2 mm Hg, TRB = 35.4 +/- 2.1 mm Hg, IC = 60.4 +/- 11.1 mm Hg; p < 0.02) and percent change in oxygenation (SC = -20.6% +/- 10.3%, TRB = -24.2% +/- 9.5%, IC = -57.1% +/- 6.2%; p < 0.03). None of the other variables demonstrated significant differences.

CONCLUSIONS

Thromboxane receptor blockade prevents the pulmonary hypertension and the decline in oxygenation seen in an experimental model of acute lung injury that mimics adult respiratory distress syndrome.