Neutrophils in reexpansion pulmonary edema

This study investigated the possible contribution of neutrophils to development of reexpansion pulmonary edema (RPE) in rabbits. Rabbits' right lungs were collapsed for 7 days and then reexpanded with negative intrathoracic pressure for 2 h before study, a model that creates unilateral edema in the reexpanded lungs but not in contralateral left lungs. Two hours after lung reexpansion, significant increases in lavage albumin concentration (17-fold), percent neutrophils (14-fold), and total number of neutrophils (7-fold) recovered occurred in the reexpanded lung but not in the left. After 2 h of reexpansion increased leukotriene B4 was detected in lavage supernatant from right lungs (335 +/- 33 pg/ml) compared with the left (110 +/- 12 pg/mg, P less than 0.01), and right lung lavage acid phosphatase activity similarly increased (6.67 +/- 0.35 U/l) compared with left (4.73 +/- 0.60 U/l, P less than 0.05). Neutropenia induced by nitrogen mustard (17 +/- 14 greater than neutrophils/microliters) did not prevent RPE, because reexpanded lungs from six neutropenic rabbits were edematous (wet-to-dry lung weight ratio 6.34 +/- 0.43) compared with their contralateral lungs (4.97 +/- 0.04, P less than 0.01). An elevated albumin concentration in reexpanded lung lavage from neutropenic rabbits (8-fold) confirmed an increase in permeability. Neutrophil depletion before reexpansion did not prevent unilateral edema, although neutrophils were absent from lung sections and alveolar lavage fluid from neutropenic rabbits.     

Effect of mean airway pressure on gas exchange during high-frequency oscillatory ventilation

We studied the effect of mean airway pressure (Paw) on gas exchange during high-frequency oscillatory ventilation in 14 adult rabbits before and after pulmonary saline lavage. Sinusoidal volume changes were delivered through a tracheostomy at 16 Hz, a tidal volume of 1 or 2 ml/kg, and inspired O2 fraction of 0.5. Arterial PO2 and PCO2 (PaO2, PaCO2), lung volume change, and venous admixture were measured at Paw from 5 to 25 cmH2O after either deflation from total lung capacity or inflation from relaxation volume (Vr). The rabbits were lavaged with saline until PaO2 was less than 70 Torr, and all measurements were repeated. Lung volume change was measured in a pressure plethysmograph. Raising Paw from 5 to 25 cmH2O increased lung volume by 48-50 ml above Vr in both healthy and lavaged rabbits. Before lavage, PaO2 was relatively insensitive to changes in Paw, but after lavage PaO2 increased with Paw from 42.8 +/- 7.8 to 137.3 +/- 18.3 (SE) Torr (P less than 0.001). PaCO2 was insensitive to Paw change before and after lavage. At each Paw after lavage, lung volume was larger, venous admixture smaller, and PaO2 higher after deflation from total lung capacity than after inflation from Vr. This study shows that the effect of increased Paw on PaO2 is mediated through an increase in lung volume. In saline-lavaged lungs, equal distending pressures do not necessarily imply equal lung volumes and thus do not imply equal PaO2.     

Inhaled nitric oxide attenuates acute lung injury via inhibition of nuclear factor-kappa B and inflammation.

The effect of inhaled nitric oxide (NO) on inflammatory process in acute lung injury (ALI) is unclear. The aims of this study were to 1) examine whether inhaled NO affects the biochemical lung injury parameters and cellular inflammatory responses and 2) determine the effect of inhaled NO on the activation of nuclear factor-kappa B (NF-kappa B) in lipopolysaccharide (LPS)-induced ALI. Compared with saline controls, rabbits treated intravenously with LPS showed increases in total protein and lactate dehydrogenase in the bronchoalveolar lavage (BAL) fluid, indicating ALI. LPS-treated animals with NO inhalation (LPS-NO) showed significant decreases in these parameters. Neutrophil numbers in the BAL fluid, the activity of reactive oxygen species in BAL cells, and the levels of interleukin (IL)-1 beta and IL-8 in alveolar macrophages were increased in LPS-treated animals. In contrast, neutrophil numbers and these cellular activities were substantially decreased in LPS-NO animals, compared with LPS-treated animals. NF-kappa B activation in alveolar macrophages from LPS-treated animals was also markedly increased, whereas this activity was effectively blocked in LPS-NO animals. These results suggest that inhaled NO attenuates LPS-induced ALI and pulmonary inflammation. This attenuation may be associated with the inhibition of NF-kappa B activation.     

Pulmonary responses to acute ozone exposure in fasted mice: effect of leptin administration.

Leptin is a satiety hormone that also has proinflammatory effects, including augmentation of ozone-induced pulmonary inflammation. The purpose of this study was to determine whether reductions in endogenous levels of leptin can attenuate pulmonary responses to ozone. To reduce serum leptin, we fasted mice overnight before ozone exposure. Fasting caused a marked reduction in serum leptin to approximately one-sixth the levels observed in fed mice, and continuous infusion of leptin via Alzet micro-osmotic pumps restored serum leptin to, but not above, fed levels. Ozone exposure (2 ppm for 3 h) caused a significant, approximately 40% increase in pulmonary resistance (P < 0.01) and increased airway responsiveness in fasted but not in fed mice. The increased effect of ozone on pulmonary mechanics and airway responsiveness in fasted mice was not observed when leptin was restored via continuous infusion. Ozone exposure caused pulmonary inflammation, as evident by increases in bronchoalveolar lavage cells, protein, and soluble tumor necrosis factor receptors. There was no effect of fasting status on ozone-induced changes in the bronchoalveolar lavage inflammatory profile, and leptin treatment did not alter these responses. Our results indicate that fasting augments ozone-induced changes in pulmonary mechanics and airway responsiveness in mice. These effects of fasting are the result of declines in serum leptin. The mechanistic basis for this protective effect of leptin in fasted mice remains to be determined but is not related to effects on ozone-induced inflammation.     

血管活性肠肽、降钙素基因相关肽及其受体在气道高反应动物肺内的时空分布

为探讨内源性神经肽在气道高反应性形成中的作用,我们以臭氧应激损伤动物气道上皮细胞,建立气道高反应性动物模型,并观察臭氧应激不同时间肺内血管活性肠肽(vasoactive intestinal peptide,VIP)、降钙素基因相关肽(calcitonin gene-related CGRP)含量变化以及VIP受体(VIPR1)、CGRP受体(GRPR1)mRNA在肺内表达、分布的改变。实验观察到,臭氧应激组动物吸入乙酰甲胆碱后气道阻力高于正常对照组,肺内呈现明显的炎症改变。随臭氧应激时间延长,肺组织匀浆中VIP、CGRP浓度呈先增高后降低的双向改变,CGRP达峰值时间早于VIP。VIPR1、CGRPR1 mRNA表达亦经历了双向过程,VIPR1峰值持续时间长于CGRPR1。在无应激对照组动物,肺间质、支气管上皮细胞、血管内皮细胞、平滑肌细胞均有VIPR1、CGRPR1 mRNA表达。随应激时间的延长,阳性细胞呈斑片状集中于气管、血管周围,染色强度增加,至臭氧应激第8天,阳性染色细胞减少。因此我们推测,臭氧应激可以诱导动物气道高反应性的形成。在炎症的早期,以CGRP的作用为主,与肺损伤早期炎症信号的传递有关,以清除刺激原、及时终止致病原的作用;炎症后期以保护机体、促进修复、减轻损伤为主,VIP发挥主要作用。     

Enhanced nitric oxide and reactive oxygen species production and damage after inhalation of silica

In previous reports from this study, measurements of pulmonary inflammation, bronchoalveolar lavage cell cytokine production and nuclear factor-kappa B activation, cytotoxic damage, and fibrosis were detailed. In this study, we investigated the temporal relationship between silica inhalation, nitric oxide (NO), and reactive oxygen species (ROS) production, and damage mediated by these radicals in the rat. Rats were exposed to a silica aerosol (15 mg/m(3) silica, 6 h/day, 5 days/wk) for 116 days. We report time-dependent changes in 1) activation of alveolar macrophages and concomitant production of NO and ROS, 2) immunohistochemical localization of inducible NO synthase and the NO-induced damage product nitrotyrosine, 3) bronchoalveolar lavage fluid NO(x) and superoxide dismutase concentrations, and 4) lung lipid peroxidation levels. The major observations made in this study are as follows: 1) NO and ROS production and resultant damage increased during silica exposure, and 2) the sites of inducible NO synthase activation and NO-mediated damage are associated anatomically with pathological lesions in the lungs.     

Analytical subcellular fractionation of alveolar macrophages from normal and BCG-vaccinated rabbits with particular reference to heterogeneity of hydrolase-containing granules.

Macrophages were obtained by pulmonary lavage from normal rabbits or rabbits that had developed pulmonary granulomas after receiving intravenous BCG vaccine 2-3 weeks earlier. The cells were disrupted in iso-osmotic sucrose and a low-speed supernatant was fractionated by isopycnic centrifugation on a linear sucrose density gradient. Three populations of hydrolase-containing granules (putative lysosomes) were found in both normal and BCG-induced macrophages. They were distinguished by their different distributions in the gradient and different sensitivities to disruption by digitonin and were termed:type A, containing lysozyme; type B, containing N-acetyl-beta-glucosaminidase, beta-glactosidase, beta-glucuronidase and possibly some lysozyme; type C, containing cathepsin D. Acid phosphatase appeared to be about equally distributed between type B and C granules. Type A and B granules from BCG-induced macrophages showed markedly greater equilibrium density than did those from normal macrophages. Beta-glucuronidase and acid phosphatase had greater specific activity in the induced cells.     

Reactive absorption of nitrogen dioxide by pulmonary epithelial lining fluid

In a previous study (J. Appl. Physiol. 68: 594-603, 1990) in isolated rat lungs, we suggested that the rate of pulmonary air space absorption of inhaled NO2 is limited, in part, by chemical reaction(s) rather than by physical solubility. Because the initial site of primary absorption interactions involves the epithelial lining fluid (ELF), we investigated whether ELF-NO2 interactions could account for pulmonary NO2 reactive absorption. Rat ELF, obtained by bronchoalveolar lavage (BAL), was compared with a model chemical system (reduced glutathione, GSH). In vitro exposures (NO2-air) used constant gas flow and planar gas-liquid interfaces. 1) Solvent pH notably altered NO2 uptake by GSH but to a lesser extent by BAL. 2) Uptake displayed [GSH]-dependent saturation. [ELF] in BAL was augmented by sequential lavage (lavagate reuse) of multiple lungs. Uptake was proportional to [ELF] but did not saturate under these exposure conditions. 3) The uptake rate exhibited [NO2] dependence. However, relative to increasing [NO2], fractional uptakes decreased for BAL and 1 mM GSH but not for 10 mM GSH. 4) Altered convective gas flow produced nonlinear increments in uptake (10 mM GSH) and substantial decrements in fractional uptake. 5) Arrhenius plots [ln(r) vs. 1/T, where r is reaction rate and T is absolute temperature (degree K)] for BAL and 1 mM GSH yielded respective activation energies of 4,952 and 4,149 kcal.g-1.mol-1 and degree of change in the rate of NO2 uptake per 10 degrees C (Q10) of 1.32 and 1.25. These results imply that the rate of NO2 uptake into rat ELF, like intact lung, is limited, in part, by chemical reaction(s).(ABSTRACT TRUNCATED AT 250 WORDS)     

Lung injury in a surfactant-deficient lung is modified by indomethacin

Repetitive total lung lavage in adult rabbits leads to a reproducible severe surfactant-deficient lung injury. Hypoxemia requiring mechanical ventilation occurs, accompanied by a substantial pulmonary hypertension, a large intra-alveolar protein leak, peripheral neutropenia, and pathological features of marked neutrophil infiltration with extensive hyaline membrane formation. Pretreatment with indomethacin abolishes postlavage pulmonary hypertension, preserves a slightly better lung function with higher arterial PO2, and prevents the postlavage peripheral neutropenia found in untreated animals. Pretreatment with a thromboxane A2 receptor blocker (L 655,240, Merck Frosst, Canada) also completely attenuated pulmonary hypertension, providing evidence that thromboxane A2 mediates pulmonary arterial hypertension after lung lavage. However, specific thromboxane receptor blockade had no other long-lasting beneficial effects on the ongoing injury in this model.     

Differential effects of inhaled nitric oxide and hyperoxia on pulmonary dysfunction in newborn guinea pigs

This study tested the hypothesis that inhaled nitric oxide (NO) and combined NO and hyperoxia will result in less pulmonary dysfunction and delay onset of respiratory signs compared with hyperoxia-exposed newborn guinea pigs (GPs). GPs were exposed to room air (n = 14), 95% O(2) (n = 36), 20 parts per million (ppm) NO (n = 14), or combined 20 ppm NO and 95% O(2) (NO/O(2), n = 13) for up to 5 days. Data evaluated included latency interval for onset of respiratory distress, pressure volume curves, lung histology, and bronchoalveolar lavage (BAL) polymorphonuclear cells (PMNs), proteolytic activity, and total protein. NO-exposed GPs did not develop respiratory distress and had no evidence of pulmonary dysfunction. O(2)-exposed GPs developed respiratory distress after 1-5 days (median 4.0) vs. 3-5 days (median 5.0) for NO/O(2) exposure (P < 0.05). BAL from O(2)-exposed GPs showed increased PMNs compared with NO/O(2)-exposed GPs. O(2)- and NO/O(2)-exposed GPs had comparable reduced lung volumes, lung histology, and increased BAL proteinase activity and total protein. In summary 1) O(2) exposure resulted in multiple measures of pulmonary dysfunction in newborn GPs, 2) 5-day exposure to NO produced no noticeable respiratory effects and pulmonary dysfunction, and 3) short-term exposure (相似文献   

Effects of mechanical ventilation at low lung volume on respiratory mechanics and nitric oxide exhalation in normal rabbits.

Lung mechanics, exhaled NO (NOe), and TNF-alpha in serum and bronchoalveolar lavage fluid were assessed in eight closed and eight open chest, normal anesthetized rabbits undergoing prolonged (3-4 h) mechanical ventilation (MV) at low volume with physiological tidal volumes (10 ml/kg). Relative to initial MV on positive end-expiratory pressure (PEEP), MV at low volume increased lung quasi-static elastance (+267 and +281%), airway (+471 and +382%) and viscolelastic resistance (+480 and +294%), and decreased NOe (-42 and -25%) in closed and open chest rabbits, respectively. After restoration of PEEP, viscoelastic resistance returned to control, whereas airway resistance remained elevated (+120 and +31%) and NOe low (-25 and -20%) in both groups of rabbits. Elastance remained elevated (+23%) only in closed-chest animals, being associated with interstitial pulmonary edema, as reflected by increased lung wet-to-dry weight ratio with normal albumin concentration in bronchoalveolar lavage fluid. In contrast, in 16 additional closed- and open-chest rabbits, there were no changes of lung mechanics or NOe after prolonged MV on PEEP only. At the end of prolonged MV, TNF-alpha was practically undetectable in serum, whereas its concentration in bronchoalveolar lavage fluid was low and similar in animals subjected or not subjected to ventilation at low volume (62 vs. 43 pg/ml). These results indicate that mechanical injury of peripheral airways due to their cyclic opening and closing during ventilation at low volume results in changes in lung mechanics and reduction in NOe and that these alterations are not mediated by a proinflammatory process, since this is expressed by TNF-alpha levels.     

Hyperbaric oxygen toxicity: role of thromboxane.

Exposing rabbits for 1 h to 100% O2 at 4 atm barometric pressure markedly increases the concentration of thromboxane B2 in alveolar lavage fluid [1,809 +/- 92 vs. 99 +/- 24 (SE) pg/ml, P less than 0.001], pulmonary arterial pressure (110 +/- 17 vs. 10 +/- 1 mmHg, P less than 0.001), lung weight gain (14.6 +/- 3.7 vs. 0.6 +/- 0.4 g/20 min, P less than 0.01), and transfer rates for aerosolized 99mTc-labeled diethylenetriamine pentaacetate (500 mol wt; 40 +/- 14 vs. 3 +/- 1 x 10(-3)/min, P less than 0.01) and fluorescein isothiocyanate-labeled dextran (7,000 mol wt; 10 +/- 3 vs. 1 +/- 1 x 10(-4)/min, P less than 0.01). Pretreatment with the antioxidant butylated hydroxyanisole (BHA) entirely prevents the pulmonary hypertension and lung injury. In addition, BHA blocks the increase in alveolar thromboxane B2 caused by hyperbaric O2 (10 and 45 pg/ml lavage fluid, n = 2). Combined therapy with polyethylene glycol- (PEG) conjugated superoxide dismutase (SOD) and PEG-catalase also completely eliminates the pulmonary hypertension, pulmonary edema, and increase in transfer rate for the aerosolized compounds. In contrast, combined treatment with unconjugated SOD and catalase does not reduce the pulmonary damage. Because of the striking increase in pulmonary arterial pressure to greater than 100 mmHg, we tested the hypothesis that thromboxane causes the hypertension and thus contributes to the lung injury. Indomethacin and UK 37,248-01 (4-[2-(1H-imidazol-1-yl)-ethoxy]benzoic acid hydrochloride, an inhibitor of thromboxane synthase, completely eliminate the pulmonary hypertension and edema.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of improvement in pulmonary gas exchange during isovolemic hemodilution.

Severe anemia is associated with remarkable stability of pulmonary gas exchange (S. Deem, M. K. Alberts, M. J. Bishop, A. Bidani, and E. R. Swenson. J. Appl. Physiol. 83: 240-246, 1997), although the factors that contribute to this stability have not been studied in detail. In the present study, 10 Flemish Giant rabbits were anesthetized, paralyzed, and mechanically ventilated at a fixed minute ventilation. Serial hemodilution was performed in five rabbits by simultaneous withdrawal of blood and infusion of an equal volume of 6% hetastarch; five rabbits were followed over a comparable time. Ventilation-perfusion (VA/Q) relationships were studied by using the multiple inert-gas-elimination technique, and pulmonary blood flow distribution was assessed by using fluorescent microspheres. Expired nitric oxide (NO) was measured by chemiluminescence. Hemodilution resulted in a linear fall in hematocrit over time, from 30 +/- 1.6 to 11 +/- 1%. Anemia was associated with an increase in arterial PO(2) in comparison with controls (P < 0.01 between groups). The improvement in O(2) exchange was associated with reduced VA/Q heterogeneity, a reduction in the fractal dimension of pulmonary blood flow (P = 0.04), and a relative increase in the spatial correlation of pulmonary blood flow (P = 0. 04). Expired NO increased with anemia, whereas it remained stable in control animals (P < 0.0001 between groups). Anemia results in improved gas exchange in the normal lung as a result of an improvement in overall VA/Q matching. In turn, this may be a result of favorable changes in pulmonary blood flow distribution, as assessed by the fractal dimension and spatial correlation of blood flow and as a result of increased NO availability.     

Ibuprofen attenuates cardiopulmonary dysfunction by modifying vascular tone in endotoxemia.

Ibuprofen, a cyclooxygenase inhibitor, improves pulmonary and cardiovascular injury in endotoxemia. We studied the mechanism of the beneficial effects of ibuprofen in relation to production of inflammatory mediators which influence vascular tone in endotoxemia. Rats were randomly assigned to one of three groups: (1) control, (2) endotoxemia alone; and (3) ibuprofen pretreatment and endotoxemia. Plasma and lung lavage concentrations of tumor necrosis factor, thromboxane B2 (TXB2), leukotriene (LT) C4,D4,E4 and nitric oxide (NO) were determined over a 2 h period. Pretreatment with ibuprofen resulted in increased survival, and attenuation of pulmonary and cardiovascular dysfunction when compared to the rats receiving endotoxin alone. The marked elevation in plasma TXB2 concentration in endotoxemic rats was prevented by pretreatment with ibuprofen. Similarly, pretreatment with ibuprofen prevented the decrease in lung lavage NO levels in endotoxemic rats. The improved survival and cardiopulmonary protection in endotoxemic rats pretreated with ibuprofen appears to be related to decreased thromboxane production and preservation of endothelial production of nitric oxide.     

Postnatal episodic ozone results in persistent attenuation of pulmonary and peripheral blood responses to LPS challenge

Early life is a dynamic period of growth for the lung and immune system. We hypothesized that ambient ozone exposure during postnatal development can affect the innate immune response to other environmental challenges in a persistent fashion. To test this hypothesis, we exposed infant rhesus macaque monkeys to a regimen of 11 ozone cycles between 30 days and 6 mo of age; each cycle consisted of ozone for 5 days (0.5 parts per million at 8 h/day) followed by 9 days of filtered air. Animals were subsequently housed in filtered air conditions and challenged with a single dose of inhaled LPS at 1 yr of age. After completion of the ozone exposure regimen at 6 mo of age, total peripheral blood leukocyte and polymorphonuclear leukocyte (PMN) numbers were reduced, whereas eosinophil counts increased. In lavage, total cell numbers at 6 mo were not affected by ozone, however, there was a significant reduction in lymphocytes and increased eosinophils. Following an additional 6 mo of filtered air housing, only monocytes were increased in blood and lavage in previously exposed animals. In response to LPS challenge, animals with a prior history of ozone showed an attenuated peripheral blood and lavage PMN response compared with controls. In vitro stimulation of peripheral blood mononuclear cells with LPS resulted in reduced secretion of IL-6 and IL-8 protein in association with prior ozone exposure. Collectively, our findings suggest that ozone exposure during infancy can result in a persistent effect on both pulmonary and systemic innate immune responses later in life.     

In Vivo Inactivation of Lysozyme by Ozone

Bronchial mucus contains large quantities of the antibacterial enzyme lysozyme. Acutely exposing mice or rabbits to ozone reduced the amount of active lysozyme obtainable by bronchopulmonary lavage. The effect was proportional to ozone concentration, as well as to duration of exposure. Enzyme activity returned to normal levels during the 12 hr following exposure. Ozone (5 muliters/liter; 5 parts per million) for 3 hr reduced lysozyme levels approximately 30%. Studies of the release of lysozyme by alveolar cells support the theory that loss of activity is due to in vivo oxidation of lysozyme.     

Hyaluronan fragments contribute to the ozone-primed immune response to lipopolysaccharide

Hyaluronan is a high-molecular mass component of pulmonary extracelluar matrix, and lung injury can generate a low-molecular mass hyaluronan (HA) fragment that functions as endogenous ligand to cell surface receptors CD44 and TLR4. This leads to activation of intracellular NF-κB signaling and proinflammatory cytokine production. Based on previous information that ozone exposure causes increased HA in bronchial alveolar lavage fluid and ozone pre-exposure primes immune response to inhaled LPS, we hypothesized that HA production during ozone exposure augments the inflammatory response to LPS. We demonstrate that acute ozone exposure at 1 part per million for 3 h primes the immune response to low-dose aerosolized LPS in C57BL/6J mice, resulting in increased neutrophil recruitment into the airspaces, increased levels of protein and proinflammatory cytokines in the bronchoalveolar lavage fluid, and increased airway hyperresponsiveness. Intratracheal instillation of endotoxin-free HA (25 μg) enhances the biological response to inhaled LPS in a manner similar to ozone pre-exposure. In vitro studies using bone marrow-derived macrophages indicate that HA enhances LPS responses measured by TNF-α production, while immunofluorescence staining of murine alveolar macrophages demonstrates that HA induces TLR4 peripheralization and lipid raft colocalization. Collectively, our observations support that ozone primes macrophage responsiveness to low-dose LPS, in part, due to HA-induced TLR4 peripheralization in lung macrophages.     

Surface tension and pulmonary compliance in premature rabbits

In vitro surface properties of pulmonary surfactant thought to be essential to its ability to increase pulmonary compliance include minimum surface tension less than 10 dyn/cm and large surface tension variability and hysteresis. We tested four surface-active agents (Tween 20, a detergent; and FC-100, FC-430, and FC-431, industrial fluorocarbons), all lacking these properties, for their ability to increase pulmonary compliance in surfactant-deficient premature rabbits. Fetal rabbits were delivered by cesarean section at 27 days (full term = 31 days) and injected via tracheostomy with 50% lactated Ringer solution, adult rabbit surfactant, or one of the four experimental agents. Dynamic compliance was measured using 1 h of mechanical ventilation followed by alveolar lavage. Each experimental agent produced a dynamic compliance significantly higher than 50% lactated Ringer solution and statistically equal to or greater than natural surfactant. Equilibrium surface tension of the agents and minimum and equilibrium surface tension of the alveolar washes each correlated with compliance (P less than 0.05). This suggests that some surface properties of pulmonary surfactant believed to be essential are not, although surface tension does seem to play a role in pulmonary compliance.     

NO inhalation reduces pulmonary arterial pressure but not hemorrhage in maximally exercising horses.

In horses, the exercise-induced elevation of pulmonary arterial pressure (Ppa) is thought to play a deterministic role in exercise-induced pulmonary hemorrhage (EIPH), and thus treatment designed to lower Ppa might reasonably be expected to reduce EIPH. Five Thoroughbred horses were run on a treadmill to volitional fatigue (incremental step test) under nitric oxide (NO; inhaled 80 ppm) and control (N(2), same flow rate as per NO run) conditions (2 wk between trials; order randomized) to test the hypothesis that NO inhalation would reduce maximal Ppa but that this reduction may not necessarily reduce EIPH. Before each investigation, a microtipped pressure transducer was placed in the pulmonary artery 8 cm past the pulmonic valve to monitor Ppa. EIPH severity was assessed via bronchoalveolar lavage (BAL) 30 min postrun. Exercise time did not differ between the two trials (P > 0.05). NO administration resulted in a small but consistent and significant reduction in peak Ppa (N(2), 102.3 +/- 4.4; NO, 98.6 +/- 4.3 mmHg, P < 0.05). In the face of lowered Ppa, EIPH severity was significantly higher in the NO trial (N(2), 22.4 +/- 6.8; NO, 42.6 +/- 15.4 x 10(6) red blood cells/ml BAL fluid, P < 0.05). These findings support the notion that extremely high Ppa may reflect, in part, an arteriolar vasoconstriction that serves to protect the capillary bed from the extraordinarily high Ppa evoked during maximal exercise in the Thoroughbred horse. Furthermore, these data suggest that exogenous NO treatment during exercise in horses may not only be poor prophylaxis but may actually exacerbate the severity of EIPH.