Simultaneous measurement of O2 radicals and pulmonary vascular reactivity in rat lung

The role of endogenous radicals in the regulation of pulmonary vascular tone was evaluated by simultaneous measurement of pulmonary artery pressure and lung radical levels during exposure of isolated rat lungs to varying inspired O2 concentrations (0-95%) and angiotensin II. Lung radical levels, measured "on-line" using luminol and lucigenin-enhanced chemiluminescence, decreased in proportion to the degree of alveolar hypoxia. Radical levels fell during hypoxia before the onset of pulmonary vasoconstriction and promptly returned to basal levels with restoration of normoxic ventilation. Mild alveolar hypoxia (10% O2), which failed to decrease chemiluminescence, did not trigger pulmonary vasoconstriction. Although chemiluminescence tended to decrease more as the hypoxic response strengthened, there was not a simple correlation between the magnitude of the change in chemiluminescence induced by hypoxia and the strength of the hypoxic pressor response. Normoxic chemiluminescence was largely inhibited by superoxide dismutase but not catalase. Superoxide dismutase also increased normoxic pulmonary vascular tone and the strength of the pressor response to hypoxia and angiotensin II. Thus the predominant activated O2 species in the lung, during normoxia, was the superoxide anion or a closely related substance. Alteration of endogenous radical levels can result in changes in vascular tone. It remains uncertain whether the decrease in lung radical production during hypoxia caused pulmonary vasoconstriction or was merely associated with hypoxic ventilation.     

Vascular reactivity is increased in rat lungs injured with alpha-naphthylthiourea

We investigated the effects of lung injury due to alpha-naphthylthiourea (ANTU) on pulmonary vascular reactivity. Rats were treated with ANTU (10 mg/kg ip) or the vehicle Tween 80. Four hours later, lungs from ANTU-treated rats had increased wet-to-dry weight ratios, bronchial lavage protein concentrations, and perivascular edema. To test vascular reactivity, lungs were isolated and perfused with blood at constant flow rate, while mean pulmonary arterial pressure was monitored. ANTU-treated lungs vasoconstricted earlier than Tween-treated lungs in response to severe airway hypoxia (fractional inspired O2 0%). ANTU-treated lungs vasoconstricted in response to 10% O2, while Tween-treated lungs failed to respond to 10% O2, indicating that the threshold for hypoxic vasoconstriction was decreased by ANTU. ANTU also decreased the threshold for and increased the magnitude of angiotensin II pressor responses, indicating that the increased vasoreactivity was not specific for hypoxia. Addition of meclofenamate to perfusates increased the rate and magnitude of responses to 0% O2 in Tween-treated lungs, but did not change the responses of ANTU-treated lungs. Light microscopy of ANTU-treated lungs showed no pulmonary arterial obstruction, and electron microscopy revealed mild capillary endothelial cell injury. We conclude that enhanced pulmonary vascular reactivity accompanies the increased-permeability pulmonary edema caused by ANTU. A similar increase in vasoreactivity might contribute to pulmonary hypertension observed in patients with the adult respiratory distress syndrome.     

NO and reactive oxygen species are involved in biphasic hypoxic vasoconstriction of isolated rabbit lungs

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion with ventilation but may also result in chronic pulmonary hypertension. It has not been clarified whether acute HPV and the response to prolonged alveolar hypoxia are triggered by identical mechanisms. We characterized the vascular response to sustained hypoxic ventilation (3% O(2) for 120-180 min) in isolated rabbit lungs. Hypoxia provoked a biphasic increase in pulmonary arterial pressure (PAP). Persistent PAP elevation was observed after termination of hypoxia. Total blockage of lung nitric oxide (NO) formation by N(G)-monomethyl-L-arginine caused a two- to threefold amplification of acute HPV, the sustained pressor response, and the loss of posthypoxic relaxation. This amplification was only moderate when NO formation was partially blocked by the inducible NO synthase inhibitor S-methylisothiourea. The superoxide scavenger nitro blue tetrazolium and the superoxide dismutase inhibitor triethylenetetramine reduced the initial vasoconstrictor response, the prolonged PAP increase, and the loss of posthypoxic vasorelaxation to a similar extent. The NAD(P)H oxidase inhibitor diphenyleneiodonium nearly fully blocked the late vascular responses to hypoxia in a dose that effected a decrease to half of the acute HPV. In conclusion, as similarly suggested for acute HPV, lung NO synthesis and the superoxide-hydrogen peroxide axis appear to be implicated in the prolonged pressor response and the posthypoxic loss of vasorelaxation in perfused rabbit lungs undergoing 2-3 h of hypoxic ventilation.     

Pulmonary vascular reactivity and hemodynamic changes in elastase-induced emphysema in hamsters.

Changes in pulmonary hemodynamics and vascular reactivity in emphysematous hamsters were studied in an isolated lung preparation perfused at constant flow with blood and 3% dextran. Hamsters were treated with intratracheal porcine pancreatic elastase at 70 days of age, and experimental studies were conducted at 1, 3, and 8 mo after treatment. Baseline pulmonary arterial pressure in elastase-treated lungs was increased compared with saline-treated control lungs 1 mo after treatment, but this increase did not progress at 3 and 8 mo. Increases in pulmonary arterial pressure in elastase-treated lungs were temporally correlated with the morphological development of emphysema and right ventricular hypertrophy; both of these were evident at 1 mo after treatment and showed little change thereafter. Pressor responses to hypoxia and angiotensin II were not different between elastase-treated and control lungs at 1 and 3 mo. At 8 mo, however, pressor responses in emphysematous lungs to 0% O2 (but not to angiotensin II) were significantly increased. This was the result of a lack of the normal age-related fall in the hypoxic pressor response. Our results suggest that the right ventricular hypertrophy found in these emphysematous animals results from a chronically increased pulmonary vascular resistance. Furthermore, increases in pulmonary vascular resistance in the early development of emphysema are likely a result of the loss of vascular beds and supporting connective tissue.     

Decreased pulmonary vascular responsiveness in rats raised on an essential fatty acid deficient diet

Leukotriene C₄ is produced during hypoxic pulmonary vasoconstriction and leukotriene inhibitors preferentially inhibit the hypoxic pressor response in rats. If lipoxygenase products are important in hypoxic vasoconstriction, then an animal deficient in arachidonic acid should have a blunted hypoxic pressor response. We investigated if vascular responsiveness was decreased in vascular rings and isolated perfused lungs from rats raised on an essential fatty acid deficient diet (EFAD) compared to rats raised on a normal diet. Rats raised on the EFAD diet had decreased esterified plasma arachidonic acid and increased 5-, 8-, 11- eicosatrieonic acid compared to rats raised on the normal diet (control). Compared to the time matched responses in control isolated perfused lungs the pressor responses to angiotensin II and alveolar hypoxia were blunted in lungs from the arachidonate deficient rats. This decreased pulmonary vascular responsiveness was not affected by the addition of indomethacin or arachidonic acid to the lung perfusate. Similarly, the pulmonary artery rings from arichidonate deficient rats demonstrated decreased reactivity to norepinephrine compared to rings from control rats. In contrast, the tension increases to norepinephrine were greater in aortic rings from the arachidonate deficient rats compared to control. Stimulated lung tissue from the arachidonate deficient animals produced less slow reacting substance and platelet activating factor like material but the same amount of 6-keto-PGF_1α and TXB₂ compared to control lungs. Thus there is an associated between altered vascular responsiveness and impairment of stimulated production of slow reacting substance and platelet activating factor like materiali rat raised on an EFAD diet.     

Decreased pulmonary vascular responsiveness in rats raised on an essential fatty acid deficient diet

Leukotriene C4 is produced during hypoxic pulmonary vasoconstriction and leukotriene inhibitors preferentially inhibit the hypoxic pressor response in rats. If lipoxygenase products are important in hypoxic vasoconstriction, then an animal deficient in arachidonic acid should have a blunted hypoxic pressor response. We investigated if vascular responsiveness was decreased in vascular rings and isolated perfused lungs from rats raised on an essential fatty acid deficient diet (EFAD) compared to rats raised on a normal diet. Rats raised on the EFAD diet had decreased esterified plasma arachidonic acid and increased 5-, 8-, 11-eicosatrienoic acid compared to rats raised on the normal diet (control). Compared to the time matched responses in control isolated perfused lungs the pressor responses to angiotensin II and alveolar hypoxia were blunted in lungs from the arachidonate deficient rats. This decreased pulmonary vascular responsiveness was not affected by the addition of indomethacin or arachidonic acid to the lung perfusate. Similarly, the pulmonary artery rings from arachidonate deficient rats demonstrated decreased reactivity to norepinephrine compared to rings from control rats. In contrast, the tension increases to norepinephrine were greater in aortic rings from the arachidonate deficient rats compared to control. Stimulated lung tissue from the arachidonate deficient animals produced less slow reacting substance and platelet activating factor like material but the same amount of 6-keto-PGF1 alpha and TXB2 compared to control lungs. Thus there is an association between altered vascular responsiveness and impairment of stimulated production of slow reacting substance and platelet activating factor like material in rats raised on an EFAD diet.     

Catalase pretreatment attenuates oleic acid-induced edema in isolated rabbit lung

Because reactive O2 metabolites have been demonstrated to be potent mediators of vascular dysfunction and are synthesized by lung tissue, their involvement as mediators of oleic acid (OA)-induced pulmonary edema in the isolated Krebs-perfused rabbit lung was assessed. Injection of OA (0.1 ml) into the pulmonary artery after vehicle pretreatment induced marked increases in lung weight [50.4 +/- 13.9 vs. 4.2 +/- 2.0 (SE) g 45 min after OA or vehicle, respectively, P less than 0.05], an index of pulmonary edema, and airway pressure. OA also caused a significant though minimal increase in pulmonary arterial pressure. Pretreatment with catalase (1,000 U/ml), a scavenger of H2O2, significantly (P less than 0.05, Friedman's) attenuated the increases in lung weight (50.4 +/- 13.9 vs. 15.1 +/- 4.9 g), airway pressure, and pulmonary arterial pressure. In contrast to catalase, pretreatment with Cu-tryptophan (40 microM), a lipid-soluble scavenger of superoxide, provided no protective effect by itself, nor was there any potentiation of protection when combined with catalase. Further evidence implicating O2 metabolites in OA-induced edema was obtained by electron paramagnetic resonance (EPR) spectroscopy of perfusate samples to which the spin trap, sodium 3,5-dibromo-4-nitrosobenzenesulfonate (10 mM), was added. Analysis of these samples revealed the presence of free radicals after OA. Pretreatment with catalase (1,000 U/ml) and superoxide dismutase (250 U/ml) attenuated the EPR signal, indicating that proximal formation of O2 free radicals was in part responsible for the signal. These results suggest that reactive O2 metabolites are mediators of OA-induced pulmonary edema in the isolated perfused rabbit lung.     

Perinatal history of hypoxia leads to lower vascular pressures and hyporeactivity to angiotensin II in isolated lungs of adult rats

The most dramatic changes in pulmonary circulation occur at the time of birth. We hypothesized that some of the effects of perinatal hypoxia on pulmonary vessels are permanent. We studied the consequences of perinatal exposure to hypoxia (12 % O2 one week before and one week after birth) in isolated lungs of adult male rats (approximately 12 weeks old) perfused with homologous blood. Perfusion pressure-flow relationship was tilted towards lower pressures in the perinatally hypoxic as compared to the control, perinatally normoxic rats. A non-linear, distensible vessel model analysis revealed that this was due to increased vascular distensibility in perinatally hypoxic rats (4.1 +/- 0.6 %/mm Hg vs. 2.3 +/- 0.4 %/mm Hg in controls, P = 0.03). Vascular occlusion techniques showed that lungs of the perinatally hypoxic rats had lower pressures at both the pre-capillary and post-capillary level. To assess its role, basal vascular tone was eliminated by a high dose of sodium nitroprusside (20 microM). This reduced perfusion pressures only in the lungs of rats born in hypoxia, indicating that perinatal hypoxia leads to a permanent increase in the basal tone of the pulmonary vessels. Pulmonary vasoconstrictor reactivity to angiotensin II (0.1-0.5 microg) was reduced in rats with the history of perinatal-hypoxia. These data show that perinatal hypoxia has permanent effects on the pulmonary circulation that may be beneficial and perhaps serve to offset the previously described adverse consequences.     

Oxygen radical scavengers protect against eosinophil-induced injury in isolated perfused rat lungs.

The protective effect of oxygen radical scavengers on lung injury induced by activated eosinophils was examined in isolated perfused rat lungs. Eosinophils were obtained by bronchoalveolar lavage from rats infected with Toxocara canis and activated with phorbol myristate acetate (PMA). There were no changes in pulmonary vascular (RT) and airway (Raw) resistances and only minimal changes in vascular permeability assessed using the capillary filtration coefficient (Kf,c) in PMA control lungs and nonactivated eosinophil-treated lungs. In lungs receiving 3 x 10(6) PMA-activated eosinophils, there were significant increases from baseline of 7.3-fold in RT at 30 min, primarily due to the constriction of small arteries and veins; 3.6-fold in Kf,c at 90 and 130 min; and 2.5-fold in Raw. The lungs also became markedly edematous. Both superoxide dismutase and catalase pretreatment prevented the significant increase in Kf,c and lung wet-to-dry weight ratios and partially attenuated the increase in Raw, but did not significantly inhibit the increase in RT induced by activated eosinophils. Heat-inactivated catalase did not attenuate the eosinophil-induced increases in Kf,c, Raw, or RT. Thus, activated eosinophils acutely increased microvascular permeability primarily through production of oxygen free radicals. The free radical scavengers superoxide dismutase and catalase partially attenuated the bronchoconstriction but had no significant effect on the vasoconstriction induced by activated eosinophils.     

Acute and long-term TNF-alpha administration increases pulmonary vascular reactivity in isolated rat lungs.

Tumor necrosis factor-alpha (TNF-alpha) causes pulmonary hypertension and arterial hypoxemia, but the mechanisms are unknown. We conducted two experiments to test the hypothesis that TNF-alpha alters pulmonary vascular reactivity, which in turn could cause either pulmonary hypertension or arterial hypoxemia. In experiment 1, rats were given acute or long-term injections of TNF-alpha (recombinant human) in vivo. Rats treated acutely received either saline or TNF-alpha (40 micrograms/kg iv in saline) 3 min (TNF-3 min; n = 8), 20 min (TNF-20 min; n = 8), or 24 h (TNF-24 h; n = 5) before the lungs were isolated. Rats treated chronically received injections of either saline or TNF-alpha (250 micrograms/kg ip in saline) two times per day for 7 days (TNF-7 days; n = 9). Lungs were isolated and perfused with Earle's salt solution (+2 g/l NaHCO3 + 4 g/100 ml Ficoll), and vascular reactivity was tested with acute hypoxia (3 min; 3% O2) and angiotensin II (ANG II; 0.025-0.40 micrograms). Pulmonary pressor responses to hypoxia were greater (P less than 0.05) in TNF-20 min and TNF-7 day groups. ANG II responses were increased (P less than 0.05) in TNF-7 day rats. In experiment 2, lungs were isolated and perfused and received direct pulmonary arterial injections of TNF-alpha (0.2, 2.0, and 20 micrograms) or saline, after stable responses to hypoxia and ANG II (0.10 microgram) were attained. Reactivity was not different between control and TNF-alpha rats before the injections, but TNF-alpha increased (P less than 0.05) responses to hypoxia and ANG II.(ABSTRACT TRUNCATED AT 250 WORDS)     

Downregulation of hypoxic vasoconstriction by chronic hypoxia in rabbits: effects of nitric oxide

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion to ventilation for optimizing pulmonary gas exchange. Chronic alveolar hypoxia results in vascular remodeling and pulmonary hypertension. Previous studies have reported conflicting results of the effect of chronic alveolar hypoxia on pulmonary vasoreactivity and the contribution of nitric oxide (NO), which may be related to species and strain differences as well as to the duration of chronic hypoxia. Therefore, we investigated the impact of chronic hypoxia on HPV in rabbits, with a focus on lung NO synthesis. After exposure of the animals to normobaric hypoxia (10% O(2)) for 1 day to 10 wk, vascular reactivity was investigated in ex vivo perfused normoxic ventilated lungs. Chronic hypoxia induced right heart hypertrophy and increased normoxic vascular tone within weeks. The vasoconstrictor response to an acute hypoxic challenge was strongly downregulated within 5 days, whereas the vasoconstrictor response to the thromboxane mimetic U-46619 was maintained. The rapid downregulation of HPV was apparently not linked to changes in the lung vascular NO system, detectable in the exhaled gas and by pharmacological blockage of NO synthesis. Treatment of the animals with long-term inhaled NO reduced right heart hypertrophy and partially maintained the reactivity to acute hypoxia, without any impact on the endogenous NO system being noted. We conclude that chronic hypoxia causes rapid downregulation of acute HPV as a specific event, preceding the development of major pulmonary hypertension and being independent of the lung vascular NO system. Long-term NO inhalation partially maintains the strength of the hypoxic vasoconstrictor response.     

Protective effects of O2 radical scavengers and adenosine in PMA-induced lung injury

We have previously shown that phorbol myristate acetate (PMA) produces acute lung injury in blood-perfused lungs but not in plasma-dextran-perfused lungs. This is compatible with the concept that its major mechanism of injury is the stimulation of O2 radicals by neutrophils, which in turn increase permeability by damaging the endothelial cells. In this study we measured vascular permeability and resistance before and 1 h after PMA in five groups of blood-perfused dog lungs: PMA alone in one group and pretreatment with catalase, superoxide dismutase, deferoxamine, and adenosine each in four other groups. By the use of two indexes of permeability, the filtration coefficient and the isogravimetric capillary pressure, we found that, compared with PMA alone, catalase, deferoxamine, and adenosine provided significant protection, whereas the results with superoxide dismutase were variable. These four drugs also significantly attenuated the marked increased resistance seen with PMA alone. Although the effects seen with the first three can be explained by their scavenging of O2 radicals, adenosine appears to provide protection through a separate mechanism.     

Hypoxic pulmonary vasoconstriction is unaltered by creatine depletion induced by dietary beta-guanidino propionic acid

It has been suggested that a specific phosphagen pool might serve a sensor function, allowing direct detection of alveolar hypoxia by the pulmonary vascular smooth muscle. The possibility that phosphocreatine (PCr) levels could serve as such a sensor was assessed in isolated rat lungs. Pulmonary vascular reactivity to angiotensin II and alveolar hypoxia was assessed in lungs from control and PCr-depleted rats. PCr depletion was accomplished by feeding rats a diet containing 2% beta-guanidino propionic acid (beta-GPA), an competitive inhibitor of creatine uptake. Total creatine was depleted in beta-GPA lungs, compared to control lungs (p less than 0.05). Lung PCr levels were undetectable by the available 31P NMR spectroscopy system. PCr and creatine were depleted in hearts from beta-GPA rats relative to control hearts (p less than 0.001). Normoxic pulmonary artery pressure and the pressor responses to angiotensin II and hypoxia were not qualitatively or quantitatively altered by the diet indicating either that PCr is not a critical participant in hypoxic pulmonary vasoconstriction or that the degree of PCr depletion achieved was inadequate to expose its role in the hypoxic pressor response.     

Endogenous and exogenous catalase in reoxygenation lung injury.

Reexpansion pulmonary edema parallels reperfusion (reoxygenation) injuries in other organs in that hypoxic and hypoperfused lung tissue develops increased vascular permeability and neutrophil infiltration after reexpansion. This study investigated endogenous lung catalase activity and H2O2 production during hypoxia (produced by lung collapse) and after reoxygenation (resulting from reexpansion), in addition to assessing the effects of exogenous catalase infusion on the development of unilateral pulmonary edema after reexpansion. Lung collapse resulted in a progressive increase in endogenous catalase activity after 3 (14%) and 7 days (23%), while activities in contralateral left lungs did not change (normal left lungs averaged 180 +/- 11 units/mg DNA). Tissue from control left lungs released H2O2 into the extracellular medium at a rate calculated to be 242 +/- 34 nmol.h-1.lung-1. No significant change in extracellular release of H2O2 occurred after 7 days of right lung collapse. However, after reexpansion of the previously collapsed right lungs for 2 h, H2O2 release from both reexpanded right and contralateral left lungs significantly increased (88 and 60%, respectively) compared with controls. Infusion of exogenous catalase significantly increased plasma and lung catalase activities. Exogenous catalase infusion prevented neither the increase in lung permeability nor the infiltration with neutrophils that typically occurs in reexpanded lungs. These data indicate that lung hypoxia/reoxygenation, induced by sequential collapse and reexpansion, has specific effects on endogenous lung catalase activity and H2O2 release. However, exogenous catalase does not prevent reexpansion pulmonary edema, eliminating extracellular (but not intracellular) H2O2 as an important mediator of unilateral lung injury in this model.     

Bicarbonate-dependent superoxide release and pulmonary artery tone

Pulmonary vasoconstriction is influenced by inactivation of nitric oxide (NO) with extracellular superoxide (O2-*). Because the short-lived O2-* anion cannot diffuse across plasma membranes, its release from vascular cells requires specialized mechanisms that have not been well delineated in the pulmonary circulation. We have shown that the bicarbonate (HCO3-)-chloride anion exchange protein (AE2) expressed in the lung also exchanges O2-* for HCO3-. Thus we determined whether O2-* release involved in pulmonary vascular tone depends on extracellular HCO3-. We assessed endothelium-dependent vascular reactivity and O2-* release in the presence or absence of HCO3- in pulmonary artery (PA) rings isolated from normal rats and those exposed to hypoxia for 3 days. Lack of extracellular HCO3- in normal PA rings significantly attenuated endothelial O2-* release, opposed hypoxic vasoconstriction, and enhanced acetylcholine-mediated vasodilation. Release of O2-* was also inhibited by an AE2 inhibitor (SITS) and abolished in normoxia by an NO synthase inhibitor (NG-nitro-L-arginine methyl ester). In contrast, hypoxia increased PA AE2 protein expression and O2-* release; the latter was not affected by NG-nitro-l-arginine methyl ester or other inhibitors of enzymatic O2-* generation. Enhanced O2-* release by uncoupling NO synthase with geldanamycin was attenuated by hypoxia or by HCO3- elimination. These results indicate that O2-* produced by endothelial NOS in normoxia and unidentified sources in hypoxia regulate pulmonary vascular tone via AE2.     

Angiotensin is not required for hypoxic constriction in salt solution-perfused rat lungs

It has been reported that angiotensin II is specifically required for hypoxic vasoconstriction in rat lungs perfused with physiological salt solution. However, studies with other preparations indicate that angiotensin II does not play a necessary role in the mechanism of hypoxic vasoconstriction. In an attempt to resolve this disagreement I investigated in salt solution-perfused rat lungs whether vasoactive agents other than angiotensin II would induce hypoxic vasoconstriction, and, if so, whether the effect was due to selective action on the hypoxic mechanism or to a nonspecific increase in vascular reactivity. The results showed the development of hypoxic pressor responses after addition to perfusate of plasma, angiotensin II, KCl, vanadate, 4-aminopyridine, or norepinephrine plus propranolol. In contrast, addition of saline (control), ouabain, or tetraethylammonium chloride did not induce hypoxic vasoconstriction. Saralasin inhibited the effect of angiotensin II, but not that of plasma. Induction of responsiveness to hypoxia was associated with an increase in normoxic perfusion pressure and with potentiation of pressor responses to KCl. These results suggest that angiotensin II does not play a unique, integral role in the hypoxic mechanism, but instead is only one of many substances that will induce hypoxic pressor reactivity by reversing the vascular hyporeactivity of salt solution-perfused rat lungs.     

Reversal of pulmonary hypoxic vasoconstriction with pentoxifylline and aminophylline in isolated lungs

Pentoxifylline (Pent) is a xanthine known to improve erythrocyte deformability and thought to have little effect on smooth muscle tone. In this study I examined the direct effects of Pent on the pulmonary vasculature of isolated lungs and compared them with the effects of aminophylline. The object was to study whether Pent can reverse the hypoxic pressor response (HPR) by its hemorheological property. Changes in pulmonary arterial pressure (Pa) of isolated lungs (pigs and rats) perfused at constant flow rate were monitored to reflect changes in vascular resistance. During normoxia, injection of Pent (5 mg/kg animal weight) in pig lungs depressed the Pa from 12.8 +/- 1.8 to 8.1 +/- 0.8 mmHg (1 mmHg = 133.3 Pa); whereas during hypoxia, Pa was depressed from 34.0 +/- 2.3 to 12.3 +/- 1.4 mmHg. To identify the mechanism of this vasodepressor effect (being either vasodilation or improved erythrocyte deformability), I tested the effect of Pent in lungs perfused with cell-free perfusate. In these plasma-perfused lungs, the vasodepressor effects of Pent were similar to those observed during blood perfusion (slight depression in Pa during normoxia, but large during hypoxia). Similar experiments in blood and plasma perfused pig lungs revealed that aminophylline (5 mg/kg) also produced similar vasodepressor responses. The effects of Pent in rat lungs were comparable; no effect during normoxia, but a depressor effect during hypoxia. Vasoconstriction in pig lungs induced by angiotensin infusion was also abolished by Pent.(ABSTRACT TRUNCATED AT 250 WORDS)     

Polyethylene glycol-attached antioxidant enzymes decrease pulmonary oxygen toxicity in rats

When exposed continuously to hyperoxia (100% O2, 760 Torr barometric pressure), rats pretreated with polyethylene glycol (PEG)-attached superoxide dismutase and catalase (PEG-SOD + PEG-CAT) lived longer (79.1 + 7.6 h) than rats pretreated with saline (60.7 +/- 2.1 h) or PEG-inactivated-SOD + PEG-inactivated-CAT (62.3 +/- 1.6 h). Rats pretreated with PEG-SOD + PEG-CAT also had less hyperoxia-induced acute oxidative edematous lung injury, as assessed by increases in lung oxidized glutathione (GSSG) contents, pleural effusions, and lung lavage albumin concentrations than saline-pretreated rats. Rats pretreated with the long-lived conjugates PEG-inactivated-SOD + PEG-inactivated-CAT or PEG-albumin also had decreased acute oxidative edematous lung injury compared with rats pretreated with PEG, SOD + CAT + PEG, SOD + CAT, or saline. In vitro studies suggested that PEG itself may have contributed to protection by scavenging hydroxyl radical (.OH) but not superoxide (O2-.) or H2O2. Compared with more effective endogenous (via preexposure to hypoxia) or exogenous (via liposomes) means for increasing lung antioxidant enzymes, PEG enzymes are less protective against lung injury from continuous hyperoxia.     

Blunted pulmonary pressor responses to hypoxia in blood perfused, ventilated lungs isolated from oxygen toxic rats: Possible role of prostaglandins

To determine the effects of high oxygen (O₂) tension on pulmonary vascular reactivity, we exposed rats either to 100% O₂ for 48 hrs or 40% O₂ for 3 to 5 weeks. Lungs from all rats were isolated, blood perfused and ventilated, and pressor responses to airway hypoxia and to infused angiotensin II were measured. We found that chronic subtoxic hyperoxia did not augment subsequent hypoxic vasoconstriction, and that 48 hrs of 100% O₂ markedly blunted hypoxic vasoconstriction. Meclofenamate restored hypoxic vasoconstriction to control levels in the lungs with blunted responses. Evidence for O₂ toxicity in the lungs exposed to 100% O₂ included interstitial swelling with alveolar exudates seen by light microscopy, and lung edema by water content calculations. We conclude that 1) chronic subtoxic hyperoxia does not influence subsequent hypoxic vasoconstriction, and 2) a dilator prostaglandin produced in the lung is a potent inhibitor of hypoxic vasoconstriction in O₂ toxic lungs.     

Pulmonary vascular resistance in adult rats exposed to hypoxia in the neonatal period

Newborn rats were exposed to hypoxia (10% O2 + N2) from 24 h to day 6 of neonatal life and then returned to room air until 45 days of age (experimental). The rats were anaesthetized, heparinized, and exsanguinated. The chest was opened and the lungs were perfused with diluted autologous blood at a constant flow rate (Q). The pulmonary arterial pressure (Pa) and venous pressure (Pv) were monitored. The properties of the pulmonary vasculature were assessed by measuring baseline vascular resistance, PVR = (Pa-Pv)/Q, segmental pressure gradients (double occlusion technique), pressure-flow relationship, hypoxic pressor response (HPR, 3% O2), and the response to 0.5 microgram bolus of angiotensin II (AII). These were compared with similar measurements on age-matched control animals never exposed to hypoxia. The perfusate hematocrit and gases were not significantly different between the two groups. The PVR normalized to body weight was 30% higher in the experimental groups (p less than 0.005). The double occlusion results (obtained at a flow rate of 13 mL/min) revealed that this increase in resistance was primarily due to the increase in the postcapillary resistance. HPR was primarily in the upstream segment in both groups but was larger in the experimental group. In contrast, the response to AII occurred in both the upstream as well as in the downstream vascular segments and did not differ between the two groups. We conclude that adult rats exposed to hypoxia in the neonatal period have elevated pulmonary vascular resistance and increased vascular reactivity to hypoxia.