Mediators Of Hypoxic Pulmonary Vasoconstriction Research Articles

AMP‐activated Protein Kinase is Necessary for Cardiorespiratory Adjustments during Hypoxia

The AMP‐activated protein kinase (AMPK) has been proposed to underpin appropriate cardiorespiratory adjustments during hypoxia (Evans et al., 2006), and has been identified as a mediator of hypoxic pulmonary vasoconstriction (HPV) (Evans et al., 2005). Our objective was to assess by non‐invasive Echo Doppler ultrasound, the effect(s) on HPV of deleting LKB1, CAMKK‐β or the AMPKα1 & α2 catalytic subunits. Our preliminary investigations on wild type and transgenic mice suggest that the LKB1‐AMPK signalling pathway, but not CAMKK‐β, are required for HPV. Moreover, deletion of AMPK catalytic subunit(s) blocked hypoxia‐induced inhibition of voltage‐gated potassium currents (Kv) in acutely isolated pulmonary arterial smooth muscle cells. Therefore, AMPK underpins the regulation of pulmonary vascular responses to hypoxia at the molecular, cellular and system level.Evans et al. (2005). J. Biol Chem., 280, 41505‐41511.Evans (2006). J. Physiol., 574, 113‐123.

CFTR and sphingolipids mediate hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) optimizes pulmonary ventilation-perfusion matching in regional hypoxia, but promotes pulmonary hypertension in global hypoxia. Ventilation-perfusion mismatch is a major cause of hypoxemia in cystic fibrosis. We hypothesized that cystic fibrosis transmembrane conductance regulator (CFTR) may be critical in HPV, potentially by modulating the response to sphingolipids as mediators of HPV. HPV and ventilation-perfusion mismatch were analyzed in isolated mouse lungs or in vivo. Ca(2+) mobilization and transient receptor potential canonical 6 (TRPC6) translocation were studied in human pulmonary (PASMCs) or coronary (CASMCs) artery smooth muscle cells. CFTR inhibition or deficiency diminished HPV and aggravated ventilation-perfusion mismatch. In PASMCs, hypoxia caused CFTR to interact with TRPC6, whereas CFTR inhibition attenuated hypoxia-induced TRPC6 translocation to caveolae and Ca(2+) mobilization. Ca(2+) mobilization by sphingosine-1-phosphate (S1P) was also attenuated by CFTR inhibition in PASMCs, but amplified in CASMCs. Inhibition of neutral sphingomyelinase (nSMase) blocked HPV, whereas exogenous nSMase caused TRPC6 translocation and vasoconstriction that were blocked by CFTR inhibition. nSMase- and hypoxia-induced vasoconstriction, yet not TRPC6 translocation, were blocked by inhibition or deficiency of sphingosine kinase 1 (SphK1) or antagonism of S1P receptors 2 and 4 (S1P2/4). S1P and nSMase had synergistic effects on pulmonary vasoconstriction that involved TRPC6, phospholipase C, and rho kinase. Our findings demonstrate a central role of CFTR and sphingolipids in HPV. Upon hypoxia, nSMase triggers TRPC6 translocation, which requires its interaction with CFTR. Concomitant SphK1-dependent formation of S1P and activation of S1P2/4 result in phospholipase C-mediated TRPC6 and rho kinase activation, which conjointly trigger vasoconstriction.

Ceramide inhibits Kv currents and contributes to TP-receptor-induced vasoconstriction in rat and human pulmonary arteries

Neutral sphingomyelinase (nSMase)-derived ceramide has been proposed as a mediator of hypoxic pulmonary vasoconstriction (HPV), a specific response of the pulmonary circulation. Voltage-gated K(+) (K(v)) channels are modulated by numerous vasoactive factors, including hypoxia, and their inhibition has been involved in HPV. Herein, we have analyzed the effects of ceramide on K(v) currents and contractility in rat pulmonary arteries (PA) and in mesenteric arteries (MA). The ceramide analog C6-ceramide inhibited K(v) currents in PA smooth muscle cells (PASMC). Similar effects were obtained after the addition of bacterial sphingomyelinase (SMase), indicating a role for endogenous ceramide in K(v) channel regulation. K(v) current was reduced by stromatoxin and diphenylphosphine oxide-1 (DPO-1), selective inhibitors of K(v)2.1 and K(v)1.5 channels, respectively. The inhibitory effect of ceramide was still present in the presence of stromatoxin or DPO-1, suggesting that this sphingolipid inhibited both components of the native K(v) current. Accordingly, ceramide inhibited K(v)1.5 and K(v)2.1 channels expressed in Ltk(-) cells. Ceramide-induced effects were reduced in human embryonic kidney 293 cells expressing K(v)1.5 channels but not the regulatory subunit K(v)β2.1. The nSMase inhibitor GW4869 reduced the thromboxane-endoperoxide receptor agonist U46619-induced, but not endothelin-1-induced pulmonary vasoconstriction that was partly restored after addition of exogenous ceramide. The PKC-ζ pseudosubstrate inhibitor (PKCζ-PI) inhibited the K(v) inhibitory and contractile effects of ceramide. In MA ceramide had no effect on K(v) currents and GW4869 did not affect U46619-induced contraction. The effects of SMase were also observed in human PA. These results suggest that ceramide represents a crucial signaling mediator in the pulmonary vasculature.

Increased expression and altered subcellular distribution of PKC-δ and PKC-ɛ in pulmonary arteries exposed to hypoxia and 15-HETE

15-Hydroxyeicosatetraenoic acid (15-HETE), a product of arachidonic acid (AA) catalyzed by 15-lipoxygenase (15-LOX), is an important mediator of hypoxic pulmonary vasoconstriction (HPV). We have previously reported that 15-HETE-induced pulmonary vasoconstriction occurs via protein kinase C (PKC) pathway, however, the role of PKC isoforms involved in 15-HETE-induced pulmonary vasoconstriction remains poorly understood. To examine the potential role of PKC-δ and PKC-ɛ isoforms that appear to be involved in 15-HETE-induced pulmonary artery (PA) contraction, a combination of immunofluorescence, western blotting, semi-quantitative PCR and functional contractile tension approaches on rat PA rings were utilized. We found that 15-HETE activates the translocation of PKC-δ and PKC-ɛ from the cytoplasm to the membranes of pulmonary arterial smooth muscle cells (PASMCs). However, the alteration was significantly reversed by nordihydroguairetic acid (NDGA), a 15-LOX inhibitor which blocked the formation of endogenous 15-HETE. Both endogenous and exogenous 15-HETE enhanced the expression of PKC-δ and PKC-ɛ in PASMCs exposure to hypoxia. The PKC inhibitor Gö6983 and rottlerin (PKC-δ selective), and the inhibitor selective for PKC-ɛ peptide significantly attenuated constriction effect of 15-HETE on isolated PA rings of rats maintained for 9 days in hypoxic environments (FiO 2 = 0.12) compared with siblings rats under normoxia. Thus, these findings indicate that PKC-δ and PKC-ɛ contributing to hypoxic pulmonary artery contraction elicited by 15-HETE.

Regulation of hypoxic pulmonary vasoconstriction: basic mechanisms

Hypoxic pulmonary vasoconstriction (HPV), also known as the von Euler-Liljestrand mechanism, is a physiological response to alveolar hypoxia which distributes pulmonary capillary blood flow to alveolar areas of high oxygen partial pressure. Impairment of this mechanism may result in hypoxaemia. Under conditions of chronic hypoxia generalised vasoconstriction of the pulmonary vasculature in concert with hypoxia-induced vascular remodelling leads to pulmonary hypertension. Although the principle of HPV was recognised decades ago, its exact pathway still remains elusive. Neither the oxygen sensing process nor the exact pathway underlying HPV is fully deciphered yet. The effector pathway is suggested to include L-type calcium channels, nonspecific cation channels and voltage-dependent potassium channels, whereas mitochondria and nicotinamide adenine dinucleotide phosphate oxidases are discussed as oxygen sensors. Reactive oxygen species, redox couples and adenosine monophosphate-activated kinases are under investigation as mediators of hypoxic pulmonary vasoconstriction. Moreover, the role of calcium sensitisation, intracellular calcium stores and direction of change of reactive oxygen species is still under debate. In this context the present article focuses on the basic mechanisms of hypoxic pulmonary vasoconstriction and also outlines differences in current concepts that have been suggested for the regulation of hypoxic pulmonary vasoconstriction.

Pulmonary vasodilation by acetazolamide during hypoxia is unrelated to carbonic anhydrase inhibition

Acute hypoxic pulmonary vasoconstriction can be inhibited by high doses of the carbonic anhydrase inhibitor acetazolamide. This study aimed to determine whether acetazolamide is effective at dosing relevant to human use at high altitude and to investigate whether its efficacy against hypoxic pulmonary vasoconstriction is dependent on carbonic anhydrase inhibition by testing other potent heterocyclic sulfonamide carbonic anhydrase inhibitors. Six conscious dogs were studied in five protocols: 1) controls, 2) low-dose intravenous acetazolamide (2 mg.kg(-1).h(-1)), 3) oral acetazolamide (5 mg/kg), 4) benzolamide, a membrane-impermeant inhibitor, and 5) ethoxzolamide, a membrane-permeant inhibitor. In all protocols, unanesthetized dogs breathed spontaneously during the first hour (normoxia) and then breathed 9-10% O(2) for the next 2 h. Arterial oxygen tension ranged between 35 and 39 mmHg during hypoxia in all protocols. In controls, mean pulmonary artery pressure increased by 8 mmHg and pulmonary vascular resistance by 200 dyn.s.cm(-5) (P <0.05). With intravenous acetazolamide, mean pulmonary artery pressure and pulmonary vascular resistance remained unchanged during hypoxia. With oral acetazolamide, mean pulmonary artery pressure increased by 5 mmHg (P < 0.05), but pulmonary vascular resistance did not change during hypoxia. With benzolamide and ethoxzolamide, mean pulmonary artery pressure increased by 6-7 mmHg and pulmonary vascular resistance by 150-200 dyn.s.cm(-5) during hypoxia (P < 0.05). Low-dose acetazolamide is effective against acute hypoxic pulmonary vasoconstriction in vivo. The lack of effect with two other potent carbonic anhydrase inhibitors suggests that carbonic anhydrase is not involved in the mediation of hypoxic pulmonary vasoconstriction and that acetazolamide acts on a different receptor or channel.

Nitric oxide: physiology and pharmacology.

Nitric oxide: physiology and pharmacology.

Nitric Oxide

Nitric Oxide

The role of the endothelium in hypoxic pulmonary vasoconstriction.

The precise mechanisms underlying hypoxic pulmonary vasoconstriction (HPV) are still elusive. The recent discovery of K+ channels that are depressed by hypoxia in pulmonary vascular smooth muscle has provided a potential signal transduction mechanism for linking a reduction in Po2 to Ca2+ entry, but there are many reports suggesting that sustained HPV depends on the presence of the endothelium. Many endothelium-derived vasoactive factors have been investigated as possible mediators of HPV, including endothelium-derived relaxing factor (EDRF-NO), leukotrienes, prostanoids and endothelin, yet none have been found to be indispensable for HPV. They do, however, act as powerful modulators of the response to hypoxia. HPV is probably multifactorial in origin, as exemplified by the biphasic response to hypoxia seen in isolated pulmonary arteries over 40 min. The first phase is of rapid onset but transient, endothelium independent and partly related to Ca2+ release from intracellular stores. The second phase is slowly developing but sustained, dependent on the endothelium and associated with a stable elevation of cytosolic Ca2+. Since tension continues to rise while intracellular [Ca2+] remains constant, this implies Ca2+ sensitization of the contractile apparatus. This is independent of protein kinase C or pH. It is proposed that HPV depends upon both smooth muscle and endothelium resident mechanisms. Inhibition of K+ channels causes an elevation of cytosolic Ca2+, which may not be sufficient to generate substantive contraction on its own. However, release from the endothelium of an as yet unidentified mediator increases Ca2+ sensitivity of the contractile apparatus, and sustained contraction ensues.

The mechanism of acute hypoxic pulmonary vasoconstriction: the tale of two channels.

Hypoxia causes constriction in small pulmonary arteries and dilatation in systemic arteries. Hypoxic pulmonary vasoconstriction (HPV) is an important mechanism by which pulmonary blood flow is controlled in the fetus and by which local lung perfusion is matched to ventilation in the adult. HPV reduces the flow of desaturated blood through underventilated areas of lung. Even though many vasoactive substances have been examined as possible mediators of HPV, these appear more likely to be modulators than mediators. Hypoxic contraction has been demonstrated in single pulmonary vascular smooth muscle cells (PVSMC). The ability to sense changes in oxygen tension is observed in PVSMC and type 1 cells of the carotid body. In both cells, hypoxia has been shown to inhibit an outward potassium current, thus causing membrane depolarization and calcium entry through the voltage-dependent calcium channels. In both cells there is evidence to suggest that changes in the redox status of the oxygen-sensitive potassium channel or channels may control current flow, so that the channel is open when oxidized and closed when reduced. The redox status may be determined by the effects of hypoxia on mitochondrial/peroxisomal function or on the activity of an oxidase similar to NAD(P)H oxidase. More studies are needed to precisely define the individual potassium channels responsive to hypoxia and to confirm the gating mechanism. In systemic arteries hypoxia causes an increased current through ATP-dependent potassium channels and vasodilatation, whereas in the pulmonary arteries hypoxia inhibits potassium current and causes vasoconstriction.

A Study of Endothelium-dependent Pulmonary Arterial Relaxation and the Role of Nitric oxide on Acute Hypoxic Pulmonary Vasoconstriction in Rats

Backgroud: Since the demonstration of the fact that vascular relaxation by acetylcholine(Ach) results from the release of relaxing factor from the endothelium, the identity and physiology of this endothelium-derived relaxing factor(EDRF) has been the target for many researches. EDRF has been identified as nitric oxide(NO). With the recent evidences that EDRF is an important mediator of vascular tone, there have been increasing interests in defining the role of the EDRF as a potential mediator of hypoxic pulmonary vasoconstriction. But the role of EDRF in modulating the pulmonary circulation is not compeletely clarified. To investigate the endothelium-dependent pulmonary vasodilation and the role of EDRF during hypoxic pulmonary vasoconstriction, we studied the effects of -monomethyl-L-arginine(L-NMMA) and L-arginine on the precontracted pulmonary arterial rings of the rat in normoxia and hypoxia. Mothods: The pulmonary arteries of male Sprague Dawley(300~350g) were dissected free of surrounding tissue, and cut into rings. Rings were mounted over fine rigid wires, in organ chambers filled with 20ml of Krebs solution bubbled with 95 percent oxygen and 5 percent carbon dioxide and maintained at . Changes in isometric tension were recorded with a force transducer(FT.03 Grass, Quincy, USA) Results: 1) Precontraction of rat pulmonry artery with intact endothelium by phenylephrine(PE, ) was relaxed completely by acetylcholine(Ach, ) and sodium nitroprusside(SN, ), but relaxing response by Ach in rat pulmonary artery with denuded endothelium was significantly decreased. 2) L-NMMA() pretreatment inhibited Ach()-induced relaxation, but L-NMMA () had no effect on relaxation induced by SN(). 3) Pretreatment of the L-arginine() significantly reversed the inhibition of the Ach ()-induced relaxation caused by L-NMMA() 4) Pulmonary arterial contraction by PE() was stronger in hypoxia than normoxia but relaxing response by Ach() was decreased, 5) With pretreatment of L-arginine(), pulmonary arterial relaxation by Ach() in hypoxia was reversed to the level of relaxation in normoxia. Conclusion: It is concluded that rat pulmonary arterial relaxation by Ach is dependent on the intact endothelium and is largely mediated by NO. Acute hypoxic pulmonary vasoconstriction is related to the suppression on NO formation in the vascular endothelium.

Effects of nordihydroguaiuretic acid on sulfidopeptide leukotriene activity and hypoxic pulmonary vasoconstriction in the isolated sheep lung

Sulfidopeptide leukotrienes have been proposed as mediators of hypoxic pulmonary vasoconstriction. We studied the effects of nordihydroguaiuretic acid (NDGA), a lipoxygenase inhibitor, on hypoxic pulmonary vasoconstriction and on immunoreactive leukotriene C4 (i-LTC 4) levels in lung lymph, perfusate and bronchoalveolar lavage (BAL) fluid in isolated, indomethacin treated, sheep lungs perfused with blood at a constant flow of 100 ml/kg/min. The protocol consisted of two randomized periods with inspired O2 concentration either 28.2 or 4.2%. Five groups of lungs were studied with calculated NDGA concentrations of 0 to 1300 μM. At the end of each period, i-LTC 4 was measured in lung lymph, perfusate and BAL. NDGA levels were also measured. Consistent with our previous findings (10), hypoxia did not increase i-LTC 4 levels in lymph, perfusate or BAL fluid. Although NDGA decreased i-LTC 4 in perfusate in a dose-dependent manner, it did so at concentrations which were ten times lower than those causing inhibition of hypoxic vasoconstriction. These NDGA concentrations also inhibited vasomotor responses to KCl, serotonin and PGF 2α suggesting nonspecificity. These observations are not consistent with mediation of hypoxic vasoconstriction by i-LTC 4.

Hypoxic pulmonary vasoconstriction is not endothelium dependent.

Feline intrapulmonary arteries (mean diameter, 0.9 mm) were equilibrated in Earle's solution at constant tension in a chamber bubbled with an hyperoxic gas mixture (30% oxygen, 5% carbon dioxide, balance nitrogen). The endothelium was removed from half the vessels by gentle rubbing. The isometric response to the addition of acetylcholine (1*10(-6) M) was dilator in the vessels with endothelium and constrictor in those without endothelium. Intermittent exposure to a hypoxic gas mixture (0% oxygen, 5% carbon dioxide, balance nitrogen) for 20 min with five repetitions demonstrated sustained constrictor responses in the presence or absence of endothelium. Endothelial cells are, therefore, not required for the mediation of hypoxic pulmonary vasoconstriction.

Hypoxic pulmonary vasoconstriction in the adult respiratory distress syndrome

Increased pulmonary vascular resistance (PVR) and microvascular hyperpermeability resulting in lung edema and arterial hypoxemia are mainstays in the development of adult respiratory distress syndrome (ARDS). The proposed pathophysiologic mechanisms include activation of complement and polymorphonuclear leukocytes secreting lysosomal enzymes, toxic oxygen metabolites (TOM) and eicosanoids. Platelets and coagulation factors are also involved, and in the most severe cases even monocytes are activated as reflected in release of thromboplastin. The latter may elicit disseminated intravascular coagulation (DIC). Under physiologic conditions lung blood flow is diverted from poorly to better oxygenated areas by way of hypoxic pulmonary vasoconstriction (HPV), thereby counteracting a decrease in arterial oxygenation. Many vasoactive substances have been proposed and again refuted as possible mediators of HPV. In this study we have focused on the following: histamine, catecholamines, arachidonates, calcium, phosphoinositides and TOM as well as endothelium-derived relaxing and constricting factors. Whether HPV is present in ARDS and whether it is advantageous or not seems to depend on the stage and extent of disease. We discuss possible interactions between HPV and ARDS mediators and between HPV and various vasoactive agents tested for therapeutic effects. Out of the abundance of mediators released, prostacyclin, prostaglandin E1, activated complement and platelet activating factor have been shown explicitly to inhibit HPV whereas others are suspected of doing so. In therapeutical use, prostacyclin has proved to reduce PVR and at the same time enhance cardiac output and oxygen delivery. In mild to moderate ARDS, improvement of arterial oxygenation has also been obtained employing almitrine bismesylate, a potentiator of HPV. Experimentally, adenosine effectively reduces increments in PVR and microvascular permeability with modest effects on systemic circulation. However, further investigations are warranted to decide whether adenosine or more specific blockers as, for instance, monoclonal antibodies against tumor necrosis factor should be integrated in ARDS therapy in the future.

Leukotriene Synthesis Inhibition and Receptor Blockade Do Not Inhibit Hypoxic Pulmonary Vasoconstriction in Sheep

Several lines of evidence suggest that leukotrienes may be mediators of hypoxic pulmonary vasoconstriction (HPV). However, the effect of leukotriene inhibition on HPV remains controversial. The present study investigated the effect of leukotriene synthesis inhibition and receptor blockade on HPV in the halothane-anesthetized sheep. After initial baseline measurements, the pulmonary pressor response to 15 min of global hypoxia (FIO2 = 0.13) was measured. A second set of baseline measurements was obtained and the sheep then received the combined cyclooxygenase/lipoxygenase inhibitor BW755C, the selective lipoxygenase inhibitor U60257, or the leukotriene receptor antagonist LY171883. Hemodynamic measurements were obtained after drug administration and during a subsequent hypoxic challenge (FIO2 = 0.13). Initial hypoxic challenge increased pulmonary artery pressure 68% and increased pulmonary vascular resistance 104%. Pulmonary hemodynamics after recovery from hypoxia were similar to initial baseline values. Drug administration had no significant hemodynamic effect. Hypoxic challenge after drug administration resulted in a pulmonary pressor response identical to the initial hypoxic challenge. Because leukotriene synthesis inhibition and receptor blockade did not alter the response to hypoxia, we conclude that leukotrienes are not obligatory mediators of HPV. A critical review of the literature supports a modulatory rather than an obligatory role for leukotrienes in HPV.

Role of leukotriene C4 in pulmonary hypertension: platelet-activating factor vs. hypoxia

The aim of this study was to determine whether leukotriene C4 (LTC4) is a mediator of hypoxic pulmonary vasoconstriction. We hypothesized that similar increases in LTC4, detected in the lung parenchyma and pulmonary vascular compartment during cyclooxygenase blockade with indomethacin (INDO), would be observed during an equal increase in pulmonary arterial pressure caused by acute alveolar hypoxia (HYP, 100% N2) or platelet-activating factor (PAF, 10 micrograms into the pulmonary artery). Rat lungs were perfused at constant flow in vitro with an albumin-Krebs-Henseleit solution. Mean pulmonary arterial pressure (n = 6 per group) increased from a base line of 10.9 +/- 1.2 to 15.8 +/- 2.1 (HYP + INDO) and 15.5 +/- 1.9 (SE) Torr (PAF + INDO). LTC4 levels increased only in response to PAF + INDO; perfusate levels increased from 0.4 +/- 0.07 to 5.3 +/- 1.1 ng/40 ml, and lung parenchymal levels increased from 1.9 +/- 0.07 to 22.8 +/- 5.3 ng/lung. Diethylcarbamazine (lipoxygenase inhibitor) reduced PAF-induced lung parenchymal levels of LTC4 by 68% and pulmonary hypertension by 63%. We conclude that 1) LTC4 is not a mediator of hypoxic pulmonary vasoconstriction and 2) intravascular PAF is a potent stimulus for LTC4 production in the lung parenchyma.

The Effect of Methylprednisolone on Hypoxic Pulmonary Vasoconstriction in the Newborn Late-Gestation Lamb

Methylprednisolone (30 mg/kg), which inhibits a number of mediators of hypoxic pulmonary vasoconstriction derived from arachidonic acid, has been found to alleviate hypoxic pulmonary vasoconstriction in adult humans and in the isolated rat lung preparation. We studied the effect of 30 mg/kg of methylprednisolone on the pulmonary vascular response to hypoxia in six late-gestation newborn lambs. During hypoxia, pulmonary vascular resistance nearly doubled compared with the baseline hyperoxic state. This was true both before and after administration of methylprednisolone. We conclude that methylprednisolone, when administered at the dosage used in previous studies of adult humans and animals, does not affect the response of the pulmonary vascular bed to hypoxia in newborn lambs.

Does leukotriene C4 mediate hypoxic vasoconstriction in isolated ferret lungs?

To evaluate leukotriene (LT) C4 as a mediator of hypoxic pulmonary vasoconstriction, we examined the effects of FPL55712, a putative LT antagonist, and indomethacin, a cyclooxygenase inhibitor, on vasopressor responses to LTC4 and hypoxia (inspired O2 tension = 25 Torr) in isolated ferret lungs perfused with a constant flow (50 ml.kg-1.min-1). Pulmonary arterial injections of LTC4 caused dose-related increases in pulmonary arterial pressure during perfusion with physiological salt solution containing Ficoll (4 g/dl). FPL55712 caused concentration-related inhibition of the pressor response to LTC4 (0.6 micrograms). Although 10 micrograms/ml FPL55712 inhibited the LTC4 pressor response by 61%, it did not alter the response to hypoxia. At 100 microgram/ml, FPL55712 inhibited the responses to LTC4 and hypoxia by 73 and 71%, respectively, but also attenuated the vasoconstrictor responses to prostaglandin F2 alpha (78% at 8 micrograms), phenylephrine (68% at 100 micrograms), and KCl (51% at 40 mM). At 0.5 microgram/ml, indomethacin significantly attenuated the pressor response to arachidonic acid but did not alter responses to LTC4 or hypoxia. These results suggest that in isolated ferret lungs 1) the vasoconstrictor response to LTC4 did not depend on release of cyclooxygenase products and 2) LTC4 did not mediate hypoxic vasoconstriction.

Inhibition of rat lung glutathione synthesis attenuates hypoxic pulmonary vasoconstriction and the associated leukotriene C4 production.

Lipoxygenase metabolites of arachidonic acid have been proposed as possible mediators of hypoxic pulmonary vasoconstriction (HPV) in the rat. Since reduced glutathione (GSH) is a required substrate for the synthesis of the sulfidopeptide eicosanoid leukotriene C4 (LTC4), we reasoned that this specific GSH dependence of LTC4 synthesis might allow us to distinguish between the roles of sulfidopeptide leukotrienes and other 5-lipoxygenase metabolites of arachidonic acid. In the present study we have examined the effect of in vivo pretreatment with the GSH synthesis inhibitor buthionine sulfoximine (BSO) on both the hypoxic pressor response and lung leukotriene synthesis in the rat. The intraperitoneal administration of 4 mmol/kg of BSO 30, 20, and 4 h prior to lung excision significantly depleted total lung glutathione as compared to saline-pretreated controls. This depletion of glutathione was associated with a significant attenuation of HPV in isolated perfused lungs but no alteration in pressor response to angiotensin II or KCl. In addition, hypoxia-associated LTC4 levels in lung homogenates were significantly lower in animals pretreated with BSO than in saline-pretreated controls. The specificity of the effects of BSO on lung leukotriene synthesis was examined by quantitating immunoreactive leukotrienes produced by unstimulated and ionophore A23187-stimulated parenchymal lung fragments, lonophore stimulation of lung fragments from BSO-pretreated rats produced 76 +/- 12.2% as much LTC4 and 127 +/- 22.3% as much leukotriene B4 as did fragments from saline-pretreated rats. Our data demonstrating that GSH depletion caused parallel reductions in both HPV and hypoxia-associated lung LTC4 levels are therefore consistent with the hypothesis that sulfidopeptide leukotrienes are involved in this pressor response in the rat.

Evidence for Platelet-Activatinc Factor as a Mediator of Hypoxic Pulmonary Vasoconstriction

Evidence for Platelet-Activatinc Factor as a Mediator of Hypoxic Pulmonary Vasoconstriction