Hypotheses regarding the role of pericytes in regulating movement of fluid, nutrients, and hormones across the microcirculatory endothelial barrier.

A decade ago, we initiated studies to define relationship(s) between products of 5-lipoxygenase-mediated arachidonic acid metabolism and altered microvascular permeability. Patients with permeability (nonhydrostatic) pulmonary edema (adult respiratory distress syndrome) and intact animal models of permeability edema, produced with agents that required neutrophils (phorbol myristate acetate) and those that did not (ethchlorvynol), invariably revealed the presence of leukotrienes; in contrast, leukotrienes were not detected in cases of hydrostatic pulmonary edema. In isolated perfused canine lung, we identified increases in microvascular permeability coefficients in response to the injurious agent. Permeability coefficients were not increased when injurious agents were given in the presence of 5-lipoxygenase inhibitors. To define further the relationships between leukotriene generation and edema formation, we postulated that leukotrienes effected contraction of capillary pericytes, thereby increasing pore size of endothelial intercellular junctions and enhancing movement across the microvascular barrier. We isolated pericytes from bovine retinas, identified them morphologically and by staining characteristics, and, in preliminary experiments, found that they do not possess the 5-lipoxygenase enzyme; however, when cocultured with neutrophils, which possess 5-lipoxygenase but cannot synthesize sulfidopeptide leukotrienes because of their lack of glutathione S-transferase, sulfidopeptide leukotriene synthesis ensued. In view of the anatomic position of pericytes, evidence that they participate in endothelial transport, their ability to contract, and evidence of cell-to-cell communication, we propose that pericytes control the movement of fluid, solutes, hormones, and small and large molecules across the microvascular endothelium.

Vascular endothelial growth factor mRNA in pericytes is upregulated by phorbol myristate acetate.

Increased microvascular permeability, which occurs in conditions such as the adult respiratory distress syndrome and diabetes mellitus, is related to physicochemical alterations in the microvascular barrier. We postulate that, in part, capillary pericytes affect microvascular permeability via production of a vasoactive cytokine, viz, vascular endothelial growth factor (VEGF), also known as vascular permeability factor. The goal of the present study was to evaluate the effects of phorbol myristate acetate (PMA), a substance known to produce nonhydrostatic pulmonary edema in intact animals, on VEGF gene expression in pericyte cultures. Microvascular pericytes were isolated from bovine retinas using magnetic microspheres coated with 3G5 monoclonal antibody. Pericyte identity was confirmed both morphologically and by immunostaining for alpha-smooth muscle actin and 3G5 ganglioside. The cultured pericytes were stimulated with N(omega)-nitro-L-arginine methyl ester (L-NAME, 1 x 10(-4) mmol/L), angiotensin II (1 x 10(-6) mmol/L), and PMA (5 x 10(-8) mmol/L), selected because of their ability to upregulate VEGF mRNA expressions in other cell types. Northern blot analysis was performed using [32P]dCTP labeled human VEGF cDNA (Genentech). Lane-loading differences were normalized using mouse GAPDH control cDNA probe. VEGF mRNA expression was upregulated by PMA (10(-9) to 10(-6) mol/L) in a dose-dependent manner, whereas neither L-NAME nor angiotensin II affected VEGF mRNA expression in pericytes. These results support the hypothesis that pericytes increase permeability of the endothelial barrier through increased VEGF production.

Relationship of arachidonic acid release to porcine coronary artery relaxation.

In porcine coronary artery endothelium-dependent relaxation to bradykinin is in part attributed to a chemically unidentified factor, termed endothelium-derived hyperpolarizing factor (EDHF). We hypothesize that arachidonic acid, acting through a cyclooxygenase-independent mechanism, is responsible for EDHF production. To define the relationship between EDHF production and arachidonic acid release, we investigated the role of phospholipase C in bradykinin-induced relaxation and prostaglandin I2 production (an index of arachidonic acid release) in porcine coronary artery. The phospholipase C inhibitor U73122 (1 mumol/L) abolished bradykinin-induced, nitric oxide-mediated relaxation but did not inhibit either bradykinin-induced, EDHF-mediated relaxation or prostaglandin I2 production. However, when given at a larger dose (20 mumol/L) U73122 abolished both bradykinin-induced, EDHF-mediated relaxation and prostaglandin I2 production. Similarly, the calcium-ATPase inhibitor thapsigargin, given at a dose (1 mumol/L) that abolished bradykinin-induced increases in intracellular calcium concentration in cultured porcine coronary artery endothelial cells, eliminated both bradykinin-induced. EDHF-mediated relaxation and prostaglandin I2 production. Although thapsigargin abolished bradykinin-induced prostaglandin I2 production, the basal production of prostaglandin I2 was enhanced and contraction of endothelium-intact rings was attenuated. These latter responses are most likely related to enhanced basal arachidonic acid release and associated EDHF production. These observations suggest that phospholipase C activation and increased intracellular calcium concentration are required for both bradykinin-induced arachidonic acid release and EDHF production in porcine coronary artery.(ABSTRACT TRUNCATED AT 250 WORDS)

Relaxation of porcine coronary artery to bradykinin. Role of arachidonic acid.

Bradykinin-induced relaxation of precontracted, porcine coronary artery (PCA) rings is mediated by distinctly different endothelium-derived relaxing factors depending on the contractile agent used. Thus when contracted with KCl, bradykinin-induced relaxation of PCA rings is mediated solely by nitric oxide (NO), whereas when contracted with the thromboxane mimetic U46619, a small component of the relaxation is attributable to NO and a large component is attributable to a non-NO mechanism that is independent of cyclooxygenase activity. We hypothesized that the non-NO component was mediated by arachidonic acid (AA) or by a non-cyclooxygenase product of AA metabolism. Bradykinin-induced relaxations of PCA rings precontracted with U46619 in the presence of indomethacin (10 mumol/L) were moderately attenuated by the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME, 100 mumol/L), whereas when precontracted with KCl, L-NAME abolished the relaxations. AA produced endothelium-dependent relaxations of rings precontracted with U46619 that were unaffected by L-NAME, whereas AA did not relax rings precontracted with KCl. In rings precontracted with U46619, in the presence of L-NAME and indomethacin the phospholipase inhibitors quinacrine (50 mumol/L) and 4-bromophenacyl bromide (10 mumol/L) attenuated bradykinin- but not AA-induced relaxations. Inhibitors of both lipoxygenase (BW 755c [100 mumol/L] and nafazatrom [20 mumol/L]) and cytochrome P-450 (proadifen [10 mumol/L] and clotrimazole [10 mumol/L]) pathways did not eliminate bradykinin- or AA-induced relaxations, although clotrimazole partially attenuated AA-induced relaxations. These findings suggest that bradykinin-induced relaxation of PCA rings is mediated by AA through a mechanism that is not dependent on cyclooxygenase, lipoxygenase, or cytochrome P-450 pathways.

Effect of indomethacin on increases in leukotriene B4 and pulmonary edema in response to phorbol ester administration in dogs.

The administration of leukotrienes (LTs) into the pulmonary circulation results in edema formation and increased vascular permeability. We reported previously that the administration of phorbol myristate acetate (PMA, 20 micrograms/kg) to intact anesthetized dogs results in a reduction in circulating white blood cells as well as the development of pulmonary edema concomitant with the appearance of LTs in the lungs. In contrast, when a smaller dose of PMA (10 micrograms/kg) was administered, neither extravascular lung water nor LTs increased, although there was a similar reduction in circulating white blood cells. In the present study, we used a property of indomethacin, namely, its capacity to augment the formation of LTs, to examine further the relationship between LT generation and pulmonary edema formation in response to PMA administration. In intact pentobarbital-anesthetized dogs pretreated with saline (N = 9), the administration of PMA at a dose of 10 micrograms/kg, i.v., did not result in any change in extravascular lung water or in LTB4 present in bronchoalveolar lavage fluid (BALF). In contrast, in six animals pretreated with indomethacin (5 mg/kg), the administration of this dose of PMA resulted in increases in both extravascular lung water (P < 0.05) and LTB4 (P < 0.05) in BALF. These results provide support for the hypothesis that leukotrienes are requisite for PMA-induced increases in extravascular lung water.

Effects of ibuprofen on the hypoxemia of established ethchlorvynol-induced unilateral acute lung injury in anesthetized dogs.

Nonsteroidal anti-inflammatory agents, given prior to induction of unilateral acute lung injury with ethchlorvynol (ECV) in anesthetized dogs, prevent the decreases in systemic oxygen tension (PaO2) which are observed when ECV is given alone. We investigated whether ibuprofen, administered after acute lung injury, would result in improvement in arterial oxygenation. In animals not receiving ibuprofen after unilateral acute lung injury with ECV, PaO2 decreased and venous admixture increased significantly from control values at all experimental time-periods. In those animals receiving ibuprofen, significant decreases in venous admixture were noted. The decrease in PaO2 after ECV administration was significantly less than that observed in animals that did not receive ibuprofen after acute lung injury (p less than 0.05). Ibuprofen had no effect on extravascular lung water. These results demonstrate that in an ECV model of acute lung injury the administration of ibuprofen, after the acute lung injury, results in significant decreases in venous admixture.

OKY-046 prevents increases in LTB4 and pulmonary edema in phorbol ester-induced lung injury in dogs.

Thromboxanes (Txs) were implicated as possible participants in the altered microvascular permeability of acute lung injury when the Tx synthase inhibitor, OKY-046, was reported to prevent pulmonary edema induced by phorbol myristate acetate (PMA). Recently, however, we found that OKY-046, at a dose just sufficient to block Tx synthesis in intact dogs, did not prevent PMA-induced pulmonary edema but rather merely reduced it modestly. The present study was designed to explore other mechanisms whereby OKY-046 might prevent PMA-induced pulmonary edema. The finding that 5-lipoxygenase (5-LO) metabolites of arachidonic acid were increased within the lung after PMA administration, coupled with the report that OKY-046 inhibited slow-reacting substance of anaphylaxis formation, permitted formulation of the hypothesis that OKY-046, at a dose in excess of that required to inhibit Tx synthesis, inhibits the formation of a product(s) of 5-LO and, thereby, prevents edema formation. In vehicle-pretreated pentobarbital-anesthetized male mongrel dogs (n = 4), PMA (20 micrograms/kg i.v.) increased pulmonary vascular resistance (PVR) from 4.4 +/- 0.3 to 26.3 +/- 8.8 mmHg.l-1 x min (P < 0.01) and extravascular lung water from 6.7 +/- 0.5 to 19.1 +/- 6.2 ml/kg body wt (P < 0.05). Concomitantly, both TxB2 and leukotriene B4 (LTB4) were significantly increased in the lung. Pretreatment with OKY-046 (100 mg/kg i.v., n = 8) prevented PMA-induced increases in TxB2, LTB4, and pulmonary edema formation but did not prevent the increase in PVR.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of leukotriene C4 and D4 to hypoxic pulmonary vasoconstriction in dogs.

Leukotrienes C4 and D4 have been implicated as possible mediators of hypoxic pulmonary vasoconstriction. To test this hypothesis, the relationship between pulmonary leukotriene (LT) synthesis in response to hypoxia and alterations in pulmonary hemodynamics was evaluated in pentobarbital sodium-anesthetized, neuromuscular-blocked, male, mongrel dogs. A reduction in the fraction of inspired O2 (FIO2) in vehicle-treated animals (n = 12) from 0.21 to 0.10 was associated with increases in LTC4 and LTD4 in bronchoalveolar lavage fluid (BALF). After 30 min of continuous hypoxia, LTC4 and LTD4 increased from control values of 59.4 +/- 10.4 and 91.7 +/- 18.1 ng/lavage to 142.7 +/- 31.8 (P less than 0.05) and 156.3 +/- 25.3 (P less than 0.01) ng/lavage, respectively. Concomitantly, mean pulmonary arterial pressure (Ppa) and pulmonary vascular resistance (PVR) were increased over control by 67 +/- 7 (P less than 0.001) and 62 +/- 7% (P less than 0.001), respectively. In contrast, in animals treated with diethylcarbamazine (n = 5), a leukotriene A4 synthase inhibitor, identical reductions in FIO2 were not associated with increases in LTC4 and LTD4 in BALF, although at the same time period, Ppa and PVR were increased over control by 60 +/- 13 (P less than 0.05) and 112 +/- 31% (P less than 0.05), respectively. These results, therefore, do not support the contention that leukotrienes mediate hypoxic pulmonary vasoconstriction in dogs.

Effect of cyclooxygenase inhibition on ethchlorvynol-induced acute lung injury in dogs.

In anesthetized dogs ethchlorvynol (ECV, 9 mg/kg) was selectively administered into the right pulmonary circulation to produce unilateral acute lung injury (ALI) characterized by nonhydrostatic pulmonary edema and systemic hypoxemia. To investigate the hypothesis that products of cyclooxygenase activity are mediators of the arterial hypoxemia, but not the edema formation in this injury, animals were pretreated with one of two chemically dissimilar cyclooxygenase inhibitors, indomethacin (5 mg/kg), or ibuprofen (12.5 mg/kg), or vehicle (0.1 M sodium carbonate) prior to the administration of ECV. Pretreatment with either inhibitor prevented the ECV-induced systemic hypoxemia observed in animals pretreated with vehicle (P less than 0.01). Despite this protection of systemic oxygenation, there was no redistribution of blood flow to the uninjured lung following unilateral ECV administration. Cyclooxygenase inhibition prior to ALI did not attenuate the accumulation of lung water. In the ibuprofen group, left atrial pressure increased significantly following ECV administration. We conclude that a product(s) of cyclooxygenase-mediated arachidonic acid metabolism is responsible for the altered vascular reactivity and consequent systemic hypoxemia in this model, but that the edema formation following ECV is not related to cyclooxygenase activity. In addition, ibuprofen, administered prior to the induction of ALI, exhibits properties not shared by indomethacin but is not different in its capacity to attenuate hypoxemia or in its failure to limit edema formation.

Thromboxane does not mediate pulmonary hypertension in phorbol ester-induced acute lung injury in dogs.

Thromboxane (Tx) has been suggested to mediate the pulmonary hypertension of phorbol myristate acetate- (PMA) induced acute lung injury. To test this hypothesis, the relationship between Tx and pulmonary arterial pressure was evaluated in a model of acute lung injury induced with PMA in pentobarbital sodium-anesthetized male mongrel dogs. Sixty minutes after administration of PMA (20 micrograms/kg iv, n = 10), TxB2 increased 10-fold from control in both systemic and pulmonary arterial blood and 8-fold in bronchoalveolar lavage (BAL) fluid. Concomitantly, pulmonary arterial pressure (Ppa) increased from 14.5 +/- 1.0 to 36.2 +/- 3.5 mmHg, and pulmonary vascular resistance (PVR) increased from 5.1 +/- 0.4 to 25.9 +/- 2.9 mmHg.l-1.min. Inhibition of Tx synthase with OKY-046 (10 mg/kg iv, n = 6) prevented the PMA-induced increase in Tx concentrations in blood and BAL fluid but did not prevent or attenuate the increase in Ppa. OKY-046 pretreatment did, however, attenuate but not prevent the increase in PVR 60 min after PMA administration. Pretreatment with the TxA2/prostaglandin H2 receptor antagonist ONO-3708 (10 micrograms.kg-1.min-1 iv, n = 7) prevented the pressor response to bolus injections of 1-10 micrograms U-46619, a Tx receptor agonist, but did not prevent or attenuate the PMA-induced increase in Ppa. ONO-3708 also attenuated but did not prevent the increase in PVR. These results suggest that Tx does not mediate the PMA-induced pulmonary hypertension but may augment the increases in PVR in this model of acute lung injury.

Increased leukotriene C4 in ethchlorvynol-induced acute lung injury in dogs.

We investigated whether ethchlorvynol (ECV)-induced acute lung injury (ALI) is associated with an increase in leukotriene C4 (LTC4) production. In six pentobarbital sodium-anesthetized dogs, ECV (15 mg/kg iv) introduced into the pulmonary circulation resulted in a 164 +/- 31% increase in extravascular lung water 120 min after ECV administration. Concomitantly, the mean (+/- SE) concentration of LTC4 in arterial plasma measured by radioimmunoassay following 80% EtOH precipitation, XAD-7 extraction and high-pressure liquid chromatography purification was 5.0 +/- 1.3 pg/ml, unchanged from control (pre-ECV) values. In contrast, in pulmonary edema fluid 120 min post-ECV, the LTC4 concentration was 35.2 +/- 10.8 pg/ml, sevenfold greater than those values found in the arterial plasma (P less than 0.01). In six additional dogs, 120 min after unilateral ALI had been induced with ECV (9 mg/kg iv), LTC4 in the bronchoalveolar lavage (BAL) of the uninjured lung was 12.1 +/- 1.5 pg/ml, unchanged from pre-ECV values, whereas, LTC4 in the BAL of the injured lung increased from a control value of 10.2 +/- 1.6 to 24.2 +/- 3.5 pg/ml (P less than 0.01) 120 min after ECV administration. These results demonstrate that, in ECV-induced acute lung injury, LTC4 concentrations in pulmonary edema fluid are considerably greater than those found in arterial plasma in the case of bilateral acute lung injury and significantly greater in the BAL of the injured lung compared with the uninjured lung in the case of unilateral acute lung injury. The results are a necessary first step in support of the hypothesis that leukotrienes participate in the altered permeability of ECV-induced acute lung injury.

Eicosanoid balance and perfusion redistribution of oleic acid-induced acute lung injury.

We have proposed that endogenous prostacyclin opposes the vasoconstriction responsible for redistribution of regional pulmonary blood flow (rPBF) away from areas of increased regional lung water concentration (rLWC) in canine oleic acid- (OA) induced acute lung injury (D. P. Schuster and J. Haller. J. Appl. Physiol. 69: 353-361, 1990). To test this hypothesis, we related regional lung tissue concentrations of 6-ketoprostaglandin (PG) F1 alpha and thromboxane (Tx) B2 in tissue samples obtained 2.5 h after administration of OA (0.08 ml/kg iv) to rPBF and rLWC measured by positron emission tomography. After OA only (n = 16), rLWC increased in dependent lung regions. Some animals responded to increased rLWC by redistribution of rPBF away from the most edematous regions (OA-R, n = 6), whereas others did not (OA-NR, n = 10). In another six animals, meclofenamate was administered after OA (OA-meclo). After OA, tissue concentrations of 6-keto-PGF1 alpha were greater than TxB2 in all groups, but concentrations of 6-keto-PGF1 alpha were not different between OA-R and OA-NR animals. TxB2 was increased in the dependent regions of animals in both OA-R and OA-NR groups compared with controls (no OA, n = 4, P < 0.05). The tissue TxB2/6-keto-PGF1 alpha ratio was smaller in controls and OA-NR in which no perfusion redistribution occurred than in OA-R and OA-meclo in which it did occur. This TxB2/6-keto-PGF1 alpha ratio correlated significantly with the magnitude of perfusion redistribution.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced microvascular permeability of PMA-induced acute lung injury is not mediated by cyclooxygenase products.

Products of cyclooxygenase activity have been proposed to mediate the pulmonary hypertension and increased microvascular permeability associated with phorbol myristate acetate- (PMA) induced acute lung injury. Previously, we reported that thromboxane (Tx) does not mediate PMA-induced pulmonary hypertension in intact anesthetized dogs. In the present study, PMA was administered to isolated canine lungs perfused with autologous blood at constant flow to investigate a possible role for Tx in the PMA-induced increase in microvascular permeability. Changes in permeability were assessed by determining changes in the capillary filtration coefficient (Kfc). In lobes pretreated with papaverine to prevent PMA-induced increases in pulmonary vascular resistance, Kfc increased from a baseline value of 0.2 +/- 0.03 to 1.5 +/- 0.29 ml.min-1.cmH2O-1.100 g wet lobe wt-1 (P < 0.01) 30 min after PMA (5.8 x 10(-8) M, n = 10). Concomitantly, TxB2, the stable metabolite of TxA2, increased from 138 +/- 44 to 1,498 +/- 505 pg/ml (P < 0.05) in the blood. Both the selective Tx synthase inhibitor, OKY-046 (7 x 10(-4) M, n = 6), and the cyclooxygenase inhibitor, indomethacin (10(-4) M, n = 7), prevented the PMA-induced increase in TxB2, but neither compound attenuated the PMA-induced increase in Kfc. ONO-3708 (10(-6) M), a selective prostaglandin (PG) H2/TxA2 receptor antagonist, prevented the vasoconstriction resulting from administration of U-46619, a stable PGH2/TxA2 receptor agonist, but it did not prevent the PMA-induced increases in Kfc (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired release of ATP from red blood cells of humans with primary pulmonary hypertension.

Previously, we reported that in the isolated perfused rabbit lung, red blood cells (RBCs) obtained from either rabbits or healthy humans were a required component of the perfusate to unmask evidence of nitric oxide (NO) participation in regulation of the pulmonary circulation. In addition, we found that mechanical deformation of rabbit and healthy human RBCs released ATP, a known agonist for enhanced NO synthesis. In contrast, RBCs obtained from patients with cystic fibrosis (CF) did not release ATP in response to mechanical deformation. The coexistence of airway disease and alveolar hypoxia in patients with CF precluded the drawing of conclusions relating a defect in RBC ATP release with the pulmonary hypertension associated with CF. Airway disease and alveolar hypoxia are not, however, features of primary pulmonary hypertension (PPH), a human condition of unknown etiology. We postulated that a defect in NO generation might contribute to the increased pulmonary vascular resistance in PPH, and as a first step, we hypothesized that RBCs obtained from patients with PPH would not release ATP. In contrast to RBCs of healthy humans, when RBCs of PPH patients were passed through filters (average pore size 12, 8, or 5 microm), ATP was not released and the RBCs exhibited reduced deformability. Moreover, when incubated with the active cAMP analogue, Sp-cAMP (100 microM), an activator of the CF transmembrane conductance regulator, ATP was not released. These results demonstrate that RBCs obtained from patients with PPH fail to release ATP whether the stimulus is mechanical or pharmacological. Thus, failure of RBCs to release ATP in patients with PPH might be a major pathogenetic factor that accounts for the heretofore unknown etiology of their pulmonary hypertension.

The role of G protein beta subunits in the release of ATP from human erythrocytes.

Previously, we demonstrated that adenosine triphosphate (ATP) is released from human erythrocytes in response to mechanical deformation and that this release requires activation of a signal-transduction pathway involving adenylyl cyclase and the heterotrimeric G protein, Gs. Here we investigate the role of heterotrimeric G proteins of the Gi subtype in the release of ATP from human erythrocytes. In addition, we determined the profile of heterotrimeric G protein beta subunits present in these erythrocyte membranes. The activity of Gi was stimulated by incubation of erythrocytes (20% hematocrit) with mastoparin (10 microM). ATP release was measured using the luciferin/luciferase assay. Heterotrimeric G protein beta subunits present in erythrocyte membranes were resolved using gel electrophoresis and subunit specific antibodies. Incubation of human erythrocytes with mastoparan (an activator of Gi/o) resulted in a 4.1 +/- 0.6-fold increase in ATP present in the medium (P<0.01). Human erythrocyte membranes stain positively for beta subunit types 1, 2, 3 and 4, all of which been reported to activate of some isoforms of adenylyl cyclase. Activation of the heterotrimeric G protein, Gi, results in ATP release from erythrocytes. This effect is may be related to the activity of beta subunits associated with this G protein in the human erythrocyte.

Proposed role for leukotrienes in the pathophysiology of multiple systems organ failure.

The leukotrienes are a group of biologically active products of arachidonic acid metabolism that have been demonstrated to possess the capability to alter vascular reactivity as well as vascular permeability when applied topically to tissues or infused into the vascular bed of various organs. These biologic effects of the exogenous leukotrienes have led to the speculation that these arachidonic acid metabolites could be important mediators in the pathophysiologic events that culminate in the development of acute lung injury and MSOF. Increased production of inflammatory mediators is undoubtedly a contributing factor to the pathophysiologic events that culminate in MSOF. LTB4 has been shown to be a potent stimulator of neutrophil chemotaxis and adhesion. It is possible that LTB4 may be responsible for initiation or amplification of the inflammatory response in this syndrome. The sulfidopeptide leukotrienes have profound effects on cardiac function, which may be mediated through effects on both coronary blood flow and cardiac contractility. These arachidonic acid metabolites are also capable of altering blood flow to several vascular beds and, when synthesized endogenously and released, may be important in the regulation of the peripheral circulation. Endogenous leukotrienes may have physiologic or pathophysiologic effects on cardiac output and its distribution to peripheral vascular beds or on systemic blood pressure. In view of the observations that alterations in cardiac output, blood pressure, and individual organ blood flow have been demonstrated in the clinical setting of ARDs and MSOF, it is attractive to suggest that the leukotrienes may contribute to the hemodynamic alterations observed in these clinical conditions. In addition to their effects on vascular smooth muscle and the myocardium, the leukotrienes have been shown to increase the permeability of blood vessels. In the case of the sulfidopeptide leukotrienes, the enhanced vascular permeability appears to be a direct effect of these lipoxygenase metabolites. Although LTB4 may directly increase vascular permeability, several lines of evidence suggest that the recruitment and activity of neutrophils is required for edema formation to develop following application of this leukotriene. Enhanced capillary permeability, at least in the pulmonary circulation, is a hallmark of ARDS and is also frequently present in MSOF. The leukotrienes, either through direct effects on vessels or through recruitment of inflammatory cells, are likely contributors to the nonhydrostatic edema associated with these syndromes.

Vascular responses to prostaglandin F 2 alpha in isolated cat lungs.

Changes in perfusion pressure in response to graded doses of prostaglandin F 2 alpha (PGF 2 alpha) were measured during both forward and retrograde perfusion of isolated cat lungs perfused at a constant flow rate. During forward perfusion, PGF 2 alpha produced a dose-dependent increase in pulmonary artery pressure and a decrease in lung fluid volume. During retrograde perfusion, PGF 2 alpha also produced a dose-dependent increase in perfusion pressure; however, the dose required was fivefold greater than that needed to produce an identical change in pressure during forward perfusion. In addition, during retrograde perfusion, the lung fluid volume increased in response to PGF 2 alpha. These results suggest that the major site of activity of PGF 2 alpha is on the arterial side of the pulmonary vascular bed and that inactivation of PGF 2 alpha by the lung occurs primarily distal to this arterial site of vasomotion. The changes in perfusion pressure in response to PGF 2 alpha were markedly dependent on pH and oxygen tension (Po-2), being abolished by severe alkalosis and potentiated by both acidosis and hypoxia. In contrast, neither serotonin nor norepinephrine exhibited such a pH or Po-2 dependency. Since the ratio of the forward response to the retrograde response was not decreased by alterations of pH or Po-2, their influence on the responses appears to be through interaction at the site of vascular activity rather than through alteration of the rate of prostaglandin inactivation.