Inhibition of the platelet activating factor (PAF)-induced in vivo responses in rats by trans-2,5-(3,4,5-trimethoxyphenyl) tetrahydrofuran (L-652,731), a PAF receptor antagonist

Trans-2,5-bis-(3,4,5-trimethoxyphenyl)tetrahydrofuran (L652, 731), a potent and specific receptor antagonist of platelet activating factor (PAF) (Hwang et al., J. Biol. Chem. 260: 15639-15645, 1985), potently inhibits several PAF-induced in vivo responses in rats. Intravenously and p.o. administered L-652,731 gave a dose-response inhibition of PAF-induced lysosomal hydrolase secretion and extravasation with ED50 values of 1 and 3 mg/kg, respectively. Inhibitions of 87% were achieved with 50 mg/kg p.o. After a single 5-mg/kg p.o. dose, L-652,731 achieved 50 to 60% inhibition of PAF-induced lysosomal hydrolase secretion and extravasation by 0.5 to 1.5 hr with near maximum inhibition lasting through 6 hr. A 20-mg/kg p.o. dose of L-652,731 inhibited PAF-induced leukopenia and neutropenia by 96 and 73%, respectively. The most substantial inhibitions of the extravasation and lysosomal hydrolase secretion induced by PAF or soluble immune complexes were achieved by p.o. L-652,731 (20 mg/kg) with moderate inhibition by dexamethasone and little or no inhibition by antagonists/inhibitors of histamine H1 or H2 or serotonin receptors or cyclooxygenase. Intravenous infusion of a 0.4 mg of L-652,731 bolus inhibited the hypotensive responses from subsequent PAF infusions by a maximum of 72% and with a half-life duration of action of 2.5 hr. Intravenous infusion of L-652,731 results in an immediate 87% reversal of the extreme hypotension induced by a previous endotoxin injection. Thus, with its good p.o. activity, long duration of action and specificity in inhibiting PAF-induced responses in vivo, L-652,731 is a very useful tool in determining the role of PAF in mediating different pathophysiological processes.     

Effects of verapamil on thromboxane synthesis and pulmonary hypertension in sheep

Infusion of plasma containing zymosan-activated complement into sheep produces leukopenia, pulmonary leukostasis, pulmonary hypertension, hypoxia and increased plasma levels of thromboxane. We investigated the effects of the calcium channel blocking agent verapamil in this system using conscious sheep. Verapamil in 5 mg and 10 mg doses was administered by intravenous infusion prior to an infusion of autologous plasma containing zymosan-activated complement. Pretreatment with verapamil inhibited thromboxane synthesis, the rise in pulmonary artery pressure and the hypoxia without affecting the transient leukopenia. These effects are similar to those previously demonstrated with nonsteroidal antinflammatory drugs, suggesting that verapamil is acting at one or more early steps of the arachidonic acid cascade in addition to its influence on the calcium-sensitive protein interactions involved in smooth muscle function.     

Enhanced glomerular thromboxane A2 mediates some pathophysiologic effect of platelet-activating factor in rabbit nephrotoxic nephritis: evidence from biochemical measurements and inhibitor trials

Previous studies have shown that platelet-activating factor (PAF) receptor blocking has a protective effect on rabbit nephrotoxic nephritis (NTN). We examined whether arachidonic acid (AA) metabolism is altered in NTN and whether a PAF receptor antagonist has any influence on such changes. Rabbits injected with anti-glomerular basement membrane antiserum in the heterologous phase had a markedly increased glomerular thromboxane B2 (TxB2) production level, whereas no changes have been detected in glomerular 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and prostaglandin E2 (PGE2). During the autologous phase of the disease, the glomerular TxB2 level was even higher than in the heterologous phase. The level of 6-keto-PGF1 alpha was significantly lower than normal, and the level of PGE2 was unchanged in respect to the basal values. The use of L-652,731 (a specific PAF receptor antagonist) reversed the abnormal generation of AA metabolites at glomerular level both in the heterologous and autologous phase of the disease. The effect of L-652,731 on AA metabolism is likely to be an indirect result of the PAF receptor blocking, because L-652,731 given to normal rabbits had no direct effect on glomerular AA metabolism. To assess whether the beneficial effect of L-652,731 in NTN is at least in part mediated by its capability of suppressing the excessive intrarenal synthesis of thromboxane A2 (TxA2), we compared the effect of L-652,731 with that of a selective TxA2-synthase inhibitor (FCE-22178). FCE-22178 ameliorated the morphologic expression of rabbit NTN and reduced function deterioration. The protective effect of L-652,731 on proteinuria in the autologous phase and on glomerular filtration rate in both phases was superior to that of FCE-22178. We conclude that an excessive intraglomerular synthesis of TxA2 occurs in rabbit NTN that can play a role in renal function deterioration. Both a specific PAF receptor antagonist and a TxA2-synthase inhibitor reduced the exaggerated TxA2 synthesis and favorably influenced the evolution of the disease.     

Primed stimulation of isolated perfused rabbit lung by endotoxin and platelet activating factor induces enhanced production of thromboxane and lung injury.

Bacterial sepsis often precedes the development of the adult respiratory distress syndrome (ARDS) and bacterial endotoxin (LPS) produces a syndrome similar to ARDS when infused into experimental animals. We determined in isolated, buffer-perfused rabbit lungs, free of plasma and circulating blood cells that LPS synergized with platelet activating factor (PAF) to injure the lung. In lungs perfused for 2 h with LPS-free buffer (less than 100 pg/ml), stimulation with 1, 10, or 100 nM PAF produced transient pulmonary hypertension and minimal edema. Lungs perfused for 2 h with buffer containing 100 ng/ml of Escherichia coli 0111:B4 LPS had slight elevation of pulmonary artery pressure (PAP) and did not develop edema. In contrast, lungs exposed to 100 ng/ml of LPS for 2 h had marked increases in PAP and developed significant edema when stimulated with PAF. LPS treatment increased capillary filtration coefficient, suggesting that capillary leak contributed to pulmonary edema. LPS-primed, PAF-stimulated lungs had enhanced production of thromboxane B2 (TXB) and 6-keto-prostaglandin F1 alpha (6KPF). Indomethacin completely inhibited PAF-stimulated production of TXB and 6KPF in control and LPS-primed preparations, did not inhibit the rise in PAP produced by PAF in control lungs, but blocked the exaggerated rise in PAP and edema seen in LPS-primed, PAF-stimulated lungs. The thromboxane synthetase inhibitor dazoxiben, and the thromboxane receptor antagonist, SQ 29,548, similarly inhibited LPS-primed pulmonary hypertension and edema after PAF-stimulation. These studies indicate that LPS primes the lung for enhanced injury in response to the physiologic mediator PAF by amplifying the synthesis and release of thromboxane in lung tissue.     

Evidence for the role of platelet-activating factor in immune complex vasculitis in the rat.

These studies were designed to determine the role of platelet-activating factor (PAF) in the pathogenesis of immune complex (IgG) induced dermal vasculitis in the rat. In vitro, very low (pM and nM) concentrations of PAF "primed" rat neutrophils for enhanced O2-. responses to IgG immune complexes while higher concentrations were directly stimulatory. The PAF receptor antagonist, L-652,731, blocked responses (O2-. production and enzyme release) of rat neutrophils stimulated with PAF but did not block responses triggered by immune complexes, formyl chemotactic peptide or opsonized zymosan particles. When L-652,731 was added to the antibody employed in the reversed passive Arthus reaction, the injury resulting from immune complex-induced vasculitis was significantly attenuated. In order to determine if in vivo protection provided by L-652,731 was related to neutrophils, we developed a new model in which rats are systemically depleted of neutrophils by cyclophosphamide and then locally reconstituted with intact neutrophils in a manner that allows restoration of immune complex-induced vascular injury. With this model, we demonstrated that the effects of neutrophil reconstitution are substantially diminished if the cells are pretreated with L-652,731 and then washed. By priming neutrophils with substimulatory concentrations of PAF, we have also provided in vivo evidence that neutrophil priming can increase the magnitude of vascular injury. These data provide evidence that vascular injury associated with immune complex dermal vasculitis is related to availability of PAF receptors on neutrophils, suggesting a mechanism through which PAF may function as a mediator in the pathogenesis of immune complex vasculitis.     

Thrombin-induced leukopenia and thrombocytopenia are attenuated by PAF antagonist WEB 2086

Thrombin has been shown to increase pulmonary transvascular permeability in vivo. This permeability change appears to be dependent on polymorphonuclear leukocytes (PMNs). In vitro, thrombin has been demonstrated to increase PMN adherence to endothelial cells coincident with generation of platelet activating factor (PAF) by endothelial cells. These observations have led to the suggestion that PAF mediates, in part, the attachment of PMNs to endothelial cells. We examined this hypothesis in vivo and in vitro with a specific PAF receptor antagonist, WEB 2086. Prior infusion of WEB 2086 into conscious sheep significantly attenuated the drop in peripheral blood PMN counts observed during and after infusion of alpha-thrombin (30 NIH U/kg). These data suggest that WEB 2086 prevented PMN margination on endothelial cells. WEB 2086 also attenuated the thrombocytopenia seen after thrombin infusion and ameliorated the thrombin-induced hypoxemia and hemoconcentration. WEB 2086 did not affect the thrombin-induced hemodynamic response, the degree of intravascular coagulation as assessed by fibrin degradation product generation, or thromboxane B2 generation. In vitro, WEB 2086 prevented the augmented adherence of sheep PMNs to sheep endothelial cell monolayers after thrombin stimulation. The results of the present study are consistent with the hypothesis that PAF mediates, at least in part, thrombin-induced leukopenia and thrombocytopenia in vivo.     

Platelet-activating factor and arachidonic acid metabolites mediate tumor necrosis factor and eicosanoid kinetics and cardiopulmonary dysfunction during bacteremic shock

OBJECTIVE: Platelet-activating factor (PAF) and eicosanoids are putative mediators of septic shock that are associated with release of tumor necrosis factor (TNF). The purpose of this investigation was to a) examine temporal patterns of TNF and arachidonic acid metabolite release in a porcine model of bacteremic shock and b) selectively block PAF, thromboxane A2, prostacyclin, and leukotrienes to determine the relationships among these inflammatory response mediators and the alterations in cardiorespiratory dysfunction for which they are required. DESIGN: Prospective, nonrandomized, controlled trial. SETTING: Laboratory at a university medical center. SUBJECTS: Thirty-four female Yorkshire swine. INTERVENTIONS: Animals were divided into six experimental groups: five septic groups receiving an infusion of Aeromonas hydrophila at 0.2 mL/kg/hr, gradually increasing to 0.4 mL/kg/hr over 4 hrs. Each of four septic groups was pretreated with a specific mediator inhibitor (PAF receptor antagonist, n = 6; prostacyclin antibody, n = 5; leukotriene synthesis inhibitor, n = 5; and thromboxane receptor antagonist, n = 6). One septic group (n = 6) received no mediator inhibitor and served as a septic control, and one anesthesia control group (n = 6) received no intervention. MEASUREMENTS AND MAIN RESULTS: PAF receptor blockade significantly increased systemic hypotension and mixed venous oxygen saturation and decreased pulmonary artery pressure, oxygen extraction and consumption, hemoconcentration, and levels of TNF and eicosanoids. Leukotriene inhibition increased mean arterial pressure, pulmonary and systemic vascular resistance indices, and arterial and mixed venous oxygen saturation and reduced pulmonary hypertension, oxygen delivery, oxygen extraction, oxygen consumption, and all measured mediators. Thromboxane receptor blockade lowered TNF and leukotriene levels, ameliorated systemic and pulmonary vasoconstriction, and significantly increased arterial and tissue oxygenation compared with septic controls. Prostacyclin antagonism reduced prostacyclin plasma concentrations, arterial hypoxemia, and oxygen consumption during sepsis and increased circulating leukotriene B4. CONCLUSIONS: Elevations in plasma TNF predictably precede peak levels of eicosanoids in this model. PAF, leukotrienes, and thromboxane A2 are necessary for pulmonary hypertension during bacteremia. Systemic hypotension and increased vascular permeability are mediated by both leukotrienes and PAF. There are complex interactions among mediators during sepsis and further studies are required to define these relationships.     

Thromboxane synthesis inhibition reverses group B Streptococcus-induced pulmonary hypertension

Group B Streptococcus (GBS) sepsis in humans may cause the persistent pulmonary hypertension syndrome. Infusions of GBS in animals elevate pulmonary artery pressure (PAP) and resistance and are associated with elevated thromboxane levels. We investigated the hemodynamic effects of the specific thromboxane synthesis inhibitor, dazmegrel, in a piglet model of GBS-induced pulmonary hypertension. PAP rose from 22 +/- 6 to 42 +/- 11 (SD) mm Hg during infusion of heat-killed GBS; pulmonary vascular resistance increased from 1440 +/- 400 to 4000 +/- 1040 dyne X sec/cm5. No significant changes in cardiac output, mean arterial pressure, or left atrial pressure were noted. Treatment with 1 mg/kg of dazmegrel resulted in a rapid return of PAP and resistance to control values. No other hemodynamic effects of either bacteria or drug were observed despite continued infusion of GBS.     

Protective effect of BN 50739, a new platelet-activating factor antagonist, in endotoxin-treated rabbits

Platelet-activating factor (PAF) has been demonstrated in the circulation and organs of animals exposed to gram negative endotoxins, whereas PAF antagonists have been shown to exhibit some efficacy in modifying the course of endotoxemia. In this study we evaluated BN 50739, a novel specific PAF antagonist, for its capacity to block PAF or lipopolysaccharide endotoxin (LPS)-mediated effects in rabbits. Pretreatment with BN 50739 (3 and 10 mg/kg i.p.) inhibited PAF (500 pmol/kg i.v.)-induced thrombocytopenia, leukopenia and plasma thromboxane B2 elevation in a dose-dependent manner. The inhibitory effect lasted 3.5 to 4.5 hr. BN 50739 (10 mg/kg) prevented the early phase of LPS (50 micrograms/kg i.v.)-induced thrombocytopenia and thromboxane B2 elevation, and reduced the 24-hr mortality rate from 75 to 22% (P less than .05). Post-treatment with BN 50739 increased the 10-hr survival rate from 33 to 87% (P less than .05); however, it had no effect on the 24-hr mortality. BN 50739 did not affect LPS-induced leukopenia or the elevation in plasma tumor necrosis factor. Our data support possible therapeutic efficacy of PAF antagonists in septic shock despite their inability to prevent the generation of tumor necrosis factor.     

Role of platelet activating factor in endotoxemic acute renal failure in the male rat

We have developed a model of endotoxemic acute renal failure in the anesthetized male rat in which acute endotoxin infusion induces renal vasoconstriction and decreased glomerular filtration rate (GFR) in the absence of systemic hypotension. Because increased levels of platelet activating factor (PAF) have been observed in experimental models of endotoxemia, we pretreated rats with PAF receptor antagonist BN 52021 or SRI 63-675 before administering endotoxin. Compared with treatment with vehicle, treatment with BN 52021 led to significant preservation of RBF, GFR, and urine flow rate. Pretreatment with SRI 63-675 resulted in significant improvement in RBF while completely preventing the fall in GFR and urine flow rate. Intrarenal artery infusion of exogenous PAF (30 ng/kg/min) resulted in renal vasoconstriction, decreased GFR, and oliguria. These effects were also prevented by pretreatment with SRI 63-675. Thus, the adverse renal hemodynamic effects of endotoxemia were blunted or prevented by pretreatment with PAF receptor antagonists. We conclude that endogenously produced PAF is an important mediator of endotoxemic acute renal failure.     

The effect of thromboxane synthetase inhibition on cardiopulmonary function during endotoxemia in sheep

The early pulmonary hypertension seen with endotoxin (lipopolysaccharide) has been reported as resulting from the release of thromboxane A2. We studied the cardiopulmonary response to endotoxin in sheep with and without treatment with a thromboxane synthetase inhibitor, OKY-046. The animals were implanted with instruments for crystallographic dimension analysis of the left ventricle and measurement of left ventricular, aortic, left atrial, and pulmonary arterial pressures and cardiac index. Thirteen sheep received 1.0 micrograms/kg of Escherichia coli endotoxin with (n = 6) and without (n = 7) OKY-046 (10 mg/kg bolus, then 10 micrograms/kg/min). OKY-046 prevented the increase in pulmonary arterial pressure and decrease in cardiac index usually seen during the early phase of endotoxemia. Between 8 and 12 hours after the administration of endotoxin, cardiac index increased from 6.4 +/- 0.8 to 8.4 +/- 0.8 L/min/m2. Concomitantly, the end-systolic pressure/diameter relationship (a sensitive myocardial contractility index) significantly decreased from 14.7 +/- 0.6 to 7.7 +/- 0.7 mm Hg/mm. Another index of the left ventricular contractility, the maximum rate of pressure rise was also reduced. OKY-046 prevented decreases in end-systolic pressure/diameter relationship and maximum rate of pressure rise.     

Airway hyperresponsiveness induced by platelet-activating factor: role of thromboxane generation

The effect of platelet-activating factor (acetyl glyceryl ether phosphorylcholine; PAF), a potent inflammatory mediator, on airway responsiveness was studied. In six dogs airway responsiveness was determined by measuring the provocative concentration of acetylcholine aerosol that increased total pulmonary resistance (RL) by 5 cm H2O X L-1 X s, before and after inhalation of PAF (1 mg). PAF caused a 2.3-fold increase in RL (P less than .001) that lasted approximately 30 min. Airway hyperresponsiveness was maximal at 3 hr (mean 3.7-fold increase, P less than .001), persisted at 6 hr (P less than .005) and disappeared by 24 hr. Inhalation of 0.9% NaCl had no effect on RL or on responsiveness. PAF caused an 8-fold increase in neutrophil recovery in bronchoalveolar lavage fluid at 3 hr. OKY-046, a thromboxane synthetase inhibitor, inhibited PAF-induced bronchoconstriction and hyperresponsiveness but did not alter the increase in neutrophil recovery. We tested the specificity of the effect of OKY-046 on the release of cyclooxygenase products from canine neutrophils in vitro; OKY-046 suppressed PAF-induced generation of thromboxane and caused a small increase in prostaglandin F2 alpha release. The studies suggest that the airway hyperresponsiveness induced by PAF may depend on thromboxane generation.     

Site-specific effect of guanosine 3',5'-cyclic monophosphate phosphodiesterase inhibition in isolated lamb lungs

OBJECTIVE: To determine the effect of combining inhaled nitric oxide (NO) with an inhibitor of guanosine 3',5'-cyclic monophosphate-specific phosphodiesterase on total and segmental lung resistances. STUDY DESIGN: A controlled laboratory study in isolated blood-perfused lungs prepared from lambs. SETTING: Animal research facility affiliated with a university teaching hospital. SUBJECTS: Five newborn lambs at <48 hrs of life. INTERVENTIONS: Isolated blood-perfused lungs were prepared and treated with indomethacin (40 microg/mL) to inhibit prostaglandin synthesis. After a baseline period of normoxia (28% oxygen), pulmonary hypertension was induced with the thromboxane mimetic U46619 (0.1-0.4 microg/kg/min). During pulmonary hypertension, lungs were studied with inhaled NO only, with infusion of zaprinast only (0.25 mg/kg bolus and 0.05 mg/kg/min infusion), and with a combination of the two. For each study condition, the total pressure decrease across the lung was measured, and the inflow-outflow occlusion technique was used to partition the total pressure gradient measured at constant flow (100 mL/kg/min) into gradients across relatively noncompliant large arteries and veins and more compliant small arteries and veins. MEASUREMENTS AND MAIN RESULTS: U46619 infusion produced significant pulmonary vasoconstriction. The combination of inhaled NO and zaprinast decreased the total pressure decrease across the lung significantly more than NO alone. This effect was primarily attributable to a significantly greater decrease in gradient across the small artery segment after inhaled NO and zaprinast compared with NO alone. CONCLUSIONS: Guanosine 3',5'-cyclic monophosphate phosphodiesterase inhibition with zaprinast enhances the effect of inhaled NO, particularly in conditions in which small arteries represent the site of resistance. Phosphodiesterase inhibition may be a promising adjunct to inhaled NO for the treatment of persistent pulmonary hypertension.     

Biochemical and pharmacological characterization of L-659,989: an extremely potent, selective and competitive receptor antagonist of platelet-activating factor

L-659,989 [trans-2-(3-methoxy-5-methylsulfonyl-4-propoxy-phenyl)-5-(3,4,5- trimethoxyphenyl)tetrahydrofuran] is a p.o. active and extremely potent, selective and competitive platelet-activating factor (PAF) receptor antagonist. It inhibited [3H]PAF binding to either rabbit platelet or rabbit polymorphonuclear leukocyte membranes with an equilibrium inhibition constant (Kl) of 1.1 nM; whereas in human platelet, human polymorphonuclear leukocyte or human lung membranes, L-659,989 was about 10 times less potent with a Kl of 14.3 nM. The structural specificity of L-659,989 was demonstrated by the low activity of its cisisomer which was about 100 to 200 times less potent, also the (-)-L-659,989 was found to be 20- to 30-fold more potent than (+)-L-659,989. In both [3H]PAF binding and PAF-induced platelet aggregation inhibition, L-659,989 was found to be a competitive inhibitor with an equilibrium dissociation constant (KB) of 1.5 and 1.7 nM, respectively, in rabbit platelets. Even up to 6 microM concentration, L-659,989 showed no inhibition on the aggregation of rabbit platelets in plasma induced by ADP, arachidonic acid, collagen or thrombin. In an in vivo model, it inhibited guinea pig bronchoconstriction induced by i.v. infusion of 75 ng/kg of PAF with ED50 values of 13 micrograms/kg i.v. and 0.5 mg/kg p.o.     

Platelet-activating factor (PAF)-induced decreases in whole-body and skeletal muscle protein synthesis

Platelet-activating factor (PAF) has been shown to reduce rat skeletal muscle amino acid uptake, which may restrict intracellular amino acid availability for protein synthesis and amino acid oxidation during endotoxemia. We investigated in rats the effect of PAF infusion on amino acid and protein metabolism by measuring (a) whole-body and tissue leucine kinetics; (b) plasma amino acid profile; and (c) muscle RNA activity (protein synthesis efficiency) and relative abundance of myofibrillar proteins. Fasted male Sprague-Dawley rats (250+/-20 g) were given a 4-h i.v. continuous infusion of L-(1-14C)-leucine to determine leucine kinetics during the infusion of PAF (2 microg/kg PAF as a priming i.v. bolus 1 h before a 4-h i.v. infusion of 2 microg/kg/h PAF) or vehicle. PAF infusion caused sustained hypotension, hyperglycemia, hematological alterations, and hyperlacticacidemia. Whole-body protein synthesis was decreased by 24% (P < 0.05) and leucine flux oxidized was increased by 23% (P < 0.05). Leucine flux was reduced, although not significantly (P = 0.07), in PAF-treated rats (n = 8) compared with controls (n = 8). PAF significantly decreased fractional protein synthesis in the rectus abdominus (33%), soleus (30%), and extensor digitorum longus (26%) muscles, but not in the liver. Plasma branched-chain amino acid levels decreased (approximately 30%, P < 0.05) in PAF-treated rats. Muscle RNA activity was 32% lower and myosin relative abundance declined whereas actin was unchanged in PAF-treated rats. PAF induced net protein catabolism as a result of elevated leucine oxidation at the expense of protein synthesis. PAF had the cumulative effects in the skeletal muscle of (a) attenuating amino acid uptake, (b) reducing protein synthesis efficiency, (c) decreasing fractional protein synthesis rate, and (d) decreasing myosin relative abundance. Thus, PAF may be an important mediator of decreased protein synthesis in skeletal muscle during endotoxic and septic shock.     

Additive effects of inhaled nitric oxide and intravenous milrinone in experimental pulmonary hypertension

OBJECTIVE: To determine whether inhaled nitric oxide (IN0) and intravenous milrinone have additive pulmonary vasodilator effects in a rat model of pulmonary hypertension. DESIGN: Prospective, experimental study. SETTING: Animal laboratory of a university medical center. SUBJECTS: Male New Zealand White rabbits. INTERVENTIONS: Anesthetized rabbits were mechanically ventilated and instrumented for measurement of systemic mean arterial pressure (MAP), pulmonary artery pressure (PAP), left atrial pressure, and cardiac output (CO). After baseline measurements, the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (30 mg/kg iv) was administered. Pulmonary hypertension was produced by the continuous infusion of U46619, a thromboxane A2 mimetic. INO (40 ppm) was added to the inspired gas, and hemodynamic measurements were obtained before and after INO. Milrinone was administered sequentially as a 30-mg/kg bolus followed by a 3-microg/kg/min infusion, a 100-mg/kg bolus followed by a 10-microg/kg/min infusion, and a 300-mg/kg bolus followed by a 30-microg/kg/min infusion (M3). Hemodynamic measurements were obtained with and without INO at each dose of milrinone. MEASUREMENTS AND MAIN RESULTS: During U46619-induced pulmonary hypertension, INO decreased PAP and pulmonary vascular resistance (PVR) but did not affect MAP, systemic vascular resistance (SVR), or CO. Milrinone dose dependently decreased PAP, PVR, MAP, and SVR and increased CO. At each dose of milrinone, INO further decreased PVR but not SVR. M3 decreased PVR 49%, and the addition of INO decreased PVR an additional 19% so that PAP and PVR decreased to baseline values. CONCLUSIONS: Milrinone and INO both decrease pulmonary hypertension individually, and the combination produces additive effects. Combination therapy may produce potent and selective pulmonary vasodilation during the treatment of pulmonary hypertension.     

Impact of platelet activating factor on vascular responsiveness in isolated rat lungs

Platelet activating factor (PAF), a suspected mediator of acute lung injury, has been shown to potentiate contraction in isolated porcine carotid arteries. Such an action of PAF, if it occurred in the pulmonary circulation, could be of significance to the evolution of pulmonary hypertension in acute lung injury. Accordingly, we used isolated rat lungs perfused at a constant flow rate with physiologic salt solution to test the hypothesis that PAF-induced lung injury is associated with pulmonary vascular hyperresponsiveness to constrictor stimuli. PAF in concentrations of 0.1 to 10 ng/ml failed to influence pressor responses evoked by i.a. bolus injections of angiotensin II (Ang II) whereas 1 micrograms/ml of PAF significantly blunted Ang II-induced vasoconstriction. Similarly, 1 micrograms/ml, but not 0.1 ng/ml, of PAF attenuated constriction induced by ventilation with 3% O2. PAF at all concentrations tested failed to influence pressor responses evoked by i.a. bolus injections of KCI. Concentrations of PAF which blunted Ang II and hypoxic responses were associated with increased lung wet-to-dry weight ratios indicative of pulmonary edema. Another agent that provokes edema, cytochalasin B, also increased lung wet-to-dry weight ratios and blunted Ang II-, hypoxia-, and KCI-induced pressor responses. PAF delivered as i.a. bolus injections caused acute vasodilation in preparations preconstricted with Ang II but not in those preconstricted with KCI. Collectively, these observations demonstrate that PAF fails to augment and instead blunts pulmonary vascular responsiveness to pressor stimuli, possibly by mechanisms that relate to PAF-induced edema formation and/or vasodilation.     

Platelet-activating factor: evidence against a role in hypoxic pulmonary vasoconstriction

The mechanism of hypoxic pulmonary vasoconstriction (HPV) remains unknown. The platelet-activating factor (PAF) antagonist WEB 2086 attenuated HPV in the isolated lung model of the rat. We evaluated the effect of WEB 2086 on HPV in an intact animal. Pigs were anesthetized, mechanically ventilated, and had their hemodynamic variables monitored with a pulmonary artery catheter and arterial line. Cardiac output was measured by thermodilution. Initial studies determined that PAF (0.03 to 1.0 micrograms) injected iv dose-dependently increased pulmonary vascular resistance (PVR) with a 262 +/- 58% increase in PVR 5 min after a dose of 1.0 microgram. WEB 2086 (25 mg/kg iv) completely blocked the increase in PVR caused by iv PAF. Additionally, indomethacin (2 mg/kg followed by 2 mg/kg.h iv) treatment of the animals attenuated the PAF-induced increase in PVR. To evaluate the effect of WEB 2086 on HPV, animals were alternately ventilated with 21% oxygen and 10-min periods of 10% oxygen to induce HPV. After three initial control episodes of hypoxic ventilation, WEB 2086 (25 mg/kg) was injected iv and two more episodes of ventilation with 10% oxygen were given. During the three control HPV episodes the increases in PVR were 80 +/- 10%, 108 +/- 10%, and 107 +/- 22% (n = 5). After WEB 2086, the increase in PVR during two episodes of hypoxia were 96 +/- 28% and 99 +/- 19%, respectively, which was not significantly different from the control response to hypoxia. We conclude that iv PAF dose dependently increases PVR in pigs, and can be blocked by WEB 2086, that its effect is partially mediated through cyclooxygenase products, and that PAF does not appear to mediate HPV in this species.     

Nonadrenergic effects of isoproterenol in dogs with hypoxic pulmonary vasoconstriction. Possible role of prostaglandins

To determine whether the pulmonary vasodilation produced by isoproterenol is mediated solely by its beta adrenergic effects, we studied the hemodynamic responses to isoproterenol in three groups of dogs with pulmonary vasoconstriction produced by continuous ventilation with 10% oxygen: (a) hypoxia alone, (b) hypoxia and propranolol 0.3 mg/kg i.v. bolus followed by an infusion of 5 micrograms/kg per min, and (c) hypoxia after pretreatment with an inhibitor of cyclooxygenase, either indomethacin or meclofenamate 5 mg/kg s.c. twice daily for 2 d prior to study. All groups had similar values for mean pulmonary artery pressure (PAPm) and pulmonary vascular resistance (PVR) during room air and hypoxic ventilation. Isoproterenol in doses of 0.0025, 0.005, and 0.05 micrograms/kg per min produced a dose-related decline in PAPm and PVR during hypoxia in group 1. Despite beta-blockade with propranolol (group 2), isoproterenol at all three doses significantly reduced PAPm and PVR. The responses to isoproterenol were comparable in the presence or absence of propranolol; at 0.05 micrograms/kg per min the effects of isoproterenol were blunted, but not abolished, by propranolol. Similar results were observed even when five times the dose of propranolol was given. Isoproterenol at all three doses had no effect, however, on PAPm and PVR in the cyclooxygenase inhibitor-pretreated group. These data suggest that the pulmonary vasodilator effects of isoproterenol are not mediated solely by pulmonary vascular beta adrenergic receptors, and that vasodilator prostaglandins may play a role in the responses to this drug.     

Beneficial effects of TCV-309, a novel potent and selective platelet activating factor antagonist in endotoxin and anaphylactic shock in rodents.

Pharmacological profiles of a novel specific platelet activating factor (PAF) antagonist, TCV-309 (3-bromo-5-[N-phenyl-N-[2-[2- (1,2,3,4-tetrahydro-2-isoquinolycarbonyloxy)ethyl] carbamoyl]ethyl] carbamoyl]-1-propylpyridinium nitrate] and its beneficial effects in shock were examined. TCV-309 specifically inhibited PAF-induced aggregation of rabbit and human platelets, and [3H]PAF binding to rabbit platelet microsomes with IC50 values of 33, 58 and 27 nM, respectively. It was as potent as WEB 2086 and more potent than CV-6209 and CV-3988. TCV-309 did not cause hemolysis in human or rat blood due to a detergent-like action. In rats, TCV-309 selectively inhibited the PAF-induced hypotension, hemoconcentration and death with ED50 values of 2.7, 6.4 and 1.7 micrograms/kg (i.v.), respectively. TCV-309 most potently protected mice from death induced by PAF and due to anaphylactic shock with ED50 values of 2.1 and 2.6 micrograms/kg (i.v.), respectively, when compared with CV-3988, CV-6209, WEB 2086 (i.v.) and L-652731 (p.o.). TCV-309 also reversed PAF-induced hypotension and endotoxin-induced hypotension in rats with ED50 values of 3.3 and 1.2 micrograms/kg (i.v.), respectively. There was a significant linear relationship between the ability (ED50 value) of these PAF antagonists to prevent death induced by PAF and death due to anaphylactic shock in mice, and between their reversing ability (ED50 value) for the hypotension induced by PAF and endotoxin in rats. TCV-309 (100 micrograms/kg i.v.) protected rats from death induced by endotoxin. Thus, PAF may be a lethal mediator in anaphylactic shock and a hypotensive mediator in endotoxin shock in rodents.