Synthesis of dl-4-Amino-2-hydroxybutyric Acid

Male Wistar rats were given purified diets containing safflower (SAF), perilla (PER), or palm (PAL) oils with or without 1% tea polyphenols (TP) for 3 weeks, and chemical mediator releasing activity from rat peritoneal exudate cells (PEC) was measured. Histamine releasing activity was not influenced by TP, while histamine release and intracellular histamine content were significantly increased in the PAL-fed group. On the contrary, leukotriene B₄ (LTB₄) release was significantly lower in rats fed PER than in those fed SAF and PAL, and TP significantly decreased the release in all fat groups. TP also significantly inhibited the release of LTB₅, which was generated only in rats fed PER. TP significantly decreased the proportion of arachidonic acid (AA) in PEC in the SAF-fed group and that of eicosapentaenoic acid (EPA), the precursor of LTB₅ in the PER-fed group, but did not influence that of AA in the PAL- and PER-fed group. These results suggest that ingestion of TP improves type I allergic symptom through the inhibition of LT release though the inhibition by TP could not be totally explained by the reduction of substrate fatty acid.     

Dietary effects of eicosapentaenoic and docosahexaenoic acid esters on lipid metabolism and immune parameters in Sprague-Dawley rats

Sprague-Dawley rats were fed eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) ethyl esters at the 2% level for 3 weeks to clarify their effects on immune functions. In the rats fed EPA or DHA, serum cholesterol, triglyceride, and phospholipid (PL) levels were significantly lower than those in the rats fed safflower oil. In PL fractions of serum, liver, lung, splenocytes, and peritoneal exudate cells (PEC), increases in linoleic and dihomo-gamma-linolenic acid contents and a decrease in arachidonic acid (AA) content were observed in the rats fed EPA or DHA. In addition, the EPA content increased in the rats fed EPA and DHA. In the rats fed EPA or DHA, a decrease of LTB4 productivity and an increase of LTBs productivity were observed in the PEC, in response to the treatment with 5 microM calcium ionophore A23187 for 20 min. The changes in leukotriene production were more marked in EPA-fed rats than in DHA-fed rats. These results suggest that dietary EPA affects lipid metabolism and leukotriene synthesis more strongly than DHA.     

The effect of adjuvant-induced arthritis on rat leukocyte membrane phospholipid fatty acid levels following fish oil feeding.

Inflamed rats with adjuvant-induced polyarthritis showed similar incorporation of dietary eicosapentaenoic acid (EPA) into peritoneal exudate cell (PEC) membrane phospholipids compared to pair fed non-inflamed control rats. Arachidonic acid (AA) content was similar in PEC phospholipids of inflamed and control rats whereas the linoleic acid content was consistently higher in inflamed rats.     

Effect of n-3 and n-6 dietary fats on the lipoxygenase products from stimulated rat neutrophils.

Fish oil was fed to rats in combination with an equal amount of olive, sunflower or linseed (flax) oil in semisynthetic diets for 3 weeks. Following stimulation of isolated neutrophils with calcium ionophore the levels of leukotrienes (LT) were determined by HPLC. Graphical presentation of the resultant data show a direct linear relationship between LTB production and substrate concentration with no preferential conversion of n-3 or n-6 substrates. In addition the results highlighted the greater conversion of eicosapentaenoic acid (EPA) and arachidonic acid (AA) to 5-hydroxy metabolites in stimulated neutrophils. There is no suggestion in our results of inhibition of any of the enzymatic conversion steps between EPA or AA and LTB production by any of the dietary fatty acids except by altering the EPA/AA ratio in neutrophil membranes.     

Dietary Effects of Eicosapentaenoic and Docosahexaenoic Acid Esters on Lipid Metabolism and Immune Parameters in Sprague-Dawley Rats

Sprague-Dawley rats were fed eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) ethyl esters at the 2% level for 3 weeks to clarify their effects on immune functions. In the rats fed EPA or DHA, serum cholesterol, triglyceride, and phospholipid (PL) levels were significantly lower than those in the rats fed safflower oil. In PL fractions of serum, liver, lung, splenocytes, and peritoneal exudate cells (PEC), increases in linoleic and dihomo-γ-linolenic acid contents and a decrease in arachidonic acid (AA) content were observed in the rats fed EPA or DHA. In addition, the EPA content increased in the rats fed EPA and DHA. In the rats fed EPA or DHA, a decrease of LTB₄ productivity and an increase of LTB₅ productivity were observed in the PEC, in response to the treatment with 5 μM calcium ionophore A23187 for 20 min. The changes in leukotriene production were more marked in EPA-fed rats than in DHA-fed rats. These results suggest that dietary EPA affects lipid metabolism and leukotriene synthesis more strongly than DHA.     

Leukotriene B4 is an indirectly acting vasoconstrictor in guinea pig aorta via an inducible type of BLT receptor

Leukotriene B(4) (LTB(4)) is a potent leukocyte chemoattractant recently implicated in the pathogenesis of atherosclerosis. The aim of this study was to assess the effects of LTB(4) on isolated aortic preparations. Rings of guinea pig aorta were challenged with LTB(4) for recording mechanical responses and measurements of mediator release, and LTB(4) receptor (BLT(1)) expression was assessed by RT-PCR. Single concentrations of LTB(4) induced concentration-dependent contractions that were inhibited by treatment with antihistamines, indomethacin, or the thromboxane receptor antagonist BAYu3405 as well as by denudation of endothelium. In addition, LTB(4) increased the release of histamine and thromboxane in the bath. The contractions induced by LTB(4) were inhibited by either the unselective BLT receptor antagonist ONO-4057 or the selective BLT(1) receptor antagonist U-75302. Pretreatment with all-trans-retinoic acid enhanced the contractions and the release of histamine induced by LTB(4), without affecting either the contractions induced by histamine or the histamine release evoked by calcium ionophore A23187. Analysis by RT-PCR indicated the expression of a BLT(1) receptor in the guinea pig aorta and that BLT(1) receptor mRNA was upregulated after treatment with retinoic acid. These results suggest that LTB(4) contracts the guinea pig aorta via an indirect mechanism involving the release of histamine and thromboxane and that this BLT(1) receptor-mediated response can be upregulated by all-trans-retinoic acid.     

Inhibition by adenosine 3':5'-monophosphate of eicosanoid and platelet-activating factor biosynthesis in the mouse PT-18 mast cell.

A mouse spleen-derived mast cell line (PT-18) was employed to examine the mechanisms of adenosine 3':5'-monophosphate (cAMP)-mediated inhibition of antigen-induced lipid mediator biosynthesis. Specifically, we tested the hypothesis that increasing cAMP in mast cells inhibits lipid mediator biosynthesis by a mechanism independent of effects on histamine release (degranulation) or changes in cytosolic calcium concentration. Forskolin inhibited antigen-induced prostaglandin D2 (PGD2), leukotriene C4 (LTC4), and leukotriene B4 (LTB4) production by 30-50%. In contrast, forskolin had no inhibitory effect on antigen-induced increases in cytosolic calcium concentration, as monitored by the calcium indicator fura-2, or histamine release from the cells. The combination of the phosphodiesterase inhibitor isobutylmethylxanthine with forskolin inhibited the antigen-induced production of PGD2 and LTC4 by 90-100% and histamine release by about 60%. These responses were accompanied by a virtual abolition of the antigen-induced increase in cytosolic calcium. To test further the hypothesis that increasing cAMP can lead to inhibition of lipid mediator biosynthesis in the absence of effects on cytosolic calcium, we employed the calcium ionophores A23187 and ionomycin. Forskolin alone or in combination with isobutylmethylxanthine had no effect on ionophore-induced increases in cytosolic calcium but effectively inhibited leukotriene biosynthesis. In addition, increasing cyclic AMP led to an inhibition of ionophore-induced production of platelet-activating factor and liberation of arachidonic acid. These data suggest that a relatively modest increase in cAMP-dependent protein kinase activity in mast cells leads to inhibition of the lipase-catalyzed cleavage of arachidonic acid from membrane phospholipids in the absence of measurable effects on either histamine release or changes in cytosolic calcium concentration. This effect results in a selective inhibition of the biosynthesis of lipid mediators including LTC4, LTB4, PGD2, and platelet-activating factor.     

The effect of dietary eicosapentaenoic acid on arachidonic acid incorporation and metabolism in rat leukocytes

Arachidonic Acid (AA) released from membrane phospholipids by phospholipase A2 during cell activation is the major polyunsaturated fatty acid precursor in mammals for the cyclooxygenase and lipoxygenase pathways. Eicosapentaenoic acid (EPA), a major polyunsaturated fatty acid in fish oils competes with AA for these enzymes. The resulting products from EPA are generally less potent than the corresponding AA metabolites which may explain the beneficial effects of this oil in reducing thrombotic and inflammatory responses. This study compares the incorporation of 14C-AA into leukocyte phospholipids and its release and metabolism by the cyclooxygenase and lipoxygenase pathways in rats fed a 'Max EPA' fish oil rich diet (EPA group) and a hydrogenated coconut/safflower oil control diet. More than 75% of radiolabel was incorporated into leukocytes with no difference seen between dietary groups. Upon stimulation with calcium ionophore, the EPA group released significantly more radiolabelled AA than the control group. The EPA diet showed a significant increase in the formation of 5-hydroxyeicosatetraenoic acid and 6-keto-prostaglandin F1 alpha but no difference was seen in leukotriene B4 formation. The majority of radiolabel released was free AA, this being significantly higher in the EPA group than in the control. The percentage of radiolabel remaining after stimulation in phosphatidylglycerol, phosphatidylethanolamine and neutral lipids was significantly less in EPA fed rats. As the release and metabolism of endogenous AA may not be the same as 14C-AA, these results do not necessarily indicate that the mass of AA available for eicosanoid biosynthesis has been altered by the EPA diet.     

The effect of dietary eicosapentaenoic acid on arachidonic acid incorporation and metabolism in rat leukocytes

Arachidonic Acid (AA) released from membrane phospholipids by phospholipase A₂ during cell activation is the major polyunsaturated fatty acid precursor in mammals for the cyclooxygenase and lipoxygenase pathways. Eicosaspentaenoic acid (EPA), a major polyunsaturated fatty acid in fish oils competes with AA for these enzymes. The resulting products from EPa are generally less potent than the corresponding AA metabolites which may explain the beneficial effects of this oil in reducing thrombotic and inflammatory responses. This study compares the incorporation of ¹⁴C-AA into leukocyte phospholipids and its release and metabolism by the cyclooxygenase and lipoxygenase pathways in rats fed a ‘Max EPA’ fish oil rich diet (EPA group) and a hydrogenated coconut/safflower oil control diet. More than 75% of radiolabel was incorporated into leukocytes with no difference seen between dietary groups. Upon stimulation with calcium ionophore, the EPA group released significantly more radiolabelled AA than the control group. The EPA diet showed a significant increase in the formation of 5-hydroxyeicosatetraenoic acid and 6-keto-prostaglandin F_1α but no difference was seen in leukotriene B₄ formation. The majority of radiolabel released was free AA, this being significantly higher in the EPA grou than in the control. The percentage of radiolabel remaining after stimulation in phosphatidylglycerol, phosphatidylethanolamine and neutral lipids was significantly less in EPA fed rats. As the release and metabolism of endogenous AA may not be the same as ¹⁴C-AA, these results do not necessarily indicate that the mass of AA available for eicosanoid biosynthesis has been altered by the EPA diet.     

Effect of oxidized cholesterol on age-associated changes to immune parameters in spleen lymphocytes and peritoneal exudate cells derived from rats

The effects of oxidized cholesterol on immune parameters were examined by using spleen lymphocytes and peritoneal exudate cells (PEC) derived from 5-week- (Young) and 9-month-old (Adult) rats. The immunoglobulin (Ig) G and IgM production was inhibited by oxidized cholesterol in the rats of both ages when lymphocytes were exposed to 30 micrograms/ml of oxidized cholesterol for 24 hr. The intracellular IgA level was also lowered by 30 micrograms/ml of oxidized cholesterol, irrespective of age. In contrast, IgE production was significantly increased by the addition of 30 micrograms/ml of oxidized cholesterol in only young lymphocytes. Moreover, oxidized cholesterol enhanced the intracellular histamine accumulation in only adult PEC, although the total histamine level produced by PEC was similar in the rats of both ages. These results thus suggest the possibility that oxidized cholesterol can have different effects on the age-related modulation of immune functions such as Igs production and histamine release.     

The Basenji-Greyhound dog model of asthma: leukocyte histamine release, serum IgE, and airway response to inhaled antigen

To understand the immunologic mechanisms underlying the variation in airway response to inhaled Ascaris antigen (AA) in Basenji-Greyhound (BG) dogs having hyperreactive airways, we examined the relationship between leukocyte histamine release, Ascaris-specific serum IgE, changes in pulmonary resistance (RL), and decreases in dynamic compliance (Cdyn). All Ascaris-sensitive BG dogs showing airway responses to AA aerosol challenge exhibited an antigen dose-dependent release of leukocyte histamine, with total leukocyte histamine ranging from 68 to 123 ng/10(7) cells. Airway response to inhaled antigen more closely correlated with antigen dose releasing 50% total leukocyte histamine (RL, r = 0.94); Cdyn, r = 0.82), than with circulating levels of antigen-specific IgE (RL, r = 0.68; Cdyn, r = 0.69). We conclude that the airway response of sensitized BG dogs to AA inhalations is more dependent on factors affecting mediator release from pulmonary sources than circulating specific reaginic antibody.     

Phorbol ester plus calcium ionophore induces release of arachidonic acid from membrane phospholipids of a human B cell line

Binding of LA350, a lymphoblastoid human B cell line, by phorbol myristate acetate (PMA) plus a calcium ionophore, either ionomycin or A23187, produced unique alterations in the release of arachidonic acid (AA) from cellular phospholipids. After equilibrium labeling of cells with radioactive fatty acids, [14C]AA demonstrated a selective enhanced release from the cells in response to the binding of PMA plus calcium ionophore as compared to the release of [14C]stearic acid (STE), [3H]oleic acid (OLE) and [3H]palmitic acid (PAL). The major phospholipid sources of the released [14C]AA were shown to be phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. The participation of protein kinase C (PKC) in the enhanced synergistic release of [14C]AA was demonstrated by the inhibition of the release by the PKC inhibitor, staurosporine. Approximately 2-6% of the labeled AA liberated was converted to 5-hydroxyeicosatetraenoic acid by an endogenous 5-lipoxygenase. Therefore during cell activation the B cell is capable of liberating AA via a PKC-dependent mechanism, implicating AA and/or its metabolites in signal transduction.     

Phospholipase A2 activation in human neutrophils. Differential actions of diacylglycerols and alkylacylglycerols in priming cells for stimulation by N-formyl-Met-Leu-Phe

Both 1,2-diacyl- and 1-O-alkyl-2-acylglycerols are formed during stimulation of human neutrophils (PMN), and both can prime respiratory burst responses for stimulation by the chemotactic peptide, N-formyl-Met-Leu-Phe (fMLP); however, mechanisms of priming are unknown. Arachidonic acid (AA) release through phospholipase A2 activation and metabolism by 5-lipoxygenase are important activities of PMN during inflammation and could be involved in the process of primed stimulation. Therefore, we have examined the ability of diacyl- and alkylacylglycerols to act as priming agents for AA release and metabolism in human neutrophils. After prelabeling PMN phospholipids with [3H]AA, priming was tested by incubating human PMN with the diacylglycerol, 1-oleoyl-2-acetylglycerol (OAG), or its alkylacyl analog, 1-O-delta 9-octadecenyl-2-acetylglycerol (EAG) before stimulating with fMLP. fMLP (1 microM), OAG (20 microM), or EAG (20 microM) individually caused little or no release of labeled AA. However, after priming PMN with the same concentrations of either OAG or EAG, stimulation with 1 microM fMLP caused rapid (peak after 1 min) release of 6-8% of [3H]AA from cellular phospholipids; total release was similar with either diglyceride. Priming cells with OAG also enhanced conversion of released AA to leukotriene B4 (LTB4) and 5-hydroxyeicosatetraenoic acid (5-HETE) upon subsequent fMLP stimulation, but AA metabolites were not increased in EAG-primed PMN. If fMLP was replaced with the calcium ionophore A23187 (which directly causes release of AA and production of LTB4 and 5-HETE), priming by both diglycerides again enhanced release of [3H]AA, but only OAG priming increased lipoxygenase activity. Indeed, EAG pretreatment markedly reduced LTB4 and 5-HETE production. Thus, both diglycerides prime release of AA from membrane phospholipids but have opposite actions on the subsequent metabolism of AA.     

Arachidonic acid in postshock mesenteric lymph induces pulmonary synthesis of leukotriene B4.

Mesenteric lymph is the mechanistic link between splanchnic hypoperfusion and acute lung injury (ALI), but the culprit mediator(s) remains elusive. Previous work has shown that administration of a phospholipase A(2) (PLA(2)) inhibitor attenuated postshock ALI and also identified a non-ionic lipid within the postshock mesenteric lymph (PSML) responsible for polymorphonuclear neutrophil (PMN) priming. Consequently, we hypothesized that gut-derived leukotriene B(4) (LTB(4)) is a key mediator in the pathogenesis of ALI. Trauma/hemorrhagic shock (T/HS) was induced in male Sprague-Dawley rats and the mesenteric duct cannulated for lymph collection/diversion. PSML, arachidonic acid (AA), and a LTB(4) receptor antagonist were added to PMNs in vitro. LC/MS/MS was employed to identify bioactive lipids in PSML and the lungs. T/HS increased AA in PSML and increased LTB(4) and PMNs in the lung. Lymph diversion decreased lung LTB(4) by 75% and PMNs by 40%. PSML stimulated PMN priming (11.56 +/- 1.25 vs. 3.95 +/- 0.29 nmol O(2)(-)/min; 3.75 x 10(5) cells/ml; P < 0.01) that was attenuated by LTB(4) receptor blockade (2.64 +/- 0.58; P < 0.01). AA stimulated PMNs to produce LTB(4), and AA-induced PMN priming was attenuated by LTB(4) receptor antagonism. Collectively, these data indicate that splanchnic ischemia/reperfusion activates gut PLA(2)-mediated release of AA into the lymph where it is delivered to the lungs, provoking LTB(4) production and subsequent PMN-mediated lung injury.     

Green tea flavonoids inhibit the LDL oxidation in osteogenic disordered rats fed a marginal ascorbic acid in diet

Osteogenic Disorder Shionogi (ODS) rats can not synthesize ascorbic acid (AA). We have examined the capacity of green tea flavonoids (GTF) to modify low-density lipoprotein (LDL) oxidation in ODS rats with dietary AA restriction. In the first experiment, ODS rats were fed diets containing 300 (AA300 diet) or 0 (AA0 diet) mg AA/kg diets for 20 d. In comparison with the AA300 diet, the AA0 diet significantly decreased the concentrations of plasma AA and alpha-tocopherol in LDL and significantly shortened the lag time of LDL oxidation in vitro. In the second experiment, ODS rats were fed one of the following three diets: the AA300 diet, the diet containing 25 mg AA (AA25, marginal AA)/kg diet (AA25 diet), or the diet containing 25 mg AA + 8 g GTF/kg diet (AA25 + GTF diet) for 20 d. Plasma AA concentration were significantly lower in rats fed AA25 compared with AA300 but not in those fed AA25 + GTF. LDL oxidation lag time was significantly longer in rats fed AA25 + GTF compared with the other two groups. Lag time for LDL oxidation was significantly and positively correlated with LDL alpha-tocopherol (r = 0.6885, P = 0.0191). These results suggest that dietary flavonoids suppress the LDL oxidation through the sparing effect on LDL alpha-tocopherol and/or plasma AA when AA intake is marginal in the ODS rats.     

A commentary on the inhibition by retinoids of leukotriene B4 production in leukocytes

The order of potency of retinoids as inhibitors of A23187-induced production of leukotriene B4 (LTB4) in human polymorphonuclear leukocytes (PMN) was retinoic acid greater than retinal greater than retinol. However, the conversion of exogenous arachidonate (AA) to LTB4 by PMN homogenates was inhibited in the rank order retinol greater than retinal much greater than retinoic acid. The agreement between active concentrations of retinol in these two systems is consistent with this compound acting directly to inhibit AA metabolism: this is not so for the other retinoids. The order of potency for inhibition of phorbol dibutyrate (PDBu)-stimulated superoxide (O-2) production in HL60 granulocytes was retinol greater than retinoic acid much greater than retinal (inactive); neither retinol nor retinal displaced [3H]PDBu from HL60 cells. We conclude that inhibition of LTB4 production by retinoic acid and retinal is neither through inhibition of AA metabolism nor through inhibition of protein kinase C.     

In vitro effect of glutathione precursors on cytotoxicity of amino acids to human mesothelial cells.

Amino acids (AA) which were proposed as an alternative osmotically active agents in dialysates are toxic to human peritoneal mesothelial cells (HPMC) due to disturbance of the antioxidant-oxidant balance in cells by reducing level of glutathione. We assessed if the addition intracellular glutathione precursors: N-acetyl-cysteine (NAC), tioproline (TP), L--2-oxo--4-thiazolidine acid (PC), and glutathione (GSH) could reduce the cytotoxicity of AA, as measured by inhibition of cells proliferation and disorders of intracellular 86Rb transport. HPMC were obtained from omentum from nonuremic donors and cultured in in vitro conditions. The HPMC proliferation capacity was assessed indirectly by the 3H-methyl-thymidine incorporation assay. The injury to HPMC membrane integrity was assessed by the release of radioisotope molecules of 86Rb from the prelabelled cells. We have found that AA diminished the intracellular potassium (86Rb) influx. Supplementation of AA mixture with NAC enhanced the total 86Rb influx into HMC. Other precursors of intracellular glutathione (TP,PC,GSH) tested in the presence of AA significantly stimulated intracellular transport of 86Rb via Na,K-ATPase dependent channel, but the total intracellular transport of 86Rb was still lower than in control. HMC proliferation was significantly inhibited by AA what was measured by incorporation of H-metyl-tymidine. In the presence of NAC inhibition of HMC proliferation caused by AA was weaker. Our results suggest that some of intracellular glutathione precursors may reduce the disturbances of the HMC function caused by AA.     

Short-term administration of conjugated linoleic acid reduces liver triglyceride concentration and phosphatidate phosphohydrolase activity in OLETF rats

The present study explored the short-term effects of dietary conjugated-linoleic acid (CLA) on liver lipid metabolism in starved/refed Otsuka Long Evans Tokushima Fatty (OLETF) rats. Male OLETF rats (12 weeks old) were starved for 24 hours, then refed for 48 hours with either a CLA diet [7.5% CLA and 7.5% Safflower oil (SAF)] or a SAF control diet (15% SAF). The results demonstrated a 30% reduction of hepatic triglyceride (TG) concentration in the CLA group when compared to the control group. Liver cholesterol concentration was also 26% lower in the CLA fed rats. The activity of mitochondrial carnitine palmitoyltransferase, the rate-limiting enzyme of fatty acid oxidation, was moderately elevated by 1.2-fold in the livers of the CLA group when compared to the control. In contrast, phosphatidate phosphohydrolase, the rate-limiting enzyme for TG synthesis, was found to be 20% lower in the livers of the CLA-fed rats. Therefore, dietary CLA evidently lowers liver lipid concentrations through a reduced TG synthesis and enhanced fatty acid oxidation in starved/refed OLETF rats.     

Intravascular anti-IgE challenge in perfused lungs: mediator release and vascular pressor response.

Intravascular application of goat anti-rabbit immunoglobulin E (IgE) was used to stimulate parenchymal mast cells in situ in perfused rabbit lungs. Sustained pulmonary arterial pressure rise was evoked in the absence of lung vascular permeability increase and lung edema formation. Early prostaglandin (PG) D2 and histamine release into the perfusate was documented, accompanied by more sustained liberation of cysteinyl leukotrienes (LT), LTB4, and PGI2. The quantities of these inflammatory mediators displayed the following order: histamine greater than cysteinyl-LT greater than PGI2 greater than LTB4 greater than PGD2. Pressor response and inflammatory mediator release revealed corresponding bell-shaped dose dependencies. Cyclooxygenase inhibition (acetylsalicylic acid) suppressed prostanoid generation, increased LT release, and did not substantially affect pressor response and histamine liberation. BW755 C, a cyclo- and lipoxygenase inhibitor, blocked the release of cysteinyl-LT and markedly reduced the liberation of the other inflammatory mediators as well as the pressor response. The H1-antagonist clemastine caused a moderate reduction of the anti-IgE-provoked pressure rise. We conclude that intravascular anti-IgE challenge in intact lungs provokes the release of an inflammatory mediator profile compatible with in situ lung parenchymal mast cell activation. Pulmonary hypertension represents the predominant vascular response, presumably mediated by cysteinyl-LT and, to a minor extent, histamine liberation.