RANTES and macrophage inflammatory protein 1 alpha induce the migration and activation of normal human eosinophil granulocytes.

The cellular infiltrates of certain inflammatory processes found in parasitic infection or in allergic diseases consist predominantly of eosinophilic granulocytes, often in association with activated T cells. This suggests the existence of chemotactic agonists specific for eosinophils and lymphocyte subsets devoid of neutrophil-activating properties. We therefore examined four members of the intercrine/chemokine superfamily of cytokines (monocyte chemotactic peptide 1 [MCP-1], RANTES, macrophage inflammatory protein 1 alpha [MIP-1 alpha], and MIP-1 beta), which do not activate neutrophils, for their ability to affect different eosinophil effector functions. RANTES strongly attracted normal human eosinophils by a chemotactic rather than a chemokinetic mechanism with a similar efficacy as the most potent chemotactic myeloid cell agonist, C5a. MIP-1 alpha also induced eosinophil migration, however, with lower efficacy. RANTES and MIP-1 alpha induced eosinophil cationic protein release in cytochalasin B-treated eosinophils, but did not promote leukotriene C4 formation by eosinophils, even after preincubation with interleukin 3 (IL-3), in contrast to other chemotactic agonists such as C5a and formyl-methionyl-leucyl-phenylalanine (FMLP). RANTES, but not MIP-1 alpha, induced a biphasic chemiluminescence response, however, of lower magnitude than C5a. RANTES and MIP-1 alpha both promoted identical transient changes in intracellular free calcium concentration ([Ca2+]i), with kinetics similar to those induced by chemotactic peptides known to interact with G protein-coupled receptors. No cross-desensitization towards other peptide agonists (e.g., C5a, IL-8, FMLP) was observed, suggesting the presence of specific receptors. Despite its weaker eosinophil-activating properties, MIP-1 alpha was at least 10 times more potent on a molar basis than RANTES at inducing [Ca2+]i changes. Interestingly, RANTES deactivated the MIP-1 alpha-induced [Ca2+]i changes, while the RANTES response was preserved after MIP-1 alpha stimulation. MCP-1, a potent monocyte chemoattractant and basophil agonist, as well as MIP-1 beta, a peptide with pronounced homology to MIP-1 alpha, did not activate the eosinophil functions tested. Our results indicate that RANTES and MIP-1 alpha are crucial mediators of inflammatory processes in which eosinophils predominate.

Leukotrienes as mediators in tissue trauma.

A significant increase in the production of cysteinyl leukotrienes was observed after mechanical or thermal trauma in the anesthesized rat. The amount of biliary N-acetyl-leukotriene E4, which represents a suitable indicator for blood plasma leukotrienes, was used as a measure of leukotriene generation. Cysteinyl leukotrienes were rapidly eliminated from blood plasma into bile where N-acetyl-leukotriene E4 was the major metabolite. Leukotrienes were at a much lower concentration in blood plasma than in bile and differed in the pattern of metabolites. The detected amounts of leukotrienes were sufficient to induce known phenomena associated with trauma, such as tissue edema and circulatory and respiratory dysfunction. Increased leukotriene generation appears to play an important role in the pathophysiology of tissue trauma.

Dietary menhaden oil lowers plasma prostaglandins and calcium in mice bearing the prostaglandin-producing HSDM1 fibrosarcoma.

The omega 3 class of polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA, 20:5), has been shown to alter the patterns of arachidonic acid (20:4) metabolism in both in vitro and in vivo systems. To examine further the role of arachidonic acid conversion to prostaglandins (PG) in hypercalcemic mice bearing the PG-producing HSDM1 fibrosarcoma, we have performed experiments in which control and tumor-bearing animals were fed diets either low (0.1-0.2% of total fatty acid) or high (17%) in EPA. In all five experiments performed, tumor-bearing mice eating control diets had markedly elevated (average sixfold above control) plasma concentrations of 13,14-dihydro-15-keto-PGE2 (PGE2-M), while in mice bearing HSDM1 tumors and eating the EPA-enriched menhaden oil diet, the elevation was reduced to only twice control values. The increase in plasma calcium concentration (approximately 2.5 mg/dl above control) in tumor-bearing animals was also reduced significantly (P less than 0.05) to only 1.3 mg/dl above control in mice eating the diet enriched in EPA. Plasma immunoreactive hydroxy fatty acids (i12-HETE) and sulfidopeptide leukotrienes (iSRS) were not elevated in tumor-bearing mice and were unaffected by diet. The contents of PGE2, PGF2 alpha, and 6-keto-PGF1 alpha were lower in tumor tissue from animals eating the diet high in EPA, whereas the tissue contents of i12-HETE and iSRS were not altered by diet. Fatty acid analysis of liver and tumor tissue revealed marked increases in certain omega 3 fatty acids (20:5, 22:5, and 22:6) from animals eating the enriched diet. Body weights, tumor weights, and tumor histology were not significantly altered by diet. To determine whether dietary calcium played a role in the elevation of plasma calcium in mice bearing the HSDM1 tumor and the reduction of plasma calcium in animals fed EPA, we compared results in mice fed diets containing 0.80% (normal) and 0.015% (deficient) calcium. The increases in plasma calcium and PGE2-M observed in tumor-bearing mice were the same on both normal and very low calcium intakes. We conclude, in mice of the Swiss albino strain bearing the HSDM1 fibrosarcoma, that consumption of a diet enriched in EPA reduces the production of cyclooxygenase products of arachidonic acid metabolism and thereby reduces the elevation of plasma calcium concentration. Dietary enrichment with EPA did not alter the production of serologically determined lipoxygenase products of arachidonic acid.

Regulation of human eosinophil degranulation and activation by endogenous phospholipase A2.

The unique granular proteins of eosinophils may have a pathogenetic role in asthma and in the defense against parasitic infestations. However, the mechanisms regulating eosinophil degranulation are largely unknown. We examined the hypothesis that release of these proteins is regulated by endogenous activation of phospholipase A2. Human eosinophils (HE) were isolated from the peripheral blood of 42 subjects either by Percoll density separation or by negative-selection immunomagnetic fractionation. Eosinophil activation was initiated in vitro with 10(-6) M FMLP and 5 micrograms/ml cytochalasin B and was assessed by measurement of eosinophil peroxidase (EPO), leukotriene C4 (LTC4) and superoxide radical (.O2-) secretion. Treatment of HE with 100 microM mepacrine before activation blocked EPO release (2.0 +/- 0.2 vs 10.2 +/- 2.1% cell content for activated HE, P < 0.004, n = 9), .O2- generation (2.6 +/- 0.9 vs 44.2 +/- 10.8 nmol/ml per 10(6) HE, P < 0.002, n = 5), and LTC4 secretion (68.2 +/- 32.2 vs 1,125.2 +/- 526.8 pg/ml per 10(6) HE, P < 0.04, n = 8). Pretreatment of HE with 100 microM 4-bromophenacyl bromide before activation similarly blocked EPO release, .O2- generation and LTC4 secretion. Addition of AA to HE after treatment with 100 microM mepacrine and before subsequent activation reversed the inhibition of both EPO (10.4 +/- 2.2% with 1 microM AA vs 2.0 +/- 0.2% for mepacrine, n = 5, P < 0.02) and LTC4 secretion (695.1 +/- 412.9 with 10 microM AA vs 68.2 +/- 32.2 pg/ml per 10(6) HE for mepacrine, n = 8, P < 0.04), but did not reverse inhibition of .O2- generation by mepacrine. We demonstrate that secretion of preformed cytotoxic proteins and .O2- by eosinophils is regulated endogenously by phospholipase A2.

Functional characteristics of leukotriene C4- and D4-metabolizing enzymes (gamma-glutamyl transpeptidase, dipeptidase) within human plasma.

Several properties of the leukotriene C4- and leukotriene D4-metabolizing enzymes within human plasma were studied after fractionation of the plasma proteins using ammonium sulfate precipitation. Leukotriene D4-metabolizing enzymes were widely distributed among the fractions obtained. They showed different pH optima (pH 6.5, pH 7.0 and pH greater than or equal to 8.5) and revealed a different degree of thermal stability. The results indicate the presence of more than one enzyme in plasma which interacts with leukotriene D4. EDTA and L-cysteine inhibited the metabolism of leukotriene D4. Two leukotriene C4-metabolizing activities (gamma-glutamyl transpeptidases) differing in their molecular weights were detected after gel filtration. Their molecular weights were estimated to be Mr greater than or equal to 150 000 and Mr between 55 000 and 100 000.

2,3,5-Trimethyl-6-(12-hydroxy-5,10-dodecadiynyl)-1,4-benzoquinone (AA861), a selective inhibitor of the 5-lipoxygenase reaction and the biosynthesis of slow-reacting substance of anaphylaxis.

2,3,5-Trimethyl-6-(12-hydroxy-5,10-dodecadiynyl)-1,4-benzoquinone (AA861) inhibited 5-lipoxygenase of guinea pig peritoneal polymorphonuclear leukocytes (ID50, 0.8 microM). The inhibition was of competitive type. 12-Lipoxygenases and fatty acid cyclooxygenase were not affected below 10 microM. The formation of slow-reacting substance of anaphylaxis by the sensitized guinea pig lung was almost fully suppressed by the compound at 10 microM.

Metabolism of leukotriene A4 into C4 by human platelets.

Tritium-labelled leukotriene A4 is converted by a suspension of human platelets into leukotriene C4. The conversion is stimulated by reduced glutathione and is dependent on the platelet concentration. Formation of leukotriene C4 is temperature and time dependent and is destroyed by heating the platelets at 100 degrees C for 5 min. Verification of leukotriene C4 formation was obtained by conversion into leukotriene D4 during reaction of the HPLC-purified platelet-derived leukotriene C4 with commercial gamma-glutamyl transpeptidase. In separate experiments we incubated authentic tritiated leukotriene C4 with human platelets and we showed the formation of tritiated leukotriene D4, demonstrating the presence of gamma-glutamyl transpeptidase activity in these cells. This activity could be blocked by the presence of reduced glutathione in the incubation mixture. In contrast, erythrocytes converted tritiated leukotriene A4 almost exclusively into leukotriene B4. Although platelets have been reported to lack 5-lipoxygenase activity, our study demonstrates that platelets possess the necessary machinery to transform leukotriene A4 into leukotrienes C4 and D4. Our results suggest that an intracellular interaction between platelets and leukotriene A4-forming cells, e.g., polymorphonuclear leukocytes, could lead to the formation of these potent peptidolipids in the circulation.

The metabolism of leukotrienes in blood plasma studied by high-performance liquid chromatography.

The metabolism of leukotrienes (B4, C4, D4, and E4) within human plasma was studied and a simple sample preparation is presented. It was demonstrated that leukotriene E4 and leukotriene B4 were stable during incubation at 37 degrees C using the in vitro system. In contrast, leukotriene C4 was metabolized by gamma-glutamyl transpeptidase activities into leukotriene D4 which was further metabolized by dipeptidase activities of plasma into leukotriene E4. The transition state inhibitor of gamma-glutamyl transpeptidase L-serine-borate decreased the metabolism of leukotriene C4 in plasma. Dilution of plasma demonstrated that the dipeptidase was more active compared to the gamma-glutamyl transpeptidase. The metabolizing activities of plasma were functionally characterized by fractionating the plasma proteins.

An alternate mechanism for regulation of leukotriene production in leukocytes: studies with an anti-inflammatory drug, sodium diclofenac.

Sodium diclofenac, a potent cyclooxygenase inhibitor, was recently shown to inhibit arachidonic acid conversion to leukotriene products in human leukocytes. This activity was confirmed by radioimmunoassay in calcium ionophore A 23187-stimulated leukocytes isolated from the rat peritoneal cavity and human peripheral blood. Studies with rat peritoneal leukocytes revealed that this effect was not mediated by inhibition of 5-lipoxygenase or phospholipase A2, but rather through modulation of arachidonic acid uptake and release. The potency of this effect was dependent upon cell type; macrophages being more sensitive to the drug than neutrophils. In leukocytes treated with sodium diclofenac, arachidonic acid released from phospholipids in response to A 23187 challenge was reincorporated into triacylglycerols. The drug enhanced the spontaneous uptake of arachidonic acid into the cellular triacylglycerol pool and, in this manner, decreased the availability of intracellular arachidonic acid. Therefore, sodium diclofenac, in addition to inhibition of cyclooxygenase, regulates leukotriene production of inflammatory cells by a mechanism mediated in part through the redistribution of arachidonic acid in lipid pools.

Leukotriene formation by bovine polymorphonuclear leukocytes.

The leukotriene production by bovine polymorphonuclear leukocytes isolated from peripheral blood has been studied. Cells were incubated in the presence of arachidonic acid, glutathione, calcium ionophore A23187 and Ca2+. The leukotrienes then formed are leukotriene C4, leukotriene B4, two all-trans isomers of leukotriene B4 and the double dioxygenation product 12-epi-6-trans-8-cis-leukotriene B4. Leukotriene C4 is formed in such a large quantity by the bovine polymorphonuclear leukocyte that it might constitute an excellent and inexpensive source for the biosynthetic preparation of this spasmogenic leukotriene.

In vivo mediator release and degranulation of mast cells in hematoporphyrin derivative-induced phototoxicity in mice.

This study was designed to assess the role of the mast cell in the early phase of hematoporphyrin derivative (HPD)-induced phototoxicity. BALB/c mice were rendered phototoxic by i.p. injection of hematoporphyrin derivative, followed by exposure to 13.6 kJ/m2 of 400-410 nm radiation. The phototoxic response was quantified by measurement of ear thickness immediately before the irradiation, and at 0, 0.5, 1, 1.5, and 2 h after. At these time-points, determinations of serum histamine and plasma leukotriene C4 levels and histologic examination of the ears were undertaken. Mice injected i.p. with buffered saline and subsequently irradiated served as controls. In mice exposed to HPD and radiation, a maximal peak increased ear-thickness of 125.7 +/- 14.4% (mean +/- SEM) was noted at 2 h; this was associated with a net increased serum histamine of over 120% and histologic evidence of mast cell degranulation. In addition, moderate increases in plasma levels of leukotriene C4 were observed at 0 h and 1.5 h in the HPD- and irradiation-treated animals. These data provide direct evidence for the participation of mast cells in the early phase of HPD-induced phototoxicity.

Release of leukotriene C4 from human polymorphonuclear leucocytes as determined by radioimmunoassay.

Rabbits were immunized with a conjugate of leukotriene (LT) C4 and bovine serum albumin prepared by coupling the single free amino group of the hapten to the protein using gluteraldehyde. Binding of [3H]LTC4 to the antibodies obtained is inhibited by 50% with 1.5 ng LTC4. The relative cross-reaction of LTD4 is 16% and of LTC4-methyl ester 3.6%. The validity of the radioimmunoassay was demonstrated by comparison with bioassay using the isolated guinea pig ileum. Using the radioimmunoassay it could be shown that endogenous LTC4 is released in a dose-dependent manner by human polymorphonuclear leucocytes stimulated with the divalent cation ionophore A23187.

Formation of leukotriene C4 by human leukocytes exposed to monosodium urate crystals.

Monosodium urate (MSU) crystals stimulate the metabolism of arachidonic acid in mixed populations of human leukocytes. Leukocytes exposed to MSU crystals released leukotriene C4. Leukotriene C4 (LTC4) was characterized and detected by high-performance liquid chromatography (HPLC), UV absorption, bioassay with guinea pig ileum, and radioimmunoassay. Results indicate that MSU crystals stimulate the transformation of arachidonic acid and the formation of leukotriene C4 in human leukocytes; an effect inhibited by colchicine. Moreover, they suggest that LTC4 may serve as a mediator of inflammation in crystal-associated diseases.

An in vivo model for measuring antigen-induced SRS-A-mediated bronchoconstriction and plasma SRS-A levels in the guinea-pig.

1 Pharmacological modulation of antigen-induced anaphylaxis in actively sensitized guinea-pigs with intravenously administered indomethacin (10 mg/kg), pyrilamine (2.0 mg/kg) and propranolol (0.1 mg/kg) resulted in a delayed onset, slowly developing bronchoconstriction indicative of a slow-reacting substance of anaphylaxis (SRS-A) response. 2 Measurements of pulmonary mechanics on the drug-pretreated animals challenged with ovalbumin demonstrated a more prominent effect on dynamic compliance than resistance. This is consistent with the more potent effects of SRS-A on peripheral rather than central airways. 3 The slowly developing bronchoconstriction obtained after treatment with indomethacin, pyrilamine and propranolol was inhibited by the standard SRS-A antagonist, FPL 55712 and the SRS-A synthesis inhibitors, phenidone, BW 755C and nordihydroguaiaretic acid. 4 Plasma SRS-A levels were determined in guinea-pigs following antigen challenge. The appearance of SRS-A in the plasma preceded the onset of bronchoconstriction and SRS-A levels remained elevated throughout its development. Coincident with the inhibition of bronchoconstriction by the SRS-A synthesis inhibitor, phenidone, was a dose-dependent reduction in plasma SRS-A. The intravenous ED50 in each case was 4 mg/kg. 5 This model of antigen-induced SRS-A-mediated bronchoconstriction should prove useful for the in vivo evaluation and development of therapeutics which regulate the synthesis of SRS-A.

Differential inhibitory effects of auranofin on leukotriene B4 and leukotriene C4 formation by human polymorphonuclear leukocytes.

Auranofin (AF) is a newly introduced oral gold compound having antirheumatic properties, and its efficacy in the treatment of bronchial asthma is now under investigation. In this study, we examined the effects of AF on leukotriene (LT) formation by human polymorphonuclear leukocytes (PMNs) stimulated with the calcium ionophore A23187. AF inhibited LTC4 formation in a dose-dependent manner with an IC50 (concentration required to produce 50% inhibition of control) of 3.2 microM. In contrast, LTB4 formation was not prevented by AF at concentrations up to 6 microM, but it was reduced to 59 +/- 4% (mean +/- SE, N = 3) of control by an 8 microM concentration. As a next step, we explored the mechanisms of the differential inhibitory effects of AF using cell-free systems. When arachidonic acid (AA) and reduced glutathione (GSH) were used as substrates, AF inhibited LTC4 synthesis more effectively (IC50 = 14 microM) than LTB4 synthesis (IC50 = 100 microM). However, LTB4 and LTC4 syntheses from LTA4 were affected only slightly by AF within the concentrations tested (3-100 microM). These results in the cell-free systems indicate that the inhibition of LT formation was caused by a reduction of LTA4 synthesis and that the differential inhibitory effects can be ascribed to the higher Km value of glutathione S-transferase for LTA4 than that of LTA4 hydrolase in PMNs. In accordance with this hypothesis, LTC4 synthesis was more dependent than LTB4 synthesis on LTA4 concentrations within 25-100 microM, and AA-861, a 5-lipoxygenase inhibitor, caused similar differential inhibitory effects on the formation of LTs by intact PMNs. The inhibitory effect of AF on LT formation at physiological concentrations may play some role in the efficacy of this drug.

Increased plasma levels of atrial natriuretic factor, renin activity, and leukotriene C4 in chronic obstructive pulmonary disease.

We studied atrial natriuretic factor (ANF), plasma renin activity (PRA), and plasma levels of leukotrienes (LTs) B4 and C4 in 23 patients with COPD undergoing right cardiac catheterization for suspected pulmonary hypertension. Hemodynamic measurements together with concomitant ANF levels (both in venous and pulmonary artery blood and right atrial and pulmonary artery plasma levels of LTC4 and LTB4, were determined at rest (T0), after 30 min of breathing oxygen (3 L/min) (T1), and after 30 min recovering and breathing air (T2). Patients with effective exacerbation or definitive evidence of left ventricular disease, hypertension, arrhythmias, or vasodilator or diuretic therapy were excluded. Increased levels of ANF, both in peripheral venous blood (117 +/- 65 pg/ml) and the pulmonary artery (153 +/- 75 pg/ml), were found in patients with COPD, with or without pulmonary hypertension. Levels of LTC4 were also significantly increased (366 +/- 406 pg/ml) when compared with our control values. No correlations among ANF, LTC4 values, functional tests, and hemodynamic measurements were found. Brief increased levels of oxygen did not modify ANF or LTC4 plasma levels, either in patients with or without pulmonary hypertension.

Detection of leukotrienes B4, C4 and of their isomers in arterial, mixed venous blood and bronchoalveolar lavage fluid from ARDS patients.

Seven patients with the adult respiratory distress syndrome (ARDS) were studied. As a control group we used 6 surgical patients who underwent minor surgical operation (inguinal hernia). For both groups the same sample collection and analysis was used. The presence of leuktorienes (LTs) B4 and C4 and of their isomers 11-trans LTC4 and delta 6-trans-12-epi LTB4 was determined in arterial, mixed venous blood and in bronchoalveolar lavage (BAL) fluid. The samples, analysed by reverse phase high performance liquid chromatography (RP-HPLC), showed a similar chromatographic picture among ARDS patients, while the control group showed no detectable amounts of LTs in BAL or blood. The distribution of these arachidonic acid metabolites in mixed venous blood, arterial blood and BAL seems to suggest pulmonary metabolism and/or inactivation. It is suggested that these mediators act as humoral factors in pathogenesis of the ARDS.

Increased leukotriene E4 excretion during antigen-induced bronchoconstriction in allergic sheep.

The metabolism of leukotrienes (LT) in the sheep was investigated to define markers of 5-lipoxygenase involvement in allergic responses, obtainable by noninvasive techniques. Intravenous administration of 14, 15-[3H]LTC4 (0.5 microCi/kg) revealed a rapid clearance from the circulation (half time = 90 s). Circulatory metabolism was apparent, with early formation (within 1 min) of LTD4 and LTE4 shown by reverse-phase high-pressure liquid chromatography (RP-HPLC). Urinary 3H excretion comprised 10% of the original dose. [3H]LTE4 (characterized by coelution with authentic standards during RP-HPLC analysis) was observed in early urine samples. By use of a sensitive and specific RP-HPLC radioimmunoassay analysis, immunoreactive material coeluting with LTE4 was detected in urine from allergic sheep. Excretion of this material was significantly increased during antigen-induced acute bronchoconstriction in eight conscious allergic sheep [preantigen, 65.70 +/- 24.27 (SE) pg; 0-1 h postantigen, 208.00 +/- 71.10 pg, P less than 0.05], but not during late responses. However, total postantigen LTE4 excretion (37.8 - 956.1 pg/8 h) was highly correlated (r = 0.976, P less than 0.001) with the severity of bronchoconstriction (445.3 - 2,409.1% specific pulmonary resistance per hour) assessed by measurement of the area under the curve of pulmonary function plotted against time. These findings represent an important demonstration of in vivo allergen-induced peptide LT generation in a physiologically characterized animal model of prolonged allergic bronchoconstriction and further substantiate an important role for LT in this model of allergic asthma.

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.

Neurotensin elevates hematocrit and plasma levels of the leukotrienes, LTB4, LTC4, LTD4 and LTE4, in anesthetized rats.

The IV injection of neurotensin (NT) into anesthetized rats produced a marked increase in hematocrit, labored breathing and peripheral blood stasis with cyanosis. This effect could also be produced by the NT-related peptides, neuromedin-N and xenopsin; however, it was not observed when nine other biologically active peptides, including bradykinin and substance P, were tested. Associated with these responses were increases in the plasma levels of histamine (measured radioenzymatically) and the leukotrienes, LTB4, LTC4, LTD4, and LTE4 (measured by RIA and HPLC). The increment in hematocrit after varying doses of NT correlated to the increase in plasma levels of LTC4. Histamine and LTC4 were both capable of elevating hematocrit when given IV; however, LTC4 was approximately 1000 times more potent than histamine and active doses of histamine elevated LTC4 levels. Furthermore, the effects of NT on plasma LTC4 and hematocrit were reduced by pretreating animals with antagonists to histamine and serotonin. Pretreatment with the specific mast cell degranulating agent, compound 48/80, also blocked NT's ability to elevate plasma levels of histamine, LTB4 and LTC4 and prevented the increased hematocrit and cyanosis. These results indicate that NT-related peptides are very potent and specific stimulators of leukotriene release and that this action is mediated by mast cells and associated with loss of plasma volume and blood stasis. A working hypothesis is that histamine, released from mast cells in response to NT, stimulates LTC4 production by other cells.