Exogenous arachidonic acid promotes insulin release from intact or permeabilized rat islets by dual mechanisms. Putative activation of Ca2+ mobilization and protein kinase C

A number of indirect studies suggest a role for endogenous arachidonic acid (AA) in pancreatic islet function. To probe the effects of this fatty acid, AA and other polyunsaturated fatty acids were exogenously provided in Ca2+-free medium to avoid the formation of insoluble or impermeant Ca2+-arachidonate complexes. Concentrations of AA of greater than or equal to 3 microM induced potent and sustained but reversible 45Ca efflux from prelabeled intact (or digitonin-permeabilized) islets; AA also induced insulin release at somewhat higher concentrations. Other unsaturated fatty acids (erucic, oleic, linoleic, linolenic, dihomo-gamma-linolenic, eicosapentaenoic, docosahexaenoic acids) were generally less active than AA itself, indicating a structure-function relationship. The effects of AA were saturable, were inhibitable by cooling, and were not accompanied in parallel by 51Cr release or trypan blue retention, suggesting a nontoxic mechanism. At low concentrations (3.3-16 microM), at which AA does not stimulate insulin release, AA-induced 45Ca efflux was not reduced by pretreatment with ionomycin (to deplete membrane-bound Ca2+ stores), suggesting stimulation of Ca2+ extrusion through the plasma membrane. At higher concentrations (greater than or equal to 25 microM), at which AA promotes insulin release, further Ca2+ efflux was stimulated, which was blunted by pretreatment with ionomycin (as well as by trifluoperazine). Conversely, pretreatment with AA obliterated the effects of ionomycin (3 microM) on cellular Ca2+ mobilization. Thus, AA also mobilizes Ca2+ from intracellular organelles, leading to a rise in free cytosolic Ca2+ (as previously reported). AA-induced 45Ca efflux and insulin release were independent of the presence of extracellular Na+ and did not require the oxygenation of AA. Dose-response curves comparing 45Ca efflux and insulin secretion suggested that AA also stimulates hormone release by at least one other mechanism in addition to Ca2+ mobilization. This second stimulatory effect of AA could be seen in digitonin-permeabilized islets, where changes in cytosolic free Ca2+ concentration were vitiated by EGTA-containing buffers. Such secretion was also saturable and was inhibited by cooling or by spermine (which inhibits protein kinase C in the islet). Furthermore, AA-induced secretion from either intact or permeabilized islets was blunted by prolonged pretreatment of islets with a phorbol ester to deplete them of protein kinase C. Thus, exogenous arachidonic acid seems to be a complete secretagogue, having stimulatory effects both on Ca2+ mobilization and Ca2+-related secretory processes, putatively the activation of protein kinase C.

Effect of inhibitors of eicosanoid metabolism in murine collagen-induced arthritis

The dual inhibitors of arachidonic acid metabolism, Smith Kline & French (SK&F) 86002, SK&F 104351, and phenidone; the corticosteroid, dexamethasone; and the selective cyclooxygenase inhibitors, ibuprofen, indomethacin, naproxen, and piroxicam were evaluated for their antiarthritic potency in the murine, collagen-induced arthritis model. The ability of these compounds to alter the severity of arthritic lesions and to reduce serum levels of the acute-phase reactant, serum amyloid P component (SAP) were monitored. Serum concentrations of SAP were found to correlate strongly (r = 0.985) with disease severity at day 35 postimmunization. Treatment with SK&F 86002, SK&F 104351, phenidone, or dexamethasone significantly reduced disease severity, as judged by clinical score (55%, 72%, 41%, and 45% inhibition, respectively) and SAP levels (62%, 94%, 52%, and 94% inhibition, respectively) in arthritic mice. This profile of activity was not shared by the selective cyclooxygenase inhibitors, which did not uniformly inhibit disease activity by both parameters. The results suggest that dual inhibitors of 5-lipoxygenase and cyclooxygenase may prove more effective than selective cyclooxygenase inhibitors as anti-arthritic agents.

Autoradiographic observation of platelets in cerebrovascular injuries induced by arachidonic acid and its prevention by ticlopidine

The behavior of circulating 111In-labeled platelets in cerebrovascular injuries induced by arachidonic acid injection was studied. Fourteen rabbits were pretreated with the antiplatelet agent ticlopidine, and 10 rabbits were used for controls. Arachidonic acid (AA, 0.7 mg/kg) was injected into the internal carotid artery. Prior to the injection, platelets labeled with 111Inoxine were injected for autoradiography of the brain. Evans blue was injected as an indicator of blood-brain barrier disturbance. Nine control animals showed marked blue staining, and one showed slight blue staining. Seven out of the 14 pretreated animals showed slight or no staining, while 7 showed intensive staining. The distributions of the blue staining and the radioactivity showed high correlation. In the rabbits whose platelet aggregability was depressed by ticlopidine, lower blue staining as well as lower radioactivity was observed. Our findings suggest that activated platelets have an important role in the genesis of cerebrovascular injuries.

The role of macrophages and eicosanoids in the pathogenesis of experimental allergic neuritis. Serial clinical, electrophysiological, biochemical and morphological observations

Experimental allergic neuritis (EAN) can be prevented or ameliorated by global blockade of macrophages. How these cells damage peripheral nervous tissue in this autoimmune demyelinating polyneuropathy is not fully understood. Since macrophages exert a number of their inflammatory actions by the release of arachidonic acid-derived eicosanoids, we investigated the possible role of these mediators in the pathogenesis of EAN. Lewis rats with myelin-induced EAN were treated before and after onset of clinical signs. Administration of corticosteroids or of the cyclo-oxygenase inhibitors indomethacin and BW755c before the onset of neurological signs suppressed the disease, as judged by clinical assessment, serial electrophysiological testing and histological examination, while initiation of drug treatment on day 13 postimmunization still markedly attenuated the course of EAN. The selective lipoxygenase blocker nafazatrom had only a slight effect. Determination of the production by macrophages ex vivo of eicosanoids corroborated the predicted site of action of the pharmacological compounds applied. We infer that macrophage-derived proinflammatory arachidonic acid metabolites significantly contribute to functional and tissue damage in EAN. Our results may be relevant to future pharmacological treatment of the acute Guillain-Barré syndrome.

Antigen-induced contraction of guinea-pig isolated trachea: studies with novel inhibitors and antagonists of arachidonic acid metabolites

1. Responses of antigen-challenged isolated trachea from sensitized guinea-pigs were pharmacologically characterized by use of some novel inhibitors and antagonists of arachidonic acid metabolites. 2. The cyclo-oxygenase inhibitor, indomethacin, prolonged without altering the maximum response to antigen in the absence of the anti-muscarinic agent, atropine, and/or the H1-receptor blocker, mepyramine. In the presence of mepyramine, indomethacin both prolonged and increased the magnitude of the response. The selective (SQ-29548) and non-selective (L-640,035) thromboxane A2 (TXA2) antagonist and the TXA2 synthetase inhibitor, OKY-046, were essentially inactive. 3. Two novel inhibitors of 5-lipoxygenase product formation, AA-861 and L-651,896 produced complete inhibition of the response to antigen on tissues treated with atropine and mepyramine, with or without indomethacin. 4. Equimolar concentrations of the leukotriene D4 (LTD4) receptor antagonists LY-171883 greater than L-649,923 greater than or equal to L-648,051 greater than or equal to FPL-55712 blocked part of the response to antigen on tissues treated with atropine, mepyramine and indomethacin. All compounds tended to block a larger component of the response in the absence of indomethacin. A similar tendency was observed with the potent phosphodiesterase inhibitor, isobutylmethylxanthine (IBMX) but not the less potent phosphodiesterase inhibitor theophylline. 5. These results are consistent with the hypothesis that 5-lipoxygenase products acting on LTD4 receptors play only a minor role in the mediation of the contraction of guinea-pig trachea to antigen challenge. The nature of the residual contractile mediator is unknown; however, it can be completely blocked by the 5-lipoxygenase inhibitors AA-861 and L-651,896 and non-selectively blocked by the phosphodiesterase inhibitor, IBMX and non-selective LTD4 receptor antagonists, such as LY-171883.

Prostaglandins, leukotrienes, and other arachidonic acid metabolites in the pathogenesis of otitis media

Among the various inflammatory mediators of otitis media (OM), metabolites of arachidonic acid (AA) such as prostaglandins (PGs) and leukotrienes (LTs) appear to play an important role in the pathogenesis of otitis media. In an effort to investigate the role of AA metabolites on the pathogenesis of otitis media, concentrations of AA metabolites were measured in middle ear effusion (MEE) from human and paralleling animal models of otitis media and the effects of inhibitors of AA metabolism, antibiotics, and tympanostomy tube (TT) on the outcome of animal models of OM were studied. Concentrations of AA metabolites in MEE were higher in the younger age group. Levels of PGE2 and LTB4 in MEE seem to represent the degree of inflammation of OM best. Lipoxygenase products seem to be associated with the mucoid type of MEE. In the study of animal models of OM, combined models and ears with TT showed more inflammation than single models and ears without TT. Study of the therapeutic use of inhibitors of AA metabolism, penicillin, and TT showed that lipoxygenase products may be more important in the pathogenesis of OM than the cyclo-oxygenase products, and that the use of a combination of penicillin and corticosteroid produces the best results. It is clear from these studies that arachidonic acid metabolites are important inflammatory mediators in the pathogenesis of otitis media.

Why single daily dose of aspirin may not prevent platelet aggregation

The effect of different doses of aspirin on the synergistic activity of sodium arachidonate plus platelet activating factor (paf) ADP or collagen in platelet aggregation was studied in human volunteers. Aggregation studies in platelet rich plasma (PRP) showed that aspirinated platelets, unresponsive to arachidonate, when stirred with threshold concentrations of paf, ADP or collagen, reacted differently according to the dose of aspirin and the time elapsed since ingestion. After a single or daily 50 mg dose for 7-10 days independent of elapsed time until blood withdrawal, a complete synergistic activity was obtained. In PRP samples obtained 24 hours after the last aspirin intake, a complete synergistic aggregation was achieved after a single dose or after 7-10 days of 500 mg aspirin ingestion; synergistic effect did not appear when blood was drawn 2.5 hours after intake. The thromboxane B2 concentrations were very low in all samples after PRP stimulation with sodium arachidonate or paf or both. As rationale is that platelet activation in vivo occurs in response to several stimuli, the therapeutic implications of our results is that aspirin may not prevent the agonist potentiation effect when low dose or daily high dose (500mg) are administrated. This may explain the erratic results of most aspirin trials in which this drug was used to suppress platelet function.

Extract of a spice--omum (Trachyspermum ammi)-shows antiaggregatory effects and alters arachidonic acid metabolism in human platelets

An ethereal extract of omum (Trachyspermum ammi; Hindustani: ajwan)--a frequently consumed spice--was found to inhibit platelet aggregation induced by arachidonic acid (AA), epinephrine and collagen; in this respect it was most effective against AA-induced aggregation. Inhibition of aggregation by omum could be explained by its effect on platelet thromboxane production as suggested by the following experimental observation. (i) Omum reduced TxB2 formation in intact platelet preparations from added arachidonate, and (ii) it reduced the formation of TxB2 from AA-labelled platelets after stimulation with Ca2+-ionophore A23187 by a direct action on cyclooxygenase as it did not affect the release of AA from labelled platelets. An increased formation of lipoxygenase-derived products from exogenous AA in omum-treated platelets was apparently due to redirection of AA from cyclooxygenase to the lipoxygenase pathway.

Insulin directly reduces platelet sensitivity to aggregating agents. Studies in vitro and in vivo

The aim of this study was to investigate the influence of insulin on platelet function, both in vitro and in vivo. For the in vitro investigation, we evaluated whether insulin affects platelet function at a physiological hormone concentration by incubating the platelet-rich plasma (PRP) of fasting subjects with human regular insulin at the final concentration of 40 microU/ml for 30 min; we observed a significant reduction of platelet sensitivity to all the aggregating agents employed, i.e., ADP, platelet-activating factor (PAF), epinephrine, collagen, and Na+ arachidonate. To investigate whether the insulin effect on platelets is dose dependent, we incubated the PRP of fasting subjects with different concentrations of human regular insulin (40, 80, 120, and 160 microU/ml) for 5 min, and we observed that the insulin-induced reduction of platelet sensitivity to aggregating agents is a dose-dependent phenomenon. Furthermore, the comparison between the platelet responses after 5 and 30 min of incubation with insulin showed that the insulin effect on platelet aggregation is time dependent. The lack of specificity of its inhibiting activity suggests that insulin does not interfere with the initial binding of each aggregating agent at specific sites but does influence a common step of platelet aggregation. Our study rules out the possibility that insulin reduces platelet-function-modifying intraplatelet cAMP levels or thromboxane A2 production, because this hormone decreases the platelet concentrations of cAMP--a phenomenon that, per se, promotes platelet aggregation--and does not modify collagen or Na+ arachidonate--induced platelet production of thromboxane A2, measured by radioimmunoassay of its stable-metabolite thromboxane B2.(ABSTRACT TRUNCATED AT 250 WORDS)

A new 5-lipoxygenase selective inhibitor derived from Artocarpus communis strongly inhibits arachidonic acid-induced ear edema

Natural compounds isolated from the Indonesian plant, Artocarpus communis, inhibit 5-lipoxygenase of cultured mastocytoma cells. One of five compounds, AC-5-1, strongly inhibits 5-lipoxygenase with a half-inhibition dose of 5 +/- 0.12 X 10(-8) M. However, prostaglandin synthesizing activity is not inhibited until 10(-5) M. AC-5-1 is a highly selective inhibitor for 5-lipoxygenase. The AC-5-1 at 10(-5) M inhibits 96% of leukotriene C4 synthesis of mouse peritoneal cells facilitated by calcium-ionophore. Arachidonic acid-induced ear edema of mice, an in vivo inflammatory model, involving leukotriene induction, is strongly inhibited by AC-5-1 in a dose-dependent manner. The inhibition is the strongest of any inhibitors of 5-lipoxygenase reported previously. Since the natural compound AC-5-1 can selectively inhibit 5-lipoxygenase and affect in vivo inflammation, it will be interesting to investigate the role of leukotrienes on inflammation and other physiological processes.

Modulation of arteriolar blood flow by inhibitors of arachidonic acid oxidation after thermal injury: possible role for a novel class of vasodilator metabolites

To examine the contribution of arachidonic acid (AA) metabolites to the maintenance of cutaneous vasomotor tone after thermal injury, enzyme inhibitors were topically applied to the hamster cheek pouch before and after a spot burn. By use of video microscopy, blood flow was measured in adjacent arterioles that supplied the injured site. Ringer's solutions containing no drug (vehicle), indomethacin (cyclooxygenase inhibitor), BW755c (cyclooxygenase/lipoxygenase inhibitor), or ketoconazole (lipoxygenase/cytochrome P450 inhibitor) continuously suffused the entire tissue. There were no effects of these drugs on preburn blood flow at concentrations that blocked the vascular effects evoked by topical AA. In all groups, blood flow transiently increased after burn and thereafter decreased to levels that were altered by treatment. These results could not be attributed to alterations in vascular reactivity because neither the burn nor the drugs altered the vasodilation evoked by adenosine or prostacyclin. Relative to Ringer's, indomethacin had no effect, BW755c caused vasodilation, and ketoconazole caused vasoconstriction, which suggests that cytochrome P450 products might be vasoactive mediators in injured tissue. Therefore, purified synthetic compounds were compared with known vasodilators. The potency was prostacyclin greater than 12R-hydroxyeicostetraenoic acid greater than adenosine = 5,6 epoxyeicosatrienoic acid greater than AA, which supports the hypothesis that AA can be the source of a novel class of nonprostaglandin vasodilator compounds. In addition, at least one of the vasodilator responses was stereospecific. Nevertheless, the exact explanation for the differential effects of AA inhibitors on postburn blood flow is unknown.

[Inhibition of arachidonic acid-induced ear edema in the mouse with lipoxygenase-, cyclo-oxygenase- and dual inhibitors]

The ear edema of the mouse induced by local application of arachidonic acid is suitable for in vivo differentiation of lipoxygenase (LOX) and cyclooxygenase (COX) inhibitors. LOX blockers inhibit initially and plateau-like during the first hour of the course of the edema, while COX-blockers do so mostly only initially. For practice we recommend the measurement about 60 min following edema provocation. The dual blocker of LOX and COX, BW 755 c, acts like a LOX inhibitor.

Inhibitory effect of glycyrrhetinic acid derivatives on arachidonic acid-induced mouse ear oedema

The inhibitory effects of glycyrrhetinic acid and its derivatives were examined on arachidonic acid (AA)-induced ear oedema in mice. Of the compounds, dihemiphthalate derivatives of 18 beta-olean-12-ene-3 beta, 30-diol (IId, IId'), 18 beta-olean-9(11)12-diene-3 beta, 30-diol (IIIa, IIIa') and olean-11, 13(18)-diene-3 beta, 30-diol (IVa, IVa') showed a strong inhibition of ear oedema on both tropical (ID50, 1.9, 2.8 and 1.7 mg/ear, respectively) and oral (ID50, 90, 130 and 88 mg kg-1, respectively) administration. Topical ID50 values were approximately the same potency as nordihydroguaiaretic acid (ID50, 2.1 mg/ear). Given topically these compounds were also capable of inhibiting PGE2 and LTC4 formation at an early stage of AA-induced ear oedema. However, glycyrrhetinic acid (Ia) and deoxoglycyrrhetol (IIa), the fundamental skeletons of the derivatives, showed no detectable inhibition of oedema at a dose of 1 mg/ear (topical) or 200 mg kg-1 (oral). The most effective time for the topical administration of the compound IId against ear oedema was 0-30 min before AA application; this is different from dexamethasone which requires a time lag for reaction. The results suggest that the inhibitory effect of the hemiphthalate compounds (IId, IId', IIIa, IIIa', IVa and IVa') is a direct action, and does not involve the anti-inflammatory action of steroids mediated by the secondary formation of a reactive protein.

Activation of human peripheral blood monocytes by lipoproteins

Activation of human peripheral blood monocytes could enhance their attachment and or migration into the arterial intima and their various secretory and other functions, thus influencing the pathogenesis of atherosclerosis. In these experiments the authors have explored the role of lipoproteins in the activation of human blood monocytes. Monocytes were purified from citrated blood by Histopaque density gradient centrifugation and countercurrent centrifugal elutriation and cultured in DMEM in the presence of 20% acid-treated autologous serum or 100 micrograms/ml each of VLDL, LDL, Ac-LDL, and HDL. Secretion of beta-glucuronidase activity into the media was measured as a marker of activation. All of the lipoprotein density classes as well as serum stimulated secretion of beta-glucuronidase activity, with LDL and Ac-LDL having a greater influence than serum, VLDL, or HDL. Serum and LDL also stimulated secretion of prostaglandin E into the culture medium. Incubation of monocytes with serum or LDL in the presence of inhibitors of arachidonate metabolism (NDGA and indomethacin) resulted in a significant decrease in secreted and intracellular beta-glucuronidase activity, indicating a role for products of arachidonate metabolism in the activation of monocytes by lipoproteins.

Modulation of interleukin-1 activity on murine thymocytes by various inhibitors of arachidonic acid oxygenation

A variety of cyclooxygenase (FCO) and 5-lipoxygenase (5-LPO) inhibitors were tested for their ability to modulate murine thymocyte proliferation induced by IL-1 and suboptimal levels of the mitogen phytohemagglutinin (PHA). The contribution of drug toxicity to inhibition of 3H-thymidine incorporation was estimated by measuring MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) formazan production in the thymocyte cultures at the end of the assay. Cyclosporin A and dexamethasone, two positive control compounds, potently inhibited thymocyte proliferation at extremely low concentrations (0.01 and 0.001 micrograms/ml respectively), although activity roughly paralleled toxicity. In contrast, 5-LPO inhibitors (AA-861, BW-755c, and ETYA), but not selective FCO inhibitors (ibuprofen and indomethacin), suppressed lymphoproliferation at nontoxic concentrations, suggesting that products of the 5-LPO pathway may mediate the thymocyte proliferative response induced by IL-1/PHA. Attempts to counteract the suppressive activity of 5-LPO inhibitors by addition of leukotriene (LT) B4, LTC4, LTE4, 5-HETE, and 15-HETE were unsuccessful.

Human PC-3 prostate cell line DNA synthesis is suppressed by eicosatetraynoic acid, an in vitro inhibitor of arachidonic acid metabolism

When human PC-3 cells derived from a metastatic prostatic adenocarcinoma were incubated for 15 min to 4 h with the in vitro inhibitor of eicosanoid biosynthesis, eicosatetraynoic acid (ETYA) at 10-80 microM, DNA synthesis was suppressed. No reduction in cellular viability occurred, as judged by exclusion of trypan blue or unaltered release of 51Cr-labeled proteins, and the inhibition was partially reversible. Indomethacin (to 12.5 micrograms/ml) did not inhibit DNA synthesis or alter the suppression of DNA synthesis by ETYA, suggesting a role for a lipoxygenase product in this effect. Addition of leukotriene B4 (LTB4) at 10(-8) M did not reverse the inhibition of DNA synthesis produced by ETYA, nor did arachidonic acid (10(-5) - 10(-9) M) incubated with control cells mimic the effect of that agent. 3H-arachidonic acid incubated with PC-3 cells was rapidly incorporated into phospholipids and this labeling was differentially inhibited by ETYA. Positive modulation of PC-3 cellular DNA synthesis by lipoxygenase products and inhibition of their synthesis by ETYA is one attractive hypothesis with which to account for these results. Other consequences of producing a selective deficiency of arachidonic acid in cellular membrane phospholipids and even the probable substitution of ETYA for arachidonic acid could also contribute to the inhibition of DNA synthesis by ETYA.

Hydrogen-peroxide-induced arachidonic acid metabolism in the rat alveolar macrophage

Mounting evidence suggests that reactive oxygen metabolites can initiate the release and metabolism of arachidonic acid (AA). We therefore examined the effects of hydrogen peroxide (H2O2), a biologically relevant oxygen metabolite, on AA release and cyclooxygenase metabolism by the rat alveolar macrophage (AM). At concentrations between 10(-4) and 10(-3) M, which were largely noncytotoxic as assessed by chromium release, H2O2 exposure for 30 min caused a steep dose-dependent increase in AA release that peaked at approximately 5-fold stimulation at 10(-3) M H2O2. AA release induced by H2O2 was inhibited by the H2O2 scavenger catalase, but not by inactivated catalase or by scavengers of superoxide anion, hydroxyl radical, or ferric iron. An evaluation of cyclooxygenase metabolite formation by specific radioimmunoassays and high performance liquid chromatography demonstrated a greater than 2-fold increment in thromboxane (Tx)A2 (measured as TxB2) synthesis at 10(-4) M H2O2, but no increment in prostaglandin (PG) E2 synthesis. H2O2-induced TxB2 synthesis was cyclooxygenase-dependent, since it was inhibited by indomethacin (1 microM). There was no significant degradation of either PGE2 or TxB2 in AM cultures by H2O2 at concentrations to 10(-2) M. The effect of H2O2 on agonist-induced cyclooxygenase metabolism was also examined. H2O2 at 10(-4) M inhibited PGE2 synthesis induced by zymosan and A23187, whereas agonist-induced TxB2 synthesis was either unaffected (zymosan) or augmented (A23187) by H2O2. These findings suggest inhibition by H2O2 of PGE2 synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Nature of endothelium-derived relaxing factor: are there two relaxing mediators?

The role of arachidonic acid in forming endothelium-derived relaxing factor remains controversial. This controversy may be explained if more than one factor exists. To test this hypothesis, the effects of various inhibitors of arachidonic acid metabolism were studied. The perfusate from canine femoral arteries with endothelium was bioassayed with coronary artery rings without endothelium. Treatment of the perfused segment (but not the bioassay ring) with inhibitors of phospholipase A2 (quinacrine) or cytochrome P450 (metyrapone) had no effect on the basal relaxing activity of the effluent; treatment with the inhibitor of lipoxygenase, nordihydroguaiaretic acid, significantly depressed it. With increasing concentrations of acetylcholine, a biphasic concentration-relaxation curve was obtained; the enzyme inhibitors depressed or prevented the first phase but did not affect the second phase. Infusion of arachidonic acid or soybean lipoxidase directly on the bioassay ring did not cause relaxation; together, they evoked concentration-dependent relaxations. These data suggest that acetylcholine can trigger the release of two chemically different relaxing mediators from the endothelium of the canine femoral artery. One factor may be a product of lipoxygenase (or epoxigenase). The second factor is not a metabolite of arachidonic acid and may be released under basal conditions. The existence of two (or more) chemically different endothelium-derived mediators may help to explain the controversial data regarding the nature of the factor(s).

Modulation of acetylcholine release from guinea-pig Ileum myenteric plexus by arachidonic acid cascade inhibitors

Effects of arachidonic acid and mepacrine on ACh release from guinea-pig ileum myenteric plexus were investigated. Mepacrine (1-8 microM) inhibited the ACh release in a concentration-dependent manner. Arachidonic acid counteracted the inhibitory effect of mepacrine, but PGE2 did not. The inhibition induced by a combination of mepacrine and indomethacin on nicotine-induced ACh release was prevented by arachidonic acid, while that on spontaneous ACh release was prevented by arachidonic acid and PGE2 added simultaneously. The roles of arachidonic acid and PGs in the ACh release will be discussed.

C3-mediated release of prostaglandin from human monocytes: evidence for independent regulation of thromboxane and prostaglandin synthesis

Human mononuclear phagocytes (HMP) lose the capacity to respond to C3b, exogenous arachidonic acid (AA), and other stimuli of arachidonic acid (AA) metabolism when precultured for 24 hours prior to the addition of stimulus. Data presented in this report suggest that this reduced capacity is not due to loss of the cyclooxygenase enzyme, to autoinactivation of cyclooxygenase, or to release by the cells of inhibitors of PGE and TxB2 production. Additional metabolism of PGE or a shift of AA metabolism from the cyclooxygenase pathway to the lipoxygenase pathways does not occur and hence cannot account for the observation. HMP precultured for 24 hours with gamma interferon (IFN-gamma) retain the capacity to metabolize AA in response to subsequent exposure to C3b and LPS. HMP precultured with LPS or C3b respond to exogenous AA, but not to restimulation with LPS or C3b, by releasing PGE and to a lesser extent TxB2. The amounts of PGE and TxB2 produced under these conditions are reversed when compared to HMP treated with IFN-gamma LPS, or C3b at the initiation of culture. These data suggest that the production of TxB2 and PGE may be independently regulated in HMP and that IFN-gamma affects HMP AA metabolism by different mechanisms than LPS or C3b.

Antigen- and histamine H1 receptor-mediated relaxation of guinea pig isolated trachea

We have investigated the effect of challenge in vitro with specific antigen (ovalbumin) on actively sensitized guinea pig tracheal rings maximally precontracted with methacholine. Ovalbumin relaxed the trachea in a concentration-dependent fashion with a negative log ED50 value (g/ml) of 7.0 +/- 0.3. In 16 experiments, the maximum antigen-induced relaxation was 26 +/- 3% of complete relaxation induced by 10(-3) M papaverine (mean +/- S.E.M.). Antigen-induced relaxations were selectively antagonized by diphenhydramine. Similarly, histamine relaxed the precontracted tracheal smooth muscle with a negative log molar ED50 of about 4.5 and a maximum effect of 28 +/- 3% (mean +/- S.E.M., n = 20). Histamine-induced relaxations were antagonized by diphenhydramine and mepyramine but were unaffected by cimetidine, metiamide or burimamide. Dimaprit (10(-5)-10(-3) M) did not relax the precontracted trachea. Indomethacin significantly inhibited relaxation induced by both antigen and histamine. In contrast, phenidone or 5,8,11,14-eicosa-tetraynoic acid had no effect on relaxation but reversed the inhibition by indomethacin. Neither propranolol (10(-6) M) nor removing the tracheal epithelium inhibited histamine-induced relaxation. These results suggest that antigen-induced relaxation of guinea pig tracheal smooth muscle involves activation of histamine H1 receptors and can occur independently of arachidonic acid metabolism, beta-adrenoceptor activation or airway epithelium.

AD6 (8-monochloro-3-beta-diethylamino-ethyl-4-methyl-7-ethoxycarbonyl-meth oxy coumarin) inhibits the release of arachidonic acid in human platelets stimulated by thrombin

The coumarin derivative AD6 is known to inhibit platelet aggregation and release and it possesses vasodilatory properties on coronary arteries of laboratory animals. Furthermore, the inhibition of the production of TxB2 from endogenous substrates after stimulation of human platelets with collagen has been demonstrated. The present report demonstrates that AD6 inhibits the production of labeled arachidonic acid and diglycerides from phospholipids of platelets stimulated with thrombin. This effect is dose-dependent and is already evident at a concentration of the drug (25 microM) which is unable to prevent the aggregation. Apparently, AD6 inhibits the release of arachidonic acid from phosphatidylinositol and choline phosphoglycerides which are the main sources of the substrate for the synthesis of prostaglandins and thromboxanes.

Antiplatelet effect of butylidenephthalide

Butylidenephthalide inhibited, in a dose-dependent manner, the aggregation and release reaction of washed rabbit platelets induced by collagen and arachidonic acid. Butylidenephthalide also inhibited slightly the platelet aggregation induced by PAF and ADP, but not that by thrombin or ionophore A23187. Thromboxane B2 formation caused by collagen, arachidonic acid, thrombin and ionophore A23187 was in each case markedly inhibited by butylidenephthalide. Butylidenephthalide inhibited the aggregation of ADP-refractory platelets, thrombin-degranulated platelets, chymotrypsin-treated platelets and platelets in the presence of creatine phosphate/creatine phosphokinase. Its inhibition of collagen-induced aggregation was more marked at lower Ca2+ concentrations in the medium. The aggregability of platelets inhibited by butylidenephthalide could be recovered after the washing of platelets. In human platelet-rich plasma, butylidenephthalide and indomethacin prevented the secondary aggregation and blocked ATP release from platelets induced by epinephrine. Prostaglandin E2 formed by the incubation of guinea-pig lung homogenate with arachidonic acid could be inhibited by butylidenephthalide, indomethacin and aspirin. It is concluded that the antiplatelet effect of butylidenephthalide is mainly due to an inhibitory effect on cyclo-oxygenase and may be due partly to interference with calcium mobilization.