内皮源性缩血管因子花生四烯酸代谢物的作用多样性

Vascular endothelium releases vasocontracting and/or vasorelaxing substances. Here, we report the diversity of endothelium-derived vasocontracting factors (EDCFs), arachidonic acid metabolites, and discuss the pathophysiological significance. In the canine basilar artery and the rabbit intrapulmonary artery, acetylcholine-induced contractions (ACh-induced EDC) are due to endothelial thromboxane A2 (TXA2) (TXA2-type). The ACh-induced EDC in the rabbit coronary artery is due to endothelial leukotrienes (LTs) (LTs-type). In addition, in the rat coronary artery, nicotine and noradrenaline (NAd)-induced EDCs are due to endothelial COX-metabolites (COX metabolite-type). These arachidonic acid metabolites derived from endothelium (activation by vasoactive substances including ACh, NAd and nicotine) cause a contraction of vascular smooth muscle cells and may disturb the local circulation. These EDCFs (TXA2, LTs and COX-metabolites) may be involved in the pathophysiology of cardiovascular immuno-inflammatory diseases.     

Involvement of endothelial cyclo-oxygenase metabolites in noradrenaline-induced contraction of rat coronary artery

1. Noradrenaline (NA; 0.3 micromol/L) caused a contraction of the rat coronary artery that markedly increased in the presence of the nitric oxide synthase (NOS) inhibitor N(G)-nitro-l-arginine methyl ester (L-NAME; 100 micromol/L) and arachidonic acid (1 micromol/L; P < 0.05). 2. The present experiments attempted to elucidate the endothelium dependency of the contraction and to pharmacologically characterize the factors involved in the contraction induced by NA (0.3 micromol/L) in the presence of L-NAME and arachidonic acid in ring preparations of the rat coronary artery. 3. The NA (0.3 micromol/L)-induced contraction was attenuated by a chemical remover of the endothelium (saponin at concentrations of 0.1 and 0.4 mg/mL) in a concentration-dependent manner (P < 0.05). 4. The cyclo-oxygenase (COX)-1 inhibitor flurbiprofen (0.01-1 micromol/L) and the COX-2 inhibitor nimesulide (0.01-1 micromol/L) attenuated the NA-induced contraction in a concentration-dependent manner and the inhibitory effect of flurbiprofen was significantly more potent than that of nimesulide (P < 0.05). The 5-lipoxigenase inhibitor ZM-230487 (1 micromol/L) did not affect the NA-induced contraction. 5. The thromboxane A2 (TXA2) synthetase inhibitor OKY-046 (30 micromol/L) and the TXA2 antagonist S-1452 (0.1-10 micromol/L) did not attenuate the NA-induced contraction. 6. These results indicate that the contraction induced by NA in the rat coronary artery in the presence of L-NAME and arachidonic acid is endothelium dependent and is due to endothelial COX metabolites of arachidonic acid.     

Noradrenaline-induced contraction mediated by endothelial COX-1 metabolites in the rat coronary artery

Noradrenaline-induced contraction of the rat coronary arteries was significantly augmented by the presence of NG-nitro-L-arginine methyl ester (L-NAME) and arachidonic acid. The experiments in the study presented here were undertaken to characterize pharmacologically the augmented noradrenaline-induced contraction in ring preparations of rat coronary arteries. The contraction was stopped by a chemical remover of endothelium (saponin). Oxygen radical scavengers, superoxide dismutase and catalase, significantly attenuated the contraction. Cyclooxygenase-1 inhibitors (flurbiprofen, 10(-7) M) attenuated the noradrenaline-induced contraction and cyclooxygenase-2 (nimesulide, 10(-7) M) slightly attenuated the contraction. A thromboxane A2 (TXA2) synthetase inhibitor (OKY-046) and a TXA2 receptor antagonist (S-1452) did not affect the contraction. Based on these results, it was suggested that the contraction induced by noradrenaline in the rat coronary artery in the presence of L-NAME and arachidonic acid is endothelium-dependent, and that it involves reactive oxygen species and endothelial cyclooxygenase-1 metabolites of arachidonic acid.     

Are endothelium-derived hyperpolarizing and contracting factors isoprostanes?

The endothelium releases many vasoactive substances, including prostacyclin, nitric oxide and endothelin, in addition to several other factors about which little is known. The latter are referred to as 'endothelium-derived hyperpolarizing factors' (EDHFs) and 'endothelium-derived contracting factors' (EDCFs). Although there is much debate about the identities of EDHFs and EDCFs, a prevailing hypothesis is that they are cyclooxygenase-independent metabolites of arachidonic acid and many researchers associate them with free radicals. These properties are shared with isoprostanes. In this article, I compare the properties of EDHFs and EDCFs with those of the isoprostanes and propose novel experiments that might identify isoprostanes as candidate molecules for EDHFs and EDCFs.     

An endothelium-dependent contraction in canine mesenteric artery caused by caffeine.

1. We examined whether or not caffeine caused an endothelium-dependent contraction (EDC) in canine mesenteric artery and whether the endothelium-dependent contracting factors (EDCF) were arachidonic acid metabolites. 2. Caffeine (1, 3 and 10 mM) caused a transient contraction in endothelium-intact arterial strips. Removal of the endothelium significantly attenuated the caffeine (1 and 3 mM)-induced contraction. 3. Caffeine (1 mM)-induced EDC was not affected by quinacrine and manoalide (phospholipase A2 inhibitors), indomethacin and aspirin (cyclo-oxygenase inhibitors), ONO-3078 and S-1452 (thromboxane A2 antagonists) or AA-861 and TMK-777 (lipoxygenase inhibitors). 4. Caffeine (1 mM)-induced EDC was also unaffected by 50-235 (an endothelin A receptor antagonist). In addition, catalase combined treatment with superoxide dismutase, or allopurinol (antioxidant) did not affect the EDC. 5. Gro-PIP and NCDC (phospholipase C inhibitors) did not affect the caffeine-induced EDC. However, wortmannin (a phospholipase D inhibitor) and staurosporine (a protein kinase C inhibitor) attenuated the caffeine-induced EDC. 6. The present experiments demonstrate that caffeine causes an EDC in canine mesenteric artery and suggest that the EDCF mediating this response is probably not arachidonic acid metabolites, endothelin or superoxide. Instead, caffeine-induced EDC may be due to activation of the phospholipase D pathway.     

Endothelium-dependent contraction in intrapulmonary arteries: mediation by endothelial NK1 receptors and TXA2.

1. We have examined whether three natural tachykinins, substance P (SP), neurokinin A (NKA) and neurokinin B (NKB) induce an endothelium-dependent contraction (EDC) in the rabbit isolated intrapulmonary artery. 2. Removal of the endothelium almost abolished the contraction induced by SP (10(-8) M) while it did not attenuate the contraction induced by SP (10(-7) M), NKA (10(-9) - 10(-7) M) or NKB (10(-8) and 10(-7) M). 3. The EDC induced by SP (10(-8) M) was abolished by NK1 antagonists (FK-888, CP-96345, CP-99994 and SR-140333) but not by an NK2 antagonist (SR-48968). 4. The EDC induced by SP was attenuated by cyclo-oxygenase inhibitors (aspirin and indomethacin), thromboxane A2 (TXA2) synthetase inhibitors (OKY-046, KY-234 and KY-063) and a TXA2 antagonist (S-1452). 5. The rank order of potency causing endothelium-independent contraction (EIC) was NKA > NKB > SP. The EIC induced by SP (10(-7) M) was attenuated by an NK2 antagonist but not by NK1 antagonists, cyclo-oxygenase inhibitors, TXA2 synthetase inhibitors or a TXA2 antagonist. 6. In conclusion, SP at 10(-8) M induces EDC via endothelial NK1 receptors and TXA2 production, and SP at 10(-7) M induces EIC via NK2 receptors in the rabbit intrapulmonary artery.     

Mechanisms of the ergonovine-induced vasoconstriction in the rabbit main coronary artery

Summary The mean membrane potential of smooth muscle cells of the rabbit main coronary artery was-60.3 mV and an evoked action potential could be recorded in response to acetylcholine (ACh). Ergonovine or 5-hydroxytryptamine (5-HT) slightly depolarized the membrane and methysergide, a relatively selective antagonist for the 5-HT receptor, had a slight inhibitory action on these depolarizations. 5-HT produced larger contractions than ergonovine, and the concentration-effect relationships obtained for both agents shifted to higher concentrations following pre-equilibration with methysergide. ACh (10^–11 M) slightly hyperpolarized the membrane and relaxed the tissue, and high concentrations of ACh (>10^–8 M) depolarized the membrane, increased the membrane resistance and produced a contraction. ACh but not ergonovine or 5-HT, produced a contraction in Ca-free EGTA-containing solution. Following a 60 min pre-equilibration with indomethacin, the ergonovine-induced contraction was markedly enhanced but the 5-HT-or ACh-induced contractions were not. Removal of the endothelium by rubbing the vascular lumen enhanced the ergonovine-or ACh-induced contractions, but not those to 5-HT.The results obtained can be summarized as follows: ergonovine probably accelerates Ca influx and thereby produces contraction in the rabbit main coronary artery. This contraction is due to activation of the 5-HT receptor as an agonist, but the ergonovine-induced contraction is attenuated due to activation of the endothelium from which inhibitory prostanoid substances may be released. Ergonovine, therefore, may produce greater contractions in coronary arteries with damaged endothelium than in intact tissues.     

Transformation of arachidonic acid and prostaglandin endoperoxides by the guinea pig heart. Formation of RCS and prostacyclin.

The metabolism of arachidonic acid (AA) was studied in perfused isolated hearts from guinea pigs. The coronary effluent was continuously bioassayed for prostaglandin-like substances (PLS) using the cascade technique of Vane. Injections of AA in doses between 1--50 microgram into the perfusion fluid prior to the heart produced vasodilatation of the coronary vascular bed followed by a contraction of the rat stomach strip (RSS), chick rectum (CR) and rat colon (RC) as well as relaxation of the bovine coronary artery (BCA). At the higher doses of AA there was also contraction of the rabbit aorta (RbA). The same pattern of effects on the bioassay tissues was seen when prostaglandin endoperoxide (PGH2) was perfused through the heart. The response of the bank of superfused tissues provided evidence for the formation of prostacyclin (PGX or PGI2), PGE2 and PGF2alpha. Chromatographic studies showed that 6-oxo-PGF1alpha together with other prostaglandins was present in the perfusate after acidification, which suggested that the bovine coronary relaxing substance consists mainly of PGI2. Moreover, the rabbit aorta contracting substance (RCS) released in the perfusate was due to prostaglandin endoperoxides and not to thromboxane (TXA2). The formation of PLS from AA was completely blocked after treatment of the heart with the cyclo-oxygenase inhibitors, indomethacin or meclofenamic acid. Pretreatment of the heart with 15-hydroperoxyarachidonic acid (15-HPAA), a selective inhibitor of prostacyclin synthetase, inhibited the effect of AA on the coronary vasculature and diverted the metabolic transformation of AA towards PGE2 and PGF2alpha.     

Endothelium-derived contractile factors

1. Vascular endothelium releases different substances (endothelium-derived contractile factors, EDCFs), which mediate vasoconstrictor responses induced by several agents. 2. Clear differences have been reported in endothelium-dependent contractions, which suggest at least three distinct EDCFs, named EDCF1, EDCF2 and EDCF3, respectively. 3. EDCF1 is a cyclooxygenase metabolite(s) of arachidonic acid. EDCF2 is a polypeptide released from cultured endothelial cells. It has been isolated and identified as a 21-amino acid peptide called endothelin, which is described as the most potent vasoconstrictor agent known to date. EDCF3 is an unidentified contractile factor(s), which is neither EDCF1 nor EDCF2. 4. The physiological role of these endothelial contractile factors is not yet clear. However, they have been implicated in the local mechanisms involved in blood flow regulation, as well as in some pathological conditions, such as hypertension or cerebral vasospasm.     

Endothelium-dependent vasocontraction in response to noradrenaline in the canine cerebral artery

In the canine basilar artery, noradrenaline-induced contraction was markedly decreased by intimal rubbing. Scanning electron microscopic studies showed that the rubbing procedure had scrapped away the endothelial cells from the intimal surface of the artery. Prazosin (10(-7) M) reduced the noradrenaline-induced contraction in intact arteries, but did not significantly affect the contraction in the scrapped arteries. Yohimbine (10(-7) M) strongly inhibited the contraction in both intact and scrapped arteries. The endothelium-dependent vasocontraction produced by noradrenaline was inhibited by aspirin (5 X 10(-5) M), OKY-046 (10(-5) M) and ONO-3708 (5 X 10(-9) M). The present experiments provided evidence for endothelium-dependence of the vasocontraction produced by noradrenaline in canine basilar arteries, and they suggested that the endothelium-derived contracting factors might be arachidonic acid metabolites such as TXA2; they also suggested that alpha 1 adrenoceptors might be preferentially distributed on the endothelium, while alpha 2 adrenoceptors are preferentially located in smooth muscle.     

Redirection of arachidonic acid metabolism by ICI D1542: effects on thrombus formation in the coronary artery of the anaesthetized dog.

1. The effects of simultaneous redirection of arachidonic acid metabolism, by inhibition of thromboxane A2 (TXA2) synthase and blockade of the platelet thromboxane A2 receptor (TP-receptor), was examined on the rate of thrombus formation in a stenosed coronary artery with damaged endothelium in an anaesthetized dog. 2. Redirection of arachidonic acid metabolism was achieved by intravenous doses of ICI D1542, a selective and potent inhibitor of TXA2 synthase and the TP-receptor. 3. Redirection of arachidonic acid metabolism was demonstrated in whole blood, stimulated ex vivo by collagen. The ED50 for inhibition of TXB2 production was 7.1 micrograms kg-1, i.v.; there were corresponding increases in the production of the eicosanoids prostaglandin D2 (PGD2), PGE2 and PGF2 alpha. 4. Thrombus formation was inhibited by D1542 (ED50 0.55 micrograms kg-1, i.v.), but could be restarted by an intravenous infusion of adrenaline (0.2-38 micrograms kg-1 min-1, i.v.). In the presence of the maximum effective dose of D1542 (1 mg kg-1, i.v.) a 190 fold increase in the infusion rate of adrenaline was required to restore thrombus formation. 5. In the presence of D1542, removal of endoperoxide metabolites by inhibition of cyclo-oxygenase with aspirin (5 mg kg-1, i.v.) caused thrombus formation to restart, indicating the ability of the endoperoxide metabolites to inhibit thrombus formation in vivo. 6. These results indicate that, in the stenosed and damaged coronary artery of the dog, redirection of arachidonic acid metabolism by D1542 is more effective at preventing thrombus formation than inhibition of cyclo-oxygenase by aspirin.     

Gender differences and endothelium- and platelet-derived factors in the coronary circulation

1. Experiments were designed to determine whether or not interactions of platelets with coronary arteries are affected by gender or oestrogen-status. 2. Platelets and right coronary arteries were isolated from sexually mature male, female and ovariectomized pigs. Arteries were suspended in organ chambers for the measurement of isometric force. Responses of rings, with and without endothelium, were evaluated to aggregating platelets and the platelet products 5-hydroxytryptamine (5-HT) and adenosine diphosphate (ADP). 3. Release of 5-HT, thromboxane A2 (TXA2) and prostacyclin were measured from platelets. 4. Platelets caused relaxations of rings with endothelium from all pigs. However, in rings without endothelium, consistent contractions were observed only in rings from male pigs. 5. The release of 5-HT and prostacyclin was greatest from platelets of ovariectomized pigs compared with male and female pigs. Release of TXA2 was greatest from platelets of male pigs. 6. Endothelium-dependent relaxations to ADP and contractions to 5-HT were similar among the three groups. 7. These results suggest that there may be gender-specific differences in vasomotor responses to autogenous platelets but not to the platelet-derived products 5-HT and ADP. Furthermore, there are gender differences in platelets in the release of cyclo-oxygenase metabolites of arachidonic acid and 5-HT. These products could contribute to gender differences in response to injury in the coronary circulation.     

Pharmacological nature of nicotine-induced contraction in the rat basilar artery: involvement of arachidonic acid metabolites

The pharmacological nature of nicotine-induced contraction in the rat basilar artery is poorly understood. The purpose of this study was to investigate the endothelium dependency and involvement of arachidonic acid metabolites in nicotine-induced contraction in the rat basilar artery. The rat basilar artery was removed from the brain and cut into a spiral preparation. Nicotine (3x10(-5) to 10(-2) M) induced the concentration-dependent contraction in the rat basilar artery, and the maximal contraction was obtained at 3x10(-3) M. The contraction induced by nicotine (3x10(-3) M) was significantly attenuated by the presence of saponin (0.05 mg/ml, 15 min). Phospholipase C (PLC) inhibitors (NCDC and U-73122), calcium-independent phospholipase A(2) (iPLA(2)) inhibitor (BEL), cyclooxygenase-2 (COX-2) inhibitors (nimesulide, L-745,337 and celecoxib), and a 5-lipoxygenase (5-LOX) inhibitor (ZM-230487) concentration-dependently attenuated the nicotine-induced contraction. A cytosolic phospholipase A(2) (cPLA(2)) inhibitor (AACOCF3), secretory phospholipase A(2) (sPLA(2)) inhibitor (indoxam), and cyclooxygenase-1 (COX-1) inhibitors (flurbiprofen and ketoprofen) did not affect the nicotine-induced contraction. From these results, it was suggested that nicotine-induced contraction in the rat basilar artery is endothelium-dependent and is due to arachidonic acid metabolites.     

Hydrogen peroxide is an endothelium-dependent contracting factor in rat renal artery

In addition to endothelium-derived relaxing factor and hyperpolarizing factor, vascular endothelium also modulates smooth muscle tone by releasing endothelium-derived contracting factor(s) (EDCF), but the identity of EDCF remains obscure. We studied here the involvement of hydrogen peroxide (H2O2) in endothelium-dependent contraction (EDC) of rat renal artery to acetylcholine (ACh). ACh (10(-6), 10(-5), and 10(-4) M) induced a transient contraction of rat renal artery with intact endothelium in a concentration-related manner, but not in the artery with endothelium removed. In phenylephrine-precontracted renal arteries, ACh induced an endothelium-dependent relaxation response at lower concentrations (10(-8)-10(-6) M), and a relaxation followed by a contraction at higher concentrations (10(-5) M). Inhibition of nitric oxide synthase by N(omega)-nitro-L-arginine (10(-4) M) enhanced the EDC to ACh. Catalase (1000 U ml(-1)) reduced the EDC to ACh. H2O2 (10(-6), 10(-5), and 10(-4) M) induced a similar transient contraction of the renal arteries as ACh, but in an endothelium-independent manner. Inhibition of NAD(P)H oxidase and cyclooxygenase by diphenylliodonium chloride and diclofenac greatly attenuated ACh-induced EDC, while inhibition of xanthine oxidase (allopurinol) and cytochrome P450 monooxygenase (17-octadecynoic acid) did not affect the contraction. Antagonist of thromboxane A2 and prostaglandin H2 receptors (SQ 29548) and thromboxane A2 synthase inhibitor (furegrelate) attenuated the contraction to ACh and to H2O2. In isolated endothelial cells, ACh (10(-5) M) induced a transient H2O2 production detected with a fluorescence dye sensitive to H2O2 (2',7'-dichlorofluorescein diacetate). The peak concentration of H2O2 was 5.1 x 10(-4) M at 3 min and was prevented by catalase. Taken together, these results show that ACh triggers H2O2 production through NAD(P)H oxidase activation in the endothelial cells, and that ACh and H2O2 share the same signaling pathway in causing smooth muscle contraction. Therefore, H2O2 is most likely the EDCF in rat renal artery in response to ACh stimulation.     

A comparison of basal and agonist-stimulated release of endothelium-derived relaxing factor from different arteries

1. The release of endothelium-derived relaxing factor (EDRF) from rabbit aorta and pig coronary artery vessels in response to acetylcholine (ACh), substance P (SP) and the calcium ionophore A23187 has been studied by means of a bioassay cascade system. 2. A technique is described which allows the quantification of EDRF release rates from vessels of different sizes, perfused at different flow rates and with different donor-recipient transient times. 3. Rabbit aorta and pig coronary arteries, perfused at flow rates which equalize endothelial shear stress, released EDRF at a similar basal rate. 4. In response to ACh, rabbit aortic endothelium released EDRF at a significantly greater maximum rate than pig coronary artery endothelium. 5. In response to SP, both endothelium types released EDRF; SP was a significantly more potent agonist in pig coronary artery than in rabbit aorta, but maximum SP-induced EDRF release from rabbit aorta was twice that of pig coronary artery. 6. These data indicate that different endothelium types can release EDRF at widely different rates, according to the agonist used, and that the previously obtained lack of relaxant response to ACh in pig coronary artery was due to a lack of EDRF release rather than concomitant smooth muscle constriction.     

Contraction of the rat isolated aorta caused by Clostridium perfringens alpha toxin (phospholipase C): evidence for the involvement of arachidonic acid metabolism.

1. Alpha toxin produced by Clostridium perfringens contracted the rat isolated aorta and stimulated release of arachidonic acid in the tissue. 2. Quinacrine did not inhibit contraction caused by the toxin. 3. Indomethacin blocked contraction caused by the toxin in a dose-dependent manner and markedly increased levels of arachidonic acid released by the toxin. 4. The toxin-induced contraction was blocked by the thromboxane synthetase inhibitor OKY-046 and the thromboxane A2 (TXA2) antagonist ONO-3708. 5. The toxin stimulated production of TXB2 and this was blocked by pretreatment with either indomethacin or OKY-046. 6. Toxin-induced contraction was diminished by pretreating aorta with collagenase or by rubbing the intimal surface to remove the endothelium. 7. These data suggest that the contractile response to the toxin is associated with stimulation of TXA2 production from arachidonic acid released by the toxin in the endothelial cells of the aorta.     

Inhibition of thromboxane A2-induced vasocontraction by KF4939, a new anti-platelet agent, in rabbit mesenteric and dog coronary arteries

The effect of a new anti-platelet agent, KF4939, on thromboxane A2 (TXA2)-induced vasocontraction was studied in superfused rabbit mesenteric and dog coronary arteries, in comparison with the effects on the contractions evoked by KCl, noradrenaline, serotonin, angiotensin II and histamine. The calcium sources involved in the TXA2-induced vasocontraction were also examined. The TXA2-induced contraction of the rabbit mesenteric artery was partly attenuated after exposure to the calcium-free medium, but was not attenuated by nifedipine. The TXA2-induced contraction of the dog coronary artery was markedly attenuated by nifedipine. These results indicate that TXA2 utilizes both intracellularly stored calcium and an extracellular source of calcium for its vasocontraction, and the voltage-dependent calcium channel plays an important role in the dog coronary artery, but in the rabbit mesenteric artery. KF4939 inhibited the TXA2-induced contraction in both arteries. In the rabbit mesenteric artery, three times and more higher concentration than that to inhibit TXA2-induced one were required to inhibit other agonist induced contractions, KF4939 caused no alteration in the KCl-induced contraction of both arteries. Thus, KF4939 seems to be a selective inhibitor of TXA2-induced vasocontraction, and the receptor-linked mechanism may be a possible site of the TXA2 antagonistic action of KF4939.     

Experimental coronary artery thrombosis in the absence of thromboxane A2 synthesis: evidence for alternate pathways for coronary thrombosis

The actions of the thromboxane synthetase inhibitor, U-63557A, were evaluated in vivo in anesthetized open-chest dogs by inducing left circumflex coronary artery (LCCA) thrombosis with low amperage electrical stimulation (100 microA for 6 h) of the intimal surface of the vessel, and ex vivo by assessing platelet aggregation and TXB2 production. U-63557A, 10 mg/kg + 5 mg/kg/h i.v., reduced ex vivo platelet aggregation in response to arachidonic acid (0.65 mM) by 93 +/- 2% (p less than 0.05, means +/- SEM), whereas the concurrent formation of TXB2 was decreased by 78 +/- 8% (p less than 0.05). TXB2 concentration also was reduced significantly in vivo as measured from coronary sinus blood samples; however, 6-keto-PGF1 alpha concentration was unchanged from predrug values. Despite the significant inhibition of platelet aggregation and TXB2 production, thrombus mass was not reduced: control, 32.0 +/- 5.9 mg (n = 7); U-63557A, 30.8 +/- 12.0 mg (n = 5, p = NS). These results suggest that U-63557A effectively inhibits TXA2 synthetase, but lacks antithrombotic activity in our experimental model. Therefore, substances other than TXA2 may be capable of mediating occlusive coronary artery thrombosis.     

Pharmacology of prostaglandins; their profile and characterization in the body

Arachidonic acid metabolites, prostaglandins (PGs), thromboxanes (TXs), and leukotrienes (LTs), are classified as a type of autacoids. They are not stored in the cells, but stored as a precursor acid, arachidonic acid, in membrane phospholipids. Various physiological stimuli activate phospholipase A2 and release arachidonic acid, which is converted to 1 or 2 products by related enzymes within a few minutes, depending upon individual cell functions. The metabolites are readily inactivated in aqueous solution and in the body. The structures of the various metabolites are quite similar, but their pharmacological actions vary from metabolite to metabolite and some exert opposite actions to those of others. The metabolites play some pivotal roles in physiological or pathophysiological responses such as pain sensation, fever, plasma leakage and skin erythema. Thus, non-steroidal anti-inflammatory drugs, like aspirin, exert their actions through cyclooxygenase inhibition at low doses. PGE2 can be detected in the exudate of acute inflammatory models, and the simultaneous release of PGE2 with bradykinin induces a large increase in plasma leakage and triggers exudate accumulation. LTB4 induces extravasation of PMN leukocytes at the microcirculatory level and is generated in the reperfused area of infarcted cardiac tissue after ligation of rat coronary artery, but in the latter case, the leukocyte migration was not solely induced by LTB4, which was replaced by a complement component (C5a). LTC4 may be involved in ethanol injury of the gastric mucosa and endogenous PGE2 prevents this injury. The real roles of individual arachidonic acid metabolites have been gradually disclosed, but most of the roles are still yet to be clarified.     

Heterogeneous distribution of muscarinic receptors in the rabbit saphenous artery.

1. The properties of the muscarinic receptors in the rabbit saphenous artery were determined from electrical and mechanical responses of smooth muscle cells produced by acetylcholine (ACh). The inhibitory action of atropine and pirenzepine on the ACh-induced responses was also studied. 2. ACh produced a transient hyperpolarization of the membrane and inhibited the noradrenaline (NA)-induced contraction. These effects of ACh were apparent only when the endothelial cells were intact. 3. The ACh-induced transient hyperpolarization was antagonized by atropine or pirenzepine, with similar potencies (the ID50 values were about 2 x 10(-8) M for both antagonists). 4. The ACh-induced inhibition of the contraction to NA was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 2 x 10(-8) M for atropine and 10(-6) M for pirenzepine). 5. The excitatory junction potential (e.j.p.) evoked by perivascular nerve stimulation was inhibited by ACh (above 10(-8) M). The ACh-induced inhibition of the e.j.p. was antagonized by atropine more preferentially than by pirenzepine (the ID50 values were 3 x 10(-8) M for atropine and 6 x 10(-6) M for pirenzepine). 6. It is concluded that in the rabbit saphenous artery, two subtypes of muscarinic receptor (M1 and M2) are located on the endothelial cells. Stimulation of each subtype releases a different substance, i.e., a hyperpolarizing substance (M1-subtype) or a relaxant substance (M2-subtype). In prejunctional nerve terminals, the muscarinic receptors responsible for inhibiting the release of transmitter substances are of the M2-subtype.