Vasomotor effects of leukotrienes C4 and D4 on cavian pulmonary artery and aorta. Characterization and mechanisms

5-lipoxygenase expression and activity in aorta from streptozotocin-induced diabetic rats

We previously reported an activation of the 5-lipoxygenase pathway in aorta from streptozotocin-induced diabetic rats. The aim of this study was to investigate whether this activation was associated with an increased expression of 5-lipoxygenase, an increased cysteinyl leukotriene (CysLT) production in response to arachidonic acid or calcium ionophore A23187 and/or a hypersensitivity of the aorta to CysLTs in streptozotocin-induced diabetic rats. In aorta from diabetic and control rats, reverse transcriptase-PCR and western blot analysis with a specific 5-lipoxygenase antibody provided evidence for the presence of 5-lipoxygenase in aorta. However, the expression of 5-lipoxygenase was not significantly different between diabetic and control rats. Challenge by A23187 (10 microM) and arachidonic acid (10 microM and 0.1 mM) with or without A23187 (10 micromol/l) induced a significant increase of CysLT release (measured by enzyme immunoassay) that was in the same range in aorta from control and diabetic rats. In contrast, aortas from diabetic rats showed a greater sensitivity to LTC4 and LTD4 contractile effects. These data suggested that the activation of the 5-lipoxygenase pathway previously reported in streptozotocin-induced diabetic rats could be explained by an augmented sensitivity to CysLTs of the diabetic aorta.

International Union of Pharmacology XXXVII. Nomenclature for leukotriene and lipoxin receptors

The leukotrienes and lipoxins are biologically active metabolites derived from arachidonic acid. Their diverse and potent actions are associated with specific receptors. Recent molecular techniques have established the nucleotide and amino acid sequences and confirmed the evidence that suggested the existence of different G-protein-coupled receptors for these lipid mediators. The nomenclature for these receptors has now been established for the leukotrienes. BLT receptors are activated by leukotriene B(4) and related hydroxyacids and this class of receptors can be subdivided into BLT(1) and BLT(2). The cysteinyl-leukotrienes (LT) activate another group called CysLT receptors, which are referred to as CysLT(1) and CysLT(2). A provisional nomenclature for the lipoxin receptor has also been proposed. LXA(4) and LXB(4) activate the ALX receptor and LXB(4) may also activate another putative receptor. However this latter receptor has not been cloned. The aim of this review is to provide the molecular evidence as well as the properties and significance of the leukotriene and lipoxin receptors, which has lead to the present nomenclature.

Involvement of cysteinyl leukotrienes in angiotensin II-induced contraction in isolated aortas from transgenic (mRen-2)27 rats

OBJECTIVES

We have previously reported that 5-lipoxygenase-derived products, and particularly the cysteinyl leukotrienes (CysLTs), were involved in angiotensin II (Ang II)-induced contractions in isolated aortas from spontaneously hypertensive rats.

DESIGN

The aim of this study was to assess the role of CysLTs in the vascular response to Ang II in an Ang II-dependent model of hypertension, the (mRen-2)27 transgenic rats (TGs).

METHODS

Intact aortic rings from TG and normotensive Sprague-Dawley rats (SDs) were suspended in organ chambers for isometric tension development in response to Ang II. In addition, the release of CysLTs in response to Ang II (0.3 micromol/l) was measured by enzyme immunoassay.

RESULTS

In isolated aortas from TG rats, pretreatment with the 5-lipoxygenase inhibitor (AA861, 10 micromol/l) or the CysLT1 receptor antagonist (MK571, 1 micromol/l) significantly (P < 0.05) reduced Ang II-induced contractions by 52 and 42%, respectively. In addition, Ang II induced a 2.6-fold increase in CysLT release (pg/mg dry weight tissue: 58.3 +/- 17.9 (Ang II, n = 7) versus 22.5 +/- 5.9 (basal, n = 7) P < 0.05), which was inhibited by the AT1 receptor antagonist losartan (1 micromol/l). In contrast, in aortas from SD rats, pretreatment with AA861 or MK571 did not alter Ang II-induced contraction and CysLT production remained unchanged after exposure to Ang II.

CONCLUSION

These data suggest that CysLTs are involved in the contractile responses to Ang II in isolated aortas from TG but not from SD rats.

Functional studies of leukotriene receptors in vascular tissues

The paradoxical effects of cysteinyl-leukotrienes, namely contraction and relaxation, are now well documented in a number of vascular preparations from various species. The vascular smooth muscle contractions are associated with activation of a single receptor subtype and in some vascular smooth muscles with activation of two receptor subtypes. However, the receptors implicated in the contraction of vessels such as pig pulmonary arteries and veins, dog inferior vena cava, and dog splenic and mesenteric veins remain to be established. There are sufficient data concerning some vascular tissues to suggest that relaxations induced by cysteinyl-leukotrienes are via the stimulation of specific receptors present on the endothelium. The endothelium in human pulmonary arteries has one receptor (CysLT2) and activation induced the release of NO. However, in isolated human pulmonary veins two receptors are present, CysLT1 and CysLT2 (Figure 1). Activation of the former induced the release of a contractile factor whereas activation of the CysLT2 receptor released NO. In guinea pig pulmonary artery and guinea pig thoracic aorta, one receptor has been demonstrated since the relaxations are blocked by ICI-198615. These data suggest the presence of a CysLT1 receptor. Activation of this receptor leads to the release of a relaxant factor, namely, nitric oxide. In contrast, in human pulmonary arteries and veins activation of a receptor that is resistant to ICI-198615 is associated with NO release. These results suggest that there may be species differences even when analogous vascular preparations are examined. While the cysteinyl-leukotrienes are known to relax vascular smooth muscle in a variety of preparations from different species, there are presently two pathways known to be involved in this response. One involves the metabolites of arachidonic acid via the cyclooxygenase enzymatic pathway and the other implicates products of the L-arginine enzymatic pathway. Although both pathways may be present and active in the endothelium of the vascular preparations only one of these enzymatic pathways may be dominant and responsible for the relaxations observed. Ortiz and coworkers have demonstrated that in pulmonary veins the dominant pathway for cysteinyl-leukotriene relaxations is the NO pathway. There are some reports from animal studies that support a dominant role for NO in pulmonary veins. In contrast, Allen and co-workers demonstrated that the LTC4-induced relaxations in isolated human saphenous veins were not modified by treatment of tissues with an NO inhibitor but were significantly enhanced after treatment with indomethacin. These authors suggested that a contracting factor derived from the arachidonic acid pathway was released in preparations challenged with LTC4. In addition, these investigators demonstrated that the NO inhibitor had no effect on the LTC4 relaxations. Together, these results suggest that cysteinyl-leukotriene effects in human pulmonary veins are dominated by the NO pathway whereas in human systemic veins these mediator effects are modified by metabolites of the cyclooxygenase pathway. Unfortunately, most studies involving the actions of cysteinyl-leukotrienes on vessels have been performed in the presence of indomethacin, making interpretation of the relative contribution of the cyclooxygenase and NO pathways difficult. In any event, the cysteinyl-leukotrienes may have a prominent role in the activation of these pathways and the receptors involved have not been clearly established.

Leukotriene receptors on human pulmonary vascular endothelium

1. Cysteinyl-leukotrienes cause contractions and/or relaxations of human isolated pulmonary vascular preparations. Although, the localization and nature of the receptors through which these effects are mediated have not been fully characterized, some effects are indirect and not mediated via the well-described LT1 receptor. 2. In human pulmonary veins (HPV) with an intact endothelium, leukotriene D4 (LTD4) induced contraction above basal tone. This response was observed at lower concentrations of LTD4 in the presence of nitric oxide synthase inhibitor N omega-nitro-L-arginine (L-NOARG). Contractions (in the absence and presence of L-NOARG) were partially blocked by the LT1 antagonists (MK 571 and ICI 198615). 3. LTD4 relaxed HPV previously contracted with noradrenaline. This relaxation was potentiated by LT1 antagonists, but was abolished by removal of the endothelium. LTD4 also relaxed human pulmonary arteries (HPA) precontracted with noradrenaline but this effect was not modified by LT1 antagonists. 4. The results suggest that contraction of endothelium-intact HPV by LTD4 is partially mediated via LT1 receptors. Further, in endothelium-intact HPV, this contraction was opposed by a relaxation induced by LTD4, dependent on the release of nitric oxide, which was mediated, at least in part, via a non-LT1 receptor. In addition, LTD4 relaxation on contracted HPA was not mediated by LT1 receptors. 5. The mechanical effects of LTD4 on human pulmonary vasculature are complex and involve both direct and indirect mechanisms mediated via at least two types of cysteinyl-leukotriene receptors.

Response to anti-human IgE in human pulmonary arteries. Regulation by endothelium

Initial reports concerning anaphylactic reactions in the lung have demonstrated that histamine is released, and this mediator may be responsible for the severe hypotension observed in vivo in sensitized animals. However, those mechanisms involved in the antigen-vascular interactions have not been elucidated. Human isolated pulmonary arterial preparations relaxed when challenged with anti-human IgE (a-IgE). This response was associated with a release of histamine and PGI2. Both the relaxation and the release of PGI2 were attenuated by removal of the endothelium or by prior treatment of the tissues with chlorpheniramine. Indomethacin also significantly reduced the relaxations produced by a-IgE. In addition, L-NOARG in the presence of indomethacin blocked the a-IgE-dependent relaxation. Stimulation of these tissues with histamine also induced relaxations which were endothelium dependent and blocked by chlorpheniramine and L-NOARG in the presence of indomethacin. These results suggest that the relaxations via products of the cyclooxygenase and NO pathways were mediated by histamine release which stimulated the endothelium.

Effect of removing the endothelium on the vascular responses induced by leukotrienes C4 and D4 in guinea-pig isolated heart

The coronary vascular endothelium of the guinea-pig isolated perfused heart was removed by treatment with 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS), a zwitterionic detergent. After CHAPS treatment of the heart the vasoconstrictor responses of leukotriene (LT) C4, LTD4 and angiotensin II (AII) were significantly attenuated whereas the vascular actions of U46619, a thromboxane (Tx) A2 mimetic, and endothelin-1 (ET-1) were unaltered. The endothelium-dependent vasoconstrictor response of LTC4 and LTD4 could not be attributed to the release of TxA2, platelet-activating factor or AII since indomethacin, WEB 2086 and captopril had no effect on LT actions. However, in the presence of cromakalim, a potassium channel activator, the vasoconstrictor effects induced by LTC4, LTD4 and AII were significantly attenuated to a greater extent than the responses of U46619 and ET-1. The results suggest that in the coronary vasculature of the guinea-pig isolated heart the vasoconstrictor responses of LTC4, LTD4 and AII are endothelium-dependent and may involve a cromakalim-sensitive mechanism.

Identification of arginine as a precursor of endothelium-derived relaxing factor

Nitric oxide (NO) is a major endothelium-derived relaxing factor (EDRF) released in response to vasodilating amines, peptides, proteins, ionophores, and nucleotides. EDRF is an important regulator of smooth muscle tone and platelet aggregation and adhesion. Histamine and acetylcholine relax the intact norepinephrine-constricted guinea pig pulmonary artery by an EDRF-dependent mechanism in a medium free of amino acids. N omega-Monomethylarginine (N-MeArg; 0.25 mM) inhibited this relaxation by 64-73%. Inhibition by N-MeArg developed rapidly and was immediately and completely reversed by excess L-arginine but not by D-arginine or by citrulline. N-MeArg did not diminish relaxation induced by nitroprusside, an NO-generating agent, indicating that N-MeArg acts on endothelium rather than on smooth muscle. These observations strongly suggest that, in the intact guinea pig pulmonary artery, EDRF originates from enzymatic action on the guanido nitrogen(s) of an endogenous pool of arginine. This is strikingly similar to the origin of reactive nitrogen intermediates in activated macrophages.