Cardiovascular effects of leukotriene D4 in SHR and WKY rats

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.

Prostacyclin is neither sufficient alone nor necessary to cause pulmonary dysfunction: results from infusions of prostacyclin and antiprostacyclin antibody in porcine septic shock

OBJECTIVE

This study evaluated whether prostacyclin is a necessary mediator of inflammation in graded bacteremia or is sufficient alone in pathophysiologic concentrations to cause the pulmonary derangement of bacteremic shock.

DESIGN

Experimental.

SETTING

Laboratory.

SUBJECTS

Twenty-three anesthetized adult swine. INTERVENSIONS: Swine were studied in four groups for 4 hrs: a) an anesthesia control group (n = 6); b) a septic control group (n = 6), in which 1010/mL Aeromonas hydrophila was infused intravenously at 0.2 mL.kg-1.hr-1 and increased to 4.0 mL.kg-1.hr-1 over 3 hrs; c) a prostacyclin infusion group (n = 6), which received prostacyclin infusion to match septic control plasma concentrationsclm without bacteremia; and d) an antiprostacyclin antibody group (n = 5), which received continuous Aeromonas hydrophila infusion plus antiprostacyclin antibody infusion.

MEASUREMENTS AND MAIN RESULTS

Pulmonary hemodynamics, arterial blood gases, and plasma concentrations of arachidonate metabolites were measured hourly over a 4-hr period. In the septic control group and antiprostacyclin antibody group, elevated pulmonary vascular resistance index and pulmonary artery pressure with decreased Pao2, as well as lower pH, were documented after 1 and 3 hrs of graded bacteremia compared with the anesthesia control group and prostacyclin infusion group (p <.05). Thromboxane B2 concentration increased significantly in all groups during septic shock. In the antiprostacyclin antibody group, leukotriene B4 increased immediately after starting antiprostacyclin antibody infusion and reached significance at 3 hrs compared with the septic control group (p <.05). The prostacyclin infusion group had consistently lower concentrations of leukotrienes C4, D4, and E4 than all other groups.

CONCLUSIONS

Prostacyclin does not mediate blood gas changes, alterations of pulmonary hemodynamics, or platelet abnormalities in porcine septic shock, because antiprostacyclin antibody infusion did not change the pulmonary hypertension and hypoxemia, and infusion of prostacyclin to pathophysiologic blood concentrations did not reproduce such changes. Antiprostacyclin blockade during bacteremia significantly increased concentrations of leukotrienes C4, D4, and E4 and leukotriene B4, whereas prostacyclin infusion suppressed concentrations of leukotrienes C4, D4, and E4, suggesting that endogenous prostacyclin may blunt leukotriene release.

Metabolic and hemodynamic effects of peptide leukotriene C4 and D4 in man

The time course of the effects of intravenous or intracoronary administration of peptide leukotrienes on metabolic parameters and on systemic and coronary hemodynamics was evaluated in 15 patients with normal coronary arteries. Peptide leukotriene C4 (2 nmol given as a bolus intravenous injection) induced an early fall (at 2 min) in mean arterial pressure (P < 0.02) associated with a rise in heart rate (P < 0.001) and in plasma levels of epinephrine (P < 0.05) and norepinephrine (P < 0.005), but without significant changes in coronary blood flow or coronary vascular resistance. Mean arterial pressure, heart rate, norepinephrine, and epinephrine returned to baseline values 10 min after leukotriene C4 administration. In contrast, at 10 min post leukotriene C4, with coronary blood flow and myocardial oxygen consumption unchanged, an increase in coronary vascular resistance (P < 0.05) and in myocardial oxygen extraction (P < 0.01) was observed, which returned to baseline values at 20 min post leukotriene C4. Peptide leukotriene D4 (3 nmol, given in the left coronary artery) induced an early (20 s) and transient fall in mean arterial pressure (P < 0.001) paralleled by a rise in heart rate and plasma levels of epinephrine and norepinephrine, all of which returned to baseline at 10 min. Coronary vascular resistance increased at 10 and 15 min (P < 0.02 and P < 0.05, respectively) and myocardial oxygen extraction at 15 min (P < 0.02). These results suggest that small doses of peptide leukotrienes induce both an early and transient fall in mean arterial pressure associated with secondary sympathoadrenergic activation, and a late increase in small coronary arteriolar resistance.

Effect of the LTD4 receptor antagonist LY203647 on endotoxic shock sequelae in the rat

This study investigated the effect of a novel LTD4 receptor antagonist LY203647 on Salmonella enteritidis endotoxin-induced shock sequelae in anesthetized rats. LY203647 (30 mg/kg i.v.) or vehicle was given 10 min prior to endotoxin (10 mg/kg i.v.) or its vehicle, and the hematocrit, mean arterial pressure and circulating leukocyte counts were determined. LY203647 significantly inhibited endotoxin-induced hemoconcentration up to 90 min post-endotoxin (46.7 +/- 1.3 vs. 51.9 +/- 2.4% at 30 min post-endotoxin, 45.9 +/- 1.1 vs. 53.1 +/- 1.4% at 90 min post-endotoxin, N = 8-9, P less than 0.05). The endotoxin-induced decreases in mean arterial pressure were also attenuated by LY203647, -29 +/- 5 vs. -56 +/- 9 mm Hg at 60 min post-endotoxin and -42 +/- 4 vs. -60 +/- 9 mm Hg at 90 min post-endotoxin (N = 9-10, P less than 0.05). LY203647 also attenuated endotoxin-induced decreases in leukocyte count in arterial blood. A study of differential counts in circulating leukocytes (N = 3) showed that endotoxin induced complete disappearance in circulating neutrophils. The circulating lymphocyte count was decreased by 30 +/- 10 and 41 +/- 1% at 15 and 30 min post-endotoxin, respectively. LY203647 inhibited endotoxin-induced lymphopenia (P less than 0.05) but failed to alter endotoxin-induced neutropenia. These data suggest that LTD4 may play an important role in mediating hemoconcentration, hypotensive and lymphocytopenic sequelae of endotoxin shock.

Interaction of leukotriene C4 and Chinese hamster lung fibroblasts (V79A03 cells). 1. Characterization of binding

A novel, specific, and potent biological action of leukotriene C4 (LTC4) was demonstrated in the Chinese hamster lung fibroblast cell line V79A03 (V79 cells), namely the confirment of protection against subsequent gamma-irradiation. Consequently, studies were conducted to determine whether LTC4-conferred radioprotection could be attributed to a receptor-mediated phenomenon. Specific binding sites for leukotriene C4 (LTC4) were identified and characterized using intact V79 cells incubated at 4 degrees C in the presence of serine-borate, during which time conversion of LTC4 to LTD4 or LTE4 was undetectable. Binding was maximal in a broad region between pH 6.2 and 8.8. Ca2+, Mg2+, and Na+ were not required for binding, and binding was not altered by GTP, ATP, or cAMP, by leukotrienes B4, D4, or E4, or by the leukotriene end point antagonists LY 171883, FPL 55712, or Revlon 5901-5. Scatchard analyses and kinetic experiments indicated the presence of high-affinity [Kd = 2.5 +/- 0.63 nM, approximately 9.9 x 10(5) sites/cell] and low-affinity [Kd = 350 +/- 211 nM, approximately 2.7 x 10(6) sites/cell] binding sites. The observed binding characteristics of LTC4 to V79 cells are consistent with a receptor-mediated phenomenon. In a companion communication which follows this report, we report the subcellular distribution of LTC4 binding to V79 cells and demonstrate that this binding is unlikely to be attributed principally to interaction with glutathione-S-transferase.

Coronary reperfusion in dogs inhibits endothelium-dependent relaxation: role of superoxide radicals

Previous studies indicate that release of superoxide radicals during coronary reperfusion following occlusion may relate to the loss of endothelium-dependent coronary arterial relaxation. We examined coronary arterial ring relaxation in dogs subjected to temporary circumflex (Cx) coronary artery occlusion and treated with saline or the superoxide radical scavenger superoxide dismutase (SOD). In dogs treated with saline, Cx coronary ring relaxation in response to leukotriene D4 (LTD4) and acetylcholine (ACh) was attenuated (p less than 0.01), but coronary relaxation in response to nitroglycerin was preserved, suggesting loss of endothelium-dependent relaxation following coronary reperfusion. In contrast, Cx coronary relaxation in response to LTD4 and ACh was preserved in the SOD-treated dogs (p less than 0.01 compared to saline-treated dogs). To further examine the role of superoxide radicals in the loss of endothelium-dependent relaxation, normal nonischemic canine coronary artery and rat aortic rings were exposed to a superoxide radical generating system of xanthine and xanthine oxidase in vitro. Xanthine plus xanthine oxidase treatment caused a significant (p less than 0.01) decrease in the relaxant effects of ACh. Pretreatment of rat aortic rings with SOD protected against the loss of ACh-induced relaxation. These observations suggest that release of superoxide radicals during reperfusion is the basis of loss of endothelium-dependent coronary arterial relaxation. Treatment with superoxide radical scavengers prior to coronary reperfusion protects against this loss.

Release of leukotrienes and histamine by the isolated anaphylactic heart

The purpose of this study was to demonstrate the ability of heart tissue to release the mediators of anaphylaxis after antigenic challenges. Guinea pigs were sensitized with ovalbumin. Hearts were excised, perfused in a langendorff apparatus, and challenged with a bolus injection of ovalbumin. Analysis of the perfusates demonstrated the presence of histamine as determined by radioenzymatic assay. Histamine release was observed to be maximum after 2 min (8 +/- 1 nmol) of perfusion, then decreased to baseline level. The heart also released LTB4, LTC4, LTD4, and LTE4 as determined by high performance liquid chromatography and bioassays. The release of LTC4 occurred rapidly, reaching maximum after 2 min (4.2 +/- 1 pmol) and then returned to baseline level. Although the release of LTD4 paralleled the release of LTC4, it reached a maximum after 5 min (7.7 +/- 2 pmol). LTE4 was detected after 10 min and was undetectable after 15 min. Maximum release of LTB4 was observed after 5-10 min (15 +/- 3 pmol) and was no longer detectable after 15 min. These results indicate that the isolated sensitized heart undergoing antigenic challenge releases leukotrienes and histamine suggesting the cardiac anaphylaxis might occur by the locally released mediators.

Role of thromboxane, prostaglandins and leukotrienes in endotoxic and septic shock

Intravenous bolus endotoxin elicits a marked but transient increase in plasma TxB2 and 6-keto-PGF1 alpha in a large number of species. A smaller, delayed and more prolonged increase in TxB2 and 6-keto-PGF1 alpha are reported in animals with septic shock, i.e., those with fecal peritonitis or cecal ligation. Thromboxane synthetase inhibitors or antagonists attenuate endotoxin-induced acute cardiopulmonary changes, the delayed increase in serum lysosomal enzymes, fibrin/fibrinogen degradation products and the thrombocytopenia in a number of species. While these drugs increase survival of rats or mice following endotoxin they do not alter survival of rats in septic shock. These results support the hypothesis that TxA2 exerts a pathophysiologic effect in shock following bolus endotoxin. In contrast, nonsteroidal antiinflammatory drugs (NSAID) and dietary essential fatty acid deficiency increase survival of rats subjected to endotoxin shock, and survival time in models of septic shock. These results also suggest that some other cyclooxygenase product(s) is involved in septic shock due to fecal peritonitis or cecal ligation. Preliminary experimental studies indicate salutary effects of leukotriene inhibitors and antagonists in endotoxin shock and in models of acute pulmonary injury. Clinical studies have demonstrated elevated plasma TxB2 and 6-keo-PGF1 alpha concentrations in patients with septic shock, and elevated LTD4 in pulmonary edema fluid of patients with the adult respiratory distress syndrome. In view of these clinical and experimental results, clinical trials of NSAID and/or leukotriene inhibitors/antagonists should be considered.

Overall and regional hemodynamic effects of leukotriene D4 in spontaneously hypertensive rats

Leukotriene D4, a constituent of slow-reacting substance of anaphylaxis, elicits a pressor response followed by hypotensive shock in spontaneously hypertensive rats but not in other rats. Hemodynamic mechanisms underlying this pattern in spontaneously hypertensive rats, pithed and vagotomized to eliminate circulatory reflexes, were studied using radiolabeled microspheres. One minute after leukotriene D4 administration (20 micrograms/kg i.v.), mean arterial pressure increased by 54 mm Hg, total peripheral resistance index increased by 68%, heart rate decreased by 34 beats/minute, and cardiac index was unchanged. Profound reductions of blood flow and increases of vascular resistance in the hepatosplanchnic area, skeletal muscles, and skin also occurred. Five minutes later, mean arterial pressure remained elevated (+35%), hematocrit rose (+17%), and total peripheral resistance index increased, which offset 40% decreases in cardiac and stroke volume indices. Ten minutes after leukotriene D4 administration, during hypotension, cardiac and stroke volume indices and blood flow to all vascular beds declined further while total peripheral resistance index and hematocrit (+28%) continued to rise. In Wistar-Kyoto rats, administration of leukotriene D4 caused less of a pressor response (+34 mm Hg) because vascular resistance was increased only in skeletal muscles, which was followed by a slight hypotension without any significant changes in cardiac and stroke volume indices, total or regional vascular resistance, and hematocrit. Thus, in spontaneously hypertensive rats the leukotriene D4-induced pressor response appears to be caused by generalized vasoconstriction, and the subsequent hypotension appears to result not from vascular collapse but from reduced cardiac output.

Comparative study of the pulmonary effects of intravenous leukotrienes and other bronchoconstrictors in anaesthetized guinea pigs

Pulmonary responses to intravenous leukotrienes C4, D4 and E4 administered as a bolus injection and by continuous infusion were studied in anesthetized guinea pigs. LTD4, LTC4 and LTE4 (respective ED50 of 0.21 +/- .1, 0.64 +/- .2 and 2.0 +/- .1 microgram kg-1) produced dose-dependent increases in insufflation pressure when given as a bolus injection to anesthetized guinea pigs (Konzett-Rössler). Bronchoconstriction was antagonized by FPL-55712 (50-200 micrograms kg-1), and indomethacin (50-200 micrograms kg-1) but was not significantly altered by mepyramine (1.0 mg kg-1), methysergide (0.1 mg kg-1), intal (10 mg kg-1) mepacrine (5 mg kg-1) or dexamethasone (10 mg kg-1). The beta adrenoceptor blocker, timolol (5 micrograms kg-1) produced a significantly greater potentiation of the responses to the leukotrienes than to arachidonic acid, histamine and acetylcholine. Responses to bolus injection of LTE4 but not LTD4 or LTC4 were partially antagonized by atropine (100 micrograms kg-1) and bilateral vagotomy. In experiments of a different design, continuous infusion of LTD4 and LTE4 (2.8-3.2 micrograms kg-1 min-1) into indomethacin-treated animals produced slowly developing increases in pulmonary resistance and decreases in compliance. The increase in resistance produced by LTE4 and LTD4 was partly reversed by intravenous FPL-55712 (1.0 mg kg-1) and atropine (100 micrograms kg-1) but was almost completely reversed by FPL-55712 (3 - 10 mg kg-1). These findings indicate that leukotrienes can produce bronchoconstriction in guinea pigs through cyclooxygenase-dependent and cyclooxygenase independent mechanisms both of which are blocked by FPL-55712. Cholinergic mechanisms are involved in the mediation of part of the response to bolus injection of LTE4 as well as a small part of the initial response to continuous infusion of LTD4 and LTE4. Intrinsic beta adrenoceptor activation serves to down modulate responses to the leukotrienes to a greater extent than responses to arachidonic acid, histamine and acetylcholine.

[Eicosanoids and phospholipases]

Prostaglandins, thromboxanes, and leukotrienes have been implicated to play an important role in physiology as well as in a growing list of pathophysiologic conditions. These oxidation products of 8.11.14-eicosatrienoic-, 5.8.11.14.-eicosatetraenoic-, and 5.8.11.14.17.-pentaenoic acids have been collectively designated eicosanoids. Many clinically important diseases are associated with altered eicosanoid biosynthesis. Furthermore, a series of hormones are known to induce acutely formation of eicosanoids, suggesting a crucial role in a multitude of tissue responses including phenomena such as secretion, platelet aggregation, chemotaxis, and smooth muscle contraction. The major precursor for the eicosanoids seems to be 5.8.11.14.-eicosatetraenoic acid or arachidonic acid. Virtually all of arachidonic acid however is present in esterified form in complex glycerolipids. Since cyclooxygenase and the lipoxygenases utilize arachidonic acid in its free form, a set of acylhydrolases is required to liberate arachidonic acid from membrane lipids before eicosanoid formation can occur. It became only recently apparent that a minor acidic phospholipid, phosphatidylinositol, comprising only 5%-10% of the phospholipid mass in mammalian cells, plays an important role in arachidonic acid metabolism. Phosphatidylinositol--after phosphorylation to phosphatidylinositolphosphate and phosphatidylinositolbisphosphate--appears to be hydrolyzed by specific phospholipases C generating 1-stearoyl-2-arachidonoyl-diglyceride. Diglyceride serves as substrate for diglyceride lipase to form monoglyceride and free fatty acid. Alternatively diglyceride is phosphorylated by diglyceride kinase yielding phosphatidic acid, which is believed to be reincorporated into phosphatidylinositol. In addition to phosphatidylinositol phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid may contribute to arachidonic acid release. These phospholipids are substrates for phospholipases A2 generating free arachidonic acid and the respective lysophospholipid. Understanding of the biochemistry of arachidonic acid liberation may be critical in developing strategies of pharmacological intervention in a variety of pathological conditions.

Differential hemodynamic effects of leukotriene D4 in anesthetized rats: evaluation by directional pulsed Doppler technique

Leukotriene D4 (LTD4) is the major constituent of slow-reacting substance of anaphylaxis (SRS-A). Cardiovascular depression and hypotensive shock represent the major manifestations that attend systemic anaphylaxis. To further evaluate the hemodynamic effects of LTD4, we measured blood pressure (BP), heart rate (HR) and blood flow (BF) (directional pulsed Doppler flowmeter) to different vascular beds (hindquarter, mesenteric and renal) of the urethane-anesthetized rat. LTD4 (3, 10 and 30 micrograms/kg, i.v.) caused a dose-dependent increase in BP: 15 +/- 3, 20 +/- 4 and 24 +/- 2 mm Hg, respectively, which was maximum after 2 min and returned to control level at 10 min; HR was not significantly altered. BF to different vascular beds was differentially altered: mesenteric (-59%) greater than hindquarter (-38%) greater than renal (-10%). Vascular resistance (VR) increased by 195, 85 and 40% in mesenteric, hindquarter and renal beds, respectively. Thyrotropin-releasing hormone (TRH) (2-5 mg/kg, i.v.) injected after LTD4 increased BP, reversed the decrease in BF and the increase in VR in the mesenteric and hindquarter vascular beds. These data suggest that LTD4 receptors are unevenly distributed in various vascular beds and that the splanchnic area is particularly vulnerable to anaphylaxis-induced ischemia. Furthermore, Thyrotropin Releasing Hormone (TRH) might be useful to antagonize the hemodynamic consequences mediated by SRS-A or leukotriene.

Cardiovascular effects of leukotrienes in neonatal piglets. Role in hypoxic pulmonary vasoconstriction?

We investigated the effects of exogenous leukotriene D4, synthesis inhibitors, and a leukotriene receptor antagonist upon chloralose anesthetized, mechanically ventilated, neonatal piglets with constant left pulmonary blood flow and upon piglets with uncontrolled pulmonary blood flow. Leukotriene D4 (100-10,000 ng, intravenously) caused dose-dependent increases in peak tracheal pressure, pulmonary vascular resistance, and systemic arterial pressure. In a limited number of experiments, cardiovascular responses to exogenous leukotriene C4 were qualitatively similar but quantitatively less than those to leukotriene D4. Neither treatment with diethylcarbamazine or the lipoxygenase inhibitor nordihydroguaiaretic acid, nor with the leukotriene receptor antagonist, FPL55712, altered any baseline cardiovascular parameter measured, suggesting the absence of any influence of leukotrienes on resting hemodynamics. Hypoxia or hypoxia combined with mild hypercapnia caused pulmonary vasoconstriction. Neither treatment with diethylcarbamazine or the lipoxygenase inhibitor nordihydroguaiaretic acid, nor with the leukotriene receptor antagonist FPL55712, altered the pulmonary vasoconstrictor response to hypoxia or combined hypoxia/hypercapnia. We conclude that endogenous leukotrienes do not appear to have an influence on resting cardiovascular function, neither do they appear to be necessary for hypoxic pulmonary vasoconstriction in the neonatal piglet, although exogenous leukotrienes are capable of producing cardiovascular effects.

Nifedipine on cardiovascular leukotriene D4 actions in the anaesthetized dog

Open-chest dogs were used to study the effects of intracoronary leukotriene (LTD4; 0.5 microgram/kg) on haemodynamics, coronary blood flow in the left circumflex artery and coronary resistance in the absence or presence of nifedipine (10 micrograms/kg, i.v.). LTD4 increased ventricular filling pressure by 133% (P less than 0.001), the S-T segments and coronary resistance by 490% (P less than 0.001) and abolished coronary flow for 3 min. Nifedipine pretreatment inhibited the cessation of coronary flow. No change in filling pressure and no ischemia signs in the peripheral ECG were noted. However, nifedipine did not inhibit the LTD4-induced decrease in left ventricular dP/dtmax. Haemodynamic LTD4 actions thus could be partially reversed by i.v. nifedipine, suggesting that leukotriene effects on the coronary and peripheral circulation are mediated by calcium release.

Intracerebroventricular and preoptic injections of leukotrienes C4, D4, and E4 in the rat: lack of febrile effect

Experiments were performed to ascertain the effect on core temperature in the rat of central administration of 3 products of the lipoxygenase pathway of arachidonate metabolism. The agents tested were leukotrienes C4, D4, and E4 (LTC4, LTD4, LTE4). In one series of rats, the leukotrienes were injected into the ventral aspect of the third cerebral ventricle (5 microliter injection volume). Each rat received, in separate experimental sessions, an injection of a control solution, of 1 microgram of prostaglandin E1 (PGE1) and of 1 microgram of LTC4, LTD4, or LTE4. In another series of rats, bilateral 1 microliter injections into the tissue of the preoptic region were made. Each animal received a control solution, 40 ng PGE1 (20 ng/side) and 400 ng LTC4, LTD4, or LTE4 (200 ng/side). Neither the intraventricular nor the preoptic injections of any of the leukotrienes produced a significant increase in colonic temperature. However, PGE1 injected intraventricularly or into the preoptic region produced a large, rapidly developing core temperature rise. The strong febrile response to PGE1 and the results of dye distribution studies indicate that the lack of effect of the leukotrienes was not due to incorrect injection cannula placement. The ineffectiveness of the leukotrienes also cannot be attributed to loss of biological activity of these agents during storage. Near the end of the study, samples of each leukotriene were assayed using the guinea pig tracheal strip method and were found to be highly active. The results suggest that, at least in the rat, these 3 arachidonate metabolites are not likely to be important mediators of fever.

Cardiovascular and sympathetic effects of leukotriene D4 in renovascular and spontaneously hypertensive rats

Leukotriene D4 (LTD4), a constituent of slow-reacting substance of anaphylaxis (SRS-A), elicits a pressor response followed by prolonged hypotension in spontaneously hypertensive (SHR) but not in Wistar-Kyoto (WKY) rats. In order to investigate whether the depressor response to LTD4 in SHR rats is related to hypertension itself, we have studied the cardiovascular and sympathetic effects of LTD4 in 1-kidney, 1-clip hypertensive rats and 1-kidney, normotensive rats. In all groups of conscious rats, intra-arterial administration of LTD4 (0.2-20 micrograms/kg) caused dose-dependent pressor responses of similar degree except in WKY rats, which responded less. Only SHR rats developed a significant and progressive hypotension (-58 +/- 5 mm Hg) following pressor phase. In all but WKY rats, the response phase was attended by an increase in plasma levels of norepinephrine and epinephrine. Only SHR rats showed marked and persistent hemoconcentration following pressor effect. Thus, depressor response of SHR rats to systemic administration of LTD4 does not appear to be merely due to the magnitude of blood pressure elevation and may in part result from microcirculatory changes not present in other hypertensive rats.

Leukotriene D4-induced hypotension is reversed by thyrotropin-releasing hormone

Injection of leukotriene D4 (LTD4, 20 micrograms/kg, i.a.) to conscious spontaneous hypertensive (SHR) rats produces a short-lasting pressor and tachycardic response followed by prolonged hypotension and bradycardia. Plasma norepinephrine and epinephrine were elevated at the peak pressor/tachycardic phase as well as at the hypotensive phase. Injection of thyrotropin-releasing hormone (TRH, 2 or 5 mg/kg) at the peak of the LTD4-induced hypotension resulted in prompt reversal of the hypotension and bradycardia in a dose-related manner. Naloxone (5 mg/kg) had no effect on blood pressure and heart rate of LTD4-treated SHR rats. Pretreatment with TRH (5 mg/kg) did not prevent the depressor effect of LTD4, but attenuated the bradycardic effect of this leukotriene. In addition, TRH had no effect on LTD4-induced hypotension in the pithed SHR rat. These results suggest that TRH might exert beneficial effects in hypotensive states mediated by leukotrienes or other mediators of anaphylactic reactions.

Effects of FPL-55712 or indomethacin on leukotriene-induced coronary constriction in the intact pig heart

Intracoronary leukotriene D4, 0.1-3.0 micrograms (0.2-6.0 nmol), produced dose-dependent decreases in coronary flow of anesthetized pigs. Pretreatment with intracoronary FPL-55712 (0.1, 0.3 and 1.0 mg) reduced coronary constriction due to 1.0 micrograms leukotriene D4 by up to 77%. FPL-55712 did not produce sustained alterations in coronary flow, left ventricular end-diastolic pressure, systemic arterial pressure, or heart rate. Indomethacin pretreatment (6 mg/kg i.v.) had no effect on leukotriene-induced coronary constriction. Inhibition produced by FPL-55712 may be useful in disease states involving leukotriene-mediated coronary constriction.

Coronary constriction by leukotriene C4, D4, and E4 in the intact pig heart

Leukotrienes are naturally occurring vasoactive metabolites of arachidonic acid that increase during inflammatory reactions and anaphylaxis. Coronary constriction and reduced myocardial contractility after leukotriene C4 and D4 administration were demonstrated in the isolated guinea pig heart. To explore the effects of leukotrienes in the in situ, blood-perfused heart, we administered leukotrienes C4, D4, and E4 into the coronary artery of the domestic pig. Increasing doses (0.1, 0.3, 1.0, and 3.0 micrograms) of leukotrienes C4, D4, and E4 were injected into the left anterior descending coronary artery of 8 open-chest domestic pigs. Significant dose-related reduction in coronary blood flow was observed after each leukotriene administration. Three micrograms of each leukotriene produced the following maximal decreases (mean +/- standard error); C4 = 80 +/- 9%, p less than 0.001; D4 = 81 +/- 3%, p less than 0.001; E4 = 64 +/- 12%, p less than 0.005. In several instances, surface electrograms recorded from the myocardial region exposed to leukotrienes showed signs of focal myocardial ischemia, sometimes accompanied by ventricular arrhythmia. Significant elevation of left ventricular end-diastolic pressure was observed after large doses (1 or 3 micrograms) of leukotrienes C4 and D4. Minimal (5 to 10%) decreases in mean arterial pressure and no change in heart rate were observed after leukotriene injection. We conclude that leukotrienes C4, D4, and E4 are extremely potent coronary constrictors in the in situ heart. The intensity of response and associated electrocardiographic signs of ischemia suggest that constriction is mainly due to a primary effect on vascular smooth muscle. However, coronary flow reduction may also reflect consequences of a primary negative inotropic action. Leukotrienes may play a significant role in the pathogenesis of a variety of cardiac disorders, particularly those associated with extensive inflammatory changes.

Alteration of leukotriene D4 hypotension by thyrotropin releasing hormone

Leukotriene D4 (LTD4), a component of slow reacting substance of anaphylaxis (SRS-A)1, produces marked hypotension in laboratory animals2,3, implicating it as a potential mediator of anaphylactic shock. It has been demonstrated that naloxone reverses the hypotension associated with endotoxaemia4-6, hypovolaemia7,8 and spinal injury9,10, presumably through blockade of endogenous opioid systems11. More recently, it has also been shown that thyrotropin releasing hormone (TRH) improves experimental shock12 and spinal injury13, and it has been postulated that TRH acts by 'physiological' antagonism of endogenous opioid systems in these conditions. Here we report that TRH both blocked and reversed leukotriene-induced hypotension in the unanaesthetized guinea pig, whereas naloxone had no effect. LTD4 hypotension was also reversed by intracerebroventricular (i.c.v.) administration of TRH at a dose that had no effect when given systemically. LTD4 administration was associated with sympatho-adrenomedullary activation, and TRH further augmented this response. Peripheral cholinergic blockade with methylatropine did not alter the leukotriene hypotension. These data demonstrate the first dissociation of TRH and naloxone in experimental shock and suggest that the beneficial effects of TRH in this model result from central interactions which are not mediated by endogenous opioids. The findings further provide a potential link between the physiological (that is, cardiovascular) effects of a peptide (TRH) and those of a leukotriene; this may have implications with regard to the pathophysiology and therapy of anaphylaxis.

Thyrotropin-releasing hormone reverses the hypotension and bradycardia produced by leukotriene D4 in unanesthetized guinea pigs

We have utilized an unanesthetized guinea pig model to study the cardiovascular effects of leukotriene D4 (LTD4), a compound implicated in slow reacting substance of anaphylaxis (SRS-A). Intravenous (IV) administration of LTD4 at 5 micrograms/kg produced profound hypotension, bradycardia, hypoxia and acidosis. The hypotension and bradycardia were rapidly and completely reversed by IV thyrotropin-releasing hormone (TRH) at a dose of 2 mg/kg. In contrast, TRH treatment had no effect on the hypoxia or acidosis induced by LTD4. These data provide a potential physiological link between two distinct classes of endogenous substances and may have implications for the therapy of anaphylactic shock.