Potentiation of vagal contractile response by thromboxane mimetic U-46619

We studied the effect of the thromboxane mimetic U-46619 on tracheal smooth muscle contraction caused by bilateral stimulation of the vagus nerves in 14 mongrel dogs in situ. The parasympathetic contractile response was studied isometrically after beta-adrenergic blockade with 2 mg/kg iv propranolol plus 20 micrograms X kg-1 X min-1 continuous intravenous infusion and blockade of endogenous prostaglandin synthesis with 5 mg/kg iv indomethacin. An initial frequency-response curve was generated by electrical stimulation of the caudal ends of cut cervical vagi over the range of frequencies 2-25 Hz (constant 25 V) at 15-s intervals. In five dogs, 10(-10) to 10(-8) mol of the thromboxane mimetic (15S)-hydroxyl-11 alpha,9 alpha-(epoxymethano)prosta-5Z,13E-dienoic acid (U-46619) was injected selectively into the tracheal arterial circulation, causing a transient contractile response (less than or equal to 10 g/cm). Additional frequency response studies were generated 7 min before and 1, 15, 30, 45, and 60 min after U-46619. Substantial augmentation of tracheal contraction to efferent vagal stimulation was observed after U-46619 for all frequencies greater than 4 Hz (P less than 0.02). Augmentation of vagally mediated contraction was not observed in four other dogs after equivalent tracheal contraction was elicited without U-46619. Similarly, in four separate dogs, augmentation of tracheal contraction was not observed when acetylcholine was given instead of vagal stimulation after U-46619. We conclude that the thromboxane analogue, U-46619, causes augmentation of tracheal contractile response induced by efferent vagus nerve stimulation. Potentiation is caused by a prejunctional action of U-46619 and is not induced by nonspecific precontraction with another agonist.     

Positive inotropic effect of the thromboxane analog U-46619 on guinea pig left atrium: mediation by specific receptors and association with increased phosphoinositide turnover

U-46619, a stable epoxymethano analog of thromboxane A2 elicited a direct positive inotropic effect on guinea pig left atrium paced at a constant rate (EC50 = 2.5 nM). This novel observation contrasts with previous reports of a decrease in myocardial contractility by thromboxane mimetic compounds in coronary-perfused preparations, an action recognized as secondary to vasoconstriction. The positive inotropic effect of U-46619 was competitively antagonized by the specific thromboxane receptor blocker L-655,240 (pA2 = 8.02; identical to that reported in smooth muscle), but was unaffected by blockers of alpha 1-, beta 1-, and H1-receptors and by cyclooxygenase and lipoxygenase inhibitors. Increased tissue levels of inositol phosphates, but not cAMP, were associated with the positive inotropic action of U-46619, in analogy to the actions of alpha 1- and H1-receptor agonists. However, the inotropic effect of U-46619 and the concomitant increase in phosphoinositide breakdown were both selectively antagonized by L-655,240. Thus, U-46619 acts on specific thromboxane receptors in guinea pig left atrium and elicits a positive inotropic effect that probably results from an increase in phosphoinositide metabolism.     

Prostacyclin (PGI2) and U-46619 stimulate coronary arteries from diabetic dogs and their action is influenced by inhibitors of prostaglandin biosynthesis

Isolated coronary arteries from diabetic dogs presented different contractile response to U-46619 to prostacyclin (PGI₂) and to arachidonic acid (AA) than those of normal dogs. The stimulatory effect of the synthetic endoperoxide analogue U-46619, was significantly higher in the diabetic condition than in preparations from normal animals. On the other hand, while PGI₂ evoked a dose-dependent relaxation of normal coronary arteries, diabetic vessels were not relaxed by low concentration of PGI₂ whereas higher ones produced a distinct constrictor effect. Additionally, inhibitors of prostaglandins and thromboxane (TX) biosynthesis such as corticosterone, indomethacin, acetylsalicylic acid, imidazole and L-8027, abolished the stimulatory effect of PGI₂ in coronary arteries from diabetic dogs. AA relaxed coronaries from normal dogs and constricted those from diabetic animals, this action being inhibited by imidazol and L-8027.The present results suggests that: a) coronary vessels from diabetic dogs are more reactive to an endoperoxide analogue than normal preparations and b) PGI₂ and AA probably contract diabetic coronary arteries via the participation of a TX like material. It is then plausible that this effect could be tentatively ascribed to the production of a prostaglandin constricting substance including als the probable generation of a TXA₂-like agonist.     

Hypoxia induces hypersensitivity and hyperreactivity to thromboxane receptor agonist in neonatal pulmonary arterial myocytes

PPHN, caused by perinatal hypoxia or inflammation, is characterized by an increased thromboxane-prostacyclin ratio and pulmonary vasoconstriction. We examined effects of hypoxia on myocyte thromboxane responsiveness. Myocytes from 3rd-6th generation pulmonary arteries of newborn piglets were grown to confluence and synchronized in contractile phenotype by serum deprivation. On the final 3 days of culture, myocytes were exposed to 10% O2 for 3 days; control myocytes from normoxic piglets were cultured in 21% O2. PPHN was induced in newborn piglets by 3-day hypoxic exposure (Fi(O2) 0.10); pulmonary arterial myocytes from these animals were maintained in normoxia. Ca2+ mobilization to thromboxane mimetic U-46619 and ATP was quantified using fura-2 AM. Three-day hypoxic exposure in vitro results in increased basal [Ca2+]i, faster and heightened peak Ca2+ response, and decreased U-46619 EC50. These functional changes persist in myocytes exposed to hypoxia in vivo but cultured in 21% O2. Blockade of Ca2+ entry and store refilling do not alter peak U-46619 Ca2+ responses in hypoxic or normoxic myocytes. Blockade of ryanodine-sensitive or IP3-gated intracellular Ca2+ channels inhibits hypoxic augmentation of peak U-46619 response. Ca2+ response to ryanodine alone is undetectable; ATP-induced Ca2+ mobilization is unaltered by hypoxia, suggesting no independent increase in ryanodine-sensitive or IP3-linked intracellular Ca2+ pool mobilization. We conclude hypoxia has a priming effect on neonatal pulmonary arterial myocytes, resulting in increased resting Ca2+, thromboxane hypersensitivity, and hyperreactivity. We postulate that hypoxia increases agonist-induced TP-R-linked IP3 pathway activation. Myocyte thromboxane hyperresponsiveness persists in culture after removal from the initiating hypoxic stimulus, suggesting altered gene expression.     

Thromboxane receptors can modulate gastric acid secretion in the rat

The effects of PGE2 and the thromboxane A2 mimetic, U-46619, have been investigated on gastric secretion in the rat isolated gastric mucosa. Both compounds produced concentration-related inhibitions of histamine-induced secretion whereas only U-46619 inhibited methacholine-stimulated and basal secretion, and neither compound had any effect on the secretory response to dbcAMP. Indomethacin had no effect on the antisecretory activity of PGE2 but markedly reduced the potency of U-46619 suggesting that endogenous prostaglandins play a role in the U-46619 responses. However, direct inhibitory effects of U-46619 were seen at high concentrations. The thromboxane receptor antagonist AH23848, at concentrations selective for thromboxane receptors, had no effect on responses to PGE2 but markedly inhibited the effects of U-46619. We conclude that the antisecretory profile of U-46619 differs from that of PGE2. U-46619 has both direct and indirect antisecretory effects and these are mediated via thromboxane receptors in the rat gastric mucosa.     

Chronic hypoxia increases MCA contractile response to U-46619 by reducing NO production and/or activity.

Chronic hypoxia alters contractile sensitivity of isolated arteries to alpha-adrenergic stimulation and other agonists. However, most studies have been performed in thoracic aortas or other large vessels making little contribution to vascular resistance in their respective circulations. To determine the effect of chronic hypoxia on the vasoconstrictor response in a small, resistance-sized vessel, we studied second and third generation middle cerebral arteries (MCA; approximately 75-microm internal diameter before mounting). MCA were isolated from normoxic (inspired oxygen = 125 Torr) and hypoxic (8 wk at 3,960 m; inspired oxygen = 90 Torr) guinea pigs, and their vasoconstrictor responses were determined to the thromboxane mimetic U-46619 by using dual-pipette video microscopy. Arteries from hypoxic animals had greater contractile sensitivity to U-46619 compared with those of the normoxic animals (-log EC50 = 7.86 +/- 0.11 vs. 7.62 +/- 0.06, respectively, P < 0.05). Addition of the nitric oxide (NO) inhibitor nitro-L-arginine (200 microM) to the vessel bath eliminated the differences in contractile sensitivity between the MCA from the normoxic and chronically hypoxic groups. Supplementation with L-arginine in the drinking water sufficient to raise plasma L-arginine levels 41% reduced MCA contractile sensitivity to U-46619 in the normoxic group (-log EC50 = 7.22 +/- 0.31, P < 0.05 compared with the nonsupplemented normoxic group) but not in the chronically hypoxic group. These results show that chronic hypoxia increases the sensitivity of the MCA to the vasoconstrictor U-46619, likely because of a reduction in NO production and/or activity.     

Close-arterial administration of the thromboxane mimetic U-46619 induces damage to the rat gastric mucosa

The pro-ulcerogenic actions of the thromboxane mimetic, U-46619 on the rat gastric mucosa have been investigated, utilizing a novel technique which allows administration directly into the left gastric artery. Local intra-arterial infusion of U-46619 (100-500 ng/kg/min for 10 min) induced dose-dependent macroscopic damage in both the corpus and antral regions, characterized as vasocongestion, disruption and haemorrhage, with deep penetrating ulcers in the antral mucosa. Vascular congestion, epithelial cell and glandular disruption was observed histologically in both corpus and antral regions. Local intra-arterial infusion of lower doses of U-46619 (25-100 ng/kg/min) significantly disrupted the mucosa in the presence of 10% ethanol in a concentration which itself did not induce macroscopic damage. The damaging actions of U-46619 were substantially reduced by pretreatment with the thromboxane-receptor antagonist, BM 13,177 (5mg/kg i.v.) or 16,16-dimethyl PGE2 (5 micrograms/kg s.c.). These findings support the role of endogenous thromboxane A2 as a local mediator of gastric injury.     

Influence of the thromboxane-mimetic U-46619 and the prostacyclin metabolite 6-keto prostaglandin E1 on vasoconstriction induced by noradrenaline and 5-HT in rat mesenteric vasculature

The aim of this study was to investigate the effects of U-46619, a thromboxane A2-mimetic, and 6-keto prostaglandin E1 (6-keto PGE1) a biologically active metabolite of prostacyclin, on vasoconstrictor responses to noradrenaline and 5-hydroxytryptamine (5-HT). In vitro, U-46619 (3-100 nmol/l) amplified responses to both noradrenaline and 5-HT in a concentration-dependent manner. This effect was not caused by an increase in the affinity of the alpha-adrenoceptor for noradrenaline because U-46619 (100 nmol/l) did not alter the pA2 of phentolamine. In vivo, U-46619 (100 nmol/l) induced vasoconstriction and consequently significantly shifted the log-concentration-effect curves to noradrenaline and 5-HT upward in an additive manner. 6-Keto PGE1 (1 mumol/l) did not affect either perfusion pressure or vasoconstriction in response to noradrenaline in vivo. The study highlights some differences in responses between in vitro- and in vivo-perfused mesentery.     

Rho activation in excitatory agonist-stimulated vascular smooth muscle

Small GTPase Rho and its downstream effector, Rho kinase, havebeen implicated in agonist-stimulated Ca²⁺ sensitization of20-kDa myosin light chain (MLC₂₀) phosphorylation andcontraction in smooth muscle. In the present study we demonstrated forthe first time that excitatory receptor agonists induce increases inamounts of an active GTP-bound form of RhoA, GTP-RhoA, in rabbit aorticsmooth muscle. Using a pull-down assay with a recombinant RhoA-bindingprotein, Rhotekin, we found that a thromboxane A₂ mimetic,U-46619, which induced a sustained contractile response, induced asustained rise in the amount of GTP-RhoA in a dose-dependent mannerwith an EC₅₀ value similar to that for the contractile response. U-46619-induced RhoA activation was thromboxaneA₂ receptor-mediated and reversible. Other agonistsincluding norepinephrine, serotonin, histamine, and endothelin-1 (ET-1)also stimulated RhoA, albeit to lesser extents than U-46619. Incontrast, ANG II and phorbol 12,13-dibutyrate failed to increaseGTP-RhoA. The tyrosine kinase inhibitor genistein substantiallyinhibited RhoA activation by these agonists, except for ET-1. Thusexcitatory agonists induce Rho activation in an agonist-specificmanner, which is thought to contribute to stimulation ofMLC₂₀ phosphorylation Ca²⁺ sensitivity.

     

Amplification effect and mechanism of action of ET-1 in U-46619-induced vasoconstriction in pig skin

The aim of this study was to investigate if a low concentration of endothelin-1 (ET-1; 8 x 10(-10) M) may amplify the skin vasoconstrictor effect of other vasoactive substances in the pathogenesis of skin vasospasm. Pig skin flaps (6 x 16 cm) were perfused with Krebs buffer equilibrated with 95% O(2) and 5% CO(2) at 37 degrees C and pH 7.4. Skin perfusion pressure measured by a pressure transducer and skin perfusion assessed by the dermofluorometry technique were used for assessment of skin vasoconstriction. We observed that ET-1 (8 x 10(-10) M) significantly amplified the concentration-dependent (10(-7)-10(-5) M) skin vasoconstrictor effect of norepinephrine. More importantly, we observed for the first time that this low concentration of ET-1 also amplified the concentration-dependent (10(-8)-10(-6) M) skin vasoconstrictor effect of the thromboxane A(2) mimetic U-46619, and this amplification effect of ET-1 was completely blocked by the protein kinase C (PKC) inhibitor chelerythrine (5 x 10(-6) M). Conversely, the PKC activator phorbol 12,13-dibutyrate (10(-7) M) amplified the vasoconstrictor effect of U-46619. Furthermore, the sensitivity of the skin vasculature to the vasoconstrictor effect of extracellular Ca(2+) in U-46619-induced skin vasoconstriction was significantly enhanced in the presence of 8 x 10(-10) M ET-1. Finally, the cyclooxygenase inhibitor indomethacin (5 x 10(-6) M) did not affect the amplification effect of ET-1 on U-46619-induced skin vasoconstriction. We conclude that a low concentration of ET-1 can amplify the skin vasoconstrictor effect of U-46619 independent of endogenous cyclooxygenase products, and the mechanism may involve activation of PKC and increase in sensitivity of the contractile apparatus to Ca(2+) in smooth muscle cells.     

Role of serotonin in thromboxane A2-induced coronary chemoreflex

We reported previously that the thromboxane A(2) (TxA(2)) mimetic U-46619 stimulates cardiac vagal afferent nerves, eliciting a reflex decrease in heart rate (HR) and arterial blood pressure (ABP). The present experiments were designed to test the hypothesis that TxA(2) evokes these changes via the release of serotonin [5-hydroxytryptamine (5-HT)] and activation of the 5-HT(3) receptor. Injections of the 5-HT(3) antagonist tropisetron (1 mg of 3-tropanyl-indole-3-carboxylate or ICS-205-930) attenuated the decreases in HR and ABP induced by left atrial injections of U-46619 (20 microg). Tropisetron administration also eliminated the U-46619-induced increase in impulse frequency in a majority of cardiac, vagal afferent units tested. Measurement of serum 5-HT levels revealed an elevation in serum 5-HT levels after U-46619 injection in those rabbits that displayed a significant HR change following injection of U-46619. These results indicate that although other factors may also contribute to these reflex responses, the release of 5-HT and stimulation of the 5-HT(3) receptor plays a significant role in coronary reflexes induced by TxA(2).     

Receptor-mediated effect of a synthetic thromboxane-analogue on cytosolic calcium in isolated proximal tubules.

The aims of this study were to measure cytosolic calcium concentration -[Ca2+]i- under resting conditions in isolated renal proximal tubules and to analyze the effect of U-46619 (stable analogue of thromboxane A2/PGH2 on [Ca2+]i in a mammalian epithelium. Proximal tubules were dissected out from male New Zealand rabbits (2.5 to 3.0 kg). After isolation they were washed twice and resuspended in 2 ml phosphate buffer solution (PBS). Tubules were loaded with Quin 2-AM (25 microM) for 15 min. After washing with PBS to eliminate the excess of extracellular Quin 2, fluorescence was measured at 340 nm excitation and 490 emission, under resting conditions and after stimulation. U 46619 (from 10 nm to 10 mM) increased [Ca2+]i in a concentration-dependent pattern. Exposure to an antagonist of the thromboxane receptor (S-145) blocked the response to U-46619. Removal of external calcium abolished the response to U-46619. Change of PBS for Ringer-choline blunted the response to thromboxane analogue. Our results indicate that U-46619 increases cytosolic calcium through a receptor-mediated mechanism that requires external calcium to operate. Blockade of the response in the absence of external sodium suggests that Na+/Ca2+ exchanger participates in this response.     

Characterization of thromboxane and prostacyclin effects on pulmonary vascular resistance.

Although thromboxane and prostacyclin (PGI2) have long been described as major controllers of pulmonary vascular resistance, little has been reported on the characteristics of the interactions between the two arachidonic acid products. The current study uses segmental vascular resistance and compliance measurements to evaluate the actions of thromboxane and PGI2 in isolated blood-perfused rat lung. The thromboxane analogue U-46619 increases pulmonary vascular resistance by increasing only small artery resistance and decreases pulmonary vascular compliance in the middle compartment. Among the vascular effects of U-46619 are a maximum increase in resistance (RmaxU-46619) of 60.3 +/- 15.6 cmH2O.l-1.min.100 g-1 and a concentration required for 50% of maximum increase (K0.5,U-46619) of 1.60 +/- 0.85 nM for small artery resistance, a minimum vascular compliance (CminU-46619) of -0.93 +/- 0.58 cmH2O, and a K0.5,U-46619 of 1.10 +/- 1.60 nM for middle compartment compliance. Similar results were obtained for total resistance and total compliance. The effects of PGI2 on thromboxane-induced resistance and compliance changes were evaluated using K0.5,PGI2, RmaxPGI2, and CmaxPGI2 at each dose of thromboxane. PGI2 was more effective in reversing the thromboxane constriction at higher concentrations of thromboxane. These data show that the absolute concentration of PGI2 and thromboxane and not a simple ratio of thromboxane to PGI2 determines vascular tone.     

Spasmogenic activity of C5adesArg anaphylatoxin on guinea pig lung parenchymal strips: Sensitivity of the leukotriene-mediated component to cyclooxygenase inhibitors

The leukotriene-dependent component of C5a_desArg-induced contractile activity on guinea pig lung parenchymal strips is inhibited by cyclooxygenase inhibitors. Indomethacin simultaneously increased leukotriene release while inhibiting both cyclooxygenase-dependent mediator release and the contractile force generated. Tissue responses to LTC₄ and LTD₄ are also inhibited by cyclooxygenase blockade, while contractions induced by the thromboxane A₂ analog, U-46619, histamine or acetylcholine are not affected. These data indicate a functional role for cyclooxygenase metabolites in leukotriene-induced contractile responses in lung.     

Cellular mechanisms of thromboxane A2-mediated contraction in pulmonary veins

Our objectives were to identify the relative contributions of [Ca2+]i and myofilament Ca2+ sensitivity in the pulmonary venous smooth muscle (PVSM) contractile response to the thromboxane A2 mimetic U-46619 and to assess the roles of PKC, tyrosine kinases (TK), and Rho-kinase (ROK) in that response. We tested the hypothesis that U-46619-induced contraction in PVSM is mediated by both increases in [Ca2+]i and myofilament Ca2+ sensitivity and that the PKC, TK, and ROK signaling pathways are involved. Isometric tension was measured in isolated endothelium-denuded (E-) canine pulmonary venous (PV) rings. In addition, [Ca2+]i and tension were simultaneously measured in fura-2-loaded E- PVSM strips. U-46619 (0.1 nM-1 microM) caused dose-dependent (P < 0.001) contraction in PV rings. U-46619 contraction was attenuated by inhibitors of L-type voltage-operated Ca2+ channels (nifedipine, P < 0.001), inositol 1,4,5-trisphosphate-mediated Ca2+ release (2-aminoethoxydiphenylborate, P < 0.001), PKC (bisindolylmaleimide I, P < 0.001), TK (tyrphostin A-47, P = 0.014), and ROK (Y-27632, P = 0.008). In PV strips, U-46619 contraction was associated with increases in [Ca2+]i and myofilament Ca2+ sensitivity. Both Ca2+ influx and release mediated the early transient increase in [Ca2+]i, whereas the late sustained increase in [Ca2+]i only involved Ca2+ influx. Inhibition of both PKC and ROK (P = 0.006 and P = 0.002, respectively), but not TK, attenuated the U-46619-induced increase in myofilament Ca2+ sensitivity. These results suggest that U-46619 contraction is mediated by Ca2+ influx, Ca2+ release, and increased myofilament Ca2+ sensitivity. The PKC, TK, and ROK signaling pathways are involved in U-46619 contraction.     

Significance of thromboxane generation in ozone-induced airway hyperresponsiveness in dogs

To determine whether thromboxane A2 may be involved in ozone (O3)-induced airway hyperresponsiveness, we studied the effect of a thromboxane synthase inhibitor (OKY-046, 100 micrograms X kg-1 X min-1 iv) in five dogs exposed to O3. Airway responsiveness was assessed by determining the provocative concentration of acetylcholine aerosol that increased total pulmonary resistance by 5 cmH2O X l-1 X s. O3 (3 ppm) increased airway responsiveness as demonstrated by a decrease in acetylcholine provocative concentration from 2.42 (geometric SEM = 1.64) to 0.14 mg/ml (geometric SEM = 1.30). OKY-046 significantly inhibited this effect without altering pre-O3 responsiveness or the O3-induced increase in neutrophils and airway epithelial cells in bronchoalveolar lavage fluid. To further examine the role of thromboxane A2, we studied the effect of a thromboxane A2 mimetic, U-46619, on airway responsiveness in five additional dogs. U-46619 in subthreshold doses (i.e., insufficient to increase base-line pulmonary resistance) caused a fourfold increase in airway responsiveness to acetylcholine. Subthreshold doses of histamine had no effect. These results suggest that thromboxane A2 may be an important mediator of O3-induced airway hyperresponsiveness.     

Platelet aggregation increases cholinergic neurotransmission in canine airway

To determine whether thromboxane A2 released from aggregating platelets increases the contractile response of airway smooth muscle to cholinergic nerve stimulation and, if so, what the mechanism of action is, we studied in vitro bronchial segments from dogs under isometric conditions. The contractile responses to electrical field stimulation at 30 s and 1 min after the addition of autologous platelets were increased by 11.1 +/- 3.2 (SD) and 20.7 +/- 5.4%, respectively, and were accompanied by the release of thromboxane A2. These effects were inhibited either by pretreatment of platelets with indomethacin or by addition of the thromboxane A2 receptor antagonist SQ 29548. Likewise, the thromboxane A2 mimetic U 46619, in subthreshold doses (i.e., insufficient to increase base-line tension), increased electrical field stimulation-induced contraction by 18.7 +/- 4.8%. The increase was greater in the presence of a concentration of physostigmine that did not cause spontaneous contraction and was blocked by SQ 29548 but not by hexamethonium or by phentolamine. Methacholine-induced contractions were unaffected by U 46619. These results indicate that aggregating platelets, by releasing thromboxane A2, increase the airway contractile response to neural stimulation probably by the accelerated release of acetylcholine.     

5-Oxo-ETE regulates tone of guinea pig airway smooth muscle via activation of Ca2+ pools and Rho-kinase pathway

5-Oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE) is a proinflammatory mediator, but its effects on airway smooth muscle (ASM) have never been assessed. Tension measurements performed on guinea pig ASM showed that 5-oxo-ETE induced sustained concentration-dependent positive inotropic responses (EC50 = 0.89 microM) of somewhat lower amplitude than those induced by carbamylcholine and the thromboxane A2 (TXA2) agonist U-46619. Transient inotropic responses to 5-oxo-ETE were recorded in Ca2+-free medium, suggesting mobilization of intracellular Ca2+. Meanwhile, the sustained contraction, which required Ca2+ entry, was partially blocked by 1 microM nifedipine (an L-type Ca2+ channel blocker) but relatively insensitive to 100 microM Gd3+. The 5-oxo-ETE responses were also inhibited by indomethacin and SC-560 [a cyclooxygenase (COX-1) inhibitor] pretreatments but not by NS-398 (a selective COX-2 inhibitor). The contractile effects of 5-oxo-ETE on ASM were inhibited by the selective TXA2 receptor (TP receptor) antagonist SQ-29548 (-75%) and by 2-(p-amylcinnamoyl) amino-4-chlorobenzoic acid pretreatment, a phospholipase A2 inhibitor (-66%), suggesting that the major part of its effect is mediated by the release of TXA2. ASM responses to 5-oxo-ETE were also blocked by the Rho-kinase inhibitor Y-27632, which also partially inhibited the response to the TP receptor agonist U-46619, suggesting that the contractile response is due in part to Ca2+ sensitization of ASM cell myofilaments.     

Different mechanisms between thromboxane A2- and leukotriene D4-induced nasal blockage in guinea pigs

Although thromboxane (TX)A2 is involved in allergic rhinitis, the mechanisms inducing nasal blockage have not been elucidated. We evaluated the roles of nasal mucosal vascular changes following intranasal instillation of the TXA2 analog U-46619 or leukotriene (LT)D4 to induce nasal blockage in a guinea pig model of allergic rhinitis. Both U-46619- and LTD4-induced nasal blockages in sensitized animals were swiftly and completely suppressed by a vasoconstrictor, naphazoline. The nitric oxide synthase inhibitor N(omega)-nitro-l-arginine methyl ester relieved LTD4-induced nasal blockage, but not U-46619-induced nasal blockage. Although both agonists produced vasodilatation of nasal mucosa in vivo, LTD4 caused vasodilatation while U-46619 caused vasoconstriction in vitro. Both LTD4- and U-46619-induced nasal blockages in vivo should depend on vasodilatation of nasal mucosa. LTD4-induced nasal blockage is induced by direct vasodilatation via nitric oxide. In contrast, U-46619-induced nasal blockage may be associated with contraction of a certain vein that should exist at the exit of capacitance vessels, leading to congestion of the nasal mucosa.