Prostaglandins potentiate U-46619-induced pulmonary microvascular dysfunction.

The induction of cyclooxygenase is an important event in the pathophysiology of acute lung injury. The purpose of this study was to examine the synergistic effects of various cyclooxygenase products (PGE(2), PGI(2), PGF(2alpha)) on thromboxane A(2) (TxA(2))-mediated pulmonary microvascular dysfunction. The lungs of Sprague-Dawley rats were perfused ex vivo with Krebs-Henseleit buffer containing indomethacin and PGE(2) (5 x 10(-8) to 1 x 10(-7) M), PGF(2alpha) (7 x 10(-9) to 5 x 10(-6) M), or PGI(2) (5 x 10(-8) to 2 x 10(-5) M). The TxA(2)-receptor agonist U-46619 (7 x 10(-8) M) was then added to the perfusate, and then the capillary filtration coefficient (K(f)), pulmonary arterial pressure (Ppa), and total pulmonary vascular resistance (RT) were determined. The K(f) of lungs perfused with U-46619 was twice that of lungs perfused with buffer alone (P = 0.05). The presence of PGE(2), PGF(2alpha), and PGI(2) within the perfusate of lungs exposed to U-46619 caused 118, 65, and 68% increases in K(f), respectively, over that of lungs perfused with U-46619 alone (P < 0.03). The RT of lungs perfused with PGE(2) + U-46619 was approximately 30% greater than that of lungs exposed to either U-46619 (P < 0.02) or PGE(2) (P < 0.01) alone. When paired measurements of RT taken before and then 15 min after the addition of U-46619 were compared, PGI(2) was found to attenuate U-46619-induced increases in RT (P < 0.01). These data suggest that PGE(2), PGI(2), and PGF(2alpha) potentiate the effects of TxA(2)-receptor activation on pulmonary microvascular permeability.     

A thromboxane mimetic, U-46619, produces plasma exudation in airways of the guinea pig.

Thromboxane A2 (TxA2) has been implicated in airway responses to allergen and in the bronchial hyperresponsiveness observed in asthma. Furthermore a TxA2 receptor antagonist and a TxA2 synthase inhibitor inhibit plasma exudation in airways induced by inhaled platelet-activating factor. To evaluate whether TxA2 has any direct effect on plasma exudation in the airways, we studied the effect of a stable TxA2 mimetic (U-46619; 2, 20, and 200 nmol/kg iv) on lung resistance (RL) and Evans blue dye extravasation (marker of plasma albumin; 20 mg/kg iv) at the airway levels of trachea, main bronchi, and proximal and distal intrapulmonary airways in anesthetized, tracheostomized, and mechanically ventilated guinea pigs. Injection of U-46619 produced an immediate and marked dose-dependent increase in RL, which peaked at approximately 30 s. At the highest dose of U-46619, we also observed a later increase in RL, starting at approximately 3 min and reaching a second peak at approximately 8 min. Mean systemic blood pressure increased in a dose-dependent manner [maximum 82 +/- 8 (SE) mmHg]. U-46619 also produces dose-dependent plasma exudation, measured as Evans blue dye extravasation, at all airway levels as well as into the tracheal lumen. Airway responses to U-46619 (200 nmol/kg iv) were abolished in animals pretreated with the TxA2 receptor antagonist ICI-192605 (0.5 mg/kg iv). We conclude that U-46619, despite being a vasoconstrictor, is potent in inducing plasma exudation in airways and that this effect is mediated via a TxA2 receptor.     

Pregnancy reduces RhoA/Rho kinase and protein kinase C signaling pathways downstream of thromboxane receptor activation in the rat uterine artery

During pregnancy, reduced vascular responses to constrictors contribute to decreased uterine and total vascular resistance. Thromboxane A(2) (TxA(2)) is a potent vasoconstrictor that exerts its actions via diverse signaling pathways, and its biosynthesis increases in preeclampsia. In this study, we hypothesized that maternal vascular responses to TxA(2) will be attenuated via Rho kinase, PKC, p38 MAPK, and ERK1/2 signaling pathways. Isolated ring segments of uterine and small mesenteric arteries from late pregnant (19-21 days) and virgin rats were suspended in a myograph, and isometric force was measured. Pregnancy did not affect uterine and mesenteric artery responses to the TxA(2) analog U-46619 (10(-9)-10(-5) M), but transduction signals associated with these contractions were different between pregnant and nonpregnant rats. Inhibition of Rho kinase (10(-6) M Y-27632) reduced sensitivity to U-46619 in virgin uterine vessels but did not inhibit these contractions in pregnant uterine arteries and had no effect on mesenteric vessels. Treatment of arterial segments with a PKC inhibitor (10(-6) M bisindolylmaleimide I) reduced U-46619-induced contractions in virgin uterine and mesenteric arteries and in pregnant mesenteric arteries. Pregnant uterine arteries, however, were unresponsive to PKC inhibition. Inhibition of ERK1/2 (10(-5) M PD-98059) and p38 MAPK (10(-5) M SB-203580) reduced U46619-induced contractions in nonpregnant vessels and in pregnant uterine and mesenteric vessels. These data suggest that normal pregnancy does not affect uterine and mesenteric contractile responses to TxA(2) but reduces the contribution of Rho kinase and PKC signaling pathways to these contractions in the uterine vasculature. In contrast, the role of ERK1/2 and p38 MAPK in U-46619-induced uterine contractions remains unchanged with pregnancy. TxA(2)-associated transduction signals and its regulators might present potential targets for the development of new treatments for preeclampsia and other pregnancy-associated vascular diseases.     

Daltroban blocks thromboxane responses in the pulmonary vascular bed of the cat.

The influence of daltroban (BM13.505; SK&F 96148), a thromboxane (Tx) A2-receptor-blocking agent, on responses to the TxA2 mimics U-46619 and U-44069 was investigated in the pulmonary vascular bed of the intact-chest cat under constant-flow conditions. Daltroban (5 mg/kg iv) had no significant effect on mean baseline vascular pressures but significantly decreased responses to the TxA2 mimics without altering responses to prostaglandin (PG) F2 alpha or PGD2 or the PGD2 metabolite 9 alpha, 11 beta-PGF2. Dose-response curves for U-46619 and U-44069 were shifted to the right in a parallel manner, and daltroban had no significant effect on responses to norepinephrine, serotonin, angiotensin II, BAY K 8644, endothelin-(ET) 1, ET-2, or platelet-activating factor (PAF). After administration of daltroban, responses to U-46619 returned to 50% of control in 90 min and responses to the PG and TxA2 precursor arachidonic acid were decreased significantly. These results suggest that daltroban selectively antagonizes TxA2-receptor-mediated responses in a competitive and reversible manner. These data provide support for the hypothesis that discrete TxA2 receptors unrelated to receptors stimulated by PGF2 alpha, PGD2, or 9 alpha, 11 beta-PGF2 are present in the pulmonary vascular bed of the cat. The present data suggest that pulmonary vasoconstrictor responses to PAF and ET peptides are not dependent on activation of TxA2 receptors in the cat.     

Myocardial ischemia-mediated excitatory reflexes: a new function for thromboxane A2?

Clinical and experimental evidence has shown that myocardial ischemia activates cardiac spinal afferents that mediate sympathoexcitatory reflex responses. During myocardial ischemia, thromboxane A2 (TxA2) is released in large quantities by activated platelets in the coronary circulation of patients with coronary artery disease. We hypothesized that endogenous TxA2 contributes to sympathoexcitatory reflexes during myocardial ischemia through stimulation of TxA2/prostaglandin endoperoxide (TP) receptors. Regional myocardial ischemia was induced by occlusion of a diagonal branch of left anterior descending coronary artery of anesthetized cats. Hemodynamic parameters and renal sympathetic nerve activity were recorded after sinoaortic denervation and bilateral vagotomy. Regional myocardial ischemia evoked significant increases in mean blood pressure (122+/-10 vs. 139+/-12 mmHg, before vs. ischemia), aortic flow (153+/-18 vs. 167+/-20 ml/min), first derivative of left ventricular pressure at 40-mmHg developed pressure (2,736+/-252 vs. 2,926+/-281 mmHg/s), systemic vascular resistance (0.6+/-0.1 vs. 0.9+/-0.12 peripheral resistance units), and renal sympathetic nerve activity (by 22%). The reflex nature of the excitatory responses was confirmed by observing its disappearance after blockade of cardiac nerve transmission with intrapericardial 2% procaine treatment. Moreover, application of U-46619 (2.5-10 microg), a TxA2 mimetic, on the heart caused graded increases in mean arterial pressure and renal nerve activity, responses that were abolished 3 min after local blockade of cardiac neural transmission with intrapericardial procaine. BM 13,177 (30 mg/kg iv), a selective TP receptor antagonist, eliminated the reflex responses to U-46619 and significantly attenuated the excitatory responses during brief (5 min) regional myocardial ischemia. The sympathoexcitatory reflex responses to U-46619 were unchanged by blockade of histamine H1 receptors with pyrilamine and serotonin 5-HT3 receptors with tropisetron, indicating specificity of this TP receptor agonist. These data indicate that endogenous TxA2 participates in myocardial ischemia-mediated sympathoexcitatory reflex responses through a TP receptor mechanism.     

The F2-isoprostane, 8-epi-prostaglandin F2 alpha, a potent agonist of the vascular thromboxane/endoperoxide receptor, is a platelet thromboxane/endoperoxide receptor antagonist.

F2-isoprostanes are a recently discovered series of prostaglandin (PG)F2-like compounds that are produced in vivo in humans by nonenzymatic free radical catalyzed peroxidation of arachidonic acid. One of the compounds that can be produced in abundance by this mechanism is 8-epi-PGF2 alpha. 8-epi-PGF2 alpha is a potent vasoconstrictor in the rat, an effect that has been shown to be mediated via interaction with vascular thromboxane (TxA2)/endoperoxide (PGH2) receptors. In an effort to further understand the biological properties of this prostanoid in relation to its ability to interact with TxA2/PGH2 receptors, we examined its effects on human and rat platelets. At concentrations of 10(-6) M and 10(-5) M, 8-epi-PGF2 alpha induced only a shape change in human platelets and at higher concentrations (10(-4) M) induced reversible but not irreversible aggregation. Both the shape change and reversible aggregation were unaffected by indomethacin but were inhibited by the TxA2/PGH2 receptor antagonist SQ29548. Conversely, 8-epi-PGF2 alpha inhibited platelet aggregation induced by the TxA2/PGH2 receptor agonists U46619 (10(-6) M) and IBOP (3.3 x 10(-7) M) with an IC50 of 1.6 x 10(-6) M and 1.8 x 10(-6) M, respectively. 8-epi-PGF2 alpha also inhibited platelet aggregation induced by arachidonic acid. Similarly, in rat platelets, 8-epi-PGF2 alpha alone induced only modest reversible aggregation but completely inhibited U46619-induced aggregation.     

Enhanced microvascular permeability of PMA-induced acute lung injury is not mediated by cyclooxygenase products.

Products of cyclooxygenase activity have been proposed to mediate the pulmonary hypertension and increased microvascular permeability associated with phorbol myristate acetate- (PMA) induced acute lung injury. Previously, we reported that thromboxane (Tx) does not mediate PMA-induced pulmonary hypertension in intact anesthetized dogs. In the present study, PMA was administered to isolated canine lungs perfused with autologous blood at constant flow to investigate a possible role for Tx in the PMA-induced increase in microvascular permeability. Changes in permeability were assessed by determining changes in the capillary filtration coefficient (Kfc). In lobes pretreated with papaverine to prevent PMA-induced increases in pulmonary vascular resistance, Kfc increased from a baseline value of 0.2 +/- 0.03 to 1.5 +/- 0.29 ml.min-1.cmH2O-1.100 g wet lobe wt-1 (P < 0.01) 30 min after PMA (5.8 x 10(-8) M, n = 10). Concomitantly, TxB2, the stable metabolite of TxA2, increased from 138 +/- 44 to 1,498 +/- 505 pg/ml (P < 0.05) in the blood. Both the selective Tx synthase inhibitor, OKY-046 (7 x 10(-4) M, n = 6), and the cyclooxygenase inhibitor, indomethacin (10(-4) M, n = 7), prevented the PMA-induced increase in TxB2, but neither compound attenuated the PMA-induced increase in Kfc. ONO-3708 (10(-6) M), a selective prostaglandin (PG) H2/TxA2 receptor antagonist, prevented the vasoconstriction resulting from administration of U-46619, a stable PGH2/TxA2 receptor agonist, but it did not prevent the PMA-induced increases in Kfc (n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)     

Amplification of sumatriptan-induced contraction in rabbit saphenous vein but not in basilar artery

The modulation of serotonin (5-HT(1B/1D)) receptor-induced vascular contractility by histamine and U-46619 was compared in the rabbit basilar artery and saphenous vein. In the saphenous vein, histamine (5 x 10(-7) M) significantly increased the potency (from pEC(50) of 6.0 to 6.8) and efficacy (from 52.3% to 88.2%) of sumatriptan. Likewise, U-46619 (5 x 10(-9) M) also increased the potency (from pEC(50) of 5.9 to 6.6) and efficacy (from 51.8% to 92.1%) of sumatriptan in the saphenous vein. In contrast, equieffective concentrations of histamine (10(-7) M) and U-46619 (3 x 10(-9) M) failed to amplify contraction to sumatriptan in the basilar artery. Contraction to sumatriptan was inhibited by nitrendipine (10(-7) M) in the basilar artery but not in the saphenous vein, suggesting that different contractile signaling mechanisms are operating in these tissues. Furthermore, U-46619- and thrombin-induced contractility in the basilar artery were also not amplified by histamine, suggesting that lack of amplification of contraction in the basilar artery was not restricted to sumatriptan but was rather a characteristic of this cerebral vessel. These data suggest that in the in vivo plasma milieu sumatriptan will more markedly contract the peripheral saphenous vein than the basilar artery, a cerebral blood vessel.     

Thromboxane A2-induced arrhythmias in the anesthetized rabbit

Experiments were conducted in the anesthetized rabbit to investigate mechanisms for arrhythmias that occur after left atrial injection of the thromboxane A(2) (TxA(2)) mimetic U-46619. Arrhythmias were primarily of ventricular origin, dose dependent in frequency, and TxA(2) receptor mediated. The response was receptor specific since arrhythmias were absent after pretreatment with a specific TxA(2) receptor antagonist (SQ-29548) and did not occur in response to another prostaglandin, PGF(2alpha). Alterations in coronary blood flow were unlikely the cause of these arrhythmias because coronary blood flow (as measured with fluorescent microspheres) was unchanged after U-46619, and there were no observable changes in the ECG-ST segment. In addition, arrhythmias did not occur after administration of another vasoconstrictor (phenylephrine). The potential involvement of autonomic cardiac efferent nerves in these arrhythmias was also investigated because TxA(2) has been shown to stimulate peripheral nerves. Pretreatment of animals with the beta-adrenergic receptor antagonist propranolol did not reduce the frequency of these arrhythmias. Pretreatment with atropine or bilateral vagotomy resulted in an increased frequency of arrhythmias, suggesting that parasympathetic nerves may actually inhibit the arrhythmogenic activity of TxA(2). These experiments demonstrate that left atrial injection of U-46619 elicits arrhythmias via a mechanism independent of a significant reduction in coronary blood flow or activation of the autonomic nervous system. It is possible that TxA(2) may have a direct effect on the electrical activity of the heart in vivo, which provides significant implications for cardiac events where TxA(2) is increased, e.g., after myocardial ischemia or administration of cyclooxygenase-2 inhibitors.     

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).     

Effects of platelet-activating factor and thromboxane A2 on isolated perfused guinea pig liver

Lipid mediators, thromboxane A2 (TxA2) and platelet-activating factor (PAF), are potent vasoconstrictors, and have been implicated as mediators of liver diseases, such as ischemic-reperfusion injury. We determined the effects of a TxA2 analogue (U-46619) and PAF on the vascular resistance distribution and liver weight (wt) in isolated guinea pig livers perfused with blood via the portal vein. The sinusoidal pressure was measured by the double occlusion pressure (P(do)), and was used to determine the pre- (R(pre)) and post-sinusoidal (R(post)) resistances. U-46619 and PAF concentration-dependently increased the hepatic total vascular resistance (R(t)). The minimum concentration at which significant vasoconstriction occurs was 0.001 microM for PAF and 0.1 microM for U-46619. Moreover, the concentration of U-46619 required to increase R(t) to the same magnitude is 100 times higher than PAF. Thus, the responsiveness to PAF was greater than that to U-46619. Both agents increased predominantly R(pre) over R(post). U-46619 caused a sustained liver weight loss. In contrast, PAF also caused liver weight loss at lower concentrations, but it produced liver weight gain at higher concentrations (2.5 +/- 0.3 per 10g liver weight at 1 microM PAF), which was caused by substantial post-sinusoidal constriction and increased P(do). In conclusion, both TxA2 and PAF contract predominantly the pre-sinusoidal veins. TxA2 causes liver weight loss, while PAF at high concentrations increases liver weight due to substantial post-sinusoidal constriction in isolated guinea pig livers.     

Progesterone regulation of vascular thromboxane A(2) receptors in rhesus monkeys

We hypothesized that progesterone regulates thromboxane A(2) receptor (TxA(2)R) density in primate vascular muscle and that TxA(2)R density correlates with coronary reactivity in vivo and in vitro. Reactivity to serotonin + U-46619 was determined by angiography in surgically postmenopausal [ovariectomized (Ovx)] rhesus monkeys without progesterone replacement and after 2-wk progesterone treatment (1-2 ng/ml). In untreated Ovx animals, 100 micromol/l serotonin + 1 micromol/l U-46619 (syringe concentrations) provoked vasospasm-like constrictions in six of six monkeys; zero of six progesterone-treated monkeys developed vasospasms. Sustained Ca(2+) responses in vascular muscle cells isolated from Ovx coronaries (208 +/- 63% of basal 20 min after stimulation) treated with serotonin + U-46619 contrasted with transient Ca(2+) responses (143 +/- 18% of basal and decreasing 5 min after stimulation) in progesterone-treated monkeys. The maximum density of [1S-(1I,2J(5Z),3I(1E,3R*),4I)]-7-[3-(3-hydroxy-4-(4'-[(125)I]iodophenoxy)- 1-butenyl)-7-oxabicyclo[2.2.1]heptan-2-yl]-5-heptenoic acid ([(125)I]-BOP) binding was greater (P < 0.01) in carotid arteries and aortic membranes from Ovx (109 +/- 11 fmol/mg) compared with progesterone-treated (43 +/- 15 fmol/mg) monkeys. TxA(2)R immunolabeling revealed greater coronary TxA(2)R labeling in Ovx compared with progesterone-treated monkeys. The results suggest that progesterone can decrease arterial TxA(2)R in Ovx monkeys.     

Leukotriene D4 reduces coronary blood flow in the anesthetized dog

We studied the effects of intracoronary administration of leukotriene (LT)D4 on coronary blood flow and myocardial function in chloralose anesthetized dogs. For comparison, the effects of injections of U-46619 were examined in the same dogs. Both LTD4 and U-46619 decreased coronary blood flow, left ventricular dP/dt and cardiac output. LTD4 was ten times more potent than U-46619 in decreasing coronary blood flow. The effects of neither drug were different after indomethacin administration.     

Melatonin potentiates contractile responses to serotonin in isolated porcine coronary arteries

The present study was designed to determine the effects of melatonin on coronary vasomotor tone. Porcine coronary arteries were suspended in organ chambers for isometric tension recording. Melatonin (10(-10)-10(-5) M) itself caused neither contraction nor relaxation of the tissues. Serotonin (10(-9)-10(-5) M) caused concentration-dependent contractions of coronary arteries, and in the presence of melatonin (10(-7) M) the maximal response to serotonin was increased in rings with but not without endothelium. In contrast, melatonin had no effect on contractions produced by the thromboxane A(2) analog U-46619 (10(-10)-10(-7) M). The melatonin-receptor antagonist S-20928 (10(-6) M) abolished the potentiating effect of melatonin on serotonin-induced contractions in endothelium-intact coronary arteries, as did treatment with 1H-[1, 2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10(-5) M), methylene blue (10(-5) M), or N(G)-nitro-L-arginine (3 x 10(-5) M). In tissues contracted with U-46619, serotonin caused endothelium-dependent relaxations that were inhibited by melatonin (10(-7) M). Melatonin also inhibited coronary artery relaxation induced by sodium nitroprusside (10(-9)-10(-5) M) but not by isoproterenol (10(-9)-10(-5) M). These results support the hypothesis that melatonin, by inhibiting the action of nitric oxide on coronary vascular smooth muscle, selectively potentiates the vasoconstrictor response to serotonin in coronary arteries with endothelium.     

Leukotriene D4 reduces coronary blood flow in the anesthetized dog

We studied the effects of intracoronary administration of leukotriene (LT)D₄ on coronary blood flow and myocardial function in chloralose anesthetized dogs. For comparison, the effects of injections of U-46619 were examined in the same dogs. Both LTD₄ and U-46619 decreased coronary blood flow, left ventricular dP/dt and cardiac output. LTD₄ was ten times more potent than U-46619 in decreasing coronary blood flow. The effects of neither drug were different after indomethacin administration.     

Characterization of thromboxane receptor blocking effects of SQ 29548 in the feline pulmonary vascular bed.

The effects of SQ 29548, a thromboxane (Tx) A2 receptor blocking agent, on responses to the TxA2 mimic U46619 were investigated in the pulmonary vascular bed of the intact-chest cat under constant-flow conditions. The administration of SQ 29548 in doses of 0.25-1 mg/kg iv reduced vasoconstrictor responses to U-46619; however, responses to prostaglandins (PG) F2 alpha and D2 and to serotonin were also decreased. After administration of SQ 29548 in doses of 0.05-0.1 mg/kg iv, responses to U-46619 and U-44069 were reduced significantly, and the dose-response curves for these TxA2 mimics were shifted to the right in a parallel manner at a time when responses to PGF2 alpha and PGD2 were not altered. The low doses of the TxA2 receptor blocking agent significantly reduced responses to the PG and TxA2 precursor arachidonic acid but were without significant effect on vasoconstrictor responses to serotonin; histamine; norepinephrine; angiotensin II; the major PGD2 metabolite 9 alpha,11 beta-PGF2; BAY K 8644, an agent that enhances calcium entry; and endothelin-1. The present data show that at low doses SQ 29548 selectively blocks TxA2 receptor-mediated responses in a competitive and reversible manner in the pulmonary vascular bed. These data suggest that responses to arachidonic acid are mediated in large part by the formation of TxA2 and provide evidence in support of the hypothesis that a discrete TxA2 receptor unrelated to PGF2 alpha or PGD2 receptors is present in undefined resistance vessel elements in the feline pulmonary vascular bed.     

Influence of SQ 30741 on thromboxane receptor-mediated responses in the feline pulmonary vascular bed.

The effects of SQ 30741, a thromboxane A2 (TxA2) receptor blocking agent, on responses to the TxA2 mimic, U-46619, were investigated in the pulmonary vascular bed of the intact-chest cat under constant-flow conditions. The administration of SQ 30741 in doses of 1-2 mg/kg iv markedly reduced vasoconstrictor responses to U-46619 without altering responses to prostaglandin (PG) F2 alpha or PGD2 and serotonin. SQ 30741 had no significant effect on mean vascular pressures in the cat, and the dose-response curve for U-46619 was shifted to the right in a parallel manner with a similar apparent maximal response. In addition to not altering responses to PGF2 alpha, PGD2 alpha, or serotonin, SQ 30741 (2 mg/kg iv) was without significant effect on pulmonary vasoconstrictor responses to the PGD2 metabolite 9 alpha, 11 beta-PGF2, norepinephrine, angiotensin II, BAY K 8644, endothelin 1, or endothelin 2. Although responses to vasoconstrictor agents, which act through a variety of mechanisms, were not altered, responses to the PG and TxA2 precursor, arachidonic acid, were reduced significantly. The duration of the TxA2 receptor blockade was approximately 30 and 75 min at the 1- and 2-mg/kg iv doses of the antagonist, respectively. The present data show that SQ 30741 selectively blocks TxA2 receptor-mediated responses in a competitive and reversible manner in the pulmonary vascular bed. These data suggest that responses to arachidonic acid are due in large part to the formation of TxA2 and that discrete TxA2 receptors unrelated to receptors activated by PGD2 or PGF2 alpha are most likely located in resistance vessel elements in the feline pulmonary vascular bed.     

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.     

The effects of vasoconstricting prostanoids on the coronary and hindlimb vascular beds of the conscious sheep

Vasoconstricting prostaglandins were injected, in bolus doses, into the lower abdominal aorta on the left circumflex coronary artery (LCCA) of conscious sheep. Local blood flow, mean arterial pressure (MAP), heart rate (HR) and ECG were continuously monitored. Thromboxane B2 had no effect on either vascular bed in doses up to 100 micrograms. PGF2 alpha produced mild vasoconstriction in both vascular beds with no systemic response. The endoperoxide analogues, U-44069 and U-46619, produced complex responses in both vascular beds. Initial vasodilation was followed rapidly by prolonged vasoconstriction. In the coronary circulation, vasoconstriction was temporarily masked by a hyperaemic phase. The U-compounds also affected MAP, possibly as a result of pulmonary and systemic vasoconstriction.     

A novel loop diuretic, torasemide, inhibits thromboxane A2-induced contraction in the isolated canine coronary artery.

We examined the effect of a novel antihypertensive diuretic, torasemide, on the vasoconstriction induced by TXA2 in the isolated canine coronary artery. Carbocyclic thromboxane A2 (CTA2), a stable analogue of the potent coronary vasoconstrictor thromboxane A2, exhibited a slow onset and progressive contraction of isolated canine coronary arteries at 2 x 10(-8) M. Torasemide (10(-7) approximately 10(-4) M) elicited a dose-dependent vasodilating action in the isolated canine coronary arteries contracted by CTA2, whereas indapamide or furosemide had little effect on this preparation. The maximum vasodilating response to torasemide was 45 +/- 12% of vasodilating effect induced by 10(-4) M papaverine. These results suggest that torasemide is a promising antihypertensive agent with a coronary protective effect.