The pharmacology of L-670,596, a potent and selective thromboxane/prostaglandin endoperoxide receptor antagonist

Preterm prelabor rupture of the membranes: A disease of the fetal membranes

Preterm prelabor rupture of the membranes (pPROM) remains a significant obstetric problem that affects 3-4% of all pregnancies and precedes 40-50% of all preterm births. pPROM arises from complex, multifaceted pathways. In this review, we summarize some old concepts and introduce some novel theories related to pPROM pathophysiology. Specifically, we introduce the concept that pPROM is a disease of the fetal membranes where inflammation-oxidative stress axis plays a major role in producing pathways that can lead to membrane weakening through a variety of processes. In addition, we report microfractures in fetal membranes that are likely sites of tissue remodeling during gestation; however, increase in number and morphometry (width and depth) of these microfractures in pPROM membranes suggests reduced remodeling capacity of membranes. Microfractures can act as channels for amniotic fluid leak, and inflammatory cell and microbial migration. Further studies on senescence activation and microfracture formation and their role in maintaining membrane homeostasis are needed to fill the knowledge gaps in our understanding of pPROM as well as provide better screening (biomarker and imaging based) tools for predicting women at high risk for pPROM and subsequent preterm birth.

Inverse agonism of SQ 29,548 and Ramatroban on Thromboxane A2 receptor

G protein-coupled receptors (GPCRs) show some level of basal activity even in the absence of an agonist, a phenomenon referred to as constitutive activity. Such constitutive activity in GPCRs is known to have important pathophysiological roles in human disease. The thromboxane A2 receptor (TP) is a GPCR that promotes thrombosis in response to binding of the prostanoid, thromboxane A2. TP dysfunction is widely implicated in pathophysiological conditions such as bleeding disorders, hypertension and cardiovascular disease. Recently, we reported the characterization of a few constitutively active mutants (CAMs) in TP, including a genetic variant A160T. Using these CAMs as reporters, we now test the inverse agonist properties of known antagonists of TP, SQ 29,548, Ramatroban, L-670596 and Diclofenac, in HEK293T cells. Interestingly, SQ 29,548 reduced the basal activity of both, WT-TP and the CAMs while Ramatroban was able to reduce the basal activity of only the CAMs. Diclofenac and L-670596 showed no statistically significant reduction in basal activity of WT-TP or CAMs. To investigate the role of these compounds on human platelet function, we tested their effects on human megakaryocyte based system for platelet activation. Both SQ 29,548 and Ramatroban reduced the platelet hyperactivity of the A160T genetic variant. Taken together, our results suggest that SQ 29,548 and Ramatroban are inverse agonists for TP, whereas, L-670596 and Diclofenac are neutral antagonists. Our findings have important therapeutic applications in the treatment of TP mediated pathophysiological conditions.

BQ-153, a novel endothelin (ET)A antagonist, attenuates the renal vascular effects of endothelin-1

Endothelin (ET)-l, leukotriene D4 and the thromboxane analogue, U-44069, were all shown to produce dose-dependent reductions in renal blood flow after direct injection into the renal artery of anaesthetized pigs. The effects of ET-1 differed from the other two mediators in that ET-1 caused a transient vasodilator followed by a prolonged vasoconstrictor response. The pressor response was not mediated by the secondary release of either leukotriene D4 or thromboxane A2 as evidenced by the lack of effect of appropriate receptor antagonist MK571 (3-{-2(7-chloro-2 quinolinyl) ethenyl}phenyl{3-(dimethylamino-3-oxopropyl)thio}methyl thio propionic acid) and L-670,596 respectively. This response, however, could be inhibited in a dose-dependent fashion by the selective ETA antagonist, BQ-153 (cyclo-d-sulphalanine-l-Pro-d-Val-l-Leu-d-Trp-). Following blockade by BQ-153 the vasodilator response was unaffected and a residual pressor response remained, suggesting that either or both of these effects were mediated either through an ETB or a novel, as yet undefined, endothelin receptor.

Design, synthesis and biological evaluation of a sulfonylcyanoguanidine as thromboxane A2 receptor antagonist and thromboxane synthase inhibitor

The synthesis and the structure of N-isopropyl-N′-[2-(3′-methylphenylamino)-5-nitrobenzenesulfonyl] urea (14) was drawn from two thromboxane A2 receptor antagonists structurally related to torasemide. Compound 14 showed an IC50 value of 22 nm for the thromboxane A2 (TXA2) receptor of human washed platelets. Compound 14 prevented platelet aggregation induced by arachidonic acid (0.6 mm) and U-46619 (1 μm) with an IC50 value of 0.45 and 0.15 μm, respectively. Moreover, 14 relaxed the rat isolated aorta and guinea-pig trachea precontracted by U-46619, a TXA2 agonist. Its efficacy (IC50) was 20.4 and 5.47 nm, respectively. Finally, 14 (1 μm) completely inhibited TXA2 synthase of human platelets. The pKa value and the crystallographic data of 14 were determined and used to propose an interaction model between the TXA2 antagonists related to torasemide and their receptor.

Thromboxane A(2) receptors mediate pulmonary hypertension in 60% oxygen-exposed newborn rats by a cyclooxygenase-independent mechanism

Endothelin-1 (ET-1) mediates the development of pulmonary hypertension (PHT) in newborn rats exposed to 60% O(2) for 14 days, a model for human chronic neonatal lung injury. ET-1 production by d-14 rat pulmonary artery smooth muscle cells in vitro was markedly increased by thromboxane (TX) A(2) receptor agonists and inhibited by a competitive antagonist. We hypothesized that stimulation of the TX A(2) receptor contributed to O(2)-mediated PHT in vivo. Newborn rat pups received daily intraperitoneal injections of L670596, a competitive TX A(2) receptor antagonist, or 5,5-dimethyl-3-(3-fluorophenyl)4-(4-methylsulfonyl)phenyl-2(5H)-furanone (DFU), a cyclooxygenase-2 inhibitor, during 14 days of 60% O(2) or air exposure. L670596, but not DFU, prevented 60% O(2)-mediated right ventricular and small pulmonary vessel smooth muscle hypertrophy. Lung ET-1 content was significantly reduced by L670596 in 60% O(2)-exposed animals. We conclude that TX A(2) receptor activation, though not by TX A(2), caused upregulation of ET-1 and PHT in this model. A likely mediator is the stable lipid peroxidation product, 8-iso-prostane, which acts as an incidental ligand of the TX A(2) receptor and is a potent inducer of ET-1 production by cultured d-14 rat pulmonary artery smooth muscle cells in vitro.

Role of thromboxane in retinal microvascular degeneration in oxygen-induced retinopathy

Microvascular degeneration is an important event in oxygen-induced retinopathy (OIR), a model of retinopathy of prematurity. Because oxidant stress abundantly generates thromboxane A2 (TxA2), we tested whether TxA2 plays a role in retinal vasoobliteration of OIR and contributes to such vascular degeneration by direct endothelial cytotoxicity. Hyperoxia-induced retinal vasoobliteration in rat pups (80% O2 exposure from postnatal days 5-14) was associated with increased TxB2 generation and was significantly prevented by TxA2 synthase inhibitor CGS-12970 (10 mg x kg(-1) x day(-1)) or TxA2-receptor antagonist CGS-22652 (10 mg x kg(-1) x day(-1)). TxA2 mimetics U-46619 (EC50 50 nM) and I-BOP (EC50 5 nM) caused a time- and concentration-dependent cell death of neuroretinovascular endothelial cells from rats as well as newborn pigs but not of smooth muscle and astroglial cells; other prostanoids did not cause cell death. The peroxidation product 8-iso-PGF2, which is generated in OIR, stimulated TxA2 formation by endothelial cells and triggered cell death; these effects were markedly diminished by CGS-12970. TxA2-dependent neuroretinovascular endothelial cell death was mostly by necrosis and to a lesser extent by apoptosis. The data identify an important role for TxA2 in vasoobliteration of OIR and unveil a so far unknown function for TxA2 in directly triggering neuroretinal microvascular endothelial cell death. These effects of TxA2 might participate in other ischemic neurovascular injuries.

Variable responses to prostaglandin E(2) in human non-pregnant myometrium

Cumulative concentration-effect curves for prostaglandin E(2), sulprostone and butaprost were constructed in matched strips of human non-pregnant myometrium from 14 different donors. All samples were obtained from the mid-lateral wall of the uterus. Prostaglandin E(2) produced four types of concentration-effect curves: monophasic inhibitory (n = 7), monophasic excitatory (n = 2), biphasic consisting of an excitatory phase followed by an inhibitory phase (n = 4), and biphasic consisting of an inhibitory phase followed by an excitatory phase (n = 1). Sulprostone produced excitation of spontaneous contractile activity in all tissues (mean pEC(50) = 9.1+/-0.2, range 8.1-10.1, n = 14). Butaprost produced relaxation of cloprostenol-stimulated contractile activity in all tissues (mean pEC(50) = 5.7 +/- 0.1, range 5.0-6.9, n = 14). The mean pEC(50) value for sulprostone was significantly higher in tissues where prostaglandin E(2) caused some excitation (pEC(50) = 9.4 +/- 0.2, n = 7) compared to those where prostaglandin E(2) caused only inhibition (pEC(50) = 8.8 +/- 0.2, n = 7). Mean pEC(50) values for butaprost were not significantly different between these groups. These data suggest that (a) variability in EP receptor-mediated responses exists within a single anatomical site; (b) both excitatory and inhibitory EP receptor-mediated pathways are always operative in human non-pregnant myometrium, regardless of the type of tissue response to prostaglandin E(2); and (c) regulation of EP receptor-mediated responses occurs predominantly in the excitatory (EP(3) or EP(1) receptor) pathway rather than the inhibitory (EP(2) receptor) pathway.

Operational correlates of prostanoid TP receptor expression in human non-pregnant myometrium are unaffected by excision site or menstrual cycle status of the donor

1. Cumulative concentration-effect curves for the selective prostanoid TP receptor agonist, U46619, were constructed in strips of human non-pregnant myometrium grouped according to tissue excision site (top, lateral wall, lower uterine segment, sub-serosal or sub-endometrial), tissue orientation (strips cut either parallel or perpendicular to the serosa) and donor menstrual status (proliferative or secretory phase). 2. U46619 was excitatory in all tissues. There was no significant difference in either pEC50 or maximum response between groups (P<0.05). The range of pEC50 values was 6.8+/-0.1 (lateral wall, proliferative phase, n=5) to 7.1+/-0.3 (lateral wall, secretory phase, n=5). The range of maximum response values was 0.9+/-0.8 N cm-2 (lateral wall, proliferative phase, n=5) to 3.1+/-1.0 N cm-2 (lateral wall, secretory phase, n=5). 3. Saturation binding analyses were conducted using the radiolabelled TP receptor agonist, [125I]-BOP. Binding parameters were estimated for membranes prepared from human non-pregnant myometrium excised from the lateral wall and grouped according to donor menstrual status. 4. There were no significant differences in the mean pKd and [R]tot values for [125I]-BOP binding between the two groups (proliferative phase: pKd=8.3+/-0.3, [R]tot=412+/-319 fmol mg protein-1, n=5; secretory phase: pKd=8.5+/-0.4, [R]tot=369+/-192 fmol mg protein-1, n=6; P<0.05). 5. These data indicate that U46619-mediated responses in human non-pregnant myometrium are not influenced by tissue excision site, tissue orientation or donor menstrual status and that [125I]-BOP binding is not influenced by donor menstrual status. This suggests that the TP receptor population is homogeneous throughout the human non-pregnant myometrium, and not subject to hormonal regulation.

Inhibitory prostanoid EP receptors in human non-pregnant myometrium

Prostanoid EP receptor agonists relaxed cloprostenol-stimulated contraction of human non-pregnant myometrium in vitro with pEC50 values of (n = 4): prostaglandin E2, 7.8+/-0.2 > 1-OH prostaglandin E1, 7.2+/-0.3 > misoprostol, 6.6+/-0.1 > 16,16-dimethyl prostaglandin E2, 6.3+/-0.7 > butaprost, 5.7+/-0.3 > 11-deoxy prostaglandin E1, 5.5+/-0.2 = AH13205 ((+)-trans-2-[4-(1hydroxyhexyl)phenyl]-5-oxocyclopentaneheptano ic acid), 5.5+/-0.2. The EP4 receptor antagonist AH23848B ([1alpha(z), 2beta5alpha]-(+/-)-7-[5-[[(1,1'-biphenyl)-4-yl]methoxy]-2-(4-morph olinyl)-3-oxo-cyclopentyl]-4-heptenoic acid) (29 microM) had no effect on concentration-effect curves to the EP receptor agonists. The mixed prostanoid receptor antagonist AH6809 (6-isopropoxy-9-oxaxanthene-2-carboxylic acid) competitively antagonised prostaglandin E2 with a pA2 of 5.6+/-0.2. AH6809 (42 microM) antagonised misoprostol, 11-deoxy prostaglandin E1, and the prostanoid DP receptor agonist BW245C (5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl)hydantoin) with apparent pA2 values of 5.6+/-0.3, 5.1+/-0.9 and 5.9+/-0.4 (n = 4), respectively, but was ineffective against the IP receptor agonist cicaprost (n = 4). The prostanoid DP receptor antagonist BW A868C (3-benzyl-5-(6-carboxyhexyl)-1-(2-cyclohexyl-2-hydroxyethylamino)h ydantoin) (50 nM) had no effect on responses to prostaglandin E2 or misoprostol. The presence of an AH6809-sensitive, AH23848B- and BW A868C-insensitive mechanism is consistent with the hypothesis that inhibitory EP receptor agonists cause relaxation of human non-pregnant myometrium by an EP2 receptor-mediated process.

A novel mechanism for vasoconstrictor action of 8-isoprostaglandin F2 alpha on retinal vessels

Using a video-imaging technique, we characterized the effects of 8-isoprostaglandin F2 alpha (8-iso-PGF2 alpha) on retinal vasculature from piglets. 8-Iso-PGF2 alpha potently contracted (EC50 = 5.9 +/- 0.5 nM) retinal vessels. These effects were completely antagonized by the cyclooxygenase inhibitor indomethacin, the thromboxane synthase blocker CGS-12970, the thromboxane receptor antagonist L-670596, and the putative inhibitor of the non-voltage-dependent receptor-operated Ca2+ pathway SKF-96365; constrictor effects of 8-iso-PGF2 alpha were also partly attenuated by the ETA-receptor blocker BQ-123 and an inhibitor of endothelin-converting enzyme, phosphoramidon, but was negligibly affected by the L-type voltage-gated Ca2+ channel blocker nifedipine. Correspondingly, 8-iso-PGF2 alpha elicited endothelin release from retinal preparations, which was markedly reduced by SKF-96365. 8-Iso-PGF2 alpha also increased thromboxane production in the retina and cultured endothelial cells, but not on retinovascular smooth muscle cells; these effects of 8-iso-PGF2 alpha were blocked by indomethacin, CGS-12970, SKF-96365, and EGTA, but not by nifedipine. 8-Iso-PGF2 alpha also increased Ca2+ transients in retinal endothelial cells, which were inhibited by SKF-96365 and EGTA, but not by nifedipine, whereas in smooth muscle cells U-46619, but not 8-iso-PGF2 alpha, stimulated a rise in Ca2+ transients. Finally, H2O2 + FeCl2 (in vitro) and anoxia followed by reoxygenation (in vivo) stimulated formation of 8-iso-PGF2 alpha in the retina. In conclusion, 8-iso-PGF2 alpha-induced retinal vasoconstriction is mediated by cyclooxygenase-generated formation of thromboxane and, to a lesser extent, by endothelin after Ca2+ entry into cells, possibly through receptor-operated channels. Retinal vasoconstriction to 8-isoprostanes might play a role in the genesis of ischemic retinopathies.

Key role for cyclooxygenase-2 in PGE2 and PGF2alpha receptor regulation and cerebral blood flow of the newborn

Ibuprofen, a cyclooxygenase (COX) inhibitor nonselective for either COX-1 or COX-2 isoform, upregulates cerebrovascular prostaglandin E2 (PGE2) and PGF2alpha receptors in newborn pigs. COX-2 was shown to be the predominant form of COX and the main catalyst of prostaglandin synthesis in the newborn brain. We proceeded to establish direct evidence that COX-2-generated prostaglandins govern PGE2 and PGF2alpha receptor density and function in the cerebral vasculature of the newborn. Hence, we determined PGE2 and PGF2alpha receptor density and functions in brain vasculature by using newborn pigs treated with saline, ibuprofen, COX-1 inhibitor (valerylsalicylate), or COX-2 inhibitors (DUP-697 and NS-398). Newborn brain PGE2 and PGF2alpha concentrations were significantly reduced by ibuprofen, DUP-697, and NS-398 but not by valerylsalicylate. In newborn pigs treated with DUP-697, NS-398, and ibuprofen, PGE2 and PGF2alpha receptor densities in brain microvessels were increased to adult levels; there was also a significant increase in inositol 1,4,5-trisphosphate (IP3) production and cerebral vasoconstrictor effects of 17-phenyl trinor PGE2 (EP1 receptor agonist), M&B-28767 (EP3 receptor agonist), PGF2alpha, and fenprostalene (PGF2alpha analog). Treatment with ibuprofen or DUP-697 also increased the upper blood pressure limit of cerebral cortex and periventricular blood flow autoregulation from 85 to > or = 125 mmHg (uppermost blood pressure studied). However, valerylsalicylate treatment did not affect cerebrovascular PGE2 and PGF2alpha receptors, IP3 production, or vasoconstrictor effects in newborn animals. These in vivo and in vitro observations indicate that COX-2 is mainly responsible for the regulation of PGE2 and PGF2alpha receptors and their functions in the newborn cerebral vasculature.

Effects of the isoprostane, 8-epi-prostaglandin F2 alpha, on the contractility of the human myometrium in vitro

8-epi-prostaglandin F2 alpha stimulated contraction of human myometrial strips obtained from five different donors at the time of hysterectomy with a pEC50 value of 6.3 +/- 0.5. In paired strips from the same donors the pEC50 value for the selective TP receptor agonist U46619 ([1R-[1a,4a,5b(Z),6a(1E,3S*)]]-7-[6-(3- hydroxy-1-octenyl)-2-oxabicyclo[2.2.1]hept-5-yl]-5-heptenoic acid) was 8.3 +/- 0.4. In strips from four other donors 8-epi-prostaglandin F2 alpha was ineffective whereas the pEC50 for U46619 was 6.9 +/- 0.3. Responses to 8-epi-prostaglandin F2 alpha were unaffected by the selective DP receptor antagonist BW A868C (3-benzyl-5-(6-carboxyhexyl)-1-(2- cyclohexyl-2-hydroxyethylamino)hydantoin) at 50 nM but were blocked by the selective TP receptor antagonist L670596 ((-)6,8-difluoro-9-p-methylsulfonyl benzyl-1,2,3,4- tetrahydrocarbazol-1-yl-acetic acid) at 50 nM. The pIC50 values obtained when the TP receptor antagonists GR 32191 ([1R- [1 alpha(Z),2 beta,3 beta,5 alpha]]-(+)-7-[5-[[(1,1'-biphenyl)-4- yl]methoxy]-3-hydroxy-2-(1-piperidinyl)cyclopentyl]-4-heptenoic acid), ICI D1542 ((4(Z)-6-[(2S,4S,5R)-2-[1-methyl-1-(2-nitro-4-tolyloxy)ethyl]- 4-(3-pyridyl)-1,3-dioxan-5-yl]hex-4-enoic acid), ICI 192605 (4(Z)-6-[(2,4,5-cis)-2-(2-chlorophenyl)-4-(2-hydroxyphenyl)-1,3- dioxan-5-yl]hexenoic acid), L670596 and SQ 29548 ([1S-(1 alpha,2 beta(5Z),3 beta,4 alpha]]-7- [3-[[2-[(phenylamino)carbonyl]hydrazino]methyl]-7- oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic acid) were added cumulatively to strips pre-contracted with an EC80 concentration of 8-epi-prostaglandin F2 alpha were not significantly different from those obtained when an EC80 concentration of U46619 was used. The effects of 8-epi-prostaglandin F2 alpha on strips pre-contracted with an EC80 concentration of U46619 were not different from those of U46619 itself. It is concluded that in the non-pregnant human myometrium 8-epi-prostaglandin F2 alpha is a medium potency contractile agonist acting predominantly at the TP receptor.

Functional characterization of the prostanoid DP receptor in human myometrium

Spontaneous contractile activity of strips of human myometrium obtained from non-pregnant donors at the time of hysterectomy was inhibited by the selective prostanoid DP receptor agonists BW 245C (5-(6-carboxyhexyl)-1-(3-cyclohexyl-3-hydroxypropyl)hydantoin) and ZK110841 ((5Z,13E)-(9R,11R,15S)-9 beta-chlor-15-cyclohexyl-11,15-dihydroxy-16,17,18,19, 20-pentanor-5,13-prostadienoic acid) with pEC50 values of 8.4 and 7.3 respectively but prostaglandin D2 produced a biphasic effect. In the presence of the TP receptor antagonist L670596 ((-)-6,8-difluoro-9-p-methylsulfonyl benzyl-1,2,3,4-tetrahydrocarbazol-1-yl-acetic acid), contractile activity induced by the FP receptor agonist, cloprostenol ([1R-[1 alpha(Z),2 beta(1E,3R),3 alpha,5 alpha]]-7-[2-[4-(3- chlorophenoxy)-3-hydroxy-7-butenyl]-3,5-dihydroxycyclopentyl]-5-he ptenoic acid), was inhibited by BW 245C (pEC50 = 7.5), ZK110841 (pEC50 = 6.7) and prostaglandin D2 (pEC50 = 6.3). Under these conditions both prostaglandin J2 and 9 alpha,11 beta-prostaglandin F2 were inhibitory partial agonists. All compounds were antagonized by the selective DP receptor antagonist BW A868C (3-benzyl-5-(6-carboxyhexyl)-1-(2-cyclohexyl-2-hydroxyethylamino)h ydantoin), but the pKB values were both concentration-dependent (pKB versus BW 245C at 10 nM = 9.1, at 50 nM = 8.3) and agonist-dependent (pKB at 10 nM versus BW 245C = 9.1, versus ZK110841 = 7.4). Both agonist and antagonist potencies support the existence of DP receptors in human myometrium. The concentration and agonist dependence of the action of BW A868C suggests that putative DP receptor agonists relax human myometrium by more than one mechanism. These observations may be explained by the existence of subtypes of DP receptor in human myometrium.

Sensory neuropeptide-mediated bronchoconstriction of the guinea pig lung by diamide; a comparison to hydrogen peroxide

The effect of the thiol oxidizing agent diamide on airway conductance, dynamic compliance and perfusion flow of isolated perfused and ventilated guinea pig lungs was investigated. When infused in the pulmonary circulation, diamide (100 microM) induced bronchoconstriction, but no effect on perfusion flow was observed. Although diamide exposure induced the formation of thromboxane A2, the thromboxane/prostaglandin endoperoxide receptor antagonist L-670,596 did not affect the decrease in conductance and compliance induced by diamide. Diamide induced the release of the sensory neuropeptide calcitonin gene-related peptide. The bronchoconstriction and the release of calcitonin gene-related peptide induced by diamide were abolished by capsaicin pretreatment of the guinea pigs. Combined pretreatment with the NK1 and NK2 receptor antagonists, CP-96,345 and SR-48968, attenuated the effect of diamide. Hydrogen peroxide-induced vaso- and bronchoconstriction was not affected by capsaicin-pretreatment, nor did hydrogen peroxide induce detectable release of calcitonin gene-related peptide. The results indicate that diamide activates sensory nerves and induces neuropeptide release and neurokinin receptor-mediated bronchoconstriction in the isolated perfused and ventilated guinea pig lung.

Hydrogen peroxide-induced broncho- and vasoconstriction in the isolated perfused and ventilated guinea pig lung

The effect of hydrogen peroxide on perfusion flow, airway conductance (Gaw) and dynamic compliance (Cdyn of isolated perfused and ventilated guinea pig lungs was investigated. Hydrogen peroxide (50 microM in the perfusion buffer) induced a decrease in Gaw and Cdyn and perfusion flow during 5 min. of exposure. Hydrogen peroxide also caused an increase in the levels of thromboxane in the perfusate of the lung. The constrictor effects as well as the formation of thromboxane were inhibited by the cyclooxygenase inhibitor ibuprofen (50 microM). The thromboxane/prostaglandin endoperoxide receptor antagonist L-670,596 (1 microM) abolished the effects of hydrogen peroxide on perfusion flow, Gaw and Cdyn, but did not affect the formation of thromboxane. The thromboxane-synthetase inhibitor carboxyheptylimidazole (100 microM) reduced both the hydrogen peroxide-induced formation of thromboxane and vaso- and bronchoconstriction, suggesting a predominant role for thromboxane A2 versus prostaglandin H2 in these effects. A role for platelet-activating factor in mediating the effect of hydrogen peroxide could not be supported, as the platelet-activating factor receptor antagonist WEB 2086 (10 microM) did not affect hydrogen peroxide induced vaso- and brochoconstriction. The results of this study show that hydrogen peroxide induces thromboxane A2 mediated vaso- and bronchoconstriction in the isolated perfused and ventilated guinea pig lung. Platelet-activating factor does not appear to play a significant role in the hydrogen peroxide-induced vaso- and bronchoconstriction. Our results also suggest that the perfused guinea pig lung is more sensitive to hydrogen peroxide than the perfused rat lung.

Role of endothelin ETB receptors in bronchoconstrictor and vasoconstrictor responses in guinea-pigs

In anesthetized and ventilated guinea-pigs, intravenous injections of endothelin (ET)-1 (0.5 nmol/kg), ET-3 (0.5 nmol/kg), and [Ala1,3,11,15]ET-1 (20 nmol/kg), an ETB-selective receptor agonist, induced bronchoconstrictor and transient vasoconstrictor responses. Only the ET-1-induced transient vasoconstriction was followed by a secondary sustained pressor response. The ETA-selective receptor antagonist, BQ-123 (1 mg/kg i.v.), attenuated only the sustained pressor response. These results indicate that the bronchoconstrictor and transient vasoconstrictor responses to endothelins in guinea-pigs are mediated by ETB receptors, whilst the sustained pressor response is mediated by ETA receptors. The thromboxane A2 receptor antagonist, L-670,596 (0.5 mg/kg, i.v.) and a high dose of BQ-123 (30 mg/kg i.v.) abolished the bronchoconstriction only without affecting the transient pressor response to endothelin isopeptides. These results suggest that the ETB-mediated bronchoconstriction depends on thromboxane A2 formation. The different sensitivity of these ETB-mediated transient responses to BQ-123 suggests the possible existence of distinct ETB receptor subtypes.

FLAP: a novel drug target for inhibiting the synthesis of leukotrienes

FLAP (5-lipoxygenase-activating protein) is a novel 18 kDa membrane protein that together with 5-lipoxygenase is required for cellular leukotriene biosynthesis. Tony Ford-Hutchinson describes a novel class of leukotriene biosynthesis inhibitor, exemplified by MK0886, that can bind to FLAP with high affinity, prevent the activation of 5-lipoxygenase by preventing the translocation of the enzyme from the cytosol to the membrane and inhibit leukotriene production. The term 'translocation inhibitors' may be used for inhibitors such as MK0886 and may represent a new class of drug.

Dual antagonism by L-636,499 of serotonin and thromboxane A2 induced aggregation of canine platelets

We evaluated the ability of L-636,499 (3-carboxyl-dibenzo-[b,f] thiepen-5,5-dioxide), a compound structurally similar to cyproheptadine, to antagonize U46619 (a TXA2/PGH2 mimetic) and serotonin (5-hydroxytryptamine; 5HT)-induced aggregation of canine platelets in vitro. L-636,499 antagonized competitively and dose-dependently aggregation induced by both 5HT and U46619, with pA2 values of 5.8 +/- 0.6 and 4.8 +/- 0.2, respectively. L-670,596, a potent TXA2/PGH2 receptor antagonist, and ketanser in, a potent 5HT2 receptor antagonist, yielded pA2 values of 7.0 +/- 0.3 and 9.0 +/- 0.2 vs. their respective agonists. These results show that despite its low potency vs. TXA2- and 5HT2-induced aggregation, L-636,499 antagonizes both physiologic mediators comparably.

Predominant functional roles for thromboxane A2 and prostaglandin E2 during late nephrotoxic serum glomerulonephritis in the rat

While much is known regarding acute nephrotoxic serum (NTS)-induced glomerular injury, the glomerular dynamics and pathophysiologic mediators of the more relevant chronic autologous phase remain poorly defined. Studies were performed in rats 14 d after injection of rabbit serum (n = 6), NTS in the absence (n = 6), or presence, of a cyclooxygenase inhibitor, ibuprofen (n = 6) or a thromboxane A2 (TxA2) receptor antagonist, L-670,596 (n = 5). A mesangial macrophage/monocyte infiltrate was noted with equal intensity in all NTS-treated rats. Glomerular generation rates of prostaglandin (PG) E2, PGF2a, and TxA2 in nephritic kidneys were dramatically increased as compared to controls. 2 wk after NTS, there was an increase in glomerular plasma flow rate (SNPF), attainment of filtration pressure disequilibrium, and augmentation of net transcapillary hydraulic pressure difference (delta P). Glomerular filtration rate (GFR), however, was reduced, due to a marked fall in the glomerular capillary ultrafiltration coefficient (Kf). Cyclooxygenase inhibition resulted in normalization of glomerular eicosanoid generation rates, amelioration of proteinuria, afferent vasoconstriction, and normalization of SNPF, delta P, Kf, and GFR. Selective antagonism of TxA2 also led to preservation of Kf, but was without effect on SNPF, thereby leading to elevated values for GFR. Thus, in contrast to the pathophysiologic role of arachidonate-lipoxygenase products in the early heterologous phase, PG-mediated vasodilatation and TxA2-induced reductions in Kf and GFR underlie glomerular functional changes during autologous mesangioproliferative glomerulonephritis.

Effects of leukotrienes on susceptibility of the rat stomach to damage and investigation of the mechanism of action

The ability of various leukotrienes to alter the susceptibility of the rat gastric mucosa to injury by 20% ethanol and the possible mechanism of action were examined using an ex vivo gastric chamber preparation. Intraarterial infusions of leukotriene B4 or N-acetyl leukotriene E4 (0.01-1.0 microgram/kg per min for 10 min) had no significant effect on the extent of damage induced by topically applied 20% ethanol. However, infusion of leukotriene C4, D4, or E4 (1.0 micrograms/kg per min) significantly increased ethanol-induced damage, as measured macroscopically, histologically, and functionally. Lower doses of leukotriene C4, D4, or E4 were without significant effect in this model. The increase in damage induced by these three leukotrienes could be blocked by pretreatment with either of two structurally unrelated leukotriene D4 antagonists (L-649,923 or L-660,711). The augmentation of damage by leukotriene C4 was not affected by pretreatment with indomethacin or with a specific thromboxane A2-receptor antagonist (L-670,596). At the dose that increased ethanol-induced damage, none of the leukotrienes tested significantly altered gastric vascular permeability, as measured by Evan's blue leakage. However, using laser-Doppler flowmetry, leukotrienes C4 and D4 were found, when administered intraarterially at doses in the 0.05-1.0 micrograms/kg per min range, to produce dose-dependent reductions of gastric blood flow while N-acetyl leukotriene E4 was without effect and leukotriene B4 induced slight increases. The effects of leukotrienes C4 and D4 on gastric blood flow could be inhibited by the two leukotriene D4 antagonists but not by the thromboxane antagonist. These results demonstrate that although they do not produce damage by themselves, leukotrienes C4, D4, and E4 are capable of augmenting ethanol-induced injury to the gastric mucosa. Changes in vascular permeability do not appear to play a role in the mechanism of action of the leukotrienes, while their effects on gastric blood flow are likely to be important. Under certain condition it is therefore possible that local release of leukotrienes could, at least in part through reducing vascular perfusion, predispose the surrounding tissue necrosis.