Heterogeneity of thromboxane A2 (TP-) receptors: evidence from antagonist but not agonist potency measurements

A Review of Prostanoid Receptors: Expression, Characterization, Regulation, and Mechanism of Action

Prostaglandin signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain to cell survival. Disruption of normal prostanoid signaling is implicated in numerous disease states. Prostaglandin signaling is facilitated by G-protein-coupled, prostanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of prostanoid receptors with particular emphasis on human isoforms.

Human thromboxane A2 receptor genetic variants: in silico, in vitro and "in platelet" analysis

Thromboxane and its receptor have emerged as key players in modulating vascular thrombotic events. Thus, a dysfunctional hTP genetic variant may protect against (hypoactivity) or promote (hyperactivity) vascular events, based upon its activity on platelets. After extensive in silico analysis, six hTP-α variants were selected (C⁶⁸S, V⁸⁰E, E⁹⁴V, A¹⁶⁰T, V¹⁷⁶E, and V²¹⁷I) for detailed biochemical studies based on structural proximity to key regions involved in receptor function and in silico predictions. Variant biochemical profiles ranged from severe instability (C⁶⁸S) to normal (V²¹⁷I), with most variants demonstrating functional alteration in binding, expression or activation (V⁸⁰E, E⁹⁴V, A¹⁶⁰T, and V¹⁷⁶E). In the absence of patient platelet samples, we developed and validated a novel megakaryocyte based system to evaluate human platelet function in the presence of detected dysfunctional genetic variants. Interestingly, variant V80E exhibited reduced platelet activation whereas A160T demonstrated platelet hyperactivity. This report provides the most comprehensive in silico, in vitro and “in platelet” evaluation of hTP variants to date and highlightscurrent inherent problems in evaluating genetic variants, with possible solutions. The study additionally provides clinical relevance to characterized dysfunctional hTP variants.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

Prostanoid receptor antagonists: development strategies and therapeutic applications

Identification of the primary products of cyclo-oxygenase (COX)/prostaglandin synthase(s), which occurred between 1958 and 1976, was followed by a classification system for prostanoid receptors (DP, EP(1), EP(2) ...) based mainly on the pharmacological actions of natural and synthetic agonists and a few antagonists. The design of potent selective antagonists was rapid for certain prostanoid receptors (EP(1), TP), slow for others (FP, IP) and has yet to be achieved in certain cases (EP(2)). While some antagonists are structurally related to the natural agonist, most recent compounds are 'non-prostanoid' (often acyl-sulphonamides) and have emerged from high-throughput screening of compound libraries, made possible by the development of (functional) assays involving single recombinant prostanoid receptors. Selective antagonists have been crucial to defining the roles of PGD(2) (acting on DP(1) and DP(2) receptors) and PGE(2) (on EP(1) and EP(4) receptors) in various inflammatory conditions; there are clear opportunities for therapeutic intervention. The vast endeavour on TP (thromboxane) antagonists is considered in relation to their limited pharmaceutical success in the cardiovascular area. Correspondingly, the clinical utility of IP (prostacyclin) antagonists is assessed in relation to the cloud hanging over the long-term safety of selective COX-2 inhibitors. Aspirin apart, COX inhibitors broadly suppress all prostanoid pathways, while high selectivity has been a major goal in receptor antagonist development; more targeted therapy may require an intermediate position with defined antagonist selectivity profiles. This review is intended to provide overviews of each antagonist class (including prostamide antagonists), covering major development strategies and current and potential clinical usage.

Prostanoid EP(1)- and TP-receptors involved in the contraction of human pulmonary veins

1. To characterize the prostanoid receptors (TP, FP, EP(1) and/or EP(3)) involved in the vasoconstriction of human pulmonary veins, isolated venous preparations were challenged with different prostanoid-receptor agonists in the absence or presence of selective antagonists. 2. The stable thromboxane A(2) mimetic, U46619, was a potent constrictor agonist on human pulmonary veins (pEC(50)=8.60+/-0.11 and E(max)=4.61+/-0.46 g; n=15). The affinity values for two selective TP-antagonists (BAY u3405 and GR32191B) versus U46619 were BAY u3405: pA(2)=8.94+/-0.23 (n=3) and GR32191B: apparent pK(B)=8.25+/-0.34 (n=3), respectively. These results are consistent with the involvement of TP-receptor in the U46619 induced contractions. 3. The two EP(1)-/EP(3)- agonists (17-phenyl-PGE(2) and sulprostone) induced contraction of human pumonary veins (pEC(50)=8.56+/-0.18; E(max)=0.56+/-0.24 g; n=5 and pEC(50)=7.65+/-0.13; E(max)=1.10+/-0.12 g; n=14, respectively). The potency ranking for these agonists: 17-phenyl-PGE(2) > sulprostone suggests the involvement of an EP(1)-receptor rather than EP(3). In addition, the contractions induced by sulprostone, 17-phenyl-PGE(2) and the IP-/EP(1)- agonist (iloprost) were blocked by the DP-/EP(1)-/EP(2)-receptor antagonist (AH6809) as well as by the EP(1) antagonist (SC19220). 4. PGF(2alpha) induced small contractions which were blocked by AH6809 while fluprostenol was ineffective. These results indicate that FP-receptors are not implicated in the contraction of human pulmonary veins. 5. These data suggest that the contractions induced by prostanoids involved TP- and EP(1)-receptors in human pulmonary venous smooth muscle.

Prostanoid receptor assays

Prostanoids, which include the prostaglandins (PGs) and thromboxanes (TXs), interact with a specific family of G-protein coupled receptors, of which there are known to be five distinct types, DP, EP, FP, IP and TP, each particularly sensitive to one of the five natural prostanoids, PGD₂, PGE₂, PGF₂(, PGI₂ and TXA₂, respectively. Of these, it is known that the EP receptor comprises four well-characterized subtypes: EP₁, EP₂, EP₃ and EP₄. These receptor subtypes are widely distributed throughout mammals and other species, and show particularly high levels of expression in smooth muscle and blood platelets. Despite the fact that few of these preparations express a single receptor type/subtype in isolation, a range of useful smooth muscle and platelet assays for the various prostanoid receptors are available and are presented in this unit.

Distinction between relaxations induced via prostanoid EP(4) and IP(1) receptors in pig and rabbit blood vessels

1. Our study shows that the prostacyclin analogues AFP-07 and cicaprost are moderately potent agonists for prostanoid EP(4) receptors, in addition to being highly potent IP(1) receptor agonists. Both activities were demonstrated on piglet and rabbit saphenous veins, which are established EP(4) preparations. 2. On piglet saphenous vein, PGE(2) was 6.1, 24, 96, 138, 168 and 285 times respectively more potent than AFP-07, cicaprost, PGI(2), iloprost, carbacyclin and TEI-9063 in causing relaxation. Another prostacyclin analogue taprostene did not induce maximum relaxation (21 - 74%), and did not oppose the action of PGE(2). The EP(4) receptor antagonist AH 23848 (30 microM) blocked relaxant responses to PGE(2) (dose ratio=8.6+/-1.3, s.e.mean) to a greater extent than cicaprost (4.9+/-0.7) and AFP-07 (3.8+/-0.8), had variable effects on TEI-9063-induced relaxation (3.7+/-1.5), and had no effect on taprostene responses (<2.0). 3. On rabbit saphenous vein, AH 23848 blocked the relaxant actions of PGE(2), AFP-07, cicaprost, iloprost and carbacyclin to similar extents. 4. AFP-07, cicaprost and TEI-9063 showed high IP(1) relaxant potency on piglet carotid artery, rabbit mesenteric artery and guinea-pig aorta, with AFP-07 confirmed as the most potent IP(1) agonist reported to date. AH 23848 did not block cicaprost-induced relaxation of piglet carotid artery. EP(3) contractile systems in these preparations can confound IP(1) agonist potency estimations. 5. Caution is urged when using AFP-07 and cicaprost to characterize IP(1) receptors in the presence of EP(4) receptors. Taprostene may be a lead to a highly selective IP(1) receptor agonist.

Disturbance of peristalsis in the guinea-pig isolated small intestine by indomethacin, but not cyclo-oxygenase isoform-selective inhibitors

1. Since the cyclo-oxygenase (COX) isoform-nonselective inhibitor indomethacin is known to modify intestinal motility, we analysed the effects of COX-1 and COX-2 inhibition on intestinal peristalsis. 2. Peristalsis in isolated segments of the guinea-pig small intestine was triggered by a rise of the intraluminal pressure and recorded via the pressure changes associated with peristalsis. 3. The COX-1 inhibitor SC-560, the COX-2 inhibitor NS-398 (both at 0.1 -- 1 microM) and the isoform-nonselective inhibitors flurbiprofen (0.01 - 10 microM) and piroxicam (0.1 - 50 microM) were without major influence on peristalsis, whereas indomethacin and etodolac (0.1 -- 10 microM) disturbed the regularity of peristalsis by causing nonpropulsive circular muscle contractions. 4. Radioimmunoassay measurements showed that SC-560, NS-398, indomethacin and etodolac (each at 1 microM) suppressed the release of 6-keto-prostaglandin F(1 alpha) (6-keto-PGF(1 alpha)) from the intestinal segments. 5. Reverse transcription - polymerase chain reaction tests revealed that, relative to glyceraldehyde-3 phosphate dehydrogenase ribonucleic acid, the expression of COX-1 mRNA increased by a factor of 2.0 whereas that of COX-2 mRNA rose by a factor of 7.9 during the 2 h experimental period. 6. Pharmacological experiments indicated that the action of indomethacin to disturb intestinal peristalsis was unrelated to inhibition of L-type calcium channels, adenosine triphosphate-sensitive potassium channels or phosphodiesterase type IV. 7. These results show that selective inhibition of COX-1 and COX-2 does not grossly alter peristaltic motor activity in the guinea-pig isolated small intestine and that the effect of indomethacin to disturb the regular pattern of propulsive motility in this species is unrelated to COX inhibition.

FP-receptor mediated trophic effects of prostanoids in rat ventricular cardiomyocytes

The aim of this study was to characterize the receptor subtype involved in cardiac effects of prostanoids. For this purpose we determined in neonatal and adult rat cardiomyocytes effects of prostanoids on inositol phosphate (InsP)-formation (assessed as accumulation of total [(3)H]-InsP's in myo-[(3)H]-inositol pre-labelled cells) and on rate of protein synthesis (assessed as [(3)H]-phenylalanine incorporation), and on contractile force in left ventricular strips of the rat heart. For comparison, effects of prostanoids on InsP-formation and contractile force were determined in rat thoracic aorta, a classical TP-receptor containing tissue. Prostanoid increased InsP-formation and rate of protein synthesis in neonatal as well as adult rat cardiomyocytes; the order of potency was in neonatal (PGF(2alpha)>PGD(2)> or =PGE(2)> or =U 46619>PGE(1)) and adult (PGF(2alpha)>PGD(2)> or =PGE(2)>U 46619) rat cardiomyocytes well comparable. Moreover, in electrically driven left ventricular strips PGF(2alpha) caused positive inotropic effects (pD(2) 7.5) whereas U 46619 (up to 1 microM) was uneffective. In contrast, in rat thoracic aorta U 46619 was about 100 times more potent than PGF(2alpha) in increasing InsP-formation and contractile force. The TP-receptor antagonist SQ 29548 only weakly antagonized prostanoid-induced increases in rate of protein synthesis (pK(B) about 6) in rat cardiomyocytes but was very potent (pK(B) about 8-9) in antagonizing prostanoid-induced increases in InsP-formation and contractile force in rat aorta. We conclude that, in cardiomyocytes of neonatal and adult rats, the prostanoid-receptor mediating increases in InsP-formation and rate of protein synthesis is a FP-receptor. Moreover, stimulation of these cardiac FP-receptors can mediate increases in contractile force.

Characterization of prostanoid receptors mediating inhibition of histamine release from anti-IgE-activated rat peritoneal mast cells

1. Prostanoid receptors mediating inhibition of anti-IgE induced histamine release from rat peritoneal mast cells have been characterized pharmacologically. PGD2 and the specific DP receptor agonists BW 245C and ZK 118182 were the most potent inhibitors with half-maximal concentrations of 0.26, 0.06 and 0.02 microM respectively. The maximum inhibition attainable was 60-65% with 10(-5) M BW 245C and ZK 118182. 2. Among several EP receptor agonists investigated, only PGE2 and the EP2/EP3 receptor agonist misoprostol induced significant inhibition (46.8 +/- 4.7% at 10(-4) M and 18.7 +/- 6.8% at 10(-5) M respectively). The IP receptor agonists cicaprost and iloprost were both less potent than the DP agonists in inhibiting histamine release (45.2 +/- 3.3% and 35.1 +/- 2.5% inhibition respectively at 10(-5) M), whereas PGF2 alpha and the TP receptor agonist U-46619 were only marginally effective. 3. The EP4/TP receptor antagonist AH 23848 failed to affect the inhibitory actions of PGD2 or PGE2 even at 10(-5) M, whereas the DP/EP1/EP2 receptor antagonist AH 6809 slightly enhanced the effect of PGD2 at 10(-6) M. 4. At concentrations of 3 x 10(-6) to 10(-5) M, the putative DP receptor antagonist ZK 138357 dose-dependently suppressed the inhibitory activities of the DP agonists, PGE2 and cicaprost. The antagonism of ZK 138357 against the DP receptor agonists appeared to be competitive with pA2 values of around six. 5. In conclusion, these data support our earlier proposal that an inhibitory DP receptor is the predominant prostanoid receptor in rat peritoneal mast cell. The properties of this receptor in relation to putative DP receptor subtypes reported in the literature are discussed.

The cyclo-oxygenase-dependent regulation of rabbit vein contraction: evidence for a prostaglandin E2-mediated relaxation

1. Arachidonic acid (0.01-1 microM) induced relaxation of precontracted rings of rabbit saphenous vein, which was counteracted by contraction at concentrations higher than 1 microM. Concentrations higher than 1 microM were required to induce dose-dependent contraction of vena cava and thoracic aorta from the same animals. 2. Pretreatment with a TP receptor antagonist (GR32191B or SQ29548, 3 microM) potentiated the relaxant effect in the saphenous vein, revealed a vasorelaxant component in the vena cava response and did not affect the response of the aorta. 3. Removal of the endothelium from the venous rings, caused a 10 fold rightward shift in the concentration-relaxation curves to arachidonic acid. Whether or not the endothelium was present, the arachidonic acid-induced relaxations were prevented by indomethacin (10 microM) pretreatment. 4. In the saphenous vein, PGE2 was respectively a 50 and 100 fold more potent relaxant prostaglandin than PGI2 and PGD2. Pretreatment with the EP4 receptor antagonist, AH23848B, shifted the concentration-relaxation curves of this tissue to arachidonic acid in a dose-dependent manner. 5. In the presence of 1 microM arachidonic acid, venous rings produced 8-10 fold more PGE2 than did aorta whereas 6keto-PGF1alpha and TXB2 productions remained comparable. 6. Intact rings of saphenous vein relaxed in response to A23187. Pretreatment with L-NAME (100 microM) or indomethacin (10 microM) reduced this response by 50% whereas concomitant pretreatment totally suppressed it. After endothelium removal, the remaining relaxing response to A23187 was prevented by indomethacin but not affected by L-NAME. 7. We conclude that stimulation of the cyclo-oxygenase pathway by arachidonic acid induced endothelium-dependent, PGE2/EP4 mediated relaxation of the rabbit saphenous vein. This process might participate in the A23187-induced relaxation of the saphenous vein and account for a relaxing component in the response of the vena cava to arachidonic acid. It was not observed in thoracic aorta because of the lack of a vasodilatory receptor and/or the poorer ability of this tissue than veins to produce PGE2.

Characterization of a prostanoid EP3-receptor in guinea-pig aorta: partial agonist action of the non-prostanoid ONO-AP-324

Contraction of guinea-pig isolated aorta induced by the prostaglandin E analogue sulprostone (1-400 nM) has a lower maximum response (40%) than that of phenylephrine or U-46619 (TP-receptor agonist). A prostanoid EP3-receptor subtype is involved based on agonist potency ranking: equi-effective molar ratios (EMR) are sulprostone (EC50 approximately equal to 23 nM) 1.0, SC-46275 0.11, misoprostol 2.2, gemeprost 3.3, PGE2 5.4, 17-phenyl PGE2 6.0, GR-63799 8.9. GR-63799, which contains a bulky ester group, is relatively more potent on neuronal EP3 preparations than on the aorta. ONO-AP-324, a relative of the non-prostanoid prostacyclin mimetic series, behaves as an EP3 partial agonist on the aorta, inhibiting sulprostone responses but acting synergistically (in a similar manner to sulprostone) with phenylephrine; it may be a useful pharmacological tool for studying EP3-receptors. Sulprostone contractions are markedly suppressed in zero-Ca2+ bathing fluid containing either 2 mM EDTA or 50 microM EGTA, and by Cd2+ (500 microM), but are usually unaffected by nifedipine (0.3 microM) and verapamil (4.44 microM). Influx of Ca2+, but not through L-type Ca2+-channels, appears to be the major contractile mechanism. The guinea-pig aorta is a valuable addition to the vascular EP3 preparations available and may increase our knowledge of the mechanisms whereby Gi-coupled receptors mediate vasoconstriction (c.f. 5-HT1B/D- and alpha2-receptors). The possibility of certain EP3 agonists distinguishing EP3-receptor isoforms is discussed.

Prostanoid action on the human pulmonary vascular system

1. Four types of prostanoid receptor are present on pulmonary arterial vessels of man. Thromboxane (TP) receptors mediate constriction and are blocked by antagonists such as BAY u-3405, GR 32,191 and EP 169. Prostaglandin (PG) EP3 receptors also mediate constriction, the agonist potency ranking being SC 46,275 > sulprostone > misoprostol > or = PGE2; this action needs to be borne in mind when PGE analogues are used therapeutically. 2. Prostaglandin E2 causes relaxation in a few pulmonary artery preparations: an EP2 receptor may be involved. Prostacyclin, acting through i.p. receptors, consistently produces relaxation and studies are in progress to determine the contribution made by K(+)-channel opening. Agonist potencies of stable prostacyclin analogues and non-prostanoid prostacyclin mimetics, such as BMY 45,778 and the novel diphenylindole CU 23, on human pulmonary artery and platelets are well correlated. Interestingly, the non-prostanoid mimetics show persistent relaxant effects in vitro, which may be related to their high lipophilicities. 3. Prostacyclin and iloprost are being used to treat severe pulmonary hypertension; further study of the pharmacodynamic and pharmacokinetic properties of other i.p. receptor agonists could produce improved therapy.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

Pharmacological evidence for thromboxane receptor heterogeneity--implications for the eye

The pharmacological activity of two novel thromboxane A2 (TxA2)-mimetics, AGN191976 and AGN192093, was investigated in vitro, using standard organ bath assays and human platelets, to determine potency and selectivity at various prostanoid (PG-) receptors. The effects of these compounds on intraocular pressure in Beagle dogs were then compared with U-46619, a widely employed and structurally different TP-receptor agonist. AGN191976 and AGN192093 were highly potent TP-receptor agonists in the rat aorta (EC50 of 0.32 and 1.3 nM, respectively) and human myometrium. Both compounds were approximately 10 to 50 fold more potent than U-46619. These contractile responses could be blocked with a potent TP-receptor antagonist, SQ29548. In human platelets, AGN191976 (EC50 = 16.3 nM) and U-46619 (EC50 = 538.3 nM) potently stimulated aggregation (TP-receptor mediated effect), whereas AGN192093 was a much weaker agonist (EC50 = 37.9 microM). AGN192093 was not a partial agonist in platelets, since it did not antagonize aggregation induced by AGN191976, U-46619, arachidonic acid or ADP. These results provide evidence for a subdivision of TP-receptors, and AGN192093 appears to be able to distinguish between TP-receptors in smooth muscle and platelets. In the Beagle dog eye, both AGN191976 and AGN192093 were highly potent and efficacious ocular hypotensives. Single 2.5 micrograms doses of drug decreased IOP by 11.4 (AGN191976) and 7.7 mm Hg (AGN192093) relative to the contralateral control eye. In contrast, U-46619 did not lower IOP. AGN191976, but not U-46619, increased outflow facility in these animals, which is consistent with their effects on IOP. Neither compound caused miosis which is FP-receptor mediated in the dog. These studies suggest the existence of heterogeneous populations of TP-receptors. AGN191976 and AGN192093, two novel TP-receptor agonists, appear to be useful tools for the pharmacological distinction of TP-receptor subtypes.

Evidence for human thromboxane receptor heterogeneity using a novel series of 9,11-cyclic carbonate derivatives of prostaglandin F2 alpha

1. The pharmacological activity of a novel series of 9,11-cyclic carbonate derivatives of prostaglandin F2 alpha (PGF2 alpha) was investigated in various isolated smooth muscle preparations possessing different prostanoid receptor subtypes as well as in human platelets. Since subdivision of thromboxane (TP-) receptors into vascular/smooth muscle and platelet subtypes is a controversial subject, our studies included a human smooth muscle preparation (myometrium) in addition to the widely used rat aorta and human platelets as TP-receptor preparations. 2. Two members of that series, AGN191976 and AGN192093 were found to be highly potent and selective thromboxane-mimetics. AGN191976 and AGN192093 contracted isolated tissues of the rat thoracic aorta with EC50 values of 0.32 +/- 0.08 and 1.30 +/- 0.53 nM, respectively. Both agonists were at least 10 times more potent than the benchmark TP-agonist, U-46619, in this preparation, whilst being at least 500 times less potent at other prostanoid receptors (DP, EP1, EP3, FP, IP) in vitro. 3. In human myometrial strips from pregnant and non-pregnant donors, both AGN191976 and AGN192093 were potent contractile agonists. The rank order of potency in myometrium of AGN191976 > AGN192093 > U-46619 correlated well with that in the rat aorta. In human platelet-rich plasma (PRP), however, AGN191976 had potent proaggregatory activity (EC50 = 16.3 +/- 1.4 nM), which is a TP-receptor-mediated event, whereas AGN192093 was a much weaker agonist (EC50 = 37.9 +/- 2.0 microM). AGN192093 did not behave as an antagonist in the platelets, since it did not antagonize platelet aggregation induced by ADP, arachidonic acid, U-46619 or AGN191976. In human washed platelets, the activity profile of AGN191976 (EC50 = 4.15 +/- 0.52 nM) and AGN192093 (no aggregation up to 10 microM) was similar to that obtained in PRP. 4. The involvement of TP-receptors was verified with the potent TP-antagonist, SQ29548. SQ29548 (0.1 microM in myometrium; 1 microM in aorta; 1 microM and 10 microM in platelets) antagonized responses to U-46619, AGN191976 and AGN192093 as expected. 5. In conclusion, AGN191976 and AGN192093, both 9,11-cyclic carbonate derivatives of PGF2 alpha, were found to be highly potent and selective thromboxane-mimetics in rat vascular and human myometrial smooth muscle. However, only AGN 191976 was a potent agonist at TP-receptors in human platelets. The differential activity of AGN192093 on TP-receptor-mediated events in platelets and smooth muscle provides further evidence for a subdivision of TP-receptors. AGN192093 appears to be a useful tool for the pharmacological distinction of TP-receptor subtypes.

Pharmacological studies on prostanoid receptors in the rabbit isolated saphenous vein: a comparison with the rabbit isolated ear artery

1. In isolated ring preparations of the rabbit saphenous vein, prostaglandin E2 (PGE2) caused well-defined, stable and concentration-dependent relaxations of KCl contracted tissues with a mean potency (p[A50]) of 9.39. 2. The prostanoid EP-receptor agonists, PGE1, 11-deoxy PGE1, 16,16-dimethyl PGE2 and misoprostol were all full agonists in this preparation. The EP2-receptor selective agonists, butaprost and AH13205, and the EP1/EP3-receptor selective agonist, sulprostone, also relaxed this tissue but were at least 300 times less potent than PGE2. 3. Prostaglandin D2 (PGD2), the DP-receptor agonist, BW245C, and the IP-receptor agonist, cicaprost, caused concentration-dependent relaxations of the rabbit saphenous vein but were at least 60 times less potent than PGE2. 4. The selective EP4-receptor antagonist, AH23848B (30 microM), antagonized the PGE2 concentration-effect (E/[A]) curves yielding a pA2 estimate of 4.96. The EP1/DP-receptor antagonist, AH6809 (10 microM), had no effect on the location of PGE2 E/[A] curves. 5. The stable thromboxane A2-mimetic, U46619, elicited concentration-dependent contractions of the rabbit saphenous vein (p[A50] = 8.01) however, PGE2 and prostaglandin F2 alpha (PGF2 alpha) were unable to produce a contractile response. The response to U46619 was competitively antagonized by the TP-receptor antagonist, GR32191B, yielding a pKB estimate of 7.08. 6. In the rabbit isolated ear artery, PGE2, misoprostol and AH13205 relaxed tissues pre-contracted with phenylephrine. PGE2 (p[A50] = 7.04) and misoprostol were equipotent, whereas AH13205 was some 40 fold less potent. AH23848B (30 microM) and AH6809 (1 and 10 microM) caused no significant shift in the location of PGE2 E/[A] curves. 7. These data suggest that the rabbit isolated saphenous vein contains prostanoid, EP-, DP-, IP- and TP-receptors. Based on antagonist affinity information and agonist potency orders, the rabbit saphenous vein contains an inhibitory prostanoid EP-receptor different from that in the rabbit ear artery, but comparable to the recently described EP4-receptor.

Thromboxane A2 receptor-mediated signal transduction in rabbit aortic smooth muscle cells

1. 9,11-Epithio-11,12-methenothromboxane A2 (STA2), a stable analogue of thromboxane A2 (TXA2), contracted rabbit aortic smooth muscles (RASM) and accumulated [3H]inositol phosphates in cultured RASM cells. The contraction and phosphoinositide hydrolysis were competitively inhibited by TXA2 receptor antagonists, including ONO NT-126, S-145, SQ29548, KW3635, GR32191B and ONO3708. 2. STA2 inhibited [3H]ONO NT-126 binding in a concentration-dependent manner in membranes derived from cultured aortic smooth muscle cells, but GTP gamma S, a stable GTP analogue, did not affect STA2-induced inhibition of [3H]ONO NT-126 binding. 3. The time course analysis revealed that STA2 rapidly decreased inositol phosphate level and therefter increased. Pertussis toxin did not attenuate but rather increased STA2-induced phosphoinositide hydrolysis. 4. TXA2 receptor stimulation results in at least two signaling pathways in RASM cells: stimulation and inhibition of phosphoinositide hydrolysis.

Characterization of the prostanoid receptors mediating constriction and relaxation of human isolated uterine artery

1. This study was undertaken to characterize pharmacologically the prostanoid receptor subtypes mediating constriction and relaxation of human isolated uterine artery. 2. U-46619 was a potent constrictor agonist on human uterine artery (EC50 [95% CL] = 3.5 [1.8-6.7] nM). Prostaglandin E2 (PGE2), PGF2 alpha, PGD2 and PGI2 only weakly constricted the uterine artery, being at least 100 times less potent than U-46619. The PGE2 and PGI2 constrictor effects may be modified by the potent dilator effects of these compounds. A number of agonists which show selectivity for FP-, DP- and EP-receptors including ICI 81008, BW 245C, sulprostone, rioprostil and butaprost, failed to cause any constriction at concentrations up to 30 microM. 3. Constrictor responses induced by all agonists tested were reduced or abolished by the TP-receptor blocking drugs, GR 32191 and EP 092. pA2 estimates for both antagonists versus U-46619 were 8.50, values which are consistent with their affinities at TP-receptors. 4. In preparations pre-constricted with phenylephrine (1 microM) both PGI2 and PGE2 were potent relaxant agonists. The selective IP-receptor agonists, cicaprost and iloprost, also dilated human uterine artery and were approximately 10 fold more potent than PGI2. The EP2-receptor agonists, butaprost and rioprostil and the selective DP-agonist, BW 245C, were at least 100 fold weaker than PGI2 and PGE2 suggesting that neither DP- nor EP2 receptors were involved. 5. We conclude that TP-receptors mediate constriction, whereas IP- and possibly EP4-receptors mediate relaxation of human uterine artery.

Aspects of the thromboxane receptor system

1. The aim of this review is to establish what is known about the thromboxane (TP) receptor, and to identify where future research is headed. In addition, the impact of the recent advances at the molecular level on resolving pharmacological controversies, such as possible subtypes of the TP receptor, is discussed and what molecular information is known about the TP receptor presented. 2. The clinical status of TP receptor antagonists is considered particularly in relation to the potential role of epi prostaglandins. 3. Basic information about TP agonists, antagonists and signal transduction pathways is also given.

Comparison of the EP receptor subtypes mediating relaxation of the rabbit jugular and pig saphenous veins

A fourth PGE receptor subtype, the EP4 receptor, has recently been described in the pig saphenous vein (PSV). Similar to the EP2 receptor, it mediates relaxation and is linked to stimulation of adenylate cyclase. The aim of this study was to determine whether or not the EP receptor present in the rabbit jugular vein (RJV), currently classified as an atypical EP2 receptor, is of the EP4 subtype. The relaxant activities of four EP2 agonists, 11-deoxy PGE1, 16,16-dimethyl PGE2, butaprost, and AH 13205, on the RJV and PSV have been examined, and the effect of the EP4 receptor antagonist AH 23,848B studied. The EP2 agonists showed a similar order of potency on the two preparations. 11-Deoxy PGE1 and 16,16-dimethyl PGE2 were potent agonists on the EP4 receptors of the PSV and on the RJV giving approximately equi-effective concentration ratios (EECs) of 2.0-6.6 and 2.8-9.9, respectively, compared to PGE2 (EEC = 1), and so do not discriminate between EP2 and EP4 receptors. Butaprost was less active on these preparations (EEC 42-43) than on classical EP2 receptors, and AH 13205 was much less active (EEC 3100-2780). While these results suggest that the EP receptors on the RJV are of the EP4 subtype, this was not confirmed using the EP4 receptors antagonist AH 23,848B.

Actions of the E2-isoprostane, 8-ISO-PGE2, on the platelet thromboxane/endoperoxide receptor in humans and rats: additional evidence for the existence of a unique isoprostane receptor

D2/E2-isoprostanes, are a recently discovered series of novel prostaglandin-like compounds that are produced in vivo as products of free radical-catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. One of the E-ring compounds expected to be produced in abundance by this mechanism, 8-iso-prostaglandin E2 (8-iso-PGE2), is a potent renal vasoconstrictor in the rat, and this effect can be abrogated by the thromboxane/endoperoxide (TxA2/PGH2) receptor antagonist SQ29548, suggesting that 8-iso-PGE2 exerts these effects by interaction with this receptor in the vasculature. Nonetheless, it has recently been suggested that 8-iso-PGE2 induces vasoconstriction by interaction with a unique receptor similar to, but distinct from, the TxA2/PGH2 receptor. Because this issue has not been resolved, we carried out studies to further examine the interaction of this compound with the TxA2/PGH2 receptor on human and rat platelets. Only at concentrations of 10(-5) M or greater did 8-iso-PGE2 induce human platelet aggregation. The aggregation was unaffected by indomethacin but was inhibited by the TxA2/PGH2 receptor antagonist SQ29548. Conversely, 8-iso-PGE2 inhibited the thromboxane receptor agonists U46619 (10(-6) M) and IBOP (3.3 x 10(-7) M) with an IC50 of 5 x 10(-7) M and 5 x 10(-6) M, respectively. 8-iso-PGE2 also inhibited platelet aggregation induced by arachidonic acid but not by ADP. Similarly in rat platelets, 8-iso-PGE2 alone.

Potent contractile actions of prostanoid EP3-receptor agonists on human isolated pulmonary artery

1. In 13 of 15 experiments, prostaglandin E2 (PGE2) and sulprostone (a prostanoid EP1/EP3-receptor agonist) contracted isolated rings of human pulmonary artery at low concentrations (> or = 5 and > or = 0.5 nM respectively). Tissue was obtained from patients undergoing surgery mainly for carcinoma of the lung. Characterization of the receptors involved was complicated by loss of sensitivity to the contractile PGE action over the experimental period. In contrast, contractile responses to KCl, phenylephrine and the specific thromboxane (TP-) receptor agonist, U-46619, did not decrease with time. 2. The relative contractile potencies for seven PGE analogues, measured during the first few hours after setting up the preparations, were as follows: sulprostone > misoprostol = gemeprost > or = PGE2 > or = GR 63799X > 17-phenyl-omega-trinor PGE2 > or = 11-deoxy PGE1. This ranking indicates that an EP3-receptor is involved. 3. The contractile action of sulprostone was not blocked by the TP-receptor antagonists, EP 169 and GR 32191, and the EP1-receptor antagonist, AH 6809. 4. In two experiments, PGE2 (50 nM) reduced basal tone and sulprostone was a weak contractile agent. Phenylephrine-induced tone was also inhibited by PGE2 (EC50 = 5-20 nM), 11-deoxy PGE1 and butaprost (a selective EP2-receptor agonist); the latter prostanoids were about 2 and 4 times less potent than PGE2 respectively. Interactions with phenylephrine were different in experiments where PGE2 alone was contractile: PGE2 induced contraction superimposed on the phenylephrine response and 11-deoxy PGE1 induced either further contraction or had no effect. Butaprost produced relaxation at high concentrations;this may not be an EP2 action since preparations were highly sensitive to relaxant actions of prostacyclin (IP-) receptor agonists (cicaprost and TEI-9063).5 The study has shown that in the majority of experiments on the human isolated pulmonary artery,the contractile EP3 system outweighed the relaxant EP2 system. However, in two experiments the reverse was true. It is not clear to what extent these differences are due to disease processes affecting the tissues.The findings are discussed in relation to the adverse cardiovascular responses occasionally encountered during treatment of postpartum haemorrhage with sulprostone, and more generally to the clinical use of EP-receptor agonists in man.

Ipratropium bromide protects against bronchoconstriction during bronchoscopy

Pulmonary function is reportedly impaired by fiberoptic bronchoscopy. We investigated the effect of two anticholinergic agents, intramuscular atropine and inhaled ipratropium bromide, on bronchoconstriction in 29 patients who were undergoing diagnostic bronchoscopy. The patients were divided into three groups; the first received 0.5 mg of atropine intramuscularly; the second took four puffs of 0.02 mg ipratropium bromide aerosolized by a metered-dose inhaler, and the third inhaled four puffs of a placebo. Fifteen minutes later a standardized topical anesthetic, lidocaine, was administered, and a bronchoscopic examination was performed. Pulmonary function was measured before and 15 minutes after each step. Pulmonary function was not affected by the treatment with anticholinergics or the placebo. In the placebo and the atropine groups, the topical anesthesia produced significant reductions in forced expiratory volume in 1 second (FEV1) and peak expiratory flow rate (PEFR); further reductions in these values were observed after bronchoscopy. In the group treated with ipratropium bromide there were no significant changes in FEV1 and PEFR after topical anesthesia. Bronchoscopy induced significant reductions in FEV1 and PEFR, but the changes were significantly smaller than those seen in the placebo and atropine groups. The results suggest that the deleterious effect of bronchoscopy on pulmonary function is due to topical lidocaine anesthesia and to the bronchoscopic examination itself. Inhaled ipratropium bromide protects against these deleterious effects, whereas intramuscular atropine does not.

Halogen-substituted trimetoquinol analogs as thromboxane A2 receptor antagonists in platelets and aorta

Trimetoquinol (TMQ) is a non-prostanoid compound that blocks prostaglandin H2/thromboxane A2 (TXA2) receptor-mediated responses initiated by a prostaglandin (PG) H2 analog, U46619, in human platelets and rat aorta. Ring fluorine-substituted TMQ analogs selectively antagonized PG-dependent human platelet activation induced by U46619, arachidonic acid, collagen, ADP or epinephrine; and were about 300-fold less potent as inhibitors of PG-independent responses mediated by thrombin or bacterial phospholipase C. For each inducer of the PG-dependent pathway, the rank order of inhibitory potency was identical (TMQ > 8-fluoro-TMQ > 5-fluoro- TMQ). Iodine substitution yielded a similar rank order of antagonism against U46619-induced platelet activation (TMQ > 8-iodo-TMQ > 5-iodo-TMQ), and all TMQ analogs inhibited platelet aggregation in whole blood as well as in platelet-rich plasma. Inhibition of specific [3H]SQ 29,548 binding by TMQ analogs was highly correlated with inhibition of functional responses to U46619. Radioligand binding experiments using TMQ analogs with rat platelets showed no interspecies difference in comparison with human platelets. The rank order of inhibitory potencies for the fluorinated (but not iodinated) TMQ analogs changed in rat thoracic aorta with 8-fluoro-TMQ > TMQ > or = 5-fluoro-TMQ as antagonists of U46619-induced vascular contraction. These findings demonstrate that the primary mechanism of antiplatelet action of TMQ analogs is related to a blockade of TXA2 receptor sites, and ring-halogenated TMQ analogs distinguish between TXA2-mediated functional responses in vascular smooth muscle and platelets.

Hyperpolarization induced by K+ channel openers inhibits Ca2+ influx and Ca2+ release in coronary artery

The vasodilating mechanisms of the K+ channel openers--cromakalim, pinacidil, nicorandil, KRN2391, and Ki4032--were examined by measurement of the cytoplasmic Ca2+ concentration ([Ca2+]i) using the fura-2 method in canine or porcine coronary arterial smooth muscle. The five K+ channel openers all produced a reduction of [Ca2+]i in 5 and 30 mM KCl physiological salt solution (PSS), the effects of which were antagonized by tetrabutylammonium (TBA) or glibenclamide, but failed to affect [Ca2+]i in 45 and 90 mM MCl-PSS. Cromakalim and Ki4032 only partially inhibited the 30 mM KCl-induced contractures, whereas pinacidil, nicorandil, and KRN2391 nearly abolished contractions produced by high KCl-PSS. The increased [Ca2+]i and force produced by a thromboxane A2 analogue, U46619, were inhibited by K+ channel openers and verapamil. In the absence of extracellular Ca2+, U46619 induced a transient increase in [Ca2+]i with a contraction, which is effectively inhibited by cromakalim and Ki4032. Their inhibitory effects were blocked by TBA and counteracted by 20 mM KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca2+ release. Cromakalim reduced U46619-induced IP3 production and TBA blocked this inhibitory effect. Thus, cromakalim and Ki4032 are more specific K+ channel openers than pinacidil, nicorandil, and KRN2391. The vasodilation related with a reduction of [Ca2+]i produced by K+ channel openers is due to the hyperpolarization of the plasma membrane resulting in not only the closure of voltage-dependent Ca2+ channels but also inhibition of the production of IP3 and Ca2+ release from intracellular stores related to stimulation of the thromboxane A2 receptor.

Potentiation of aggregation and inhibition of adenylate cyclase in human platelets by prostaglandin E analogues

1. The 16-phenoxy prostaglandin E analogue sulprostone consistently potentiates primary aggregation waves induced by adenosine 5'-diphosphate (ADP), PAF and 11,9-epoxymethano PGH2 (U-46619) in platelet-rich plasma from human donors. The effect is not blocked by the TP-receptor antagonists, EP 092 and GR 32191. The high potency of sulprostone (threshold concentration = 4-10 nM) and the weak block of sulprostone potentiation by the EP1-receptor antagonist, AH 6809 (pA2 = 4.3) suggest the involvement of EP3-receptors as opposed to EP1- or EP2-subtypes. 2. Eight prostaglandin E (PGE) analogues were compared against sulprostone for their effects on PAF-induced aggregation in human platelet-rich plasma (PRP) in the presence of GR 32191 and the DP-receptor antagonist, BW A868C. PGE2 and 11-deoxy PGE2-1-alcohol showed evidence of both potentiating and inhibitory actions and butaprost showed only inhibitory activity at high concentrations. The remaining analogues always elicited potentiation, with the following potency ranking: sulprostone = 16,16-dimethyl PGE2 > MB 28767 > misoprostol > GR 63779X = 17-phenyl-omega-trinor PGE2. The results again indicate that EP3- rather than EP1- or EP2-receptors are involved. However, relative potentiating potency could be affected by differences in plasma protein binding and the very high sensitivity of the human platelet to prostacyclin (IP)-receptor-mediated inhibition (IC50 for the specific IP-receptor agonist cicaprost = 0.8 nM). 3. On human washed platelet suspensions the PGE analogues, with the exception of butaprost,inhibited the rise in adenosine 3':5'-cyclic monophosphate (cyclic AMP) induced by cicaprost (8 nM).PGE2 produced a monophasic inhibition curve (IC50 = 5.4 nM, 92% inhibition at 600 nM). The potency ranking was 16,16-dimethyl PGE2> sulprostone>MB 28767 = PGE2> misoprostol> GR 63778X>17-phenyl-w-trinor PGE2> 1 1-deoxy PGE2-1-alcohol. AH 6809 inhibited the effect of sulprostone and 17-phenyl-c-trinor PGE2 with pA2 values of 5.75 and 5.32 respectively; these values are at least one log unit lower than those found for EP1-receptor block in smooth muscle.4. There is a statistically significant correlation between IC50 values for the PGE analogues on the human platelet cyclic AMP assay and the guinea-pig vas deferens (standard EP3 preparation): slope =1.00, r = 0.80, P <0.05. However the correlation is far from ideal and GR 63779X in particular has a lower potency in the cyclic AMP assay. At this time we suggest that it is prudent to describe the human platelet receptor as 'EP3-like'.5. We believe that our results provide further evidence for linking PGE-induced potentiation of aggregation to inhibition of adenylate cyclase. Sulprostone is a suitable agonist for further study of this system and in particular the nature of the G-protein linkage(s) involved. In addition the necessity to consider potentiation of platelet aggregation in -relation to the clinical use of PGE analogues in man is emphasised.

The thromboxane receptors of rat and guinea-pig lung

The thromboxane (TXA2) receptors on rat and guinea-pig lung strips were compared using TXA2 agonists and TXA2 receptor antagonists. On rat lung strip several TXA2 mimetics were full agonists whilst the primary prostanoids lacked agonist activity. On guinea-pig lung strip the same agonists displayed markedly different efficacies. Both preparations contained homogeneous populations of TXA2 receptors as evidenced by BAY u3405 giving comparable pA2 values against four TXA2 mimetics. However, the observed pA2's of nine different TXA2 receptor antagonists, determined against U46619, did not correlate between the two preparations. These results point to the existence of TXA2 receptor subtypes.

K+ channel openers, cromakalim and Ki4032, inhibit agonist-induced Ca2+ release in canine coronary artery

The effects of K+ channel openers, cromakalim and an acetoxyl derivative of KRN2391 (Ki4032), were studied on force of contraction, increases in intracellular calcium concentration ([Ca2+]i) measured by fura-2 and inositol 1,4,5-trisphosphate (IP3) production induced by the thromboxane A2 analogue, U46619, in canine coronary arteries. Upon single dose applications of U46619 at 300 nmol/l, phasic and tonic increases in [Ca2+]i and force were seen, which were almost abolished by cromakalim (10 mumol/l) and Ki4032 (100 mumol/l). In the absence of extracellular Ca2+, U46619 induced a transient increase in [Ca2+]i with a contraction. Cromakalim (0.01-10 mumol/l) and Ki4032 (0.1-100 mumol/l) concentration-dependently inhibited the increases in [Ca2+]i and contraction. The inhibitory effects of cromakalim and Ki4032 were blocked by the K+ channel blocker tetrabutylammonium (TBA) and counteracted by 20 mmol/l KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca2+ release. Cromakalim reduced U46619-induced IP3 production significantly and TBA blocked this inhibitory effect. These results suggest that the hyperpolarization of the plasma membrane by K+ channel openers inhibits the production of IP3 and Ca2+ release from intracellular stores related to stimulation of the thromboxane A2 receptor.

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.

Characterization of the thromboxane receptor mediating prostacyclin release from cultured endothelial cells

The thromboxane A2 (TXA2) mimetic, 9,11-dideoxy-11,9-epoxymethano-prostaglandin F 2 alpha (U46619), mobilized calcium in the bovine aortic endothelial cell line AG4762 and stimulated release of prostacyclin from these cells. The U46619-stimulated release of prostacyclin could be inhibited by TXA2 antagonists with the order of potency [Is-[1 less than a, 2 less than b(5z), 3 less than b, 4 less than a]]-7-[3-[[2-[(phenylamino)carbonyl]hydrazino]methyl]-7-oxabicyclo- [2.2.1]hept-2-yl]-5- heptenoic acid (SQ29548) greater than 4-[2-(4-chlorobenzene-sulphonamido) ethyl]phenylacetic acid (BM13505) greater than 4-[2-(phenylsulphonamido)-ethyl]phenoxyacetic acid (BM13177), which was consistent with release being mediated by a TXA2 (TP) receptor. The TP receptor ligands, [3H]SQ29548 and 9,11-dimethylmethano-16(3-[125I]iodo-4-hydroxyphenyl)-13,14-dih ydr o-13-aza- 15-omega-o-tetranor-thromboxane ([125I]-PTA-OH), both appeared to bind to a homogenous population of sites in AG4762 cell membranes. The affinities of [3H]SQ29548 and [125I]PTA-OH were approximately 10 nM and approximately 0.3 nM, respectively, and the density of sites labelled by either ligand was approximately 25 fmol/mg protein. Under conditions where equilibrium was approached, the specific binding of [3H] SQ29548 or [125I]PTA-OH was displaced by SQ29548, BM13505 and BM13177 with the same order of potency and similar apparent affinities as in the functional assay, suggesting that these binding sites represent bona fide TP receptors.

Investigation of the prostaglandin E (EP-) receptor subtype mediating relaxation of the rabbit jugular vein

1. Prostaglandin E2 (PGE2) relaxes circular smooth muscle of the rabbit isolated jugular vein at very low concentrations (mean pIC50 against histamine-induced contraction = 9.34). This effect is not blocked by the EP1-receptor antagonist, AH 6809 (2 microM). 2. From a group of prostaglandin E analogues examined, 16,16-dimethyl PGE2, misoprostol, 11-deoxy PGE2-1-alcohol and 11-deoxy PGE1 were highly potent relaxant agents, whereas 17-phenyl-omega-trinor PGE2, MB 28767 and butaprost had low potency and sulprostone and oxoprostol were virtually inactive. 3. Comparison of the jugular vein data with published data for inhibitory agonist potencies on the cat trachea (EP2 preparation) and the field-stimulated guinea-pig vas deferens (EP3) indicates that the EP-receptor in the rabbit jugular vein is closest to the EP2 subtype. However, the correlation is not entirely convincing. For example, butaprost, 16,16-dimethyl PGE2 and 11-deoxy PGE1 are of similar potency on the cat trachea, whereas butaprost is about 300 times less potent than the other two analogues on the jugular vein. The existence of more than one EP2-receptor appears possible. 4. It was felt that the activity of butaprost required further investigation in view of the claim that it is a specific EP2-receptor agonist. We have shown that butaprost has very low inhibitory activity on the guinea-pig vas deferens, a very sensitive EP3-receptor containing preparation. However, on the chick ileum, the original EP3 preparation, butaprost showed potent contractile activity (pEC25 approximately 8.0).In addition, its maximum response was lower than that of PGE2; lower maxima were also found for sulprostone, MB 28767 and oxoprostol, but not for ICI 80205, 16,16-dimethyl PGE2 and 17-phenyl-omega-trinor PGE2. The maximal response to a combination of either sulprostone and butaprost or sulprostone and PGE2 was similar to that achieved by PGE2 alone. Analysis of the interaction between sulprostone and PGE2 appears to exclude a partial agonist action for sulprostone. Furthermore neither sulprostone nor butaprost appear to have inhibitory activity on the ileum. AH 6809 at 2 pM produced only a small shift of the PGE2 log concentration-response curve.5. It is likely that contraction of the longitudinal smooth muscle of the chick ileum is mediated by (at least) two EP-receptor subtypes; activation of only one receptor system does not induce the maximum response (i.e. the acetylcholine maximum) of the preparation. One receptor could be an EP3 subtype, at which sulprostone exerts a selective agonist action. The other receptor is unlikely to be an EP, subtype, because of the high agonist potency of butaprost, the low agonist potency of iloprost, and the low antagonist potency of AH 6809. An alternative hypothesis is that the chick ileum contains a novel EP-receptor subtype in addition to an EP3-receptor.

Characterization of receptors involved in the direct and indirect actions of prostaglandins E and I on the guinea-pig ileum

1. A study of the effects of prostaglandin E2 (PGE2) and eleven synthetic analogues on the guinea-pig isolated ileum preparation has revealed three distinct contractile actions, each associated with a different prostaglandin E (EP-) receptor subtype. In addition, PGI2 (prostacyclin) and its stable analogues can activate prostaglandin I (IP-) receptors to elicit both contraction and relaxation of the ileum. 2. Two of the PGE actions involve direct stimulation of the smooth muscle, being unaffected by 1 microM morphine treatment. One action is blocked by AH 6809 at micromolar concentrations and ICI 80205 and 16,16-dimethyl PGE2 are particularly potent agonists. Activation of EP1-receptors appears to be involved. The second action is unaffected by AH 6809; sulprostone and MB 28767 are potent agonists. Comparison with agonist potency rankings on the guinea-pig vas deferens indicates that EP3-receptors may be involved. 3. The third PGE effect and the stimulant PGI effect are blocked by morphine, indicating enteric neurones and/or sensory nerve terminals as sites of action. EP2-receptors may be involved in the PGE action, in view of the marked effect of morphine on the contractile actions of misoprostol, 11-deoxy PGE2-1-alcohol, 11-deoxy PGE1 and butaprost, all of which show some selectivity for EP2-receptors. The PGI action is most easily studied with cicaprost (EC25 = 1.3 nM), since iloprost, carbacyclin and to a lesser extent PGI2 also have agonist activity at EP1-receptors. 4. The contractile action of 17-phenyl-omega-trinor PGE2 on the ileum is unaffected by morphine. Since this analogue shows only weak agonist activity on the rabbit jugular vein (EP2 preparation) and guinea-pig vas deferens (EP3), it may be a more useful standard agonist than PGE2 in EPl1-receptor studies.5. In the presence of morphine and AH 6809, cicaprost inhibits histamine-induced contractions (IC25 = 22 nM). PGI2 and iloprost show mixed inhibitory/potentiating actions, whereas carbacyclin only potentiates histamine contractions. This IP-receptor-mediated inhibition may account for the bell-shaped log concentration-response curve of cicaprost (no inhibitors present) and the very marked block of iloprostinduced contractions by AH 6809.6. We have found no evidence for either IP-receptors mediating direct contraction or EP-receptors mediating inhibition of the ileum longitudinal smooth muscle, as has been suggested in the literature.7. In view of the complexity of prostanoid action on the guinea-pig ileum we feel that the preparation must be used with caution to ascertain the EPl agonist or antagonist potencies of novel compounds.

BAY u3405, a potent and selective thromboxane A2 receptor antagonist on airway smooth muscle in vitro

1. BAY u3405 (3(R)-[[(4-fluorophenyl) sulphonyl]amino]-1,2,3,4- tetrahydro-9H-carbazole-9-propanoic acid) has been evaluated on airway smooth muscle, from a number of species including man, for its thromboxane A2 (TXA2) antagonist activity. 2. BAY u3405 was a potent, and competitive, antagonist of the TXA2-mimetic U46619-induced contractions of human, guinea-pig, rat and ferret airway smooth muscle with pA2 values between 8.0 and 8.9 and with no inherent contractile activity (10(-9)-10(-4) M). 3. The TXA2 antagonist activity of BAY u3405 was stereoselective. Its (S)-enantiomer, BAY u3406, was approximately 50 fold less effective against U46619 on guinea-pig and human airway smooth muscle. 4. BAY u3405 also competitively antagonized contractions of guinea-pig airway smooth muscle induced by prostaglandin D2 (PGD2) or its metabolite 9 alpha, 11 beta-PGF2. On human and ferret airway smooth muscle it abolished contractions induced by PGD2, PGF2 alpha and 16,16-dimethyl-PGE2. 5. A high concentration (10(-6) M) of BAY u3405 had no effect on the contraction, or relaxation, of airway smooth muscle induced by a range of other agonists, nor did BAY u3405 have any effect on other prostanoid receptor types (DP, EP2, FP or IP). 6. BAY u3405, in contrast to some other TXA2 antagonists, is a potent and selective antagonist on a wide range of airways including human. This high affinity, and the oral activity of the compound described elsewhere, suggest it may be an appropriate tool to investigate the role of prostanoids in airway diseases such as asthma.