A common binding site for primary prostanoids in vascular smooth muscles: a definitive discrimination of the binding for thromboxane A2/prostaglandin H2 receptor agonist from its antagonist

Endothelial cyclooxygenase-1 paradoxically drives local vasoconstriction and atherogenesis despite underpinning prostacyclin generation

Endothelial cyclooxygenase-1-derived prostanoids, including prostacyclin, have clear cardioprotective roles associated with their anti-thrombotic potential but have also been suggested to have paradoxical pathological activities within arteries. To date it has not been possible to test the importance of this because no models have been available that separate vascular cyclooxygenase-1 products from those generated elsewhere. Here, we have used unique endothelial-specific cyclooxygenase-1 knockout mice to show that endothelial cyclooxygenase-1 produces both protective and pathological products. Functionally, however, the overall effect of these was to drive pathological responses in the context of both vasoconstriction in vitro and the development of atherosclerosis and vascular inflammation in vivo. These data provide the first demonstration of a pathological role for the vascular cyclooxygenase-1 pathway, highlighting its potential as a therapeutic target. They also emphasize that, across biology, the role of prostanoids is not always predictable due to unique balances of context, products, and receptors.

The Local Regulation of Vascular Function: From an Inside-Outside to an Outside-Inside Model

Our understanding of the regulation of vascular function, specifically that of vasomotion, has evolved dramatically over the past few decades. The classic conception of a vascular system solely regulated by circulating hormones and sympathetic innervation gave way to a vision of a local regulation. Initially by the so-called, autacoids like prostacyclin, which represented the first endothelium-derived paracrine regulator of smooth muscle. This was the prelude of the EDRF-nitric oxide age that has occupied vascular scientists for nearly 30 years. Endothelial cells revealed to have the ability to generate numerous mediators besides prostacyclin and nitric oxide (NO). The need to classify these substances led to the coining of the terms: endothelium-derived relaxing, hyperpolarizing and contracting factors, which included various prostaglandins, thromboxane A2, endothelin, as well numerous candidates for the hyperpolarizing factor. The opposite layer of the vascular wall, the adventitia, eventually and for a quite short period of time, enjoyed the attention of some vascular physiologists. Adventitial fibroblasts were recognized as paracrine cells to the smooth muscle because of their ability to produce some substances such as superoxide. Remarkably, this took place before our awareness of the functional potential of another adventitial cell, the adipocyte. Possibly, because the perivascular adipose tissue (PVAT) was systematically removed during the experiments as considered a non-vascular artifact tissue, it took quite long to be considered a major source of paracrine substances. These are now being integrated in the vast pool of mediators synthesized by adipocytes, known as adipokines. They include hormones involved in metabolic regulation, like leptin or adiponectin; classic vascular mediators like NO, angiotensin II or catecholamines; and inflammatory mediators or adipocytokines. The first substance studied was an anti-contractile factor named adipose-derived relaxing factor of uncertain chemical nature but possibly, some of the relaxing mediators mentioned above are behind this factor. This manuscript intends to review the vascular regulation from the point of view of the paracrine control exerted by the cells present in the vascular environment, namely, endothelial, adventitial, adipocyte and vascular stromal cells.

Thromboxane hypersensitivity in hypoxic pulmonary artery myocytes: altered TP receptor localization and kinetics

Hypoxia-induced neonatal persistent pulmonary hypertension (PPHN) is characterized by sustained vasospasm and increased thromboxane (TxA2)-to-prostacyclin ratio. We previously demonstrated that moderate hypoxia induces myocyte TxA2 hypersensitivity. Here, we examined TxA2 prostanoid receptor (TP-R) localization and kinetics following hypoxia to determine the mechanism of hypoxia-induced TxA2 hypersensitivity. Primary cultured neonatal pulmonary artery myocytes were exposed to 10% O2 (hypoxic myocytes; HM) or 21% O2 (normoxic myocytes; NM) for 3 days. PPHN was induced in neonatal piglets by in vivo exposure to 10% FiO2 for 3 days. TP-R was studied in whole lung sections from pigs with hypoxic PPHN- and age-matched controls; intracellular localization was studied by immunocytochemistry. TP-R affinity was studied in cultured myocytes by saturation binding kinetics using 3H-SQ-29548 and competitive binding kinetics by coincubation with U-46619. Phosphorylation and coupling were examined in immunoprecipitated TP-R. We report distal propagation of TP-R expression in PPHN, extending to pulmonary arteries <50 microm. In HM, intracellular TP-R moves towards the perinuclear region, mirroring a change in endoplasmic reticulum (ER) morphology. TP-R kinetics also alter in HM membranes, with decreased Kd and Bmax (maximal binding sites). Additionally, in hypoxia, 3H-SQ-29548 is displaced at lower concentration of U-46619 than in normoxia, suggesting increased agonist affinity. Phosphorylation of serine residues on HM TP-R was significantly decreased compared with NM; this difference correlated with increased Galphaq coupling in hypoxia and was ablated by incubation with PKA. We conclude that the TP-R is normally desensitized in the neonatal pulmonary circuit by PKA-mediated regulatory phosphorylation, decreasing ligand affinity and coupling to Galphaq; this protection is lost following hypoxic exposure. Also, the appearance of TP-R in resistance arteries after development of hypoxic PPHN may contribute to increased pulmonary arterial pressure.

Channel modulators affect PGE(2) binding to bovine aortic endothelial cells

PGE(2), PGF(2alpha) and the thromboxane agonist U-46619 bind to bovine aortic endothelial cells and compete on the same binding site with similar affinity. In addition, binding remains unaffected by prolonged exposure to the ligand. These characteristics differ significantly from those of any known G-coupled prostaglandin receptor. Binding of PGE(2) to the cells is reduced in the presence of the cyclic nucleotides cGMP and cAMP, and is unaffected by protein kinase inhibitors. Removal of permeable cyclic nucleotides from the cell medium results in a fast and complete restoration of PGE(2) binding to the cells, suggesting that both cyclic nucleotides reduce PGE(2) binding by a reversible interaction with the prostaglandin-binding site, without the involvement of second messenger-activated protein kinases. Our data further show that binding of prostaglandins to bovine aortic endothelial cells is sensitive to heavy metals and to activators and blockers of calcium, ATP-sensitive K(+) and chloride channels. Nickel, a specific cyclic nucleotide-gated (CNG) channel activator, decreases PGE(2) binding and so do the CNG channel activators Rp-8-Br-PET-cGMPS and Sp-8-Br-PET-cGMPS. On the other hand, the calcium channel blockers pimozide, diltiazem as well as LY-83,583, a guanylate cyclase inhibitor, which were reported to block CNG channels, enhance PGE(2) binding. The sensitivity of PGE(2) binding to selective CNG channel modifying agents, as well as the rapid and reversible interaction with cyclic nucleotides, may suggest that the common low-affinity prostanoid-binding site on bovine aortic endothelial cells is associated with a molecular entity, which possess several properties of a CNG channel.

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.

Regulation of a common, low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

Bovine aortic endothelial cells contain a prostaglandin site which binds with similar low-affinity PGE2, PGF2alpha and the thromboxane agonist U-46619. Treatment of the cells with agents that increase the level of cellular cAMP such as forskolin, a direct activator of adenylate cyclase or IBMX, a phosphodiesterase inhibitor, decreased the binding of PGE2 to the cells. Addition of dibutyryl cAMP to intact cells caused a quick reduction in PGE2 binding with a half time of less than 2 min. The reduction in PGE2 binding was completely reversible after removing the dibutyryl cAMP. The reduction in PGE2 binding after addition of dibutyryl cAMP to the intact cells was also observed after a mechanical disruption of the cells or after permeabilization with digitonin. Incubation of the cells with myristoylated PKI(14-22) amide, a specific protein kinase A inhibitor, resulted in partial suppression of the reduction of PGE2 binding by dibutyryl cAMP. Pretreatment of intact cells for 24 h with 10(-6) M PGE2 or a PKC activator did not reduce the specific binding of [3H]-PGE2. These results suggest that PKA, but not PKC, is involved in a fast reversible regulation of the common prostanoid receptor on bovine endothelial cells.

Intracellular Ca2+ elevation and contraction due to prostaglandin F2alpha in rat aorta

Prostaglandin F2alpha was tested to determine (a) whether its effect on intracellular Ca2+ levels ([Ca2+]i) and force in vascular smooth muscle was mediated through activation of the thromboxane A2 and/or prostaglandin receptor, and (b) the relative roles of Ca2+ influx via L-type and non-L-type Ca2+ channels in prostaglandin receptor-mediated contraction. [Ca2+]i and force were measured simultaneously in fura-2-loaded rat aortic strips. The thromboxane A2 receptor antagonist, SQ29548 ([1S]-1a,2b(5Z),3b,4a-7-(3-[2-[(phenylamino)carbonyl] hydrazinomethyl)-7-oxobicyclo-[2.2.1]hept-2-yl-5-heptenoic acid), prevented the prostaglandin F2alpha-induced plateau [Ca2+]i elevation and force by 80-90%, while abolishing these responses due to the thromboxane A2 receptor agonist, U46619 (9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F2alpha). Prostaglandin F2alpha (+ SQ29548)-induced plateau [Ca2+]i elevation and force were not inhibited by verapamil. Ni2+, a non-selective cation channel blocker, in the presence of verapamil, abolished the prostaglandin F2alpha (+ SQ29548)-elevated [Ca2+]i, while the contraction was only partially inhibited. These results suggest that, in rat aorta, (1) elevated [Ca2+]i and force due to high prostaglandin F2alpha concentrations largely results from thromboxane A2 receptor activation, and (2) the prostaglandin component of the prostaglandin F2alpha-induced contraction is dependent on Ca2+ influx via non-L-type channels.

Low-affinity thromboxane receptor mediates proliferation in cultured vascular smooth muscle cells of rats

We examined the binding properties and mitogenic effects of U46619, using cultured vascular smooth muscle cells (VSMCs), by ligand-binding assay, measuring [3H]thymidine and [3H]leucine incorporation, checking with flow cytometry, and counting the cell number. The U46619-activated mitogenic signal-transduction pathway was assessed by measuring formation of inositol monophosphate (IP); [Ca2+]i; mitogen-activated protein kinase (MAPK), MAPK kinase (MAPKK), and p74raf-1 activities; and GTP-bound Ras. [3H]U46619 bound to cultured VSMCs from Wistar-Kyoto (WKY) rats at a single class of site (Kd: 15.5 +/- 2.6 nmol/L). However, it bound to VSMCs from spontaneously hypertensive rats (SHRs) at two classes of sites (Kd: 2.3 +/- 0.6 nmol/L and 1.4 +/- 0.5 mumol/L). U46619 increased DNA and protein synthesis, cell number, IP formation, [Ca2+]i, and MAPK and MAPKK activities, with EC50 values close to its Kd value for the low-affinity binding site in VSMCs from SHR. Prostaglandin (PG) E2 and PGF2 alpha showed little of such mitogenic effects. All these effects of U46619 were inhibited by SQ29548, staurosporine, or pretreatment of VSMCs with phorbol 12-myristate 13-acetate for 24 hours. However, U46619 stimulation did not lead to a significant increase in the Ras-GTP complex or p74raf-1 activity. In conclusion, the mitogenic effect of U46619 appears to be mediated via the activation of low-affinity thromboxane binding sites that trigger phosphoinositide hydrolysis and activate the MAPK pathway, leading to DNA synthesis and cell proliferation.

A common low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

[3H]PGE2 and [3H]PGF2 alpha were shown to bind with similar binding capacity and dissociation constants to bovine aorta endothelial cells. The similarity in the binding parameters suggests that both agonists may bind to the same binding site. Displacement of [3H]PGE2 performed with PGE2, PGF2 alpha or U-46619, a thromboxane agonist, shows that all three prostanoids displaced the bound [3H]PGE2 with comparable potency (IC50 = 10(-7) M). These results indicated that the three different prostanoids, which serve as specific agonists to different prostanoid receptors, also compete for the same binding site in bovine endothelial cells with similar affinity. Comparison of the displacement of [3H]PGE2 or [3H]PGF2 alpha by a number of prostaglandin agonists and antagonists further supports the notion that the natural prostanoids bind with similar affinities to the same binding site. Thus, sulprostone, an EP1/EP3 agonist, displaced bound [3H]PGE2 and [3H]PGF2 alpha with IC50 of about 10(-7) M. On the other hand, thromboxane antagonists (BAY u-3405 and GR-32191B), EP1 specific antagonist (SC-19220) EP1/DP antagonist (AH-6809) and iloprost, a stable prostacyclin agonist, failed to displace bound [3H]PGE2 or [3H]PGF2 alpha at a concentration range of 10(-9)-10(-6) M. Gradual increase of sodium fluoride (NaF), a general activator of G binding proteins, or incubation of permeabilized cells with GTP gamma S resulted in a decrease in [3H]PGE2 binding, suggesting that the binding site represents a low-affinity common prostanoid receptor which, similar to other prostanoid receptors, is probably coupled with G binding proteins.

Characterization of the thromboxane (TP-) receptor subtype involved in proliferation in cultured vascular smooth muscle cells of rat

1. The effects of the thromboxane A2 (TxA2)-mimetic, U-46619, on the proliferation of vascular smooth muscle cells (VSMCs) were examined in a clonal smooth muscle cell line, A10, which was derived from foetal rat aorta. 2. [3H]-U-46619 bound to A10 cells of passages 18-20 (p18-20) with two classes of sites. The high affinity site showed a Bmax of 3.0 +/- 1.8 fmol mg-1 protein with a KD value 1.0 +/- 0.1 nM, while the low affinity site showed a Bmax of 43.0 +/- 6.0 fmol mg-1 protein and KD value of 129.0 +/- 7.9 nM. However, [3H]-U-46619 bound to A10 cells from passages 28-30 (p28-30) at a single class of site with a Bmax 111.0 +/- 9.0 fmol mg-1 protein and a KD value of 175.4 +/- 22.0 nM. 3. Cinnamophilin and SQ29548 inhibited specific [3H]-U-46619 binding to p18-20 A10 cells in a concentration-dependent manner with Ki values of 390.0 +/- 3.2 and 4.6 +/- 1.0 nM, respectively at a high affinity site, and 2.6 +/- 0.2 microM and 310.0 +/- 6.4 nM, respectively at the low affinity site. 4. U-46619 produced isometric contractions of rat aorta in a concentration-dependent manner with an EC50 7.0 +/- 1.2 nM. Cinnamophilin and SQ29548 antagonized U-46619-induced aortic contractions with pA2 values 6.3 +/- 0.1 and 8.2 +/- 0.2, respectively. 5. U-46619 increased [3H]-thymidine incorporation into DNA of p18-20 and p28-30 A10 cells in aconcentration-dependent manner with EC50 values 362.7 +/- 27.0 and 302.5 +/- 20.1 nm, respectively. The U-46619-induced increase of [3H]-thymidine incorporation into DNA of p28 -30 AO0 cells was potentiatedby PDGF (1 ng ml-1) and FCS (1%) and was inhibited by cinnamophilin (10 microM) and SQ29548 (1 microM)with estimated pKB values 5.4 +/- 1.2 and 6.3 +/- 0.9, respectively.6. Cell cycle analysis revealed that U-46619-increased cell cycle progression was primarily due to a rapidtransition from the DNA synthetic (S) to the G2/mitotic (M) phase. Moreover, U-46619 also increasedprotein synthesis and cell numbers in VSMC. All these effects of U-46619 were inhibited bycinnamophilin and SQ29548.7. U-46619 caused phosphoinositide breakdown and increased the intracellular Ca2+ concentration inVSMC, effects which were blocked by cinnamophilin and SQ29548.8 These data indicate there are two U-46619 binding sites in AlO VSMC. The high affinity site is correlated to U-46619-induced vasoconstriction while the low affinity site is correlated to U-46619-mediated VSMC proliferation. These data also reveal that U-46619 stimulates the cell cycle progression in VSMC primarily through a rapid transition from S to G2/M. Since cinnamophilin inhibits TPreceptor-mediated VSMC proliferation, it may thus hold promising potential for the prevention of atherosclerosis or vascular diseases.

Inhibitory effect of vapiprost on contractile responses of isolated dog renal arteries to thromboxane A2 analogue, U46619

1. Cumulative administrations of U46619, a thromboxane A2 analogue, and prostaglandin (PG) F2 alpha produced concentration-dependent contractions of isolated dog renal arterial preparations, which were significantly and concentration-dependently inhibited by vapiprost. 2. A bolus administration of U46619 or PGF2 alpha produced sustained contracture of these preparations, which was concentration-dependently relaxed by cumulative vapiprost. 3. Results indicate that vapiprost inhibits U46619- and PGF2 alpha-induced dog renal arterial contractions through antagonism for so-called TP receptors.

Potentiation of norepinephrine-induced contraction by primary prostaglandin receptor activation in rat aorta

This study investigates the role of primary prostaglandin receptor activation in the modulation of agonist-induced vascular smooth muscle contraction. Prostaglandin F2 alpha induced a concentration-dependent contraction of the rat aorta that was nearly abolished by the thromboxane A2 receptor antagonist, SQ29548. Prostaglandin F2 alpha in the presence of SQ29548 induced leftward shifts of the norepinephrine and KCl concentration-response curves. Nifedipine abolished the leftward shift of the norepinephrine concentration-response curve observed in the presence of prostaglandin F2 alpha and SQ29548. These results suggest that a function of primary prostaglandin receptor activation may be to potentiate agonist-induced contraction. The potentiation is dependent upon the opening of dihydropyridine-sensitive Ca2+ channels.

Vasoconstrictor reaction to prostaglandins E1 and E2 in the isolated rat hindlimb mediated through serotonin release

The vasoconstrictor effects of prostaglandins E (PGEs) were studied in the isolated perfused hindlimbs of the rat. Doses of PGE1 in the range of 0.01-10 micrograms caused dose-dependent increases in perfusion pressure with rapid development of tachyphylaxis. The pressure increases were inversely temperature-dependent, being 4.8 +/- 1.2 mm Hg at 37 degrees C and 22.2 +/- 1.8 mm Hg at 27 degrees C. The vasoconstriction was limited to PGE1 and PGE2. Other prostaglandins tested, PGF2 alpha, PGD2, PGA1, 7-oxo-PGI2 and arachidonic acid, were without any effect at 27 degrees C. Increased serotonin concentrations were found in the effluent after the administration of 1 microgram of PGE1. The pressor response to PGE1 was blocked by 5-HT2 antagonists, ketanserin (2 micrograms) and naftidrofuryl (100 micrograms), while the alpha-adrenoceptor antagonist phentolamine (6.7 micrograms) was without effect. Rapid tachyphylaxis of the development of vasoconstriction was observed after the repeated administration of PGE1 and PGE2. PGE1 (1 microgram) induced desensitization with more than 60 min duration. All the findings suggest that serotonin was liberated in the rat hindlimbs by PGEs.

The existence of distinct classes of prostaglandin E2 receptors mediating adenylate cyclase and phospholipase C pathways in osteoblastic clone MC3T3-E1

We have reported previously that PGE2 evoked an increase in intracellular calcium level ([Ca2+]i) in mouse osteoblastic cells (1). Here, we investigated the effects of PGE1 and PGF2 alpha on cAMP production and [Ca2+]i in comparison with those of PGE2. In osteoblastic clone, MC3T3-E1 cells, PGE1 stimulated cAMP production, but had no effect on [Ca2+]i, whereas PGF2 alpha evoked only [Ca2+]i increase. In contrast, PGE2 not only stimulated cAMP production, but also increased [Ca2+]i. From the Scatchard plot analysis of PGE2 it was confirmed that there were two classes of PGE2 binding sites (Kd value, 9.2 nM; binding site, 29 fmole/mg protein, and Kd value, 134 nM; binding site, 148 fmole/mg protein). As the increase in [Ca2+]i was caused by PGF2 alpha and PGE2, but not by PGE1, we investigated the displacement of [3H]-PGF2 alpha binding. The displacement capacity of unlabeled PGE2 was about 110 of that of PGF2 alpha, while that of PGE1 was very low even at 500-fold excess. These data indicate the possibility that the dual action of PGE2 is mediated by distinct receptor systems.

Blockade of thromboxane/endoperoxide receptor-mediated responses in the pulmonary vascular bed of the cat by sulotroban

The effects of sulotroban (BM13.177; SK & F 95587), a thromboxane (TX) A2/endoperoxide (PGH2) receptor blocking agent on responses to the TXA2/PGH2 mimics, U46619 and U44069, were investigated in the pulmonary vascular bed of the intact-chest cat under constant flow conditions. Injections of U46619 and U44069 directly into the perfused lobar artery caused dose-related increases in lobar arterial pressure without altering left atrial pressure. Following administration of sulotroban in a dose of 5 mg/kg i.v., dose-response curves for U46619 and U44069 were shifted to the right in a parallel manner. The duration of the blocking effect of sulotroban was investigated, and responses to U46619 returned to approximately 50% of control in 120 min and were not significantly different from control 240 min after administration of the receptor antagonist. Sulotroban was without significant effect on responses to prostaglandin (PG) D2 or F2 alpha or serotonin, histamine, norepinephrine, angiotensin II or BAY K8644, an agent which enhances calcium entry. Sulotroban was without effect on responses to endothelin (ET)-1, sarafotoxin (S) 6a or S6c and platelet-activating factor (PAF). Sulotroban did not alter baseline vascular pressures in the cat and responses to the PG and TXA2/PGH2 precursor, arachidonic acid, were reduced. The present data show that sulotroban selectively blocks TXA2/PGH2 receptor-mediated responses in a competitive and reversible manner in the pulmonary vascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional differences in the prostanoid receptors mediating prostaglandin F2 alpha-induced contractions of cat isolated arteries

U46619, a stable thromboxane A2 (TXA2) mimetic, and prostaglandin F2 alpha (PGF2 alpha) contracted helical strips of cat coronary, renal and mesenteric arteries in a concentration-dependent manner. The EC50 values for U46619 did not differ significantly in these arteries, but those for PGF2 alpha were in the order of coronary less than renal less than mesenteric arteries. Contractions induced by U46619 were antagonized by S-145, a selective TXA2 receptor antagonist, with similar activity in these arteries. On the other hand, contractions induced by low concentrations of PGF2 alpha (10(-9) to 10(-7) M) were not influenced by treatment with S-145 in coronary arteries, although those induced by high concentrations (5 x 10(-7) to 10(-5) M) were partially attenuated. These contractions resistant to the TXA2 antagonist were antagonized by diphloretin phosphate (DPP), a non-selective PG antagonist. Contractions induced by PGF2 alpha (5 x 10(-7) to 5 x 10(-5) M) in mesenteric arteries were inhibited by S-145 in a concentration-dependent manner. Contractions induced by PGF2 alpha in renal arteries were partially inhibited by S-145. The inhibitory activity of S-145 to PGF2 alpha-induced contractions at EC50 was in the order of coronary less than renal less than mesenteric arteries. Treatment with indomethacin slightly potentiated the contractions induced by PGF2 alpha in mesenteric arteries. Removal of the endothelium did not affect the contractile responses induced by PGF2 alpha and the inhibitory activity of S-145 in the arteries. These results suggest that the contractile responses induced by low concentrations of PGF2 alpha (up to 10(-7) M) are associated with their action via PG receptor(s), which is different from TXA2 receptor, and those induced by high concentrations of PGF2 alpha (5 x 10(-7) M or higher) interact with TXA2 receptors in cat vascular smooth muscles. It appears that the functional expression of this PG receptor(s) is greater in coronary arteries than in renal arteries, and that it is not found in mesenteric arteries.

Receptor-mediated mitogenic effect of thromboxane A2 in vascular smooth muscle cells

The effects of thromboxane A2 (TXA2) on the proliferation of vascular smooth muscles cells (VSMC) were examined using primary cultures of VSMC from rat aorta. U46619, a stable TXA2 mimetic, stimulated DNA synthesis of VSMC only in the presence of insulin. The effect was concentration-dependent with a half-maximal effect obtained at approximately 1 x 10(-8) M. The mitogenic effect of U46619 was larger than that of endothelin, another mitogen derived from endothelium. Among several TXA2/PGH2 analogs, the proliferative activity was detected only in the agonists, and not in the antagonists or in the metabolite of TXA2. A series of TXA2/PHG2 receptor antagonists completely suppressed the U46619-stimulated DNA synthesis as well as the [3H]SQ29,548 binding to the TXA2/PGH2 receptors in VSMC. The rank order of binding affinities to the receptors among the respective antagonists correlated well with the potencies for suppression of the proliferative effects of U46619. The mitogenic effects of U46619 were also attenuated by the presence of calcium antagonists. U46619 caused activation of phospholipase C with the production of inositol trisphosphate, leading to increases in the intracellular free Ca2+ concentration as measured with the fluorescent indicator fura-2. These results suggest that TXA2 induces mitogenic effects on VSMC through binding to its specific receptors. This effect of TXA2 on the proliferation of VSMC may be related to the development of atherosclerosis.