The mouse prostaglandin E receptor EP2 subtype: cloning, expression, and northern blot analysis

A functional cDNA clone for the mouse prostaglandin (PG) E receptor EP2 subtype was isolated from a mouse cDNA library. The mouse EP2 receptor consists of 362 amino acid residues with seven putative transmembrane domains. [3H]PGE2 bound specifically to the membrane of Chinese hamster ovary cells stably expressing the cloned receptor. This binding was displaced by unlabeled prostanoids in the order of PGE2 = PGE1 >> iloprost, a stable PGI2 agonist > PGF2 alpha > PGD2. Binding was also inhibited by butaprost (an EP2 agonist) and to a lesser extent by M&B 28767 (an EP3 agonist), but not by sulprostone (an EP1 and EP3 agonist) or SC-19220 (an EP1 antagonist). PGE2 and butaprost increased the cAMP level in the Chinese hamster ovary cells in a concentration-dependent manner. Northern blot analysis revealed that EP2 mRNA is expressed most abundantly in the uterus, followed by the spleen, lung, thymus, ileum, liver, and stomach.

Identification of Glu173 as the critical amino acid residue for the ADP-ribosyltransferase activity of Clostridium botulinum C3 exoenzyme

Clostridium botulinum C3 exoenzyme specifically ADP-ribosylates rho-p21 in eukaryotic cells. Trp18 and Glu173 of this enzyme were substituted with other amino acids via site-directed mutagenesis. All substitutions at Glu173 caused a significant reduction in affinity for NAD and diminished ADP-ribosyltransferase activity. On the other hand, the activity of enzymes with the substitution at Trp18 remained intact. Swiss 3T3 cells treated with the enzyme with the Trp18 substitution showed the typical morphologic changes of the C3 exoenzyme phenotype. In contrast, no changes were found in cells incubated with the Glu173-substituted enzyme. These results indicate that the Glu173 residue of the C3 exoenzyme plays a key role in interacting with NAD and in expression of ADP-ribosyltransferase activity, which is essential for the phenotypic change by C3 exoenzyme treatment.

Copresence of prostaglandin EP2 and EP3 receptors on gastric enterochromaffin-like cell carcinoid in African rodents

BACKGROUND & AIMS

Prostaglandins (PGs) have important roles in the regulation of gastric acid secretion. The aim of this study was to examine the possible presence of PG receptors on the gastric enterochromaffin-like (ECL) carcinoid of Mastomys natalensis, which might be a useful model of normal ECL cells.

METHODS

A [3H]PGE2 binding experiment was performed by using the ECL tumor membrane, and intracellular signal transduction was studied in the cells. In addition, Northern blot analysis using EP2 and EP3 receptor complementary DNAs was conducted.

RESULTS

[3H]PGE2 specifically bound to the tumor cell membrane, and the binding was displaced by various PGs with a potency order of PGE1 = PGE2 > enprostil > PGF2 alpha. Although PGE1 and PGE2 stimulated 5'-cyclic adenosine monophosphate (cAMP) production, neither PGF2 alpha nor enprostil had any effect. On the other hand, all of PGE1, PGE2, PGF2 alpha, and enprostil attenuated the forskolin-induced cAMP production. Moreover, enprostil inhibited histamine release induced by forskolin. However, on pertussis toxin treatment, PGE2 paradoxically enhanced the forskolin-induced increase of cAMP production. Finally, the presence of EP2 and EP3 receptor messenger RNAs was confirmed by RNA blot analysis.

CONCLUSIONS

The ECL carcinoid tumor cells of Mastomys seem to possess two subtypes of PGE receptor: EP2 linked to cAMP production and EP3 coupled with inhibitory guanosine 5'-triphosphate-binding proteins mediating the inhibition of cAMP production.

The effect of inhaled anesthetics on the platelet aggregation and the ligand-binding affinity of the platelet thromboxane A2 receptor

The mechanism by which anesthetics suppress platelet aggregation has not been elucidated. We determined the effects of halothane, enflurane, and isoflurane on human platelet aggregation induced by adenosine diphosphate (ADP), epinephrine, and a thromboxane A2 (TXA2) analog, and on ligand binding to the platelet TXA2 receptor. Halothane (2.6 mM) strongly suppressed ADP- and epinephrine-induced secondary aggregation of platelets, without significant alteration of primary aggregation. Platelet aggregation induced by a specific TXA2 agonist, (+)-9,11-epithia-11,12-methano-TXA2 (STA2), was suppressed by halothane, enflurane, and isoflurane in a concentration-dependent manner; the concentration of halothane, enflurane, and isoflurane which induced 50% inhibition (IC50) were 3.2, 12.3, and 15.7 mM, respectively (or 4.7, 9.8, and 24 minimum alveolar anesthetic concentration [MAC], respectively). The binding of a specific TXA2 receptor antagonist, 3H-S145, was significantly reduced by halothane (14-28 mM), but not by enflurane (20 mM) and isoflurane (20 mM). Scatchard analysis revealed that halothane (14 mM) increased Kd from 0.53 nM to 14.3 nM but did not alter Bmax significantly. These results indicate that halothane has a stronger suppressive effect on platelet aggregation than enflurane and isoflurane, and that the effect of halothane on platelet aggregation is due to reduction of the ligand-binding affinity of the platelet TXA2 receptor.

Lysophosphatidic acid-induced activation of protein Ser/Thr kinases in cultured rat 3Y1 fibroblasts. Possible involvement in rho p21-mediated signalling

Renaturation kinase assay was used to detect protein kinases activated by lysophosphatidic acid (LPA) in cultured rat 3Y1 fibroblasts. LPA activated several Ser/Thr protein kinases with apparent molecular weights of 145K, 85K, 64-65K (a doublet), and 60K (each named p145, p85, p64165 and p60, respectively) in addition to p43 mitogen activated protein (MAP)-kinase. Experiments using pertussis toxin and botulinum C3 exoenzyme showed that p145, p85, and p64165 kinases were activated by a pertussis toxin-insensitive rho p21-dependent pathway and that the activation of MAP-kinase was mediated by both the pertussis toxin-sensitive rho p21-independent and the pertussis toxin-insensitive rho p21-dependent pathways.

The small GTP-binding protein, rho p21, is involved in bone resorption by regulating cytoskeletal organization in osteoclasts

Rho protein (rho p21), a p21ras-related small guanine nucleotide binding protein, regulates cytoskeletal organization in a number of different types of cells. Evidence has indicated that Clostridium botulinum-derived ADP-ribosyltransferase (C3 exoenzyme) specifically ADP-ribosylates rho p21 at Asn41 and renders it functionally inactive. In this study, we examined the involvement of rho p21 in osteoclastic bone resorption using the C3 exoenzyme. When osteoclast-like multinucleated cells obtained from cocultures of mouse osteoblastic cells and bone marrow cells were placed on dentine slices, they formed ringed structures of podosomes containing F-actin (corresponding to the clear zone) within 8 hours. Many resorption pits were formed on dentine slices after culture for 24 hours. The C3 exoenzyme at 0.15-10 micrograms/ml added to the culture medium disrupted the ringed structure of podosomes in osteoclast-like cells in a dose-dependent manner. Correspondingly, pit formation by osteoclast-like cells on dentine slices was dose-dependently inhibited also by adding the C3 exoenzyme. Microinjection of the C3 exoenzyme into osteoclast-like cells placed on culture dishes completely disrupted the ringed podosome structure within 20 minutes. The amount of the rho p21 which was ADP-ribosylated by the C3 exoenzyme in vitro was much greater in purified osteoclast-like cells than in osteoblastic cells. Prior exposure of the purified osteoclast-like cell preparation to the C3 exoenzyme in vivo markedly decreased the amount of unribosylated rho p21. This indicated that the C3 exoenzyme incorporated into osteoclast-like cells effectively ADP-ribosylates rho p21 in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of prostaglandin E receptor 'EP2' cloned from mastocytoma cells EP4 subtype

We previously cloned a cDNA for a mouse PGE receptor positively coupled to adenylate cyclase from mouse mastocytoma cells, and reported it as EP2 subtype of PGE receptor [Honda, A., Sugimoto, Y., Namba, T., Watabe, A., Irie, A., Negishi, M., Narumiya, S. and Ichikawa, A. (1993) J. Biol. Chem. 268, 7759-7762]. However, it is not sensitive to one of the EP2 agonists, butaprost. Recently another subtype of PGE receptor coupled to adenylate cyclase has been identified pharmacologically and named EP4. These findings have led us to examine whether the cloned receptor is the EP4 subtype. AH23848B, a selective EP4 antagonist, not only displaced the [3H]PGE2 binding to the cloned receptor but antagonized the PGE2-stimulated cAMP formation in the receptor. In contrast, EP2 specific agonist, butaprost and 19(R)OH-PGE2 neither bound to the receptor nor stimulated the cAMP formation. These results suggest that this receptor previously reported as 'EP2' subtype is identical to the pharmacologically defined EP4 subtype and not of EP2 subtype.

Structural organization and chromosomal assignment of the human prostacyclin receptor gene

Prostacyclin receptor is a member of the prostanoid receptor family in the G protein-coupled receptor superfamily with seven transmembrane domains. We report here the isolation and structural organization of the human prostacyclin receptor gene. Southern blot analysis demonstrated a single copy of the human prostacyclin receptor gene in the human genome. The human prostacyclin receptor gene spanned approximately 7.0 kb and was composed of three exons separated by two introns. The first intron occurred in the 5'-untranslated region, 13 bp upstream to the ATG start codon. The second intron was located at the end of the sixth transmembrane domain, thereby separating it from the downstream coding region and the 3'-untranslated region. By primer extension analysis, the transcription initiation sites were mapped 870-872 bp upstream to the ATG start codon. The 1.2-kb human prostacyclin receptor 5'-flanking region lacked conventional TATA and CCAAT boxes, but it contained several cis-acting regulatory elements including an inverted CCAAT box (Y box) and two copies of SP-1 binding sites. Using human-rodent somatic hybrid cell DNA, the human prostacyclin receptor gene was assigned to human chromosome 19. The present study helps establish the genetic basis for prostacyclin receptor research and provides further insight into the molecular mechanisms underlying the prostanoid receptor family.

Involvement of thromboxane A2-thromboxane A2 receptor system of the hepatic sinusoid in pathogenesis of cold preservation/reperfusion injury in the rat liver graft

This study was designed to investigate the possible involvement of the thromboxane A2 (TXA2)-TXA2 receptor (TXA2R) system of the hepatic sinusoid in cold preservation/reperfusion injury in liver grafts. Rat livers were preserved in cold University of Wisconsin solution for either 6 or 24 hr. The number of TXA2Rs in sinusoidal endothelial cells isolated from 0-, 6-, and 24-hr preserved liver specimens was 22.50 +/- 1.80 x 10(3)/cell, 12.66 +/- 1.00 x 10(3)/cell, and 4.17 +/- 0.65 x 10(3)/cell, respectively. Kd and Bmax at 0 hr, 6 hr, and 24 hr of preservation were 8.54 +/- 1.26 nM and 37.34 +/- 3.01 fmol/10(6) cells, 7.08 +/- 1.14 nM and 12.66 +/- 1.00 fmol/10(6) cells, and 1.91 +/- 0.10 nM and 3.88 +/- 0.59 fmol/10(6) cells, respectively. The administration of OKY-046 (inhibitor of TXA2 synthesis) to the University of Wisconsin solution suppressed this reduction in TXA2R number. Furthermore, the concentration of TXA2 in hepatic sinusoid was decreased by OKY-046. In a reperfusion experiment, liver tissue preserved for 24 hr exhibited a higher reperfusion pressure, and effluent levels of both aspartate aminotransferase and lactate dehydrogenase were markedly elevated. The addition of OKY-046 to the preservation solution, however, prevented the rise in reperfusion pressure almost completely and the increase in effluent enzyme levels. This study showed that the TXA2Rs in sinusoidal endothelial cells were internalized through binding with TXA2 during cold preservation, causing activation of the TXA2-TXA2R system. This activation apparently induces an increase in reperfusion pressure, possibly due to sinusoidal contraction, resulting in microcirculatory disturbances.(ABSTRACT TRUNCATED AT 250 WORDS)

Do structural changes of T cell receptor complex occur in tumor-bearing state?

T cells in tumor-bearing mice and cancer patients were recently shown to be devoid of CD3-zeta chain, a signal-transducing invariant chain in T cell receptor (TCR) complex, and p56lck tyrosine kinase. In the present study, we investigated the structure and function of TCR complex in T cells from BALB/c mice bearing CSA1M fibrosarcoma. The expressions of TCR chains and p56lck in a T cell-enriched population from spleen were analyzed. Almost complete loss of CD3-zeta and p56lck was observed in the preparation from tumor-bearing mice as assessed by immunoblotting analysis using whole cell lysates, whereas the amounts of other TCR chains were relatively unchanged. However, these changes were due to the increase of contaminating Mac-1+ cells in the spleen of tumor-bearing mice because: 1) the removal of Mac-1+ cells led to the restoration of CD3-zeta and p56lck; and 2) CD3-zeta was clearly present when the preparation was solubilized with ionic detergent. Fc receptor gamma chain detected in the preparation from tumor-bearing mice disappeared along with the removal of Mac-1+ cells. These observations were further supported by the finding that addition of Mac-1+ cells from tumor-bearing mice to normal T cells resulted in loss of CD3-zeta, leaving CD3-epsilon largely intact. When T cells from tumor-bearing mice were highly purified by depletion of Mac-1+ cells, these T cells contained normal amounts of CD3-zeta at mRNA, protein, and surface levels, and expressed the properly assembled TCR complex on their cell surface. Moreover, stimulation of the TCR in these T cells by anti-TCR antibodies resulted in a comparable Ca2+ mobilization to that observed in normal T cells. These results suggest that no structural changes occur in TCR complex in our tumor-bearing mice, and that complete depletion of Mac-1+ cells in important to assess the structure of TCR complex.

Guanosine 5'-3-O-(thio)triphosphate stimulates tyrosine phosphorylation of p125FAK and paxillin in permeabilized Swiss 3T3 cells. Role of p21rho

Addition of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) to streptolysin O-permeabilized Swiss 3T3 cells induced tyrosine phosphorylation of M(r) 110,000-130,000 and 70,000-80,000 bands. Specifically, GTP gamma S stimulated tyrosine phosphorylation of both focal adhesion kinase (p125FAK) and paxillin. GTP gamma S induced tyrosine phosphorylation was dose-dependent (EC50 of 2.5 microM) and reached maximum levels after 1.5 min for the M(r) 110,000-130,000 band and 2 min for the M(r) 70,000-80,000 paxillin band. Guanosine 5'-O-(2-thiodiphosphate) inhibited GTP gamma S-induced tyrosine phosphorylation with an IC50 of 100 microM. Protein kinase C did not mediate GTP gamma S-induced tyrosine phosphorylation. Varying the Ca2+ concentration from 0 to 6 microM did not increase tyrosine phosphorylation above basal levels and did not affect the ability of GTP gamma S to induce tyrosine phosphorylation. GTP gamma S was able to stimulate tyrosine phosphorylation in the presence of nanomolar concentrations of Mg2+. Furthermore, 30 microM AlF4- only weakly induced tyrosine phosphorylation in permeabilized cells. Pretreatment with the Clostridium botulinum C3 exoenzyme which inactivates p21rho, markedly reduced the ability of GTP gamma S to stimulate tyrosine phosphorylation of M(r) 110,000-130,000 and 70,000-80,000 bands including p125FAK and paxillin in permeabilized Swiss 3T3 cells. Furthermore, a peptide of p21rho (p21rho17-44) inhibited GTP gamma S-induced tyrosine phosphorylation in a dose-dependent manner (IC50 1 microM). This peptide also inhibited tyrosine phosphorylation of p125FAK and paxillin. In contrast, 20 microM p21ras17-44 peptide failed to inhibit GTP gamma S-induced tyrosine phosphorylation. Using permeabilized cells, our findings demonstrate that GTP gamma S stimulates tyrosine phosphorylation of p125FAK and paxillin and that a functional p21rho is implicated in this process.

Molecular evolution of receptors for eicosanoids

The amino acid sequences of the receptors for various prostaglandins, thromboxane and lipoxin, which belong to the rhodopsin family, were aligned, and a phylogenetic tree was constructed to infer the evolutionary history of the arachidonic acid cascade. The obtained tree suggested that the origin of the cyclooxygenase pathway was different from that of the lipoxygenase pathway. The receptors involved in the cyclooxygenase pathway constructed an independent cluster, but the lipoxin A4 receptor, which is involved in the lipoxygenase pathway, belonged to the cluster of peptide receptors. The primitive form of the cyclooxygenase pathway had been a signal transduction system composed of prostaglandin E2 and its receptor associated with cAMP metabolism.

Inhibition of skin development by targeted expression of a dominant-negative retinoic acid receptor

Although pharmacological doses of retinoic acid (RA) have a wide variety of actions in vivo, experimental difficulties have prevented a definitive assignment of its physiological functions. We recently made a dominant-negative retinoic acid receptor (RAR) by a single amino-acid substitution which creates a dominant-negative thyroid hormone receptor. The mutated RAR efficiently inhibited the endogenous activities of RARs (alpha, beta, gamma). Thus, targeted expression of the mutated receptor should reveal RA functions during organogenesis by blocking RA signalling in the tissues concerned. To address this possibility, we expressed the dominant-negative RAR in the epidermis, a potential target organ of RA. We report here that the resultant transgenic mice exhibited dramatic suppression of epidermal maturation, demonstrating the requirement of RA in normal skin development.

Cloning and expression of a cDNA for rat prostacyclin receptor

A cDNA clone for rat prostacyclin receptor was isolated. The cDNA encodes a protein of 416 amino acid residues (M(r) 44,662) with putative seven transmembrane domains, and belongs to the G protein-coupled receptor superfamily. Specific binding of [3H]iloprost was found in membrane of COS-7 cells transfected with the cDNA (Kd = 1.3 nM) and was displaced with unlabeled prostaglandins in the order of iloprost = cicaprost > PGE1 > STA2 = PGE2 = PGD2 > PGF2 alpha. Northern blot analysis demonstrated that rat prostacyclin receptor mRNA is expressed in the lung, spleen, heart, pancreas, thymus, stomach and aorta.

Activation-induced expression of thymic shared antigen-1 on T lymphocytes and its inhibitory role for TCR-mediated IL-2 production

We have produced a hamster mAb, PRST1, which reacts with thymic shared Ag-1 (TSA-1), a product of the Ly6 gene family. By cross-blocking experiments, we found that TSA-1 is identical to stem cell Ag-2 (Sca-2). Using PRST1, the changes of TSA-1/Sca-2 expression on mature T cells during the activation process were analyzed. Although freshly isolated T cells did not express detectable TSA-1 on their cell surface, in vitro stimulation of T cells with concanavalin A induced a marked increase of surface TSA-1 expression. The increased expression of TSA-1 on T cells was detected from 12 h after stimulation and was associated with the increase of TSA-1 mRNA. In vivo injection of mice with staphylococcal enterotoxin B (SEB) resulted in the enhanced TSA-1 expression in splenic V beta 8+ T cells. This antigen-specific induction of TSA-1 expression in vivo preceded a detectable increase in numbers of V beta 8- T cells after SEB injection. Functionally, whereas anti-TSA-1 mAb was not mitogenic to T cells, it inhibited anti-CD3-induced IL-2 production by T cell hybridomas. These results indicate that TSA-1/Sca-2 is a unique marker for T cell activation and a signal through this molecule may have a negative feedback role to limit IL-2 production from activated T cells stimulated through the TCR.

Prostaglandin E2 protects cultured cortical neurons against N-methyl-D-aspartate receptor-mediated glutamate cytotoxicity

The effects of prostaglandin (PG) E2 on glutamate-induced cytotoxicity were examined using primary cultures of rat cortical neurons. The cell viability was significantly reduced when cultures were briefly exposed to either glutamate or N-methyl-D-aspartate (NMDA) then incubated with normal medium for 1 h. Similar cytotoxicity was observed with the brief application of ionomycin, a calcium ionophore, and S-nitrosocysteine, a nitric oxide (NO)-generating agent. PGE2 at concentrations of 0.01-1 microM dose-dependently ameliorated the glutamate-induced cytotoxicity. PGE1, butaprost, an EP2 receptor agonist, and 8-bromo-cAMP were also effective in protecting cultures against glutamate cytotoxicity. By contrast, neither 17-phenyl-omega-trinor-PGE2, an EP1 receptor agonist, nor M&B 28767, an EP3 receptor agonist, affected glutamate-induced cytotoxicity. NMDA-induced cytotoxicity was ameliorated by PGE2, butaprost, MK-801, N-omega-nitro-L-arginine, a NO synthase inhibitor, and hemoglobin, which binds NO. These agents excluding MK-801 ameliorated the ionomycin-induced cytotoxicity. The cytotoxicity induced by S-nitrosocysteine was prevented only by hemoglobin but not by the other agents including PGE2. These findings indicate that PGE2 protects cultured cortical neurons against NMDA receptor-mediated glutamate neurotoxicity via EP2 receptors. EP2 receptor stimulation may suppress a step in NO formation triggered by Ca(2+)-influx through NMDA receptors.

Botulinum C3 exoenzyme blocks the tyrosine phosphorylation of p125FAK and paxillin induced by bombesin and endothelin

In this study we examined the role of rho p21 in neuropeptide-stimulated tyrosine phosphorylation. Intact Swiss 3T3 cells were treated with the Clostridium botulinum C3 exoenzyme which specifically ADP ribosylates and inactivates rho p21. C3 exoenzyme treatment of cells caused a marked decrease in both bombesin- and endothelin-stimulated tyrosine phosphorylation of multiple proteins, including p125 focal adhesion kinase (FAK) and paxillin. Our results suggest that rho p21 is a component of the signal transduction pathway linking seven transmembrane domain receptors with tyrosine phosphorylation and cytoskeletal events.

Molecular characterization of a mouse prostaglandin D receptor and functional expression of the cloned gene

Prostanoid receptors belong to the family of G protein-coupled receptors with seven transmembrane domains. By taking advantage of nucleotide sequence homology among the prostanoid receptors, we have isolated and identified a cDNA fragment and its gene encoding a mouse prostaglandin (PG) D receptor by reverse transcription polymerase chain reaction and gene cloning. This gene codes for a polypeptide of 357 amino acids, with a calculated molecular weight of 40,012. The deduced amino acid sequence has a high degree of similarity with the mouse PGI receptor and the EP2 subtype of the PGE receptor, which together form a subgroup of the prostanoid receptors. Chinese hamster ovary cells stably expressing the gene showed a single class of binding sites for [#H]PGD2 with a Kd of 40 nM. This binding was displaced by unlabeled ligands in the following order: PGD2 > BW 245C (a PGD agonist) > BW A868C (a PGD antagonist) > STA2 (a thromboxane A2 agonist). PGE2, PGF2 alpha, and iloprost showed little displacement activity at concentrations up to 10 microM. PGD2 and BW 245C also increased cAMP levels in Chinese hamster ovary cells expressing the receptor, in a concentration-dependent manner. BW A868C showed a partial agonist activity in the cAMP assay. Northern blotting analysis with mouse poly(A)+ RNA identified a major mRNA species of 3.5 kb that was most abundantly expressed in the ileum, followed by lung, stomach, and uterus.

Functional reconstitution of platelet thromboxane A2 receptors with Gq and Gi2 in phospholipid vesicles

The partially purified thromboxane (TX) A2 receptor was reconstituted with two species of purified heterotrimeric G proteins, Gq and Gi2, in phospholipid vesicles. The receptors reconstituted with Gq and Gi2 showed a single class of [3H]S-145 binding with Kd values of 9.6 +/- 0.7 and 12.1 +/- 1.0 nM, respectively; binding was displaced by GR32191, 9,11-epithio-11, 12-methano-thromboxane A2 (STA2), and U46619, with almost identical Ki values for each compound in the two types of reconstituted vesicles. When the receptor and Gq were reconstituted, the agonist STA2 stimulated guanosine-5'-O-(3-[35S]thio)triphosphate binding. This stimulation was half-maximal at 80 nM and reached a plateau at 1 microM STA2 stimulated the initial rate by 20-30-fold, compared with the basal rate. The stimulation of guanosine-5'-O-(3-[35S]thio)triphosphate binding to Gi2 by the agonist-liganded receptor was seen in the presence of GDP. Under these conditions, 10 microM STA2 stimulated the initial rate by 1.5-2-fold, compared with the basal rate. This effect was half-maximal at 150 nM and reached a plateau at 1 microM. The agonist-liganded receptor also stimulated the GTPase activities of the reconstituted G proteins. The steady state rates of STA2-stimulated [32P]Pi release from [gamma-32P]GTP were 2.21/min.receptor and 0.87/min.receptor in the Gq- and Gi2-reconsituted vesicles, respectively, and the Kcat values of Gq and Gi2 in the presence of STA2 were 0.87 +/- 0.21 min-1 and 2.41 +/- 0.12 min-1, respectively. These results clearly show that the TXA2 receptor functionally couples to both Gq and Gi2. Consistent with this finding, STA2, by acting on the TXA2 receptor in intact platelets, inhibited prostaglandin I2-induced cAMP elevation.