A possible role of endogenously formed cerebral prostaglandins in the development of adaptive protection against cerebral hypoxia/ischemia in mice

We found recently that exogenously administered PGD2, PGE1 and PGI2 showed a protective effect against cerebral hypoxia/ischemia in mice. In the present study, to find out whether these PGs play a pathophysiological role in cerebral hypoxia/ischemia, we examined a possible role of PGs in the development of adaptive protection against cerebral hypoxia/ischemia. Mice were pretreated with a sublethal dose of KCN, hypoxic gas mixture and electroshock 10-120 min before tests. Ten to thirty min after pretreatment with a sublethal dose of KCN, mice proved to be significantly protected against cerebral hypoxia/anoxia in all models studied: KCN-induced anoxia, normobaric hypoxia and decapitation-induced gasping. Similar results were observed when hypoxic gas and electroshock were used as pretreatments. These facts indicate that the protective effect does not depend on how cerebral hypoxia/anoxia is induced but on the substances formed in the brain after hypoxia/anoxia as well as electroshock. Brain concentrations of cyclooxygenase products markedly increased subsequent to hypoxia/anoxia as well as electroshock. The increase in PGs formation as well as resistance to hypoxia was prevented by pretreatment with indomethacin. These findings suggest that endogenously formed PGs at least including the three PGs, PGD2, PGE1 and prostacyclin in mouse brain during or after hypoxia/ischemia are responsible for the increase of resistance to hypoxia/ischemia.     

Enhancement by prostacyclin of the contractility of the guinea-pig airways smooth muscle

Aerosolized prostacyclin (PGI2) potentiated the increase in pulmonary resistance to inflation induced by serotonin, prostaglandin F2 alpha (PGF2 alpha), acetylcholine and histamine in the guinea-pig. This was not due to a reflex, nor to further production of PG cyclooxygenase derivatives. PGI2 and PGF2 alpha induced contraction of the parenchyma lung strip of the guinea-pig, which could be inhibited by polyphloretin phosphate and by PGE1. Since PGF2 alpha failed to potentiate the bronchial responses to acetylcholine, histamine or serotonin, under conditions where PGI2 was effective, the in vitro similarities between the two PGs cannot explain the in vivo results. The ability of PGI2 to potentiate bronchial responses was not shared by the other PGs. Since the latter are either bronchoconstrictor agents by themselves (PGF2 alpha and PGD2), or bronchodilators (PGE1, PGE2), our hypothesis is that PGI2 potentiates the responses of the bronchi to various agonists by a mechanism similar to that which accounts for the potentiation of acute inflammation and pain by PGE1 and PGE2, the latter being ineffective in enhancing the bronchial responses because of the associated bronchodilator activity.     

Comparison of the effects of different arachidonic acid metabolites on cyclic nucleotide accumulation in human peripheral lymphocytes

The effect of PGE1, PGE2, PGD2, PGF2 alpha, PGI2, PGG2, PGA1, 12L-HETE, arachidonic acid, 15- HPETEa and linolenic acid on the accumulation of cyclic AMP in human peripheral lymphocytes was studied. PGE1, PGE2 and PGD2 were essentially equipotent as stimulators of cyclic AMP accumulation (threshold at about 10(-8)M and EC50 about 0.15 microM), PGF2 alpha was about 20 times less potent, while PGG2, 12L-HETE, 15-HPETE, PGA1 and linolenic acid were inactive. PGI2 caused a weak stimulation between 5 and 600 nM and a secondary stimulation above 3 microM. Arachidonic acid had no effect on cyclic AMP levels up to 100 microM. PGE1, PGD2, PGI2 and PGF2 alpha increased cyclic GMP in the concentrations that produced a rise in cyclic AMP, but the cyclic GMP increase was of smaller magnitude. Exogenous arachidonic acid was converted mainly to 12L-HETE, HHT and thromboxane B2 by lymphocyte suspensions. This conversion could be accounted for by contamination with blood platelets. The results show that the degree of cyclic AMP accumulation in human lymphocytes following stimulation of arachidonic acid metabolism will be critically dependent upon which prostaglandins are in fact formed by cells surrounding the lymphocytes.     

Interactions of thromboxane A2 analogs and prostaglandins in isolated dog arteries

Interactions of ONO3708, a thromboxane (TX) A2 analog, and epithio-methano-TXA2 (sTXA2) or prostaglandins (PGs) were investigated in helical strips of dog cerebral, coronary, renal, and mesenteric arteries. In these arterial strips sTXA2 (10(-10) to 10(-7) M) produced a dose-dependent contraction, whereas ONO3708 up to 10(-6) M failed to contract the arteries but antagonized the contractile response to sTXA2. The inhibition tended to be greater in renal arteries than in the other arteries. Contractions induced by PGF2 alpha, PGE2, and PGD2 were also suppressed by treatment with low concentrations (3 X 10(-9) and 10(-8) M) of ONO3708. The attenuations of the response to sTXA2, PGF2 alpha, PGE2, and PGD2 did not appreciably differ. Norepinephrine-induced contractions were not influenced by ONO3708 up to 2 X 10(-7) M. On the other hand, relaxant responses to PGI2 of cerebral and renal arteries were not reduced by ONO3708. Prostaglandin H2 produced a transient contraction followed by a relaxation in cerebral and renal arteries. The contractile response was abolished by 10(-7) M ONO3708, and the relaxation was potentiated. It may be concluded that ONO3708 selectively antagonizes the vasoconstrictor action of TXA2, its analogs, and PGs but does not alter the action of vasodilator PGs. At least in part, sTXA2, PGF2 alpha, PGE2, and PGD2 appear to share the same receptive site responsible for vascular contraction.     

Enrichment and characterization of specific [3H]PGE2 binding sites in the porcine gastric mucosa

[3H]Prostaglandin E2 (PGE2) binding sites were 10-fold enriched from a porcine fundic mucosal homogenate by differential centrifugation and subsequent discontinuous sucrose gradient separation. PGE2 bound with an activation energy of 66 kJ/mol to a single class of sites with an affinity of 1.5 +/- 0.4 nM and a capacity of 274 +/- 76 fmol/mg protein. There was no indication for any cooperativity between the binding sites. Kinetic analysis revealed a kon of 3 x 10(5) M x s-1 at 30 degrees C and pH 5.5. Dissociation was biphasic with an initial rapid (koff = 10(-3) s-1) and a subsequent slower phase (koff = 4 X 10(-5) s-1), presumably reflecting the existence of two interchangeable forms of [3H]PGE2 binding sites. The rank order of affinity for other prostanoids (PGE2 greater than PGE1 greater than PGA2 greater than iloprost (PGI2 derivative) greater than PGF2 alpha greater than PGB2 greater than PGD2) is discussed against the background of the recently postulated E-type receptors in the stomach.     

Central nervous system effects of arachidonic acid, PGE2, PGF2 alpha, PGD2 and PGI2 on gastric secretion in the rat.

The effects of arachidonic acid, prostaglandin E2 (PGE2), PGF2 alpha, PGD2 and PGI2 on gastric secretion (acid, pepsin and volume) after intracerebroventricular administration were investigated in conscious, pylorus-ligated rats. Arachidonic acid 30-1000 micrograms had no effect on gastric secretion. PGE2 3 and 10 micrograms, reduced gastric secretion as measured 1 hour after injection, although the inhibition induced by 3 micrograms disappeared by 2 h. PGF2 alpha 10 and 30 micrograms, inhibited gastric secretion as measured after 1 h, whereas no change was observed in the gastric contents collected 2 h after 10 micrograms of PGF2 alpha. Intramuscular injection of 30 micrograms of PGF2 alpha had no effect on gastric secretion. Intracerebroventricular administration of 1-30 micrograms of PGD2 or PGI2 had no effect on gastric secretion. The results indicate that PGE2 and PGF2 alpha have a potent central antisecretory action in conscious, pylorus-ligated rats, whereas arachidonic acid, PGD2 and PGI2 do not have any central effects on gastric secretion. It is suggested that PGE2 and PGF2 alpha may be involved in the central nervous system control of gastric secretion.     

Response of peripheral collateral arteries in the dog to prostaglandins E1, E2, F2 alpha, and I2 (prostacyclin)

Prostaglandins (PG) E2, E2, E2 alpha, and I2 [prostacyclin (PGI2)] were tested in vitro on collateral arteries that enlarge following chronic occlusion of the femoral artery in the dog. After contraction with an ED50 dose of KCl, serial doses of a PG were added. Collateral arteries relaxed significantly to PGI2 (10(-7)--10(-5) M). Normal, similarly sized branch arteries did not relax. The contractile response of collateral arteries to PGE1, PGE2, and PGF2 alpha was qualitatively similar to that of branch arteries, but the magnitude of the responses for collateral arteries was PGF2 alpha = PGE2 greater than PGE1. The magnitude of the response for branch arteries was PGF2 alpha much greater than PGE2 greater than PGE1. The effects of the PGs on anterior tibial arteries from the contralateral limb and on anterior tibial arteries exposed to low pressure below the occlusion did not differ. Thus the lowered pressure in the limb with the occluded femoral artery was not responsible for differences in the effects of the PGs on collateral arteries. Collateral arteries are more sensitive to the relaxant effects of PGI2 in high doses and are less sensitive to the contractile effects of other PGs, particularly PGF2 alpha, than similarly sized arteries. This suggests that therapeutic doses of PGI2 may increase blood flow to ischemic areas.     

The effect of a synthetic 7-thiaprostaglandin E1 derivative, TEI-6122, on monocyte chemoattractant protein-1 induced chemotaxis in THP-1 cells.

1 The ability of various prostaglandins (PGs) to inhibit monocyte chemotaxis induced by monocyte chemoattractant protein-1 (MCP-1) was investigated with a human monocytic leukaemia cell line, THP-1. Moreover, to investigate the mechanism of the inhibitory action of PGs the involvement of either intracellular adenosine 3': 5'-cyclic monosphosphate (cyclic AMP) accumulation or intracellular Ca2+ mobilization was studied. 2 TEI-6122, a synthetic 7-thia-PGE1 derivative, inhibited chemotaxis of THP-1 cells induced by MCP-1 with an IC50 of 1.5 pM. Its inhibitory activity was 1000 fold more than that of PGE1 and PGE2 (IC50 = 2.8 nM and 0.9 nM, respectively), which were more potent than other PGs such as PGA1, PGA2, PGF2 alpha and PGI2 (IC50 > or = 1 microM). 3 With respect to the effect on intracellular cyclic AMP accumulation in THP-1 cells, TEI-6122 was as potent as PGE1 and PGE2, which were approximately 100 to 1000 fold more potent than the other PGs such as PGA1, PGA2 and PGI2. The minimum concentration of TEI-6122 required to increase intracellular cyclic AMP accumulation in THP-1 cells was 1 nM. 4 TEI-6122 and PGE1 (4 microM) transiently increased intracellular calcium levels in THP-1 cells. When added prior to MCP-1, both PGs partially suppressed the increased in Ca2+ caused by this cytokine. There were no significant differences between the activity of TEI-6122 and PGE1 in either respect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism underlying relaxations caused by prostaglandins and thromboxane A2 analog in isolated dog arteries

In helical strips of dog cerebral, coronary, mesenteric, and renal arteries treated with ONO3708, an inhibitor of vasoconstricting prostaglandin (PG) receptors, and previously contracted with serotonin, PGF2 alpha, PGD2 and epithio-methano thromboxane A2 (sTxA2), a TxA2 analog, caused a relaxation. The cerebral arterial relaxation was suppressed by treatment with indomethacin and abolished by diphloretin phosphate (DPP), a PG antagonist. On the other hand, the relaxation of mesenteric arteries was not influenced by indomethacin but was markedly attenuated by DPP. Removal of endothelium did not alter the relaxation. Relaxations of coronary and renal arteries by PGF2 alpha were suppressed by indomethacin and DPP, whereas the PGD2-induced relaxation was not affected by indomethacin but was abolished by DPP. Concentration--relaxation curves for PGI2 were shifted to the right by treatment with DPP. It is concluded that after ablation of the constrictor response, dog cerebral arteries relax in response to PGs and TxA2, probably due mainly to the release of PGI2-like substance from the arterial wall and to the action of PGI2 receptive sites, whereas the mesenteric arterial relaxation appears to be associated with their action on PGI2 receptors in smooth muscle cells. PGF2 alpha-induced relaxations in coronary and renal arteries may result from the release of PGI2, and relaxations by PGD2 from the action on PGI2 receptors.     

Regional differences in the responses to prostanoids of circular muscle from guinea-pig isolated intestine

The effects of prostaglandins (PGs) D2, E2, F2 alpha, an epoxymethano analogue of PGH2 (U-46619), prostacyclin (PGI2), 6-keto-PGF1 alpha and thromboxane (TX) B2 were tested on spirally-cut strips of guinea-pig isolated ileum or colon. In the ileum no prostanoid exerted a marked effect on the resting tissue, but PGD2, PGE2 or PGI2 1 ug ml-1 inhibited submaximal contraction to KC1. U-46619 1 ug ml-1 either inhibited or increased contractions in KC1, but PGF2 alpha, 6-keto-PGF1 alpha or TXB2 1 ug ml-1 had no significant effect. PGE2 relaxed colonic strips whereas the other prostanoids caused contraction, except for TXB2 which had no effect. The PG antagonist SC-19220 blocked colonic contractions to the prostanoids, and the residual inhibitory effect of PGD2, U-46619 or PGI2 was demonstrated by the reduction of submaximal contractions to acetylcholine. Our results suggest that prostanoid receptors mediating inhibitory responses of circular muscle predominate in the ileum, whereas in the colon both excitatory and inhibitory prostanoid receptors occur.     

Prostaglandin E2, prostaglandin I2 and the vascular changes of inflammation.

1. Plasma exudation and blood flow changes induced by intradermal injection of prostaglandins E2 (PGE2), I2 (PG12), D2 (PGD2) and F2 alpha (PGF2 alpha) were measured in rabbit dorsal skin by the use of [131I]-albumin and 133Xe. 2. Little plasma exudation was produced by any of the prostaglandins when injected alone. 3. Both PGE2 and PGI2 were potent at increasing blood flow, whereas PGF2 alpha and PGD2 produced an increase only at high doses. 4. All of the prostaglandins studied potentiated the plasma exudation induced by bradykinin. PGE2 and PGI2 had similar potent potentiating activity, whereas PGD2 and PGF2 alpha had activity at doses too high to be of biological significance. 5. Intradermal injections of arachidonate alone resulted in little plasma exudation but produced an increase in blood flow. Arachidonate potentiated bradykinin-induced plasma exudation. 6. Locally-injected indomethacin had no effect on basal blood flow and little effect on the exudation produced by bradykinin, but indomethacin did inhibit the vasodilatation and exudation potentiation produced by arachidonate. 7. PGE2 and PGI2 had similar potency in producing marked potentiation of plasma exudation induced by intradermal injection of zymosan. 8. In the reaction to zymosan, it is concluded that vasodilatation is the result of the release of arachidonate, which is subsequently coverted to either PGE2 or PGI2. These substances regulate the plasma exudation induced by independently-released vascular permeability-increasing mediators.     

Effects of prostaglandins E1, E2, F2 alpha and I2 on adrenergic transmission in guinea pig pulmonary arteries

Effects of some prostaglandins (PGs) on adrenergic transmission were studied in guinea pig pulmonary arteries preloaded with 3H-norepinephrine. PGE1 and PGE2 at 0.1 to 100 nM concentration-dependently inhibited 3H-release and contraction evoked at 5 Hz. This inhibition was antagonized by diphloretin phosphate. PGF2 alpha at 1 to 100 nM had no effect on evoked 3H-release and contraction. PGI2 at 1 to 100 nM also failed to modify evoked 3H-release, but markedly and concentration-dependently decreased evoked contraction. There exist presynaptic inhibitory PGE1 and PGE2 receptive sites on adrenergic neurons innervating guinea pig pulmonary arteries, whereas PGF2 alpha and PGI2 produced no effect on the adrenergic neurons.     

Prostaglandin D2-induced catalepsy in rats: role of 5-hydroxytryptamine

Prostaglandin D2 (PGD2), the major PG in the rat brain, induced a dose-related catalepsy in rats on intracerebroventricular (i.c.v.) administration. This cataleptic response was significantly attenuated following the i.c.v. administration of pharmacological agents that decrease rat brain 5-hydroxytryptamine (5-HT) activity. PGE1 synergized but PGF2 alpha antagonized the catalepsy induced by PGD2. PGD2 and PGE1 have previously been shown to augment rat brain 5-HT activity, whereas PGF alpha inhibited it. It is therefore likely that the observed effects of these PGs on catalepsy involve a central 5-HT mechanism.     

Evidence that prostaglandins activate calcium channels to enhance basal and stimulation-evoked catecholamine release from bovine adrenal chromaffin cells in culture

The effects of prostaglandins (PGs) on catecholamine (CA) secretion and Ca2+ fluxes were studied in a primary culture of bovine chromaffin cells. PGD2, PGF2 alpha and PGE2 induced CA release from cultured bovine chromaffin cells in a concentration dependent manner (0.03-3 microM). PGD2, PGF2 alpha and PGE2 at 3 microM elicited maximum CA release of 0.043 +/- 0.001, 0.059 +/- 0.008, 0.062 +/- 0.002 micrograms/10(6) cells, respectively. Three micromolar of PGD2, PGF2 alpha and PGE2 enhanced CA release induced by acetylcholine (ACh) in a degree of 186 +/- 10, 206 +/- 6, 150 +/- 4% of control respectively. PGs also enhanced CA release induced by 20 mM K+, veratridine and A23187. In Ca2+-free medium, PGs failed to affect basal and caffeine (50 mM)-induced CA release. PGF2 alpha increased 45Ca uptake and showed additive effect with ACh on 45Ca uptake. Nicardipine (0.1-10 microM) suppressed CA release and 45Ca uptake induced by PGF2 alpha, while diltiazem and verapamil failed to affect these responses to PGF2 alpha. BAY K 8644 (1 microM) potentiated CA release and 45Ca uptake evoked by PGF2 alpha. These results suggest that PGs enhance basal and stimulation-evoked CA release from chromaffin cells possibly through facilitation of Ca2+ influx. The mechanisms of action of PGs in adrenal medulla are discussed.     

Pinane thromboxane A2 analogues are non-selective prostanoid antagonists in rat and human stomach muscle.

1 Pinane thromboxane A2 (PTxA2) and its epi-OH isomer were studied on rat and human stomach longitudinal muscle. 2 PTxA2 (0.5 micrograms/ml) usually caused a slight contraction of rat gastric fundus. Contractions to PGE2, PGF2 alpha, PGI2 and epoxymethano analogues of PGH2 (U-46619 and U-44069) were substantially inhibited, whereas those to PGD2 and acetylcholine were only slightly reduced. 3 In human stomach, PTxA2 0.5 micrograms/ml rarely stimulated the muscle. Contractions to PGE2, PGF2 alpha and U-46619 were antagonized, with little effect on those to acetylcholine. 4 epi-PTxA2 (0.5 micrograms/ml) did not affect rat gastric tone. It was moderately potent against PGI2 on rat gastric fundus, but was less effective than PTxA2 against U-44069.     

Separation of prostaglandin A2 and prostaglandin B2 by ion-exchange liquid chromatography

2 closely related prostaglandins (PGs), A2 and B2, were separated by ion-exchange liquid chromatography. PGA2 and PGB2 are an isomeric pair of PGs which show little resolution on thin-layer chromatography; the pair can be resolved as gas-liquid chromatography, but use of this method requires protection of the C9-carbonyl group. Ion-exchange liquid chromatography, in contrast, requires no protective derivatization to achieve separation of PGA2 and PGB2 from PGE2. 2 different column types were used, and their results compared favorably. In 1 experiment, a triethylaminoethyl cellulose column resolved the 2 species; in another experiment, a strong anion-exchange pellicular support was successful. Both columns were stable and could be used to monitor the stability of PGE2 by following the appearance of PGA2 and PGB2, using the peak height method (i.e., a plot of micrograms injected vs. peak height). Triethylaminoethyl cellulose gave somewhat better resolution of PGA2 and PGB2 as compared with the pellicular strong anion-exchange column, but the advantage of this effect is offset by the tediousness of packing columns of triethylaminoethyl cellulose.     

Pathophysiological roles of the prostanoids in the cardiovascular system: studies using mice deficient in prostanoid receptors

Prostanoids, consisting of the prostaglandins (PGs) and thromboxanes (TXs), exert various actions through activation of their specific receptors. They include the DP, EP, FP, IP, and TP receptors for PGD2, PGE2, PGF2alpha, PGI2, and TXA2, respectively. Moreover, EP receptors are classified into four subtypes, the EP1, EP2, EP3 and EP4 receptors. Using mice lacking prostanoid receptors, we intended to clarify in vivo roles of prostanoids under pathophysiological conditions of the cardiovascular system, which include ischemia-induced cardiac injury, pressure overload-induced cardiac hypertrophy, renovascular hypertension, tachycardia during systemic inflammation and thromboembolism. The results demonstrated that 1) PGI2 plays an important role in attenuating the ischemic injury and the pressure overload-induced hypertrophy of the hearts, and also contributes to the development of renovascular hypertension; 2) PGE2 plays a cardioprotective role against the ischemic injury via both the EP3 and EP4, and also participates in acute thromboembolism via the EP3; and 3) both PGF2alpha and TXA2, which have been produced during systemic inflammation, are responsible for tachycardia.     

Profile of prostaglandins generated in the detrusor muscle of rat urinary bladder: effects of adenosine triphosphate and adenosine

The generation and release of PGE2, PGF2 alpha, PGD2, TXB2 and 6-keto-PGF1 alpha in the rat detrusor muscle were studied by means of radioimmunoassays. The effect of ATP (0.1 mmol/1) and adenosine (0.1 mmol/1) on the content and profile of PGs in the incubation medium was investigated. It was found that PGE2 and 6-keto-PGF1 alpha accounted for more than 80% of the total PG activity. ATP increased the amounts of PGs in the incubation medium (percentage change of the control values, N = 6: PGE2 54.53 +/- 12.69, PGF2 alpha 31.01 +/- 8.82, PGD2 44.52 +/- 12.36, TXB2 17.29 +/- 10.45, 6-keto-PGF1 alpha 36.62 +/- 5.0) but did not change their profile. Adenosine had no effect on either content or profile of the PGs. The results suggest that ATP but ot adenosine may activate PG biosynthesis via P2-purinoceptor-mediated mechanisms.     

Inhibition of adenosine uptake into rat brain synaptosomes by prostaglandins, benzodiazepines and other centrally active compounds

A number of compounds have been tested for their abilities to inhibit the rapid uptake of adenosine by rat cerebral cortical synaptosomes. Prostaglandins PGI2, PGA2, and PGE1 and PGE2 were potent inhibitors of adenosine uptake with IC20 values in the 10(-7) M-10(-6) M range. PGA1, PGD2 and PGF2 alpha also inhibited uptake but were less active. The benzodiazepine antagonist Ro 15-1788 inhibited adenosine uptake and failed to antagonize the effects of diazepam. Another antagonist, ethyl-beta-carboline-3-carboxylate, was a weak inhibitor of adenosine uptake. Ro 5-4864, the so-called peripheral benzodiazepine ligand, inhibited adenosine uptake. Hydroxyzine and tracazolate, two anxiolytic agents, inhibited uptake as did flunarizine, a coronary vasodilator. Two calmodulin antagonists, W7 and R 24571, were effective inhibitors of adenosine uptake. Their IC50 values were comparable to those at which they have been demonstrated to inhibit calmodulin-mediated reactions in other systems. These observations suggest that adenosine uptake may be a calmodulin-regulated process.     

Acetylcholine-induced endothelium-dependent contractions in the SHR aorta: the Janus face of prostacyclin

In the spontaneously hypertensive rat (SHR) and aging Wistar-Kyoto rats (WKY), acetylcholine releases an endothelium-derived contracting factor (EDCF) produced by endothelial cyclooxygenase-1, which stimulates thromboxane A2 receptors (TP receptors) on vascular smooth muscle. The purpose of the present study was to identify this EDCF by measuring changes in isometric tension and the release of various prostaglandins by acetylcholine. In isolated aortic rings of SHR, U 46619, prostaglandin (PG) H2, PGF2alpha, PGE2, PGD2, prostacyclin (PGI2) and 8-isoprostane, all activate TP receptors of the vascular smooth muscle to produce a contraction (U 46619>8-isoprostane=PGF2alpha=PGH2>PGE2=PGD2>PGI2). The contractions produced by PGH2 and PGI2 were fast and transient, mimicking endothelium-dependent contractions. PGI2 did not relax isolated aortic rings of WKY and SHR. Acetylcholine evoked the endothelium-dependent release of thromboxane A2, PGF2alpha, PGE2, PGI2 and most likely PGH2 (PGI2>PGF2alpha>or=PGE2>TXA2>8-isoprostane, PGD2). Dazoxiben abolished the production of thromboxane A2, but did not influence the endothelium-dependent contractions to acetylcholine. The release of PGI2 was significantly larger in the aorta of SHR than in WKY, and the former was more sensitive to the contractile effect of PGI2 than the latter. The inhibition of PGI-synthase was associated with an increase in PGH2 spillover and the enhancement of acetylcholine-induced endothelium-dependent contractions. Thus, in the aorta of SHR and aging WKY, the endothelium-dependent contractions elicited by acetylcholine most likely involve the release of PGI2 with a concomitant contribution of PGH2.