Methyl xanthine phosphodiesterase inhibitors behave as prostaglandin antagonists in a perfused rat mesenteric artery preparation.

The methyl xanthines, theophylline, caffeine and 3-isobutyl-1 methyl xanthine (MIX) inhibited the pressure responses to noradrnealine, angiotensin II and potassium ions in the isolated perfused mesenteric vascular bed of the male rat. The ID50s for inhibition of responses to noradrenaline were 1.85 mug/ml (0.83 x 10(-5) M) for MIX, 18 mug/ml (1 x 10(-4)M) for theophylline and 133 mug/ml (6.8 x 10(-4) M) for caffeine. Similar ID50 concentrations were found for responses to angiotensin II and potassium. We have previously found that substances which inhibit the three pressor agents equally may be prostaglandin (PG) synthesis inhibitors or PG antagonists. Xanthine itself, cyclic AMP and dibutyrl cyclic AMP had no inhibitory effects on the preparation up to concentrations of 10-2 M. Partial inhibition of PG synthesis by indomethacin shifted the % inhibition/log concentration curve to the left, while addition of exogenous PGE2 shifted it to the right. In preparations completely inhibited by sufficient indomethacin added to the perfusate to block PG synthesis, and then restored by adding 1 or 5 ng/ml PGE2 in addition to the indomethacin, the methyl xanthines again inhibited responses suggesting that they were PG antagonists rather than inhibitors of synthesis or release. In preliminary experiments MIX also inhibited effects of PGF2alpha on rat uterus and PGE1 on guinea pig ileum. Effective concentrations of theophylline were similar to the therapeutic levels in human plasma. PG antagonists may be a major action of methyl xanthines requiring reinterpretation of many experiments which have attributed their effects to PDE inhibition. PGs may also be involved in regulating PDE action.     

Methyl xanthine phosphodiesterase inhibitors behave as prostaglandin antagonists in a perfused rat mesenteric artery preparation

The methyl xanthines, theophylline, caffeine and 3-isobutyl-1 methyl xanthine (MIX) inhibited the pressure responses to noradrenaline, angiotensin II and potassium ions in the isolated perfused mesenteric vascular bed of the male rat. The ID50s for inhibition of responses to noradrenaline were 1.85 μg/ml (0.83 × 10⁻⁵M) for MIX, 18 μg/ml (1 × 10⁻⁴M) for theophylline and 133 μg/ml (6.8 × 10⁻⁴M) for caffeine. Similar ID50 concentrations were found for responses to angiotensin II and potassium. We have previously found that substances which inhibit the three pressor agents equally may be prostaglandin (PG) synthesis inhibitors or PG antagonists. Xanthine itself, cyclic AMP and dibutyryl cyclic AMP had no inhibitory effects on the preparation up to concentrations of 10⁻²M. Partial inhibition of PG synthesis by indomethacin shifted the % inhibition/log concentration curve to the left, while addition of exogeneous PGE2 shifted it to the right. In preparations completely inhibited by sufficient indomethacin added to the perfusate to block PG synthesis, and then restored by adding 1 or 5 ng/ml PGE2 in addition to the indomethacin, the methyl xanthines again inhibited responses suggesting that they were PG antagonists rather than inhibitors of synthesis or release. In preliminary experiments MIX also inhibited effects of PGF2α on rat uterus and PGE1 on guinea pig ileum. Effective concentrations of theophylline were similar to the therapeutic levels in human plasma. PG antagonism may be a major action of methyl xanthines requiring reinterpretation of many experiments which have attributed their effects to PDE inhibition. PGs may also be involved in regulating PDE action.     

Indomethacin inhibits responses to all vasoconstrictors in the rat mesenteric vascular bed: Restoration of responses by prostaglandin E2

Indomethacin added to the perfusing buffer inhibited pressor responses to noradrenaline, angiotensin II, arginine vasopressin, histamine, serotonin, calcium ions and potassium ions in the male rat mesenteric vascular bed. For every pressor agent the indomethacin concentration which inhibited response amplitude by 50% was about 7 microg/ml (2.1 × 10^?5 M). With every pressor agent, prostaglandin (PG) E2 could restore normal responsiveness in indomethacin-blocked preparations even while the indomethacin was still present in the buffer. The concentration of PGE2 required was proportional to the concentration of indomethacin. Preparations completely inhibited by indomethacin needed about 5ng/ml PGE2 for complete restoration of normal responses. Aspirin and mefenamic acid could also inhibit responses to all pressor agents tested but with these drugs only a partial restoration could be achieved by PGE2.     

Rat mammary preadipocytes in culture produce a trophic agent for mammary epithelia-prostaglandin E2

Cell strains and cell lines rat mammary (Rama) 350-353 have been isolated from the slowly adherent stromal fraction of enzymatically digested rat mammary glands. Primary cultures of this fraction yield fat cels on extended culture. Their proportion can be increased with horse serum or growth hormone in the medium, and this increase is associated with a 100-fold or more increase in the release of radioimmunoassayable prostaglandins of the E type (PGE). The stromal cell strains and lines that are capable of yielding fat cells also secrete elevated levels (greater than 100 ng/mg/24 hr) of PGE; the fast-sticking epithelial fraction in primary cultures and the epithelial cell lines derived from it secrete 10-100 times less. Chromatography and radioisotopic labeling of the culture media from Rama 352 cells identify the PG as PGE2. PGE2 with insulin and hydrocortisone maximally stimulates [3H]DNA synthesis of epithelial cell lines and primary cultures from normal and tumorous glands by 2-4-fold at concentrations (10-20 ng/ml) well below those released by the preadipocytic stromal cells (20-100 ng/ml). Medium exposed to most cultured cells stimulates [3H]DNA synthesis of one epithelial cell line, Rama 25, by 2-4-fold. Prevention of the synthesis of PGE2 in Rama 352 cultures with indomethacin or flurbiprofen abolishes the mitogenic activity present in the culture medium, and the PG receptor antagonist polyphloretin phosphate inhibits completely the mitogenic activity for Rama 25 cells. Myoepithelial-like cell lines normally secrete moderate levels of PGE (10-100 ng/mg/24 hr) but the mitogenic activity for Rama 25 cells released from one such line, Rama 29, is not abolished by preventing the synthesis of PG's nor by PG-receptor antagonists.     

Effects of prostaglandin E2 and lipopolysaccharide on osteoclastogenesis in RAW 264.7 cells

INTRODUCTION: Prostaglandins (PGs) can act on both hematopoietic and osteoblastic lineages to enhance osteoclast formation. METHODS: We examined PGE2 stimulated osteoclastogenesis in RAW 264.7 cells and the role of endogenous PGE2 in lipopolysaccharide (LPS) stimulated osteoclastogenesis. RESULTS: RANKL (1-100 ng/ml) increased formation of osteoclasts, defined as tartrate resistant acid phosphatase multinucleated cells, with peak effects at 30 ng/ml. Addition of PGE2 (0.01-1.0 microM) to RANKL (30 ng/ml) dose dependently increased osteoclast number 30-150%. Use of NS-398 (0.1 microM) or indomethacin (Indo, 1.0 micro M) to block endogenous PG synthesis had little effect on the response to RANKL alone but significantly decreased the response to PGE2. Addition of LPS (100 ng/ml) to RANKL increased osteoclast number 50%, and this response was significantly decreased by NS-398 and Indo. RANKL and PGE2 produced small, additive increases in COX-2 mRNA levels, while LPS produced a larger increase. PG release into the medium was not increased by RANKL and PGE2 but markedly increased by LPS. CONCLUSION: We conclude that RANKL stimulated osteoclastogenesis can be enhanced by PGE2 and LPS though direct effects on the hematopoietic cell lineage and that these effects may be mediated in part by induction of COX-2 and enhanced intracellular PG production.     

Copper inhibits pressor responses to noradrenaline but not potassium. Interactions with prostaglandins E1, E2, and I2 and penicillamine.

Low concentrations of copper inhibited responses to norepinephrine and angiotensin (IC50 3 X 10(-6) M) but not to potassium in rat mesenteric vascular preparations perfused either with buffer or indomethacin and prostaglandin (PGE2). The dose-response curve was not shifted by indomethacin, imidazole, or PGE2 but was moved to the right by 2.8 X 10(-11) M PGE1 and to the left by 2.8 X 10(-7) M PGE1. These effects of copper are similar to the effects of PGI2 in the preparation. Copper moved the PGI2 dose-response curve against noradrenaline in parallel to the left, suggesting that the two were interacting at some point. Penicillamine, which may stimulate PGE1 synthesis, had PGE1-like interactions with the copper effect, suggesting that its value in Wilson's disease may be partly due to antagonism of the biological action of copper as well as to its copper-chelating properties.     

Corticosteroids inhibit prostaglandin release from perfused mesenteric blood vessels of rabbit and from perfused lungs of sensitized guinea pig.

Infusion of norephinephrine (NE) (1 - 3 mug/ml/min) into the isolated mesenteric vascular preparation of rabbit resulted in a rise in perfusion pressure, which was associated with the release of prostaglandin E-like substance (PGE) at a concentration of 2.81 +/- 0.65 ng/ml in terms of PGE2. Indomethacin (3 mug/ml) abolished the NE-induced release of PGE. Arachidonic acid (0.2 mug/ml) in the presence of indomethacin did not restore the NE-induced release of PGE. Hydrocortisone (10 - 30 mug/ml) and dexamethasone (2 - 5 mug/ml) also inhibited the NE-induced release of PGE. The inhibitory action of both corticosteroids was abolished by arachidonic acid (0.2 mug/ml). Antigen-induced release of a prostaglandin-like substance (PGs) (43.1 +/- 3.8 ng/ml in terms of PGE2 and a rabbit aorta contracting substance (RCS) from perfused lungs of sensitized guinea pigs was completely abolished by indomethacin (5 mug/ml) or by hydrocortisone (100 mug/ml). Indomethacin, however, increased histamine release up to 280% of the control level, which was 470 +/- 54 ng/ml, while hydrocortisone diminished histamine release down to 30% of the control level. A superimposed infusion of arachidonic acid (1 mug/ml) into the pulmonary artery reversed the hydrocortisone-induced blockade of the release of RCS and PGs. It may be concluded that corticosteroids neither inhibit prostaglandin synthetase nor influence prostaglandin transport through the membranes but they do impair the availability of the substrate for the enzyme.     

Bradykinin and electrical stimulation increase prostaglandin production in the rat vas deferens

The epididymal portion of the rat vas deferens produced prostaglandins (PG) E(2), F(2alpha)and 6-keto F(1alpha). Electrical stimulation (ES, 0.1 Hz, 1 ms) increased such production by 100%, and similar results were obtained in the presence of 1.0 microM bradykinin (Bk). When both stimuli were applied simultaneously, the increases in PG production were 1100% for PGE(2), 800% for PGF(2alpha)and 400% for PG6-keto F(1alpha). Prazosin abolished the effect of ES on PG production. A selective Bk B(2)-receptor antagonist abolished the increase in PG production induced by Bk, both in non-stimulated and in ES tissues. Bk (1.0 microM) elicited contractile responses in non-stimulated as well as in ES tissues, responses that were not modified in the presence of 10 microM indomethacin. In conclusion, the effects of Bk on prostaglandin production appears to depend on the activation of B(2) receptors, while the increase in prostaglandin release induced by ES, and the effects observed with both stimuli simultaneously, should be mediated by the release of noradrenaline and the subsequent activation of alpha(1) adrenoceptors.     

Prostaglandin F and 13,14-dihydro-15-keto prostaglandin F in the endometrium and uterine flushings of sheep before implantation

From 22 women undergoing hysterectomy at various stages of the menstrual cycle, strip preparations were dissected from the outer, longitudinal and the inner, circular smooth muscle layers of the ampullary-isthmic junction (AIJ). The strips were mounted in organ baths, and isometric tension was recorded. Spontaneous contractions were recorded mainly in circular muscle strips. Contractions were elicited by 127 mM-K+, 10(-6) M-noradrenaline and 10(-6) M-PGF-2 alpha. Potassium induced biphasic responses that were slightly different in the two tissues. In circular muscle strips, noradrenaline and PGF-2 alpha induced phasic contractions superimposed on a rise in tone. In longitudinal muscle specimens, the two compounds produced tonic responses. All types of mechanical activity were inhibited by removal of extracellular calcium. K+-induced responses and phasic contractions produced by noradrenaline and PGF-2 alpha could be abolished by 10(-6) M-nifedipine whereas the tonic contractions in the circular and longitudinal muscle were more resistant to the calcium antagonist. The results suggest that K+-induced responses in circular and longitudinal muscle of the human AIJ, and the phasic contractions in circular muscle, depend on calcium influx via potential-sensitive membrane channels. Receptor-operated calcium channels seem to be involved in the tonic contractions observed mainly in the longitudinal smooth muscle.     

Effects of gonadotropin-releasing hormone agonists on meiotic maturation of follicle-enclosed oocytes in rabbits

The effects of GnRH agonists on in vitro maturation of rabbit follicle-enclosed oocytes were studied. Rabbit preovulatory follicles were cultured with or without hCG (10(2) ng/ml), buserelin (10(2)-10(5) ng/ml), or leuprolide (10(2)-10(5) ng/ml) for 14 hours in vitro. GnRH agonists induced the resumption of meiosis in the follicle-enclosed oocytes in a dose-dependent manner. The percentage of oocytes achieving GVBD following treatment with 10(5) ng/ml buserelin (87.9 +/- 6.3%) or 10(5) ng/ml leuprolide (86.0 +/- 4.1%) did not differ significantly from hCG-treated control (87.3 +/- 3.8%). Mature oocytes initially were detected within 2 hours of GnRH agonist exposure. Concomitant addition of a GnRH antagonist at 10(4) ng/ml significantly blocked the stimulatory effect of GnRH agonist on oocyte maturation. GnRH agonists significantly stimulated both prostaglandin (PG) E2 (PGE2) and PGF2 alpha production by preovulatory follicles (p less than 0.01), but secreted prostanoid levels did not differ significantly among different concentrations of GnRH agonists. Meiotic maturation of follicle-enclosed oocytes following GnRH agonist exposure began 2 hours earlier than production of PGs. PG production stimulated by GnRH agonists was reduced significantly by indomethacin. However, oocyte maturity in the presence of GnRH agonist plus indomethacin did not differ significantly from that of GnRH agonist alone. GnRH agonistic analogues induce the resumption of meiosis in follicle-enclosed oocytes in rabbits by a mechanism other than PG stimulation.     

Cholecystokinin (CCK) down regulates PGE2 and PGI2 release in inflamed Guinea pig gallbladder smooth muscle cell cultures

This study examines the hypothesis that cholecystitis down-regulates Guinea pig gallbladder (GPGB) smooth muscle cholecystokinin (CCK)-stimulated prostaglandin (PG) release. Guinea pig gallbladder from Control and 48 h bile duct ligated (BDL) animals were placed in cell culture and grown to confluence. The cultures underwent Western Blot analysis for smooth muscle cell content of COX-1, COX-2, Prostacyclin Synthase (PS), or were incubated with CCK at 10(-8)M or 10(-6)M with and without indomethacin for 1h and analyzed for release of 6-keto-PGF1alpha, PGE2 and TxB2 by EIA. BDL increased Guinea pig gallbladder cell culture basal PGE2 and PGI2 release which was in part due to increased COX-2 content. CCK incubation down-regulated BDL Guinea pig gallbladder cell culture release of 6-keto-PGF1alpha and PGE2 and down-regulated COX-2 content but did not alter the Control group. The decrease in CCK-mediated BDL cell Guinea pig gallbladder release may be an endogenous mechanism to limit physiologic derangements induced by increased endogenous gallbladder PG synthesis during early acute cholecystitis.     

Prostaglandins--modulation of adrenergic nervous system.

Prostaglandins (PGs) affect vascular tone by a direct action on the vascular smooth muscle and by influencing vascular reactivity to adrenergic simuli and several vasoactive substances. Thus, in the isolated Tyrode's perfused rabbit renal, mesenteric and splenic vasculature PGE2 inhibited adrenergically induced vasoconstriction. Since the vasoconstrictor responses to renal nerve stimulation were enhanced by the blockade of PG synthesis and were reduced by stimulation of PG synthesis with arachidonic acid, this suggests that PGE2 functions as an inhibitory modulator of the adrenergic nervous system. However, our demonstration that PGE2 enhanced adrenergically induced vasoconstriction in the renal and mesenteric vasculature of the rat, but had opposite effects in the rat splenic vasculature indicates that the modulatory-effect of PGE-compounds on the adrenergic neuromuscular junction is species dependent and varies in different vascular beds within the same species. Prostaglandins, the release of which is evoked by several vasoactive substances including angiotensins, kinins, and adenine nucleotides, may also contribute to the regulation of vascular tone by either opposing or amplifying the vascular actions of vasoactive substances.     

Potentiation of bradykinin-induced vascular permeability increase by prostaglandin E2 and arachidonic acid in rabbit skin.

The activity of prostaglandins (PG) in producing vascular permeability was quantitated by dye extraction method in skin of anaesthetized rabbits. PGE1 and PGE2 (0.01-10 mug) produced increase in vascular permeability. Activity was approximately equal to that of histamine (Hist) and 1/20 of that of bradykinin (BK) on a weight basis. The activity of PGF1alpha and PGF2alpha was only 1/20 of that of PGE1 or PGE2. In spite of the relatively low potency of PGE1 and PGE2 in the rabbit, near threshold doses (0.1 or 1 mug) of PGE2 could potentiate permeability responses to bradykinin (0.1 mug) by 10 or 100-fold, respectively. Equivalent doses (0.1 or 1 mug) of histamine could not potentiate the bradykinin responses. Arachidonic acid (AA) at 1 mug, produced a 10-fold potentiation in the permeability response to bradykinin (0.1 mug). Pretreatment of the rabbits with indomethacin (20 mg/kg, i.p.) reduced the responses of BK (0.1 mug) + AA (1 mug) down to a similar magnitude of those seen with bradykinin alone. However, indomethacin did not block responses to either, BK alone, BK + PGE2, or BK + Hist. Various doses (1, 10, 100 and 300 mug) of arachidonic acid alone also produced increase in cutaneous vascular permeability, although its potency was only 1/3-1/8 of that of PGE2. This activity of arachidonic acid was attributed in part to its bioconversion to PGE2, since its activity was significantly reduced by the prostaglandin antagonist, diphloretin phosphate (DPP) (60 mg/kg, i.v.) and by indomethacin (20 mg/kg, i.p.), which blocks conversion of arachidonic acid to prostaglandins. Arachidonic acid may owe some of its permeability increasing effects to histamine release, since its effects were also reduced by the anti-histamine, pyrilamine (2.5 mg/kg, i.v.).     

Effect of vasodilator prostaglandins on the vascular renin-angiotensin system

The interaction of prostaglandin (PG) with the vascular renin-angiotensin (R-A) system was examined by studies on the effects of PGI2, PGE2 and the inhibitor of PG synthesis, indomethacin, on the release of angiotensin II (Ang II) from isolated rat mesenteric arteries. The Ang II released from the vasculature was measured after its concentration in a Sep-Pak C18 cartridge connected to the perfusion system. After perfusion with drugs, the specific vascular renin activity inhibited by anti-renin antibody was determined. The basal perfusion pressure was constant (19.6 +/- 1.1 mmHg) at a flow rate of 4.5 ml/min, and was not changed by any of these drugs. The basal levels of Ang II release and vascular renin activity were 44 +/- 5 pg/30 min and 113 +/- 8 pg Ang I/mg protein/hr, respectively. Infusion of PGI2 (10(-6) M) significantly decreased both Ang II release (p less than 0.01) and vascular renin activity (p less than 0.05) as compared with the control levels. Infusion of PGE2 (10(-6) M) decreased Ang II release significantly (p less than 0.05) and vascular renin activity slightly. Infusion of indomethacin (10(-6)M) increased vascular renin activity significantly (p less than 0.01). Pretreatment with indomethacin (10 mg/kg, ip) for 2 days also increased vascular renin activity (p less than 0.01). These results indicate that in contrast to their effects on the renal R-A system, PGs suppress the vascular R-A system and that these two local vasoactive factors interact to regulate vascular tone.     

Calcium channel blockers and prostaglandin generation by gastric surface epithelial cells.

We investigated the effects of calcium channel blockers on generation of prostaglandin (PG) E2 and 6-keto PGF1 alpha by gastric mucosal surface epithelium. Surface epithelial cells (SEC) isolated from rat gastric mucosa were incubated with either verapamil (1 or 10 micrograms/ml), diltiazem (2.5 or 25 micrograms/ml) or nifedipine (2.5 or 25 micrograms/ml) for 30 min at 37 degrees C in calcium containing or calcium-free medium. Verapamil (both doses) significantly increased PGE2 and 6-keto PGF1 alpha generation by the surface epithelial cells but only in calcium containing medium. Diltiazem did not affect PG generation in calcium containing nor calcium-free medium. Nifedipine 25 micrograms/ml decreased PGE2 but increased 6-keto PGF1 alpha generation. The inhibitory effect of nifedipine on PGE2 generation was abolished in calcium-free medium, while the calmodulin antagonist did not affect verapamil-induced increase in PG generation.     

Effects of platelet-activating factor on the release of arachidonic acid and prostaglandins by rabbit iris smooth muscle. Inhibition by calcium channel antagonists.

Addition of physiological concentrations (10(-12)-10(-8)M) of platelet-activating factor (PAF) to rabbit iris muscle induced a rapid release (in 15s) of prostaglandin (PG)E2 and 6-oxo-PGF1 alpha, measured by radioimmunoassay and rapid release of 14C-labelled arachidonate and PGE2 in muscle prelabelled with [14C]arachidonic acid, measured by radiochromatography. These PAF actions are concentration- and time-dependent. The effect of PAF on PG release is not mediated through the cyclo-oxygenase pathway. The studies on the properties and mechanism of arachidonate release from phosphatidylinositol and other phospholipids in prelabelled irides by PAF suggest the involvement of a phospholipase A2. This conclusion is supported by the findings: (a) that both the removal of arachidonate and formation of lysophosphatidylinositol, from phosphatidylinositol, by PAF occur concomitantly in a time-dependent manner, (b) that Ca2+ is required for the agonist-induced release of arachidonate and PGE2, and (c) that in contrast to the rapid release of [3H]myo-inositol phosphates by carbachol and other Ca2+-mobilizing agonists previously reported in the iris muscle [Akhtar & Abdel-Latif (1984) Biochem. J. 224, 291-300], PAF (10(-12)-10(-8)M) did not appreciably enhance the release of [14C]myo-inositol phosphates and 32P labelling of phosphatidate and phosphatidylinositol in this tissue. Ca2+-channel antagonists, such as nifedipine, verapamil, diltiazem and manganese inhibited PAF-induced arachidonate and PGE2 release in a dose-dependent manner. K+ depolarization, which causes influx of extracellular Ca2+ in smooth muscle, did not increase the release of arachidonate and PGE2. The ability of Ca2+ antagonists to inhibit arachidonate release by PAF in this tissue probably reflects interference with PAF binding to its receptor. The PAF-induced release of arachidonate and PGE2 occur independently of the cyclo-oxygenase and lipoxygenase pathways. Whether the PAF-induced release of arachidonate and PG in the iris muscle is involved in the pathogenesis of inflammatory and/or physiological reactions in the eye, and how much the inhibitory effects of Ca2+-entry blockers on the PAF actions contribute to the therapeutic use of these drugs, remain to be established.     

The possible involvement of prostaglandin F2 alpha in the stimulation of muscle protein synthesis by insulin

The addition of insulin (8 ng/ml) in vitro to muscles from fasted rabbits increased protein synthesis (+80%) to a value similar to that found in muscles from fed donors. The addition of either indomethacin or meclofenamate completely blocked this effect of insulin. Muscles from fasted rabbits released less prostaglandin (PG)F2 alpha into the medium and the presence of insulin increased and indomethacin and meclofenamate reduced PGF2 alpha release. Other conditions (work load and leucocyte pyrogen) which increase protein synthesis in muscle also stimulate PGF2 alpha release. As both arachidonic acid and PGF2 alpha in themselves increase protein synthesis we suggest that accelerated phospholipolysis and PG synthesis have a general role in the control of muscle protein turnover.     

Effect of angiotensin ii and norepinephrine on release of prostaglandins E2 and I2 by the perfused rat mesenteric artery

Prostaglandin E2 (PGE2) and 6 keto-PGF1 alpha, the stable metabolite of prostacyclin (PGI2), have been measured in the effluent of perfused rat mesenteric arteries by the use of a sensitive and specific radioimmunoassay (RIA) method. The PGE2 and 6 keto-PGF1 alpha were continuously released by the unstimulated mesenteric artery over a period of 145 min. After 100 min of perfusion the release of PGE2 and 6 keto-PGF1 alpha was 45.1 +/- 8.4 pg/min and 254 +/- 75 pg/min respectively, which is in accord with the general belief that PGI2 is the major PG synthesized by arterial tissue. Angiotensin II (AII) (5 ng/ml) induced an increase of PGE2 and 6 keto-PGF1 alpha release without changing the perfusion pressure. The effect of norepinephrine (NE) injections on release of PGs depended on the duration of the stabilization period. The changes of perfusion pressure induced by NE were not related to changes in release of PGs. Thus, it seems that the increase of PG release induced by AII and NE was due to a direct effect of the drugs on the vascular wall. This may represent an important modulating mechanism in the regulation of vascular tone.