Prostacyclin production by the deendothelialized rabbit aorta

Cyclooxygenase Activity in the Thoracoabdominal Aorta After 24 Hours of Intraaortic Balloon Counterpulsation: An Assessment of the Effects of Localized Mechanical Trauma

Using a canine model, the authors evaluated the effects of twenty-four hours of intraaortic balloon counterpulsation (IABCP) on the morphology of the thoracoabdominal aorta and its spontaneous and arachidonate stimulated pro duction of prostacyclin (PGI2). The treated group was counter-pulsed at a 1:1 ratio with a 40 cc balloon. The control group had the balloon inserted and left deflated across the thoracoabdominal aorta.

There were no differences between the IABCP or sham groups in prostanoid production either in the thoracic or abdominal aorta. Baseline PGI2 production in the control versus treated group of thoracic aortas was 1880 ± 1140 pg/cm2/ min vs 1992 ± 1125. The stimulated response in the control group, 9180 ± 1198, was statistically different from baseline (P < .001). The stimulated in crease in the IABCP group, 6269 ± 5240, relative to baseline was also signifi cant (P < .01). There were no significant differences, either between or within groups in the abdominal aortas. Poststimulation values (30 min) were signifi cantly below baseline in each group in each section of the aorta (P < .05). Light microscopy showed a statistically significant difference in endothelial preserva tion between the control and the balloon group, 66 ± 25% vs 19 ± 17% (P < 0.05) .

The authors conclude that IABCP results in severe endothelial destruction but that spontaneous and stimulated PGI2 production is unimpaired. It remains to be determined whether the aorta may sustain long-term damage due to the repair process after IABCP.

Role of vascular adrenergic mechanisms in the haemodynamic and PGI2 stimulating effects of angiotensin in diabetic dogs

In aware of the well-known altered vascular responsiveness in the diabetic vasculature, this study aimed to compare the haemodynamic and PGI2 releasing effects of angiotensin in metabolically healthy (12) and alloxan-(560 umol/kg) diabetic (12) dogs as well as to analyze the role of vascular adrenoceptors in this. In vivo the effect of intracoronarially administered angiotensin (63-125-250-500-1000 pmol/kg/min) on coronary blood flow, mean arterial blood pressure, myocardial contractile force and heart rate was investigated without and with pretreatment of 2 umol/kg phentolamine. In vitro PGI2 release by isolated coronary rings was induced by 50 nmol/l angiotensin before and after pretreatment with 5 umol/l phentolamine and measured by radioimmunoassay. Angiotensin enhances dose-dependently both the mean arterial blood pressure and coronary blood flow, while it provokes a considerable (p < 0.05) increase of PGI2 formation by isolated coronary arterial rings. These alterations could be prevented by phentolamine administration both in vivo and in vitro, while this drug did not affect the angiotensin-induced enhancement of diabetic coronary blood flow. On the other hand the increase of blood pressure by angiotensin was found to be more (p < 0.05) expressed in diabetes and it could be further potentiated by phentolamine. PGI2 synthesis by isolated diabetic coronary rings could not be modified either by angiotensin alone or in combination with phentolamine. On the basis of above data, the lack of stimulated vascular PGI2 formation mediated by alpha-adrenergic mechanisms is supposed to causatively contribute to the diminished sensitivity of diabetic coronary arteries to vasodilation.

Acute stress in rats produces a rapid and sustained increase in prostacyclin production in aortic tissue: dependence on corticosterone

Immobilization stress in rats is accompanied by a fast and transient increase in plasma catecholamines (CA) and a slow but more sustained increase in plasma corticosterone (CORT) levels. CA are reported to increase prostacyclin (PGI2) secretion in endothelial cells in culture and in isolated vascular tissue. The present study was undertaken to determine a) whether short term immobilization stress in rats affects PGI2 synthesis in aortic tissue and b) whether such effects are dependent on the maintenance of an intact hypothalamic-pituitary-adrenal axis. Male Sprague Dawley rats were immobilized for 5, 15 and 30 min., killed by a blow to the head and aortic rings immediately prepared and incubated in oxygenated Krebs-Ringer buffer at 37 degrees C. Secretion of PGI2 was assessed by measuring the amount of 6-keto-PGF1 alpha, the stable hydrolysis product of PGI2, formed in the incubation medium during 90 min. Rats immobilized for 15 min demonstrated a six to seven times increase in plasma ACTH and a more than ten times increase in plasma CORT levels. Aortic rings from stressed rats also demonstrated a significant and sustained (2 hours) increase in PGI2 secretion when compared with tissue from unstressed rats (68 pg 6-keto-PGF1 alpha/mg tissue per min vs 35 pg 6-keto-PGF1 alpha/mg tissue per min during the first hour). Shorter (5 min) or longer (30 min) periods of stress gave comparable results. Adrenalectomy (ADX) carried out 7 days prior to immobilization, did not affect the baseline secretion of the eicosanoid, but completely prevented the increase in PGI2 by immobilization stress. Administration of 2 micrograms/kg and 200 micrograms/kg i.a. of CORT to ADX-rats produced plasma levels of the hormone (36 +/- 12 ng/ml and 934 +/- 366 ng/ml respectively) comparable to those seen in unstressed and stressed rats. However only the higher dose of CORT when administered immediately prior to stress, restored the effect of immobilization stress on PGI2 synthesis in aortic tissue of ADX-rats. Administration of the protein synthesis inhibitor cycloheximide (1 mg/kg) prior to stress did not affect the increase in PGI2 secretion. In contrast to restraint stress, co-administration of ACTH and NE which also raised plasma levels of ACTH, CORT and NE to peak stress levels, did not increase prostacyclin production in aortic tissue. In conclusion, our findings demonstrate that short periods of immobilization stress produce a rapid and sustained increase in PGI2 synthesis in the rat aorta, provided plasma levels of CORT are elevated.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of von Willebrand factor and PGI2 in platelet binding to a partially denuded endothelial monolayer

Platelet binding to an endothelial monolayer was examined after denudation. The binding increased for up to 10 min and thereafter declined gradually. Antibodies against von Willebrand factor (vWF) inhibited the binding. Production of prostacyclin (PGI2) occurred 10 min after endothelial denudation. This study suggests that vWF is involved in the binding during the first 10 min and that PGI2 suppresses the binding thereafter.

Inhibitory effect of clentiazem (TA-3090), a new calcium antagonist, on balloon catheter-induced intimal thickening of rabbit aorta

Male Japanese white rabbits were fed a restricted amount (100 g/head/day) of an atherogenic diet containing 0.2% cholesterol and 6% peanut oil during an 8-week experimental period. Atherosclerotic lesions, characterized by intimal thickening with lipid deposition, were produced by de-endothelialization of the rabbit aorta with a 4 F balloon catheter halfway through the experiment. Clentiazem (TA-3090), a new calcium antagonist, was administered at an oral dose of 30 mg/kg/day for 4 weeks starting on the day of deendothelialization. Clentiazem significantly depressed the intimal thickening without any effect on serum lipid levels. Clentiazem (1, 3, and 10 microM) significantly and dose-dependently inhibited the in vitro proliferation of smooth muscle cells that had been explanted from the neointima of the deendothelialized aorta. At a higher concentration, this drug markedly inhibited collagen-induced aggregation of rabbit platelets. Diltiazem also showed similar effects, but the effects of clentiazem were more potent than those of diltiazem. These results suggest that clentiazem exhibits an antiatherogenic effect, at least partly through prevention of smooth muscle cell proliferation in atheromatous lesions, in addition to its hypotensive action.

20-Hydroxyeicosatetraenoic acid is an endogenous vasoconstrictor of canine renal arcuate arteries

Recent studies have indicated that renal arteries can produce 20-hydroxyeicosatetraenoic acid (20-HETE) and suggest the potential involvement of a P450 metabolite of arachidonic acid in the myogenic activation of canine renal arteries. In the present study, the effects of 20-HETE on isolated canine renal arcuate arteries were studied. Administration of 20-HETE to the bath or the lumen at concentrations of 0.01-1 microM produced a graded reduction in the diameter of these vessels. In contrast, 19(R)-HETE was a vasodilator, whereas 19(S)-HETE was relatively inactive. The vasoconstrictor response to 20-HETE was not altered by the cyclooxygenase inhibitor indomethacin, endoperoxide/thromboxane receptor antagonist SQ29548, or combined blockade of the cyclooxygenase, lipoxygenase, and P450 pathways using indomethacin, baicalein, and 7-ethoxyresorufin. The response to 20-HETE was associated with depolarization and a sustained increase in the intracellular calcium concentration in renal vascular smooth muscle cells. Patch-clamp studies indicated that 20-HETE significantly reduced mean open time, the open-state probability, and the frequency of opening of a 117-pS K+ channel recorded from renal vascular smooth muscle cells in the cell-attached mode. Microsomes prepared from the renal cortex of dogs produced 20-HETE and 20-carboxyarachidonic acid when incubated with [14C]arachidonic acid. These results indicate that 20-HETE is an endogenous constrictor of canine renal arcuate arteries. The vasoconstrictor response to 20-HETE resembles the myogenic activation of these vessels after elevations in transmural pressure and suggests a potential role for this substance in the regulation of renal vascular tone.

Endothelial cell seeding of damaged native vascular surfaces: prostacyclin production

Endothelial cell seeding has been successful in reducing the thrombogenicity of prosthetic vascular grafts in animal and clinical studies. The reduction in thrombogenicity may be attributed to the intrinsic properties of endothelial cells themselves, and their ability to produce anti-thrombogenic mediators such as prostacyclin, and endothelium-derived relaxing factor. Endothelial seeding of damaged vascular surfaces produced during percutaneous transluminal angioplasty and endarterectomy is an attractive possibility due to the excellent attachment characteristics of the sub-endothelial tissue exposed during these procedures. The ability of endothelial seeded damaged vascular surfaces to produce prostacyclin was measured in an in vitro model of vascular injury. Endothelial-seeded damaged surfaces produced significantly higher prostacyclin release than did vessels damaged by balloon dilatation (265.5 pg cm-2 min-1 and 87.5 pg cm-2 min-1 respectively). This study provides evidence that endothelial seeding of damaged native vascular surfaces is technically feasible and that seeding may reduce the thrombogenicity of vascular surfaces following balloon dilatation.

Role of endothelium on alpha-adrenoceptor responsiveness and prostacyclin release from the mesenteric arterial bed of the rat

1 Removal of endothelium from the isolated perfused mesenteric arterial bed (MAB) of the rat was associated with an increase in both basal release of prostacyclin and the pressor response to administered noradrenaline (NA). Under these conditions, the NA-stimulated release of prostacyclin was not altered. 2 In this preparation, prazosin, an alpha 1-adrenoceptor antagonist, caused a dose-dependent (10(-10)-10(-8) M) decrease in NA-stimulated prostacyclin production, whereas rauwolscine, an alpha 2-adrenoceptor antagonist (10(-8)-10(-7) M) had no significant effect. In addition, prazosin inhibited the NA-induced pressor responses (10(-10)-10(-8) M) while rauwolscine was only effective at a concentration of 10(-7) M. 3 L-NG mono-methyl-arginine (L-NMMA), an inhibitor of endothelial nitric oxide (NO) synthesis, was used to assess whether elimination of this substance was responsible for the increased basal release of prostacyclin and/or increased pressor responses observed after endothelium removal in the MAB. Concentrations of 3 x 10(-7)-3 x 10(-6) M of L-NMMA were without effect on either prostacyclin release or pressor responses to NA in the intact MAB. 4 These results indicate that in the endothelial denuded MAB, NA-induced release of prostacyclin is, mainly, alpha 1-adrenoceptor-mediated, in contrast to the intact MAB where it is alpha 2-adrenoceptor-dependent. It therefore appears that, in the MAB of the rat, the presence of endothelium obscures the alpha 1-mediated release of prostacyclin from vascular smooth muscle cells. In this preparation the endothelium may regulate both the release of prostacyclin and the contractile responses of the underlying smooth muscle via mechanisms independent of NO formation.

Method of removal of aortic endothelium affects arachidonic acid metabolism and vascular reactivity

To assess endothelium-dependent responses of blood vessels in vitro, endothelial cells are removed by a variety of mechanical means. We sought to determine if the method of removal of the endothelium affected arachidonic acid metabolism and vascular reactivity of isolated strips of rabbit aorta. Thoracic aorta of New Zealand White rabbits were excised and sectioned into strips with a sharp razor blade. The luminal surface of the vessel was then gently stroked (denuded-1) or forcefully rubbed (denuded-2) with a moist cotton swab. Vessels were then either fixed in 3% glutaraldehyde and processed for electron microscopy, incubated with [14C]arachidonic acid and 20 microM A23187 for determination of arachidonic acid metabolism, incubated with 20 microM A23187 for measurement of endogenous release of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and 12-hydroxyeicosatetraenoic acid (12-HETE) by specific radioimmunoassays, or suspended in an organ chamber filled with Krebs bicarbonate solution for vascular reactivity experiments. Electron micrographs showed that denuded-1 vessels lacked an endothelial cell layer and had slight degeneration of the smooth muscle cells. Additionally, these vessels had a diminished capacity to produce 6-keto-PGF1 alpha as compared to control vessels (214 +/- 25 vs. 360 +/- 36 pg/mg of tissue, P less than 0.05). Denuded-2 vessels contained severe degeneration and rupture of smooth muscle cells in addition to the loss of the endothelial cell layer. While the 6-keto-PGF1 alpha concentration (168 +/- 23 pg/mg) was less in denuded-2 vessels, HPLC indicated that the production of [14C]12-HETE was markedly increased in these vessels as compared to control or denuded-1 vessels.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of the endothelial production of prostacyclin by inhibitors of protein synthesis

1. Pretreatment of bovine aortic endothelial cells with cycloheximide enhanced their capacity to release prostacyclin in response to adenosine 5'-triphosphate (ATP) and bradykinin. 2. The action of cycloheximide was time-dependent; it became detectable after a 1 h exposure to the cells and was maximal after 3 h. 3. Puromycin mimicked the effect of cycloheximide. For these two agents, the enhancement of prostacyclin release was obtained at concentrations producing a partial inhibition of protein synthesis. 4. Cycloheximide increased the mobilization of free arachidonic acid induced by ATP in bovine aortic endothelial cells. 5. In conclusion, the synthesis of new proteins is not involved in the stimulatory action of ATP and bradykinin on prostacyclin production by bovine aortic endothelial cells. Despite the short half-life of prostaglandin H synthase in endothelial cells, cycloheximide and puromycin enhanced the release of prostacyclin induced by agonists. Our data suggest that this release might be under the control of rapidly turning-over phospholipase inhibitory proteins.

Role of endothelium-derived relaxing factor and prostaglandins in responses of coronary arteries to thromboxane in vivo

We examined the relative contribution of endothelial and vascular smooth muscle-derived prostaglandins and endothelium-derived relaxing factor in modulating both the large coronary artery and resistance vessel responses to thromboxane in vivo. Vascular responses to the thromboxane analogue U46619 were measured in four separate experimental protocols: 1) The vascular responses were measured in the presence and absence of intact endothelium to examine the role of endothelium-derived vasodilators. 2) Responses were measured in the presence of intact endothelium before and after inhibition of cyclooxygenase with indomethacin to examine the role of endothelial and vascular smooth muscle-derived prostaglandins. 3) Responses were measured after endothelial removal before and after indomethacin to examine the role of vascular smooth muscle-derived prostaglandins. 4) Responses were measured after indomethacin and before and after removal of endothelium to examine the role of endothelium-derived relaxing factor. In anesthetized dogs (n = 41) that underwent constant pressure perfusion of the left anterior descending coronary artery (LAD), LAD diameter was measured with sonomicrometer crystals, and coronary flow was measured with an electromagnetic flow probe. Intracoronary infusion of U46619 (0.01-1.0 microgram/min) produced a dose-dependent constriction of LAD. Constriction of the LAD was augmented after endothelial removal, after indomethacin treatment in both the presence and absence of endothelium, and after removal of the endothelium in the presence of indomethacin. Inhibition of prostaglandin synthesis had the greatest effect of augmenting constriction of LAD to thromboxane. Coronary flow was decreased by U46619 only in the presence of indomethacin.(ABSTRACT TRUNCATED AT 250 WORDS)

Irradiation of rats abolishes susceptibility of PGI2 synthesis in blood vessels to peroxidative agents

Thoracic and abdominal aortas were obtained from rats after irradiation and used for the estimation of the synthesis of prostacyclin (PGI2) determined as 6-keto PGF1 alpha. Twenty four h after exposure to 7.0 Gy an increase was noted in the amount of PGI2 released, and 4 weeks later its level significantly decreased. The 24 h value did not increase with the further radiation dose increment (9, 12.5, 15 Gy). Cysteine or H2O2 intensified prostacyclin synthesis in control vessels but decreased it in vessels from the animals irradiated 24 h earlier. Later after the exposure cysteine or H2O2 were no longer effective.

Influence of transluminal angioplasty on the prostanoid release from the arterial wall

Vasospasm and thrombosis complicate percutaneous transluminal angioplasty (PTA). To study if the release of the prostanoids PGI2 and TxA2 are affected by PTA, the following experiment was undertaken: In ten rabbits, the upper or lower half of the aorta was randomised either to transluminal angioplasty or control segment. After excision the segments were simultaneously but separately perfused ex vivo with Hank's balanced salt solution for five consecutive 15 min periods. Arachidonic acid was added to the perfusate for the last 15 min period. PGI2 and TxA2 were measured by radioimmunoassay in the perfusate as the stable degradation products 6-keto-PGF1 alpha and TxB2. After perfusion, the two aortic segments were prepared for scanning electron microscopy (SEM). Angioplasty decreased the basic release of PGI2 as well as the response to arachidonic acid. This is likely to be due to endothelial denudation as seen by SEM. The release of TxA2 from the vessel wall was very low and was not increased by dilatation. The influence of angioplasty on the prostanoid system may be of importance in the complications of vasospasm and thrombosis.

Prostacyclin and vascular function: implications for hypertension and atherosclerosis

Prostacyclin and endothelium-derived relaxing factor (or nitric oxide) are unstable mediators produced by the vascular endothelium, that are important for local regulation of platelet behavior and blood flow. This review focuses on the basic biochemistry and pharmacology of prostacyclin, its interactions with nitric oxide and nitrovasodilator drugs, and the implications of disturbances in this system for vascular disease, particularly hypertension and atherosclerosis. Prostacyclin and its stable analogs are also finding limited therapeutic applications in preservation of platelet function, pulmonary hypertension, and investigation into the cytoprotective and antiatherosclerotic properties is continuing.

Effects of amiloride analogues on the production of prostacyclin by aortic endothelial cells

1. The release of prostacyclin (PGI2) from bovine aortic endothelial cells stimulated by adenosine 5'-triphosphate (ATP) was decreased by amiloride analogues bearing alkyl groups on the 5-amino nitrogen atom, like 5-(N-ethyl-N-isopropyl)amiloride (EIPA), which are inhibitors of the Na+/H+ exchanger. Analogues substituted on a terminal guanidino nitrogen atom were not inhibitory. 2. The release of PGI2 induced by ATP was not significantly depressed in a Na+-poor medium or in a medium acidified to pH 6.9, two conditions known to inhibit the Na+/H+ exchanger. 3. Cytoplasmic alkalinization by ammonium chloride did not suppress the inhibitory action of EIPA. By itself, ammonium chloride decreased the response of endothelial cells to ionophore A23187 and ATP, whereas sodium acetate had no effect. 4. EIPA did not decrease the mobilization of free arachidonic acid induced by ATP. It inhibited the conversion of exogenous arachidonate into PGI2 and prostaglandin E2 (PGE2). 5. Although the intracellular pH was not measured in this study, it seems unlikely that cytoplasmic alkalinization via the activation of the Na+/H+ exchanger plays a significant role in the stimulatory action of ATP on the release of PGI2 from endothelial cells. The inhibition of that release by EIPA and other amiloride analogues might involve a direct effect on cyclo-oxygenase, although an action on the reacylation of free arachidonic acid cannot be excluded.

Palmitoyl-L-carnitine increases the release of prostacyclin from vascular endothelial cells

Prostacyclin biosynthesis is dramatically increased in patients with acute myocardial infarction. As palmitoylcarnitine accumulates during myocardial ischemia, the action of this metabolite on the endothelial production of prostacyclin was studied. Palmitoyl-L-carnitine (10-100 microM) enhanced the release of prostacyclin and free arachidonic acid from bovine aortic endothelial cells. This action was mimicked by lysophosphatidylcholine, but by none of the following compounds: acetylcarnitine, carnitine, palmitic acid, sphingosine, dihydrosphingosine and N-stearoyl-dihydrosphingosine. In addition to mobilizing free arachidonate, palmitoylcarnitine induced the release of free choline and phosphorylcholine presumably via the activation of phospholipases C and D. Palmitoyl-L-carnitine had also a cytotoxic effect on the endothelial cells. These data suggest that the increased biosynthesis of prostacyclin in myocardial infarction might be partially explained by the accumulation and release of palmitoyl-L-carnitine.

Effects of polycations on pulmonary vascular permeability in conscious sheep

The role of charged sites on the permeability characteristics of the pulmonary microvascular barrier were investigated using chronically instrumented unanesthetized sheep. In one series of experiments we studied the effects of the cationic amphiphile, dodecyl trimethylamine (DTA; 297 mol wt), and the anionic amphiphile, SDS (288 mol wt), on lung lymph flow rates (Ql), lung lymph to plasma protein ratios (L/P), pulmonary hemodynamics, and systemic hemodynamics. DTA significantly increased both Ql and L/P, whereas SDS had a more modest and transient effect on these variables. In a second series of experiments the polycations polybrene and poly-l-lysine were found to have very similar effects as those of DTA. In another series of experiments we tested the pretreatment inhibition potential of chlorpheniramine (an H1 receptor antagonist), dibutyryl-cyclic AMP (db-cAMP), and the calcium channel antagonists verapamil and nifedipine on polybrene-induced lung injury. We found that only verapamil and db-cAMP significantly attenuated the permeability effects of polybrene. We conclude that both cationic amphiphiles and polycations cause hemodynamic and permeability alterations in the pulmonary circulation of unanesthetized sheep. In addition, the permeability alterations induced by polybrene can be modulated by intracellular calcium and/or cAMP levels.

Rapid development of atherosclerotic lesions in the rabbit carotid artery induced by perivascular manipulation

A new rabbit model of atherosclerosis is described in which several of the features seen in early human atherosclerosis are generated within a period of 7 days. The positioning of a hollow silastic collar around the carotid artery of a cholesterol-fed rabbit results in macrophage and smooth muscle cell infiltration into the arterial subendothelium, foam cell formation and the deposition of extracellular lipid. A time-dependent accumulation of extracellular cholesteryl ester occurs within the arterial wall. Each of these changes occurs in the presence of a morphologically intact endothelium as assessed using light microscopy, scanning and transmission electron microscopy. A high cholesterol diet did not affect the extent of proliferation but exacerbated cholesteryl ester accumulation. It is proposed that the changes induced by the collar may be mediated by obstruction of the adventitial vasa vasorum with the creation of a localised ischaemic region.

Effect of various plasminogen activators on prostacyclin synthesis in cultured vascular cells

In this study, we examined the effects of various plasminogen activators on arachidonic acid release and prostacyclin biosynthesis in cultured rat aortic smooth muscle cells and bovine pulmonary artery endothelial cells. Prostacyclin was the major product formed from arachidonic acid in aortic smooth muscle cells and endothelial cells. When intact cells were incubated with streptokinase, one of the plasminogen activators, a significant stimulatory effect on prostacyclin biosynthetic activity in cells was evident without any cellular damage at all concentrations used (1-5,000 units/ml). Streptokinase also caused a marked release of arachidonic acid. However, when it was incubated with cell-free homogenates and [3H]arachidonic acid, it did not show any effects on prostacyclin biosynthesis. The addition of urokinase and tissue-type plasminogen activator had no effect on prostacyclin biosynthesis. Urokinase stimulated the release of arachidonic acid from cells, but it did not show any effect on prostacyclin release at any concentration of urokinase (1-5,000 units/ml). The release of arachidonic acid and the increased prostacyclin synthesis were not observed when tissue-type plasminogen activator was added. These results indicate that, among various plasminogen activators investigated, only streptokinase causes the release of arachidonic acid and prostacyclin. This could be a beneficial effect in thrombolytic therapy.

Relationship between vascular adrenergic receptors and prostaglandin biosyntheses in canine diabetic coronary arteries

Before the onset of histologically detectable alterations of diabetic arteries, a considerable decrease of vasodilation ability develops. The role of an altered prostaglandin biosynthesis in this phenomenon was investigated in connection to the altered vascular adrenergic mechanisms. The effect of phenylephrine on prostacyclin production of isolated coronary arterial rings (100 mumol/l) as well as on conductivity of the coronary arterial bed (7.5-15-30-60 pmol. kg-1.min-1) were compared in 12 metabolically healthy and 12 alloxan-diabetic (560 mumol/kg) dogs. Furthermore, the effect of phentolamine (5 mumol/l) on the prostacyclin and thromboxane productions of the isolated vessels (coronary, femoral and basilar arteries) was investigated by radioimmunoassay. Although the basal prostacyclin amounts synthesized by healthy and diabetic coronary vessels were not different (5.1 +/- 1.6 and 4.9 +/- 1.4 pg/mg vessel/30 min), similarly to femoral and basilar arteries, the diabetic arterial rings produced significantly (p less than 0.05) more thromboxane than the control rings. The alpha-adrenergic blockade by phentolamine did not influence the prostacyclin production in the healthy arteries, but considerably (p less than 0.05) increased it in the diabetic coronary arteries. Phentolamine normalised the thromboxane synthesis in the diabetic group (p less than 0.01) and enhanced (p less than 0.05) it in the metabolically healthy group. Phenylephrine was ineffective (98 +/- 6%) on the prostacyclin production in vitro versus the stimulated (150 +/- 22%) prostacyclin synthesis detected in the metabolically healthy group; and in vivo induced a more significant (p less than 0.05) decrease in the coronary conductivity in diabetic than in control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of static incubation versus physiologic perfusion techniques for quantitation of luminal release of prostacyclin and thromboxane in canine arteries and veins

Intraluminal release of 6-keto-PGF1 alpha and TxB2 in ex vivo canine arteries and veins was assessed during five consecutive 15-min periods using static incubation and physiologic perfusion techniques. Arterial segments were perfused with 90 ml/min pulsatile flow at 100 mm Hg and vein segments with 90 ml/min nonpulsatile flow at 7 mm Hg. During the final 15-min period vessels were stimulated with arachidonic acid (AAS). Perfusion of vein segments caused a higher release of 6-keto-PGF1 alpha during the first 30 min (P less than 0.05) and following AAS (P less than 0.05) than did static incubation. Perfused arterial segments exhibited a higher release than did incubated segments of 6-keto-PGF1 alpha for 45 min (P less than 0.01) as well as following AAS (P less than 0.01). TxB2 release was higher during the entire observation period in perfused arteries and veins compared to incubated vessels (P less than 0.01 and less than 0.05, respectively). There was no correlation between the amounts of 6-keto-PGF1 alpha or TxB2 released when comparing values obtained by one technique to values obtained by the other (P greater than 0.1). These data suggest that flow related shear stress alters vascular prostanoid production, and that such should be accounted for when interpreting results of studies on prostacyclin and thromboxane release from intact vessels.

Activated endothelial cells in culture suppress baroreceptors in the carotid sinus of dog

Vascular stretch increases the activity of arterial baroreceptors along with the production and release of substances from the endothelium. We hypothesized that endothelial factors modulate the sensitivity of baroreceptors during increases in arterial pressure. Baroreceptor activity was recorded from single fibers innervating the isolated carotid sinus of dogs anesthetized with chloralose after removal of the endothelium (balloon denudation) and after replacing into the denuded sinus bovine aortic endothelial cells cultured on microcarrier beads. The endothelial cells were activated with either the calcium ionophore A23187 (2 microM) or bradykinin (10 microM). The threshold pressure (n = 7) determined with a slow ramp increase in static pressure averaged 73 +/- 7 (SEM) mm Hg during exposure to naked beads and was increased significantly (96 +/- 18 mm Hg; p less than 0.05) during exposure to endothelial cell cultures. During stepwise increases in pressure, activity (n = 6) averaged 14 +/- 5, 40 +/- 8, and 54 +/- 8 spikes/sec at 75, 125, and 175 mm Hg during exposure to naked beads and decreased significantly to 2 +/- 2, 30 +/- 11, and 35 +/- 12 spikes/sec at equivalent pressures during exposure to the cell cultures. The activity was restored after replacement of the cell cultures with naked beads. The suppressed activity was not caused by changes in carotid sinus diameter or strain (sonomicrometers) or by the chemical activators that were also added to the naked beads. The results indicate that chemically activated endothelial cells release an inhibitory factor that suppresses baroreceptor activity.

Novel mechanism of action of a prostacyclin enhancing agent in haemorrhagic shock

Prostacyclin (PGI2) is a potent vasodilator, an inhibitor of platelet aggregation, and a membrane stabilizing agent with beneficial effects in ischemia and shock. We studied defibrotide, a new agent which enhances PGI2 release from vascular tissue, to determine its effects in a murine model of hemorrhagic shock. Hemorrhaged rats treated with defibrotide maintained post-reinfusion mean arterial blood pressure (MABP) at significantly higher values compared to rats receiving the vehicle (final MABP, 100 +/- 3 vs. 69 +/- 7 mm Hg, p less than 0.01). Defibrotide attenuated the release of the lysosomal hydrolase cathepsin D (p less than 0.02), and the plasma accumulation of free amino-nitrogen groups (p less than 0.02). The plasma activity of a myocardial depressant factor (MDF) was significantly lower in defibrotide treated shocked rats than in the vehicle group (29 +/- 4 vs. 61 +/- 8 U/ml, p less than 0.01). Moreover, plasma i6-keto-PGF1 alpha concentrations increased 3-fold above haemorrhaged rats receiving only the vehicle. This, as well as the improved MABP, was abolished by indomethacin. Additionally, defibrotide exerts an anti-proteolytic action in pancreatic homogenates, and a lysosomal stabilizing effect in large granule fractions of rat liver homogenates. Moreover, defibrotide enhanced the recovery from norepinephrine induced vasoconstriction in rat aortic rings having an intact endothelium (p less than 0.01 from vehicle), and augmented the release of i6-keto-PGF1 alpha, the stable metabolite of PGI2, from isolated rat aortae. Our results indicate that enhancement of endogenous vascular PGI2 release coupled with direct, or PGI2 mediated antiproteolytic and membrane stabilizing actions may be important physiological mechanisms counteracting the deleterious effects of hemorrhagic shock.

Release of prostacyclin from the dog saphenous vein by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) stimulated the release of prostacyclin (PGI2) from strips of the dog saphenous vein. This stimulation was maintained after mechanical removal of the endothelium. The effect of 5-HT was mimicked by alpha-methyl-5-hydroxytryptamine (alpha-CH3-5-HT) but not by 5-carboxamidotryptamine (5-CT) and it was inhibited by ritanserin. It is concluded that, in the dog saphenous vein, the stimulation of PGI2 release by 5-HT involves 5-HT2 receptors distinct from the '5-HT1-like' receptors that mediate contraction of the vessel.

Role of protein kinase C in the control of vascular prostacyclin: study of phorbol esters effect in bovine aortic endothelium and smooth muscle

In bovine aortic endothelial cells, phorbol 12-myristate, 13-acetate induced a smaller stimulation of prostacyclin release than ionophore A23187: the combination of both agents was highly synergistic. The responses of the bovine aortic smooth muscle were very different in the 2 preparations studied. In media explants cultured for short periods, neither phorbol 12-myristate, 13-acetate, nor A23187, alone or in combination, were able to increase prostacyclin release, whereas serotonin was an effective stimulus. In cultured smooth muscle cells, outgrown from the explants, phorbol 12-myristate, 13-acetate increased prostacyclin release to the same levels as A23187 or serotonin. It is concluded that increased cytosolic Ca++ level and protein kinase C activity induce a synergistic stimulation of endothelial prostacyclin. On the other hand, the phenotypic modulation of the arterial smooth muscle, from a contractile to a synthetic state, seems to be associated with a profound change in the control of prostacyclin.

Cultured bovine coronary arterial endothelial cells synthesize HETEs and prostacyclin

Arachidonic acid metabolism was examined in endothelial cells cultured from bovine coronary arteries. In culture, these cells exhibit specific characteristics of endothelial cells. They form a contact-inhibited monolayer with a cobblestone appearance, contain immunoreactive von Willebrand's factor antigen, and have angiotensin I converting enzyme activity. Prostacyclin was the major prostaglandin synthesized from exogenous and endogenous arachidonic acid in these cells. In addition, exogenous arachidonic acid was metabolized to small amounts of prostaglandin E2 (PGE2) and several relatively nonpolar metabolites including 12-, 15-, and 11-hydroxyeicosatetraenoic acids (12-, 15-, and 11-HETE). Histamine, bradykinin, and thrombin increased PGI2 synthesis in these bovine coronary endothelial cells. Of these agonists, bradykinin was the most potent, increasing basal PGI2 release by fourfold. More vigorous stimulation of the cells with mechanical disruption of the cell monolayer, melittin, or A23187 resulted in release of both PGI2 and PGE2. Pretreatment of cells with exogenous arachidonic acid (10(-5) M) abolished their responsiveness to subsequent stimulation by arachidonic acid or vasoactive agents, but not PGH2. Furthermore, treatment of cells with 15-HPETE (10(-7)-10(-4) M), but not 15-HETE, specifically inhibited basal as well as A23187-stimulated PGI2 release. PGE2 release was increased slightly after 15-HPETE treatment. These studies indicate that bovine coronary endothelial cells can metabolize arachidonic acid to several biologically active products and that PGI2 synthesis by these cells is specifically related to the type of vasoactive agent employed. Both the qualitative pattern and quantity of eicosanoids synthesized by bovine coronary endothelial cells differ substantially from endothelial cells isolated from noncardiac vascular beds.

Adrenergic stimulation of vascular prostacyclin: role of alpha 1-receptors in smooth muscle cells

Epinephrine and norepinephrine (1-10 microM) stimulated the release of prostacyclin (PGI2) from the rabbit aorta in vitro. The stimulation was maintained for at least 2 h in the continuous presence of epinephrine. Phenylephrine mimicked this effect, whereas the selective alpha 2-agonist UK-14,304 was completely ineffective. The action of epinephrine was abolished by prazosin (1 microM) and was maintained in the presence of yohimbine. Epinephrine or phenylephrine neither increased the basal release of PGI2 from bovine aortic endothelial cells nor potentiated the stimulatory action of adenine nucleotides, which is mediated by P2-purine receptors. The response to epinephrine was lost in freshly deendothelialized strips of rabbit aorta, possibly because of cyclooxygenase self-inactivation. The response recovered however following overnight incubation of these strips in a cell culture medium. The response to epinephrine was mimicked by neither phorbol 12-myristate,13-acetate nor ionophore A23187. It was not inhibited by pretreatment with pertussis toxin. It is concluded that adrenergic agents stimulate the vascular production of PGI2, by activating alpha 1-receptors located on smooth muscle cells.

Influence of atherosclerosis on vascular responsiveness in isolated rabbit vascular smooth muscle

Coronary arteries and aortic rings were isolated from rabbits fed either a control diet or a high cholesterol (1 to 2%) diet for 8 to 11 weeks and studied for their vasoactive properties to a variety of vasoconstrictor and vasodilator agents. Perfused coronary arteries without intact endothelium constrict markedly to a thromboxane A2 agonist (i.e., carbocyclic thromboxane A2, CTA2) and dilate markedly to iloprost, a prostacyclin analog. No differences occurred between the coronary arteries isolated from control or atherosclerotic rabbits. Additional studies were conducted on rabbit aortic vascular smooth muscle rings containing functionally intact endothelium and in rings denuded of their endothelium. Acetylcholine (20 to 2000 ng/ml) neither constricted nor dilated control aortic rings without endothelium, and markedly dilated aortic rings with intact endothelium in a concentration dependent manner. In atherosclerotic aortic rings, acetylcholine constricted preparations without endothelium, and dilated rings with endothelium to a much lesser extent than that observed in control rings. Similar reductions in responsiveness occurred with adenosine diphosphate (ADP), another endothelium-dependent vasodilator, but not with iloprost, a nonendothelium-dependent dilator. No differences were observed in constrictor responses to norepinephrine. Aortae from atherosclerotic rabbits produced less prostacyclin in response to arachidonic acid than control aortae. These data point to an important role of the endothelium in modulating the vascular response to vasodilators in atherosclerotic rabbit arterial vessels.

Stimulation of prostacyclin production in blood vessels by the antithrombotic drug suloctidil

Suloctidil is a calcium antagonist with vascular relaxing activity and an antithrombotic agent: its antiplatelet action has been demonstrated in vivo, but is difficult to reproduce in vitro and the mechanism of this effect remains unknown. We have observed that suloctidil (10 microM) stimulated the release of prostacyclin (PGI2) from the rabbit aorta, the dog vena cava and the dog portal vein, in vitro. This effect could be explained by an increased mobilization of free arachidonic acid. Neither the inactive congener CP894S, nor the two calcium channel antagonists, verapamil and flunarizine, reproduced the stimulatory effect of suloctidil. Suloctidil acted selectively on the vascular endothelium: it stimulated the release of PGI2 from bovine aortic and human umbilical vein endothelial cells, but neither from the de-endothelialized rabbit aorta nor from the bovine aortic media. The stimulatory effect of suloctidil on the release of the platelet inhibitor PGI2 from the vascular endothelium might contribute to the known antiplatelet and antithrombotic activity of this drug.

Dipyridamole and vascular prostacyclin production

The action of dipyridamole on the vascular production of prostacyclin (PGI2) has been investigated. Dipyridamole (1-100 microM) did not induce a significant stimulation of PGI2 release in any of the following experimental models: rings of rabbit aorta, cultured endothelial cells from bovine aorta or human umbilical vein, cultured explants of bovine aortic smooth muscle. The activity of known stimuli of PGI2 release (ADP, suloctidil, serotonin) and the capacity of dipyridamole to inhibit adenosine uptake into endothelial cells were carefully checked. Pretreatment of the rabbit aorta with dipyridamole (10-100 microM) prolonged the transient stimulation of PGI2 release induced by mechanical deendothelialization: this effect was probably due to a partial protection of the cyclooxygenase against oxidative self-inactivation. Our largely negative results are consistent with the current theory that the antiplatelet action of dipyridamole is mediated by adenosine and not by PGI2.

Effects of dexamethasone on prostacyclin biosynthesis in rabbit mesothelial cells

We investigated whether glucocorticoids reduce the formation of arachidonic acid metabolites in a non myeloid cell type, the mesothelial cell, which is functionally and embryologically related to the vascular endothelial cell and which forms almost exclusively prostacyclin from arachidonic acid. Preincubation of rabbit mesothelial cells with 2.5 microM dexamethasone suppressed basal as well as bradykinin- or thrombin-stimulated prostacyclin biosynthesis. In further experiments bradykinin was selected as stimulus. The inhibition by dexamethasone was dose-dependent between 0.025 and 2.5 microM. The minimum contact period required for expression of this effect was 30 min and after a contact period of 60 to 120 min the inhibition reached a maximum, but was never complete. After 240 min, sufficient activity was secreted in the extracellular medium for inhibition of the prostacyclin formation in untreated cells. Experiments with cycloheximide were somewhat confused by its direct effects on prostacyclin biosynthesis, but still suggested that the anti-prostacyclin effect of dexamethasone required de novo protein biosynthesis. Our experiments indicate that glucocorticoids induce the formation of lipocortin-like factor(s) in non-phagocytic mesothelial cells, thereby suppressing the formation of prostacyclin, their main arachidonic metabolite.

Prostacyclin-stimulating drugs: new prospects

SKF 525-A (proadifen), a well-known inhibitor of drug metabolism and cytochrome P-450 activity, stimulated the release of prostacyclin (PGI2) from the rabbit aorta in vitro. The PGI2-stimulating activity of SKF 525-A was characterized by specific structural requirements: activity was abolished by the deletion of the terminal propyl chain and increased by its elongation into an isobutyl chain; chlorination of the phenyl rings increased the potency. SKF 525-A increased the production of PGI2 by cultured endothelial cells from bovine aorta and human umbilical vein, but had no effect on cultured smooth muscle from the bovine aortic media. In human platelets, SKF 525-A inhibited prostaglandin and thromboxane production induced by A23187, thrombin and ADP. Simultaneous stimulation of endothelial PGI2 and inhibition of platelet TxA2 represents an original pharmacological profile: SKF 525-A might thus constitute the prototype of a new class of antiplatelet drugs.

Prostacyclin production by the bovine aortic smooth muscle

It is well known that cultured aortic smooth muscle cells, the phenotype of which has modulated from contractile to synthetic, are able to release prostacyclin (PGI2). We have studied the release of PGI2 from cultured explants of bovine aortic media, which represent an homogeneous population of smooth muscle cells with a contractile phenotype. These explants released spontaneously huge amounts of PGI2, which was the major eicosanoid produced. PGI2 release was stimulated by serum and by serotonin. This experimental model seems useful to evaluate the contribution of smooth muscle to the biosynthesis of PGI2 by the arterial wall.

Production of eicosanoids by deendothelialized rabbit aorta: interaction between platelets and vascular wall in the synthesis of prostacyclin

Production of eicosanoids by deendothelialized aorta in response to continuous infusions of arachidonic acid and platelet suspensions was determined in a rabbit aorta perfusion model. 6-keto-PGF1 alpha production was stimulated by AA infusion in a dose-related manner. Infusion of AA at 4 micrograms/ml/min led to an initial production rate of 0.64 +/- 0.29 ng/min which gradually increased to 0.93 +/- 0.11 ng/min at the 20th min of infusion. When the concentration of AA infusion was increased to 10 micrograms/ml/min, 6-keto-PGF1 alpha production increased to 1.14 +/- 0.86 ng/min initially but declined with time. PGE2 production in response to AA 10 micrograms/min/ml was steady at around 5 ng/min while PGF2 alpha and TXB2 production were only slightly above the control. Perfusion of rabbit washed platelet suspensions at a rate of 3 X 10(8) plt/ml/min raised 6KPGF1 alpha production. The production was further increased when platelets were pretreated with 1-benzylimidazole (5 mM), along with a concurrent reduction in TXB2 release. Pretreatment of platelets with aspirin, on the other hand, abolished the increase in 6KPGF1 alpha production. Our data indicated that the vascular smooth muscle cells can efficiently utilize PGH2 produced by platelets to synthesize PGI2.