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.

Covalent binding of eicosanoids to platelet proteins

Following an incubation of washed human platelets with 14C-arachidonic acid, a small fraction of the radioactivity became tightly bound to the protein pellet. Three criteria suggested that it was actually a covalent binding: it was not removed by exhaustive extractions with solvents of various polarities, it was not dialysable against SDS-buffer and it corresponded to the labeling of several protein bands after SDS-polyacrylamide gel electrophoresis. The use of several pharmacological agents (indomethacin, eicosatetraynoic acid, dazoxiben, diamide) has allowed us to divide this binding into three components: the first one, independent from both cyclooxygenase and lipoxygenase, the second one dependent on cyclooxygenase products and finally the third one, dependent on lipoxygenase products.

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.

Metabolism of arachidonic acid in rabbit iris and retina

Both the iris and the retina of the rabbit released prostaglandins (PG) E2, F2 alpha, 6-keto-F1 alpha and thromboxane (Tx) B2, when incubated in vitro. PGE2 was the major cyclooxygenase product formed by each tissue. The kinetics of PGE2 release by the iris and the retina were similar: high initial output followed by a decline to a steady-state value. The production of PGE2 was inhibited by indomethacin and stimulated by ionophore A23187. The iris and the retina converted exogenous arachidonic acid into 12- and 15-hydroxy-eicosatetraenoic acid (HETE): inhibition by eicosatetraynoic acid (ETYA) indicated the involvement of lipoxygenase enzymes. This lipoxygenase activity was important relatively to cyclooxygenase in the retina, but was only a minor pathway in the iris. Leukotriene (LT) B4 was released by the iris and the retina in amounts smaller than PGE2, but compatible with a biological activity: ionophore A23187 stimulated LTB4 production in both tissues. Our data support the hypothesis that PGE2 and LTB4 could play a role in the initiation of ocular inflammation.

Prostacyclin production by the deendothelialized rabbit aorta

The acute effect of in vitro deendothelialization on the production of prostacyclin (PGI2) by the rabbit aorta has been investigated. The effectiveness of removing endothelium by rubbing it against filter paper or scraping it with a scalpel was demonstrated by scanning electron microscopy and en face examination after silver staining. Endothelium removal produced an immediate stimulation of PGI2 release, resulting in 408% of the control after rubbing and 367% of the control after scraping, during the first 30-min period of incubation. This increased production of PGI2 gradually declined over time to reach values similar to the control after 2h. At that time, the deendothelialized aorta was totally unresponsive to the stimuli that increase PGI2 release in the intact aorta (acetylcholine, ADP, ionophore A23187, and arachidonic acid). The enhanced production of PGI2 in the deendothelialized aorta was associated with an increased release of free arachidonic acid (353% of the control): in contrast with PGI2, this stimulation was maintained for at least 150 min. A transient exposure of the deendothelialized aorta to ibuprofen (250 microM) was followed by a rebound of PGI2 production, which was also prolonged by BW-755C (3-10 microM). In conclusion, removal of the endothelium triggered an immediate and sustained mobilization of free arachidonic acid in the rabbit aorta: the resulting increase of PGI2 production was short-lived, probably as a consequence of cyclooxygenase self-inactivation. Our results indicate that the subendothelium has a significant capacity to produce PGI2, but that this capacity is expressed only briefly.

Administration of dazoxiben, a selective thromboxane synthetase inhibitor, in the adult respiratory distress syndrome

Experimental studies suggest that thromboxane A2 could play a role in the pulmonary hypertension of the adult respiratory distress syndrome (ARDS). We therefore investigated the hemodynamic and gasometric effects of dazoxiben, a selective thromboxane synthetase inhibitor, in seven patients who had developed ARDS. The patients were studied for 120 minutes after a single intravenous bolus of 1.5 mg of dazoxiben per kilogram of body weight. During this period, there was no change in pulmonary hemodynamics, a moderate increase in arterial oxygen pressure, and a slight decrease in venous admixture. Therefore, administration of dazoxiben in patients with ARDS does not decrease pulmonary hypertension. This study does not support the role of thromboxane A2 as an important mediator in pulmonary hypertension in human ARDS, at least once the syndrome has been recognized.

Prostaglandin-9-ketoreductase activity in erythrocytes of normal and hypertensive subjects

The PGE2/PGF2 alpha balance, controlled in part by prostaglandin-9-ketoreductase, seems to be involved in the regulation of sodium excretion by the kidney. A decreased PGE2/PGF2 alpha ratio has been observed in the urine of hypertensive subjects. This suggests that an alteration of prostaglandin metabolism might be involved in the pathogenesis of essential hypertension. In order to test this hypothesis, prostaglandin-9-ketoreductase (PG-9-KR) and prostaglandin-15-dehydrogenase (PG-15-DH) activities were measured in erythrocytes of normotensive controls and patients with essential hypertension. The two enzyme activities were highly correlated in the two groups, supporting the hypothesis that they are alternate expressions of a single enzyme. These two enzyme activities were not significantly different in hypertensive subjects as compared to controls. Human essential hypertension does not appear to be linked to a generalized defect of prostaglandin catabolic enzymes.

Physiological concentrations of ADP stimulate the release of prostacyclin from bovine aortic endothelial cells

ADP (0.2-200 microM) stimulated the synthesis of prostacyclin (PGI2), as reflected by the release of 6-keto-prostaglandin F1 alpha (6-K-PGF1 alpha), in endothelial cells cultured from bovine aorta. This effect of ADP was mimicked by ATP, whereas AMP and adenosine were completely inactive. The release of 6-K-PGF1 alpha triggered by ADP was rapid and onset (within 5 min), transient (10 min) and followed by a period of refractoriness to a new ADP challenge. Growing and confluent cells were equally responsive to ADP. ADP stimulated the release of free arachidonic acid from the endothelial cells. ADP could thus exert two opposite actions on platelet aggregation in vivo: a direct stimulation and an inhibition mediated by PGI2. This last action might contribute to limit thrombus formation to areas of endothelial cell damage.

Lack of prostaglandin involvement in the mitogenic effect of TSH on canine thyroid cells in primary culture

The production of prostaglandin E2 (PGE2) by cultured dog thyroid cells was high in a serum-containing medium and low in a serum-free, completely defined medium. Thyrotropin (TSH) and epidermal growth factor (EGF), two mitogenic factors for these cells, did not stimulate PGE2 release. Indomethacin, at a concentration which completely inhibited PGE2 production, had no effect on thyroid cell multiplication and DNA synthesis stimulated by TSH and EGF. It is concluded that cyclooxygenase products are not involved in the proliferation of canine thyroid cells and its control by TSH.

Cholinergic stimulation of vascular prostacyclin synthesis

The production of prostacyclin by rings of rabbit aorta was assessed by the radioimmunoassay of 6-keto-PGF1 alpha. In steady-state conditions, the rings released 11 ng 6-K-PGF1 alpha per 100 mg tissue in 30 min. Acetylcholine increased this output: a significant effect was detected at 1 microM and at 10 microM the amplitude of stimulation was 10-fold. The production of PGE2 and PGF2 alpha was also increased, but to a lesser extent. The stimulatory action of acetylcholine was mimicked by carbamylcholine and inhibited by atropine; it was abolished in a calcium-free medium. Dog and rat aorta also produced more 6-K-PGF1 alpha in response to cholinergic agonists. A short rubbing of the intimal surface of the aorta removed the layer of endothelial cells and completely abolished the cholinergic effect. It is concluded that in the aorta, cholinergic agonists, acting on a muscarinic receptor, stimulate the production of prostacyclin by endothelial cells.

Stimulation of vascular prostacyclin synthesis by extracellular ADP and ATP

Extracellular ADP and ATP stimulated the synthesis of prostacyclin - as reflected by the release of 6-keto-PGF1 alpha - in the rabbit aorta, the rabbit pulmonary artery and the rat aorta. A doubling of 6-keto-PGF1 alpha output was produced by 3 microM ADP. Adenosine had no effect and the stimulation by ADP was blocked by quinidine, but not by theophylline. This stimulation was abolished by indomethacin and lost after mechanical removal of the endothelium. Stimulation of vascular prostacyclin synthesis by ADP released from aggregating platelets could help localize thrombus formation to areas of vascular damage.

Metabolic disposition of prostaglandin E1 in man

Metabolism of [17, 18-3H]prostaglandin E1 was investigated in three healthy male volunteers during intravenous infusion. The infusion rate was 5.0 ng/kg per min. Blood samples were obtained before the end of the infusion as well as 5, 10, 20, 40, 90 and 180 min afterwards; urine and feces were collected until 96 and 72 h, respectively, after the experiment. All samples were analyzed for radioactivity. Urine was further chromatographed, including by high-pressure liquid chromatography, and subsequently analyzed by gas chromatography-mass spectrometry. Radioactivity in plasma rapidly declined during the first 10 min after termination of the infusion, and then was eliminated exponentially with a mean half-life of 181 min, probably reflecting slow excretion of one or more metabolite. 12% of the administered radioactivity could be recovered from feces and 88% from urine. From the radioactive material obtained from urine the following metabolites could be identified (each number represents data of one volunteer): 7 alpha-hydroxy-5,11-diketotetranor-prostane-1,16-dioic acid (10.4, 20.4 and 30.1%), 7 alpha-hydroxy-5,11-diketotetranor-prostanoic acid (8.2, 6.9 and 9.3%), 5 alpha, 7 alpha-dihydroxy-11-ketotetranor-prostane-1,16-dioic acid and its delta-lactone (together accounting for 4.1, 2.1 and 3.8%).

Effects of prostaglandins F alpha on dog thyroid cyclic AMP level and function

Prostaglandins F1 alpha and F2 alpha, at high concentrations (greater than or equal to 28 microM) enhanced cyclic AMP accumulation in dog thyroid slices. At lower concentrations, they inhibited the cyclic AMP accumulation induced by thyrotropin (TSH), prostaglandin E1, and cholera toxin. This effect was rapid in onset and of short duration, calcium-dependent and suppressed by methylxanthines. Prostaglandin F alpha also inhibited TSH-induced secretion and activated iodide binding to proteins. These characteristics are similar to those of carbamylcholine action, except that prostaglandins F did not enhance cyclic GMP accumulation. The effect of prostaglandin F alpha was not inhibited by atropine, phentolamine and adenosine deaminase and can therefore not be ascribed to an induced secretion of acetylcholine, norepinephrine or adenosine. It is suggested that prostaglandins F act by increasing influx of extracellular Ca2+. Arachidonic acid also inhibited the TSH-induced cyclic AMP accumulation. However this effect was specific for TSH, it was enhanced in the absence of calcium and was not inhibited by methylxanthines or by indomethacin at concentrations which completely block its conversion to prostaglandin F alpha. Arachidonic acid action is sustained. This suggests that arachidonic acid inhibits thyroid adenylate cyclase at the level of its TSH receptor and that this effect is not mediated by prostaglandin F alpha or any other cyclooxygenase product.

Novel transformations of arachidonic acid by the rat thyroid in vitro

The transformation of arachidonic acid by the rat thyroid in vitro has been investigated. At least two metabolites have been partially characterized: they differed from known metabolites of arachidonic acid in terms of retention volume in liquid chromatography, ultraviolet spectrophotometry and pharmacology (formation not inhibited by indomethacin and enhanced by eicosatetraynoic acid). The analysis by chemical ionization mass spectrometry suggested that these metabolites might be diketo-monohydroxy- and monoketo-dihydroxy-compounds. The conversion of arachidonic acid into these compounds was stimulated by ionophore A23187, decreased by the peroxidase inhibitor methimazole and potentiated by iodide, suggesting that this pathway is under the control of Ca2+ and of a peroxidase product.

Stimulus-specificity of the chemotactic deactivation of human neutrophils by lipoxygenase products of arachidonic acid

Preincubation of human neutrophils with chemotactic concentrations of 5(S)-hydroxy-eicosatetraenoic acid (5-HETE) or 5(S), 12(R)-dihydroxy-6,14 cis-8,10 trans-eicosatetraenoic acid (leukotriene B4) induces a state of preferential chemotactic unresponsiveness to the homologous factor, termed deactivation, and less suppression of the responses to other chemotactic stimuli. The ratio of the concentrations required for maximal chemotactic deactivation of neutrophils to that which stimulates chemotaxis optimally is greater for 5-HETE and leukotriene B4 than for peptide and protein factors. In contrast to other chemotactic factors, 5-hydroperoxy-eicosatetraenoic acid (5-OOHETE) induces neutrophil chemotactic deactivation that is independent of the nature of the subsequent stimulus and is more slowly reversible after elimination of the fluid-phase deactivating factor. The unique characteristics of the chemotactic deactivation of human neutrophils by 5-OOHETE may be attributable in part to its endogenous metabolism to potent deactivating factors or to covalent derivatization of subcellular structures of the neutrophils by the highly reactive 5-OOHETE.

In vitro stimulation of prostaglandin synthesis in the rat pancreas by carbamylcholine, caerulein and secretin

Rat pancreas pieces spontaneously released PGE2 (2.3 ng/100 mg x 45 min) and PGF2 alpha (7.6 ng/100 mg x 45 min). This release corresponds probably to a neo-synthesis since it was abolished by indomethacin. Carbamylcholine (greater than or equal to 10 microM), caerulein (greater than or equal to 10 nM) and secretin (greater than or equal to 10 nM) stimulated the release of PGE2 and PGF2 alpha: the concentrations of stimulators required to increase PGs release were thus much higher than those which trigger enzyme secretion. Atropine specifically inhibited the cholinergic stimulation, whereas indomethacin blocked the stimulatory effects of all secretagogues. Stimulation of PGE2 and PGF2 alpha release was reduced in a Ca++-free medium, abolished by EGTA and mimicked by the ionophore A23187, underscoring the crucial role of Ca++ in the regulation of PGs synthesis by the pancreas. Neither PGE2 nor PGF2 alpha stimulated enzyme secretion in this system and indomethacin did not inhibit the secretory effect of carbamylcholine. Increased synthesis of prostaglandins in response to pancreatic secretagogues does not appear to be involved in the process of enzyme secretion.

Iodination of docosahexaenoic acid by lactoperoxidase and thyroid gland in vitro: formation of an lodolactone

In the presence of iodide, hydrogen peroxide and lactoperoxidase, docosahexaenoic acid (22:6 omega 3) was converted into iodinated compounds. The major product was identified as 5-iodo-4-hydroxy-7, 10, 13, 16, 19-docosapentaenoic acid, gamma-lactone, on the basis of 125 I incorporation, mass spectrometry, chemical modifications and proton nuclear magnetic resonance spectroscopy. Iodolactonization of docosahexaenoic acid occurred in the rat thyroid in vitro and was inhibited by the peroxidase inhibit or methimazole. These data indicate that formation of an idolactone constitutes one pathway of docosahexaenoic acid metabolism which could be expressed in tissues containing an iodide peroxidase.

Lactoperoxidase-catalyzed iodination of arachidonic acid: formation of macrolides

In the presence of iodide and hydrogen peroxide, lactoperoxidase catalyzed the conversion of arachidonic acid into several iodinated products; the major one was previously identified as an iodo-delta-lactone. Two minor and less polar products have now been characterized as 15-iodo-14-hydroxyeicosatrienoic acid, omega-lactone and 14-iodo-15-hydroxyeicosatrienoic acid, omega-lactone, on the basis of 125I incorporation, mass spectrometry, proton magnetic resonance spectroscopy and chemical modifications.

Theoretical analysis of the consequences of cyclic nucleotide phosphodiesterase negative co-operativity. Amplification and positive co-operativity of cyclic AMP accumulation

Most tissues contain multiple forms of cyclic nucleotide phosphodiesterases (3':5'-cyclic-nucleotide 5' nucleotidohydrolase, EC 3.1.4.17). Consequently, in most, if not in all, tissues, substrate-velocity curves deviate from Michaelian kinetics and exhibit an apparent negative co-operativity. We have studied the possible theoretical consequences of this property on the quantitative features of cyclic AMP accumulation in response to activation of adenylate cyclase. Negative co-operativity of phosphodiesterases tends to generate a "positively co-operative" cyclic AMP accumulation curve. It amplifies the stimulation of cyclic AMP accumulation as compared with the stimulation of cyclic AMP synthesis. It enhances the sensitivity of cyclic AMP accumulation to slight variation of phosphodiesterase maximal velocity. It tends to shift the cyclic AMP accumulation curve to higher concentrations of stimulator as compared with the adenylate cyclase activation curve. This accounts for much of the data in the literature of hormonal effects on phosphodiesterase activity. It shows that the characteristics of cyclic nucleotide phosphodiesterases are as important as those of adenylate cyclase in determining the response of the system.

Transformation of arachidonic acid into an iodolactone by the rat thyroid

In the presence of iodide and hydrogen peroxide, lactoperoxidase, an enzyme model for thyroid peroxidase, catalyzed the conversion of arachidonic acid into several iodinated products. The major product was identified as 6-iodo-5-hydroxy-eicosatrienoic acid, delta-lactone (iodolactone), on the basis of 125I incorporation, mass spectrometry, proton magnetic resonance spectroscopy, and chemical modifications. Using this compound as a standard, two methods were developed to establish and quantitate the production of iodolactone by the rat thyroid in vitro: 125I labeling followed by reversed phase high pressure liquid chromatography and combined gas chromatography-mass spectrometry. Addition of iodide and arachidonic acid to rat thyroid lobes resulted in the formation and release of the iodolactone, which was inhibited by methimazole. These data suggest that peroxidases capable of oxidizing halides could provide a new pathway of arachidonic acid metabolism, besides cyclooxygenase and lipoxygenases.