A. Van Coevorden

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.

P2-purinergic receptors in vascular endothelial cells: from concept to reality.

ATP exerts at least 2 actions on arterial endothelial cells: it stimulates the release of endothelium-derived relaxing factor, a still unidentified vasodilator, and of prostacyclin, a potent inhibitor of platelet aggregation. A study of agonist specificity indicates that these responses are mediated by P2-purinergic receptors. We have now demonstrated that in these cells, the P2-receptors are coupled to a phospholipase C hydrolysing phosphatidylinositol-bisphosphate and that this coupling involves a pertussis toxin-sensitive GTP-binding regulatory protein.

Cushing's syndrome with intermittent ectopic ACTH production.

A case of ectopic ACTH syndrome with intermittent secretion in a 72-yr-old woman is described. Plasma and urinary Cortisol levels were obtained at frequent intervals for a period of more than 10 months and varied erratically from the normal range to extremely high values. Nonsuppression by high doses of dexamethasone was documented during a period of hypersecretion. Normal circadian rhythmicity and normal responses to hypoglycemia were observed during an interval of dormance of the ectopic secretion. Hypokalemia did not develop. These findings, together with the occult nature of the primary tumor, resulted in unusual diagnostic difficulties. Liver masses were detected by echography and CT scan. Pathological examination of liver biopsies suggested a neuroendocrine tumor of foregut origin. While a multicentric primary apudoma secreting ACTH was a putative diagnosis, detailed and extensive microscopic post-mortem studies revealed a more likely primary tumor site in the pancreatic tail.

Stimulation of aortic smooth muscle prostacyclin by serotonin: role of distinct receptors in contractile and synthetic states.

It has been shown previously that serotonin stimulates the production of prostacyclin by bovine aortic smooth muscle cells in culture, via 5-HT2 receptors (Coughlin SR, Moskowitz MA, Antoniades HN, Levine L. Proc Natl Acad Sci USA 1981;78:7134-7138). These cells express a synthetic phenotype, whereas the majority of the smooth muscle cells in the media from adult arteries are in a contractile state. We have now compared 5-HT stimulated prostacyclin production in bovine aortic media explants, a preparation of contractile smooth muscle, with cultured smooth muscle cells derived from the explants. In the 1-10 microM range, serotonin stimulates the release of prostacyclin from the explants of bovine aortic media, cultured for a short period. In the presence of cocaine (30 microM), 1 microM was sufficient to produce a maximal effect. The stimulatory action of serotonin was sustained with time and did not induce a lasting desensitization. The effect of serotonin on the explants was inhibited only partially (+/- 30%) by ketanserin, a selective and potent 5-HT2 antagonist. It was mimicked by 5-carboxamido-tryptamine, a 5-HT1 agonist, but was only weakly inhibited by methiothepin, a 5-HT1 antagonist. As expected, in cultured smooth muscle cells, 5-carboxamido-tryptamine was only a weak agonist in stimulating prostacyclin production. In conclusion, it appears that the serotonin effect on prostacyclin production is mediated by different receptors in media explants from bovine aortic media and cultured cells obtained by outgrowth from these explants: a 5-HT2 receptor in the smooth muscle cells in culture and a receptor presenting some similarities with 5-HT1 receptors in the explants.

Stimulation of vascular prostacyclin by SKF525-A (proadifen) and related compounds☆

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 threshold concentration producing a detectable effect was 20 microM; the time course of SKF 525-A action exhibited particular features--progressive onset, long duration and slow reversibility--distinct from those of other stimuli (ADP, ionophore A23187 f.i.). The PGI2-stimulating activity of SKF 525-A was characterized by specific structural requirements: activity was abolished by the deletion of the terminal propyl group and increased by its elongation into an isobutyl group; chlorination of the phenyl groups 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. Stimulation of PGI2 release could be explained by an increased availability of free arachidonic acid, which was probably independent from cytochrome P-450 inhibition. 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-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.

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.

Control of Prostacyclin Production by Vascular Cells : Role of Adenine Nucleotides and Serotonin

The in vivo production of prostacyclin (PGI2) in man is normally very low. The use of specific analytical methods has shown that the concentration of PGI2 in peripheral blood is less than 3 pg/ml (10 pM), a concentration too low to inhibit platelets (Blair et al., 1982). The measurement of the main urinary metabolite of PGI2, prostaglandin 2,3-dinor-6-keto F1α, has allowed to estimate that the rate of PGI2 secretion into the circulation of normal man is ± 0.1 ng/kg × min, whereas studies with exogenous PGI2 suggest that infusion rates of 2–4 ng/kg × min are required to achieve the threshold for inhibition of platelet function (FitzGerald et al., 1981). Several diseases involving the intravascular activation of platelets are associated with an increased biosynthesis of PGI2 which might represent a compensatory mechanism : severe atherosclerosis of the lower limbs (FitzGerald et al., 1984), systemic sclerosis complicated by Raynaudfs phenomenon (Reilly et al., 1986), unstable angina, during the episodes of chest pain and acute myocardial infarction (Fitzgerald et al., 1986).