Comparisons of prostaglandin vasoactive effects and interactions in the in vivo microcirculation of the rat urinary bladder

A combination of techniques for in vivo transillumination, topical application of vasoactive agents, and direct microscopic observation of microcirculatory responses was utilized to evaluate the vasomotor actions of prostaglandins (PGs) E₁, E₂, F_1α, F_2α, A₁, and A₂ on rat urinary bladder arterioles and venules. The effects of PGE₁ and histamine (HIS) on arteriolar responsiveness to norepinephrine (NE), serotonin (5-HT), and PGF_2α were measured. Histochemical studies were completed to determine the primary site of prostaglandin (PG) metabolic deactivation in the urinary bladder. Arteriolar dilatation occurred with HIS, PGE₁, PGE₂, PGA₁, PGA₂, and PGF_1α, all of which (with the exception of HIS) demonstrated significant dose-related responses. Overall, PGE₁ and PGE₂ were of greatest potency. Significant dose-related arteriolar constriction occurred with NE > PGF_2α > 5-HT (in order of decreasing potency). HIS, PGE₁, PGE₂, and PGA₁ produced significant venular dilatation; PGE₁ and HIS were dose related. Only NE resulted in significant venoconstriction. Arteriolar responsiveness to NE and PGF_2α decreased after pretreatment with PGE₁ but was unchanged by HIS pretreatment, whereas application of 5-HT following pretreatment with PGE₁ or HIS produced equivalent levels of arteriolar constriction. The primary site of deactivation of PGE₁ was histochemically localized to bundles of smooth muscle fibers in the muscular coat of the rat urinary bladder wall.     

Prostaglandins of the one,two, and three series affect blood pressure in the American bullfrog, Rana catesbeiana

The effects of prostagandins (PGs), and three potential prostaglandin precursors, were studied on blood pressure and heart rate of the American bullfrog, Rana catesbeiana. Bullfrogs were chronically cannulated with a T cannula in the right sciatic artery. The mean systemic arterial blood pressure (SAP) prior to infusion was 20.4 ± 1.1 mm Hg. Mean preinfusion systolic and diastolic pressures were 23.9 ± 1.4 and 17.1 ± 0.9 mm Hg, respectively. Mean preinfusion heart rate was 41.1 ± 0.5 beats/min. Of the PGs tested, PGI₂ was a potent hypotensive agent, with effects at 0.03 μg/kg bw. PGE₂ was more potent than PGE₃, and PGE₁. PGA₁ and PGA₂ were the least potent, and were ineffective at doses below 100 μg/kg bw. PGF_2α was the most potent hypertensive agent tested, with thromboxane B₂ less potent. All compounds tested elevated heart rate, with PGE₂ the most effective. The prostaglandin precursors, eicosatrienoic acid, arachidonic acid, and eicosapentaenoic acid (2000 μg/kg bw) all decreased blood pressure by approximately 25%. The decrease was attenuated by indomethacin (4 mg/kg bw). These results indicate that the bullfrog utilizes all three hypotensive activity. The ability of the bullfrog to utilize several substrates makes it a good choice for comparative studies on prostaglandin synthesis.     

C-reactive protein impairs coronary arteriolar dilation to prostacyclin synthase activation: Role of peroxynitrite

Endothelium-derived vasodilators, i.e., nitric oxide (NO), prostacyclin (PGI₂) and prostaglandin E₂ (PGE₂), play important roles in maintaining cardiovascular homeostasis. C-reactive protein (CRP), a biomarker of inflammation and cardiovascular disease, has been shown to inhibit NO-mediated vasodilation. The goal of this study was to determine whether CRP also affects endothelial arachidonic acid (AA)-prostanoid pathways for vasomotor regulation. Porcine coronary arterioles were isolated and pressurized for vasomotor study, as well as for molecular and biochemical analysis. AA elicited endothelium-dependent vasodilation and PGI₂ release. PGI₂ synthase (PGI₂-S) inhibitor trans-2-phenyl cyclopropylamine blocked vasodilation to AA but not to serotonin (endothelium-dependent NO-mediated vasodilator). Intraluminal administration of a pathophysiological level of CRP (7 μg/mL, 60 min) attenuated vasodilations to serotonin and AA but not to nitroprusside, exogenous PGI₂, or hydrogen peroxide (endothelium-dependent PGE₂ activator). CRP also reduced basal NO production, caused tyrosine nitration of endothelial PGI₂-S, and inhibited AA-stimulated PGI₂ release from arterioles. Peroxynitrite scavenger urate failed to restore serotonin dilation, but preserved AA-stimulated PGI₂ release/dilation and prevented PGI₂-S nitration. NO synthase inhibitor L-NAME and superoxide scavenger TEMPOL also protected AA-induced vasodilation. Collectively, our results suggest that CRP stimulates superoxide production and the subsequent formation of peroxynitrite from basal released NO compromises PGI₂ synthesis, and thus endothelium-dependent PGI₂-mediated dilation, by inhibiting PGI₂-S activity through tyrosine nitration. By impairing PGI₂-S function, and thus PGI₂ release, CRP could promote endothelial dysfunction and participate in the development of coronary artery disease.     

Prostanoids and the Cough Reflex

Prostanoids such as prostaglandin (PG) D₂, PGE₂, PGF_2α, prostacyclin (PGI₂), and thromboxane (Tx) A₂ act via five classes of receptors named DP, EP, FP, IP, and TP, respectively, and mediate a diverse range of physiological effects. Prostanoids are commonly associated with many diseases as a proinflammatory mediator; however, in the lung, prostanoids, particularly PGE₂, seem to have a protective role. Inhaled PGE₂ has been shown to be anti-inflammatory and a bronchodilator but causes cough. This has hindered the development of prostanoids for the treatment of airway inflammatory diseases. We discuss here the extensive research into the role of prostanoids in the airways and their modulation of the cough reflex.     

Current concepts for a drug-induced inhibition of formation and action of thromboxane A2

Summary Urinary and plasma metabolites of thromboxane A₂ (TxA₂) indicate an increased TxA₂ synthesis in a number of diseases, whereby TxA₂ is assumed to contribute to the underlying pathomechanisms by its profound effects on platelet aggregation and smooth muscle contraction. In some clinical situations the increment in TxA₂ biosynthesis is accompanied by an increased formation of prostacyclin (PGI₂) which is one of the most potent inhibitors of platelet activation and smooth muscle contraction. Therefore, drugs are being developed which suppress the formation or action of TxA₂ without interfering with its functional antagonist PGI₂.Low doses of acetylsalicylic acid (ASA) preferentially inhibit cyclooxygenase activity in platelets and the synthesis of TxA₂ in vivo. However, neither low doses (approximately 300 mg/day) nor very low doses spare the formation of PGI₂ completely. Despite its limited selectivity, very low dose ASA (approximately 40 mg/day) provides an attractive perspective in TxA₂ pharmacology.Although thromboxane synthase inhibitors selectively suppress TxA₂ biosynthesis PGH₂ can accumulate instead of TxA₂ and substitute for TxA₂ at their common TxA₂/PGH₂ receptors. Thromboxane synthase inhibitors can only exert platelet-inhibiting and vasodilating effects if PGH₂ rapidly isomerizes to functional antagonists like PGI₂ that can be formed from platelet-derived PGH₂ by the vessel wall.TxA₂/PGH₂ receptor antagonists provide a specific and effective approach for inhibition of TxA₂. These inhibitors do not interfere with the synthesis of PGI₂ and other prostanoids but prevent TxA₂ and PGH₂ from activating platelets and inducing smooth muscle contractions. Most of the available TxA₂/PGH₂ receptor antagonists produce a competitive antagonism that can be overcome by high agonist concentrations. Since in certain disease states very high local TxA₂ concentrations are to be antagonized, non-competitive receptor antagonists may be of particular interest. Some recent TxA₂/PGH₂ receptor antagonists produce such a non-competitive type of inhibition due to their low dissociation rate constant. As a consequence, agonists like TxA₂ or PGH₂ only reach a hemiequilibrium state at their receptors, previously occupied by those antagonists.A combination of a thromboxane synthase inhibitor with a TxA₂/PGH₂ receptor antagonist presents a very high inhibitory potential that utilizes the dual activities of the synthase inhibitor to increase PGI₂ formation and of the receptor antagonist to antagonize PGH₂ and TxA₂. Such combinations or dual inhibitors, combining both moieties in one compound, prolong the skin bleeding time to a greater extent than thromboxane synthase inhibitors and even more than low dose ASA or TxA₂/PGH₂ receptor antagonists.     

Effects of third ventricle injection of prostaglandins on gonadotropin secretion in goldfish,Carassius auratus

The effects of injection of prostaglandin (PG) E₁, PGE₂, and PGF_2α into the third ventricle on serum gonadotropin (GTH) concentrations in the goldfish were tested. Blood samples were taken at 30 min postinjection for radioimmunoassay of serum GTH. PG dosages of 0.5 and 1.0 μg were ineffective. However, PGE₂ and PGF_2α at the 2.0-μg dosage significantly suppressed serum GTH. PGE₁ at a 2.0-μg dosage had no effect. There were no effects on serum GTH when 2.0 μg of PGE₁, PGE₂, or PGF_2α were injected intraperitoneally. The results indicate that PGE₂ and PGF_2α suppress gonadotropin secretion by some action, presumably on the hypothalamus. However, action of PGE₂ and PGF_2α by diffusion from the site of injection to some other brain site or the pituitary cannot be eliminated in the present study.     

Binding and inactivation of prostacyclin (PGI2) by human erythrocytes

Summary . Prostacyclin (PGI₂), the most potent inhibitor of platelet aggregation known, is rapidly hydrolysed in aqueous solution at neutral pH to its inactive derivative 6-keto-PGF_1α. In previous studies (Willems et al, 1979), we found that PGI₂ is stabilized by plasma. Yet, PGI₂ is rapidly inactivated in vivo. These findings prompted us to study the fate of PGI₂ when incubated in whole human blood. After 20 min of incubation (at 37°C, pH 7·8), 29 ± 10% of the amount of PGI₂, added to whole blood, remained in the supernatant plasma whereas, under the same conditions, 80 ± 3% and 86 ± 2% were recovered when PGI₂ was added to platelet-rich plasma or cell-free plasma, respectively. When PGI₂ was incubated with washed erythrocytes resuspended in plasma, 20 ± 15% of the added PGI₂ remained in the supernatant after 10 min of incubation. These findings indicate that PGI₂, when incubated with whole blood, is preferentially bound to erythrocytes. To study the kinetics of binding in more detail, [³H]PGI₂ was incubated with washed erythrocytes resuspended in plasma. The binding was time- and concentration-dependent. The observed binding of label represented binding of [³H]PGI₂, because (1) acid-treated label did not bind to erythrocytes; (2) no substantial binding of [³H]6-keto-PGF_1α occurred, and (3) changes in the specific activity of the PGI₂ preparation did not alter the binding percentage of labelled PGI₂. The binding of [³H]PGI₂ was not influenced by PGE₁ or 6-keto-PGF_1α. Repeated incubations of erythrocytes with PGI₂ revealed that PGI₂ was rapidly degraded into a biologically inactive non-binding substance, presumably 6-keto-PGF_1α. Binding and metabolism of PGI₂ by erythrocytes may explain the apparent instability of PGI₂ in whole blood and provides an explanation for the rapid loss of the biological effects of PGI₂ on termination of the infusion.     

Synthesis of prostaglandins by cultured rat hearts myocytes and cardiac mesenchymal cells

Ischemia, trauma and hormonal stimulation elicit the release of prostaglandins (PGs) from the heart. Although PGI₂ is synthesized by coronary arteries, the capacities for PG synthesis of individual types of cells comprising the heart have not been elucidated. Accordingly, synthesis of prostaglandins by cultured rat cardiac myocytes and mesenchymal cells was evaluated by radiochromatography of products obtained by incubating cells with [1-¹⁴C]arachidonate, and verified by assessing the effects of cell incubation medium on platelet aggregation. Cultured mesenchymal cells synthesized PGs E₂, F_2α and 6-keto-F_1α, a metabolite of PGI₂ (2076 ± 183, 1284 ± 158 and 1194 ± 152 dpm/mg protein/30 min, respectively). Medium from mesenchymal cells inhibited platelet aggregation, an effect abolished by preincubating the cells with indomethacin, further indicating that these cells synthesized PGI₂. Cardiac myocytes synthesized PGE₂ and PGF_2α (952 ± 227 and 287 ± 104 dpm/mg protein/30 mins, respectively), but no PGI₂. Medium from myocytes did not inhibit platelet aggregation. Prostaglandins D₂, A₂ and thromboxane were not synthesized by either type of cell. Thus, PGI₂ is synthesized by cardiac mesenchymal cells and the hitherto uncharacterized sources of PGE₂ and PGF_2α found in coronary sinus effluent may include cardiac myocytes as well as mesenchymal cells.     

Effects of prostacyclin (PGI2) and adenosine (ASN) on total and regional blood flow of isolated,collateralized dog hearts

Summary Dogs were subjected to chronic occlusion of the left circumflex and the right coronary artery by ameroid-type constrictors 4–5 weeks before the experiments. The hearts were isolated, fibrillated and perfused with blood (100 mmHg) from a support dog. Total and regional myocardial blood flow as well as peripheral coronary pressure (circumflex artery) were determined before and during infusion of PGI₂ and ASN into the isolated hearts. Both drugs increased total and regional blood flow to all parts of the myocardium in a dose-dependent manner, PGI₂ being 2–3 times more potent than ASN. The perfusion pressure for the collateral-dependent myocardium, the peripheral coronary pressure, decreased following high doses of both drugs. For PGI₂ a dose level existed where total flow was increased, while peripheral coronary pressure remained unaffected. After termination of the infusion of PGI₂, peripheral coronary pressure rose above predrug level with the total blood flow still being elevated. These findings indicate a PGI₂-induced dilatation of collateral vessels. In general, the hemodynamic profile within the isolated hearts and the support dogs was characterized by the pronounced vasodilatory effects of PGI₂ on arterioles. However, these effects did not deteriorate further the nonhomogenous blood flow distribution in the collateralized portions of the myocardium.The findings suggest that PGI₂ is not a specific coronary vasodilator.     

Effects of prostaglandin H2, prostaglandin E2, and arachidonic acid on parathyroid hormone and antidiuretic hormone activation of rat kidney adenylate cyclase

These experiments examine interactions of arachidonic acid; the substrate for prostaglandin cyclooxygenase, prostaglandin (PG)H₂, a key endoperoxide intermediate in prostaglandin synthesis; and prostaglandin (PG)E₂, an important prostaglandin produced within the kidney; with adenylate cyclase activity in renal cortex, outer medulla, and inner medulla. In addition, the effects of arachidonic acid, PGH₂, and PGE₂ on parathyroid hormone (PTH) activation of adenylate cyclase in cortex, and of antidiuretic hormone (ADH) activation of that enzyme in outer and inner medulla are examined. Arachidonic acid elicited a concentration-dependent inhibition of basal and PTH-stimulated adenylate cyclase activity in renal cortex. Concentration-dependent inhibition by arachidonic acid of basal and ADH-stimulated adenylate cyclase activity was observed in outer and inner medulla. PGH₂ inhibited basal activity in all three areas of the kidney. There was also inhibition by PGH₂ of medullary ADH and cortical PTH stimulation. PGE₂ stimulated adenylate cyclase in all three areas. PGE₂ had no effect upon PTH stimulation in cortex and was additive with ADH in outer and inner medulla. PGE₂ stimulation was inhibited by arachidonic acid, and this inhibition seemed competitive. Inhibition by both arachidonic acid and PGH₂ was not destructive. Experiments with [1-¹⁴C]arachidonic acid and indomethacin suggest that the inhibition by arachidonic acid was actually mediated by arachidonic acid and not a metabolite. Both PGH₂ and arachidonic acid inhibition was independent of phosphodiesterase. This activation by product, PGE₂, and inhibition by its precursors, arachidonic acid and PGH₂, provide a possible mechanism by which the prostaglandin system could modulate adenylate cyclase responsiveness to hormonal activation.     

Repeated bronchoconstriction attenuates the cough response to bronchoconstriction in naïve guinea pigs

BackgroundCough variant asthma (CVA) is recognized as a precursor of bronchial asthma (BA). However, the cough response to bronchoconstriction differs between these similar diseases. Repeated bronchoconstriction and the resulting imbalance of endogenous lipid mediators may impact the cough response.MethodsWe investigated the influence of repeated bronchoconstriction on the cough response to bronchoconstriction using naïve guinea pigs. Bronchoconstriction was induced for 3 consecutive days and changes in the cough response and lipid mediators, such as PGE₂, PGI₂, and cysteinyl-LTs (Cys-LTs), in BAL fluid (BALF) were assessed. We investigated the effect of endogenous PGI₂ on the cough response by employing a PGI₂ receptor antagonist. In order to investigate the cough response over a longer period, we re-evaluated the cough response 2 weeks after repeated bronchoconstriction.ResultsThe number of coughs induced by bronchoconstriction were significantly decreased by repeated bronchoconstriction. The levels of PGE₂, PGI₂, and Cys-LTs, and the ratio of PGI₂/PGE₂ were significantly increased, following repeated bronchoconstriction. This decrease in the cough response was suppressed by pretreatment with a PGI₂ receptor antagonist. Two weeks after repeated bronchoconstriction, the cough response returned to the same level as before repeated bronchoconstriction along with a concomitant return of lipid mediators, such as PGE₂, PGI₂, and Cys-LTs and the ratio of PGI₂/PGE₂.ConclusionsOur results suggest that repeated bronchoconstriction and the resulting imbalance of endogenous lipid mediators contribute to the difference in cough responses to bronchoconstriction in CVA and BA.     

Prostacyclin analogs stimulate receptor-mediated cAMP synthesis and ATP release from rabbit and human erythrocytes

Objectives: The purpose of this study was to establish that the prostacyclin (PGI₂) receptor (IP receptor) is present on rabbit and human erythrocytes and that its activation stimulates cyclic adenosine monophosphate (cAMP) synthesis and adenosine triphosphate (ATP) release. Methods: The effect of incubation of erythrocytes with the active PGI₂ analogs, iloprost or UT‐15C, on cAMP levels and ATP release was determined in the absence and presence of the IP receptor antagonist, CAY10441. Western analysis was used to determine the presence of the IP receptor on isolated membranes. To establish that effects of PGI₂ analogs were not due to prostaglandin E₂(PGE₂) receptor activation, the effect of PGE₂ on cAMP levels and ATP release was determined. Results: Rabbit and human erythrocytes possess IP receptors. Iloprost and UT‐15C stimulated increases in cAMP and ATP release that were prevented by the IP receptor antagonist, CAY10441. PGE₂ did not stimulate cAMP accumulation or ATP release and did not inhibit iloprost‐induced increases in cAMP. Conclusions: This study establishes that the IP receptor is present on rabbit and human erythrocytes and that its activation results in increases in cAMP and ATP release. These results suggest a novel mechanism by which PGI₂ and its active analogs, when administered pharmacologically, could produce vasodilation.     

Regulation of Prostacyclin Generation by Angiotensin Converting Enzyme Related Substances in Cultured Human Vascular Endothelial Cells

The regulation of prostacyclin (PGI₂) generation by angiotensin I-converting enzyme (ACE) related substances was investigated using cultured human vascular endothelial cells. Angiotensin I (AI) or bradykinin (BK) increased PGI₂ generation and ACE activity, while the ACE inhibitor, captopril decreased both of them, and angiotensin II (AII) did not show any effect. The increasing rate of PGI₂ generation induced by AI or BK was not affected by the pretreatment with captopril. These results suggest that the accumulation of AI or BK via the inhibition of ACE by captopril did not cause the enhancement of PGI₂ generation. Rather, it was proposed that the enhanced PGI₂ generation by AI or BK might be regulated by ACE activation derived from these substances, as an autoregulation mechanism.     

Role of prostanoids in experimental duodenal ulcer in rat

The purposes of this study were to determine whether inhibition of cyclooxygenase is a mechanism by which cysteamine and mepirizole produce duodenal ulcers, identify qualitative or quantitative differences in prostanoid production between gastric mucosa and duodenum, and determine whether differences in cyclooxygenase sensitivity to inhibition by aspirin exist between these two tissues. In fed female rats, gastric mucosal prostaglandin E₂ (PGE₂) and prostacyclin (PGI₂) generation was 235±25 and 832±40 ng/g/min, respectively, whereas full-thickness duodenal PGE₂ and PGI₂ generation was 665±46 and 662±49 ng/g/min, respectively. Over an intraperitoneal dose range of 0– 25 mg/kg, aspirin-induced cyclooxygenase inhibition was dose-dependent and similar for the two tissues. Duodenal ulceration (16.7 mm²) produced by cysteamine, 425 mg/kg, was associated with a 46% reduction in duodenal PGE₂ generation, while having no effect on PGI₂ generation; however, cysteamine, 213 mg/kg, produced no visible duodenal mucosa injury yet reduced duodenal PGE₂ generation 39% compared to control values. In fed male rats, gastric mucosal PGE₂ and PGI₂ generation was 179± 18 and 813± 61 ng/g/min, respectivley, whereas duodenal PGE₂ and PGI₂ generation was 321± 27 and 454± 38 ng/g/min, respectively. Duodenal ulceration (7.7 ± 2.3 mm²) produced by oral mepirizole was associated with a 63% reduction in duodenal PGE₂ generation compared to control values, while having no effect on PGI₂ generation. Subcutaneous aspirin, 100 mg/kg, which reduced duodenal PGE₂ generation to a greater degree than either ulcerogen, given in conjunction with pentagastrin, did not produce visible duodenal ulceration. It therefore seems unlikely that reduced PGE₂ generation within the duodenum is the primary mechanism of gross injury associated with these two ulcerogens.Supported by grant AM 17328 from NIADDK.     

Differential Effect of Prostaglandins on Gallstone-Free and Gallstone-Containing Human Gallbladder

Nonsteroidal antiinflammatory drugs, inhibitors of prostaglandin synthesis, have different effects on gallbladder contractility in normal and diseased human gallbladders in vivo. We investigated this differential effect by comparing the effects of prostaglandins PGE₂ and PGF₂, the thromboxane A₂ mimetic U46619, and PGI₂ on in vitro contractility in gallstone-free and gallstone-containing human gallbladders. Isometric tension was measured in gallbladder muscle strips mounted in organ baths. EC₅₀ was calculated for each agonist. The rank order of potency in gallstone-free gallbladders was PGE₂ > CCK > U46619 > PGF₂ and in gallstone-containing gallbladders was U46619 > PGE2 > CCK > PGF₂. PGI₂ produced contraction of gallstone-free gallbladder and relaxation of gallstone-containing gallbladder in the basal state. Further, PGI₂ produced no relaxation in gallstone-free muscle strips precontracted with CCK, but significant relaxation in CCK precontracted gallstone-containing strips. PGE₂, PGF₂, and U46619 are potent contractors of gallstone-free and gallstone-containing gallbladders, whereas PGI₂ relaxes only gallstone-containing gallbladders. Since gallbladders containing cholesterol-supersaturated bile produce increased PGI₂, this PGI₂-induced relaxation may be a determinant of the impaired gallbladder motility of gallstone disease.     

Inhibition of histamine-sensitive adenylate cyclase from guinea pig fundic gastric mucosa by arachidonic acid and by an analog of prostaglandin endoperoxide PGH2

The effects of prostaglandin precursors, namely an analog of prostaglandin endoperoxide PGH₂ [(15S)-hydroxy-9α,11α-(epoxymethano)prosta-5, 13-dienoic acid] and arachidonic acid, were assessed on gastric adenylate cyclase activity from cell-free preparations of guinea pig fundic mucosa. The two precursors were tested against basal adenylate cyclase activity and that stimulated by histamine (10^?4M), by PGE₂(10^?4M), by 5′-guanylylimidodiphosphate [Gpp(NH)p] (10^?4M) and by NaF(10^?2M). PGH₂ analog (10^?4M) and arachidonic acid (10^?4M) both inhibited to a similar extent adenylate cyclase stimulated by histamine or by NaF, but not that stimulated by PGE₂ or by Gpp(NH)p. Neither agent significantly affected basal adenylate cyclase levels. In the presence of indomethacin (10^?4M), basal adenylate cyclase activity remained unchanged but the inhibitory effect of arachidonic acid was almost entirely abolished, suggesting that such inhibitory effect may be caused by prostaglandin endoperoxides generated from arachidonic acid in the course of assay. Moreover, indomethacin did not attenuate PGH₂ inhibition of histamine action. Unlike arachidonic acid, which is a natural metabolic precursor of PGE₂, arachidic acid did not significantly influence histamine-stimulated adenylate cyclase activity. These results suggest that the prostaglandin endoperoxides may have an inhibitory effect on histamine-sensitive ciclic AMP generation in gastric mucosa.     

Relaxation of human isolated pulmonary arteries by prostacyclin (PGI2)

The effects of indomethacin (IND) and PGI₂ on the tone of human isolated pulmonary arteries were studied. Baseline tone of the vessels was increased by IND, electric stimulation (ES), norepinephrine (NE), prostaglandin F_2α (PDF_2α) or K⁺-excess. This high tone was decreased by PGI₂ in a concentration dependent manner. IC₅₀ values (molar concentrations producing 50% relaxation) for PGI₂ were in the same concentration range (10 to 58.8 nmoles/l). The potency of the relaxant effect of PGI₂ was inversely related to the magnitude of tone induced prior to the addition of PGI₂ and independent of the type of tone inducer. It is suggested that the relaxant effect of prostacyclin on human pulmonary artery may be of clinical importance in the treatment of conditions associated with a rise in pulmonary vascular resistance.     

Prostaglandins,other eicosanoids and endothelial cells

Summary Endothelial cells are an important source of eicosanoid formation in the cardiovascular systems. All major pathways of eicosanoid production have been demonstrated in endothelial cells, yielding significant amounts of prostacyclin (PGI₂), PGE₂, PGF₂, thromboxane A₂, leukotrienes and a number of hydroxy fatty acids. The regulation of eicosanoid formation by endothelial cells is poorly understood. There is evidence that precursors, such as arachidonic acid or prostaglandin endoperoxides, may also be provided by other cell types. Endothelial cell-derived eicosanoids are involved in the regulation of local vessel tone, intravascular platelet activation, cell locomotion and, eventually, cell proliferation. Most of the available information considers PGI₂. This compound is the quantitatively dominating eicosanoid in endothelial cells. Major actions of PGI₂ include inhibition of platelet activation and aggregation, relaxation of arterial vessels and inhibition of growth-factor release. There is probably a tight interaction with other biologically active mediators which needs further evaluation. This also applies to the clinical significance of eicosanoid-related pathways for the mechanism of action of cardiovascular drugs, such as organic nitrates or acetylsalicylic acid. The unique property of the cicosanoid system to become activated only in response to stimulation, the local nature of this reaction, the multiplicity of products formed and the short half-time of most of them are currently the most significant obstacles to define the role of endothelial cell-derived eicosanoids in clinical practice.     

The significance of platelet-vessel wall prostaglandin equilibrium during exercise-induced stress

Alterations in platelet-generated thromboxane A₂ (TXA₂) and vessel wall-generated prostacyclin (PGI₂) have been assoclated with myocardial ischemia. To examine TXA₂ - PGI₂ equilibrium at rest and during exercise stress, we studied 13 normal subjects and 15 coronary artery disease patients. Plasma TXB₂ and 6-keto-PGF_1α were measured as stable metabolites of TXA₂ and PGI₂, respectively, by radioimmunoassay. In normal subjects, plasma TXB₂ levels increased 24% during exercise from 135 ± 30 to 168 ± 42 pg/ml (p = NS). Plasma 6-keto-PGF_1α levels increased 224% from 54 ± 17 to 175 ± 57 pg/ml (p < 0.05). In coronary artery disease patients, although resting plasma TXB₂ levels (mean 136 ± 43 pg/ml) were comparable to levels in normal subjects, a greater increase (82%) occurred during exercise (mean 248 ± 70 pg/ml; p < 0.02 compared to resting levels). Resting plasma 6-keto-PGF_1α levels (mean 94 ± 28 pg/ml) were also similar to normal subjects but increased only by 43% during exercise (mean 134 ± 53 pg/ml; p = NS compared to resting levels). These data suggest that: in normal subjects TXA₂ and PGI₂ increase during exercise, PGI₂ increasing more than TXA₂, and although coronary disease patients have resting TXA₂ and PGI₂ levels in the normal range, TXA₂ levels increase more than PGI₂ levels during exercise. These observations may have a bearing on the mechanism of exercise-induced angina pectoris in certain coronary artery disease patients.     

Thromboxane A2 and Prostacyclin Release in Bleeding Time Blood during Primary Haemostasis in Healthy Individuals

ABSTRACT It is generally believed that prostacyclin (PGI₂) generation is greatly stimulated when blood vessels are injured, even by minor trauma, such as venepuncture. The Simplate technique for measuring skin bleeding time was adapted to quantify by radioimmunoassay PGI₂ and thromboxane A₂ (TXA₂) in the emerging blood, as the stable degradation products 6-keto-prostaglandin F_1α (6-keto-PGF_1α) and thromboxane B₂ (TXB₂), both of which were measured in venous plasma as well as in serum (clotted at 37°C for 1 h). During bleeding, when platelets aggregate to occlude the injured vessels, the median TXB₂ level in the emerging bleeding time blood was 1.7 ng/ml. The median TXB₂ level in plasma was < 1 ng/ml and in serum 275 ng/ml. The levels of immunoreactive 6-keto-PGF_1α were always below determination limit in bleeding time blood (0.2 ng/ml) and in plasma (0.1 ng/ml), whereas in serum the levels ranged between 0.26 and 0.47 ng/ml. The fact that enhanced PGI₂ production in primary haemostasis in skin incisions could not be demonstrated calls for further investigations of possible PGI₂ production with more sensitive assays or in injured large vessels.