Vasoconstrictor effects of leukotrienes C4 and D4 in the feline mesenteric vascular bed

The effects of leukotriene C₄ (LTC₄) and leukotriene D₄ (LTD₄) in the feline mesenteric vascular bed were investigated under conditions of controlled blood flow so that changes in perfusion pressure directly reflect changes in vascular resistance. Intra-arterial injections of LTC₄ and LTD₄ (0.3–3.0 μg) increased perfusion pressure in a dose-related fashion. Vasoconstrictor responses to LTC₄ and LTD₄ were similar to norepinephrine (NE) whereas mesenteric vasoconstrictor response to the thromboxane analog, U46619, was markedly greater than were responses to LTC₄ and LTD₄. Meclofenamate in a dose that greatly attenuated the systemic depressor response to arachidonic acid was without effect on vasoconstrictor responses to LTC₄ and LTD₄, NE and U46619 in the mesenteric vascular bed. The present data show that LTC₄ and LTD₄ possess significant vasoconstrictor activity in the feline mesenteric vascular bed. In addition, the present data suggest that products of the cyclooxygenase pathway do not mediate vasoconstrictor responses to LTC₄ and LTD₄ in the intestinal circulation of the cat.     

Differential effects of acute thermal injury on rat splanchnic and renal blood flow and prostanoid release

This study examines the hypothesis that acute thermal injury decreases renal and splanchnic blood flow which correlates with altered endogenous vasodilator eicosanoid release. Anesthetized male Wistar rats were subjected to sham or a non-resuscitated 30% total body surface area burn. At 1, 2, 4, 8, and 24 h post-burn mean arterial pressure as well as superior mesenteric and renal artery in vivo blood flow were measured. The superior mesenteric and renal arteries were cannulated and perfused in vitro with their end organs with Krebs buffer (pH 7.4, 37°C). Renal and splanchnic 6-keto-PGF_1α (PGI₂), PGE₂, and thromboxane B₂ (TXB₂) release were measured by EIA at 15 min of perfusion. Renal and superior mesenteric artery blood flow decreased by 40% or more at 1 and 2 h post-burn despite mean arterial pressure remaining unchanged. The major eicosanoids released were PGI₂ from the splanchnic bed and PGI₂ and PGE₂ from the kidney. Splanchnic PGI₂ and TXB₂ release and renal TXB₂ increased 2–3 fold at 1 h post-burn but returned to the sham level at 2 h post-burn. By 24 h post-burn the vasodilator eicosanoids were increased in both the splanchnic and renal vascular beds. These data show that decreased renal and splanchnic blood flow was associated with increased endogenous release of the potent vasoconstrictor TXB₂. By 2 h post-burn, renal and splanchnic blood flow began returning toward the sham level as endogenous release of TXB₂ from both organs fell to sham levels. These data suggest that increased endogenous release of TXB₂ may contribute to the short-term decrease in renal and splanchnic blood flow in the immediate post-burn period and thus may contribute to ischemia of both vascular beds.     

Effects of misoprostol on human circulation

Prostaglandins (PGs) of the E-type are potent vasodilators in most species and in most vascular beds. However, vasoconstrictor effects of PGEs have also been noted at selected sites. This study examined the effects of misoprostol, a PGE₁ analog with antiulcer activity, on the human cardiovascular system. Twenty healthy subjetcs participated in this double-blind, placebo-controlled, parallel group study. Following a 12 hour fast, heart rate, arterial blood pressure, light reflex plethysmography of the finger, resting blood flow volume in the lower arm and leg and peripheral vascular resistance were measured at 10 min. intervals for 1 hour prior to drug administration, to permit calculating baseline values. Misoprostol (400 mcg) or its matching placebo were administered orally, and the measurements were repeated at 10 min. intervals over the next 2 hours. A decrease in leg blood flow volume and a corresponding increase in leg peripheral vascular resistance were noted in the misoprostol group. A statistically significant decrease in heart rate between the two treatment groups was also noted. These small changes were not considered to be of clinical importance. No adverse experiences were reported. In conclusion, a single dose of misoprostol (400 mcg) has no clinically significant vasoconstrictive or vasodilative properties in man.     

Effect of somatostatin on organ blood flow in anaesthetized cats

Effect of somatostatin infusion (100 ng/min-61 pmol/min) on organ blood flow was studied in anaesthetized cats. Total blood flow in the superior mesenteric, left renal, lienal, inferior caval veins and the sagittal sinus was measured by the H2-clearance method. Vascular resistance decreased in the small intestine, in the hind limb and in the kidney due to somatostatin infusion. Somatostatin seems to have direct vasodilatory effect in different vascular beds.     

Correlation of prostaglandin-induced mitochondrial calcium release with contraction in bovine intrapulmonary vein

The effects of prostaglandins A₂, A₁, F_2α, E₂, E₁, F_1β and an analog of PGH₂ upon calcium release from mitochondria isolated from bovine intrapulmonary vein and contraction of helical strips of the same tissue were determined. The order of activity of the prostaglandins for calcium release was similar to that for contraction with the exception of the PGH₂ analog. It is suggested that prostaglandin A₂, F_2α, E₂ and A₁ induced release of mitochondrial calcium may influence the contractile state of bovine intrapulmonary vein. However, the PGH₂ analog has a subcellular mechanism other than or in addition to mitochondrial calcium release and is different from the other prostaglandins.     

Investigation of the vascular actions of arachidonate lipoxygenase and cyclo-oxygenase products on the isolated perfused stomach of rat and rabbit

The vascular actions of several prostanoids and arachidonate lipoxygenase products were investigated on the gastric circulation of rat and rabbit perfused with Kreb's solution. Under resting conditions, prostacyclin and PGE₂ produced small decreases in perfusion pressure with prostacyclin being the more potent. During vasoconstriction induced by infusion of noradrenaline, vasopressin or angiotensin II, prostacyclin was 20–40 times as active as PGE₂ as a gastric vasodilator in rat or rabbit stomach. PGF_2α was a less potent vasoconstrictor than noradrenaline, while the epoxy-methano endoperoxide analogue produced a long-lasting vasoconstriction. The putative metabolite, 6-oxo-PGE₁ was less active than prostacyclin as a vasodilator, having comparable activity to PGE₁, whereas 6-oxo-PGF_1α had very little activity. The endoperoxide, PGH₂ reduced perfusion pressure, this effect being inhibited by concurrent infusion of 15-HPETE. The vasodilation induced by arachidonic acid was likewise reduced by 15-HPETE, and abolished by indomethacin infusion. The arachidonate lipoxygenase hydroperoxides were vasodilator in the gastric circulation, the rank order of potency being 12-HPETE > 11-HPETE > 5-HPETE > 15-HPETE in both rat and rabbit stomach. It is possible that such vasoactive lipoxygenase products, may play modulator roles in the gastric mucosa.     

Prostaglandin endoperoxides IV. Effects on smooth muscle

The effect on smooth muscle of the endoperoxides PGG₂ and PGH₂, which are intermediates in prostaglandin biosynthesis, was studied in different systems $\text{in vitro}$ and $\text{in vivo}$. On gastrointestinal smooth muscle (gerbil colon, rat stomach) PGG₂ and PGH₂ produced contractions comparable to those of PGE₂ and PGF_2a whereas contractions elicited on vascular (rabbit aorta) and airway (guinea-pig trachea) smooth muscle were considerably greater than those of PGE₂ and PGF_2a respectively. On intravenous injection into guinea-pigs PGG₂ and PGH₂ caused a triphasic change in blood pressure and were 8–10 times more effective than PGF_2a in producing an increase in tracheal insufflation pressure. When given as aerosols the unstable endoperoxides were less effective than PGF_2a. It is concluded that the endoperoxides are potent smooth muscle stimulants and that they are more effective than their degradation products (PGD₂, PGE₂, PGF_2a) in some systems.     

Effects of 13, 14-dehydroprostacyclin methyl ester on the feline intestinal vascular bed

The effects of a stable PGI₂ analog, 13, 14-dehydro-PGI₂ methyl ester and several vasoactive hormones were compared in the feline intestinal vascular bed under conditions of controlled blood flow so that changes in perfusion pressure directly reflect changes in vascular resistance. The PGI₂ analog decreased perfusion pressure in a dose-dependent fashion when injected in the range of dose of 0.03–3 μg and was quite similar to PGE₂ whereas isoproterenol was somewhat more potent as a vasodilator in the feline intestinal vascular bed. The present data show that 13, 14-dehydro-PGI₂ methyl ester has potent vasodilator activity in the intestinal vascular bed.     

Metabolism and effect of prostaglandin H2 in adipose tissue

Prostaglandin H₂ (PGH₂) inhibited noradrenaline induced cyclic AMP accumulation in isolated rat fat cells in a dose-dependent manner. IC₅₀ was 10 – 25 ng/ml both in the absence and in the presence of theophylline. The degree of inhibition produced by PGH₂ increased with time of incubation. A stable PGH₂ analog did not inhibit cyclic AMP accumulation. PGH₂ was rapidly converted by isolated fat cells to PGD₂, PGE₂ and PGH_2α, but no formation of thromboxane B₂ was found either or . PGE₂ was a more potent inhibitor than PGH₂ of noradrenaline induced cyclic AMP accumulation. PGD₂ enhanced cyclic AMP accumulation in a limited concentration interval, while PGF_2α was essentially uneffective.Our results suggest that PGH₂ is an inhibitor of cyclic AMP formation in isolated rat fat cells only after conversion to PGE₂. A physiological role for PGH₂ as a modulator of lipolysis is considered unlikely.     

Evidence for separate PGD2 and PGF2α receptors in the canine mesenteric vascular bed

Potential interactions between PGD₂ and PGF_2α in the mesenteric and renal vascular beds were investigated in the anesthetized dog. Regional blood flows were measured with electromagnetic flow probes. PGD₂, PGF_2α and Norepinephrine (NE) were injected as a bolus directly into the appropriate artery, and responses to these agents were obtained before, during and after infusion of either PGD₂ or PGF_2α into the left ventricle. In each case, the infused prostaglandin caused vascular effects of its own. Left ventricular infusion of PGD₂ reduced responses to local injections of PGD₂ in the intestine, and a similar effect was observed for PGF_2α, suggesting significant receptor or receptor-like interactions for each of the prostanoids. However, systemic infusion of prostaglandin F_2α (20–100 ng/kg/min) had no effect on renal or mesenteric vascular responses to local injection of prostaglandin D₂. Similarly, PGD₂ administration (100 ng/kg/min) did not affect responses to PGF_2α in the intestine. The present results therefore suggest that these prostaglandins, i.e., D₂ and F_2α, act through separate receptors in the mesenteric and renal vascular beds. In addition, increased prostaglandin F_2α levels produced by infusion of F_2α reduced mesenteric but not renal blood flow, suggesting that redistribution of cardiac output might participate in side effects often observed with clinical use of this prostaglandin, such as nausea and abdominal pain.     

Thromboxane synthase from bovine lung — Solubilization and partial purification

Prostaglandin endoperoxide synthase and thromboxane synthase were both localized mainly in the microsomal fraction of bovine lung. The capacity to convert prostaglanding H₂ into TXB₂ (thromboxane synthase activity) exceeded the capacity to transform arachidonic acid into products. Thromboxane synthase of lung microsomes was solubilized with Triton X-100 and partially purified by DEAE cellulose chromatography. The preparation thus obtained catalyzed the conversion of PGH₂ to a mixture of TXB₂ and HHT, whereas PGH₁ was predominantly converted to HHD.     

Modulation of autonomic neurotransmission by PGD2: Comparison with effects of other prostaglandins in anesthetized cats

Experiments with anesthetized cats were done to study possible roles of different prostaglandins (PGs) in modulating sympathetic neuroeffector transmission. We recorded contractions of the nictitating membrane (n.m.), blood flow in the carotid artery, heart rate and blood pressure, both under control conditions and while stimulating the cut cervical sympathetic nerve. Intra-carotid arterial injection (i.a.) of PGD₂ depressed sympathetic transmission to the n.m. without depressing the effects of exogenous norepinephrine (NE). In contrast, PGE₂ enhanced the effects of nerve transmission or exogenous NE on the stimulated n.m. PGI₂ had similar but shorter effects to PGE₂. PGF_2α or a stable PGH₂ analog, contracted the n.m. smooth muscle with no detected effect on nerve transmission. Carotid blood flow was increased by PGD₂, PGE₂ and PGI₂. PGD₂ and PGI₂ caused bradycardia that could be blocked by atropine. This ability of PGD₂ to modulate autonomic nerve activity is of particular interest because of recent reports that nerve tissue synthesizes PGD₂.     

Effects of prostaglandin endoperoxide analogs on contractile elements in lung and gastrointestinal tract

Prostaglandin endoperoxides are formed in the lung as intermediate compounds in the biosynthesis of prostaglandins and thromboxanes. The effects of different doses of two analogs of prostaglandin endoperoxide PGH₂ were compared with those of PGF_2α and PGE₂ on superfused preparations of isolated trachea, bronchiole, peripheral lung, pulmonary artery and gastrointestinal smooth-muscle tissues. Endoperoxide analogs induced contraction of all smooth-muscle structures in the lung and airways. Compared to PGF_2α, analog I was approximately 71 times as potent on guinea-pig trachea, 214 times as potent on guinea-pig lung, and 57 times as potent on guinea-pig polmunary artery. Analog II was moderately less potent on all tissues than analog I. On gastrointestinal smooth-muscle organs, the PGH₂ analogs were generally closer in activity to PGF_2α and PGE₂, or even weaker. The findings show that PG endoperoxide vessels, and suggest that the naturally occurring compounds may participate in the mediation of bronchoconstriction and pulmonary vasoconstriction in disease states.     

Prostacyclin stimulation of dog arterial cyclic AMP levels

Prostacyclin (PGI₂) dose-dependently increases the adenosine 3′,5′-cyclic monophosphate (cyclic AMP) levels in canine femoral, carotid, and canine and bovine coronary arteries. The prostacyclin-stimulation is enhanced by phosphodiesterase inhibitors, and is readily measurable after 60 sec incubation. The prostaglandin endoperoxide PGH₂, but not PGH₁, also elevates cAMP levels in femoral arteries. Inhibition of arterial prostacyclin synthetase with 28 μM 9,11-azoprosta-5,13-dienoic acid (azo analog I) blocks the PGH₂-stimulation of cAMP accumulation. Azo analog I does not attenuate a direct PGI₂ stimulation, indicating that the PGH₂ dependent elevation of cAMP is due to conversion of PGH₂ to PGI₂ by the artery. PGI₂ and PGE₁ increase cyclic AMP levels and relax dog femoral and bovine coronary arteries, while PGE₂, which actually contracts bovine coronary arteries, has no effect on arterial cyclic AMP levels. The significance of the PGI₂-stimulation of arterial cyclic AMP is not known, but it is probably related to relaxation of arterial strips.     

Some biological effects of prostaglandin endoperoxide analogs.

The effects of two methano-epoxy analogs of the prostaglandin endoperoxides PGG₂ and PGH₂ were tested on human platelets and rabbit aorta strips. One of these analogs, 9α, 11α-methano-epoxy-15- hydroxy-prosta-5, 13-dienoic acid, was 3.7 times more potent than the endoperoxide, PGG₂, as aggregating agent and was 6.2 times more active than PGH₂ in eliciting contractions of the isolated rabbit aorta. The analog initiated the platelet release reaction, but was less active than the endoperoxide in this respect. Furthermore, the release of ¹⁴C-serotonin induced by this analog was inhibited by indomethacin, which indicated that generation of endoperoxide was required.The corresponding 9α, 11α, epoxy-methano-analog was less active than the 9α, 11α, methano-epoxy analog in the test systems employed.     

Effect of the thromboxane receptor antagonist EP 092 on endotoxin shock in the sheep

The thromboxane receptor antagonist EP 092 inhibits the acute pulmonary vascular response to endotoxin in the anaesthetized, closed-chest sheep. The increase in the TXB₂ level in arterial blood was not suppressed by EP 092. Intravenous infusion of the thromboxane mimetic 11,9-epoxymethano PGH₂, but not PGF_2α, raises pulmonary artery pressure and lowers arterial pO₂ similar to the endotoxin. Isolated strips of lobar pulmonary veins but not lobar arteries are contracted by low concentrations of 11,9-epoxymethano PGH₂ - the effects are potently inhibited by EP 092.     

Metabolism of prostaglandin endoperoxide in animal tissues

The endoperoxide PGH₂ serves as a common intermediate for the enzymatic production of prostaglandins (PGEs and PGFs), thromboxanes (Tx) and prostacyclin (PGI₂). These compounds have quite different physiological activities and apparently perform important regulatory functions in various tissues and organs. We have obtained information on the distribution of individual enzymes responsible for the bioconversion of PGH₂ into these compounds in various tissue preparations. [1-C¹⁴] PGH₂ was incubated with a membrane fraction from each tissue homogenate. The products were isolated and identified by radiometric TLC and gas chromatography-mass spectrometry. Short life intermediates were detected by their specific biological activities. With this approach, we have demonstrated the formation of thromboxanes in rhesus monkey platelets, spleen and bone marrow, guinea pig lung and spleen, rabbit lung, human platelets and thioglycollate stimulated peritoneal macrophage from rat. On the other hand, the membrane preparation of bovine and mare corpus luteum, uteri from rabbit, monkey and human, rat stomach and small intestine, and rabbit lung produced predominantly prostacyclin. In addition, a PGH₂ to PGD₂ isomerase was found in the homogenate of rat brain and polymorphonuclear leukocytes. In those tissues which possess more than one enzyme catalyzing the metabolism of prostaglandin endoperoxide, substrate availability appeared to be one factor controlling the metabolic fate of the endoperoxide. The wide occurrence of thromboxane and prostacyclin synthetases suggests that their biological roles are not limited to the cardiovascular system.     

PGD2 formation in the vasculature: Characteristics of rat tail vein prostaglandin endorperoxide-D isomerase

Rat tail vein homogenates, microsome and high speed supernatant fractions were incubated with [1-¹⁴C]prostaglandin endoperoxide (PGH₂) and products separated and identified by radio-thinlayer chromatography. PGI₂ synthase was localized to the microsomal fraction, but exhibited low activity compared to rat tail arteries prepared in the same manner. PGH-D isomerase was maximally active in the presence of reduced glutathione at pH 7.5–8.0, exhibited a K_m for PGH₂ of 33 μM, and was inhibited sulfhydryl-directed reagents. The similarities of this enzyme to PGD synthase of the rat cerebral microvasculature are discussed.     

PGI2 production by rat blood vessels: Diminished prostacyclin formation in veins compared to arteries

Homogenates of eleven different blood vessels from normal Sprague-Dawley rats varied in their ability to produce PGI₂ (i.e., 6-keto-PGF_1α) from [1−¹⁴C]PGH₂. The most notable difference was seen between arteries and veins. Arterial tissues produced more 6-keto-PGF_1α from exogenous PGH₂ than veins at all enzyme (i.e., protein) concentrations tested. Similar results were obtained utilizing different homogenization techniques or arterial and venous rings, indicating this difference was real and not due to homogenization artifacts. In addition, the thoracic segment of the inferior vena cava was more active in converting added [1−¹⁴C]PGH₂ to 6-keto-PGF_1α than the abdominal segment of added inferior vena cava suggestive of a possible segmental distribution of the enzyme activity in blood vessels. These results may be interpreted as indicating that PGI₂ may have a vasomotor function for blood vessels in addition to its proposed antithrombotic role.     

2-(6-carboxyhexyl)cyclopentanone hexylhydrazone: A potent inhibitor of the blood platelet cyclo-oxygenase

Previous studies have demonstrated that 13-azaprostanoic acid (13-APA) is a potent and specific antagonist of thromboxane A₂/prostaglandin H₂ (TXA₂/PGH₂) at the platelet receptor level. In the present study we evaluated the effects of a new azaprostanoid, 2-(6-carboxyhexyl) cyclopentanone hexylhydrazone (CPH), on human platelet function. This hydrazone was found to completely inhibit arachidonic acid (AA)-induced platelet aggregation at 1 uM CPH. On the other hand, CPH was not an effective inhibitor of PGH₂-induced aggregation. Furthermore, 100 uM CPH was completely ineffective in blocking platelet aggregation stimulated by adenosine diphosphate (ADP) or the stable prostaglandin endoperoxide analog U46619 (which presumably acts at the TXA₂/PGH₂ receptor). Measurement of platelet thromboxane B₂ (TXB₂) production demonstrated that the primary site-of-action of CPH is at the cyclo-oxygenase level. Thus, CPH inhibited TXB₂ formation from AA in a dose-dependent manner (0.1 uM–100 uM CPH)². In contrast, CPH blocked TXB₂ production from PGH₂ only at the highest CPH concentration tested, i.e., 100 uM. These results indicate that relative to 13-APA, addition of a second nitrogen at C₁₄ and a double bond between the 12- and 13- positions results in a loss of receptor activity but produces a high affinity for the platelet cyclo-oxygenase.