Enhanced formation of PGI2, a potent hypotensive substance, by aortic rings and homogenates of the spontaneously hypertensive rat.

Intact rings and homogenates of aorta from spontaneously hypertensive rats (SHR) contain enhanced capacity over normal rats (NR) to convert arachidonic acid into PGI2. The PGI2 synthetic system in SHR is stimulated to a greater extent than NR by norepinephrine. Indomethacin blocks this stimulation. PGE2 and PGF2alpha were detected in much smaller amounts in homogenates (undetected in rings) but their formation was not enhanced by the hypertensive tissue. The identity of PGI2 was based on 1) direct pharmacological assay on the rat blood pressure. In this system identical vasodepressor responses to PGI2 are observed after intracarotid and intrajugular administration 2) indirectly as 6-keto PGF1alpha isolated after incubation of aortic homogenates with tritiated arachidonic acid and 3) indirectly by GC-MS assay of PGE2, PGF2alpha and 6-keto PGF1alpha formed during incubation of aortic homogenates with excess unlabeled arachidonic acid. These results provide additional support to our recent hypothesis that PGI2, of aortic origin, might actively participate in the regulation of systemic blood pressure. Its enhanced formation by intact hypertensive vascular tissue reflects an increase in the number of enzyme molecules immediately available to the substrate. This could probably be an adaptive response to the elevated levels of catecholamines in the circulation.     

Prostaglandin I2 has more potent hypotensive properties than prostaglandin E2 in the normal and spontaneously hypertensive rat

The blood pressure lowering effects of PGI₂ in the normal and spontaneously hypertensive rat are described. Comparison of dose response curves for PGI₂ and PGE₂ indicate that PGI₂ is twice as potent as PGE₂ in the normal rat and 3–4 times more active in the spontaneously hypertensive rat. Furthermore PGI₂ is equiactive through intracarotid and intrajugular administration indicative of the complete lack of pulmonary inactivation. These findings supported by evidence of enhanced PGI₂ synthesis in aorta during hypertension support the notion that PGI₂ could participate in blood pressure control mechanisms.     

Effect of ibuprofen on systolic blood pressure and aortic PGI2 production in spontaneously hypertensive rats (SHR)

Ibuprofen, at doses of 90, 180, and 360 mg/kg, subcutaneously, produced no statistically significant changes in systolic blood pressure in spontaneously hypertensive rats (SHR) measured over a period of 5–240 minutes, or at 24 hours. Seven-day treatment of SHR with ibuprofen (180 mg/kg, S.C. 2 × day) also had no significant effect. Production of PGI₂-like substance by isolated aortic rings from SHR was inhibited (20–75%) by the drug in concentrations of 1–10 μg/ml. Thus, in doses known to be anti-inflammatory and/or inhibitory of prostaglandin biosynthesis, ibuprofen appears to lack an effect on systolic blood pressure in this paradigm of essential hypertension. This is in contrast to findings indicating other inhibitors of prostaglandin biosynthesis may exacerbate the hypertensive state in SHR and man.     

PGH2, TxA2 and PGI2 have potent and differentiated actions on human uterine contractility

The contractile response to three different prostanoids of the isolated human myometrium and the different layers of the utero-tubal junction (UTJ) was studied $\text{in vitro}$. The prostaglandin endoperoxide, PGH₂, stimulated contractility of both the myometrium and the outer and inner muscle layers of the UTJ, whereas the intermediate layer of the UTJ was inhibited. Thromboxane A₂ generated from PGH₂ and a thromboxane synthase preparation caused a stimulation of both the myometrium and all three layers of the UTJ. The stimulatory response to TxA₂ occurred at concentrations as low as 50–70 pg/ml. The sodium salt of PGI₂ was found to relax both the myometrium and all the layers of the UTJ. Intravenous administration of PGI₂ in repeated doses between 2–8 μg induced facial flushing and headache but had little if any effect on $\text{in vivo}$ uterine contractility. At least under $\text{in vitro}$ conditions, these short-lived prostanoids and/or their metabolites apparently have a specific action on uterine contractility, an action which is manifested at comparatively low concentrations.     

Enhancement of PGI2 formation by a new vasodilator, 2-nicotinamidoethyl nitrate in the coupled system of platelets and aortic microsomes

Exogenous arachidonate addition to the coupled system of platelets and aortic microsomes resulted in production of TXA₂ and PGI₂ (detected as the stable degradation products, TXB₂ and 6-keto PGF_1α, respectively). Imidazole, papaverine and dipyridamole increased PGI₂ and decreased TXA₂ in the coupled system. All of these agents inhibited TXA₂ formation by platelets from arachidonate. Nitroglycerin did not show any effect on PGI₂ and TXA₂ formation in the coupled system and on TXA₂ formation by platelets. In contrast with these compounds, in spite of showing no inhibitory effect on TXA₂ formation by platelets alone, 2-nicotinamidoethyl nitrate (SG-75) increased PGI₂ and decreased TXA₂ in the coupled system. It is suggested that SG-75 accelerated the conversion of PGH₂ to PGI₂ so that smaller amounts of TXA₂ was produced in the coupled system.     

PGI2-specific antibodies administered in vivo suggest against a role for endogenous PGI2 as a circulating vasodepressor hormone in the normotensive and spontaneously hypertensive rat

Immunoglobulins raised against 5,6-dihydro PGI₂ crossreact with PGI₂. When infused $\text{in vivo}$ into the rat, these immunoglobulins are capable of I) neutralising the vasodepressor effects (bolus or continuous infusion) of exogenous PGI₂, 2) blocking the catabolism of exogenous ³H-PGI₂ and prolonging its life-time in the circulation (t$\text{1}\text{2}$ approx 60 min) while that of ³H-PGE₂ is unaffected, 3) trapping an endogenously produced substance which after extraction from blood and dissociation from the ligand-antibody complex, is immunoreactive with 6-keto PGF_1α-specific antiserum. Yet the anti-5,6-dihydro PGI₂ immunoglobulins have no effect on resting arterial blood pressure both in the normotensive and spontaneously hypertensive rat. These experiments indicate that endogenously produced PGI₂ does not play a significant $\text{systemic}$ role in blood pressure control although in combination with other vasodilators it could still participate in the regulation of vascular tone at a local level.     

Estrogen increases male rat aortic endothelial cell (RAEC) PGI2 release

Estrogen has been proposed as a negative risk factor for development of peripheral vascular disease yet mechanisms of this protection are not known. This study examines the hypothesis that estrogen stimulates rat aortic endothelial cell (RAEC) release of PGI₂. Male Sprague-Dawley rat abdominal aortic 1-mm rings were placed on 35 mm matrigel plates, and incubated for 1 week. The cells were transferred to a Primaria 60-mm dish and maintained from passage 3 in RAEC complete media and experiments performed between passages 4–10. Cells were incubated with Krebs-Henseleit buffer (pH 7.4) containing carrier or increasing concentrations of β-estradiol or testosterone for 60 min. The effluent was analyzed for eicosanoid release of 6-keto-PGF_1α (6-keto, PGI₂ metabolite), PGE₂ and thromboxane B₂ (TXB₂) by EIA (hormone stimulated — basal). Cells were analyzed for total protein by the Bradford method and for cyclooxygenase-1 (COX-1) and prostacyclin synthase (PS) content by Western blot analysis and densitometry. Testosterone did not alter RAEC 6-keto-PGF_1α release, whereas estrogen increased RAEC 6-keto-PGF_1α release in a dose-related manner. Estrogen preincubation (10 ng/ml) decreased COX-1 and PS content by 40% suggesting that the estrogen-induced increase in male RAEC PGI₂ release was not due to increased synthesis of COX-1 or PS. These data support the hypothesis that estrogen stimulation can increase endogenous male RAEC release of PGI₂.     

Effects of aldose reductase inhibitors on prostacyclin (PGI2) synthesis by aortic rings from rats with streptozotocin-induced diabetes

The effects of aldose reductase inhibitors (ARIs) on the synthesis of prostacyclin (PGI2) by aortic rings from diabetic rats were examined. The ARIs studied were ONO-2235 and isoliquiritigenin, a new compound extracted from glycyrrhizae radix. The content of sorbitol in the sciatic nerve of diabetic rats induced by streptozotocin was significantly increased as compared with that of controls. This increase was significantly inhibited by the administration of an ARI. On the other hand, there was a marked decrease in the synthesis of PGI2 by the diabetic rats compared with the control rats. The decrease in PGI2 synthesis was significantly reversed by the administration of an ARI. Furthermore, the synthesis of PGI2 by the aortic rings was inversely correlated with the content of sorbitol in sciatic nerves. Those observations suggest that an ARI may have a beneficial effect on the vascular synthesis of PGI2 in diabetes mellitus.     

Effect of taurine on arterial, uterine and cardiac PGI2 and TXA2 synthesis in the rat

The influence of taurine (in drinking water for 6 weeks) on PGI₂ and TXA₂ synthesis by some female rat organs was investigated using radioimmunoassay and platelet antiaggregatory bioassay. Taurine 100 and 200 mg/kg/day increased aortic PGI₂ release from 0.59 ± 0.04 (control) to 0.85 ± 0.05 and 1.01 ± 0.06 ng/mg, respectively and that by the myometrium from 0.24 ± 0.02 (control) to 0.38 ± 0.01 and 0.50 ± 0.04 ng/mg wet tissue, respectively (P < 0.05, n = 6). It did not affect PGI₂ and TXA₂ production in the heart or TXA₂ in the aorta. Taurine 200 mg/kg depressed uterine TXA₂ synthesis from 148.6 ± 9.8 (control) to 85.4 ± 6.8 pg/mg (P < 0.05, n = 6). Furthermore taurine 0.4 and 0.8 mM in vitro stimulated PGI₂ released by the myometrial and aortic tissues from pregnant rats. The stimulant effect of taurine on PGI₂ may be related to its antioxidant effect whereas its inhibitory effect on uterine TXA₂ may result from direction of synthesis towards PGI₂. It is concluded that endogenous taurine may participate in regulation of PGs synthesis and that prostanoids may contribute to its known actions. On broad basis, taurine-induced release of PGI₂ may prove of potential value in those ailments characterised by deficiency in PGI₂ release.     

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.     

A comparison of the effects of PGI2, PGE2 and PGH2 on the cyclic nucleotide levels in rat anterior pituitary glands

Rat anterior pituitary explants were incubated with PGI₂, PGH₂ and PGE₂ in the presence of theophylline (1mM) and the production of cyclic AMP was measured. PGE₂ was found to be about 20 times more potent than PGI₂ while PGH₂ was slightly more effective than PGI₂. The results suggest that PGI₂ does not play a physiological role in cyclic AMP mediated events in the rat anterior pituitary.     

The cardiovascular pharmacology of prostacyclin (PGI2) in the rat

Physiological roles have been suggested for prostacyclin in the cardiovascular system. Prostacyclin was administered by intravenous infusion to unanesthetized rats. Over a 24 hr period, 0.32 mg/kg/day caused only flushing of the ears. Larger doses (0.56 and 1 mg/kg/day) caused hypothermia, behavioral depression, and swelling of the paws. Cumulative dose-response curves for its depressor action were determined in both unanesthetized and anesthetized, vagotimized, ganglion-blocked rats. In unanesthetized rats, the threshold dose was about 0.1 μg/kg/min. Respiratory depression precluded doses larger than 1 μg/kg/min. In anesthetized rats, the threshold dose was about 0.001 μg/kg/min, and the maximally effective dose was about 0.1 μg/kg/min. At 0.032 μg/kg/min, blood pressure first fell and then rose slightly. This compensatory rise did not occur in nephrectomized rats, suggesting renin release as the mechanism. Intravenous infusion of 0.1 but not 0.01 μg/kg/min in unanesthetized rats doubled plasma renin activity. In saline-loaded unanesthetized rats, urine volume and urinary sodium excretion were decreased by 0.1 μg/kg/min of prostacyclin.     

Enhanced release of a ‘prostacyclin-like’ substance from aortic strips of spontaneously hypertensive rats

The release of an endogenous ‘prostacyclin-like’ substance from aortic strips of 8 male Wistar rats of the New Zealand genetically hypertensive strain (GH) was compared with that of 8 weight, age and sex matched normotensive Wistar control rats. The amount of ‘prostacyclin-like’ substance released by the aortic strips into tris buffer, under the influence of mechanical stimulation, was measured by its ability to inhibit human platelet aggregation as compared to the inhibitory effect of standard prostacyclin sodium salt. It was shown that generation of this substance increased with incubation time and that a significantly greater amount was produced by GH rats.     

Release of prostacyclin (PGI2) by the layers of corpus of rat stomach

Release of PGI₂ by slices of muscularis and mucosa layers of rat corpus stomach was investigated. An anti-aggregatory substance that was released by slices of muscularis was identified as PGI₂ in various bioassay systems including anti-serum against PGI₂ as well as by stimulation of its generation with AA or PHG₂ and by inhibition of this generation with indomethacin or tranylcypromine, respectively. PGI₂ was the major PGs released from slices of muscularis. The release of PGI₂ from muscularis surpasses a similar release of PGI₂ from mucosa by a factor of 10. On the other hand, degradation of exogenous PGI₂ was 4 times faster by mucosa than by muscularis slices. Our conclusion is that in the stomach corpus wall of rats, muscularis is the main source of PGI₂, which may play a role in regulation of mucosal blood flow.     

PGE2 is a more potent vasodilator of the lamb ductus arteriosus than is either PGI2 or 6 keto PGFα

It has been shown in vitro that the lamb ductus arteriosus forms prostaglandins PGE₂, PGF_2α, 6 keto PGF_1α (and its unstable precursor PGI₂). In this study the relative potencies of these endogenous prostaglandins were investigated on isolated lamb ductus arteriosus preparations contracted by exposure to elevated PO₂ and indomethacin. All the prostaglandins (except PGF_2α) relaxed the vessel. This is consistent with the hypothesis that endogenous prostaglandins inhibit the tendency of the vessel to contract in response to oxygen. Only PGE₂, however, relaxed the vessel at concentrations below 10⁻⁸M. PGI₂ and 6 keto PGF_1α had approximately 0.001 and 0.0001 times the activity of PGE₂. Although PGE₂ has been observed to be a minor product of prostaglandin production in the lamb ductus arteriosus, the tissue's marked sensitivity to PGE₂ might make it the most significant prostaglandin in regulating the patency of the vessel.     

Dual actions of prostacyclin (PGI2) on the rat pregnant uterus

Using strips of rat pregnant uterus, treated with indomethacin to suppress spontaneous contractility, the oxytocic activity of prostacyclin was compared with other prostaglandins. A prostacyclin concentration of 32 ng/ml elicited uterine contractions in all experiments. In this respect prostacyclin was 80 times more active than 6-oxo-PGF_1α but less active than PGE₂ or PGF_2α. Apart from a direct stimulant effect, prostacyclin also exhibited an indirect potentiating action. In threshould concentrations prostacyclin caused a 3-fold potentiation of threshold doses of oxytocin. A lesser 1.5-fold potentiation of PGF_2α was also observed. The implications of these findings in relation to prostacyclin playing a role in parturition are discussed.     

EG-626: Not a thromboxane A2 antagonist, but a PGI2 potentiator in platelet aggregation

Intracerebroventricular administration of PGI₂ or PGE₂ reduced aconitine-induced cardiac arrhythmia in rats. PGF_2α had no antiarrhythmic effect under the same conditions. The ED₅₀ values of PGI₂ and E₂ were 0.25 μg/kg and 1.1 μg/kg, respectively. Central mechanisms may participate in the antiarrhythmic effect of these PGs.     

Potent hypotensive activity of 1-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphocholine in spontaneously hypertensive rat

Chemically synthesized 1-O-hexadecyl-2-O-acetyl-$\text{sn}$-glycero-3-phosphocholine possessed the most potent hypotensive activity compared with bradykinin, prostagrandin E₂ and I₂ when 5 nano moles/kg body weight of each drug were administered intravenously in spontaneously hypertensive rat. The potency and the duration of hypotensive activity of 1-O-hexadecyl-2-O-acetyl-$\text{sn}$-glycero-3-phosphocholine were dose dependent. Exogenous norepinephrine or angiotensin II showed pressor activity during the hypotensive action of 1-O-hexadecyl-2-O-acetyl-$\text{sn}$-glycero-3-phosphocholine, but did not disturb the hypotensive pattern of this ether lipid. These may suggest that 1-O-alkyl-2-O-acetyl-$\text{sn}$-glycero-3-phosphocholine plays an important role for the regulation of blood pressure.     

PGI2: A potential mediator of inflammation

PGI₂, but not its metabolite 6oxoPGF_1α, is equivalent in potency to PGE₁ as a potentiator of carrageenan, histamine and bradykinin-induced rat paw oedemas. PGI₂ must, therefore, be considered as a potential mediator of inflammatory processes.     

A comparison of some pharmacological actions of prostaglandin E1, 6-OXO-PGE1 and PGI2

Some pharmacological actions of prostaglandin E₁ (PGE₁), 6-oxo-PGE₁ and PGI₂ have been studied. 6-oxo-PGE₁ and PGI₁ relaxed guinea-pig tracheal muscle in vitro and increased nasal patency in normal volunteers and in subjects with vasomotor rhinitis whereas PGI₂ produced opposite effects. All three compounds produced bronchodilatation in the anaesthetised guinea-pig and relaxed human respiratory tract muscle in vitro.PGI₂ was several times more potent than either 6-oxo-PGE₁ or PGE₁ against ADP-induced aggregation of human and baboon platelets in vitro. Intravenous 6-oxo-PGE₁ in the baboon caused an ex vivo inhibition of platelet aggregation, but the EC₅ was 7.8 times that of PGI₂. As a vasodepressor in the baboon 6-oxo-PGE₁ and PGE₂ were equipotent. Thus with the exception of the vasodepressor effect, the actions of 6-oxo-PGE₁ qualitatively and quantitatively resembled those of the structurally related PGE₁ rather than those of PGI₂.