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 PGF_2α 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 PGF_1α isolated after incubation of aortic homogenates with tritiated arachidonic acid and 3) indirectly by GC-MS assay of PGE₂, PGF_2α and 6-keto PGF_1α 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.     

The actions of prostaglandins I2 and E2 on arrhythmias produced by coronary occlusion in the rat and dog

Prostaglandins E₂ and I₂ were compared with known antiarrhythmics for their actions against arrhythmias produced by occlusion of the left anterior descending coronary artery in the anaesthetised rat while PGI₂ was also examined in the dog. PGI₂ in the dog suppressed early arrhythmias produced during occlusion but did not influence those produced by occlusion-release or those occurring 24 hours after a permanent occlusion; none of the A,B,C or D series prostaglandins tested markedly reduced 24 hour arrhythmias. In the rat PGE₂ was antiarrhythmic against early occlusion arrhythmias (30 minutes occlusion) in a dose related manner (infusions of 1–4 μg/kg/min) whereas PGI₂ infusions potentiated the arrhythmogenic effect of occlusion. PGE₂ was as effective an antiarrhythmic as 10mg/kg Org. 6001 which was more effective in this test situation than d1-propranolol. No obvious mechanisms for the actions of PGE₂ or PGI₂ were apparent although both agents lowered blood pressure and reduced the size of the occluded zone produced by ligation.     

Release of prostaglandins I2 and E2 from the perfused mesenteric arterial bed of normotensive and hypertensive rats. Effects of sympathetic nerve stimulation and norepinephrine administration

The spontaneous output of prostaglandin (PG) I₂ from the perfused mesenteric arterial bed in vitro was significantly higher in hypertensive rats than in normotensive rats. Sympathetic nerve stimulation (at 10Hz) of the mesenteric arterial bed from normotensive rats caused a rapid and short-lived (< 4 min) two-fold increase in PGI₂ output and a smaller increase in PGE₂ output. Sympathetic nerve stimulation (at 10Hz) of the mesenteric arterial bed from hypertensive rats failed to increase PGI₂ and PGE₂ output. It is not possible to conclude whether this lack of response is a cause or a result of hypertension. Surprisingly, norepinephrine administration stimulated PGI₂ and PGE₂ release from the mesenteric arterial bed of both normotensive and hypertensive rats. Obviously, differences exist in the responsiveness of rat mesenteric arteries to endogenous and exogenous norepinephrine concerning PG release between the normotensive and hypertensive states.     

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.     

Actions of prostacyclin (PGI2) and its product, 6-oxo-PGF1α on the rat gastric mucosa in vivo and in vitro

The effects of prostacyclin (PGI₂) and its breakdown product 6-oxo-PGF_1α on various aspects of gastric function were investigated in the rat. PGI₂ increased mucosal blood flow when infused intravenously. PGI₂ was a more potent inhibitor of gastric acid secretion in vivo than PGE₂. Like PGE₂, PGI₂ inhibited acid secretion from the rat stomach in vitro. PGI₂ had comparable activity to PGE₂ in inhibiting indomethacin-induced gastric erosions. Thus prostacyclin shares several of the activities of PGE₂, and may be involved in the regulation of gastric mucosal function.     

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.     

The hyperalgesic effects of prostacyclin and prostaglandin E2

Hyperalgesia induced in rat paws or dog knee joints by prostacyclin (PGI₂) and prostaglandin E₂ was measured by a modification of the Randall-Selitto method (1) of by the degree of incapacitation (2). In both species PGI₂ induced an immediate hyperalgesic effect but the effect of PGE₂ had a longer latency. Low doses of PGI₂ caused a short lasting effect but PGE₂, large doses of PGI₂ or successive administration of small doses of PGI₂ caused a long lasting effect.It is suggested that prostacyclin mediates rat paw hyperalgesia induced by carrageenin. The long lasting hyperalgesic effect of PGE₂ and high doses of PGI₂ is possibly an indirect effect caused by stimulation of a sensory nerve sensitising mechanism.     

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

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.     

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.     

Prostanoids and hemostasis in chickens: Anti-aggregating activity of the prostaglandins E1 and E2, but not of prostacyclin and prostaglandin D2

Aggregation of chicken thrombocytes was studied in whole blood using an electronic aggregometer. Serotonin (5-hydroxytryptamine, 5HT), arachidonic acid (AA) and collagen, but not adenosinediphosphate (ADP) induced aggregation. Prostaglandin (PG) endoperoxides were essential for arachidonic acid-induced aggregation, but were not involved in 5HT-induced aggregation, as indicated by inhibitory studies with indomethacin. Similar experiments indicated that biosynthesis of endogenous PG endoperoxides contributed to the aggregation induced by low concentrations of collagen, but was of little importance when high collagen doses were employed. PGE₁ and PGE₂ could abolish all types of aggregation studied, whereas prostacyclin (PGI₂) and PGD₂ were without any anti-aggregatory activity at 1 μg/ml. Between 1 and 100 ng/ml PGE₁ and PGE₂ inhibited arachidonic acid- and 5HT-induced aggregation dose-dependently.The lack of any hemostatic function of PGI₂ in chickens was also indicated by the absence of biosynthesis of endogenous PGI₂ in chicken aorta. PGI₂ was assessed as anti-aggregating activity, released by aortic fragments stirred in rabbit platelet rich plasma. Still, the presence of chicken aortic tissue i chicken whole blood inhibited 5HT-, but not arachidonic acid-induced aggregation. This inhibition was not affected by pretreatment of the aortic fragments with indomethacin or pargyline.     

Contractile responses to prostacyclin (PGI2) of isolated human saphenous and rat venous tissue

Prostacyclin (PGI₂), in a wide concentration range, produced neither contraction nor relaxation of isolated human saphenous vein. Isolated portal veins and vena cava from normal and spontaneously hypertensive rats (SHR) responded only with an increase in contractile tension when exposed to PGI₂. This constrictor effect was absent in a calcium-free buffer. PGI₂ failed to relax KCI contracted vena cava. The constrictor effect of PGI₂ on portal vein was attenuated in a glucose-free, oxygen deficient buffer. No tachyphylaxis or tolerance to the constrictor effect of PGI₂ was noted. Results emphasize that PGI₂ may produce differing effects on vascular smooth muscle tension depending on species and type of blood vessel studied.     

Effects of prostaglandins E2, I2 and F2α, arachidonic acid and indomethacin on pressor responses to norepinephrine in conscious rats

The effects of prostaglandins E₂ (PGE₂), I₂ (PGI₂) and F₂α (PGF₂α), arachidonic acid and indomethacin on pressor responses to norepinephrine were examined in conscious rats. Intravenously infused PGE₂ (0.3, 1.25 μg/kg/min), PGI₂ (50, 100 ng/kg/min), PGF₂α (1.8, 5.4 μg/kg/min) and arachidonic acid (0.7, 1.4 mg/kg/min) did not change the basal blood pressure. Both PGE₂ and PGI₂ significantly attenuated pressor responses to norepinephrine, whereas PGF₂α significantly potentiated them. Arachidonic acid, a precursor of the prostaglandins (PGs), significantly attenuated pressor responses to norepinephrine. Since the attenuating effect of arachidonic acid was completely abolished by the pretreatment with indomethacin (5 mg/kg), arachidonic acid is thought to exert an effect through its conversion to PGs. On the contrary, intravenously injected indomethacin (0.2–5.0 mg/kg) facilitated pressor responses to norepinephrine in a dose-related manner without any direct effect on the basal blood pressure. These results suggest that endogenous PGs may participate in the regulation of blood pressure by modulating pressor responses to norepinephrine in conscious rats.     

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.     

Effect of angiotensin II and norepinephrine on release of prostaglandins E2 and I2 by the perfused rat mesenteric artery

Prostaglandin E₂ (PGE₂) and 6 keto-PGF_1α, the stable metabolite of prostacyclin (PGI₂), have been measured in the effluent of perfused rat mesenteric arteries by the use of a sensitive and specific radioimmunoadday (RIA) method. The PGE₂ and 6-keto-PGF_1α were continuousyl released by the unstimulated mesenteric artery over a period of 145 min. After 100 min of perfusion the release of PGE₂ and 6-keto-PGF_1α was 4.5 ± 8.4 pg/min and 254 ± 75 pg.min respectively, which is in accord with the general belief that PGI₂ is the major PG synthesized by arterial tissue. Angiotensin II (AII) 5 ng/ml) induced an increased of PGE₂ and 6-keto-PGF_1α release without changing the perfusion pressure. The effect of norepinephrine (NE) injections on release of PGs depended on the duration of the stabilization period. The changes of perfusion pressure induced by NE were not related to changes in release of PGs. Thus, it seems that the increase of PG release induced by AII and NE was due to a direct effect of the drugs on the vascular wall. This may represent an important modulating mechanism in the regulation of vascular tone.     

Comparison of the effects of prostacyclin (PGI2), prostaglandin E1 and D2 on platelet aggregation in different species

The activity of prostacyclin (PGI₂), PGE₁ or PGD₂ as inhibitors of platelet aggregation in plasma from human, dog, rabbit, rat, sheep and horse was investigated. Prostacyclin was the most potent inhibitor in all species. PGD₂ was a weak inhibitor in dog, rabbit and rat plasma whereas PGE₁ and prostacyclin were highly active. Theophylline or dipyridamole potentiated the inhibition of human platelet aggregation by prostacyclin, PGE₁ or PGD₂. Compound N-0164 abolished the inhibition by PGD₂ of human platelet aggregation but did not inhibit the effects of PGE₁ or prostacyclin. The results suggest that prostacyclin and PGE₁ act on similar sites on platelets which are distinct from those for PGD₂.     

Agonist-specific desensitization of PGI2-stimulated cyclic AMP accumulation by PGE1 in human foreskin fibroblasts

Agonist-specific desensitization of prostaglandin I₂-stimulated (PGI₂)¹ adenosine 3′:5′-monophosphate (cyclic AMP) accumulation can be demonstrated in intact human foreskin fibroblasts (HFF) following a single exposure to PGE₁ or a stable PGI₂ analog (nitrilo-PGI₂). A single PGI₂-stimulation of HFF cells does not result in desensitization. Continuous re-addition of PGI₂ over a 4 hr period does induce desensitization to subsequent PGI₂-stimulation. HFF cells that are desensitized to PGI₂ are also desensitized to PGE₁ or nitrilo-PGI₂ stimulation indicating that these agonists share a common adenylate cyclase complex. Desensitization to PGI₂ can be measured after a 60 min, but not after a 30 min, exposure to PGE₁ or nitrilo-PGI₂. Once HFF cells are desensitized, a 12–24 hr period is required for the recovery of PGI₂ sensitivity.The adenylate cyclase in membranes prepared from intact cells that were preincubated with PGE₁ is also desensitized to subsequent PGI₂-stimulation. Preincubation of cells with PGI₂ does not induce desensitization of PGI₂-stimulated adenylate cyclase. These data suggest that HFF cells must be constantly exposed to a biologically active prostaglandin for desensitization to occur. The intrinsic chemical lability of PGI₂ may be a biochemical protection mechanism against desensitization in cells that normally respond to PGI₂.     

The effect of PGI2 on canine renal function and hemodynamics

The effect of prostaglandin I₂ (prostacyclin) on renal and intrarenal hemodynamics and function was studied in mongrel dogs to elucidate the role of this novel prostaglandin in renal physiology. Starting at a dose of 10^?8 g/kg/min, PGI₂ decreased renal vascular resistance and redistributed the blood flow away from the outer cortex (zone 1) and towards the juxtamedullary cortex (zone 4). At 3 × 10^?8 g/kg/min, the renal vascular resistance decreased even further, but at this dose the mean arterial blood pressure also declined 13% indicating recirculation of this prostaglandin. PGI₂ infusion at a vasodilatory dose resulted in natriuresis and kaliuresis. With a decline in filtration fraction, these changes were most likely secondary to the hemodynamic effects of this prostaglandin. Unlike PGE₂, PGI₂ had no direct effect on free water clearance indicating lack of activity at the collecting duct. PGI₂ may be the important renal prostaglandin involved in modulating renal vascular resistance and intrarenal hemodynamics as well as influencing systemic blood pressure.     

Effect of PGI2, PGE2 and 6-keto PGF1α on canine gastric blood flow and acid secretion

Prostaglandins (PG)I₂, PGE₂ and 6-keto PGF₁α were infused directly into the gastric arterial supply at 10⁻⁹, 10⁻⁸ and 10⁻⁷ g/kg/min during an intra-gastric artery pentagastrin infusion in anesthetized dogs. 6-keto PGF₁α was also infused at 10⁻⁶ g/kg/min. Gastric arterial blood flow was measured continuously with a non-cannulating electromagnetic flow probe and gastric acid collected directly from the stomach. PGI₂ and PGE₂ produced similar dose-dependent increases in blood flow with an increase of more than four-fold at the highest dose. Both PGs inhibited acid output over this dose range with PGE₂ having 10 times the potency of PGI₂. 6-keto PGF₁α was at least 1000 times less active than PGI₂ or PGE₂ at increasing blood flow and failed to inhibit acid output even at 10⁻⁶ g/kg/min.     

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.     

The effects of prostacyclin (PGI2) on tissues which detect prostaglandins (PG'S)

The effects of prostacyclin (PGI₂) and its stable metabolite 6-oxo-PGF_1α on various bioassay tissues are compared with those of PGE₂ and PGF_2α, using the cascade superfusion method. On vascular smooth muscle, PGI₂ caused relaxation of all tissues tested except the rabbit aorta. PGE₂ relaxed rabbit coeliac and mesenteric artery but contracted bovine coronary artery and had no effect on rabbit aorta. 6-oxo-PGF_1α was ineffective at the concentrations tested.On gastro-intestinal smooth muscle, PGI₂ contracted strips of rat and hamster stomach and the chick rectum. It was less potent than PGE₂ or PGF_2α. None of these substances contracted that cat terminal ileum. 6-oxo-PGF_1α was inactive on these tissues at the doses tested.PGI₂ was less active than PGE₂ or PGF_2α in contracting guinea-pig trachea and rat uterus; 6-oxo-PGF_1α was active only on the rat uterus. Thus, PGI₂ can be distinguished from the other stable prostaglandins using the cascade method of superfusion.