Role of endogenous prostacyclin in gastric ulcerogenic and healing responses--a study using IP-receptor knockout mice.

Endogenous prostaglandins (PGs) play an important role in the cytoprotective and healing responses in the stomach, by altering various functions, i.e., an increase of the mucosal blood flow, yet the role of prostacyclin (PGI(2)) and its receptor (IP-receptor) in these responses remains unclarified. In the present study, we used IP-receptor knockout mice [IP (-/-)] and examined the importance of IP-receptors in gastric ulcerogenic, cytoprotective and healing responses in these animals. The studies included the ulcerogenic response to cold-restraint stress, the cytoprotective response to a mild irritant (20 mM taurocholate: TC) and capsaicin, and the healing response of chronic gastric ulcers induced by thermo-cauterization. We first checked the absence of IP-receptors by examining the effect of cicaprost (a PGI(2) agonist, topical mucosal application) on gastric mucosal blood flow and found that this agent increased the mucosal blood flow in wild-type [WT (+/+)] mice but not in IP (+/-) mice. Cold-restraint stress (4 h) induced gastric lesions in both groups of mice, but the severity of damage was significantly greater in IP (-/-) mice. Prior p.o. administration of both TC and capsaicin exhibited a marked cytoprotection against HCl/ethanol-induced gastric damage in WT (+/+) mice, both responses being significantly mitigated in the presence of indomethacin. The adaptive cytoprotection induced by TC was similarly observed in IP (-/-) mice, while the capsaicin protection was totally attenuated in the animals lacking IP receptors. On the other hand, the healing of gastric ulcers was significantly delayed by daily administration of indomethacin in WT (+/+) mice. However, this process was not altered in IP (-/-) mice. These results suggest that endogenous PGI(2) is involved in the gastric ulcerogenic response to stress, but not in the healing of pre-existing gastric ulcers. In addition, PGI(2) and its receptors may play a crucial role in capsaicin-induced gastric protection but not in the adaptive cytoprotection-induced by mild irritants.     

The effects of microsomal prostaglandin E synthase-1 deletion in acute cardiac ischemia in mice

The goal of the present study was to assess how genetic loss of microsomal prostaglandin E₂ synthase-1 (mPGES-1) affects acute cardiac ischemic damage after coronary occlusion in mice. Wild type (WT), heterozygous (mPGES-1^+/−), and homozygous (mPGES-1^−/−) knockout mice were subjected to left coronary artery occlusion. At 24 h, myocardial infarct (MI) volume was measured histologically. Post-MI survival, plasma levels of creatine phosphokinase (CPK) and cardiac troponin-I, together with MI size, were similar in WT, mPGES-1^+/− and mPGES-1^−/− mice. In contrast, post-MI survival was reduced in mPGES-1^−/− mice pretreated with I prostanoid receptor (IP) antagonist (12/16) compared with vehicle-treated controls (13/13 mPGES-1^−/−) together with increased CPK and cardiac troponin-I release. The deletion of mPGES-1 in mice results in increased prostacyclin I₂ (PGI₂) formation and marginal effects on the circulatory prostaglandin E₂ (PGE₂) level. We conclude that loss of mPGES-1 results in increased PGI₂ formation, and in contrast to inhibition of PGI₂, without worsening acute cardiac ischemic injury.     

Involvement of prostacyclin/IP receptors in decreased acid response of damaged stomachs — Mediation by somatostatin/SST2 receptors

AimsWe examined the effect of a prostacyclin (PGI₂) analog iloprost on histamine-induced acid secretion and investigated how endogenous PGI₂ mediated the decreased secretory response in the damaged stomach after exposure to taurocholate (TC).Main methodsMale C57BL/6 mice, both wild-type (WT) and IP receptor knockout (IP-KO) animals, were used after 18 h of fasting. Under urethane anesthesia, the abdomen was incised, and an acute fistula was provided in the stomach.Key findingsAcid secretion in WT and IP-KO mice was similarly and dose-dependently increased by histamine. Iloprost decreased the histamine-stimulated secretion in WT but not IP-KO mice. The inhibitory effect of iloprost in WT mice was totally abrogated by the prior administration of CYN154806, a selective somatostatin SST2 receptor antagonist. On the other hand, the acid secretion in WT mice was decreased after exposure of the stomach to 20 mM TC for 20 min, with an increase in mucosal PGI₂ content, but the decrease was significantly less marked in IP-KO mice. The decreased acid response to TC in WT mice was totally prevented by the prior administration of CYN154806 as well as indomethacin. Somatostatin contents in the stomach were reduced after the administration of iloprost or the mucosal exposure to TC, while the blood levels increased.SignificanceSomatostatin/SST2 receptors are involved in the decreased acid response of the damaged stomach, in addition to PGI₂/IP receptors. It is assumed that PGI₂ releases somatostatin from D cells, which in turn decreases acid secretion via the activation of SST2 receptors.     

Deficiency of glucocorticoid production delays the healing of acute gastric mucosal erosion and chronic ulcers in rats

Effects of glucocorticoid deficiency followed by corticosterone replacement on the healing of gastric erosions and chronic gastric ulcers have been investigated in rats. Glucocorticoid deficiency was induced by adrenalectomy performed after the formation of gastric erosions or ulcers. Gastric erosions were produced by indomethacin (35 mg/kg, i.p.) or by 6 h immobilization at temperature 8 degrees C, chronic gastric ulcers were induced by 60% acetic acid. All ulcerogenic stimuli caused an increase in corticosterone production. Adrenalectomy created corticosterone deficiency and delayed the healing of gastric erosions and chronic gastric ulcers. The effect of adrenalectomy was more evident in the indomethacin ulcerogenic model. Replacement by corticosterone prompted the healing of gastric erosions and ulcers in adrenalectomized animals. These data suggest a participation of endogenous glucocorticoids in a restoration of gastric mucosal integrity.     

Cyclooxygenase-2 selective and nitric oxide-releasing nonsteroidal anti-inflammatory drugs and gastric mucosal responses.

Occurrence of gastrointestinal damage and delayed healing of pre-existing ulcer are commonly observed in association with clinical use of nonsteroidal antiinflammatory drugs (NSAIDs). We examined the effects of NS-398, the cyclooxygenase (COX)-2 selective inhibitor, and nitric oxide (NO)- releasing aspirin (NCX-4016) on gastric mucosal ulcerogenic and healing responses in experimental animals, in comparison with those of nonselective COX inhibitors such as indomethacin and aspirin. Indomethacin and aspirin given orally were ulcerogenic by themselves in rat stomachs, while either NS-398 or NCX-4016 was not ulcerogenic at the doses which exert the equipotent antiinflammatory action with indomethacin or aspirin. Among these NSAIDs, only NCX-4016 showed a dose-dependent protection against gastric lesions induced by HCl/ethanol in rats. On the other hand, the healing of gastric ulcers induced in mice by thermal-cauterization was significantly delayed by repeated administration of these NSAIDs for more than 7 days, except NCX-4016. Gastric mucosal prostaglandin contents were reduced by indomethacin, aspirin and NCX-4016 in both normal and ulcerated mucosa, while NS-398 significantly decreased prostaglandin generation only in the ulcerated mucosa. Oral administration of NCX-4016 in pylorus-ligated rats and mice increased the levels of NO metabolites in the gastric contents. In addition, both NS-398 and NCX-4016 showed an equipotent anti-inflammatory effect against carrageenan-induced paw edema in rats as compared with indomethacin and aspirin. These results suggest that both indomethacin and aspirin are ulcerogenic by themselves and impair the healing of pre-existing gastric ulcers as well. The former action is due to inhibition of COX-1, while the latter effect may be accounted for by inhibition of COX-2 and mimicked by NS-398, the COX-2 selective NSAID. NCX-4016, despite inhibiting both COX-1 and COX-2, protects the stomach against damage and preserves the healing response of gastric ulcers, probably because of the beneficial action of NO.     

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.     

Vasomotor Reaction to Cyclooxygenase-1-Mediated Prostacyclin Synthesis in Carotid Arteries from Two-Kidney-One-Clip Hypertensive Mice

This study tested the hypothesis that in hypertensive arteries cyclooxygenase-1 (COX-1) remains as a major form, mediating prostacyclin (prostaglandin I₂; PGI₂) synthesis that may evoke a vasoconstrictor response in the presence of functional vasodilator PGI₂ (IP) receptors. Two-kidney-one-clip (2K1C) hypertension was induced in wild-type (WT) mice and/or those with COX-1 deficiency (COX-1^-/-). Carotid arteries were isolated for analyses 4 weeks after. Results showed that as in normotensive mice, the muscarinic receptor agonist ACh evoked a production of the PGI₂ metabolite 6-keto-PGF_1α and an endothelium-dependent vasoconstrictor response; both of them were abolished by COX-1 inhibition. At the same time, PGI₂, which evokes contraction of hypertensive vessels, caused relaxation after thromboxane-prostanoid (TP) receptor antagonism that abolished the contraction evoked by ACh. Antagonizing IP receptors enhanced the contraction to the COX substrate arachidonic acid (AA). Also, COX-1^-/- mice was noted to develop hypertension; however, their increase of blood pressure and/or heart mass was not to a level achieved with WT mice. In addition, we found that either the contraction in response to ACh or that evoked by AA was abolished in COX-1^-/- hypertensive mice. These results demonstrate that as in normotensive conditions, COX-1 is a major contributor of PGI₂ synthesis in 2K1C hypertensive carotid arteries, which leads to a vasoconstrictor response resulting from opposing dilator and vasoconstrictor activities of IP and TP receptors, respectively. Also, our data suggest that COX-1^-/- attenuates the development of 2K1C hypertension in mice, reflecting a net adverse role yielded from all COX-1-mediated activities under the pathological condition.     

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.     

Effects of neuropathy on high-voltage-activated Ca current in sensory neurones

Voltage-dependent Ca²⁺ channels (VDCCs) have emerged as targets to treat neuropathic pain; however, amongst VDCCs, the precise role of the Ca_V2.3 subtype in nociception remains unproven. Here, we investigate the effects of partial sciatic nerve ligation (PSNL) on Ca²⁺ currents in small/medium diameter dorsal root ganglia (DRG) neurones isolated from Ca_V2.3(−/−) knock-out and wild-type (WT) mice. DRG neurones from Ca_V2.3(−/−) mice had significantly reduced sensitivity to SNX-482 versus WT mice. DRGs from Ca_V2.3(−/−) mice also had increased sensitivity to the Ca_V2.2 VDCC blocker ω-conotoxin. In WT mice, PSNL caused a significant increase in ω-conotoxin-sensitivity and a reduction in SNX-482-sensitivity. In Ca_V2.3(−/−) mice, PSNL caused a significant reduction in ω-conotoxin-sensitivity and an increase in nifedipine sensitivity. PSNL-induced changes in Ca²⁺ current were not accompanied by effects on voltage-dependence of activation in either Ca_V2.3(−/−) or WT mice. These data suggest that Ca_V2.3 subunits contribute, but do not fully underlie, drug-resistant (R-type) Ca²⁺ current in these cells. In WT mice, PSNL caused adaptive changes in Ca_V2.2- and Ca_V2.3-mediated Ca²⁺ currents, supporting roles for these VDCCs in nociception during neuropathy. In Ca_V2.3(−/−) mice, PSNL-induced changes in Ca_V1 and Ca_V2.2 Ca²⁺ current, consistent with alternative adaptive mechanisms occurring in the absence of Ca_V2.3 subunits.     

The mechanism of enhancement by fatty acid hydroperoxides of anaphylactic mediator release

Fragments of chopped lung from indomethacin treated guinea-pigs had an anti-aggregating effect when added to human platelet rich plasma (PRP), probably due to the production of prostacyclin (PGI₂) since the effect was inhibited by 15-hydroperoxy arachidonic acid (15-HPAA, 10 μg ml⁻¹). Both 15-HPAA (1–20 μg ml⁻¹ min⁻¹) and 13-hydroperoxy linoleic acid (13-HPLA, 20 μg ml⁻¹ min⁻¹) caused a marked enhancement of the anaphylactic release of histamine, slow-reacting substance of anaphylaxis (SRS-A) and rabbit aorta contracting substance (RCS) from guinea-pig isolated perfused lungs. This enhancement was not reversed by the concomitant infusion of either PGI₂ (5 μg ml⁻¹ min⁻¹) or 6-oxo-prostaglandin F_1α (6-oxo-PGF_1α, 5 μg ml⁻¹ min⁻¹). Anaphylactic release of histamine and SRS-A from guinea-pig perfused lungs was not inhibited by PGI₂ (10 ng - 10 μg ml⁻¹ min⁻¹) but was inhibited by PGE₂ (5 and 10 μg ml⁻¹ min⁻¹). Antiserum raised to 5,6-dihydro prostacyclin (PGI₁) in rabbits, which also binds PGI₂, had no effect on the release of anaphylactic mediators. The fatty acid hydroperoxides may enhance mediator release either indirectly by augmenting thromboxane production or by a direct effect on sensitized cells. Further experiments to distinguish between these alternatives are described in the accompanying paper (27).     

Effect of bromocriptine on prostacyclin release and cyclic nucleotides on rat aortic and uterine tissues

The effect of bromocriptine mesylate on cyclic nucleotides and PGI₂ release by rat aortic and uterine tissues was investigated. Treatment of rats with bromocriptine (10 mg kg⁻¹ I.P. daily for 14 days) increased PGI₂ release by the thoracic aorta from 0.67 ± 0.02 to 1.4 ± 0.03 ng/mg wet tissue (P < 0.001; n = 6). This increase was antagonized by treatment with sulpiride (15 mg kg⁻¹). Incubation of the arterial tissue with bromocriptive (50 ug ml⁻) in vitro also stimulated PGI₂ release. Mepacrine (160 μg ml⁻) significantly decreased both basal and stimulated PGI₂ release. Incubation of myometrial tissue from pregnant rats with bromocriptine (50 μg ml⁻¹) in vitro significantly decreased PGI₂ release from 1.25 ± 0.07 to 0.60 ± 0.08 ng/mg wet tissue (P < 0.05, n = 6).It also elevated uterine cAMP from 40 ± 2 to 64 ± 3 pmoles/100 mg wet tissue. Both effects were antagonized by sulpiride. Bromocriptine did not affect uterine cGMP or the cyclic nucleotides in the aorta. It is concluded that the increase in aortic PGI₂ was mediated via activation of dopamine D-2 receptors that stimulate phospholipase A₂ enzyme. The decrease in myometrial PGI₂ release may be related to the increase in uterine cAMP resulting from activation of dopamine D-1 receptors. Previous studies suggested a role for PGI₂ in implantation in the rat. The results suggest that the inhibitory effèct on uterine PGI₂ may underlie the reported inhibition of bromocriptine on implantation. On broad basis, the decrease in uterine PGI₂ together with the reported luteolytic effect of bromocriptine point to a potential role for the compound in postcoital contraception.     

Prostaglandin synthesis by fetal rat bone in vitro: Evidence for a role of prostacyclin

Prostaglandin synthesis by fetal rat bones was examined by thin-layer chromatography of culture media after preincubation with labeled arachidonic acid. Cultures in rabbit complement (non-heat inactivated serum) were compared with cultures in heat-inactivated serum or cultures treated with indomethacin. The major complement-dependent products were PGE₂, PGF_2α and 6-keto-PGF_1α, the metabolite of prostacyclin (PGI₂). Since PGI₂ had not been previously identified in bone its ability to stimulate bone resorption was tested. Repeated addition of PGI₂ stimulated release of previously incorporated ⁴⁵Ca from fetal rat long bones in both short-term and long-term cultures at concentrations of 10⁻⁵ to 10⁻⁹M. Because of the short half life of PGI₂ in solution at neutral pH, we tested a sulfur analog, thiaprostacyclin (S-PGI₂) which was found to be a stimulator of bone resorption at concentrations of 10⁻⁵ to 10⁻⁶M. These studies suggest that endogenous PGI₂ production may play a role in bone metabolism. Since vessels produce PGI₂ it is possible that PGI₂ release may be responsible for the frequent association between vascular invasion and resorption of bone or calcified cartilage in physiologic remodeling and pathologic osteolysis.     

Effects of prostacyclin on coronary circulation, heart rate and myocardial contractile force in isolated hearts of guinea PIG and rabbit — Comparison with prostaglandin E2

Infusions of prostacyclin (PGI₂) (3 × 10⁻¹⁰ − 3 × 10⁻⁷M) into the coronary circulation of isolated hearts from guinea pigs or rabbits resulted in a concentration-dependent decrease in the coronary perfusion pressure (CPP). There was a slight decrease in left ventricular systolic pressure in the heart of the rabbit, whereas the heart rate remained unchanged. PGE₂ was without effect on the heart of the rabbit but was as potent as PGI₂ in decreasing the CPP in the guinea pig heart. 6-oxo-PGF_1α (up to 3 × 10⁻⁶ M) did not affect any of the parameters measured.     

Cytoprotective and antisecretory properties of a non-diarrheogenic and non-uterotonic prostacyclin analog: U-68, 215

U-68,215 [15-Cyclohexyl-9-deoxo-13, 14-dihydro-2′, 9a-methano-4,5,6,16,17,18,19,20-octanor-3-oxa-3,7-(′1,′3-interphenylene)-PGE₁] is a stable prostacyclin analog. When given orally to rats, it is cytoprotective for the stomach (ED₅₀: 0.8 μg/kg) and the intestine (ED₅₀: 22 μg/kg), is gastric antisecretory (ED₅₀: 35 μg/kg) and antiulcer (aspirin) (ED_{50: 5 μg/kg}). The oral antisecretory ED₅₀ in dogs in 50 μg/kg. It has a long duration of gastric cytoprotection: 8–10 hours compared to 3 hours for 16, 16-dimethyl PGE₂ Unlike most prostaglandins of the E type, it is not diarrheogenic (not enteropooling), it does not induced cellular proliferation of the gastroeintestinal mucosa, when given twice a day for eight days, it is not uterotonic (in monkeys), and it does not prevent embryo implantation in hamsters. It ihibits platelet aggregation (ED₅₀: 300 μg/kg), but does not promote bleeding from cut vessels nor from gastric ulcers. U-68,215 lowers blood pressure at a n oral dose correponding to 1–5 time the antisecretory ED₅₀ in rats and dogs, and to 150 times the cytoprotective ED₅₀ in rats. It may be of therapeutic value in the treatment of conditions where inhibition of gastric acid secretion is desirable, e.g., gastric and duodenal ulcer, and in conditions responding to cytoprotection, e.g., stress ulcers, hemorrhagic gastritis and gastric erosions associated with nonsteroidal antiinflammatory drugs.     

Comparison of gastric and intestinal antisecretory and protective effects of prostacyclin and its stable thia-amino-analogue (HOE 892) in consious rats

The effects of prostacyclin (PGI₂) and its stable thia-thimo-analogue (Hoe 892) on gastric and intestinal secretions and gastric mucosal lesions have been determined in conscious rats. Both PGI₂ and Hoe 892 given subcutaneously (s.c.) reduced dose-dependent gastric acid secretion, the ID₅₀ (dose producing 50% inhibition) being about 48.6 and 11.8 gmg/kg, respectively. In contrast, intragastric (i.g.) PGI₂ and Hoe 892 did not cause any change in gastric acid secretion at doses ranging from 1 to 100 gmg/kg. Both PGI₂ and Hoe 892 reduced significantly intestinal fluid secretion (antienteropooling activity). PGI₂ and Hoe 892 given i.g. or s.c. reduced dose-dependent gastric ulcer formation induced by acidified aspirin (ASA), Hoe 892 being somewhat less potent than PGI₂. Both PGI₂ and Hoe 892 were equally effective against mucosal necrosis induced by absolute ethanol and this effect was observed both after i.g. and s.c. administration of these agents. We conclude that stable thia-amino-PGI₂ analogue, Hoe 892, has similar gastric and intestinal antisecretory and protective activity as PGI₂ and may be useful in the prevention of gastric damage by various noxious agents.     

Actions of prostacyclin (PGI2) on adrenergic neuroeffector transmission in the rabbit kidney

In the Tyrode's perfused rabbit kidney PGI₂ (1.3 × 10⁻⁸-3.3 × 10^−7M) dose-dependently inhibited vasoconstrictor responses to sympathetic nerve stimulation, as did PGE₂. The dose-effect curve of the two compounds differed, making PGI₂ the less potent in the low concentration and the more potent in the high. PGI₂ also inhibited the vasoconstrictor response to exogenous noradrenaline, but it had no effect on transmitter release. The main metabolite of PGI₂, 6-keto-PGF_1α, was ineffective both on noradrenaline release and on vascular responses to nerve stimulation or exogenous noradrenaline. It is suggested that PGI₂,if a significant renal prostaglandin, may modulate renal neuroeffector transmission post-junctionally, thereby forming a complement to the prejunctional action of PGE₂.     

Role of prostacyclin on steroidogenesis by frog interrenal gland

The role of prostacyclin (PGI₂) on amphibian adrenal steroidogenesis was studied in perifused interrenal fragments from adult male frogs. Exogenous PGI₂ (3×10⁻⁸ M to 3×10⁻⁵ M) and, in a lesser extent, 6-keto-PGF_1α increased both corticosterone and aldosterone production in a dose-related manner. Short pulses (20 min) of 0.88 μM PGI₂ administered at 90 min intervals within the same experiment did not induce any desensitization phenomenon. A prolonged administration (6 h) of PGI₂ gave rise to an important increase in steroid production followed by a decline of corticosteroidogenesis. Indomethacin (IDM, 5 μM) induced a marked reduction of the spontaneous secretion of corticosteroid which confirmed the involvement of endogenous PGs in the process of corticosteroid biosynthesis. The IDM-induced blockade of corticosterone and aldosterone secretion was totally reversed by administration of exogenous PGI₂ in our model. Angiotensin II (AII) induced a massive release of 6-keto-PGF_1α, the stable metabolite of PGI₂. The increase of 6-keto-PGF_1α preceded the stimulation of corticosterone and aldosterone secretions. In contrast, the administration of ACTH did not modify the release of 6-keto-PGF_1α. These results indicate that PGI₂ might be an important mediator of adrenal steroidogenesis in frog. They confirm that the corticosteroidogenic actions of ACTH and AII are mediated by different mechanisms.     

Testing the “redirection hypothesis” of prostaglandin metabolism in the kidney

Furosemide increases the synthesis of two major renal eicosanoids, prostacylin (PGI₂) and thromboxane A₂ (TXA₂), by stimulating the release of arachidonic acid which in turn is metabolized to PGG₂/PGH₂, then to PGI₂ and TXA₂. PGI₂ may mediate, in part, the early increment in plasma renin activity (PRA) after furosemide. We hypothesized that thromboxane synthetase inhibition should direct prostaglandin endoperoxide metabolism toward PGI₂, thereby enhancing the effects of furosemide on renin release. Furosemide (2.0 mg.kg⁻¹ i.v.) was injected into Sprague-Dawley rats pretreated either with vehicle or with U-63, 557A (a thromboxane synthetase inhibitor, 2 mg/kg⁻¹ followed by 2 mg/kg⁻¹.hr⁻¹). Urinary 6ketoPGF₁ α and thromboxane B₂ (TXB₂), reflecting renal synthesis of PGI₂ and TXA₂, as well as PRA and serum TXB₂, were measured. Serum TXB₂ was reduced by 96% after U-63, 557A. U-63, 557A did not affect the basal PRA. Furosemide increased PRA in both vehicle and U63, 557A treated rats. However, the PRA-increment at 10, 20 and 40 min following furosemide administration was greater in U-63, 557A-treated rats than in vehicle-treated rats and urine 6ketoPGF₁ α excretion rates were increased. These effects of thromboxane synthesis inhibition are consistent with a redirection of renal PG synthesis toward PGI₂ and further suggest that such redirection can be physiologically relevant.     

Prostaglandins and renin release: II. Assessment of renin secretion following infusion of PGI2, E2 and D2 into the renal artery of anesthetized dogs

The influence of intra-renal infusions of prostaglandin (PG) I₂, PGE₂ and PGD₂ on renin secretion and renal blood flow was investigated in renally denervated, beta-adrenergic blocked, indomethacin treated dogs with unilateral nephrectomy. All three prostaglandins when infused at doses of 10⁻⁸ g/kg/min and 10⁻⁷ g/kg/min resulted in marked renal vasodilation. Renin secretory rates increased significantly with both PGI₂ and PGE₂ at the 10⁻⁸ g/kg/min and 10⁻⁷ g/kg/min infusion rates in a dose dependent manner. However, PGD₂ was inactive. At 10⁻⁷ g/kg/min, PGI₂ infusions resulted in systemic hypotension indicating recirculation of this prostaglandin. These findings suggest that PGI₂ should be included among the cyclooxygenase derived metabolites of arachidonic acid to be considered as possible mediators of renin release.     

Homocysteine decreases blood flow to the brain due to vascular resistance in carotid artery

An elevated level of Homocysteine (Hcy) is a risk factor for vascular dementia and stroke. Cysthathionine β Synthase (CBS) gene is involved in the clearance of Hcy. Homozygous individuals for (CBS−/−) die early, but heterozygous for (CBS−/+) survive with high levels of Hcy. The γ-Amino Butyric Acid (GABA) presents in the central nervous system (CNS) and functions as an inhibitory neurotransmitter. Hcy competes with GABA at the GABA_A receptor and affects the CNS function. We hypothesize that Hcy causes a decrease in blood flow to the brain due to increase in vascular resistance (VR) because of arterial remodeling in the carotid artery (CA). Blood pressure and blood flow in CA of wild type (WT), CBS−/+, CBS−/+ GABA_A−/− double knockout, and GABA_A−/− were measured. CA was stained with trichrome, and the brain permeability was measured. Matrix Metalloproteinases (MMP-2 and MMP-9), tissue inhibitor of metalloproteinase (TIMP-3, TIMP-4), elastin, and collagen-III expression were measured by real-time polymerase chain reaction (RT-PCR). Results showed an increase in VR in CBS−/+/GABA_A−/−double knockout > CBS−/+/ > GABA_A−/− compared to WT mice. Increased MMP-2, MMP-9, collagen-III and TIMP-3 mRNA levels were found in GABA_A−/−, CBS−/+, CBS−/+/GABA_A double knockout compared to WT. The levels of TIMP-4 and elastin were decreased, whereas the levels of MMP-2, MMP-9 and TIMP-3 increased, which indirectly reflected the arterial resistance. These results suggested that Hcy caused arterial remodeling in part, by increase in collagen/elastin ratio thereby increasing VR leading to the decrease in CA blood flow.