S1R agonist modulates rat platelet eicosanoid synthesis and aggregation

Sigma-1 receptor (S1R) is detected in different cell types and can regulate intracellular signaling pathways. S1R plays a role in the pathomechanism of diseases and the regulation of neurotransmitters. Fluvoxamine can bind to S1R and reduce the serotonin uptake of neurons and platelets. We therefore hypothesized that platelets express S1R, which can modify platelet function. The expression of the SIGMAR1 gene in rat platelets was examined with a reverse transcription polymerase chain reaction and a quantitative polymerase chain reaction. The receptor was also visualized by immunostaining and confocal laser scanning microscopy. The effect of S1R agonist PRE-084 on the eicosanoid synthesis of isolated rat platelets and ADP- and AA-induced platelet aggregation was examined. S1R was detected in rat platelets both at gene and protein levels. Pretreatment with PRE-084 of resting platelets induced elevation of eicosanoid synthesis. The rate of elevation in thromboxane B₂ and prostaglandin D₂ synthesis was similar, but the production of prostaglandin E₂ was higher. The concentration-response curve showed a sigmoidal form. The most effective concentration of the agonist was 2 µM. PRE-084 increased the quantity of cyclooxygenase-1 as detected by ELISA. PRE-084 also elevated the ADP- and AA-induced platelet aggregation. S1R of platelets might regulate physiological or pathological functions.

Evaluation of oxidative/nitrative stress and uterine artery pulsatility index in early pregnancy

Introduction

Increased oxidative/nitrative stress is characteristic not only in pathologic, but also in healthy pregnancy. High uterine artery pulsatility index (UtAPI) at the end of the first trimester is associated with altered placentation and elevated risk for adverse pregnancy outcomes. We aimed to examine the relationship of systemic oxidative/nitrative stress and uterine artery pulsatility index in the first trimester and their correlation to pregnancy outcomes.

Material and methods

Healthy pregnant women were recruited at 12-13th gestational week ultrasound examination; UtAPI was determined by color Doppler ultrasound. Patients were divided into high (UtAPI ≥ 2.3) (n = 30) and low (n = 31) resistance groups, and pregnancies were followed until labor. Systemic oxidative/nitrative stress was estimated by measuring total peroxide level, total antioxidant capacity and nitrotyrosine level.

Results

Plasma total peroxide level was significantly lower (2,510 ± 39 µM vs. 2,285 ± 59 µM), total antioxidant capacity was higher (781 ± 16 mM CRE vs. 822 ± 13 mM CRE) in the high UtAPI group, which were accompanied by lower birth weight (3,317 ± 64 vs. 3,517 ± 77 g, P < 0.05). Plasma total peroxide level showed a negative correlation (by Pearson) to UtAPI (P < 0.01) and positive correlation to birth weight (P < 0.05).

Conclusions

According to our results, lower systemic oxidative stress showed correlation with high UtAPI measured between the 12-13th weeks of gestation. We also found significant differences in the birth weight of healthy newborns; therefore it is worth examining this relationship in pathological pregnancies.

Prostanoid synthesis in the cerebral blood vessels of asphyxiated piglets

The aim of this study was to examine the effects of asphyxia-reventilation and hyperoxia on the cerebral blood perfusion and prostanoid production of the brain arteries and microvessels in piglets. After 10 min of asphyxia, animals were ventilated with room air, or with 100% O2. Following 4 hours of recovery, the brains were perfused, cerebral arteries were removed and microvessels were isolated from the cortex. The microvessels and the arteries were incubated with 1-14C-arachidonic acid, and the 1-14C-prostanoids were then separated by means of overpressure thin-layer chromatography and were quantitatively determined. Under control conditions, the synthesis of dilatory prostanoids dominated the arachidonate cascade both in the microvessels and in the arteries. Asphyxia and reventilation with room air did not modify the prostanoid production. O2 ventilation greatly affected the prostanoid synthesis of the microvessels, with an enhancement of PGD2 up to 247 +/- 27%. In the arteries, the production of PGI2 and of PGE2 was elevated to 272 +/- 15% and to 148 +/- 13%, respectively. These findings indicate that O2 ventilation after asphyxia substantially increases the extent of prostanoid synthesis in the cerebral blood vessels.

Platelet arachidonate cascade of migraineurs in the interictal phase

Morphological and functional alterations of platelets in migraineurs may be linked to the development of migraine. We examined the eicosanoid synthesis of platelets of untreated female migraineurs in a headache-free period and compared it to that of age- and blood group-matched healthy female volunteers. In the platelets of headache-free migraineurs significantly less amounts of anti-aggregatory prostaglandin D2 and prostacyclin, as well as of 12-L-hydroxy-5,8,10-heptadecatrienoic acid (a potent endogenous inducer of endothelial prostacyclin production) were produced, while the synthesis of platelet aggregatory thromboxane did not differ when compared to that of healthy women. These results suggest that the platelet eicosanoids of migraineurs in the headache-free period might promote the development of cellular, vascular and neurological events inducing headache.

The effects of valproate on the arachidonic acid metabolism of rat brain microvessels and of platelets

Long-term administration of the antiepileptic drug valproate can induce hematologic, hepatic and endocrine abnormalities and morphologic alterations in the brain capillaries and glial cells. Valproate elicits bone marrow suppression, reducing the number of red blood cells and platelets, and causes platelet functional abnormalities. Various data suggest that more than one mechanism of valproate-associated toxicity may exist, but the pathomechanism of cell function alterations elicited by valproate has not yet been elucidated. The reported ex vivo experiments were designed to investigate the effects of valproate on the arachidonic acid cascade of rat brain capillaries and platelets. Valproate was administered (300 mg/kg body weight/day) in the drinking water to male Wistar rats for 2 weeks. Isolated platelets and brain microvessels were labelled with [14C]arachidonic acid and the released [14C]eicosanoids were separated by overpressure thin-layer chromatography and determined quantitatively by liquid scintillation counting. Valproate treatment reduced the synthesis of cyclooxygenase and lipoxygenase products in rat platelets. In brain microvessels valproate stimulated the synthesis of lipoxygenase metabolites and attenuated the cyclooxygenase pathway. Modifications of the arachidonate cascade in platelets and brain microvessels may contribute to the cell function alterations caused by valproate.

Effects of pituitary adenylate cyclase-activating polypeptide on the cyclooxygenase pathway of rat platelets and on platelet aggregation

Several data suggest that pituitary adenylate cyclase-activating polypeptide (PACAP) is involved in the regulation of local circulation. One possible role of PACAP in the regulation of circulation is that, it may modify the cyclooxygenase pathway of the arachidonate cascade in platelets. Our study was designed to study the effect of PACAP on the cyclooxygenase pathway of rat platelets and on platelet aggregation. PACAP (10(-7) and 10(-6) M) significantly inhibited the cyclooxygenase pathway of platelets, mostly the thromboxane synthesis. Pretreatment with a PACAP receptor antagonist, PACAP(6-38), or with an inhibitor of protein kinase A, H-89, shows that the effects of PACAP on the cyclooxygenase pathway were diminished. In the aggregation studies, PACAP inhibited both the arachidonic acid-induced and the thrombin-induced platelet aggregation. It can be concluded that PACAP inhibits the cyclooxygenase pathway of rat platelets via a specific PACAP receptor-activated, cAMP-dependent pathway, and these effects of PACAP are involved in the inhibition of platelet aggregation.

Pituitary adenylate cyclase-activating polypeptide inhibits the cyclooxygenase pathway of rat cerebral microvessels

The presence of nerve endings containing pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) around cerebral microvessels suggests that these peptides have regulatory roles in the cerebral microcirculation. Prostanoids synthesized by the cerebrovascular endothelium have a determining role in the regulation of the brain circulation. In the present study, the effects of PACAP and VIP on the cyclooxygenase pathway of cerebral microvessels were investigated. The isolated microvessels were incubated with 1-14C-arachidonic acid and different concentrations of the peptides. The prostanoids formed were separated by means of overpressure thin-layer chromatography, and were quantitatively determined by liquid scintillation. Higher concentrations (10-7 and 10-6 mol L-1) of PACAP significantly inhibited the activity of the cyclooxygenase pathway, whereas VIP had no significant effect on it. As regards the cyclooxygenase metabolites, the syntheses of thromboxane A2 and prostaglandin D2 were inhibited significantly. PACAP and VIP are known to increase the intracellular cAMP level in the cerebral microvessels and in the present experiments the protein kinase A inhibitor H-89 attenuated the effect of PACAP on prostanoid synthesis. It is concluded that the cyclooxygenase pathway of rat cerebral microvessels is more sensitive to PACAP than to VIP. The inhibitory effect of PACAP on prostanoid synthesis is mediated via a cAMP-dependent pathway. By inhibiting the formation of vasoactive prostanoids, PACAP can decrease the vasoreactivity of the microvessels.

Valproate treatment and platelet function: the role of arachidonate metabolites

PURPOSE

Valproate (VPA) is an extensively used drug in the therapy of epilepsies. One of the most frequently reported side effects of VPA is hemorrhagic diathesis. Some authors emphasized the decreased platelet count as the basis of VPA-induced hemorrhagic diathesis, but some reports suggested that a significant proportion of patients with normal platelet count may still have an altered platelet function. The mechanism of the VPA-induced platelet dysfunction has not yet been elucidated. A determining element of platelet functions is the arachidonate cascade. Present ex vivo experiments were designed to determine whether a relation exists between the incidence of hemostasis caused by VPA and the effect of this drug on the arachidonate cascade of platelets.

METHODS

Platelets were isolated from patients receiving long-term VPA treatment (serum level, 36.04+/-16.12 microg/ml; n = 10) or carbamazepine (CBZ) treatment (serum level, 5.24+/-2.67 microg/ml; n = 10) and were labeled with [14C]arachidonic acid. (CBZ-treated patients were chosen as a control group, because CBZ causes blood dyscrasias similar to those elicited by VPA, but there has been no report that CBZ induces a platelet dysfunction.) The 14C-eicosanoids were separated by means of overpressure thin-layer chromatography and determined quantitatively by liquid scintillation.

RESULTS

Even when the mean plasma concentration of the drug was low, VPA treatment reduced the activity of the arachidonate cascade in platelets. VPA effectively inhibited the cyclooxygenase pathway and the synthesis of the strong platelet aggregator thromboxane A2.

CONCLUSIONS

Inhibition of the platelet arachidonate cascade may contribute to the platelet-function alterations caused by VPA.

Vasoactive substances produced by cultured rat brain endothelial cells

The vasoactive substances synthesized by primary cultures of rat brain endothelial cells were investigated and compared to those from two, immortalized cell lines, RBE4 and GP8. The vasoactivity of endothelium-derived substances was measured on isolated canine coronary artery. Vascular tone was significantly decreased by both primary and GP8, but not by RBE4 cells. Indomethacin pretreatment of primary and GP8 cells turned vasorelaxation into contraction while N(omega)-nitro-L-arginine pretreatment decreased the vasorelaxation induced by primary, but not by GP8 cells. Eicosanoid production was determined after incubation with [14C]arachidonic acid. The predominant vasoactive eicosanoid was prostaglandin E2 in both primary and GP8 cells. RBE4 cells synthetized mainly prostaglandin E2 and thromboxane B2 and significantly less prostaglandin E2 than did either primary or GP8 cells. The capacity of cerebral endothelium to regulate vascular tone by production of dilator and constrictor substances can be preserved under certain circumstances in immortalized cell lines.

Effects of inflammatory neuropeptides on the arachidonate cascade of platelets

BACKGROUND

During neurogenic inflammation, neuro-peptides (substance P, SP, calcitonin gene-related peptide, CGRP, and neurokinin A) are released from nerve endings, and these peptides initiate inflammatory reactions in the microcirculation. Platelets are one of the most important elements of the microcirculation.

METHODS

Our in vitro experiments were carried out to determine the effects of inflammatory neuropeptides (SP, and CGRP) on the arachidonate cascade of isolated rat platelets. Cells were labeled with 1-14C-arachidonic acid, then the eicosanoids were separated with overpressure thin-layer chromatography or high-performance liquid chromatography and were quantitatively determined with a liquid scintillation analyzer.

RESULTS

SP (10(-9) and 10(-8) mol/l) significantly increased the activity of the arachidonate cascade. The lipoxygenase pathway was significantly stimulated by SP (10(-11), 10(-9) and 10(-8) mol/l), while the cyclooxygenase system was inhibited by 10(-12) mol/l, and stimulated by 10(-9) mol/l SP. The dose-response curve of TxA2 to SP exhibited a similar pattern to that detected for the cyclooxygenase pathway. Among the vasodilator cyclooxygenase metabolites, only the synthesis of PGE2 was significantly elevated by SP (10(-9) mol/l ). CGRP either in low (10(-12)-10(-11) mol/l) or in high concentrations (10(-6) mol/l) activated the cyclooxygenase pathway, while it had no effect on the lipoxygenase pathway. CGRP (8-37), a specific CGRP1 receptor antagonist, inhibited the effects of CGRP.

CONCLUSIONS

Our data suggest that the arachidonate metabolites of platelets may play a role in the process of neurogenic inflammation.

The effect of substance P on the arachidonate cascade of rat platelets

The role of substance P (SP) in neurogenic inflammation is well known. Through neurokinin receptors, SP activates cells, including the arachidonate cascade of platelets. Our in vitro experiments were carried out to determine the effect of SP on the arachidonate cascade of rat platelets. The platelets were labelled with 14C-arachidonic acid, and the 14C-eicosanoids were then separated by means of overpressure thin-layer chromatography or high-performance liquid chromatography and were quantitatively determined. SP (10(-9) and 10(-8)) mol/L significantly increased the rate of the arachidonate cascade. The lipoxygenase pathway of platelets was stimulated by SP, which can result in the activation of protein kinase C mediated intracellular events. The cyclooxygenase system was inhibited by 10(-12) mol/L, and stimulated by 10(-9) mol/L SP. In our experiments SP in the physiological range of plasma concentration (10(-12) mol/L) decreased the synthesis of vasoconstrictor arachidonate metabolites (TxA2 and PGF2 alpha). These data suggest that in physiologic conditions the arachidonate cascade of platelets may play role in the vasodilator effect of SP. The formation of thromboxane in rat platelets was stimulated by higher concentration of SP (10(-9) mol/L), and therefore the SP-induced cytotoxicity against parasites might be mediated by the stimulation of thromboxane A2 synthesis.

Platelet eicosanoids and the effect of captopril in blood pressure regulation

We investigated the eicosanoid synthesis of platelets of Wistar and of Okamoto spontaneously hypertensive rats (SHR), and the effect of captopril in vitro, using [14C]arachidonic acid as a tracer substrate and chromatographic determination. Lipoxygenase activity was elevated, while the formation of cyclooxygenase products was reduced in SHR platelets, compared to those of Wistar rats. This difference might play a role in the pathomechanism of hypertension in SHR. In SHR with lower blood pressure, captopril reduced thromboxane synthesis, while in SHR with higher blood pressure thromboxane synthesis was unchanged, but the synthesis of prostaglandin D2, a potent vasodilator, and of 12-L-hydroxy-5,8,10-heptadecatrienoic acid, a stimulator of endothelial prostacyclin formation, was increased. We may conclude that, in spite of the missing angiotensin converting enzyme in platelets, a direct effect on platelet eicosanoid synthesis could contribute to the blood pressure decreasing effect of captopril.

The effect of bradykinin and substance P on the arachidonate cascade of platelets

Several peptide receptors are expressed on the surface of platelets, including B1 and NK1, through which Bk and SP might influence platelet functions including their arachidonate cascade. The metabolites of the arachidonate cascade might play a regulatory role in the inter- and intracellular functions of platelets. Platelets were separated from fresh rat blood by differential centrifugation. Platelets (10(8) ml-1 in each sample) were preincubated with Bk or SP. The arachidonate cascade was investigated with [1-14C]arachidonic acid, as tracer substrate. The synthesised [14C]eicosanoids were isolated and quantitatively determined. Bradykinin elicited a biphasic dose-response curve in the formation of the vasoconstrictor and platelet aggregating thromboxane A2 (TxA2). Bk both inhibited (10(-8) mol/l), and elevated (10(-6) mol/l) the synthesis of TxA2 in the thrombocytes. The 12-HETE synthesis was inhibited by Bk (10(-8), 10(-7), 10(-5) mol/l); 12-HETE is an endogenous regulator of prostacyclin synthesis. The formation of 12-HETE in platelets was stimulated by SP (10(-11), 10(-9), 10(-8) mol/l). The synthesis of TxA2 in platelets was either attenuated (10(-12) mol/l), or stimulated (10(-9) mol/l) by SP. According to our observations Bk and SP might play a regulatory role in the activation or deactivation of platelets.

The effect of bradykinin and its fragments on the arachidonate cascade of brain microvessels

The lipoxygenase pathway dominated the arachidonate cascade in the microvessels isolated from cerebral cortical microvessels of male rats. Bradykinin significantly reduced the lipoxygenase pathway and the synthesis of PGD2 in the microvessels of rats. The synthesis of vasoconstrictor cyclooxygenase metabolites was stimulated with Ser-Pro-Phe-Arg tetrapeptide in the cerebral capillaries while the Phe-Arg dipeptide stimulated the release of arachidonate metabolites. The arachidonate cascade was shifted to the cyclooxygenase pathway with angiotensin III in cerebral microvessels. Kallikrein stimulated the lipoxygenase pathway and the synthesis of vasoconstrictor cyclooxygenase metabolites.

The effects of somatostatin on the arachidonate cascade of platelets

Somatostatin (10(-9) M) significantly elevated the synthesis of thromboxane B2 in rat platelets. The transformation of arachidonic acid to active lipoxygenase metabolites was suppressed by somatostatin (10(-9) and 10(-8) M). The ratio of the lipoxygenase/cyclooxygenase products was significantly reduced by the polypeptide (10(-9) and 10(-8) M) in rat platelets. Higher concentrations (10(-7), 10(-6) and 10(-5) M) of somatostatin did not modify the lipoxygenase pathway of the platelets. The synthesis of the vasoconstrictor - proaggregatory cyclooxygenase products was stimulated by the polypeptide (10(-9) and 10(-8) M), while the formation of vasodilatator - antiaggregatory cyclooxygenase metabolites was induced by higher concentrations of somatostatin (10(-7) and 10(-6) M). Somatostatin might act on the deacylation process of phospholipids, reducing the free arachidonic acid substrate level, resulting in a lower lipoxygenation rate in the platelets, which could be responsible for the increased formation of thromboxane. The contradictory results reported by others concerning the action of somatostatin on the platelet function might be explained by our results that the effect of somatostatin depends on the applied dose.

Arachidonate cascade in mesenteric blood vessels and platelets of spontaneously hypertensive rats

The cyclooxygenase and lipoxygenase pathways of the arachidonate cascade are investigated in vitro in the mesenteric arteries and in the platelets of Wistar normotensive and Okamoto spontaneously hypertensive (SH) rats, using 1-14C arachidonic acid as substrate. The arachidonate cascade in the mesenteric arteries of both Wistar and Okamoto rats showed a sexual dependence. The lipoxygenase dominance in the mesenteric arteries increased almost in parallel with the blood pressure in female SH rats. The activities of cyclooxygenase and lipoxygenase were 4-12 times higher in female animals than in males. The lipoxygenase pathway dominated the arachidonate cascade in the platelets of both normotensive and hypertensive rats. The lipoxygenase pathway was more active in the platelets of SH animals than in the controls. Both in the mesenteric arteries and in the platelets, the arachidonate cascade might play some role in the pathogenesis of hypertension.

The action of peptides and proteases on the arachidonate cascade of human and rat platelets

The arachidonate cascade of human or rat platelets were found to be modified by peptides (bradykinin, angiotensin I, angiotensin II, Asp1-Val5-angiotensin II-amide, somatostatin) and proteases (trypsin, kallikrein). The lipoxygenase pathway was not altered by angiotensin I, angiotensin II, trypsin and kallikrein, while the synthesis some of the cyclooxygenase products was selectively changed by these substances. Bradykinin and somatostatin resulted in an attenuated formation of 12-HPETE and 12-HETE - U shape dose response curve, at the same time the synthesis of cyclooxygenase metabolites was increased - bell shape dose response curve. Asp1-Val5-angiotensin II-amide increased the synthesis of lipoxygenase products and diminished the formation of TxB2. At the same time this peptide selectively induced the enzymatic release of PGD2 from platelets. These peptides and proteolytic enzymes might have physiologic significance in the "Ying-Yang" balance in one hand between lipoxygenase and cyclooxygenase metabolites and on the other between the proaggregatory and antiaggregatory substances released from platelets.

Histamine-stimulated prostaglandin synthesis in rat brain microvessels

The histamine-sensitivity of prostacyclin and prostaglandin synthesis was investigated in isolated brain microvessels prepared from normal and hypoxic exercised rats. 10(-4) M histamine stimulated the in vitro synthesis of all components of arachidonate cascade. The chronic hypoxic exercise also resulted in an enhanced production of each fraction. Hypoxia and histamine showed an additive effect in the synthesis of PGE2 only. The possible molecular mechanism induced by hypoxia and histamine is discussed.

Prostacyclin and prostaglandin synthesis in isolated brain capillaries

The synthesis of prostacyclin and prostaglandins was examined in isolated blood-free brain capillaries of guinea-pigs and rats using 1-14C-arachidonic acid as a precursor. The main prostaglandins synthesized by guinea-pig microvessels were prostaglandin D2 and prostaglandin E2. Substantially less prostaglandin F2 alpha or the prostacyclin stable metabolite, 6-oxo-prostaglandin F1 alpha was synthesized. Rat capillary prostaglandin distribution differed substantially from that of the guinea-pigs although the principle prostaglandin was also PGD2. Total prostaglandin conversion was greater in guinea-pig capillaries than in the rat. Norepinephrine stimulated the prostaglandin forming capacity of blood free cerebral microvasculature of guinea-pigs. Prostacyclin and prostaglandins could be involved in the activity dependent regulation of regional cerebral blood flow and permeability.

Histamine in brain capillaries

A high amount of histamine was found in capillaries isolated by subcellular fractionation from the brain. In view of the important effects of histamine on vascular permeability in peripheral vessels, it was thought that histamine had a similar function in the cerebral vasculature. Intracarotic histamine infusion resulted in an enhanced pinocytosis of endothelial cells and te oedematous swelling of the astrocytic end-feet system. [3H]-Histamine, injected in the cerebral ventricles, accumulated in the capillary wall. Histamine and cimetidine activated hydroxyfatty acid and prostaglandin D2 synthesis in isolated brain capillaries. The possible function of the capillary histamine in the regulation of permeability of microvessels is discussed.