The effects of hepatic steatosis on thromboxane A2 induced portal hypertension / Efectos de la esteatosis hepática en la hipertensión portal inducida por tromboxano A2

Introduction and aim: Thromboxane (TX) A2 was identified as an important vasoconstrictor during Zymosan induced portal perfusion pressure (PP) increase. We aimed at investigating whether hepatic steatosis influences the extent of TXA2-induced portal hypertension. Materials and methods: Sprague-Dawley rats were randomly divided into control and steatosis (induced by the special diet) groups. PP and TXB2 (stable degradation product of TXA2) in the perfusate were measured after in situ liver perfusion with Zymosan (150μg/ml, 40-46min) or U46619 (TXA2 analog, 0.1μM/ml, 40-46min). The number of Kupffer cell (KC) was measured by immunohistochemistry with CD163. Results: Zymosan induced more TXB2 production and a higher PP increase in control group than in steatosis group despite more CD163 positive KCs in fatty livers. PP and TXB2 efflux revealed a strong correlation in control group and a moderate correlation in steatosis group. Contrary to the effect of Zymosan, U46619 induced a much higher PP increase in steatosis group than in control group. Conclusion: Severe steatosis increased number of KCs, however, PP increase and TXB2 efflux caused by Zymosan infusion in fatty livers were lower than those in healthy livers. In contrast, TXA2 analog caused higher PP increase in fatty livers. Targeting the more sensitive response to TXA2 in fatty livers might be a potential therapy of severe steatosis

Introducción y objetivos: Se ha identificado al tromboxano (TX) A2 como importante vasoconstrictor durante el aumento de la presión de perfusión portal (PP) inducida por zymosan. El objetivo ha sido analizar si la esteatosis hepática influye en el grado de hipertensión portal inducida por TXA2. Materiales y métodos: Las ratas Sprague-Dawley(R) se han dividido aleatoriamente en grupos de control y esteatosis (inducida por una dieta especial). Se midieron la PP y el TXB2 (producto de degradación estable de TXA2) en la perfusión después de la perfusión hepática in situ de zymosan (150μg/ml, minuto 40-46) o U46619 (análogo de TXA2, 0,1μM/ml, minuto 40-46). El número de células de Kupffer (CK) se midió mediante inmunohistoquímica con CD163. Resultados: Zymosan provocó más producción de TXB2 y mayor aumento de la PP en el grupo de control que en el grupo de esteatosis a pesar de hallar más CK positivas para CD163 en hígados grasos. El flujo de salida de la PP y el TXB2 reveló una fuerte correlación en el grupo de control y una correlación moderada en el grupo de esteatosis. De manera diferente al efecto de zymosan, U46619 indujo un aumento de la PP mucho mayor en el grupo de esteatosis que en el grupo de control. Conclusión: La esteatosis grave aumentó el número de CK; sin embargo, el aumento de la PP y el flujo de TXB2 provocado por la perfusión de zymosan en hígados grasos fueron menores que en los hígados sanos. En cambio, el análogo de TXA2 provocó un aumento de la PP en hígados grasos. Centrarse en la respuesta más sensible al TXA2 en hígados grasos podría convertirse en un tratamiento potencial de la esteatosis grave

The effects of hepatic steatosis on thromboxane A2 induced portal hypertension.

INTRODUCTION AND AIM: Thromboxane (TX) A2 was identified as an important vasoconstrictor during Zymosan induced portal perfusion pressure (PP) increase. We aimed at investigating whether hepatic steatosis influences the extent of TXA2-induced portal hypertension. MATERIALS AND METHODS: Sprague-Dawley rats were randomly divided into control and steatosis (induced by the special diet) groups. PP and TXB2 (stable degradation product of TXA2) in the perfusate were measured after in situ liver perfusion with Zymosan (150µg/ml, 40-46min) or U46619 (TXA2 analog, 0.1µM/ml, 40-46min). The number of Kupffer cell (KC) was measured by immunohistochemistry with CD163. RESULTS: Zymosan induced more TXB2 production and a higher PP increase in control group than in steatosis group despite more CD163 positive KCs in fatty livers. PP and TXB2 efflux revealed a strong correlation in control group and a moderate correlation in steatosis group. Contrary to the effect of Zymosan, U46619 induced a much higher PP increase in steatosis group than in control group. CONCLUSION: Severe steatosis increased number of KCs, however, PP increase and TXB2 efflux caused by Zymosan infusion in fatty livers were lower than those in healthy livers. In contrast, TXA2 analog caused higher PP increase in fatty livers. Targeting the more sensitive response to TXA2 in fatty livers might be a potential therapy of severe steatosis.

Relationship between PDK1 and contraction in carotid arteries in Goto-Kakizaki rat, a spontaneous type 2 diabetic animal model.

We studied the relationship between 3-phosphoinositide-dependent protein kinase 1 (PDK1) and contractions induced by serotonin, phenylephrine, and thromboxane A2 mimetic (U46619) in the presence of nitric oxide synthase inhibitor in the carotid arteries of Goto-Kakizaki (GK) rats, a spontaneous type 2 diabetic model, in the chronic stage of disease. Serotonin-induced contraction was greater in the GK rats than in the control Wistar rats. A specific PDK1 inhibitor (GSK2334470) decreased the serotonin-induced contraction in the GK rats but not in the Wistar rats, and the difference in such contraction was abolished with this treatment. In GK rats, phenylephrine-induced contraction exhibited a leftward shift and U46619-induced contraction was greater still. Phenylephrine- and U46619-induced contractions were reduced by GSK2334470 in both groups. These results suggest, for the first time, that the contribution of PDK1 is different among 3 vasoconstrictors and that PDK1 contributed to increased serotonin-induced contraction in the carotid arteries of GK rats.

Thromboxane A2 is Involved in Itch-associated Responses in Mice with Atopic Dermatitis-like Skin Lesions.

To investigate the mechanisms underlying itching in atopic dermatitis, we examined whether thromboxane (TX) A2, an arachidonic acid metabolite, is involved in spontaneous scratching, an itch-related response, in NC mice with atopic dermatitis-like skin lesions. The TXA2 receptor (TP) antagonist ONO-3708 inhibited the spontaneous scratching. The mRNA expression of TX synthase (TXSyn) distributed mainly in epidermis and the concentration of TXB2, a metabolite of TXA2, were increased in lesional skin. Scratching caused by the PAR2 agonist SLIGRL-NH2 was suppressed by ONO-3708. SLIGRL-NH2-induced scratching decreased approximately 75% in TP-deficient mice, compared to wild-type mice. In primary cultures of mouse keratinocytes, SLIGRL-NH2 induced the production of TXA2, as evidenced by the increased TXB2, which was inhibited by the TXSyn inhibitor sodium ozagrel and a PAR2-neutralizing antibody. Taken together, these results suggest that epidermal TXA2, which may be produced via PAR2 activation, is involved in itching in atopic dermatitis.

Sensitivity to the thromboxane A2 analog U46619 varies with inner diameter in human stem villous arteries.

INTRODUCTION: The vascular resistance of stem villous arteries is determined by the balance between different contractile and relaxant agents and in the utero-placental circulation. Thromboxane A2 (TxA2), prostaglandin F2α (PGF2α) and endothelin-1 (ET-1) are considered to be among the most important contractile factors. However, it is not known if their contractile effects are consistent along the villous tree. We hypothesized that the sensitivity to different agonists could be influenced by artery diameter and thus that their contribution to placental vascular resistance may differ. METHODS: Using an isometric wire myograph, the contractility and sensitivity (pD2) to the thromboxane A2 mimetic U46619, PGF2α and ET-1 were investigated in isolated human stem villous arteries and human uterine fundus and isthmus arteries obtained from healthy, pregnant women who had experienced uncomplicated pregnancy. RESULTS: In fetal arteries, the pD2 values for U46619 correlated positively with arterial diameter with no such dependence observed for ET-1 and PGF2α. In maternal arteries, pD2 remained constant for all the agonists tested despite highly variable vessel diameter. DISCUSSION: A selective decrease in sensitivity to TxA2 receptor stimulation was observed with decreasing vascular diameter in human stem villous arteries. The contractile factors PGF2α and ET-1 show no such relationship.

Thromboxane receptor hyper-responsiveness in hypoxic pulmonary hypertension requires serine 324.

BACKGROUND AND PURPOSE: Dysregulation of the thromboxane A2 (TP) receptor, resulting in agonist hypersensitivity and hyper-responsiveness, contributes to exaggerated vasoconstriction in the hypoxic pulmonary artery in neonatal persistent pulmonary hypertension. We previously reported that hypoxia inhibits TP receptor phosphorylation, causing desensitization. Hence, we examined the role of PKA-accessible serine residues in determining TP receptor affinity, using site-directed mutational analysis. EXPERIMENTAL APPROACH: Vasoconstriction to a thromboxane mimetic and phosphorylation of TP receptor serine was examined in pulmonary arteries from neonatal swine with persistent pulmonary hypertension and controls. Effects of hypoxia were determined in porcine and human TP receptors. Human TPα serines at positions 324, 329 and 331 (C-terminal tail) were mutated to alanine and transiently expressed in HEK293T cells. Saturation binding and displacement kinetics of a TP antagonist and agonist were determined in porcine TP, wild-type human TPα and all TP mutants. Agonist-elicited calcium mobilization was determined for each TP mutant, in the presence of a PKA activator or inhibitor, and in hypoxic and normoxic conditions. KEY RESULTS: The Ser324A mutant was insensitive to PKA activation and hypoxia, had a high affinity for agonist and increased agonist-induced calcium mobilization. Ser329A was no different from wild-type TP receptors. Ser331A was insensitive to hypoxia and PKA with a decreased agonist-mediated response. CONCLUSIONS AND IMPLICATIONS: In hypoxic pulmonary hypertension, loss of site-specific phosphorylation of the TP receptor causes agonist hyper-responsiveness. Ser324 is the primary residue phosphorylated by PKA, which regulates TP receptor-agonist interactions. Ser331 mutation confers loss of TP receptor-agonist interaction, regardless of PKA activity.

The p90 ribosomal S6 kinase (RSK) is a mediator of smooth muscle contractility.

In the canonical model of smooth muscle (SM) contraction, the contractile force is generated by phosphorylation of the myosin regulatory light chain (RLC20) by the myosin light chain kinase (MLCK). Moreover, phosphorylation of the myosin targeting subunit (MYPT1) of the RLC20 phosphatase (MLCP) by the RhoA-dependent ROCK kinase, inhibits the phosphatase activity and consequently inhibits dephosphorylation of RLC20 with concomitant increase in contractile force, at constant intracellular [Ca(2+)]. This pathway is referred to as Ca(2+)-sensitization. There is, however, emerging evidence suggesting that additional Ser/Thr kinases may contribute to the regulatory pathways in SM. Here, we report data implicating the p90 ribosomal S6 kinase (RSK) in SM contractility. During both Ca(2+)- and agonist (U46619) induced SM contraction, RSK inhibition by the highly selective compound BI-D1870 (which has no effect on MLCK or ROCK) resulted in significant suppression of contractile force. Furthermore, phosphorylation levels of RLC20 and MYPT1 were both significantly decreased. Experiments involving the irreversible MLCP inhibitor microcystin-LR, in the absence of Ca(2+), revealed that the decrease in phosphorylation levels of RLC20 upon RSK inhibition are not due solely to the increase in the phosphatase activity, but reflect direct or indirect phosphorylation of RLC20 by RSK. Finally, we show that agonist (U46619) stimulation of SM leads to activation of extracellular signal-regulated kinases ERK1/2 and PDK1, consistent with a canonical activation cascade for RSK. Thus, we demonstrate a novel and important physiological function of the p90 ribosomal S6 kinase, which to date has been typically associated with the regulation of gene expression.

Identification of a functional prostanoid-like receptor in the protozoan parasite, Trypanosoma cruzi.

Trypanosoma cruzi infection in humans and experimental animals causes Chagas disease which is often accompanied by myocarditis, cardiomyopathy, and vasculopathy. T. cruzi-derived thromboxane A2 (TXA2) modulates vasculopathy and other pathophysiological features of Chagasic cardiomyopathy. Here, we provide evidence that epimastigotes, trypomastigotes, and amastigotes of T. cruzi (Brazil and Tulahuen strains) express a biologically active prostanoid receptor (PR) that is responsive to TXA2 mimetics, e.g. IBOP. This putative receptor, TcPR, is mainly localized in the flagellar membrane of the parasites and shows a similar glycosylation pattern to that of bona fide thromboxane prostanoid (TP) receptors obtained from human platelets. Furthermore, TXA2-PR signal transduction activates T. cruzi-specific MAPK pathways. While mammalian TP is a G-protein coupled receptor (GPCR); T. cruzi genome sequencing has not demonstrated any confirmed GPCRs in these parasites. Based on this genome sequencing it is likely that TcPR is unique in these protists with no counterpart in mammals. TXA2 is a potent vasoconstrictor which contributes to the pathogenesis of Chagasic cardiovascular disease. It may, however, also control parasite differentiation and proliferation in the infected host allowing the infection to progress to a chronic state.

High resolution imaging of vascular function in zebrafish.

RATIONALE: The role of the endothelium in the pathogenesis of cardiovascular disease is an emerging field of study, necessitating the development of appropriate model systems and methodologies to investigate the multifaceted nature of endothelial dysfunction including disturbed barrier function and impaired vascular reactivity. OBJECTIVE: We aimed to develop and test an optimized high-speed imaging platform to obtain quantitative real-time measures of blood flow, vessel diameter and endothelial barrier function in order to assess vascular function in live vertebrate models. METHODS AND RESULTS: We used a combination of cutting-edge optical imaging techniques, including high-speed, camera-based imaging (up to 1000 frames/second), and 3D confocal methods to collect real time metrics of vascular performance and assess the dynamic response to the thromboxane A(2) (TXA(2)) analogue, U-46619 (1 µM), in transgenic zebrafish larvae. Data obtained in 3 and 5 day post-fertilization larvae show that these methods are capable of imaging blood flow in a large (1 mm) segment of the vessel of interest over many cardiac cycles, with sufficient speed and sensitivity such that the trajectories of individual erythrocytes can be resolved in real time. Further, we are able to map changes in the three dimensional sizes of vessels and assess barrier function by visualizing the continuity of the endothelial layer combined with measurements of extravasation of fluorescent microspheres. CONCLUSIONS: We propose that this system-based microscopic approach can be used to combine measures of physiologic function with molecular behavior in zebrafish models of human vascular disease.

I4, a new synthetic sulfonylurea compound, inhibits the action of TXA2 in vivo and in vitro on platelets and aorta vascular smooth muscle.

INTRODUCTION: 1-[4-[2-(4-Bromobenzene-sulfonamino)ethyl]phenylsulfonyl]-3-(trans-4-methylcy-clohexyl)urea(I(4), CAS865483-06-3); a totally synthetic new sulfonylurea compound, combining the hypoglycemic active structure of Glimepiride (CAS 93479-97-1) and anti-TXA(2) receptor (TP) active structure of BM-531(CAS 284464-46-6), was designed and synthesized. Its effects on TXA(2) synthesis and TP have not been reported yet. AIM: To study the inhibitory effects of I(4) and its mechanisms of action on TXA(2) and TP. METHODS: Platelet aggregation studies were performed on human platelet, rat whole blood platelet and rabbit platelet, platelets aggregation was induced by TP agonist U-46619(stable analog of TXA(2), CAS 56985-40-1). Plasma TXB(2) and 6-keto-prostaglandin F(1α) (6-keto-PGF(1α)) were used as markers to determine the effect of I(4) on thromboxane synthesis. Fluo-3-AM was used to measure the cytosolic Ca(2+) concentrations ([Ca(2+)](i)) in rabbit platelet. Aorta rings with and without endothelium were prepared and aorta contraction was induced by U-46619. A model of type 2 diabetes mellitus was established by intraperitoneal injection of low dose of streptozocin to rats fed a high-calorie diet. Both normal rats and type 2 diabetic rats were used to assay the inhibitory effect of I(4) on platelet aggregation induced by U-46619. RESULTS: I(4) exhibited a higher inhibitory potency than Glimepiride on U-46619 induced platelet aggregation in vitro and in vivo. I(4) increased the ratio of plasma PGI(2)/TXA(2) and decreased [Ca(2+)](i) release from platelet internal stores. In addition, I(4) presented a vasorelaxant activity on isolated rat aorta contraction induced by U-46619.Oral administration of I(4) (1~10mg/kg) markedly and dose-dependently inhibited platelet aggregation in both normal rats and type 2 diabetic rats. CONCLUSION: I(4) significantly inhibited platelet aggregation induced by U-46619 in vitro and in vivo, and rat aorta contraction. It probably acts by partly blocking TXA(2) action, decreasing the platelet intracellular Ca(2+), and increasing the PGI(2)/TXA(2) ratio.

Allergic inflammation induces a persistent mechanistic switch in thromboxane-mediated airway constriction in the mouse.

Actions of thromboxane (TXA(2)) to alter airway resistance were first identified over 25 years ago. However, the mechanism underlying this physiological response has remained largely undefined. Here we address this question using a novel panel of mice in which expression of the thromboxane receptor (TP) has been genetically manipulated. We show that the response of the airways to TXA(2) is complex: it depends on expression of other G protein-coupled receptors but also on the physiological context of the signal. In the healthy airway, TXA(2)-mediated airway constriction depends on expression of TP receptors by smooth muscle cells. In contrast, in the inflamed lung, the direct actions of TXA(2) on smooth muscle cell TP receptors no longer contribute to bronchoconstriction. Instead, in allergic lung disease, TXA(2)-mediated airway constriction depends on neuronal TP receptors. Furthermore, this mechanistic switch persists long after resolution of pulmonary inflammation. Our findings demonstrate the powerful ability of lung inflammation to modify pathways leading to airway constriction, resulting in persistent changes in mechanisms of airway reactivity to key bronchoconstrictors. Such alterations are likely to shape the pathogenesis of asthmatic lung disease.

Arachidonic acid epoxygenase and 12(S)-lipoxygenase: evidence of their concerted involvement in ductus arteriosus constriction to oxygen.

Oxygen promotes closure of the ductus arteriosus at birth. We have previously presented a scheme for oxygen action with a cytochrome P450 (CYP450) hemoprotein and endothelin-1 (ET-1) being, respectively, sensor and effector, and a hypothetical monooxygenase product serving as a coupling link. We have also found in the vessel arachidonic acid (AA) 12(S)-lipoxygenase (12-lipoxygenase) undergoing upregulation at birth. Here, we examined the feasibility of a sensor-to-effector messenger originating from AA monooxygenase and 12-lipoxygenase pathways. The epoxygenase inhibitor, N-methylsulfonyl-6-(2-)hexanamide, suppressed the tonic contraction of ductus to oxygen. A similar effect was obtained with 12-lipoxygenase inhibitors baicalein and PD 146176. By contrast, none of the inhibitors modified the endothelin-1 contraction. Furthermore, an AA ω-hydroxylation product, 20-hydroxyeicosatetraenoic acid (20-HETE), reportedly responsible for oxygen contraction in the systemic microvasculature, had no such effect on the ductus. We conclude that AA epoxygenase and 12-lipoxygenase jointly produce a hitherto uncharacterized compound acting as oxygen messenger in the ductus.

The function and three-dimensional structure of a thromboxane A2/cysteinyl leukotriene-binding protein from the saliva of a mosquito vector of the malaria parasite.

The highly expressed D7 protein family of mosquito saliva has previously been shown to act as an anti-inflammatory mediator by binding host biogenic amines and cysteinyl leukotrienes (CysLTs). In this study we demonstrate that AnSt-D7L1, a two-domain member of this group from Anopheles stephensi, retains the CysLT binding function seen in the homolog AeD7 from Aedes aegypti but has lost the ability to bind biogenic amines. Unlike any previously characterized members of the D7 family, AnSt-D7L1 has acquired the important function of binding thromboxane A(2) (TXA(2)) and its analogs with high affinity. When administered to tissue preparations, AnSt-D7L1 abrogated Leukotriene C(4) (LTC(4))-induced contraction of guinea pig ileum and contraction of rat aorta by the TXA(2) analog U46619. The protein also inhibited platelet aggregation induced by both collagen and U46619 when administered to stirred platelets. The crystal structure of AnSt-D7L1 contains two OBP-like domains and has a structure similar to AeD7. In AnSt-D7L1, the binding pocket of the C-terminal domain has been rearranged relative to AeD7, making the protein unable to bind biogenic amines. Structures of the ligand complexes show that CysLTs and TXA(2) analogs both bind in the same hydrophobic pocket of the N-terminal domain. The TXA(2) analog U46619 is stabilized by hydrogen bonding interactions of the ω-5 hydroxyl group with the phenolic hydroxyl group of Tyr 52. LTC(4) and occupies a very similar position to LTE(4) in the previously determined structure of its complex with AeD7. As yet, it is not known what, if any, new function has been acquired by the rearranged C-terminal domain. This article presents, to our knowledge, the first structural characterization of a protein from mosquito saliva that inhibits collagen mediated platelet activation.

Maternal levels of prostacyclin, thromboxane, ICAM, and VCAM in normal and preeclamptic pregnancies.

PROBLEM: To evaluate whether impaired endothelial function and endothelial inflammatory response occur in parallel in the women with preeclampsia. METHOD OF STUDY: Venous blood was drawn from normal (n=40) and severe preeclamptic (sPE) (n=40) pregnant women when they were admitted to the L&D Unit and 24 hrs after delivery. Plasma and serum samples were extracted and measured for 6-keto PGF1α and TXB(2) (stable metabolites of PGI2 and TXA2), and intercellular adhesion molecule (ICAM) and vascular cell adhesion molecule (VCAM) by ELISA. Data are analyzed by Mann-Whitney test and paired t-test. The statistical significance is set as P<0.05. Results Plasma 6-keto PGF1α levels were significantly reduced at admission and 24hr after delivery in sPE compared to normal pregnant controls, P<0.01. The ratio of 6-keto PGF1α and TXB(2) was significant less in sPE than that in normal pregnant controls before delivery. There was no significant difference for ICAM and VCAM levels between normal and patients with sPE before and after delivery. CONCLUSION: Maternal 6-keto PGF1α levels and the ratio of 6-keto PGF1α and TXB(2) were decreased in patients with sPE compared to normal pregnant controls. In contrast, maternal ICAM and VCAM levels were not different between the two groups. These data suggest that serum ICAM and VCAM levels may not be sensitive inflammatory biomarkers for preeclampsia.

Effects of endothelin-1 on the relaxation of rat coronary arteries.

To analyze the effects of endothelin-1 on the b-adrenergic response of the coronary circulation, 2-mm-long segments of coronary arteries from rats were prepared for isometric tension recording in organ baths. The relaxation to isoproterenol (3 x 10(-8) M), field electrical stimulation (4 Hz, 0.1-millisecond duration, 10 seconds), acetylcholine (3 x 10(-8) M), and sodium nitroprusside (10(-9) M) was recorded in arteries precontracted with U46619 (10(-7) to 5 x 10(-7) M) before and after treatment with endothelin-1 (3 3 10210 and 1029 M). The relaxation to isoproterenol was increased by treatment with endothelin-1 and with the endothelin ET(B) antagonist BQ788 (10(-6) M) but not with the endothelin ET(A) antagonist BQ123 (10(-6) M) or with the blocker of protein kinase C chelerythrine (10(-5) M). In the presence of BQ788, BQ123, or chelerythrine, endothelin-1 did not modify the relaxation to isoproterenol. Treatment with endothelin-1 did not modify the relaxation to electrical stimulation, acetylcholine, or sodium nitroprusside. These results suggest that endothelin-1 may potentiate coronary beta-adrenergic vasodilatation, at least in part due to stimulation of endothelin ET(A) receptors and activation of protein kinase C.

Behavioral abnormalities of fetal growth retardation model rats with reduced amounts of brain proteoglycans.

Fetal growth retardation (FGR) is a critical problem in the neonatal period, because a substantial population of infants born with FGR go on to develop various developmental disorders. In the present study, we produced FGR model rats by continuous administration of a synthetic thromboxane A2 analogue (STA2) to pregnant rats. The FGR pups exhibited a significant delay in postnatal neurological development. Moreover, behavioral analyses revealed the presence of a learning disability in juvenile FGR male rats. To investigate the mechanism underlying the neurological disorders, histological and biochemical analyses of the brain of FGR rats were performed. The density of neurons in the cortical plate of an FGR brain was low compared with the brains of a similarly aged, healthy rat. Consistent with this finding, the density of TUNEL-positive cells was higher in the cortical plate of FGR brains. Western blot analyses showed that the levels of three brain-specific chondroitin sulfate proteoglycans (CSPGs), neurocan, phosphacan, and neuroglycan C, were all significantly reduced in the brain of neonatal FGR rats compared with those of the control. The reduction of CSPG-levels and morphological changes in the brain may be relevant to neurological dysfunction in FGR.

Effects of centrally administered prostaglandin E(3) and thromboxane A(3) on plasma noradrenaline and adrenaline in rats: comparison with prostaglandin E(2) and thromboxane A(2).

Previously, we reported the involvement of brain omega-6 prostanoids, especially prostaglandin E(2) and thromboxane A(2), in the activation of central sympatho-adrenomedullary outflow in rats. omega-3 Prostanoids, including prostaglandin E(3) and thromboxane A(3), are believed to be less bioactive than omega-6 prostanoids, although studies on the functions of omega-3 prostanoids in the central nervous system have not been reported. In the present study, therefore, we compared the effects of centrally administered omega-3 prostanoids, prostaglandin E(3) and thromboxane A(3), with those of omega-6 prostanoids, prostaglandin E(2) and thromboxane A(2), on the plasma catecholamines in anesthetized rats. Intracerebroventricularly (i.c.v.) administered prostaglandin E(2) (0.15, 0.3 and 1.5 nmol/animal) and prostaglandin E(3) (0.3 and 3 nmol/animal) predominantly elevated plasma noradrenaline but not adrenaline, but the latter was less efficient than the former. On the other hand, U-46619 (an analog of thromboxane A(2)) (30, 100 and 300 nmol/animal, i.c.v.) and Delta(17)-U-46619 (an analog of thromboxane A(3)) (100 and 300 nmol/animal, i.c.v.) both elevated plasma catecholamines (adrenaline>>noradrenaline) to the same degree. These results suggest that centrally administered prostaglandin E(3) is less effective than prostaglandin E(2) to elevate plasma noradrenaline, and that thromboxane A(3) is almost as equipotent as thromboxane A(2) to elevate plasma catecholamines in rats.

Ca2+- and thromboxane-dependent distribution of MaxiK channels in cultured astrocytes: from microtubules to the plasma membrane.

Large-conductance, voltage- and Ca2+-activated K+ channels (MaxiK) are broadly expressed ion channels minimally assembled by four pore-forming alpha-subunits (MaxiKalpha) and typically observed as plasma membrane proteins in various cell types. In murine astrocyte primary cultures, we show that MaxiKalpha is predominantly confined to the microtubule network. Distinct microtubule distribution of MaxiKalpha was visualized by three independent labeling approaches: (1) MaxiKalpha-specific antibodies, (2) expressed EGFP-labeled MaxiKalpha, and (3) fluorophore-conjugated iberiotoxin, a specific MaxiK pore-blocker. This MaxiKalpha association with microtubules was further confirmed by in vitro His-tag pulldown, co-immunoprecipitation from brain lysates, and microtubule depolymerization experiments. Changes in intracellular Ca2+ elicited by general pharmacological agents, caffeine or thapsigargin, resulted in increased MaxiKalpha labeling at the plasma membrane. More notably, U46619, an analog of thromboxane A2 (TXA2), which triggers Ca2+-release pathways and whose levels increase during cerebral hemorrhage/trauma, also elicits a similar increase in MaxiKalpha surface labeling. Whole-cell patch clamp recordings of U46619-stimulated cells develop a approximately 3-fold increase in current amplitude indicating that TXA2 stimulation results in the recruitment of additional, functional MaxiK channels to the surface membrane. While microtubules are largely absent in mature astrocytes, immunohistochemistry results in brain slices show that cortical astrocytes in the newborn mouse (P1) exhibit a robust expression of microtubules that significantly colocalize with MaxiK. The results of this study provide the novel insight that suggests that Ca2+ released from intracellular stores may play a key role in regulating the traffic of intracellular, microtubule-associated MaxiK stores to the plasma membrane of developing murine astrocytes.

The Gq and G12 families of heterotrimeric G proteins report functional selectivity.

Receptors coupled to the G(q) and G(12) families of heterotrimeric G proteins have surfaced rarely in the context of functional selectivity and always indirectly. We explore here the differential engagement of G(q) and G(13) (of the G(12) family) by the thromboxane A(2) receptor alpha (TPalpha), via agonist-effected [(35)S]-guanosine 5'-O-(3-thio)triphosphate binding when the G proteins themselves are used as reporters. We find for TPalpha introduced into human embryonic kidney 293 cells and for the receptor expressed normally in human platelets an agonist-selective engagement of G(q) versus G(13). Pinane thromboxane A(2) (PTA(2)) activates G(q) in preference to G(13), whereas 8-iso-prostaglandin F(2alpha) activates G(13) in preference to G(q). 9,11-Dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619), in contrast, exhibits no preference. Reserve of receptor in relation to G protein and of G protein in relation to downstream events is apparent in some instances but does not have a bearing on selectivity. Activation of G proteins by PTA(2) is right-shifted from binding of the ligand to receptor, a manifestation of which is a bimodal action: PTA(2) is an antagonist at low concentrations and an agonist at higher concentrations. We posit two populations of TPalpha, or two intrinsic sites of ligand binding, with selectivity evident not only in terms of the G proteins activated but properties of antagonism versus agonism.