Role of cyclooxygenase-1-mediated prostacyclin synthesis in endothelium-dependent vasoconstrictor activity of porcine interlobular renal arteries

Prostacyclin Synthase Deficiency Leads to Exacerbation or Occurrence of Endothelium-Dependent Contraction and Causes Cardiovascular Disorders Mainly via the Non-TxA2 Prostanoids/TP Axis

BACKGROUND

Prostaglandin I2 synthesized by endothelial COX (cyclooxygenase) evokes potent vasodilation in some blood vessels but is paradoxically responsible for endothelium-dependent constriction (EDC) in others. Prostaglandin I2 production and EDC may be enhanced in diseases such as hypertension. However, how PGIS (prostaglandin I2 synthase) deficiency affects EDC and how this is implicated in the consequent cardiovascular pathologies remain largely unknown.

METHODS

Experiments were performed with wild-type, Pgis knockout (Pgis-/-) and Pgis/thromboxane-prostanoid receptor gene (Tp) double knockout (Pgis-/-Tp-/-) mice and Pgis-/- mice transplanted with unfractionated wild-type or Cox-1-/- bone marrow cells, as well as human umbilical arteries. COX-derived prostanoids were measured by high-performance liquid chromatography-mass spectrometry. Vasomotor responses of distinct types of arteries were assessed by isometric force measurement. Parameters of hypertension, vascular remodeling, and cardiac hypertrophy in mice at different ages were monitored.

RESULTS

PGF2α, PGE2, and a trace amount of PGD2, but not thromboxane A2 (TxA2), were produced in response to acetylcholine in Pgis-/- or PGIS-inhibited arteries. PGIS deficiency resulted in exacerbation or occurrence of EDC ex vivo and in vivo. Endothelium-dependent hyperpolarization was unchanged, but phosphorylation levels of eNOS (endothelial nitric oxide synthase) at Ser1177 and Thr495 were altered and NO production and the NO-dependent relaxation evoked by acetylcholine were remarkably reduced in Pgis-/- aortas. Pgis-/- mice developed high blood pressure and vascular remodeling at 16 to 17 weeks and subsequently cardiac hypertrophy at 24 to 26 weeks. Meanwhile, blood pressure and cardiac parameters remained normal at 8 to 10 weeks. Additional ablation of TP (TxA2 receptor) not only restrained EDC and the downregulation of NO signaling in Pgis-/- mice but also ameliorated the cardiovascular abnormalities. Stimulation of Pgis-/- vessels with acetylcholine in the presence of platelets led to increased TxA2 generation. COX-1 disruption in bone marrow-derived cells failed to affect the development of high blood pressure and vascular remodeling in Pgis-/- mice though it largely suppressed the increase of plasma TxB2 (TxA2 metabolite) level.

CONCLUSIONS

Our study demonstrates that the non-TxA2 prostanoids/TP axis plays an essential role in mediating the augmentation of EDC and cardiovascular disorders when PGIS is deficient, suggesting TP as a promising therapeutic target in diseases associated with PGIS insufficiency.

The vasoconstrictor activities of prostaglandin D2 via the thromboxane prostanoid receptor and E prostanoid receptor-3 outweigh its concurrent vasodepressor effect mainly through D prostanoid receptor-1 ex vivo and in vivo

Prostaglandin (PG) D2, a commonly considered vasodilator through D prostanoid receptor-1 (DP1), might also evoke vasoconstriction via acting on the thromboxane (Tx)-prostanoid receptor (the original receptor of TxA2; TP) and/or E prostanoid receptor-3 (one of the vasoconstrictor receptors of PGE2; EP3). This study aimed to test the above hypothesis in the mouse renal vascular bed (main renal arteries and perfused kidneys) and/or mesenteric resistance arteries and determine how the vasoconstrictor mechanism influences the overall PGD2 effect on systemic blood pressure under in vivo conditions. Experiments were performed on control wild-type (WT) mice and mice with deficiencies in TP (TP-/-) and/or EP3 (EP3-/-). Here we show that PGD2 indeed evoked vasoconstrictor responses in the above-mentioned tissues of WT mice, which were however not only reduced by TP-/- or EP3-/-, but also reversed by TP-/-/EP3-/- in some of the above tissues (mesenteric resistance arteries or perfused kidneys) to dilator reactions that were reduced by non-selective DP antagonism. A slight or mild pressor response was also observed with PGD2 under in vivo conditions, and this was again reversed to a depressor response in TP-/- or TP-/-/EP3-/- mice. Non-selective DP antagonism reduced the PGD2-evoked depressor response in TP-/-/EP3-/- mice as well. These results thus demonstrate that like other PGs, PGD2 activates TP and/or EP3 to evoke vasoconstrictor activities, which can outweigh its concurrent vasodepressor activity mediated mainly through DP1, and hence result in a pressor response, although the response might only be of a slight or mild extent.

Prostaglandin F2α evokes vasoconstrictor and vasodepressor activities that are both independent of the F prostanoid receptor

The F prostanoid receptor (FP), which accounts for the therapeutic effect of PGF_2α in uterine atony that leads to postpartum hemorrhage and maternal morbidity, could possibly mediate vasoconstrictor effect in small or resistance arteries to elevate blood pressure that limits the clinical use of the agent in patients with cardiovascular disorders. This study aimed to test the above hypothesis with genetically altered mice. Ex vivo and in vivo experiments were performed on control wild-type (WT) mice and mice with deficiencies in FP (FP^-/- ) or thromboxane (Tx)-prostanoid receptor (the original receptor of TxA₂ ; TP^-/- ), and/or those with an additional deficiency in E prostanoid receptor-3 (one of the vasoconstrictor receptors of PGE₂ ; EP3^-/- ). Here, we show that PGF_2α indeed evoked vasoconstrictor responses in the above-mentioned tissues of WT mice, which were however unaltered by FP^-/- . Interestingly, such contractile responses were reversed into dilations by TP^-/- /EP3^-/- . A similar pattern of results was observed with the pressor effect of PGF_2α under in vivo conditions. However, TP^-/- alone (which could largely remove the contractile responses) did not result in relaxation to PGF_2α . Also, either the ex vivo vasodilator effect or the in vivo depressor response of PGF_2α obtained after the removal of TP and EP3-mediated actions was unaltered by FP^-/- . Therefore, both the ex vivo vasoconstrictor action in small or resistance arteries and the systemic pressor effect of PGF_2α can reflect vasoconstrictor activities derived from the non-FP receptors TP and EP3 outweighing a concurrently activated dilator effect, which is again independent of FP.

Endothelium-dependent contraction: The non-classical action of endothelial prostacyclin, its underlying mechanisms, and implications

Although commonly thought to produce prostacyclin (prostaglandin I₂ ; PGI₂ ) that evokes vasodilatation and protects vessels from the development of diseases, the endothelial cyclooxygenase (COX)-mediated metabolism has also been found to release substance(s) called endothelium-derived contracting factor(s) (EDCF) that causes endothelium-dependent contraction and implicates in endothelial dysfunction of disease conditions. Various mechanisms have been proposed for the process; however, the major endothelial COX metabolite PGI₂ , which has been classically considered to activate the I prostanoid receptor (IP) that mediates vasodilatation and opposes the effects of thromboxane (Tx) A₂ produced by COX in platelets, emerges as a major EDCF in health and disease conditions. Our recent studies from genetically altered mice further suggest that vasomotor reactions to PGI₂ are collectively modulated by IP, the vasoconstrictor Tx-prostanoid receptor (TP; the prototype receptor of TxA₂ ) and E prostanoid receptor-3 (EP3; a vasoconstrictor receptor of PGE₂ ) although with differences in potency and efficacy; a contraction to PGI₂ reflects activities of TP and/or EP3 outweighing that of the concurrently activated IP. Here, we discuss the history of endothelium-dependent contraction, evidences that support the above hypothesis, proposed mechanisms for the varied reactions to endothelial PGI₂ synthesis as well as the relation of its dilator activity to the effect of another NO-independent vasodilator mechanism, the endothelium-derived hyperpolarizing factor. Also, we address the possible pathological and therapeutic implications as well as questions remaining to be resolved or limitations of our above findings obtained from genetically altered mouse models.

Endothelial cyclooxygenase-1 paradoxically drives local vasoconstriction and atherogenesis despite underpinning prostacyclin generation

Endothelial cyclooxygenase-1-derived prostanoids, including prostacyclin, have clear cardioprotective roles associated with their anti-thrombotic potential but have also been suggested to have paradoxical pathological activities within arteries. To date it has not been possible to test the importance of this because no models have been available that separate vascular cyclooxygenase-1 products from those generated elsewhere. Here, we have used unique endothelial-specific cyclooxygenase-1 knockout mice to show that endothelial cyclooxygenase-1 produces both protective and pathological products. Functionally, however, the overall effect of these was to drive pathological responses in the context of both vasoconstriction in vitro and the development of atherosclerosis and vascular inflammation in vivo. These data provide the first demonstration of a pathological role for the vascular cyclooxygenase-1 pathway, highlighting its potential as a therapeutic target. They also emphasize that, across biology, the role of prostanoids is not always predictable due to unique balances of context, products, and receptors.

Differential properties of E prostanoid receptor-3 and thromboxane prostanoid receptor in activation by prostacyclin to evoke vasoconstrictor response in the mouse renal vasculature

Vasomotor reactions of prostacyclin (prostaglandin I₂ ; PGI₂ ) can be collectively modulated by thromboxane prostanoid receptor (TP), E-prostanoid receptor-3 (EP3), and the vasodilator I prostanoid receptor (IP). This study aimed to determine the direct effect of PGI₂ on renal arteries and/or the whole renal vasculature and how each of these receptors is involved. Experiments were performed on vessels or perfused kidneys of wild-type mice and/or mice with deficiency in TP (TP^-/- ) and/or EP3. Here we show that PGI₂ did not evoke relaxation, but instead resulted in contraction of main renal arteries (from ~0.001-0.01 µM) or reduction of flow in perfused kidneys (from ~1 µM); either of them was reversed into a dilator response in TP^-/- /EP3^-/- counterparts. Also, we found that in renal arteries although it has a lesser effect than TP^-/- on the maximal contraction to PGI₂ (10 µM), EP3^-/- but not TP^-/- resulted in relaxation to the prostanoid at 0.01-1 µM. Meanwhile, TP^-/- only significantly reduced the contractile activity evoked by PGI₂ at ≥0.1 µM. These results demonstrate that PGI₂ may evoke an overall vasoconstrictor response in the mouse renal vasculature, reflecting activities of TP and EP3 outweighing that of the vasodilator IP. Also, our results suggest that EP3, on which PGI₂ can have a potency similar to that on IP, plays a major role in the vasoconstrictor effect of the prostanoid of low concentrations (≤1 µM), while TP, on which PGI₂ has a lower potency but higher efficacy, accounts for a larger part of its maximal contractile activity.

Prostaglandin E2 sequentially activates E-prostanoid receptor-3 and thromboxane prostanoid receptor to evoke contraction and increase in resistance of the mouse renal vasculature

Although recognized to have an in vivo vasodepressor effect blunted by the vasoconstrictor effect of E-prostanoid receptor-3 (EP3), prostaglandin E₂ (PGE₂ ) evokes contractions of many vascular beds that are sensitive to antagonizing the thromboxane prostanoid receptor (TP). This study aimed to determine the direct effect of PGE₂ on renal arteries and/or the whole renal vasculature and how each of these two receptors is involved in the responses. Experiments were performed on isolated vessels and perfused kidneys of wild-type mice and/or mice with deficiency in TP (TP^-/- ), EP3 (EP3^-/- ), or both TP and EP3 (TP^-/- /EP3^-/- ). Here we show that PGE₂ (0.001-30 μM) evoked not only contraction of main renal arteries, but also a decrease of flow in perfused kidneys. EP3^-/- diminished the response to 0.001-0.3 μM PGE₂ , while TP^-/- reduced that to the prostanoid of higher concentrations. In TP^-/- /EP3^-/- vessels and perfused kidneys, PGE₂ did not evoke contraction but instead resulted in vasodilator responses. These results demonstrate that PGE₂ functions as an overall direct vasoconstrictor of the mouse renal vasculature with an effect reflecting the vasoconstrictor activities outweighing that of dilation. Also, our results suggest that EP3 dominates the vasoconstrictor effect of PGE₂ of low concentrations (≤0.001-0.3 μM), but its effect is further added by that of TP, which has a higher efficacy, although activated by higher concentrations (from 0.01 μM) of the same prostanoid PGE₂ .

Molecular Dynamics Studies on COX-2 Protein-tyrosine Analogue Complex and Ligand-based Computational Analysis of Halo-substituted Tyrosine Analogues

Background:

The quest for new drug entities and novel structural fragments with applications in therapeutic areas is always at the core of medicinal chemistry.

Methods:

As part of our efforts to develop novel selective cyclooxygenase-2 (COX-2) inhibitors containing tyrosine scaffold. The objective of this study was to identify potent COX-2 inhibitors by dynamic simulation, pharmacophore and 3D-QSAR methodologies. Dynamics simulation was performed for COX-2/tyrosine derivatives complex to characterise structure validation and binding stability. Certainly, Arg120 and Tyr355 residue of COX-2 protein formed a constant interaction with tyrosine inhibitor throughout the dynamic simulation phase. A four-point pharmacophore with one hydrogen bond acceptor, two hydrophobic and one aromatic ring was developed using the HypoGen algorithm. The generated, statistically significant pharmacophore model, Hypo 1 with a correlation coefficient of r2, 0.941, root mean square deviation, 1.15 and total cost value of 96.85.

Results:

The QSAR results exhibited good internal (r2, 0.992) and external predictions (r2pred, 0.814). The results of this study concluded the COX-2 docked complex was stable and interactive like experimental protein structure. Also, it offered vital chemical features with geometric constraints responsible for the inhibition of the selective COX-2 enzyme by tyrosine derivatives.

Conclusion:

In principle, this work offers significant structural understandings to design and develop novel COX-2 inhibitors.

TP and/or EP3 receptors mediate the vasoconstrictor and pressor responses of prostaglandin F2α in mice and/or humans

The vasoconstrictor and/or pressor effects of prostaglandin (PG)F_2α participate in the development of vascular pathologies and limit the clinical use of the agent. This study aimed to determine the receptor types responsible for the vasoconstrictor activity of PGF_2α and whether they mediate the pressor response evoked by the prostanoid under in vivo conditions. Experiments were performed on genetically altered mice and/or on vessels from these mice or humans. Here we show that deletion of the thromboxane-prostanoid receptor (TP^-/-) abolished or drastically diminished the contraction to PGF_2α in isolated mouse vessels (some of which were resistance arteries) and reduced the elevation in blood pressure evoked by the prostanoid under in vivo conditions. In accordance, TP antagonism abolished the contraction in small arteries of human omentum. Further deletion of E prostanoid receptor type 3 (EP3^-/-) removed the PGF_2α-evoked contraction that remained in some TP^-/- arteries and added to the effect of TP^-/- on the elevation in blood pressure evoked by the prostanoid under in vivo conditions. In contrast, the uterine contraction to PGF_2α mediated via the F prostanoid receptor (FP) was unaltered in TP^-/-/EP3^-/- mice. These results demonstrate that the non-FP receptors TP and/or EP3 mediate the vasoconstrictor and pressor effects of PGF_2α, which are still of concern under clinical conditions.-Liu, B., Li, J., Yan, H., Tian, D., Li, H., Zhang, Y., Guo, T., Wu, X., Luo, W., Zhou, Y. TP and/or EP3 receptors mediate the vasoconstrictor and pressor responses of prostaglandin F_2α in mice and/or humans.

EP3 Blockade Adds to the Effect of TP Deficiency in Alleviating Endothelial Dysfunction in Atherosclerotic Mouse Aortas

Endothelial dysfunction, which leads to ischemic events under atherosclerotic conditions, can be attenuated by antagonizing the thromboxane-prostanoid receptor (TP) that mediates the vasoconstrictor effect of prostanoids including prostacyclin (PGI₂). This study aimed to determine whether antagonizing the E prostanoid receptor-3 (EP3; which can also be activated by PGI₂) adds to the above effect of TP deficiency (TP^-/-) under atherosclerotic conditions and if so, the underlying mechanism(s). Atherosclerosis was induced in ApoE^-/- mice and those with ApoE^-/- and TP^-/-. Here, we show that in phenylephrine pre-contracted abdominal aortic rings with atherosclerotic lesions of ApoE^-/-/TP^-/- mice, although an increase of force (which was larger than that of non-atherosclerotic controls) evoked by the endothelial muscarinic agonist acetylcholine to blunt the concurrently activated relaxation in ApoE^-/- counterparts was largely removed, the relaxation evoked by the agonist was still smaller than that of non-atherosclerotic TP^-/- mice. EP3 antagonism not only increased the above relaxation, but also reversed the contractile response evoked by acetylcholine in NO synthase-inhibited atherosclerotic ApoE^-/-/TP^-/- rings into a relaxation sensitive to I prostanoid receptor antagonism. In ApoE^-/- atherosclerotic vessels the expression of endothelial NO synthase was decreased, yet the production of PGI₂ (which evokes contraction via both TP and EP3) evoked by acetylcholine was unaltered compared to non-atherosclerotic conditions. These results demonstrate that EP3 blockade adds to the effect of TP^-/- in uncovering the dilator action of natively produced PGI₂ to alleviate endothelial dysfunction in atherosclerotic conditions.

Increased role of E prostanoid receptor-3 in prostacyclin-evoked contractile activity of spontaneously hypertensive rat mesenteric resistance arteries

This study aimed to determine whether E prostanoid receptor-3 (EP3) is involved in prostacyclin (PGI₂)-evoked vasoconstrictor activity of resistance arteries and if so, how it changes under hypertensive conditions. Mesenteric resistance arteries from Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHRs) were isolated for functional and biochemical studies. Here we show that in vessels from WKYs, PGI₂ or the endothelial muscarinic agonist ACh (which stimulates in vitro PGI₂ synthesis) evoked vasoconstrictor activity, which increased in SHRs. The thromboxane-prostanoid receptor (TP) antagonist SQ29548 partially removed the vasoconstrictor activity, and an increased contractile activity of PGI₂ resistant to SQ29548 was observed in SHRs. Interestingly, L798106, an antagonist of EP3 (whose expression was higher in SHRs than in WKYs), not only added to the effect of SQ29548 but also caused relaxation to PGI₂ more than that obtained with SQ29548. In accordance, EP3 deletion, which reduced PGI₂-evoked contraction, together with SQ29548 resulted in relaxation evoked by the agonist in mouse aortas. These results thus demonstrate an explicit involvement of EP3 in PGI₂-evoked vasoconstrictor activity in rat mesenteric resistance arteries and suggest that up-regulation of the receptor contributes significantly to the increased contractile activity evoked by PGI₂ under hypertensive conditions.

Role of E-type prostaglandin receptor EP3 in the vasoconstrictor activity evoked by prostacyclin in thromboxane-prostanoid receptor deficient mice

Prostacyclin, also termed as prostaglandin I₂ (PGI₂), evokes contraction in vessels with limited expression of the prostacyclin receptor. Although the thromboxane-prostanoid receptor (TP) is proposed to mediate such a response of PGI₂, other unknown receptor(s) might also be involved. TP knockout (TP^-/-) mice were thus designed and used to test the hypothesis. Vessels, which normally show contraction to PGI₂, were isolated for functional and biochemical analyses. Here, we showed that the contractile response evoked by PGI₂ was indeed only partially abolished in the abdominal aorta of TP^-/- mice. Interestingly, further antagonizing the E-type prostaglandin receptor EP3 removed the remaining contractile activity, resulting in relaxation evoked by PGI₂ in such vessels of TP^-/- mice. These results suggest that EP3 along with TP contributes to vasoconstrictor responses evoked by PGI₂, and hence imply a novel mechanism for endothelial cyclooxygenase metabolites (which consist mainly of PGI₂) in regulating vascular functions.

The endothelial cyclooxygenase pathway: Insights from mouse arteries

To date, cyclooxygenase-2 (COX-2) is commonly believed to be the major mediator of endothelial prostacyclin (prostaglandin I2; PGI2) synthesis that balances the effect of thromboxane (Tx) A2 synthesis mediated by the other COX isoform, COX-1 in platelets. Accordingly, selective inhibition of COX-2 is considered to cause vasoconstriction, platelet aggregation, and hence increase the incidence of cardiovascular events. This idea has been claimed to be substantiated by experiments on mouse models, some of which are deficient in one of the two COX isoforms. However, results from our studies and those of others using similar mouse models suggest that COX-1 is the major functional isoform in vascular endothelium. Also, although PGI2 is recognized as a potent vasodilator, in some arteries endothelial COX activation causes vasoconstrictor response. This has again been recognized by studies, especially those performed on mouse arteries, to result largely from endothelial PGI2 synthesis. Therefore, evidence that supports a role for COX-1 as the major mediator of PGI2 synthesis in mouse vascular endothelium, reasons for the inconsistency, and results that elucidate underlying mechanisms for divergent vasomotor reactions to endothelial COX activation will be discussed in this review. In addition, we address the possible pathological implications and limitations of findings obtained from studies performed on mouse arteries.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

Role of cyclooxygenase-1 and -2 in endothelium-dependent contraction of atherosclerotic mouse abdominal aortas

The objective of this study was to determine the role of cyclooxygenase (COX)-1 or -2 in endothelium-dependent contraction under atherosclerotic conditions. Atherosclerosis was induced in apoE knockout (apoE(-/-)) mice and those with COX-1(-/-) (apoE(-/-)-COX-1(-/-)) by feeding with high fat and cholesterol food. Aortas (abdominal or the whole section) were isolated for functional and/or biochemical analyses. As in non-atherosclerotic conditions, the muscarinic receptor agonist acetylcholine (ACh) evoked an endothelium-dependent, COX-mediated contraction following NO synthase (NOS) inhibition in abdominal aortic rings from atherosclerotic apoE(-/-) mice. Interestingly, COX-1 inhibition not only abolished such a contraction in rings showing normal appearance, but also diminished that in rings with plaques. Accordingly, only a minor contraction (<30% that of apoE(-/-) counterparts) was evoked by ACh (following NOS inhibition) in abdominal aortic rings of atherosclerotic apoE(-/-)-COX-1(-/-) mice with plaques, and none was evoked in those showing normal appearance. Also, the contraction evoked by ACh in apoE(-/-)-COX-1(-/-) abdominal aortic rings with plaques was abolished by non-selective COX inhibition, thromboxane-prostanoid (TP) receptor antagonism, or endothelial denudation. Moreover, it was noted that ACh evoked a predominant production of the prostacyclin (PGI2, which mediates abdominal aortic contraction via TP receptors in mice) metabolite 6-keto-PGF1α, which was again sensitive to COX-1 inhibition or COX-1(-/-). Therefore, in atherosclerotic mouse abdominal aortas, COX-1 can still be the major isoform mediating endothelium-dependent contraction, which probably results largely from PGI2 synthesis as in non-atherosclerotic conditions. In contrast, COX-2 may have only a minor role in such response limited to areas of plaques under the same pathological condition.

A vasoconstrictor response to COX-1-mediated prostacyclin synthesis in young rat renal arteries that increases in prehypertensive conditions

This study aimed to determine whether prostacyclin (PGI2) functions as an endothelium-derived contracting factor (EDCF) in young rat renal arteries, and, if so, we wanted to examine the underlying mechanism(s) and how it changes in prehypertensive conditions. Vessels from Wistar-Kyoto (WKY) and prehypertensive spontaneously hypertensive rats (SHRs) of 25-28 days of age were isolated for functional and biochemical analyses. Result showed that following NO synthase (NOS) inhibition PGI2 and the thromboxane-prostanoid (TP) receptor agonist U-46619 evoked contractions in young WKY renal arteries that were similar to those in prehypertensive SHRs. Meanwhile, the endothelial muscarinic receptor agonist ACh evoked an endothelium-dependent contraction under NOS-inhibited conditions and a production of the PGI2 metabolite 6-keto-PGF1α; both were sensitive to cyclooxygenase (COX) and/or COX-1 inhibition but higher in prehypertensive SHRs than in young WKYs. Interestingly, in WKY renal arteries PGI2 did not evoke relaxation even after TP receptor antagonism that diminished the contraction evoked by the agonist. Indeed, PGI2 (IP) receptors were not detected in the vessel with Western blot. Moreover, we noted that treatment with the nonselective COX inhibitor indomethacin, which was started at the prehypertensive stage, blunted the elevation of systolic blood pressure and reduced the heart-to-body ratio in SHR within 2 mo of treatment. These results demonstrate that due to scarcity of IP receptors, PGI2, which is derived mainly from COX-1-mediated metabolism, acts as an EDCF in young WKY renal arteries, and it increases in prehypertensive conditions. Also, our data revealed that COX inhibition starting from the prehypertensive stage has an antihypertensive effect in young SHRs.

COX-2 is involved in vascular oxidative stress and endothelial dysfunction of renal interlobar arteries from obese Zucker rats

Obesity is related to vascular dysfunction through inflammation and oxidative stress and it has been identified as a risk factor for chronic renal disease. In the present study, we assessed the specific relationships among reactive oxygen species (ROS), cyclooxygenase 2 (COX-2), and endothelial dysfunction in renal interlobar arteries from a genetic model of obesity/insulin resistance, the obese Zucker rats (OZR). Relaxations to acetylcholine (ACh) were significantly reduced in renal arteries from OZR compared to their counterpart, the lean Zucker rat (LZR), suggesting endothelial dysfunction. Blockade of COX with indomethacin and with the selective blocker of COX-2 restored the relaxations to ACh in obese rats. Selective blockade of the TXA2/PGH2 (TP) receptor enhanced ACh relaxations only in OZR, while inhibition of the prostacyclin (PGI2) receptor (IP) enhanced basal tone and inhibited ACh vasodilator responses only in LZR. Basal production of superoxide was increased in arteries of OZR and involved NADPH and xanthine oxidase activation and NOS uncoupling. Under conditions of NOS blockade, ACh induced vasoconstriction and increased ROS generation that were augmented in arteries from OZR and blunted by COX-2 inhibition and by the ROS scavenger tempol. Hydrogen peroxide (H2O2) evoked both endothelium- and vascular smooth muscle (VSM)-dependent contractions, as well as ROS generation that was reduced by COX-2 inhibition. In addition, COX-2 expression was enhanced in both VSM and endothelium of renal arteries from OZR. These results suggest that increased COX-2-dependent vasoconstriction contributes to renal endothelial dysfunction through enhanced (ROS) generation in obesity. COX-2 activity is in turn upregulated by ROS.

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-PGF1α 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.

Vasoconstrictor role of cyclooxygenase-1-mediated prostacyclin synthesis in non-insulin-dependent diabetic mice induced by high-fat diet and streptozotocin

This study tested the hypothesis that in diabetic arteries, cyclooxygenase (COX)-1 mediates endothelial prostacyclin (PGI2) synthesis, which evokes vasoconstrictor activity under the pathological condition. Non-insulin-dependent diabetes was induced to C57BL/6 mice and those with COX-1 deficiency (COX-1−/− mice) using a high-fat diet in combination with streptozotocin injection. In vitro analyses were performed 3 mo after. Results showed that in diabetic aortas, the endothelial muscarinic receptor agonist ACh evoked an endothelium-dependent production of the PGI2 metabolite 6-keto-PGF1α, which was abolished in COX-1−/− mice. Meanwhile, COX-1 deficiency or COX-1 inhibition prevented vasoconstrictor activity in diabetic abdominal aortas, resulting in enhanced relaxation evoked by ACh. In a similar manner, COX-1 deficiency increased the relaxation evoked by ACh in nitric oxide synthase-inhibited diabetic renal arteries. Also, in diabetic abdominal aortas and/or renal arteries, both PGI2 and the COX substrate arachidonic acid evoked contractions similar to those of nondiabetic mice. However, the contraction to arachidonic acid, but not that to PGI2, was abolished in vessels from COX-1−/− mice. Moreover, we found that 3 mo after streptozotocin injection, systemic blood pressure increased in diabetic C57BL/6 mice but not in diabetic COX-1−/− mice. These results explicitly demonstrate that in the given arteries from non-insulin-dependent diabetic mice, COX-1 remains a major contributor to the endothelial PGI2 synthesis that evokes vasoconstrictor activity under the pathological condition. Also, our data suggest that COX-1 deficiency prevents or attenuates diabetic hypertension in mice, although this could be related to the loss of COX-1-mediated activities derived from both vascular and nonvascular tissues.

The effect of bradykinin on the electrical activity of rat myenteric neurons

Bradykinin is a mediator involved in inflammatory processes in the gut. Here we investigated the effect of bradykinin on the electrical activity of rat myenteric neurons, the key players for regulation of gastrointestinal motility. Bradykinin (2 × 10(-8)mol/l) induced a biphasic increase in frequency of action potentials measured with microelectrode arrays. This increase was mirrored by a biphasic increase in cytosolic Ca(2+) concentration ([Ca(2+)]i), which was observed in about 40% of the myenteric neurons. The bradykinin B1 receptor agonist des-arg(9)-bradykinin as well as the bradykinin B2 receptor agonist hyp(3)-bradykinin induced a similar effect on [Ca(2+)]i. Immunocytochemical stainings confirmed the expression of both receptor types by myenteric ganglionic cells. Real time PCR showed that the inducible B1 receptor was upregulated during cell culture. The inhibition of cyclooxygenases with piroxicam reduced the effect of bradykinin on the electrical activity of myenteric neurons. The suppression of the glial growth on microelectrode arrays did not affect the bradykinin-induced change in frequency of action potentials. This suggests that prostaglandins, which probably mediate the effect of bradykinin, are not exclusively released from glial cells. The bradykinin-induced increase in [Ca(2+)]i was dependent on the presence of extracellular Ca(2+) and was inhibited by Co(2+), Cd(2+), and Ni(2+), blockers of voltage-dependent Ca(2+) channels, indicating a stimulation of the influx of extracellular Ca(2+) by the kinin. Consequently, bradykinin induces a Ca(2+) influx in myenteric neurons via Ca(2+) channels in the plasma membrane.

New roles for old pathways? A circuitous relationship between reactive oxygen species and cyclo-oxygenase in hypertension

Elevated production of prostanoids from the constitutive (COX-1) or inducible (COX-2) cyclo-oxygenases has been involved in the alterations in vascular function, structure and mechanical properties observed in cardiovascular diseases, including hypertension. In addition, it is well known that production of ROS (reactive oxygen species) plays an important role in the impaired contractile and vasodilator responses, vascular remodelling and altered vascular mechanics of hypertension. Of particular interest is the cross-talk between NADPH oxidase and mitochondria, the main ROS sources in hypertension, which may represent a vicious feed-forward cycle of ROS production. In recent years, there is experimental evidence showing a relationship between ROS and COX-derived products. Thus ROS can activate COX and the COX/PG (prostaglandin) synthase pathways can induce ROS production through effects on different ROS generating enzymes. Additionally, recent evidence suggests that the COX-ROS axis might constitute a vicious circle of self-perpetuating vasoactive products that have a pathophysiological role in altered vascular contractile and dilator responses and hypertension development. The present review discusses the current knowledge on the role of oxidative stress and COX-derived prostanoids in the vascular alterations observed in hypertension, highlighting new findings indicating that these two pathways act in concert to induce vascular dysfunction.

A vasoconstrictor role for cyclooxygenase-1-mediated prostacyclin synthesis in mouse renal arteries

This study was to determine whether prostacyclin [prostaglandin I2 (PGI2)] evokes mouse renal vasoconstriction and, if so, the underlying mechanism(s) and how its synthesis via cyclooxygenase-1 (COX-1) influences local vasomotor reaction. Experiments were performed on vessels from C57BL/6 mice and/or those with COX-1 deficiency (COX-1−/−). Results showed that in renal arteries PGI2 evoked contraction more potently than in carotid arteries, where COX-1 is suggested to mediate prominent endothelium-dependent contraction. A similar result was observed with the thromboxane-prostanoid (TP) receptor agonist U46619. However, in renal arteries TP receptor antagonism, which inhibited the contraction, did not result in any relaxation in response to PGI2. Moreover, we noted that the endothelial muscarinic receptor agonist ACh evoked an increase in the production of the PGI2 metabolite 6-keto-PGF1α, which was prevented by endothelial denudation or COX-1−/−. Interestingly, COX-1−/− was further found to abolish a force development that was sensitive to TP receptor antagonism and result in enhanced relaxation evoked by ACh following NO synthase inhibition. Also, in renal arteries the COX substrate arachidonic acid evoked a vasoconstrictor response, which was again abolished by COX-1−/−. Meanwhile, nonselective COX inhibition did not show any effect in vessels from COX-1−/− mice. Thus, in mouse renal arteries, high expression of TP receptors together with little functional involvement from the vasodilator PGI2 receptors results in a potent vasoconstrictor effect evoked by PGI2. Also, our data imply that endogenous COX-1-mediated PGI2 synthesis leads to vasoconstrictor activity and this could be an integral part of endothelium-derived mechanisms in regulating local renal vascular function.

Receptors and mechanisms mediating the biphasic response evoked by bradykinin in rat colonic smooth muscle

BACKGROUND

In rat duodenum, bradykinin induces a relaxation followed by a contraction. Different types of ion channels and receptors as well as non-muscle cells have been suggested to be involved in this response. As it is unclear whether these changes are observed also in rat large intestine and the mechanisms which might underlie this response, the effect of bradykinin on rat colonic motility was tested.

METHODS

Isometric contractions were measured on full-thickness preparations or preparations, from which individual layers had been dissected. The expression of bradykinin receptors was analyzed by immunohistochemistry and RT-PCR. Isolated intestinal muscle cells were investigated with Ca(2+) -imaging techniques.

KEY RESULTS

Bradykinin caused a biphasic contractile response (initial relaxation followed by contraction) in rat colon, which was resistant against tetrodotoxin. The kinin-induced relaxation was inhibited by tetrapentylammonium chloride, a blocker of Ca(2+) -activated K(+) channels. Des-arg(9) -bradykinin did not induce any effect on the native colon, although after 5 h in vitro preincubation, a contractile response was evoked by this B1 receptor agonist. The consecutive ablation of adherent layers of the intestinal wall strongly reduced the response to bradykinin in comparison with a control stimulus, i.e., carbachol, suggesting a contribution of non-muscle cells in the mediation of this response.

CONCLUSIONS & INFERENCES

Bradykinin induced a biphasic change in contractility in the rat colon. In the native intestine, only the B2 receptor is involved in this effect. Neighboring cell obviously sensitize the smooth muscle to the stimulation of these receptors.

Cyclo-oxygenase-1 or -2-mediated metabolism of arachidonic acid in endothelium-dependent contraction of mouse arteries

This study aimed to determine whether the cyclo-oxygenase (COX) substrate arachidonic acid (AA) evokes endothelium-dependent contraction and, if so, the specific COX isoform(s) involved and whether prostacyclin (prostaglandin I2; PGI2), a mediator of endothelium-derived vasoconstrictor activity, can be generated in medial smooth muscle from the intermediate COX product prostaglandin H2 (PGH2) that might diffuse from the endothelium. Aortae and/or carotid arteries were isolated from C57BL/6 mice or those lacking one of the two COX isoforms (COX-1(-/-) or COX-2(-/-)) for functional and/or biochemical analyses. Results showed that in vessels from C57BL/6 mice, exogenous AA evoked not only endothelium-dependent production of the PGI2 metabolite 6-keto-PGF1α, but also contractions reduced by thromboxane-prostanoid receptor antagonism or endothelial denudation. The minimal concentration for AA to evoke contraction was 0.3 μm, a level thought to activate only COX-2. However, neither the contraction nor 6-keto-PGF1α production was altered in vessels from COX-2(-/-) mice, while both were reduced in COX-1(-/-) counterparts. In vessels from COX-1(-/-) mice, AA also caused minor contractions that were sensitive to non-selective COX inhibition. Real-time PCR showed that like COX-1, COX-2 mainly existed in the endothelium, but it was unaltered in COX-1(-/-) mice. Also, we noted that in endothelium-denuded aortae, PGH2 generated PGI2 as in intact vessels. These results demonstrate a predominant role for COX-1 and suggest that in the given mouse arteries, metabolites from either COX isoform cause contraction. Moreover, our results imply that some of the PGI2 involved in vasoconstrictor activity of endothelial COX-mediated metabolism could possibly be generated from PGH2 in medial smooth muscle.

Effect of celecoxib on cyclooxygenase-1-mediated prostacyclin synthesis and endothelium-dependent contraction in mouse arteries

This study aimed to determine whether a cyclooxygenase-2 (COX-2) inhibitor celecoxib influences endothelium-dependent contraction independent of its action on COX-2 and, if so, the underlying mechanism(s). Abdominal aortas and/or carotid arteries from C57BL/6 mice or those with genetic COX-2 deficiency (COX-2(-/-)) were isolated for functional and/or biochemical analyses. Result showed that following NO synthase inhibition celecoxib not only reduced the contraction evoked by acetylcholine in C57BL/6 abdominal aorta, but also that in COX-2 (-/-) mice showing a comparable magnitude. Notably, the IC(50) of celecoxib obtained in COX-2 (-/-) abdominal aorta was only ~0.364 μM. Also, celecoxib exhibited a similar effect on COX-2 (-/-) carotid arteries. Interestingly, celecoxib was not only found to inhibit the production of the prostacyclin (PGI(2)) metabolite 6-keto-PGF (1α) in COX-2 (-/-) aortas, but also caused a reduction in the contraction evoked by PGI(2), by the α(1)-adrenergic agonist phenylephrine, or by 30 mM K(+)-induced depolarization in COX-2 (-/-) and/or C57BL/6 abdominal aorta. Moreover, N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (NS398), another COX-2 inhibitor, also reduced the contraction evoked by acetylcholine or by 30 mM K(+)-induced depolarization in COX-2 (-/-) mice. These results demonstrate explicitly that in mouse arteries celecoxib not only inhibits COX-1-mediated synthesis of PGI(2) and probably some other prostanoids, but also causes a reduction in vessel contractility that is independent of either COX-2 or COX-1, leading to an inhibition of COX-1-mediated endothelium-dependent contraction with an IC(50) value far below that of it considered for COX-1 . Also, our data suggest that such effects of celecoxib could be possibly shared by some other COX-2 inhibitors, such as NS398.