Characterization of cannabinoid receptors coupled to vasorelaxation by endothelium-derived hyperpolarizing factor

Cyclooxygenase metabolism mediates vasorelaxation to 2-arachidonoylglycerol (2-AG) in human mesenteric arteries

Objective

The vasorelaxant effect of 2-arachidonoylglycerol (2-AG) has been well characterised in animals. 2-AG is present in human vascular cells and is up-regulated in cardiovascular pathophysiology. However, the acute vascular actions of 2-AG have not been explored in humans.

Approach

Mesenteric arteries were obtained from patients receiving colorectal surgery and mounted on a myograph. Arteries were contracted and 2-AG concentration–response curves were carried out. Mechanisms of action were characterised pharmacologically. Post hoc analysis was carried out to assess the effects of cardiovascular disease/risk factors on 2-AG responses.

Results

2-AG caused vasorelaxation of human mesenteric arteries, independent of cannabinoid receptor or transient receptor potential vanilloid-1 activation, the endothelium, nitric oxide or metabolism via monoacyglycerol lipase or fatty acid amide hydrolase. 2-AG-induced vasorelaxation was reduced in the presence of indomethacin and flurbiprofen, suggesting a role for cyclooxygenase metabolism 2-AG. Responses to 2-AG were also reduced in the presence of Cay10441, L-161982 and potentiated in the presence of AH6809, suggesting that metabolism of 2-AG produces both vasorelaxant and vasoconstrictor prostanoids. Finally, 2-AG-induced vasorelaxation was dependent on potassium efflux and the presence of extracellular calcium.

Conclusions

We have shown for the first time that 2-AG causes vasorelaxation of human mesenteric arteries. Vasorelaxation is dependent on COX metabolism, activation of prostanoid receptors (EP₄ & IP) and ion channel modulation. 2-AG responses are blunted in patients with cardiovascular risk factors.

Silicic acid in drinking water prevents age-related alterations in the endothelium-dependent vascular relaxation modulating eNOS and AQP1 expression in experimental mice: an immunohistochemical study

The maintenance of endothelial integrity is of great importance in coping with age-related vascular alterations. Endothelium-derived nitric oxide is one of the various vasoactive substances able to regulate vascular tone and homeostasis, and whose decrease is known to be related with senescence in endothelial cells. There are reports on the efficacy of silicon, especially as silicic acid, in protecting vascular integrity during age-related vascular diseases. The aim of this study was to evaluate the ability of supplementation of silicic acid in drinking water in the maintenance of vascular health in a mouse model of early physiological aging. In particular, we evaluated the relationship between Si supplementation and endothelial nitric oxide synthase (eNOS) expression, taking into account also the aquaporin-1 (AQP-1) isoform that, as recently reported, seems to be involved in nitric oxide transport across cell membranes. Our results showed that silicic acid supplementation increased both eNOS and AQP-1 expression, suggesting that silicic acid modulation of endothelial nitric oxide synthase and aquaporin-1 could represent a potential strategy against age-related vascular senescence.

The role of the endocannabinoid system in gametogenesis, implantation and early pregnancy

Maternal use of marijuana, in which the exocannabinoid Delta(9)-tetrahydrocannabinol is the most active psychoactive ingredient, is known to have adverse effects on various aspects of reproduction including ovulation, spermatogenesis, implantation and pregnancy duration. Endogenous cannabinoids of which Anandamide is the prototype are widely distributed in the body especially in the reproductive tract and pregnancy tissues and act through the same receptors as the receptor as Delta(9)-tetrahydrocannabinol. Anandamide, has been reported to have pleiotropic effects on human reproduction and in experimental animal models. It appears to be the important neuro-cytokine mediator synchronizing the embryo-endometrial development for timed implantation, the development of the embryo into the blastocyst and transport of the embryo across the fallopian tubes. The mechanisms by which it exerts these effects are unclear but could be via direct actions on the various sites within the reproductive system or its differential actions on vascular tone dependent. In this review article we bring together the current knowledge on the role of endoccanabinoids in reproduction and postulate on the potential mechanisms on how these affect reproduction. In addition, we examine its role on the endothelium and vascular smooth muscle as a potential mechanism for adverse pregnancy outcome.

The Novel Imidazopyridine 2-[2-(4-Methoxy-pyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine (BYK191023) Is a Highly Selective Inhibitor of the Inducible Nitric-Oxide Synthase

We have identified imidazopyridine derivatives as a novel class of NO synthase inhibitors with high selectivity for the inducible isoform. 2-[2-(4-Methoxy-pyridin-2-yl)-ethyl]-3H-imidazo[4,5-b]pyridine (BYK191023) showed half-maximal inhibition of crudely purified human inducible (iNOS), neuronal (nNOS), and endothelial (eNOS) NO synthases at 86 nM, 17 microM, and 162 microM, respectively. Inhibition of inducible NO synthase was competitive with l-arginine, pointing to an interaction of BYK191023 with the catalytic center of the enzyme. In radioligand and surface plasmon resonance experiments, BYK191023 exhibited an affinity for iNOS, nNOS, and eNOS of 450 nM, 30 microM, and >500 microM, respectively. Inhibition of cellular nitrate/nitrite synthesis in RAW, rat mesangium, and human embryonic kidney 293 cells after iNOS induction showed 40- to 100-fold higher IC(50) values than at the isolated enzyme, in agreement with the much higher l-arginine concentrations in cell culture media and inside intact cells. BYK191023 did not show any toxicity in various rodent and human cell lines up to high micromolar concentrations. The inhibitory potency of BYK191023 was tested in isolated organ models of iNOS (lipopolysaccharide-treated and phenylephrine-precontracted rat aorta; IC(50) = 7 microM), eNOS (arecaidine propargyl ester-induced relaxation of phenylephrine-precontracted rat aorta; IC(50) > 100 microM), and nNOS (field-stimulated relaxation of phenylephrine-precontracted rabbit corpus cavernosum; IC(50) > 100 microM). These data confirm the high selectivity of BYK191023 for iNOS over eNOS and nNOS found at isolated enzymes. In summary, we have identified a new highly selective iNOS inhibitor structurally unrelated to known compounds and l-arginine. BYK191023 is a valuable tool for the investigation of iNOS-mediated effects in vitro and in vivo.

UCM707, a potent and selective inhibitor of endocannabinoid uptake, potentiates hypokinetic and antinociceptive effects of anandamide

To date, UCM707, N-(3-furylmethyl)eicosa-5,8,11,14-tetraenamide, has the highest potency and selectivity in vitro as inhibitor of the endocannabinoid transporter, which might make this compound useful in potentiating endocannabinoid transmission, with minimal side-effects, in the treatment of several disorders. However, there is no information about how UCM707 behaves in vivo as regards certain classic effects of endocannabinoids, such as hypomotility and antinociception. In the present work, we tested in rats the dose-response effects of UCM707 in the open-field and hot-plate tests, and, in particular, we analyzed whether this compound enhanced the hypokinetic and/or the antinociceptive actions of anandamide at a subeffective dose, using these two in vivo assays. UCM707, administered alone, had no effect on ambulatory, exploratory and stereotypic activities, time spent in inactivity and sensitivity to noxious heat, with only some small responses at the highest dose used. UCM707, administered at a dose that did not produce any effects by itself or these were very small, was, however, able to significantly potentiate the action of a dose of anandamide that did not produce any effects when it was administered alone. So, the combination of both compounds produced greater decreases in exploratory activity and, particularly in ambulation, increased the time spent in inactivity and the latency to respond to a painful stimulus. In summary, UCM707, as suggested by its in vitro properties, seems also to behave in vivo as a selective and potent inhibitor of the endocannabinoid transporter, showing negligible direct effects on the receptors for endocannabinoids but potentiating the action of these endogenous compounds. This compound is, thus, a promising tool, used alone or in combination with endocannabinoids, for the treatment of a variety of disorders.

The endogenous cannabinoid system and the basal ganglia. biochemical, pharmacological, and therapeutic aspects

New data strengthen the idea of a prominent role for endocannabinoids in the modulation of a wide variety of neurobiological functions. Among these, one of the most important is the control of movement. This finding is supported by 3 lines of evidence: (1) the demonstration of a powerful action, mostly inhibitory in nature, of synthetic and plant-derived cannabinoids and, more recently, of endocannabinoids on motor activity; (2) the presence of the cannabinoid CB(1) receptor subtype and the recent description of endocannabinoids in the basal ganglia and the cerebellum, the areas that control movement; and (3) the fact that CB(1) receptor binding was altered in the basal ganglia of humans affected by several neurological diseases and also of rodents with experimentally induced motor disorders. Based on this evidence, it has been suggested that new synthetic compounds that act at key steps of endocannabinoid activity (i.e., more-stable analogs of endocannabinoids, inhibitors of endocannabinoid reuptake or metabolism, antagonists of CB(1) receptors) might be of interest for their potential use as therapeutic agents in a variety of pathologies affecting extrapyramidal structures, such as Parkinson's and Huntington's diseases. Currently, only a few data exist in the literature studying such relationships in humans, but an increasing number of journal articles are revealing the importance of this new neuromodulatory system and arguing in favour of the funding of more extensive research in this field. The present article will review the current knowledge of this neuromodulatory system, trying to establish the future lines for research on the therapeutic potential of the endocannabinoid system in motor disorders.

Cannabinoids inhibit pre- and postjunctionally sympathetic neurotransmission in rat mesenteric arteries

The effects of cannabinoids on sympathetic neurotransmission in the rat isolated perfused mesenteric arterial bed, were investigated. Electrically evoked sympathetic neurogenic vasocontraction was inhibited by the cannabinoid receptor agonists 11-hydroxy-dimethylheptyl-Delta(8)-tetrahydrocannabinol (HU210), (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]trans-4-(3-hydroxypropyl)-cyclohexanol (CP55,940) and methanandamide, and by (+)-11-hydroxy-Delta(8)-tetrahydrocannabinol (HU211), a (+)-stereoisomer of HU210. The inhibition was unaffected by cannabinoid CB(1) and CB(2) receptor antagonists. Electrically evoked release of endogenous noradrenaline from sympathetic nerves was inhibited by HU210, but not by HU211. Inhibition was blocked by a cannabinoid CB(1), but not a CB(2), receptor antagonist. HU210 attenuated contractions to noradrenaline, and all of the cannabinoids blocked contractions to KCl. Capsaicin pre-treatment had no significant effect on HU210- and CP55,940-mediated inhibition of sympathetic neurogenic contraction, but partly blocked inhibition mediated by methanandamide. These data show that cannabinoids can inhibit, by distinct pre- and postjunctional actions, sympathetic neurotransmission in the rat mesenteric arterial bed. The pre-junctional action is mediated by a cannabinoid CB(1)-like receptor, but the postjunctional action does not appear to involve either cannabinoid CB(1) or CB(2) receptors.

Characterization of vasorelaxant responses to anandamide in the rat mesenteric arterial bed

The endogenous cannabinoid anandamide has recently been identified as a vasorelaxant but the underlying mechanisms are controversial. The vasorelaxant responses to anandamide have now been examined in the rat mesenteric arterial bed. Anandamide caused potent vasorelaxations (pD(2) = 6.24 +/- 0.06; R(max) = 89.4 +/- 2.2 %) which were unaffected by inhibition of nitric oxide synthase with N(G)-nitro-L-arginine methyl ester (L-NAME; 300 microM). The responses were also predominantly endothelium independent and were unaffected by the cannabinoid CB(1) receptor antagonist SR141716A (1 microM), although at higher concentrations (3 and 10 microM) SR141716A was inhibitory. Both 1 mM ouabain (pD(2) = 5.90 +/- 0.07; R(max) = 50.4 +/- 6.5 %) and 100 microM 18alpha-glycyrrhetinic acid (pD(2) = 6.04 +/- 0.14; R(max) = 40.9 +/- 5.8 %) opposed anandamide-induced vasorelaxation. However, the gap junction inhibitors carbenoxolone (100 microM) and palmitoleic acid (50 microM) did not affect vasorelaxation to anandamide. Relaxation to anandamide was significantly attenuated by both capsaicin pretreatment to deplete the sensory nerves of neurotransmitters (pD(2) = 5.86 +/- 0.18; R(max) = 56.3 +/- 5.2 %) and the vanilloid antagonist ruthenium red (10 microM; pD(2) = 5.64 +/- 0.09; R(max) = 33.7 +/- 3.9 %). However, these inhibitory effects were prevented by the additional presence of L-NAME, when the relaxation to anandamide was unaffected (pD(2) = 6.19 +/- 0.07; R(max) = 81.9 +/- 2.8 %). The inhibitor of neuronal nitric oxide synthase, 7-nitroindazole, also prevented capsaicin from inhibiting the responses to anandamide. The results of this study point to anandamide acting via several mechanisms, which include the involvement of sensory nerves, but only in the presence of nitric oxide.

Evidence of a novel site mediating anandamide-induced negative inotropic and coronary vasodilatator responses in rat isolated hearts

1. Cannabinoids are known to cause coronary vasodilatation and reduce left ventricular developed pressure (LVDP) in isolated hearts although the identity of the receptor(s) mediating these responses is unknown. Our objective was to pharmacologically characterize cannabinoid receptors mediating cardiac responses to the endocannabinoid, anandamide. 2. Dose-response curves for coronary perfusion pressure (CPP) and LVDP were constructed to anandamide, R-(+)-methanandamide, palmitoylethanolamide (PEA) and JWH015 in isolated Langendorff-perfused rat hearts. Anandamide dose-response curves were also constructed in the presence of antagonists selective for CB(1), CB(2) or VR(1) receptors. 3. Anandamide and methanadamide significantly reduced CPP and LVDP but the selective CB(2) receptor agonists, PEA and JWH015 had no significant effect, compared with equivalent vehicle doses. 4. Single bolus additions of the selective CB(1)-receptor agonist, ACEA (5 nmol), decreased LVDP and CPP. When combined with JWH015 (5 nmol) these responses were not augmented. 5. Anandamide-mediated reductions in CPP were significantly blocked by the selective CB(1) receptor antagonists SR 141716A (1 microM) and AM251 (1 microM) and the selective CB(2) receptor antagonist SR 144528 (1 microM) but not by another selective CB(2) receptor antagonist AM630 (10 microM) nor the vanilloid VR(1) receptor antagonist capsazepine (10 microM). 6. SR 141716A, AM281 and SR 144528 significantly blocked negative inotropic responses to anandamide that were not significantly affected by AM251, AM630 and capsazepine. 7. One or more novel sites mediate negative inotropic and coronary vasodilatatory responses to anandamide. These sites can be distinguished from classical CB(1) and CB(2) receptors, as responses are sensitive to both SR 141716A and SR 144528.

A critical review of the role of endothelial factors in the pathogenesis of hypertension

The endothelium produces a variety of substances that play important roles in regulation of the circulation and vascular wall homeostasis. The control of blood vessel wall homeostasis is achieved via production of vasorelaxants and vasoconstrictors. Among the vasorelaxants are nitric oxide (NO), prostacyclin, various endothelium-derived hyperpolarizing factors (EDHFs, such as cytochrome P-450 monooxygenase metabolites of arachidonic acid like epoxyeicosatrienoic acids, and endocannabinoids), and C-type natriuretic peptide. Among the vasoconstrictors we find endothelin-1 (ET-1) and endothelium-derived contracting factors (EDCF) that are cyclooxygenase products such as endoperoxides and thromboxanes. The endothelium, via these and other agents, also exerts a critical influence on the blood stream, particularly formed elements such as leucocytes and platelets, and on substances involved in blood coagulation. All these effects contribute to modulating the growth of the vascular wall in hypertension, and participate in the development of atherothrombotic complications associated with hypertension. Inhibition of NO production may induce elevation of blood pressure in experimental animals. However, even today, we do not have incontrovertible evidence of participation of NO, EDHFs or EDCFs, or other endothelial products, in the pathogenesis of hypertension, although there is evidence of abnormal endothelium-dependent relaxation in hypertension in many but not all hypertensives. It is unclear, however, to what extent this may precede hypertension or be a consequence of elevated blood pressure, possibly contributing to its complications. Also, it is often difficult to dissociate abnormal endothelium-dependent relaxation from confounding factors such as the presence of associated conditions like dyslipidaemia, diabetes, smoking, obesity, hyperhomocysteinaemia, and others, that are accompanied themselves by abnormal endothelium-dependent relaxation. There is some evidence for a role of ET-1 in blood pressure elevation in some experimental forms of hypertension, particularly severe, sodium-sensitive hypertension, in which it may play a role in accentuating rather than initiating blood pressure elevation. Endothelin-1 may play a similar role in human hypertension.

Changes in cannabinoid CB(1) receptors in striatal and cortical regions of rats with experimental allergic encephalomyelitis, an animal model of multiple sclerosis

Data, initially anecdotal, but recently supported on more solid experimental evidence, suggest that cannabinoids might be beneficial in the treatment of some of the symptoms of multiple sclerosis (MS). Despite this evidence, there are no data on the possible changes in cannabinoid CB(1) or CB(2) receptors, the main molecular targets for the action of cannabinoids, either in the postmortem brain of patients with MS or in animal models of this disease. The present study addressed this question using the model of experimental allergic encephalomyelitis (EAE) in Lewis rats generated by inoculation of guinea pig myelin basic protein in Freund's adjuvant. After inoculation, animals were examined daily to detect the appearance of neurological signs. The first signs appeared around day 10 after inoculation, reaching the highest degree by day 13, when animals were sacrificed and their brains removed and used for analysis of CB(1) receptor binding, mRNA levels, and activation of GTP-binding proteins. CB(1) receptor binding and mRNA levels were not affected in EAE rats in brain areas such as the hippocampus, limbic structures, and cerebellum. However, there was a marked decrease in both parameters in the caudate-putamen, both in the lateral and medial parts, although this decrease did not correspond with decreases in binding in the nuclei recipient of striatal output neurons, which suggests that changes in CB(1) receptors are exclusively located in the cell bodies of striatal neurons. In addition, CB(1) receptor binding, but not mRNA levels, also decreased in the cerebral cortex, both in the deep and the superficial layers. The analysis of [(35)S]GTPgammaS binding after activation of CB(1) receptors with WIN55,212-2, a synthetic agonist, revealed that, despite the decrease in the number of CB(1) receptors in EAE rats, these were more efficiently coupled to GTP-binding protein-mediated signaling mechanisms in both the caudate-putamen and the cerebral cortex of these animals. In summary, these data suggest that the generation of EAE in Lewis rats would be associated with changes in CB(1) receptors in striatal and cortical neurons, which might be related to the alleviation of some motor signs observed after the treatment with cannabinoid receptor agonists in similar models of MS in rodents.

Identification of two distinct vasodilator pathways activated by ATP in the mesenteric bed of the rat

Adenosine 5'-triphosphate (ATP) has important roles in the cardiovascular system, modulating vascular tone by acting as both a vasoconstrictor and a vasodilator. The dilator function of ATP is traditionally thought to be monophasic and mediated primarily by nitric oxide (NO). Here we have identified the endothelium-dependent biphasic nature of ATP-induced vasodilatation of the rat isolated mesenteric bed and investigated the two distinct pathways involved. ATP, at doses of 1x10(-11) to 1x10(-8) moles, induced transient relaxations that were inhibited by the NO synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME: 1x10(-4) M), the soluble guanylyl cyclase inhibitor, 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ: 3x10(-6) M) and KCl (6x10(-2) - 1.2x10(-1) M). At doses upwards of 1x10(-8) moles (1x10(-8) - 3x10(-7) moles), ATP also induced prolonged vasodilatations which were unaltered by L-NAME, L-NAME (1x10(-3) M) and indomethacin (1x10(-5) M), or by ODQ, but were abolished in the presence of KCl. In addition, the cannabinoid CB(1) receptor antagonist SR141716A (1x10(-5) M) was found to inhibit the second prolonged phase of vasodilatation. However, at the concentration used SR141716A is reported to be non-selective. A second CB(1) receptor antagonist, AM251 (1x10(-6) M), had a small but significant inhibitory effect on the second phase of ATP-induced vasodilatation. SR141716A, AM251 and KCl (6x10(-2) - 1.2x10(-1) M) all inhibited anandamide-induced relaxation of the isolated mesenteric bed. These observations demonstrate that ATP stimulates vasodilatation of the mesenteric bed by two distinct mechanisms involving the release of NO and an EDHF. In the absence of better pharmacological tools we can only speculate as to the involvement of an endogenous CB(1) receptor ligand in these responses.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.

Cannabinoid CB1 receptors fail to cause relaxation, but couple via Gi/Go to the inhibition of adenylyl cyclase in carotid artery smooth muscle

1. The aim of the current study was to characterize which cannabinoid receptors, if any, are present on rat carotid artery smooth muscle. Additionally, the effects of cannabinoids on carotid artery tone, on cyclic AMP accumulation and on forskolin-induced relaxation were examined in the same tissue. 2. Stimulation of carotid arteries with forskolin (10 microM) significantly increased cyclic AMP accumulation, an effect that was inhibited in a concentration-dependent manner by the cannabinoid receptor agonist, methanandamide. 3. Similar inhibition was seen with the CB1 agonist HU-210 but this inhibition was not mimicked by the CB2 agonist, WIN 55,2212-2. 4. The inhibitory effect of methanandamide on cyclic AMP accumulation was prevented by incubation of the arteries with pertussis toxin and was significantly reduced by LY320135, a selective CB1 antagonist, but not by SR 144528, a CB2-selective antagonist. 5. Methanandamide failed to relax carotid arteries pre-contracted with phenylephrine, but inhibited forskolin-induced relaxation of these arteries. This functional inhibition of relaxation by methanandamide was inhibited by CB1-selective (LY320135 and SR 141716A), but not a CB2-selective antagonist (SR 144528). 6. These data demonstrate the presence of functional G protein-linked cannabinoid receptors of the CB1 subtype in the rat carotid artery, but show that these receptors inhibit cyclic AMP accumulation rather than cause relaxation.