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Perivascular adipose tissue (PVAT) is now recognized as an active player in vascular homeostasis. The expansion of PVAT in obesity and its possible role in vascular dysfunction have attracted much interest. In terms of the regulation of vascular tone and blood pressure, PVAT has been shown to release vasoactive mediators, for instance, angiotensin peptides, reactive oxygen species, chemokines and cytokines. The secretory profile of PVAT is altered by obesity, hypertension and other cardiovascular diseases, leading to an imbalance between its pro-contractile and anti-contractile effects. PVAT adipocytes represent an important source of the mediators, but infiltrating immune cells may become more important under conditions of hypoxia and inflammation. This review describes recent advances in the effects of PVAT on the regulation of vascular tone, highlighting the evidence for a pro-contractile action in health and disease. The role of the endothelium, vascular smooth muscle, immune cells and probably perivascular nerves in PVAT function is also discussed. Linked Articles This article is part of a themed section on Molecular Mechanisms Regulating Perivascular Adipose Tissue – Potential Pharmacological Targets? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.20/issuetoc

Background and Purpose Metabolites of the endocannabinoid, 2-arachidonoylglycerol (2-AG) have been postulated to act as endogenous activators of TRPV4, a Ca2+-permeable cation channel that plays a critical role in endothelium-dependent relaxation. However, it is unclear if TRPV4 contributes to the vascular actions of 2-AG. Experimental Approach Isometric tension recording of rat small mesenteric arteries and aortae were used to assess the effect of 2-AG and the synthetic TRPV4 activator, GSK1016790A (GSK) on vascular reactivity. Changes in intracellular Ca2+ concentration and single-channel currents were measured in TRPV4-expressing human coronary endothelial cells. Key Results In mesenteric arteries, endothelium-dependent relaxation to both 2-AG and GSK was attenuated by structurally distinct TRPV4 antagonists, HC067047, RN1734 and ruthenium red. The responses were inhibited by KCa inhibitors (apamin + charybdotoxin) and a gap junction inhibitor (18α-glycyrrhetinic acid). In contrast to GSK, 2-AG elicited considerable relaxation independently of the endothelium or TRPV4. Inhibition of 2-AG metabolism via monoacylglycerol lipase and COX (by MAFP and indomethacin) caused potentiation, while cytochrome P450 and lipoxygenase inhibitors had no effect on 2-AG relaxation. In coronary endothelial cells, 2-AG (with and without MAFP) induced HC067047-sensitive increases in intracellular Ca2+ concentration. 2-AG also increased TRPV4 channel opening in inside-out patches. However, in aortae, GSK induced a relaxation sensitive to HC067047 and ruthenium red, whereas 2-AG induced contractions. Conclusions and Implications These data suggest that 2-AG can directly activate endothelial TRPV4, which partly contributes to the relaxant response to 2-AG. However, the functional role of TRPV4 is highly dependent on the vascular region.

BACKGROUND AND PURPOSE The calcimimetic, (R)-N-(3-(3-(trifluoromethyl)phenyl)propyl)-1-(1-napthyl)ethylamine hydrochloride (cinacalcet), which activates Ca2+-sensing receptors (CaR) in parathyroid glands, is used to treat hyperparathyroidism. Interestingly, CaR in perivascular nerves or endothelial cells is also thought to modulate vascular tone. This study aims to characterize the vascular actions of calcimimetics. EXPERIMENTAL APPROACH In rat isolated small mesenteric arteries, the relaxant responses to the calcimimetics, cinacalcet and (R)-2-[[[1-(1-naphthyl)ethyl]amino]methyl]-1H-indole hydrochloride (calindol) were characterized, with particular emphasis on the role of CaR, endothelium, perivascular nerves, K+ channels and Ca2+ channels. Effects of L-ornithine, which activates a Ca2+-sensitive receptor related to CaR (GPRC6A), were also tested. KEY RESULTS Cinacalcet induced endothelium-independent relaxation (pEC50 5.58 ± 0.07, Emax 97 ± 6%) that was insensitive to sensory nerve desensitization by capsaicin or blockade of large-conductance Ca2+-activated K+ channels by iberiotoxin. Calindol, another calcimimetic, caused more potent relaxation (pEC50 6.10 ± 0.10, Emax 101 ± 6%), which was attenuated by endothelial removal or capsaicin, but not iberiotoxin. The negative modulator of CaR, calhex 231 or changes in [Ca2+]o had negligible effect on relaxation to both calcimimetics. The calcimimetics relaxed vessels precontracted with high [K+]o and inhibited Ca2+ influx in endothelium-denuded vessels stimulated by methoxamine, but not ionomycin. They also inhibited contractions to the L-type Ca2+ channel activator, BayK8644. L-ornithine induced small relaxation alone and had no effect on the responses to calcimimetics. CONCLUSION AND IMPLICATIONS Cinacalcet and calindol are potent arterial relaxants. Under the experimental conditions used, they predominantly act by inhibiting Ca2+ influx through L-type Ca2+ channels into vascular smooth muscle, whereas Ca2+-sensitive receptors (CaR or GPRC6A) play a minor role.

Background and purpose: N-arachidonoyl glycine (NAGly) is an endogenous lipid that is structurally similar to the endocannabinoid, N-arachidonoyl ethanolamide (anandamide). While NAGly does not activate cannabinoid receptors, it exerts cannabimimetic effects in pain regulation. Here, we have determined if NAGly, like anandamide, modulates vascular tone. Experimental approach: In rat isolated small mesenteric arteries, the relaxant responses to NAGly were characterized. Effects of N-arachidonoyl serine and N-arachidonoyl γ-aminobutyric acid were also examined. Key results: In endothelium-intact arteries, NAGly-induced relaxation (pEC50%= 5.7 ± 0.2; relaxation at 30 µM = 98 ± 1%) was attenuated by l-NAME (a nitric oxide synthase inhibitor) or iberiotoxin [selective blocker of large conductance Ca2+-activated K+ channels (BKCa)], and abolished by high extracellular K+ concentration. Endothelial removal reduced the potency of NAGly, and the resultant relaxation was inhibited by iberiotoxin, but not l-NAME. NAGly responses were sensitive to the novel cannabinoid receptor antagonist O-1918 independently of endothelial integrity, whereas pertussis toxin, which uncouples Gi/o proteins, attenuated NAGly relaxation only in endothelium-intact arteries. Treatments with antagonists for CB1, CB2 and TRPV1 receptors, or inhibitors of fatty acid amide hydrolase and COX had no effect. The two other arachidonoyl amino acids also induced iberiotoxin- and L-NAME-sensitive relaxations. Conclusion and implications: NAGly acts as a vasorelaxant predominantly via activation of BKCa in rat small mesenteric arteries. We suggest that NAGly activates an unknown Gi/o-coupled receptor, stimulating endothelial release of nitric oxide which in turn activates BKCa in the smooth muscle. In addition, NAGly might also activate BKCa through Gi/o- and nitric oxide-independent mechanisms.

Background and purpose: The cardiovascular effects of cannabinoids can be influenced by anaesthesia and can differ in chronic hypertension, but the extent to which they are influenced by acute hypertension in conscious animals has not been determined. Experimental approach: We examined cardiovascular responses to intravenous administration of anandamide and the synthetic cannabinoid, (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55212-2), in conscious male Wistar rats made acutely hypertensive by infusion of angiotensin II (AII) and arginine vasopressin (AVP). Rats were chronically instrumented for measurement of arterial blood pressure and vascular conductances in the renal, mesenteric and hindquarters beds. Key results: Anandamide dose-dependently decreased the mean arterial blood pressure of rats made hypertensive by AII-AVP infusion, but not normotensive rats. Interestingly, acute hypertension also revealed a hypotensive response to WIN55212-2, which caused hypertension in normotensive animals. The enhanced depressor effects of the cannabinoids in acute hypertension were associated with increased vasodilatation in hindquarters, renal and mesenteric vascular beds. Treatment with URB597, which inhibits anandamide degradation by fatty acid amide hydrolase, potentiated the depressor and mesenteric vasodilator responses to anandamide. Furthermore, haemodynamic responses to WIN55212-2, but not to anandamide, were attenuated by the CB1 receptor antagonist, AM251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophen yl)-4-methyl-1H-pyrazole-3-carboxamide]. Conclusions and implications: These results broadly support the literature showing that the cardiovascular effects of cannabinoids can be exaggerated in hypertension, but highlight the involvement of non-CB1 receptor-mediated mechanisms in the actions of anandamide.

Background and purpose: The endocannabinoid N-arachidonoylethanolamide (anandamide) is co-synthesized with other N-acylethanolamides, namely N-palmitoylethanolamide (PEA) and N-oleoylethanolamide (OEA), which have been shown to potentiate anandamide responses (so-called ‘entourage effects’) in non-vascular tissues. It remains unclear whether such interactions occur in the circulation. Experimental approach: In rat isolated small mesenteric arteries, the effects of PEA and OEA on relaxation to anandamide and tissue contents of the N-acylethanolamides were examined under myographic conditions. Key results: Anandamide-induced relaxation was potentiated by pretreatment with PEA (10 μM) or OEA (1 μM), or in combination. The potentiation by PEA and OEA was endothelium-independent and abolished by treatment with capsaicin (10 μM), which desensitizes the transient receptor potential vanilloid type 1 (TRPV1) receptor system, or by the TRPV1 receptor antagonist, N-(3-methoxyphenyl)-4-chlorocinnamide (SB366791) (2 μM). It was also observed at molar ratios of anandamide and PEA (or OEA) similar to those found in mesenteric arteries. PEA and inhibition of anandamide hydrolysis by 3′-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate (URB597) (1 μM) additively potentiated anandamide responses. On the other hand, PEA and OEA also induced vasorelaxation per se (rank order of potency: anandamide>OEA>PEA), but relaxation to the three N-acylethanolamides displayed different sensitivity to treatment with capsaicin, SB366791 and URB597. For example, relaxations to anandamide and OEA, but not PEA, were attenuated by both capsaicin and SB366791. Conclusion and implications: This study shows that PEA and OEA potentiate relaxant responses to anandamide through TRPV1 receptors in rat small mesenteric arteries. The congeners also induce vasorelaxation per se, suggesting a function for the N-acylethanolamides in vascular control. British Journal of Pharmacology (2008) 155, 837–846; doi:10.1038/bjp.2008.324; published online 11 August 2008

Background and purpose: The endocannabinoids, N-arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG) are rapidly degraded by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL). Whilst these lipid mediators are known to modulate vascular tone, the extent to which they are inactivated via local metabolism in the vasculature remains unclear. Experimental approach: In rat isolated small mesenteric arteries, the regulatory role of FAAH, MGL and cyclooxygenase (COX) in relaxant responses to anandamide and 2-AG was evaluated by using inhibitors of these enzymes. Relaxations to nonhydrolysable analogues of endocannabinoids and arachidonic acid were also examined. Key results: Relaxation to anandamide but not 2-AG was potentiated by the selective FAAH inhibitor, URB597 (1 mM). In contrast, MAFP (10 mM; an inhibitor of FAAH and MGL) enhanced responses to both anandamide and 2-AG. Inhibition of COX-1 by indomethacin (10 mM) potentiated relaxations to 2-AG, whereas inhibition of COX-2 by nimesulide (10 mM) potentiated anandamide-induced relaxation. With the exception of MAFP, effects of FAAH and COX inhibitors were dependent on the endothelium. Relaxation to methanandamide and noladin ether, the non-hydrolysable analogues of anandamide and 2-AG respectively, were insensitive to the enzyme inhibitors. Conclusion and implications: This study shows that local activity of FAAH, MGL and COX, which is present largely in the endothelium, limits the vasodilator action of endocannabinoids in rat small mesenteric arteries. Despite the differential roles played by these enzymes on relaxation to anandamide versus 2-AG, our results suggest that inhibitors of these enzymes enhance the vascular impact of endocannabinoids. British Journal of Pharmacology (2007) 150, 641–651. doi:10.1038/sj.bjp.0707141; published online 22 January 2007

Three cannabinoid receptor agonists, anandamide (CB1 receptor-selective) and the aminoalkyl-indoles, JWH 015(2-methyl-1-propyl-1H-indol-3-yl)-1-napthalenylmethanone; (CB2 receptor-selective), R-(+)-WIN 55,212-2 (R-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolol[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone; slightly CB2 receptor-selective), as well as the enantiomer S-(−)-WIN 55,212-3(S-(−)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolol[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone; inactive at cannabinoid receptors), induced endothelium-independent relaxation of methoxamine-precontracted isolated small mesenteric artery of rat. KCL (60 mM) precontraction did not affect relaxation to the aminoalkylindoles, but reduced that to anandamide. SR14176A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; 3 μM; CB1 receptor antagonist) inhibited relaxation only to JWH 015 and anandamide. Neither AM 251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; CB1 antagonist) nor SR 144528 (N-[(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide; CB2 antagonist; both at 3 μM) affected any of the relaxations. Vanilloid receptor desensitisation with capsaicin reduced anandamide relaxation; addition of SR 141716A (3 μM) then caused further inhibition. SR 141716A did not affect capsaicin-induced relaxation. The aminoalkylindoles inhibited CaCl2-induced contractions in methoxamine-stimulated vessels previously depleted of intracellular Ca2+. These inhibitory effects were greatly reduced or abolished in ionomycin-(a calcium ionophore) contracted vessels. Anandamide also caused vanilloid receptor-independent, SR 141716A- (3 μM) insensitive, inhibition of CaCl2 contractions. In conclusion, the aminoalkylindoles JWH 015, R-(+)-WIN 55,212-2 and S-(−)-WIN 55,212-3 relax rat small mesenteric artery mainly by inhibiting Ca2+ influx into vascular smooth muscle. Anandamide causes vasorelaxation by activating vanilloid receptors, but may also inhibit Ca2+ entry. Relaxation to JWH 015 and anandamide was sensitive to SR 141716A, but there is no other evidence for the involvement of CB1 or CB2 receptors in responses to these compounds. British Journal of Pharmacology (2003) 139, 585–597. doi:10.1038/sj.bjp.0705280

The nonpsychoactive cannabinoid abnormal-cannabidiol (trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol) (abn-cbd) produced concentration-dependent relaxation of methoxamine-precontracted rat small mesenteric artery. Endothelial removal reduced abn-cbd potency six-fold without affecting the maximum relaxation. In endothelium-intact vessels, abn-cbd was less potent under 60 mM KCl-induced tone and inhibited by combination of L-NG-nitroarginine methyl ester (L-NAME) (nitric oxide synthase inhibitor; 300 μM), apamin (small conductance Ca2+-activated K+ channels inhibitor; 50 nM) and charybdotoxin (inhibitor of intermediate conductance Ca2+-activated K+ channels and large conductance Ca2+-activated K+ channels BKCa; 50 nM). L-NAME alone or in combination with either toxin alone had little effect. In intact vessels, relaxations to abn-cbd were inhibited by SR 141716A (cannabinoid receptor antagonist; 1 or 3 μM). Concomitant addition of L-NAME, apamin and charybdotoxin had no further effect. Other cannabinoid receptor antagonists either had little (SR 144528; 1 μM and AM 251; 1 μM) or no effect (AM 630; 10 μM and AM 281; 1 μM). Inhibition of gap junctions, Gi/o protein coupling and protein kinase A also had no effect. Endothelium-independent relaxation to abn-cbd was unaffected by L-NAME, apamin plus charybdotoxin or capsaicin (10 μM). Abn-cbd inhibited CaCl2-induced contractions in vessels with depleted intracellular Ca2+ stores and stimulated with methoxamine or KCl. This was insensitive to SR 141716A (3 μM) but greatly reduced in vessels stimulated with ionomycin (Ca2+ ionophore; 1 μM). We conclude that abn-cbd relaxes the rat small mesenteric artery by endothelium-dependent activation of K+ channels via SR 141716A-sensitive pathways, which do not involve CB1 and CB2 receptors. It also causes endothelium-independent, SR 141716A-insensitive, relaxation by inhibiting Ca2+ entry through voltage-gated Ca2+ channels. British Journal of Pharmacology (2003) 138, 1320–1332. doi:10.1038/sj.bjp.0705160

The cannabinoid arachidonyl ethanolamide (anandamide) caused concentration-dependent relaxation of 5-HT-precontracted, myograph-mounted, segments of rat left anterior descending coronary artery. This relaxation was endothelium-independent, unaffected by the fatty acid amide hydrolase inhibitor, arachidonyl trifluoromethyl ketone (10 μM), and mimicked by the non-hydrolysable anandamide derivative, methanandamide. Relaxations to anandamide were attenuated by the cannabinoid receptor antagonist, SR 141716A (3 μM), but unaffected by AM 251 (1 μM) and AM 630 (1 μM), more selective antagonists of cannabinoid CB1 and CB2 receptors respectively. Palmitoylethanolamide, a selective CB2 receptor agonist, did not relax precontracted coronary arteries. Anandamide relaxations were not affected by inhibition of sensory nerve transmission with capsaicin (10 μM) or blockade of vanilloid VR1 receptors with capsazepine (5 μM). Nevertheless capsaicin relaxed coronary arteries in a concentration-dependent and capsazepine-sensitive manner, confirming functional sensory nerves were present. In contrast, capsazepine and capsaicin did inhibit anandamide relaxations in methoxamine-precontracted rat small mesenteric arteries. Relaxations to anandamide were inhibited by TEA (1 mM) or iberiotoxin (50 nM), blockers of large conductance, Ca2+-activated K+ channels (BKCa). Gap junction inhibition with 18α-glycyrrhetinic acid (100 μM) did not affect anandamide relaxations. This study shows anandamide relaxes the rat coronary artery by a novel mechanism. Anandamide-induced relaxations do not involve the endothelium, degradation into active metabolites, or activation of cannabinoid CB1 or CB2 receptors, but may involve activation of BKCa. Vanilloid receptor activation also has no role in the effects of anandamide in coronary arteries, even though functional sensory nerves are present. British Journal of Pharmacology (2001) 134, 921–929; doi:10.1038/sj.bjp.0704333

N-arachidonoyl serine (ARA-S) is one of a number of acyl amino acids recently identified in mammalian tissues. It has been referred to as an endocannabinoid-like lipid largely based on its structural similarities with the endocannabinoid, N-arachidonoyl ethanolamide (anandamide). However, little is known about its potential physiological functions and receptor targets. In this issue of the British Journal of Pharmacology, Zhang and colleagues show that ARA-S is a potent inducer of endothelial cell proliferation and migration, and angiogenesis in vitro. Furthermore, this pro-angiogenic action is mediated, at least partly, by activation of the poorly characterized, G-protein-coupled GPR55 receptor. ARA-S, via GPR55, increases phosphorylation of extracellular signal-regulated kinases and Akt, and vascular endothelial growth factor signalling. These exciting findings highlight the endothelium as an endogenous target for ARA-S and GPR55.

There is increasing evidence that Ca2+-sensing G-protein-coupled receptors (CaR) have an important role in modulating vascular activity; both CaR-mediated contraction and dilation responses have be...