Endocannabinoid's negative feedback is responsible for diminishing agonist‐induced vasoconstriction, which may be clinically important in the treatment of arterial and pulmonary hypertension. Further research is required to establish the importance of the eCB system and its downstream signalling pathways.

Endocannabinoids (mainly 2‐arachidonoylglycerol), acting via CB 1 receptors, evoke vasodilatory effects and may modulate responses of vasoconstrictors for G q/11 protein‐coupled receptors including angiotensin II, thromboxane A 2 , phenylephrine, noradrenaline in systemic or pulmonary arteries. However, the role of the endothelium in this interaction is not well‐established, and the precise vascular location of eCB system components remains unclear, which contributes to discrepancies in the interpretation of results when describing the above‐mentioned relationship.

The endocannabinoid (eCB) system centrally and peripherally regulates cardiovascular parameters, including blood pressure, in health and disease. The relationship between G q/11 protein‐coupled receptor activation, regulation of eCBs release (mainly 2‐arachidonoylglycerol) and subsequent CB 1 receptor activation was initially observed in the central nervous system. Here, we review the latest findings from systemic physiological studies which include for the first time data from pulmonary arteries. We present evidence for direct CB 1 ‐dependent cannabinoid ligand‐induced vasorelaxation, vascular expression of eCBs along with their degradation enzymes, and indicate the location of the described interaction.

Introduction The endocannabinoid (eCB) system is composed of two classical cannabinoid receptors, type 1 (CB 1 ) and type 2 (CB 2 ), as well as non‐CB 1 /CB 2 ; their ligands, eCBs, 2‐arachidonoylglycerol (2‐AG) and anandamide (AEA); and enzymes taking part in their biosynthesis or degradation.1 A number of human and animal studies suggest that the eCB system exerts cardiovascular effects, for example in the regulation of blood pressure and cardiac function, under normal and pathological conditions, including hypertension.2 Therefore, in various forms of hypertension, the eCB system becomes tonically active. This occurs by enhancing the vasorelaxant responses to eCBs, changes in plasma eCB concentration or alterations in the vascular expression of eCB system components, including up‐regulation of CB 1 receptors.3-7 It is worth mentioning that more research concerns the possible role of the eCB system in cardiovascular regulation in hypertension.3, 6, 8 Cannabinoid signalling through the CB 1 ‐sensitive pathway can cause vasorelaxation via activation of nitric oxide synthase, increase in cyclic guanosine monophosphate and opening of potassium channels which causes membrane hyperpolarization,5, 9 see also Table 1. In addition, the metabolism of eCBs into arachidonic acid, and other vasodilator eicosanoids, comprises their other vasorelaxant property (but not CB 1 receptor‐dependent);4, 10, 11 however, we will not develop this topic in this review. Table 1. Direct CB 1 ‐dependent vasorelaxation (evoked by AEA, 2‐AG and their stable analogues) and other intercurrent mechanisms in blood vessels Species Blood vessel Agonist(s) CB 1 antagonist(s) Investigated mechanism(s) References ENDO NO Potassium channels Other Human Mesenteric artery AEA AM251 (0.1) + + − − 25 Saphenous vein 2‐AG Rimonabant (1) − + − − 46 Pulmonary artery 2‐AG AM251 (1) + − − − 16 Rat Mesenteric artery AEA Rimonabant (0.1) AM251 (0.1) + − + Endothelial cannabinoid receptor Gap junctional communication Vanilloid receptor 47 AEA Rimonabant (1) − − + − 48 AEA Rimonabant (3) − − − Vanilloid receptor 49 AEA methAEA Rimonabant (1) − − − − 50 Aorta AEA AM251 (1) + + − CB 2 51 Cerebral artery 2‐AG Rimonabant (1) − − − − 42 Renal artery AEA Rimonabant (1) − − − 52 Rabbit Aorta AEA methAEA Rimonabant (1) + + − G i/o 53 Pulmonary artery 2‐AG ether AM251 (0.3) Rimonabant (0.3) − − − MEK/ERK 1/2 G i/o 54 Mesenteric artery 2‐AG AEA Rimonabant (3) AM281 (3) − − − − 55 Cat Cerebral artery AEA Rimonabant (0.1) − − − L‐type Ca2+ channels 56 Bovine Ophthalmic artery AEA Rimonabant (0.1) AM251 (0.1) + + + − 57 It was observed that the levels of pro‐hypertensive vasoactive factors such as thromboxane A 2 (TXA 2 ), angiotensin II (Ang II),12 serotonin (5‐HT),13 catecholamines14 and endothelin‐1 (ET‐1)12 are elevated in arterial, including pulmonary, hypertension. They are ligands for the G q/11 protein‐coupled receptors prostanoid TP (TP), angiotensin 1 (AT1), serotonin (5‐HT 2 ), α 1 ‐adrenergic, endothelin A (ET A ) receptors, respectively, which are all present within the vascular wall. Thus, G q/11 protein‐coupled receptors represent targets for drugs in the treatment of arterial hypertension,15 including pulmonary hypertension,16 along with the eCB system. The relationship between activation of G q/11 protein‐coupled receptors and regulation of eCB release and later CB 1 receptors activation was initially observed in the synapses of the central nervous system (CNS).17, 18 As it was discovered later, this is a common mechanism that exists in peripheral tissues, namely in varied blood vessels, arguing in favour of its general occurrence (see Table 2). Table 2. The relationship between vasoconstrictors for G q/11 protein‐coupled receptors and endocannabinoid system in blood vessels Vasoconstrictor (and receptor) Species Artery Release of eCB(s) Engagement of CB 1 The effect of antagonists/inhibitors on the concentration–response curve References CB 1 antagonist (s)/Effect eCB(s) synthesis inhibitor/Effect eCB(s) degradation inhibitor(s)/Effect Angiotensin II (AT 1 ) Rat, mouse Aorta 2‐AG + O2050 (1)/↑ THL (DAGL; 1)/↑ JZL184 (MAGL; 1)/↓ 33 Rat, mouse Gracilis 2‐AG + O2050 (1)/↑ Rimonabant (1)/↑ AM251 (1)/↑ THL (DAGL; 1)/↑ N.A. 19 Mouse Saphenous 2‐AG + O2050 (1)/↑ THL (DAGL; 1)/↑ N.A. 19 Rat Uterine AEA N.A. N.A. N.A. JZL184 (MAGL; 1)/(‐) URB579 (FAAH; 1)/↓ 8 Uterinea AEA, 2‐AG − Rimonabant (1)/(‐) N.A. JZL184 (MAGL; 1)/↓ URB579 (FAAH; 1)/↓ 8 Pulmonary 2‐AG + AM251 (1)/↑ RHC (DAGL; 40)/↑ JZL184 (MAGL; 1)/↓ URB597 (FAAH; 1)/(‐) 16 U46619 (TP) Rat Middle cerebral AEA, 2‐AG + Rimonabant (1)/↑ AM251 (1)/↑ N.A. N.A. 26 Middle cerebral 2‐AG N.A. N.A. N.A. DETFP (FAAH, MAGL; 10)/↓ URB754 (MAGL; 10)/↓ URB579 (FAAH; 1)/(‐) VDM11 (FAAH, MAGL; 10)/↓ 42 Rat, human Pulmonary 2‐AG + AM251 (1)/↑ RHC (DAGL; 40)/↑ JZL184 (MAGL; 1)/↓ URB597 (FAAH; 1)/(‐) 16 5‐HT (5‐HT 2 ) Rat Middle cerebral − − Rimonabant (1)/(‐) N.A. N.A. 26 Pulmonary − − AM251 (1)/(‐) N.A. N.A. 16 Phenylephrine (α 1 ‐adrenergic) Mouse Aorta N.A. + O2050 (1)/↑ N.A. N.A. 33 Rat Pulmonary − − AM251 (1)/(‐) N.A. N.A. 16 Noradrenaline (α 1 ‐adrenergic) Rat Aorta N.A. + O2050 (1)/↑ N.A. N.A. 33 PGF 2α (FP) Mouse Aorta N.A. − O2050 (1)/(‐) THL (DAGL; 1)/(‐) N.A. 33 As shown in the Figure 1, the stimulation of G q/11 protein‐coupled receptors leads to phospholipase C‐β (PLC‐β) activation. PLC‐β cleaves phosphatidylinositol 4,5‐bisphosphate (PIP 2 ) into inositol 1,4,5‐trisphosphate (IP 3 ) and diacylglycerol (DAG). DAG is cleaved by diacylglycerol lipase (DAGL) into 2‐AG. Simultaneously, IP 3 binds with the IP 3 receptor at the endoplasmic reticulum and initiates the release of intracellular Ca2+ ([Ca2+] i ) stores. The [Ca2+] i leads to activation of DAGL and N‐acyltransferase (NAT), which contributes to additional 2‐AG and AEA synthesis, respectively, from the common precursors, membrane phospholipids (PL). When this occurs, it causes local increases in eCB concentrations and they can act in a paracrine or autocrine manner, with multiple distinct targets, including CB 1 receptors, to regulate vasoreactivity (Figure 1).17, 19 Astonishingly, the precise location of the components of the described mechanism is still unclear, and the role of the endothelium is not well‐established. It is worth mentioning that G protein subunits ‐q and 11 (G q/11 ) are expressed at the endothelial cell–cell junction20 and on vascular smooth muscle cells (VSMCs).21 It was shown that CB 1 receptors are expressed in both endothelial cells and VSMCs, for example of the rat aorta.22 Importantly, the vascular eCBs are produced mainly in the endothelium,23, 24 and certain vasoconstrictors can induce release of eCBs from the arterial endothelium.16 However, it was observed that eCBs cause vasodilation via mediators released from the endothelium or via activation of endothelial CB 1 receptors,25 see also Table 1. These data support the idea that this mechanism occurs, not just in VSMCs,19 but also in the endothelium.16 Taking into account the divergent expression of components of the eCB system (see Table 3) and the vascular effects of eCBs (connected with their subsequent metabolism to the vasodilatory or/and vasoconstrictor products), the clear interpretation of the results often turns out to be quite problematic. Figure 1 Open in figure viewer PowerPoint Proposed mechanism of the relationship between endocannabinoid system and G q/11 protein‐coupled receptors for vasoconstrictors. Vasoconstrictors (e.g. angiotensin II – Ang II or thromboxane A 2 – TXA 2 ) activating their G q/11 protein‐coupled receptors stimulate an increase in the intracellular calcium concentration ([Ca2+] i ), released from endoplasmic reticulum of smooth muscle cells as well as from the endothelium. It directly generates contractile effects and indirectly mediates the rapid biosynthesis of endocannabinoids (mainly 2 arachidonoylglycerol; 2‐AG) by diacylglycerol lipase (DAGL) activation. eCBs may then activate the CB 1 cannabinoid receptors located in smooth muscle and endothelial cells in an autocrine or paracrine manner, leading to vasorelaxation, which reduces agonists‐induced vasoconstriction. Table 3. Vascular expression of endocannabinoid degradation enzymes – MAGL and FAAH and level of their main endocannabinoids – 2‐AG and AEA Endocannabinoid system components Endothelium‐intact Vascular smooth muscle cells Whole vascular wall Vascular expression MAGL Small rat mesenteric artery 36 a N.A. Rat uterine artery 8 FAAH Rat uterine artery8 Human kidney endothelial cells52 Human umbilical vein58 Mouse pulmonary artery37 Human pulmonary cell line37 Skeletal muscle arteries59 Bovine coronary artery60 Human pulmonary artery11 Rat aorta6 Small mesenteric artery6 Mouse cerebral artery (microcirculation)61 Vascular level 2‐AG Bovine coronary artery24 Rat aorta23 Human umbilical vein58 N.A. Rat middle cerebral artery26 Rat mesenteric artery62 Mice aorta63b AEA N.A. Human pulmonary cell line 37 Rat middle cerebral artery26 Rat mesenteric artery62 It has been previously suggested that the synthesis of eCBs depends on the magnitude of the [Ca2+] i increase by different vasoconstrictors.26 As an example, increases in [Ca2+] i within the pulmonary arteries induced by various vasoactive substances are both time‐dependent and concentration‐dependent. Thus, 5‐HT (rat; 10 μm27) and phenylephrine (canine; 10 μm28) stimulated smaller increases in the level of [Ca2+] i compared to Ang II (rat, 10 μm29) and TXA 2 analogue (U46619; rat, 0.1 μm30). U46619 and Ang II produced much stronger vasoconstriction, in terms of the maximal response values, than either phenylephrine or 5‐HT in isolated rat pulmonary arteries (rPAs).16 This suggests that production of eCBs in rPAs is dependent on agonist‐induced contraction force.16 Nonetheless, further studies in this field are necessary to confirm these assumptions. In this review, we briefly update what is currently known about the interaction between G q/11 protein‐coupled receptors activation and eCB release which diminishes constriction in systemic and, for the first time, pulmonary arteries. Here, we present, step by step, the influence of (1) CB 1 receptors blockade; (2) eCBs synthesis and (3) degradation enzyme inhibitors (including altered levels of eCBs) on the G q/11 protein‐coupled receptor agonist‐specific contractile effects. Moreover, we attempt to indicate whether the above interaction is located in the endothelium or in VSMCs. We also consider whether this interaction may be clinically important in the treatment of hypertension, including pulmonary hypertension.

Angiotensin II and endocannabinoids The classical model of CB 1 receptor transactivation with Ang II was described in the CNS synapse in 2001.17 However, more and more data are becoming available regarding the presence and function of eCBs in peripheral tissues, including the cardiovascular system. Most data focus on Ang II and the following release of eCBs, thus we decided to discuss this vasoconstrictor first. It has been demonstrated that CB 1 and AT1 receptors physically and functionally interact when co‐expressed in recombinant as well as in endogenous systems. AT1 receptor expression may be directly regulated by CB 1 receptors in cultured VSMCs. Inhibition of CB 1 receptors by SR141716 (also known as rimonabant) and AM251 led to down‐regulation of AT1 receptor expression, whereas stimulation with the CB 1 receptor agonist CP 55 940 resulted in AT1 up‐regulation, indicating that AT1 receptor expression is directly regulated by the CB 1 receptors.31 Rozenfeld et al.32 reported that interaction with CB 1 receptors confers new signalling regulatory properties to AT1 receptors and enhanced responsiveness to Ang II, underlining the relevance of CB 1 receptor up‐regulation during chronic disease on the function and properties of other co‐expressed receptors. The blockade of CB 1 receptors resulted in the enhancement of the Ang II‐induced contraction in rPAs (by AM25116), rat uterine artery (by rimonabant8), rat and mouse gracilis artery as well as mouse saphenous artery (by O205019). Additionally, in the CB 1 ‐knockout mouse model, the inhibition of CB 1 receptors by O2050 did not enhance the Ang II‐induced vasoconstriction in isolated mouse aortas33 or saphenous arteries.19 This data demonstrated that CB 1 receptors are essential in the regulation of vascular tone in the presence of a vasoactive stimulus. Endothelium denudation abolished the modulatory effect of AM251 on Ang II‐induced vasoconstriction in rPAs, due to the lack of an endothelial source of eCBs to activate CB 1 receptors.16 However, blockade of the CB 1 receptors by rimonabant enhanced the Ang II‐induced constriction in isolated rat gracilis artery independently of endothelial function (Table 2).19 It was suggested that 2‐AG, but not AEA, predominately serves as a common paracrine signal generated through activation of G q/11 protein‐coupled receptors, which mobilizes Ca2+. It was observed that activation of the AT1 receptors by Ang II resulted in activation of CB 1 receptors via a DAGL‐dependent mechanism in Chinese hamster ovary (CHO) cells as well as in human embryonic kidney‐293 cells.34, 35 Moreover, inhibition of DAGL in rat and mouse gracilis arteriole, mouse saphenous artery,19 rat and mouse aorta33 by tetrahydrolipstatin (THL) led to the enhancement of Ang II‐induced vasoconstriction (Table 2). Another inhibitor of DAGL, RHC80267, did not modify the vasoconstrictor effect of Ang II in endothelium‐denuded, but did in endothelium‐intact, rPAs, confirming that rapid synthesis of 2‐AG, released from endothelial cells, plays a protective role against the AT1 receptor‐mediated vasoconstriction in these arteries.16 Due to blockade of 2‐AG production, CB 1 receptors are not activated, which fails to modulate the vasoconstriction. There are no any reports concerning the effect of inhibition of biosynthesis of AEA on vasoconstriction in the literature. 2‐AG is mainly hydrolysed by monoacylglycerol lipase (MAGL) and also, to a lesser extent, by fatty acid amide hydrolase (FAAH). The main enzyme responsible for AEA degradation is FAAH.1 Inhibitors of these enzymes are known to increase the concentrations of 2‐AG and AEA, respectively, thus playing a role in the hypotensive effect in systemic vessels.36 This increases the vasodilatory effect of 2‐AG or AEA on CB 1 receptors, which limits the vasoconstriction. In rat, uterine arteries from the animal model that mimics many features of preeclampsia, a disorder characterized by the onset of hypertension, both FAAH and MAGL inhibitors (URB597 and JZL184, respectively), reduced the contractile response to Ang II, but in a CB 1 receptor‐independent manner.8 JZL184 attenuated the contraction responses evoked by Ang II in rPAs16 and rat and mouse aorta.33 AEA does not play a protective role against the AT 1 receptor‐mediated vasoconstriction in the pulmonary vascular bed, likely because URB597 does not modify the Ang II‐induced response in isolated rPAs (Table 2).16 It was described previously that the 2‐AG content is higher than AEA in various tissues26, 37 and 2‐AG acts as a full agonist of the CB 1 receptor, whereas AEA is only a partial agonist of the CB 1 receptor.38 To our knowledge, there is one study in which liquid chromatography–mass spectrometry measurements were carried out to determine the endogenous eCB content in rat middle cerebral arteries (MCAs) upon activation of G q/11 protein‐coupled receptors. It was observed that U46619 increased the apparent concentrations of AEA by 3–4.8 nm and 2‐AG by 3.6–6.4 μm in endothelium‐intact and endothelium‐denuded arteries, respectively, in contrast to groups treated by 5‐HT, where the eCB levels decreased.26 In summary, 2‐AG, but not AEA, activates CB 1 receptors because of its higher endogenous content, compared to AEA, which additionally can be elevated by vasoconstrictor‐specific stimulation to the concentration that is likely to activate CB 1 receptors. These findings show that it is more likely that 2‐AG, not AEA, is commonly released from the arterial wall following activation of Ca2+‐mobilizing AT1 and activates CB 1 receptors (Figure 1). However, findings of Turu et al.35 demonstrated that CB 1 receptor transactivation by eCBs is not specific to AT1 receptors and can also be initiated in CHO cells that express other G q/11 protein‐coupled receptors. These receptors will be described in the following paragraphs.

Prostanoids – thromboxane A 2 and prostaglandin F 2α and endocannabinoids It is known that occlusion of arteries by thrombotic or atherothrombotic embolic material, accompanied by the reduction in vasodilators and excessive vasoconstriction, can cause strokes39 and is a risk factor for pulmonary arterial hypertension.16, 40 There are data providing evidence that eCBs are released from the vascular wall as a result of TP receptor activation, thus playing a substantial role in the regulation of cerebral and pulmonary vascular tone. So far, we have focused on the idea that inhibition of CB 1 receptors by AM251 enhanced the U46619‐induced vasoconstriction in endothelium‐intact but not in endothelium‐denuded human pulmonary arteries (hPAs) and rPAs (Figure 2a,c; Table 2).16 On the other hand, in endothelium‐denuded rat MCAs, the presence of rimonabant resulted in a significant increase in the U46619‐induced response but had no effect on maximum contraction. This suggests that the interaction was VSMCs‐dependent and CB 1 receptor‐mediated vasodilation is overcome when TXA 2 receptors are maximally activated. Rimonabant had no effect on arterial diameter when added alone, which suggests that in the cerebral VSMCs, and the CB 1 receptor is neither tonically activated by eCBs nor constitutively active26 (Table 2). Similarly, exogenous 2‐AG, given at one concentration of 3 μm before construction of the concentration–response curve, failed to modify the U46619‐induced reaction in hPAs and rPAs, suggesting that either the basal level of endogenous 2‐AG is low or 2‐AG may not participate in the regulation of pulmonary arterial tone in physiological conditions. In non‐preconstricted pulmonary arteries of rats, humans and mice, eCBs did not change the basal tone.16, 41 As mentioned earlier, the synthesis of eCBs depends on the magnitude of the [Ca2+] i and only after the stimulation by different vasoconstrictors, the concentration of endogenous 2‐AG is higher, while with no stimulation, that is basal tone, there is no increase. Figure 2 Open in figure viewer PowerPoint Original traces showing the cumulative administration of U46619 (0.1–300 n m ) in the presence of AM251 (a), RHC80267 (b), JZL184 (b) in endothelium‐intact (+ENDO) and in the presence of AM251 (c) and RHC80267 (c) in endothelium‐denuded (‐ENDO) human isolated pulmonary arteries. Arrows indicate U46619 application. Incubation of the endothelium‐intact vessels with RHC80267 increased contractions induced by U46619 in hPAs and rPAs, in contrast to endothelium‐denuded vessels. The experiments with RHC80267 in PAs confirmed again that the underlying mechanism of the rapid synthesis of 2‐AG, but not AEA, released from endothelial cells, plays a protective role against the TP receptor‐mediated vasoconstriction (Table 2; Figure 2b,c).16 As discussed previously, U46619 led to increase in AEA and 2‐AG levels in the isolated rat MCAs. Interestingly, AEA and 2‐AG concentrations were both significantly greater in endothelium‐denuded compared with endothelium‐intact MCAs. It is possible that the process of mechanical disruption of MCAs’ endothelium caused tissue damage that resulted in increased eCB content in endothelium‐denuded MCAs. It is also possible that mechanical disruption of the endothelium, which would eliminate the release of endothelium‐derived hyperpolarizing factors, resulted in depolarization of VSMCs. This depolarization would increase the likelihood of eCB synthesis.26 Other findings provided further support for the hypothesis that activation of TP receptors in the MCAs mobilizes the eCBs, which act via CB 1 receptor activation, and play a role in the eCB feedback inhibition of the vasoconstrictor effects of U46619. In the presence of DETFP (inhibitor of FAAH and MAGL) and URB754 (MAGL inhibitor), the U46619‐induced contraction in MCA was attenuated.42 Moreover, JZL184 (MAGL inhibitor) attenuated the U46619‐induced contraction in isolated hPAs and rPAs (Figure 2b).16 AEA does not play a protective role against the TP receptor‐mediated vasoconstriction in the PAs or in MCAs, likely because URB597 does not modify the U46619‐induced responses (Table 2).16, 42 Apart from TXA 2 , prostaglandin F 2α (PGF 2α ) is also known to be a potent vasoconstrictor derived from the prostanoids family.43 The contractile effect of PGF 2α in the aorta of the G q/11 double‐knockout mice was maintained, which demonstrates that its main signalling pathway is not G q/11 ‐dependent.33 In contrast to the effects of Ang II, PGF 2α (at a high dose of 1 μm) mobilized a smaller elevation of the [Ca2+], which was not modulated by the CB 1 receptor antagonist O2050 (Table 2). Vasoconstrictor responses in CB 1 ‐knockout mice to Ang II (see above), but not to PGF 2α , were enhanced compared to control wild‐type littermates.33 These data suggest that there is no relationship between CB 1 receptor activation and stimulation of the prostaglandin F receptor (FP) with PGF 2α . To date, reports discussing FP and the eCB system are limited.

Other vasoconstrictors – phenylephrine, noradrenaline, serotonin, endothelin‐1 and endocannabinoids Little attention has been paid to the question of whether other agonists for G q/11 protein‐coupled receptors in the vasculature might be related to mobilizing the eCBs from the vascular wall or not. The data linking other agonists of G q/11 protein‐coupled receptors such as (1) α 1 ‐adrenergic receptor agonists, phenylephrine or noradrenaline, (2) the 5‐HT 2A receptor agonist, 5‐HT, (3) the ET A receptors agonist, ET1, with the eCB system in the vasculature, ares limited. CB 1 receptor antagonist, O2050, only weakly augmented the contraction of the isolated endothelium‐intact mouse aorta using one specific concentration (1 μm) of phenylephrine and by noradrenaline (0.01 μm) in the rat aorta, while those were not affected in the CB 1 ‐knockout mice.33 Another CB 1 receptor antagonist, AM251, did not change the phenylephrine contraction response in isolated endothelium‐intact rPAs.16 There are no data concerning the influence of inhibition of eCB biosynthesis or degradation on phenylephrine‐induced or noradrenaline‐induced vasoconstriction (Table 2). It was shown that the presence of AM251 did not change the 5‐HT‐induced contraction response in isolated endothelium‐intact rPAs,16 similar to the effect of rimonabant in endothelium‐denuded rat MCAs (Table 2).26 Interestingly, incubation of the rat MCAs with 5‐HT significantly decreased AEA and 2‐AG content in both endothelium‐intact and in endothelium‐denuded MCAs, compared with control vessels incubated in buffer alone. Therefore, 5‐HT likely does not play a role in the reduction in the contraction response via CB 1 receptor activation,26 in contrast to the vasoconstrictors described above. There are no additional data regarding the influence that the inhibition of eCBs biosynthesis or degradation has on 5‐HT‐induced vasoconstriction. The interaction of ET‐1 with eCBs was investigated in the cell culture of human brain endothelial cells. It was observed that 2‐AG effectively reduced the ET‐1‐stimulated [Ca2+] i levels and formation of IP 2 and IP 3 .44 However, the relationship between ET A and CB 1 receptors in the vasculature was not examined sufficiently, and further studies are needed.

Conclusion In the vascular tissue, G q/11 protein‐coupled receptors interact with numerous hormones and mediators to affect intracellular signal transduction pathways and functional responses and thus have great potential as therapeutic targets for a wide spectrum of diseases.45 A survey of the literature helped us to consider what is currently known about the interaction between activation of G q/11 protein‐coupled receptors for select vasoconstrictors and the formation of eCBs in the vasculature. The data raise the possibility that production of eCBs in the vasculature is dependent on agonist‐induced contraction force or a downstream signalling pathway, which is characteristic. Moreover, in many cases, it is difficult to identify well‐defined eCB‐G q/11 protein‐coupled receptor interactions in the periphery, because the tissue environment is often less tightly organized, compared to the CNS synapses.17 In summary, in the vasculature (1) CB 1 receptors play a crucial role in the cross‐talk between vasoconstrictors for G q/11 protein‐coupled receptors and the eCBs system; (2) negative feedback by eCBs (mainly 2‐AG) is involved in the protective mechanism of the CB 1 ‐dependent reduction in agonist‐induced vasoconstriction, having antihypertensive properties; (3) this relationship can be both endothelium‐dependent and endothelium‐independent, according to tested animal species and tissue, as the endothelium therein plays crucial role in pulmonary arteries, but not in systemic arteries. A number of studies have been carried out in animals, and more research is required in humans to establish the importance of the eCB system4 and better understanding reciprocal connections within various cellular downstream signalling pathways.

Declarations Conflict of interest The Author(s) declare(s) that they have no conflicts of interest to disclose. Funding This work was supported by grants from the Medical University of Białystok (N/ST/MN/16/001/2213; N/ST/ZB/15/004/2213).