Endocannabinoids And The Control Of Cardiovascular Function

National Institute On Alcohol Abuse And Alcoholism

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Abstract

Our earlier studies have indicated that the endothelium-dependent vasodilator effect of the anandamide and of some atypical cannabinoid ligands, such as abnormal cannabidiol (abn-cbd) is mediated by a pertussis toxin-sensitive, G protein-coupled receptor distinct from CB1 or CB2. In collaboration with Mechoulam's group we have earlier identified a novel, endocannabinoid-like brain lipid, arachidonoyl L-serine (ARA-S). Contrary to anandamide, ARA-S has no affinity for CB1, CB2 or vanilloid TRPV1 receptors, and it produces partially endothelium-dependent vasodilation in rat isolated mesenteric artery and aorta preparations. In a more recent study just submitted for publication, we have analyzed the effect of ARA-S on Ca2+-activated K+ currents in human embryonic kidney cells stably transfected with the alpha-subunit of the human, large conductance Ca2+-activated K+ (BKCa) channel (HEK293hSlo cells). ARA-S caused relaxation of isolated, intact and denuded, small mesenteric arteries (pEC50: 5.49 and 5.14, respectively) of the rat. In both preparations the response was inhibited by 100 nM iberiotoxin. In HEK293hSlo cells, ARA-S and its enantiomer N-arachidonoyl-D-serine enhanced the whole cell outward K+ current with similar potency (pEC50: 5.63 and 5.32, respectively). The potentiation was not mediated by ARA-S metabolites, stimulation of known cannabinoid receptors, G proteins, protein kinases or Ca2+-dependent processes, and it was lost after patch excision or following membrane cholesterol depletion, but was restored after cholesterol reconstitution. BKCa currents were also enhanced by anandamide (AEA, pEC50: 5.27) but inhibited by another endocannabinoid, virodhamine (pIC50: 6.35), or by the synthetic cannabinoid O-1918, which blocks ARA-S-induced vasodilation (pIC50: 6.59). These findings indicate that (i) endocannabinoids directly modulate the activity of BKCa channels or a channel-associated component. (ii) This interaction does not involve cytosolic factors but is dependent on the presence of membrane cholesterol (iii) Direct BKCa channel activation likely contributes to the endothelium-independent component of ARA-S-induced mesenteric vasorelaxation. (iv) Depending on the structure of the head group, the effect on BKCa currents is either stimulatory or inhibitory. (v) O-1918 is a potent BKCa channel inhibitor.[unreadable] [unreadable] We have earlier reported that CB1 cannabinoid receptor antagnists, which do not affect blood pressure in healthy, normotensive rats, increase blood pressure as well as cardiac contractile function in 3 different models of experimental hypertension in rats. In contrast, a commercially available inhibitor of fatty acid amidohydrolase (FAAH), the enzyme responsible for the in vivo degradation of the endocannabinoid anandamide normalizes both the elevated blood pressure and the inappropriately high cardiac contractility in hypertensive rats. We have now extended this study by developing our own proprietary FAAH antagonist, which is more potent in producing similar effects both in anesthetized as well as in conscious, chronically instrumented animals. We further verified that these effects are mediated by CB1 receptors, as they can be prevented or acutely reversed by a CB1 receptor antagonist. The hypotensive and negative inotropic effects of the FAAH antagonist are substantially reduced following ganglionic blockade and restoration of blood pressure by vasopressin infusion, suggesting the a sympatholytic mechanism of action. Ongoing studies in spontaneously hypertensive rats are aimed to test whether chronic treatment with the FAAH antagonist can delay/prevent the development of cardiac hypertrophy.