Cannabinoids Acting on CB1 Receptors Decrease Contractile Performance in Human Atrial Muscle

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Abstract
Cannabinoids elicit hypotension mainly via activated CB1 receptors and show complex cardiovascular actions. Effects on human heart muscle have not been studied yet. Isolated human atrial heart muscle preparations were stimulated by electrical field with 1 Hz to contract isometrically at optimal length and were challenged with the endogenous cannabinoid arachidonyl ethanolamide (anandamide), the metabolically stable analogue R-methanandamide, and the potent synthetic CB1 receptor agonist HU-210. Anandamide dose-dependently decreased systolic force (82.2 ± 4.8% and 60.8 ± 6.8% of maximal systolic force for 0.1 and 1 μM, respectively, P < 0.05). The selective CB1 receptor antagonist AM-251 (1 μM, P < 0.05), but not the CB2 receptor antagonist, AM-630 (1 μM), the nitric oxide synthase inhibitor N ω-nitro-l-arginine methyl ester (l-NAME) (500 μM), or the cyclooxygenase inhibitor indomethacin (100 μM), prevented the effect. Contrary to indomethacin, l-NAME alone showed negative inotropic effects (72.1 ± 3.54%, P < 0.001). The R-methanandamide (1 μM: 50.4 ± 3.5%, P < 0.001) and HU-210 (1 μM: 60.1 ± 3.8%, P < 0.001) had similar negative inotropic effects. The existence of CB1 receptors on heart muscle was verified using Western blot analysis and immunofluorescence staining. The conclusion is that anandamide, R-methanandamide, and HU-210 decrease contractile performance in human atrial muscle via CB1 receptors.