Cannabinoid receptor type 1 activation by arachidonylcyclopropylamide in rat aortic rings causes vasorelaxation involving calcium-activated potassium channel subunit alpha-1 and calcium channel, voltage-dependent, L type, alpha 1C subunit

Cannabinoids are key regulators of vascular tone, some of the mechanisms involved include the activation of cannabinoid receptor types 1 and 2 (CB); the transient receptor potential cation channel, subfamily V, member 1 (TRPV1); and non-(CB1)/non-CB2 receptors. Here, we used the potent, selective CB1 agonist arachidonylcyclopropylamide (ACPA) to elucidate the mechanism underlying vascular tone regulation. Immunohistochemistry and confocal microscopy revealed that CB1 was expressed in smooth muscle and endothelial cells in rat aorta. We performed isometric tension recordings in aortic rings that had been pre-contracted with phenylephrine. In these conditions, ACPA caused vasorelaxation in an endothelium-independent manner. To confirm that the effect of ACPA was mediated by CB1 receptor, we repeated the experiment after blocking these receptors with a selective antagonist, AM281. In these conditions, ACPA did not cause vasorelaxation. We explored the role of K+ channels in the effect of ACPA by applying high-K+ solution to induce contraction in aortic rings. In these conditions, the ACPA-induced vasorelaxation was about half that observed with phenylephrine-induced contraction. Thus, K+ channels were involved in the ACPA effect. Furthermore, the vasorelaxation effect was similarly reduced when we specifically blocked calcium-activated potassium channel subunit alpha-1 (KCa1.1) (MaxiK; BKCa) prior to adding ACPA. Finally, ACPA-induced vasorelaxation was also diminished when we specifically blocked the calcium channel, voltage-dependent, L type, alpha 1C subunit (Cav1.2). These results showed that ACPA activation of CB1 in smooth muscle caused vasorelaxation of aortic rings through a mechanism involving the activation of KCa1.1 and the inhibition of Cav1.2.