The cellular mechanisms by which adenosine evokes release of nitric oxide from rat aortic endothelium

Abstract

Adenosine and nitric oxide (NO) are important local mediators of vasodilatation. The aim of this study was to elucidate the mechanisms underlying adenosine receptor-mediated NO release from the endothelium. In studies on freshly excised rat aorta, second-messenger systems were pharmacologically modulated by appropriate antagonists while a NO-sensitive electrode was used to measure adenosine-evoked NO release from the endothelium. We showed that A₁-mediated NO release requires extracellular Ca²⁺, phospholipase A₂ (PLA₂) and ATP-sensitive K⁺ (K_ATP) channel activation whereas A_2A-mediated NO release requires extracellular Ca²⁺ and Ca²⁺-activated K⁺ (K_Ca) channels. Since our previous study showed that A1- and A_2A-receptor-mediated NO release requires activation of adenylate cyclase (AC), we propose the following novel pathways. The K⁺ efflux resulting from A₁-receptor-coupled K_ATP-channel activation facilitates Ca²⁺ influx which may cause some stimulation of endothelial NO synthase (eNOS). However, the increase in [Ca²⁺]_i also stimulates PLA₂ to liberate arachidonic acid and stimulate cyclooxygenase to generate prostacyclin (PGI₂). PGI₂ acts on its endothelial receptors to increase cAMP, so activating protein kinase A (PKA) to phosphorylate and activate eNOS resulting in NO release. By contrast, the K⁺ efflux resulting from A_2A-coupled K_Ca channels facilitates Ca²⁺ influx, thereby activating eNOS and NO release. This process may be facilitated by phosphorylation of eNOS by PKA via the action of A_2A-receptor-mediated stimulation of AC increasing cAMP. These pathways may be important in mediating vasodilatation during exercise and systemic hypoxia when adenosine acting in an endothelium- and NO-dependent manner has been shown to be important.