Introduction and Objective: Adenosine monophosphate-activated protein kinase (AMPK) facilitates the production of nitric oxide, a major endothelium-derived relaxing factor, mainly in conduit arteries. However, in smaller arteries, which have greater contribution to the regulation of peripheral resistance than conduit arteries, endothelium-dependent hyperpolarization (EDH) plays a more important role in the regulation of vascular tone. The common characteristic of the EDH pathway is the opening of endothelial intermediate- and small-conductance calcium-activated potassium channels (IKCa and SKCa, respectively); the resultant efflux of potassium ions eventually causes hyperpolarization of the underlying smooth muscle cells leading to their relaxation. The present study aimed to examine whether or not AMPK affects EDH-mediated relaxations.

Methods: Rings of superior mesenteric arteries of male twelve-weeks old Sprague Dawley (SD) rats were suspended in conventional organ chambers for isometric tension recording. To examine the signaling pathways involved, they were incubated with different pharmacological agents during 40 minutes before exposing them to contracting and relaxing agents. After that, rings were collected to measure the enzyme activity.

Results: Both AICAR (10-4 M) and A769662 (10-4 M), AMPK activators with different structures and binding sites on the enzyme, significantly reduced EDH-like relaxations in response to acetylcholine as well as those to SKA-31 (opener of calcium-activated potassium channels) in superior mesenteric arteries of SD rats. The inhibitory effects of AICAR and A769662 were prevented by compound C (AMPK inhibitor; 10-5 M). Inhibition of IKCa or SKCa channel with their respective blocker, TRAM-34 (10-6 M) and UCL1684 (10-6 M), did not affect acetylcholine-induced relaxations; however, in the concomitant presence of AICAR, the relaxations were reduced significantly. In rings without endothelium, potassium ions and levcromakalim (opener of ATP-sensitive potassium channel) induced significant relaxations, both of which were not affected by AICAR but significantly inhibited by A769662.

Conclusions: Activation of AMPK results in inhibition of EDH-like relaxations in rat superior mesenteric arteries. While AICAR appears to reduce the generation of the endothelial signal, likely targeting calcium-activated potassium channels to produce its inhibitory effect, A769662 prevents the signaling downstream of smooth muscle hyperpolarization.