Objective: 

Adaptation of blood flow to tissue metabolic demands occurs through changes of microvascular arterial tone. The energy-sensing enzyme AMPK may be a potential regulator of smooth muscle tone in response to metabolic challenges. Thus we studied the potential vasomotor function of vascular smooth muscle AMPK in isolated small arteries.

Methods: 

Studies in intact segments as well as smooth muscle patch clamp studies were conducted in mesenteric and skeletal muscle resistance arteries (hamster and mouse; n=110). The isolated vessels, were pre-treated with indomethacin (30mM) and L-NAME (30mM). We measured simultaneously smooth muscle intracellular calcium levels [Ca²⁺]i (FURA2-AM) and vascular diameters.

Results: 

The AMPK-stimulator A769662 (A76) induced an endothelium independent vasodilation which was associated with a decrease in [Ca²⁺]i in vessels pre-constricted with norepinephrine (0.3 mM). Partial knock down of the AMPK by siRNA induced a significant rightward shift in the dose-response-curves of A76. In vessels pre-constricted by high extracellular potassium (60mM) the vasodilation was diminished and the [Ca²⁺]i decrease abolished completely. The remaining dilator effect could be completely blocked by the myosin-light-chain-phosphatase (MLCP) inhibitor calyculinA (100nM). Other AMPK-activators (metformin 0.1–3mM, 2-deoxyglucose 5mM) qualitatively mimicked the effect of A76.

Patch-clamp studies revealed activation of BKchannels by A76, which could be blocked by the specific inhibitor paxilline (500nM) and the AMPK inhibitor compoundC (100mm). Accordingly, paxilline (1mM) inhibited the A76 dilator effect of norepinephrine pre-constricted microvessels by about 30%.

Conclusions: 

AMPK augments BKchannel current and reduces calcium sensitivity of the contractile machinery of arterial microvessels. These two mechanisms may contribute to microvascular vasodilation in response to accelerated tissue metabolism.