Question: 

Blood-flow rate in the microcirculation is tightly controlled to meet the metabolic demands of the tissues. The heterotrimeric AMPK is a highly conserved sensor of cellular energy status. We therefore investigated a potential role of the AMPK in the control of microvascular tone using the novel AMPK-stimulator A769662 (A76).

Methodology: 

Simultaneous measurements of smooth muscle intracellular calcium levels [Ca²⁺]_i (FURA2-AM) and vascular diameters were performed in isolated skeletal muscle resistance arteries (hamster;n=60), pre-treated with L-NAME (30mM) and indomethacin (30mM) to exclude effects of endothelial NO and prostaglandins.

Results: 

In vessels pre-constricted with norepinephrine (0.3mM) A76 induced a dose-dependent vasodilation, which was maximal at 100mM. The dilation was associated with a decrease in [Ca²⁺]_i and endothelium-independent. Other well-known AMPK-activators (Metformin 3mM, 2-Deoxyglucose 1–10mM) mimicked the A76 effect. Partial knock down of the regulatory AMPK beta1 subunit by transfecting the microvessels with siRNA induced a significant rightward shift in the dose-response-curves. In contrast, in vessels pre-constricted by high extracellular potassium (100mM), A76 did not induce relaxation or [Ca²⁺]_i-decrease. In freshly isolated smooth muscle cells from these arteries patch-clamp studies revealed activation of a BK_Ca channel by A76, which could be blocked by the specific inhibitor Paxilline (500nM). Accordingly, Paxilline (1mM) reduced the A76 (30mM) induced [Ca²⁺]_i-decrease nearly completely and the dilation 30%, respectively.

Conclusions: 

We suggest that the AMPK is a potent regulator of microvascular tone. The dilator effect is partly due to a calcium decrease, which is induced by activation of BK_Ca channels in vascular smooth muscle.