It has been recently shown that the AMP-activated protein kinase (AMPK) could phosphorylate myosin light chain kinase in macrovascular smooth muscle, whereas studies on isolated oxygen sensing type I cells of rat carotid bodies suggest that AMPK inhibits BK_Ca channels. While the earlier mechanism, when present in vascular smooth muscle, would lead to vasodilation, the latter would result in vasoconstriction by inhibition of EDHF mediated dilation. We therefore investigated the role of AMPK in the control of microvascular tone by means of the novel activator of the AMP-activated protein kinase A769662 (A76).

Small side branches (diameter 265 4mm, n=40 in total) of the femoral artery of Golden Syrian hamsters located in the gracilis muscle region were carefully isolated and then cannulated with glass micropipettes. The smooth muscle layer was loaded with the calcium indicator Fura2-AM. Vascular diameter was simultaneously analysed by videomicroscopy. All microvesssels were pre-treated with the COX inhibitor indomethacin (30mM) and the NOS inhibitor L-NAME (30mM).

Microvessels pre-constricted with 0.3mM norepinephrine and subsequently treated with A76 (10^-6 to 3x10^-4M) showed a dose dependent vasodilatation (98.1 2.4% at 3x10^-4M) associated with a decrease of [Ca⁺⁺]_i (98.9 2.6% at 3x10^-4M). The effects observed were not dependent on the presence of intact endothelium. In arteries pre-constricted with 100mM potassium, A76 (10^-4M) induced neither relaxation nor decreased smooth muscle calcium. During an observation period of 10 min, there was no evidence for a calcium independent dilation, either. Inhibition of IK_Ca / BK_Ca potassium channels with CTX (1mM, both channels) - which has previously been shown to inhibit EDHF mediated dilations - or IBTX (0.1mM), a specific inhibitor of IK_Ca, did not alter maximal effects of A76 (10^-4M; dilation 96.5 3.9%; calcium decrease 88.0 7.2%) significantly, whereas the K_ATP inhibitor glibenclamide (0.1mM) reduced dilation to 21.1 15.5% as well as the decrease of [Ca⁺⁺]_i to 31.5  5.0%.

These results characterise AMPK activation as a potent vasodilator mechanism of small resistance arteries. The results obtained with glibenclamide suggest that K_ATP channels may be a novel target of AMPK in microvascular smooth muscle. No functional evidence has been found so far for a role of the AMPK in reducing calcium sensitivity as recently described in macrovessels.