All peripheral arteries are surrounded by perivascular adipose tissue (PVAT) which, in health, releases several factors with vasodilator properties. Recent evidence suggests that one such factor acts by opening voltage-sensitive (Kv7) K+ channels1, although our recent experiments suggest that the K+ channel-opener released is likely to be adiponectin and that it acts by opening large conductance Ca2+-dependent K+ channels (BKCa)2. In many tissues, adiponectin receptors signal via the AMP-dependent protein kinase (AMPK). Opening of BKCa channels and enhanced activity of AMPK are both linked to smooth muscle relaxation3,4,5. The present study investigated the effect of adiponectin and AMPK activation on BKCa currents in single myocytes which were freshly-isolated (enzymatically) from male Sprague-Dawley rat mesenteric arteries. BKCa currents were recorded at room temperature using the whole-cell configuration of the patch-clamp technique. Under the conditions employed (calculated intracellular calcium concentration of 0.2 μM), 500 ms steps from a holding potential of -10 mV to a series of test potentials ranging from -40 to 50 mV generated outward currents with an activation threshold of -20 mV which were inhibited by 0.1 μM iberiotoxin (a selective BKCa channel blocker). The AMPK activator A-769,662 (10 μM) significantly increased BKCa currents without modifying the activation threshold of the channel. The current induced by A-769,662 was reversibly inhibited by iberiotoxin (0.1 μM), but was insensitive to the (ATP-competitive) AMPK inhibitor dorsomorphin (2 μM). Furthermore, a less-selective AMPK activator, AICAR (1 mM), did not increase BKCa currents. Interestingly, the physiological saline solution which had bathed PVAT for 1 h (at 37°C) also increased an iberiotoxin-sensitive outward K+ current. Neither the background nor the induced current was sensitive to inhibition by either XE-991 (30 μM; a selective blocker of Kv7 channels) or dorsomorphin (2 μM). When myocytes were perfused with adiponectin (0.5 μg/ml), the BKCa current was also enhanced. Consistent with our previous findings2, under the conditions of the present study only BKCa channels were activated by a PVAT-derived factor and both adiponectin and A-769,662 were able to mimic this effect. The simplest hypothesis is that PVAT releases adiponectin which acts via AMPK to activate BKCa channels. Nevertheless, we are unable to explain why activation of BKCa by A-769,662 (which at the concentration employed is considered to be selective for AMPK), was not mimicked by AICAR or inhibited by dorsomorphin, although this might reflect the mechanisms of action of the different drugs and work is currently in progress to investigate this further.