The AMP-activated protein kinase (AMPK) is one of the key players in cellular energy regulation adapting cellular demands to nutritional and metabolic variations. Oral antidiabetic drugs like metformin and glitazones (thiazolidinediones) are known to stimulate this enzyme. Besides their established action on peripheral organs including liver and muscles, it has been claimed that these drugs may affect β-cell function. However, it is still a matter of debate whether pharmacological AMPK stimulation increases or decreases insulin secretion. To study this point and to reveal mechanisms underlying changes in insulin secretion we used the specific AMPK activator AICAR and investigated its effects on stimulus-secretion coupling. Membrane potential and currents were measured by the patch-clamp technique, [Ca (2+)]c, mitochondrial membrane potential, and NAD(P)H by fluorescence techniques and insulin secretion by a radioimmunoassay. AICAR enhanced glucose-stimulated insulin release, an effect that can be attributed to the augmentation of electrical activity and [Ca (2+)]c resulting from an AICAR-evoked inhibition of the KATP current. This latter effect was not due to a direct interaction of AICAR with the K[ATP] channels but was dependent on cell metabolism. AICAR did not affect mitochondrial membrane potential or NAD(P)H autofluorescence. Metformin mimicked the action of AICAR on electrical activity, [Ca (2+) ]c, and K[ATP] current. However, compared to AICAR the effects were less pronounced and not sufficient to stimulate insulin secretion. In conclusion, activation of AMPK augments nutrient-induced insulin secretion. Thus, targeting AMPK of β-cells may be an appropriate strategy for the treatment of disturbed glucose homeostasis..