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Data from the use of activators and inhibitors of the AMP-activated protein kinase (AMPK) suggest that AMPK increases sensitivity of glucose transport to stimulation by insulin in muscle ceils. We assayed insulin action after adenoviral (Ad) transduction of constitutively active (CA; a truncated form of AMPKα1) and dominant-negative (DN; which depletes endogenous AMPKα) forms of AMPKα (Ad-AMPKα-CA and Ad-AMPKα-DN, respectively) into C2C12 myotubes. Compared with control (Ad-green fluorescent protein), Ad-AMPK-CA increased the ability of insulin to stimulate glucose transport. The increased insulin action in cells expressing AMPK-CA was suppressed by compound C (an AMPK inhibitor). Exposure of cells to 5-aminoimidazole-4-carboxamide-1β-ᴅ-ribofuranoside (an AMPK activator) increased insulin action in uninfected myotubes and myotubes transduced with green fluorescent protein but not in Ad-AMPK-DN-infected myotubes. In Ad-AMPK-CA-transduced cells, serine phosphorylation of insulin receptor substrate 1 was decreased at a mammalian target of rapamycin (or p70 S6 kinase) target site that has been reported to be associated with insulin resistance. These data suggest that, in myotubes, activated AMPKα1 is sufficient to increase insulin action and that the presence of functional AMPKα is required for 5-aminoimidazole-4-carboxamide-1β,ᴅ-ribofuranoside-related increases in insulin action. [ABSTRACT FROM AUTHOR]