α2 Isoform-specific activation of 5′ adenosine monophosphate-activated protein kinase by 5-aminoimidazole-4-carboxamide-1- β-D-ribonucleoside at a physiological level activates glucose transport and increases glucose transporter 4 in mouse skeletal muscle

5′Adenosine monophosphate-activated protein kinase (AMPK) has been implicated in exercise-induced stimulation of glucose metabolism in skeletal muscle. Although skeletal muscle expresses both the α1 and α2 isoforms of AMPK, the α2 isoform is activated predominantly in response to moderate-intensity endurance exercise in human and animal muscles. The purpose of this study was to determine whether activation of α2 AMPK plays a role in increasing the rate of glucose transport, promoting glucose transporter 4 (GLUT4) expression, and enhancing insulin sensitivity in skeletal muscle. To selectively activate the α2 isoform, we used 5-aminoimidazole-4- carboxamide-1-β-d-ribonucleoside (AICAR), which is metabolized in muscle cells and preferentially stimulates the α2 isoform. Subcutaneous administration of 250 mg/kg AICAR activated the α2 isoform for 90 minutes, but not the α1 isoform in hind limb muscles of the C57/B6J mouse. The maximal activation of the α2 isoform was observed 30 to 60 minutes after administration of AICAR and was similar to the activation induced by a 30-minute swim in a current pool. The increase in α2 activity paralleled the phosphorylation of Thr¹⁷², the essential residue for full kinase activation, and the activity of acetyl-coenzyme A carboxylase β, a known substrate of AMPK in skeletal muscle. Subcutaneous injection of AICAR rapidly increased, by 30%, the rate of 2-deoxyglucose (2DG) transport into soleus muscle; 2DG transport increased within 30 minutes and remained elevated for 4 hours after administration of AICAR. Repeated intraperitoneal injection of AICAR, 3 times a day for 4 to 7 days, increased soleus GLUT4 protein by 30% concomitant with a significant 20% increase in insulin-stimulated 2DG transport. These data suggest that moderate endurance exercise promotes glucose transport, GLUT4 expression, and insulin sensitivity in skeletal muscle at least partially via activation of the α2 isoform of AMPK.