The insulin resistance syndrome is characterized by several risk factors for cardiovascular disease. Chronic chemical activation of AMP-activated protein kinase by the adenosine analog 5-aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside (AICAR) has been shown to augment insulin action, upregulate mitochondrial enzymes in skeletal muscles, and decrease the content of intraabdominal fat. Furthermore, acute AICAR exposure has been found to reduce sterol and fatty acid synthesis in rat hepatocytes incubated in vitro as well as suppress endogenous glucose production in rats under euglycemic clamp conditions. To investigate whether chronic AICAR administration, in addition to the beneficial effects on insulin sensitivity, is capable of improving other phenotypes associated with the insulin resistance syndrome, obese Zucker (fa/fa) rats (n ‫؍‬ 6) exhibiting insulin resistance, hyperlipidemia, and hypertension were subcutaneously injected with AICAR (0.5 mg/g body wt) daily for 7 weeks. Obese control rats were either pair-fed (PF) (n ‫؍‬ 6) or ad libitum-fed (AL) (n ‫؍‬ 6). Lean Zucker rats (fa/؊) (n ‫؍‬ 8) served as a reference group. AICAR administration significantly reduced plasma triglyceride levels (P < 0.01 for AICAR vs. AL, and P ‫؍‬ 0.05 for AICAR vs. PF) and free fatty acids (P < 0.01 for AICAR vs. AL, and P < 0.05 for AICAR vs. PF) and increased HDL cholesterol levels (P < 0.01 for AICAR vs. AL and PF). AICAR treatment also lowered systolic blood pressure by 14.6 ؎ 4.3 mmHg (P < 0.05), and AICAR-treated animals exhibited a tendency toward decreased intra-abdominal fat content. Furthermore, AICAR administration normalized the oral glucose tolerance test and decreased fasting concentrations of glucose and insulin close to the level of the lean animals. Finally, in line with previous findings, AICAR treatment was also found to enhance GLUT4 protein expression and to increase maximally insulin-stimulated glucose transport in primarily white fast-twitch muscles. Our data provide strong evidence that longterm administration of AICAR improves glucose tolerance, improves the lipid profile, and reduces systolic blood pressure in an insulin-resistant animal model. The present study gives additional support to the hypothesis that AMPK activation might be a potential future pharmacological strategy for treating the insulin resistance syndrome.

Lifestyle interventions including exercise programs are cornerstones in the prevention of obesity-related diabetes. The AMP-activated protein kinase (AMPK) has been proposed to be responsible for many of the beneficial effects of exercise on glucose and lipid metabolism. The effects of long-term exercise training or 5-aminoimidazole-4-carboxamide-1-␤-D-riboruranoside (AICAR) treatment, both known AMPK activators, on the development of diabetes in male Zucker diabetic fatty (ZDF) rats were examined. Five-week-old, prediabetic ZDF rats underwent daily treadmill running or AICAR treatment over an 8-week period and were compared with an untreated group. In contrast to the untreated, both the exercised and AICAR-treated rats did not develop hyperglycemia during the intervention period. Whole-body insulin sensitivity, as assessed by a hyperinsulinemic-euglycemic clamp at the end of the intervention period, was markedly increased in the exercised and AICAR-treated animals compared with the untreated ZDF rats (P < 0.01). In addition, pancreatic ␤-cell morphology was almost normal in the exercised and AICAR-treated animals, indicating that chronic AMPK activation in vivo might preserve ␤-cell function. Our results suggest that activation of AMPK may represent a therapeutic approach to improve insulin action and prevent a decrease in ␤-cell function associated with type 2 diabetes. Diabetes 54:928 -934, 2005

Recent studies have demonstrated that chronic administration of AICAR (5-aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside), an activator of the AMP-activated protein kinase, increases hexokinase activity and the contents of total GLUT4 and glycogen in rat skeletal muscles. To explore whether AICAR also affects insulinstimulated glucose transport and GLUT4 cell surface content, Wistar rats were subcutaneously injected with AICAR for 5 days in succession (1 mg/g body wt). Maximally insulin-stimulated (60 nmol/l) glucose uptake was markedly increased in epitrochlearis (EPI) muscle (average 63%, P < 0.001, n = 18-19) and in extensor digitorum longus muscle (average 26%, P < 0.001, n = 26-30). In contrast, administration of AICAR did not maximally influence insulin-stimulated glucose transport in soleus muscle. Studies of EPI muscle with the 4,4Ј-O-[2-[2-[2-[2-[2-[6-(biotinylamino)hexanoyl]amino]ethoxy]ethoxy] ethoxy]-4-(1-azi-2,2,2,-trifluoroethyl)benzoyl]amino-1,3-propanediyl]bis-D-mannose photolabeling techniqueshowed a concomitant increase (average 68%, P < 0.02) in cell surface GLUT4 content after insulin exposure in AICARinjected rats when compared with controls. In conclusion, 5 days of AICAR administration induces a pronounced fiber type-specific increase in insulin-stimulated glucose uptake and GLUT4 cell surface content in rat skeletal muscle with the greatest effect observed on white fast-twitch glycolytic muscles (EPI). These results are comparable with the effects of chronic exercise training, and it brings the AMP-activated protein kinase into focus as a new interesting target for future pharmacological intervention in insulin-resistant conditions. Diabetes 50:12-17, 2001 P hysical exercise augments insulin sensitivity in skeletal muscle of insulin-resistant animals (1-4) and can also improve insulin sensitivity in type 2 diabetic individuals (5,6). Exercise is therefore pivotal in the treatment of type 2 diabetes. Studies in skeletal muscle from type 2 diabetic and (7) insulin-resistant prediabetic individuals (8) have shown an increased fraction of white type 2b muscle fibers. This change toward fast-twitch fibers in skeletal muscle has further been demonstrated in hyperinsulinemic rats (9) and in fructose-fed rats (an animal model of insulin-resistance) (10). Because it is well established that fasttwitch fibers (in particular type 2b fibers) are less insulin sensitive (11,12), this change in muscle morphology might potentially play a role in the decreased insulin action seen in skeletal muscle of insulin-resistant individuals.Muscle fibers also seem to differ in their ability to increase insulin action as a response to physical exercise (1-3). The exercise intensity used during exercise programs appears to play an important role for the fiber types recruited during the work performed and thus for their capability to exhibit adaptive changes with respect to insulin action (3) and oxidative capacity (13). Several animal studies featuring treadmill-running programs have shown that predominantly fast-t...

Physical activity is known to increase insulin action in skeletal muscle, and data have indicated that 5'-AMP-activated protein kinase (AMPK) is involved in the molecular mechanisms behind this beneficial effect. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) can be used as a pharmacological tool to repetitively activate AMPK, and the objective of this study was to explore whether the increase in insulin-stimulated glucose uptake after either long-term exercise or chronic AICAR administration was followed by fiber-type-specific changes in insulin signaling and/or changes in GLUT-4 expression. Wistar rats were allocated into three groups: an exercise group trained on treadmill for 5 days, an AICAR group exposed to daily subcutaneous injections of AICAR, and a sedentary control group. AMPK activity, insulin-stimulated glucose transport, insulin signaling, and GLUT-4 expression were determined in muscles characterized by different fiber type compositions. Both exercised and AICAR-injected animals displayed a fiber-type-specific increase in glucose transport with the most marked increase in muscles with a high content of type IIb fibers. This increase was accompanied by a concomitant increase in GLUT-4 expression. Insulin signaling as assessed by phosphatidylinositol 3-kinase and PKB/Akt activity was enhanced only after AICAR administration and in a non-fiber-type-specific manner. In conclusion, chronic AICAR administration and long-term exercise both improve insulin-stimulated glucose transport in skeletal muscle in a fiber-type-specific way, and this is associated with an increase in GLUT-4 content.