Activation of AMP-activated protein kinase (AMPK) in rodent muscle by exercise, metformin, 5-aminoimidazole-4-carboxamide 1-␤-D-ribofuranoside (AICAR), and adiponectin increases glucose uptake. The aim of this study was to determine whether AICAR stimulates muscle glucose uptake in humans. We studied 29 healthy men (aged 26 ؎ 8 years, BMI 25 ؎ 4 kg/m 2 [mean ؎ SD]). Rates of muscle 2-deoxyglucose (2DG) uptake were determined by measuring accumulation of total muscle 2DG (2DG and 2DG-6-phosphate) during a primed, continuous 2DG infusion. The effects of AICAR and exercise on muscle AMPK activity/phosphorylation and 2DG uptake were determined. Whole-body glucose disposal was compared before and during AICAR with the euglycemic-hyperinsulinemic clamp. Muscle 2DG uptake was linear over 9 h (R 2 ‫؍‬ 0.88 ؎ 0.09). After 3 h, 2DG uptake increased 2.1 ؎ 0.8-and 4.7 ؎ 1.7-fold in response to AICAR or bicycle exercise, respectively. AMPK ␣ 1 and ␣ 2 activity or AMPK phosphorylation was unchanged after 20 min or 3 h of AICAR, but AMPK phosphorylation significantly increased immediately and 3 h after bicycle exercise. AICAR significantly increased phosphorylation of extracellular signal-regulated kinase 1/2, but phosphorylation of ␤-acetyl-CoA carboxylase, glycogen synthase, and protein kinase B or insulin receptor substrate-1 level was unchanged. Mean whole-body glucose disposal increased by 7% with AICAR from 9.3 ؎ 0.6 to 10 ؎ 0.6 mg ⅐ kg ؊1 ⅐ min ؊1 (P < 0.05). In healthy people, AICAR acutely stimulates muscle 2DG uptake with a minor effect on whole-body glucose disposal.

This study investigates the consequences of inhibition of adipose tissue lipolysis on skeletal muscle substrate use. Ten subjects were studied at rest and during exercise and subsequent recovery under normal, fasting conditions (control trial, CON) and following administration of a nicotinic acid analog (low plasma free fatty acid trial, LFA). Continuous [U- 13 C]palmitate and [6, H2]glucose infusions were applied to quantify plasma free fatty acid (FFA) and glucose oxidation rates and to estimate intramuscular triacylglycerol (IMTG) and glycogen use. Muscle biopsies were collected to measure 1) fiber type-specific IMTG content; 2) allosteric regulators of hormone-sensitive lipase (HSL), glycogen phosphorylase, and pyruvate dehydrogenase; and 3) the phosphorylation status of HSL at Ser 563 and Ser 565. Administration of a nicotinic acid analog (acipimox) substantially reduced plasma FFA rate of appearance and subsequent plasma FFA concentrations (P Ͻ 0.0001). At rest, this substantially reduced plasma FFA oxidation rates, which was compensated by an increase in the estimated IMTG use (P Ͻ 0.05). During exercise, the progressive increase in FFA rate of appearance, uptake, and oxidation was prevented in the LFA trial and matched by greater IMTG and glycogen use. Differential phosphorylation of HSL or relief of its allosteric inhibition by long-chain fatty acyl-CoA could not explain the increase in muscle TG use, but there was evidence to support the contention that regulation may reside at the level of the glucose-fatty acid cycle. This study confirms the hypothesis that plasma FFA availability regulates both intramuscular lipid and glycogen use in vivo in humans. muscle metabolism; intramyocellular triacylglycerol; fat oxidation; insulin resistance ELEVATED PLASMA FREE FATTY ACID (FFA) concentrations are associated with obesity and the development of insulin resistance (38,44). Combined with the fact that, in obese and/or type 2 diabetes patients, skeletal muscle shows a reduced capacity to oxidize FFA (20), this likely explains the increased intramyocellular triacylglycerol (IMTG) storage in these subjects (15,27,53). In agreement, various studies have reported a strong relationship between elevated plasma FFA levels, IMTG accretion, and insulin resistance (22,32,34). Insights from various lipid infusion studies suggest that elevated plasma FFA delivery and/or impaired fatty acid (FA) oxidation, result in intramyocellular accumulation of triacylglycerol (TG) and FA metabolites (such as fatty acyl-CoA, diacylglycerol, and ceramides), which are likely to induce defects in the insulinsignaling cascade, causing skeletal muscle insulin resistance (1,3,16,42,65).Insulin resistance can subsequently lead to the development of the hyperglycemic and hyperinsulinemic state that is associated with type 2 diabetes and accompanied by major disturbances in skeletal muscle substrate metabolism (17, 37). These disturbances produce a state of metabolic inflexibility that stimulates IMTG storage at the expense of its oxidation (20...

The AMP-activated protein kinase (AMPK) cascade is a sensor of cellular energy charge that promotes catabolic and inhibits anabolic pathways. However, the role of AMPK in adipocytes is poorly understood. We show that transgenic expression of mitochondrial uncoupling protein 1 in white fat, which induces obesity resistance in mice, is associated with depression of cellular energy charge, activation of AMPK, downregulation of adipogenic genes, and increase in lipid oxidation. Activation of AMPK may explain the complex metabolic changes in adipose tissue of these animals and our results support a role for adipocyte AMPK in the regulation of storage of body fat.

OBJECTIVE: An unexplained phenotype of mice overexpressing human UCP3 is their improved glucose homeostasis. Since overexpression of UCP3 might affect the energy charge of the cell, we investigated whether these mice have an increased AMPactivated protein kinase (AMPK) activity. METHODS: Mitochondrial localisation of UCP3 was determined by immunoelectronmicroscopy and AMPK activity was measured in medial gastrocnemius of control mice and mice overexpressing human UCP3. RESULTS: Mice overexpressing human UCP3 had 5.8 fold higher levels of UCP3 protein, for which mitochondrial localisation was confirmed by immunoelectronmicroscopy. The ATP/AMP ratio was significantly lower in mice over-expressing UCP3 compared to the wild-type (10.971.6 vs 20.471.9 AU, P ¼ 0.03). Over-expression of UCP3 resulted in increased AMPK a1 activity (1.2370.05 vs 1.0070.06 normalized values, P ¼ 0.004) and a tendency towards increased AMPK a2 activity (1.1870.08 vs 1.0070.10 normalized values, P ¼ 0.08). CONCLUSION: Increased AMPK activity provides a plausible explanation for the improved glucose tolerance characteristic for these mice.