Metformin and thiazolidinediones (TZDs) are believed to exert their antidiabetic effects via different mechanisms. As evidence suggests that both impair cell respiration in vitro, this study compared their effects on mitochondrial functions. The activity of complex I of the respiratory chain, which is known to be affected by metformin, was measured in tissue homogenates that contained disrupted mitochondria. In homogenates of skeletal muscle, metformin and TZDs reduced the activity of complex I (30 mmol/l metformin, ؊15 ؎ 2%; 100 mol/l rosiglitazone, ؊54 ؎ 7; and 100 mol/l pioglitazone, ؊12 ؎ 4; P < 0.05 each). Inhibition of complex I was confirmed by reduced state 3 respiration of isolated mitochondria consuming glutamate ؉ malate as substrates for complex I (30 mmol/l metformin, ؊77 ؎ 1%; 100 mol/l rosiglitazone, ؊24 ؎ 4; and 100 mol/l pioglitazone, ؊18 ؎ 5; P < 0.05 each), whereas respiration with succinate feeding into complex II was unaffected. In line with inhibition of complex I, 24-h exposure of isolated rat soleus muscle to metformin or TZDs reduced cell respiration and increased anaerobic glycolysis (glucose oxidation: 270 mol/l metformin, ؊30 ؎ 9%; 9 mol/l rosiglitazone, ؊25 ؎ 8; and 9 mol/l pioglitazone, ؊45 ؎ 3; lactate release: 270 mol/l metformin, ؉84 ؎ 12; 9 mol/l rosiglitazone, ؉38 ؎ 6; and 9 mol/l pioglitazone, ؉64 ؎ 11; P < 0.05 each). As both metformin and TZDs inhibit complex I activity and cell respiration in vitro, similar mitochondrial actions could contribute to their antidiabetic effects. Diabetes

The nutrient-sensitive kinase mammalian target of rapamycin (mTOR) and its downstream target S6 kinase (S6K) are involved in amino acid-induced insulin resistance. Whether the mTOR/S6K pathway directly modulates glucose metabolism in humans is unknown. We studied 11 healthy men (29 years old, BMI 23 kg/m 2 ) twice in random order after oral administration of 6 mg rapamycin, a specific mTOR inhibitor, or placebo. An amino acid mixture was infused to activate mTOR, and somatostatin-insulin-glucose clamps created conditions of low peripheral hyperinsulinemia (ϳ100 pmol/l, 0 -180 min) and prandial-like peripheral hyperinsulinemia (ϳ450 pmol/l, 180 -360 min). Glucose turnover was assessed using D-[6,6-2 H 2 ]glucose infusion (n ‫؍‬ 8). Skeletal muscle biopsies were performed at baseline and during prandial-like peripheral hyperinsulinemia (n ‫؍‬ 3). At low peripheral hyperinsulinemia, whole-body glucose uptake was not affected by rapamycin. During prandial-like peripheral hyperinsulinemia, rapamycin increased glucose uptake compared with placebo by 17% (R dԽ300 -360 min , 75 ؎ 5 vs. 64 ؎ 5 mol ⅐ kg ؊1 ⅐ min ؊1 , P ‫؍‬ 0.0008). Rapamycin affected endogenous glucose production neither at baseline nor during low or prandial-like peripheral hyperinsulinemia. Combined hyperaminoacidemia and prandial-like hyperinsulinemia increased S6K phosphorylation and inhibitory insulin receptor substrate-1 (IRS-1) phosphorylation at Ser312 and Ser636 in the placebo group. Rapamycin partially inhibited this increase in mTOR-mediated S6K phosphorylation and IRS-1 Ser312 and Ser636 phosphorylation. In conclusion, rapamycin stimulates insulin-mediated glucose uptake in man under conditions known to activate the mTOR/S6K pathway. Diabetes 56:1600-1607, 2007 T ype 2 diabetes is closely linked to obesity and insulin resistance (1-3). In addition to polygenic predisposition, environmental factors including quality and quantity of food supply, dietary behavior, and physical activity are of major importance for the development of type 2 diabetes (4,5). The availability of nutrients plays a pivotal role in the modulation of insulin action (6,7). In industrialized countries, nutrient excess comprises high intake of not only fat but also proteins (8). A chronic excess in protein intake is associated with insulin resistance, glucose intolerance, and type 2 diabetes (9 -12). We have shown that a short-term rise in plasma free fatty acids (FFAs) (13-15) or amino acids (16 -18) leads to decreased insulin-stimulated whole-body glucose disposal, which is preceded by an impaired rise in intramuscular glucose-6-phosphate concentrations and followed by reduction in rates of glycogen synthesis. These findings indicate that both FFAs and amino acids directly inhibit skeletal muscle glucose transport/phosphorylation (17).The mammalian target of rapamycin (mTOR) pathway (19) could be involved in sensing of nutrient availability and modulation of insulin action in vivo. Recent reports indicate that the activity of the mTOR pathway is increased in rodent models ...

The initial effects of free fatty acids (FFAs) on glucose transport/phosphorylation were studied in seven healthy men in the presence of elevated (1.44 +/- 0.16 mmol/l), basal (0.35 +/- 0.06 mmol/l), and low (<0.01 mmol/l; control) plasma FFA concentrations (P < 0.05 between all groups) during euglycemic-hyperinsulinemic clamps. Concentrations of glucose-6-phosphate (G-6-P), inorganic phosphate (Pi), phosphocreatine, ADP, and pH in calf muscle were measured every 3.2 min for 180 min by using 31P nuclear magnetic resonance spectroscopy. Rates of whole-body glucose uptake increased similarly until 140 min but thereafter declined by approximately 20% in the presence of basal and high FFAs (42.8 +/- 3.6 and 41.6 +/- 3.3 vs. control: 52.7 +/- 3.3 micromol x kg(-1) x min(-1), P < 0.05). The rise of intramuscular G-6-P concentrations was already blunted at 45 min of high FFA exposure (184 +/- 17 vs. control: 238 +/- 17 micromol/l, P = 0.008). At 180 min, G-6-P was lower in the presence of both high and basal FFAs (197 +/- 21 and 213 +/- 18 vs. control: 286 +/- 19 micromol/l, P < 0.05). Intramuscular pH decreased by -0.013 +/- 0.001 (P < 0.005) during control but increased by +0.008 +/- 0.002 (P < 0.05) during high FFA exposure, while Pi rose by approximately 0.39 mmol/l (P < 0.005) within 70 min and then slowly decreased in all studies. In conclusion, the lack of an initial peak and the early decline of muscle G-6-P concentrations suggest that even at physiological concentrations, FFAs primarily inhibit glucose transport/phosphorylation, preceding the reduction of whole-body glucose disposal by up to 120 min in humans.

Thiazolidinediones (TZDs) are believed to induce insulin sensitization by modulating gene expression via agonistic stimulation of the nuclear peroxisome proliferator-activated receptor-␥ (PPAR-␥). We have shown earlier that the TZD troglitazone inhibits mitochondrial fuel oxidation in isolated rat skeletal muscle. In the present study, rat soleus muscle strips were exposed to TZDs to examine whether the inhibition of fuel oxidation is mediated by PPAR-␥ activation. Our findings consistently indicated direct, acute, and PPAR-␥؊inde-pendent TZD action on skeletal muscle fuel metabolism.

Increased endogenous glucose production (EGP) could contribute to fasting hyperglycaemia in Type II (non-insulin-dependent) diabetes mellitus and could result from increased rates of glycogenolysis or gluconeogenesis (GNG) AbstractAims/hypothesis. Non-esterified fatty acids and glycerol could stimulate gluconeogenesis and also contribute to regulating hepatic glycogen stores. We examined their effect on liver glycogen breakdown in humans.Methods. After an overnight fast healthy subjects participated in three protocols with lipid/heparin (plasma non-esterified fatty acids: 2.2 0.1 mol/l; plasma glycerol: 0.5 0.03 mol/l; n = 7), glycerol (0.4 0.1 mol/l; 1.5 0.2 mol/l; n = 5) and saline infusion (control; 0.5 0.1 mol/l; 0.2 0.02 mol/l; n = 7). Net rates of glycogen breakdown were calculated from the decrease of liver glycogen within 9 h using ), respectively. Conclusion/interpretation: An increase of non-esterified fatty acid leads to a pronounced inhibition of net hepatic glycogen breakdown and increases gluconeogenesis whereas glucose production does not differ from the control condition. We suggest that this effect is not due to increased availability of glycerol alone but rather to lipid-dependent control of hepatic glycogen stores. [Diabetologia (2001) 44: 48±54]