Insulin resistance is a characteristic feature of type 2 diabetes and obesity. Insulin-resistant individuals manifest multiple disturbances in free fatty acid (FFA) metabolism and have excessive lipid accumulation in insulin target tissues. Although much evidence supports a causal role for altered FFA metabolism in the development of insulin resistance, i.e., "lipotoxicity", the intracellular mechanisms by which elevated plasma FFA levels cause insulin resistance have yet to be completely elucidated. Recent studies have implicated a possible role for mitochondrial dysfunction in the pathogenesis of insulin resistance in skeletal muscle. We examined the effect of FFA metabolites [palmitoyl carnitine (PC), palmitoyl-coenzyme A (CoA), and oleoyl-CoA] on ATP synthesis in mitochondria isolated from mouse and human skeletal muscle. At concentrations ranging from 0.5 to 2 microM, these FFA metabolites stimulated ATP synthesis; however, above 5 microM, there was a dose-response inhibition of ATP synthesis. Furthermore, 10 microM PC inhibits ATP synthesis from pyruvate. Elevated PC concentrations (> or =10 microM) inhibit electron transport chain activity and decrease the mitochondrial inner membrane potential. These acquired mitochondrial defects, caused by a physiological increase in the concentration of FFA metabolites, provide a mechanistic link between lipotoxicity, mitochondrial dysfunction, and muscle insulin resistance.

Aims/hypothesis The aim of this study was to measure mitochondrial reactive oxygen species (ROS) production directly from skeletal muscle biopsies obtained from obese insulin-resistant non-diabetic and type 2 diabetic participants. Methods Ten lean healthy, ten obese non-diabetic and ten type 2 diabetic participants received a euglycaemichyperinsulinaemic clamp to measure whole body insulin sensitivity. Mitochondria were isolated from skeletal muscle biopsies, and mitochondrial ATP synthesis and hydrogen peroxide production were measured ex vivo under conditions that maximally stimulate ATP synthesis and ROS production using chemiluminescent and fluorescent techniques, respectively. Results Compared with lean controls, both obese nondiabetic and type 2 diabetic participants were resistant to insulin, and had a reduced rate of mitochondrial ATP production. Obese insulin-resistant participants had a decreased rate of mitochondrial ROS production, while ROS production rate in participants with type 2 diabetes was similar to that in lean healthy participants. In nondiabetic participants, the rate of ROS production was strongly correlated with the rate of ATP synthesis and the glucose disposal rate measured with the euglycaemichyperinsulinaemic clamp. The ROS/ATP ratio in obese insulin-resistant participants was similar to that in lean insulin-sensitive participants, while the ratio was significantly elevated in type 2 diabetes participants.Conclusions/interpretation Since, in absolute terms, the maximal capacity for mitochondrial ROS production was not increased in either obese insulin-resistant participants or in type 2 diabetic participants, these results do not favour a role for increased mitochondrial ROS production in the pathogenesis of insulin resistance in human skeletal muscle. However, care should be taken in extrapolating these ex vivo observations to the in vivo situation.

CP, Coletta DK, Kaku K, DeFronzo RA. The relationship between fasting hyperglycemia and insulin secretion in subjects with normal or impaired glucose tolerance. Am J Physiol Endocrinol Metab 295: E401-E406, 2008. First published May 20, 2008 doi:10.1152/ajpendo.00674.2007.-To assess the relationship between the fasting plasma glucose (FPG) concentration and insulin secretion in normal glucose tolerance (NGT) and impaired glucose tolerance (IGT) subjects, 531 nondiabetic subjects with NGT (n ϭ 293) and IGT (n ϭ 238; 310 Japanese and 232 Mexican Americans) received an oral glucose tolerance test (OGTT) with measurement of plasma glucose, insulin, and C-peptide every 30 min. The insulin secretion rate was determined by plasma C-peptide deconvolution. Insulin sensitivity (Matsuda index) was measured from plasma insulin and glucose concentrations. The insulin secretion/insulin resistance (IS/IR) or disposition index was calculated as ⌬ISR/⌬G Ϭ IR. As FPG increased in NGT subjects, the IS/IR index declined exponentially over the range of FPG from 70 to 125 mg/dl. The relationship between the IS/IR index and FPG was best fit with the equation: 28.8 exp(Ϫ0.036 FPG). For every 28 mg/dl increase in FPG, the IS/IR index declined by 63%. A similar relationship between IS/IR index and FPG was observed in IGT. However, the decay constant was lower than in NGT. The IS/IR index for early-phase insulin secretion (0 -30 min) was correlated with the increase in FPG in both NGT and IGT (r ϭ Ϫ0.43, P Ͻ 0.0001 and r ϭ Ϫ0.20, P ϭ 0.001, respectively). However, the correlation between late-phase insulin secretion (60 -120 min) and FPG was not significant. In conclusion, small increments in FPG, within the "normal" range, are associated with a marked decline in glucose-stimulated insulin secretion and the decrease in insulin secretion with increasing FPG is greater in subjects with NGT than IGT and primarily is due to a decline in early-phase insulin secretion.fasting plasma glucose; ␤-cell function IMPAIRED INSULIN SECRETION plays a pivotal role in the progression from normal glucose tolerance (NGT) to impaired glucose tolerance (IGT) and to overt Type 2 diabetes mellitus (T2DM) (1-5). The impairment in ␤-cell function begins long before the onset of overt T2DM, and it is evident in the prediabetic stage, e.g., IGT and impaired fasting glucose (IFG; Refs. 2-5). Previous studies (6 -15) demonstrated that IGT is associated with a decrease in ␤-cell function. Importantly, the decline in ␤-cell function becomes evident with 2-h plasma glucose concentrations that are well within the range that is considered to represent NGT, i.e., 2-h plasma glucose (PG) ϭ 100 -139 mg/dl (13).The mechanisms that regulate the plasma glucose concentration during the postabsorptive state, e.g., fasting plasma glucose (FPG), are very different from those that regulate the plasma glucose concentration after a meal (2). Thus the FPG is primarily determined by the rate of hepatic glucose production (16), while 2-h PG after a meal is determined by the rate of gluc...

Impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) are two intermediate states in the transition from normal glucose metabolism to type 2 diabetes. Insulin clamp studies have shown that subjects with IGT have increased insulin resistance in skeletal muscle, while subjects with IFG have near normal muscle insulin sensitivity. Because of the central role of altered free fatty acid (FFA) metabolism in the pathogenesis of insulin resistance, we have examined plasma free fatty acid concentration under fasting conditions, and during OGTT in subjects with IGT and IFG. Seventy-one NGT, 70 IGT and 46 IFG subjects were studied. Fasting plasma FFA in IGT subjects was significantly greater than NGT, while subjects with IFG had similar fasting plasma FFA concentration to NGT. However, fasting plasma insulin concentration was significantly increased in IFG subjects compared to NGT while subjects with IGT had near normal fasting plasma insulin levels. The adipocyte insulin resistance index (product of fasting plasma FFA and FPI) was significantly increased in both IFG and IGT subjects compared to NGT. During the OGTT both IFG and IGT subjects suppressed their plasma FFA concentration similarly to NGT subjects, but the post-glucose loads were significantly increased in both IFG and IGT subjects. These data suggest that both subjects with IFG and IGT have increased resistance to the antilipolytic action of insulin. However, under basal conditions, fasting hyperinsulinemia in IFG subjects is sufficient to offset the adipocyte insulin resistance and maintain normal fasting plasma FFA concentration while the lack of increase in FPI in IGT subjects results in an elevated fasting plasma FFA.

Short-term physiological increase in plasma FFA concentration in lean normal glucose tolerant subjects induces insulin resistance and impairs mitochondrial membrane potential but has no significant effects on mitochondrial content, gene expression, ATP content, or citrate synthase activity.