Lactate induces insulin resistance in skeletal muscle by suppressing glycolysis and impairing insulin signaling

Endurance training increases lactate transport in male Zucker fa/fa rats

The purpose of this study was to investigate the effect of endurance training (10 weeks) on previously reported alterations of lactate exchange in obese Zucker fa/fa rats. We used sarcolemmal vesicles to measure lactate transport capacity in control sedentary rats, Zucker (fa/fa), and endurance trained Zucker (fa/fa) rats. Monocarboxylate transporter (MCT) 1 and 4 content was measured in sarcolemmal vesicles and skeletal muscle. Training increased citrate synthase activity in soleus and in red tibialis anterior, and improved insulin sensitivity measured by intraperitoneal glucose tolerance test. Endurance training increased lactate influx in sarcolemmal vesicles at 1 mM of external lactate concentration and increased MCT1 expression on sarcolemmal vesicles. Furthermore, muscular lactate level was significantly decreased after training in red tibialis anterior and extensor digitorum longus. This study shows that endurance training improves impairment of lactate transport capacity that is found in insulin resistance state like obesity and type 2 diabetes.

Effects of oxidative stress on adiponectin secretion and lactate production in 3T3-L1 adipocytes

Obesity is an increasing nutritional disorder in developed countries, and oxidative stress has been identified as a key factor in numerous pathologies such as diabetes, inflammation, and atherosclerosis, which are favored by obesity. The objective of the present study was to investigate the effects of oxidative stress in 3T3-L1 adipose cells on two parameters involved in metabolic complications associated with obesity, namely adiponectin secretion and lactate production. Differentiated 3T3-L1 adipose cells were exposed to increasing concentrations of glucose oxidase. 4-Hydroxynonenal (4-HNE), a relevant lipid peroxidation by-product which may affect several metabolic processes in making covalent adducts with various molecules; adiponectin secretion; and lactate production were measured in response to glucose oxidase exposure. Results show an inhibition of adiponectin mRNA expression by glucose oxidase and a significant inverse correlation between 4-HNE formation and adiponectin secretion. Furthermore, 4-HNE alone inhibits adiponectin production by 3T3-L1. On the other hand, glucose oxidase and 4-HNE significantly stimulated lactate production by 3T3-L1 adipocytes. These results demonstrate that adipose cells are highly sensitive to oxidative stress, with subsequent decreased adiponectin secretion and increased lactate production, two events involved in the development of insulin resistance.

Effect of thiopental, pentobarbital and diethyl ether on early steps of insulin action in liver and muscle of the intact rat

A large number of experimental studies have investigated insulin signaling in rats. In these studies different anaesthetics have been used to anaesthetize rats. However, the direct effects of anaesthetics on the regulation of the early steps of insulin action are not known. In the present study, we investigated the effect of thiopental, pentobarbital and diethyl ether on the plasma glucose disappearance rate, IR, IRS-1 and IRS-2 tyrosine phosphorylation, IRSs association with PI 3-kinase, Akt and Erk phosphorylation, in liver and muscle of rats. Fasting plasma glucose levels were higher in animals anaesthetized with ether. No differences in plasma glucose disappearance rates were observed, however. Insulin-induced IR, IRS-1 and IRS-2 tyrosine phosphorylation, association of these substrates with PI 3-kinase and Akt and ERK phosphorylation were similar in the three groups of animals in both tissues. These data suggest that both thiopental and pentobarbital may be used in studies where changes in insulin signaling are being measured and where adequate general anaesthesia is required.

Insulin suppresses PDK-4 expression in skeletal muscle independently of plasma FFA

Starvation and experimental diabetes induce a stable increase in pyruvate dehydrogenase kinase (PDK) activity in skeletal muscle, which is largely due to a selective upregulation of PDK-4 expression. Increased free fatty acid (FFA) level has been suggested to be responsible for the upregulation. Because these metabolic states are also characterized by insulin deficiency, the present study was designed to examine whether insulin has a significant effect to regulate PDK mRNA expression in rat skeletal muscle. In study 1, overnight-fasted rats received an infusion of saline or insulin for 5 h (n = 6 each). During the insulin infusion, plasma glucose was clamped at basal levels (euglycemic hyperinsulinemic clamp). A third group (n = 6) received Intralipid infusion during the clamp to prevent a fall in plasma FFA. PDK-2 mRNA level in gastrocnemius muscle was not altered by insulin or FFA (i.e., Intralipid infusion). In contrast, PDK-4 mRNA level was decreased 72% by insulin (P < 0.05), and Intralipid infusion prevented only 20% of the decrease. PDK-4 protein level was decreased approximately 20% by insulin (P < 0.05), but this effect was not altered by Intralipid infusion. In study 2, overnight-fasted rats were refed or received an infusion of saline or nicotinic acid (NA, 30 micromol/h) for 5 h (n = 5 each). During the refeeding and NA infusion, plasma FFA levels were similarly (i.e., 60-70% vs. saline control) lowered. Muscle PDK-4 mRNA level decreased 77% after the refeeding (P < 0.05) but not after the NA infusion. In conclusion, the present data indicate that insulin had a profound effect to suppress PDK-4 expression in skeletal muscle and that, contrary to previous suggestions, circulating FFA had little impact on PDK-4 mRNA expression, at least within 5 h.