Carbohydrate metabolism and lipid storage and breakdown in diabetes

1H-NMR Metabolomics Analysis of the Effect of Rubusoside on Serum Metabolites of Golden Hamsters on a High-Fat Diet

Rubusoside is a natural sweetener and the active component of Rubus suavissimus. The preventive and therapeutic effect of rubusoside on high-fat diet-induced (HFD) serum metabolite changes in golden hamsters was analyzed by ¹H-NMR metabolomics to explore the underlying mechanism of lipid metabolism regulation. ¹H-NMR serum metabolomics analyses revealed a disturbed amino acid-, sugar-, fat-, and energy metabolism in HFD animals. Animals supplemented with rubusoside can partly reverse the metabolism disorders induced by high-fat diet and exerted good anti-hypertriglyceridemia effect by intervening in some major metabolic pathways, involving amino acid metabolism, synthesis of ketone bodies, as well as choline and 4-hydroxyphenylacetate metabolism. This study indicates that rubusoside can interfere with and normalize high-fat diet-induced metabolic changes in serum and could provide a theoretical basis to establish rubusoside as a potentially therapeutic tool able to revert or prevent lipid metabolism disorders.

Surplus fat rapidly increases fat oxidation and insulin resistance in lipodystrophic mice

Objective

Surplus dietary fat cannot be converted into other macronutrient forms or excreted, so has to be stored or oxidized. Healthy mammals store excess energy in the form of triacylgycerol (TAG) in lipid droplets within adipocytes rather than oxidizing it, and thus ultimately gain weight. The ‘overflow hypothesis’ posits that the capacity to increase the size and number of adipocytes is finite and that when this limit is exceeded, fat accumulates in ectopic sites and leads to metabolic disease.

Methods

Here we studied the energetic and biochemical consequences of short-term (2-day) excess fat ingestion in a lipodystrophic (A-ZIP/F-1) mouse model in which adipose capacity is severely restricted.

Results

In wildtype littermates, this acute exposure to high fat diets resulted in excess energy intake and weight gain without any significant changes in macronutrient oxidation rates, glucose, TAG, or insulin levels. In contrast, hyperphagic lipodystrophic mice failed to gain weight; rather, they significantly increased hepatic steatosis and fat oxidation. This response was associated with a significant increase in hyperglycemia, hyperinsulinemia, glucosuria, hypertriglyceridemia, and worsening insulin tolerance.

Conclusions

These data suggest that when adipose storage reserves are saturated, excess fat intake necessarily increases fat oxidation and induces oxidative substrate competition which exacerbates insulin resistance resolving any residual energy surplus through excretion of glucose.

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In contrast to wild type mice, lipodystrophic Azip mice manifest striking metabolic inflexibility.

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Wild type mice respond to excess dietary fat by storing the surplus energy and thus gain weight.

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In contrast, Azip mice increase fat oxidation and energy expenditure in response to surplus energy intake.

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Unfortunately this apparently ‘helpful’ adaptation also results in substrate competition and worsening insulin resistance.

Future of muscle research in diabetes: a look into the crystal ball

In type 2 diabetes, skeletal muscle is not only responsible for early metabolic abnormalities, but its contractile activity also offers an efficient prevention and treatment strategy. This outlook into the coming decades summarises challenges and opportunities for translational research on skeletal muscle in diabetes and related diseases. Currently, our understanding of the interactions between myocellular networks, the master regulators of resting metabolism, and muscle's position within multi-organ crosstalk, is incomplete. In the face of an ageing population, changes within muscle tissue appear to be the predominant mechanisms responsible for sarcopenia, but the relative roles of obesity and ageing as driving forces of its development are less clear. To address these research questions, innovative approaches to optimising exercise training or minimising sedentarism will need to be devised and tested in large-scale standardised prospective studies. Finally, another major challenge will be the identification and evaluation of muscle targets to prevent and treat metabolic diseases. This is one of a series of commentaries under the banner '50 years forward', giving personal opinions on future perspectives in diabetes, to celebrate the 50th anniversary of Diabetologia (1965-2015).

Effect of the interaction between the fatty acid binding protein 2 gene Ala54Thr polymorphism and dietary fatty acids on peripheral insulin sensitivity: a cross-sectional study

BACKGROUND

The intestinal fatty acid-binding protein (FABP2) is involved in the intracellular transport and metabolism of fatty acids and may affect insulin sensitivity and glucose metabolism.

OBJECTIVE

The objective was to study the effect of interaction between the Ala54Thr polymorphism of the FABP2 gene (FABP2) and the type of dietary cooking oil used on peripheral insulin sensitivity in a population from southern Spain.

DESIGN

The study was cross-sectional. Anthropometric measurements were obtained for 1226 persons aged 18-65 y selected randomly from the municipal census of Pizarra, Spain. An oral-glucose-tolerance test was given to 1020 of these persons. Insulin resistance was measured by homeostasis model assessment. Samples of the cooking oil being used were taken from the kitchens of a random subset of 538 persons.

RESULTS

Persons who consumed sunflower oil and who also had the Thr54 variant had higher insulin resistance than did those who consumed olive oil (P = 0.01). We detected an interaction between the Ala54Thr polymorphism and the type of oil consumed that accounted for the variance in insulin resistance (P = 0.02).

CONCLUSIONS

The effect of dietary fatty acids on the populational pattern of insulin resistance is not independent of the Ala54Thr polymorphism of FABP2. An interaction existed between this polymorphism and the intake of dietary fats in a population with a high intake of monounsaturated fatty acids.

Insulin resistance and changes in plasma concentration of TNFalpha, IGF1, and NEFA in dogs during weight gain and obesity

Obesity-induced insulin resistance (IR) is a common problem in humans as well as domestic dogs. It is well-known that this syndrome is associated with many modifications but it is still unclear if the changes are alterations or adaptations. The purpose of this study was to develop obesity-induced IR in dogs, through a long-term overfeeding period, and to explore hormonal and metabolic disturbances associated with the development of this syndrome. Dogs were overfed for 7 months. Body weight increased by 43 +/- 5%, and insulin sensitivity decreased by 44 +/- 5%. Plasma insulin-like growth factor 1 (IGF1), tumour necrosis factor alpha (TNFalpha), and non-esterified fatty acids (NEFA) concentrations progressively increased during the overfeeding period (IGF1: 111 +/- 13 to 266 +/- 32 ng/ml, p < 0.001; TNFalpha: 5 +/- 5 to 134 +/- 41 pg/ml; NEFA: 0.974 +/- 0.094 to 1.590 +/- 0.127 mmol/l, p < 0.05). These metabolic and hormonal impairments are associated with IR, in obese dogs, and could explain, at least in part, the outbreak of this syndrome.

Oxidative stress and stress-activated signaling pathways: a unifying hypothesis of type 2 diabetes

In both type 1 and type 2 diabetes, the late diabetic complications in nerve, vascular endothelium, and kidney arise from chronic elevations of glucose and possibly other metabolites including free fatty acids (FFA). Recent evidence suggests that common stress-activated signaling pathways such as nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases underlie the development of these late diabetic complications. In addition, in type 2 diabetes, there is evidence that the activation of these same stress pathways by glucose and possibly FFA leads to both insulin resistance and impaired insulin secretion. Thus, we propose a unifying hypothesis whereby hyperglycemia and FFA-induced activation of the nuclear factor-kappaB, p38 MAPK, and NH2-terminal Jun kinases/stress-activated protein kinases stress pathways, along with the activation of the advanced glycosylation end-products/receptor for advanced glycosylation end-products, protein kinase C, and sorbitol stress pathways, plays a key role in causing late complications in type 1 and type 2 diabetes, along with insulin resistance and impaired insulin secretion in type 2 diabetes. Studies with antioxidants such as vitamin E, alpha-lipoic acid, and N-acetylcysteine suggest that new strategies may become available to treat these conditions.

Fatty acid binding protein 2 and insulin resistance

BACKGROUND

An A54T polymorphism of the fatty acid binding protein 2 (FABP2) gene was found to be associated with insulin resistance in nondiabetic Pima Indians. Design This is a cross-sectional study to examine the role of this polymorphism in insulin resistance in 71 healthy and normotensive Caucasian subjects with normal glucose tolerance. Insulin sensitivity (%S, ISI(M), ISI(S)) and beta-cell function (%B, dI/dG, 1stPHS, 2ndPHS) were estimated based on published models. Their genotypes were determined using a polymerase chain reaction-restriction fragment length polymorphism assay. The relationship between genotypes and phenotypes was examined.

RESULTS

After genotyping, we identified 34 AA, 32 AT and five TT subjects. The TT subjects were pooled together with the AT subjects during the analysis due to their low number. No difference was noted in gender distribution, clinical features, or fasting lipid profile between the two genotypic groups (AA vs. AT/TT). The AT/TT group had lower %S and ISI(S) than the AA group (P = 0.0118 and P = 0.0170, respectively). The difference in ISI(M) was marginal (P = 0.0544). However, no difference was noted in beta-cell function between the two groups. Multivariate analysis revealed that this polymorphism was an independent but modest determinant for %S (P = 0.0149), ISI(M) (P = 0.0489) and ISI(S) (P = 0.0175). It independently contributed 6.04% (95% CI, 0.02-20.53%), 4.28% (95% CI, 0.08-17.63%) and 4.94% (95% CI, 0.01-18.75%) of the variation of %S, ISI(M) and ISI(S), respectively.

CONCLUSIONS

We demonstrated that the A54T polymorphism at the FABP2 locus is a risk factor for insulin resistance in a Caucasian population.

The A54T polymorphism at the intestinal fatty acid binding protein 2 is associated with insulin resistance in glucose tolerant Caucasians

BACKGROUND

An A54T polymorphism at the fatty acid binding protein 2 (FABP2) locus was found to be associated with insulin resistance in non-diabetic Pima Indians. To see whether this association is present in other populations, we performed a cross sectional study to examine the role of this polymorphism on insulin resistance in 55 healthy and normotensive Caucasian subjects with normal glucose tolerance. Insulin sensitivity (%S) and beta cell function (%B) were assessed using the Homeostasis Model Assessment (HOMA). Their genotypes were determined using a polymerase chain reaction-restriction fragment length polymorphism assay. The relationship between the genotypes and the phenotypes was examined.

RESULTS

After genotyping, we identified 24 AA, 27 AT and 4 TT subjects. The TT subjects were combined with the AT subjects during the analysis due to its small sample size. No differences were noted in gender distribution, clinical features, and fasting lipid profile between the two genotypic groups (AA vs. AT/TT). The AT/TT group had a higher fasting plasma insulin concentration and a lower %S than the AA group (p = 0.0444 and p = 0.0461, respectively). However, no differences were noted in plasma glucose concentrations and %B. Univariate analysis revealed that this polymorphism explained 7.3% of the variation in %S. Multivariate analysis revealed that the polymorphism was an independent determinant for %S (p = 0.0434) and with body mass index accounted for 28.7% of the variation in %S. In contrast, this polymorphism had no impact on %B.

CONCLUSIONS

The A54T polymorphism at the FABP2 locus is a risk factor for insulin resistance in a Caucasian population.

Amelioration of insulin resistance and hypertension in a fructose-fed rat model with fish oil supplementation

In type II diabetic patients, one can detect several pathologic changes including insulin resistance and hypertension. Sprague-Dawley rats fed a fructose-rich diet (group F) exhibited these characteristic abnormalities within 2 weeks and were an excellent laboratory animal model for research on insulin action and development of hypertension. Since fish oils containing omega-3 fatty acids have a beneficial effect in preventing atherosclerotic diseases, we performed repeated experiments to test the effects of fish oil supplementation in group F rats. Compared with control rats on a normal diet (group C), group F consistently developed hypertriglyceridemia without elevated plasma free fatty acid (FFA), fasting hyperinsulinemia together with fasting hyperglycemia (insulin resistance syndrome), and systolic hypertension within 3 weeks. Insulin-stimulated glucose uptake and insulin binding of adipocytes were significantly reduced. Rats fed the same high-fructose diet but supplemented with fish oil (group O) had alleviation of all of these metabolic defects and a normalized insulin sensitivity and blood pressure. beta-Cell function as shown by plasma glucose and insulin responses to oral glucose remained intact in group F and group O. The plasma endothelin-1 (ET-1) level and ET-1 binding to adipocytes were not different among the three groups. Based on these results, we suggest that dietary high fructose induced hypertriglyceridemia and insulin resistance with normal islet function, and that the induced hypertension was not associated with plasma ET-1 abnormalities and was probably caused by other undefined pathologic changes that can be prevented by dietary omega-3 fatty acids.

Mechanisms modifying glucose oxidation in diabetes mellitus

The Glucose Fatty Acid Cycle as formulated 30 years ago and reviewed in the Minkowski lecture in 1966 described short term effects of fatty acids (minutes) to decrease uptake, glycolysis and oxidation of glucose in heart and skeletal muscles. Such short term effects have since been extended to include inhibition of glucose uptake and glycolysis and stimulation of gluconeogenesis in liver and these effects have also been convincingly demonstrated in man in vivo. More recently a longer term effect of fatty acid metabolism to decrease glucose oxidation (hours) has been shown in heart and skeletal muscle and liver. This effect increases the specific activity of pyruvate dehydrogenase kinase, which in turn results in enhanced phosphorylation and inactivation of the pyruvate dehydrogenase complex. Activity of the pyruvate dehydrogenase complex is the major determinant of glucose oxidation rate. It seems likely that longer term effects of fatty acids on this and other aspects of glucose metabolism could be important in the development of insulin resistance in diabetes mellitus in man.

Regeneration of skeletal muscle in streptozotocin-induced diabetic rats

The present study analyzes the regeneration of skeletal muscle in diabetic rats. Intravenous injection of streptozotocin (STZ) was used to induce diabetes. Six weeks later the extensor digitorum longus (EDL) muscles from diabetic rats were either transplanted into diabetic or normal hosts to initiate regeneration. Normal EDL muscle transplants in normal and diabetic hosts were also performed for comparison. One, 2, 4, and 12 weeks after transplantation, the EDL regenerates were morphologically analyzed. Regeneration and formation of neuromuscular junctions were observed in all transplants, including diabetic regenerates in diabetic hosts. The overall mass and myofiber size of the diabetic EDL regenerate in the diabetic host was significantly reduced in spite of complete regeneration. Recovery of the diabetic muscle mass and the myofiber size was observed after transplantation into normal hosts. A reduction in mass and myofiber size was observed in normal EDL muscles transplanted into diabetic hosts. It is concluded that poor recovery of diabetic muscle is related to metabolic and structural alterations in the diabetic host, rather than to innate capacity of the muscle to per se undergo regeneration and reinnervation. The observed enhancement in recovery of diabetic muscle after transplantation in a normal host and deterioration of normal muscle after transplantation in a diabetic host shows that the host environment determines the success of muscle regeneration.

The pharmacology of dichloroacetate

Dichloroacetate (DCA) exerts multiple effects on pathways of intermediary metabolism. It stimulates peripheral glucose utilization and inhibits gluconeogeneis, thereby reducing hyperglycemia in animals and humans with diabetes mellitus. It inhibits lipogenesis and cholesterolgenesis, thereby decreasing circulating lipid and lipoprotein levels in short-term studies in patients with acquired or hereditary disorders of lipoprotein metabolism. By stimulating the activity of pyruvate dehydrogenase, DCA facilitates oxidation of lactate and decreases morbidity in acquired and congenital forms of lactic acidosis. The drug improves cardiac output and left ventricular mechanical efficiency under conditions of myocardial ischemia or failure, probably by facilitating myocardial metabolism of carbohydrate and lactate as opposed to fat. DCA may also enhance regional lactate removal and restoration of brain function in experimental states of cerebral ischemia. DCA appears to inhibit its own metabolism, which may influence the duration of its pharmacologic actions and lead to toxicity. DCA can cause a reversible peripheral neuropathy that may be related to thiamine deficiency and may be ameliorated or prevented with thiamine supplementation. Other toxic effects of DCA may be species-specific and reflect marked interspecies variation in pharmacokinetics. Despite its potential toxicity and limited clinical experience, DCA and its derivatives may prove to be useful in probing regulatory aspects of intermediary metabolism and in the acute or chronic treatment of several metabolic disorders.

Cytoarchitecture of muscle in a genetic model of murine diabetes

Although diabetic neuropathy is well documented, diabetic myopathy is not, except for descriptions of diabetic patients with muscular weakness thought to be due to metabolic changes in the muscle. Muscle and nerve are dependent on each other for normal structure and function; since the peripheral nerve is damaged in diabetes, one would expect concomitant changes in the muscle. This study examines the cytoarchitecture of diabetic muscle. The extensor digitorum longus (EDL) muscles from 165-day-old C57BL/KsJ dbm mice were examined using electron microscopy. Morphological analysis of the diabetic EDL revealed that a significant number of the myofibers, examined within the midbelly region of the muscle, exhibited various degrees of degeneration, signs of denervation, and abnormal lipid stores. Both myoneural junctions and muscle spindles showed significant signs of degeneration, denervation, and abnormal structure. Thus the morphologic changes seen could account for the physiologic changes seen in diabetic muscle.

Effects of nutritional status and acute variation in substrate supply on cardiac and skeletal-muscle fructose 2,6-bisphosphate concentrations

We examined the long-term effects of nutritional status and the acute effects of changes in exogenous carbohydrate- and lipid-substrate supply and utilization on fructose 2,6-bisphosphate (Fru-2,6-P2) concentrations in heart, gastrocnemius and soleus. Starvation decreased Fru-2,6-P2 concentrations in all three muscles. The acute administration of insulin and glucose increased skeletal-muscle Fru-2,6-P2 in the fed, but not in the starved, state, but cardiac Fru-2,6-P2 was unchanged. Cardiac and skeletal-muscle Fru-2,6-P2 concentrations were unaffected by acute increases in fatty acid supply produced by the administration of corn oil plus heparin, or by acute decreases in fatty acid supply produced by inhibition of lipolysis. Differences in cardiac and skeletal-muscle Fru-2,6-P2 concentrations observed in response to starvation were not reversed by administration of glucose or glucose plus insulin, or by inhibition of lipolysis, even though changes in citrate (heart), acylcarnitine (heart) and glycogen (skeletal muscle) were observed. Concentrations remained low for at least 8 h after chow re-feeding, but the fed value was restored by 24 h.

Histochemical evidence of changes in fuel metabolism induced in red, white and intermediate muscle fibres of streptozotocin-treated rats

The present study provides histochemical evidence supporting the operation of the 'glucose-fatty acid cycle' in skeletal muscles taken 5 days after the administration of a single injection of streptozotocin. It also indicates that the cycle is more important in fast-oxidative-glycolytic (FOG) and slow-oxidative (SO) fibres than in fast-glycolytic (FG) fibres. Data from muscles taken 14 and 28 days after treatment suggest that lipid catabolism becomes progressively less important with time, and that muscles from longer-term diabetic rats rely on the aerobic and anaerobic breakdown of glucose by FOG and FG fibres to meet their cellular energy requirements. Although SO fibres appeared initially to be the least affected by streptozotocin-induced diabetes, the decline in their metabolic capabilities ultimately seemed to be greater than that in FOG fibres. Transformations in the biochemical characteristics of FOG and SO fibres occurred 14-28 days after streptozotocin treatment, in the absence of changes in actomyosin-ATPase activity. This supports the view that the division of skeletal muscle fibres into three or four distinct types on the basis of myosin- or actomyosin-ATPase activity is an oversimplification of the true situation.

Effects of starvation and exercise on concentrations of citrate, hexose phosphates and glycogen in skeletal muscle and heart. Evidence for selective operation of the glucose-fatty acid cycle

Concentrations of citrate, hexose phosphates and glycogen were measured in skeletal muscle and heart under conditions in which plasma non-esterified fatty acids and ketone bodies were physiologically increased. The aim was to determine under what conditions the glucose-fatty acid cycle might operative in skeletal muscle in vivo. In keeping with the findings of others, starvation increased the concentrations of glycogen, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in heart, indicating that the cycle was operative. In contrast, it decreased glycogen and had no effect on the concentration of citrate or the fructose 6-phosphate/fructose 1,6-bisphosphate ratio in the soleus, a slow-twitch red muscle in which the glucose-fatty acid cycle has been demonstrated in vitro. In fed rats, exercise of moderate intensity caused glycogen depletion in the soleus and red portion of gastrocnemius muscle, but not in heart. In starved rats the same exercise had no effect on the already diminished glycogen contents in skeletal muscle, but it decreased cardiac glycogen by 25-30%. After exercise, citrate and the fructose 6-phosphate/fructose 1,6-bisphosphate ratio were increased in the soleus of the starved rat. Significant changes were not observed in fed rats. The data suggest that in the resting state the glucose-fatty acid cycle operates in the heart, but not in the soleus muscle, of a starved rat. In contrast, the metabolite profile in the soleus was consistent with activation of the glucose-fatty acid cycle in the starved rat during the recovery period after exercise. Whether the cycle operates during exercise itself is unclear.

alpha-Ketoacid dehydrogenase complexes and respiratory fuel utilisation in diabetes

Activity of the pyruvate dehydrogenase complex determines the rate of glucose oxidation in animals including man. The complex is regulated by reversible phosphorylation, phosphorylation resulting in inactivation. Activity is therefore dependent upon the activities of pyruvate dehydrogenase kinase and phosphatase. Activity of the complex is reduced in diabetes and starvation as a result of insulin deficiency. The mechanism involves activation of pyruvate dehydrogenase kinase by short-term effects of products of fatty acid oxidation and by longer term effects involving specific protein synthesis; in hepatocytes the signals may include lipid fuels and glucagon. Activity of the branched chain ketoacid dehydrogenase complex determines the rate of degradation of branched chain aminoacids which is adjusted according to dietary supply. The complex is regulated by reversible phosphorylation, phosphorylation being inactivating. In liver and kidney, but not in muscles a protein activator (free E1 component) may reactivate phosphorylated complex without dephosphorylation and facilitate hepatic oxidation of branched chain ketoacids. Metabolic adjustments induced by diet and diabetes include loss of activator protein, loss of total complex activity in liver but not muscles, and enhanced inactivation by phosphorylation in liver.

Insulin-like growth factors and insulin: comparative aspects

IGF I and IGF II are two insulin-like growth factors resembling insulin in many respects. They stem from a common precursor, act through receptors similar to the insulin receptor with which they cross-react. When administered in large amounts they produce hypoglycemia. Their major effects, however, are on replication and differentiation of cells of mesodermal origin. IGF I is the major growth promoting factor in vivo. The synthesis and secretion of IGF I by the liver depend on the growth hormone status, insulin and nutrition. In contrast to insulin, the IGFs circulate in blood bound to the carrier proteins. Their half-life in man is in the order of 16 h. IGF I deficiency results in dwarfism (pygmy, Laron dwarf, toy poodle) despite normal or elevated growth hormone secretion. The anabolic actions of insulin and of the IGFs appear to complement each other as shown in Figure 7.

Regulation of lipogenesis in vivo by glucose availability and insulin secretion in maternal and foetal tissues during late gestation in the rat. Effect of glucose intubation, streptozotocin-induced diabetes and starvation

Administration of an oral load of glucose did not change the rate of lipogenesis in maternal liver during late gestation. However, streptozotocin-induced diabetes or starvation decreased maternal liver lipogenesis at 20-22 days of gestation. Glucose intubation, on the other hand, increased foetal lipogenesis at 21-22 days. In addition, maternal starvation decreased foetal lipogenesis and plasma insulin concentration. However, chronic hyperglycaemia induced by streptozotocin administration to the mother did not change foetal liver lipogenesis.

Regulation of mammalian pyruvate dehydrogenase

In mammalian tissues, two types of regulation of the pyruvate dehydrogenase complex have been described: end product inhibition by acetyl CoA and NADH: and the interconversion of an inactive phosphorylated form and an active nonphosphorylated form by an ATP requiring kinase and a specific phosphatase. This article is largely concerned with the latter type of regulation of the complex in adipose tissue by insulin (and other hormones) and in heart muscle by lipid fuels. Effectors of the two interconverting enzymes include pyruvate and ADP which inhibit the kinase, acetoin which activates the kinase and Ca2+ and Mg2+ which both activate the phosphatase and inhibit the kinase. Evidence is presented that all components of the pyruvate dehydrogenase complex including the phosphatase and kinase are located within the inner mitochondrial membrane. Direct measurements of the matrix concentration of substrates and effectors is not possible by techniques presently available. This is the key problem in the identification of the mechansims involved in the alterations in pyruvate dehydrogenase activity observed in adipose tissue and muscle. A number of indirect approaches have been used and these are reviewed. Most hopeful is the recent finding in this laboratory that in both adipose tissue and heart muscle, differences in activity of pyruvate dehydrogenase in the intact tissue persist during preparation and subsequent incubation of mitochondria.