Muscle Fatty Acid Oxidation Research Articles

AMP-activated protein kinase, a metabolic master switch: possible roles in type 2 diabetes.

Adenosine 5'-monophosphate-activated protein kinase (AMPK) now appears to be a metabolic master switch, phosphorylating key target proteins that control flux through metabolic pathways of hepatic ketogenesis, cholesterol synthesis, lipogenesis, and triglyceride synthesis, adipocyte lipolysis, and skeletal muscle fatty acid oxidation. Recent evidence also implicates AMPK as being responsible for mediating the stimulation of glucose uptake induced by muscle contraction. In addition, the secretion of insulin by insulin secreting (INS-1) cells in culture is modulated by AMPK activation. The net effect of AMPK activation is stimulation of hepatic fatty acid oxidation and ketogenesis, inhibition of cholesterol synthesis, lipogenesis, and triglyceride synthesis, inhibition of adipocyte lipolysis and lipogenesis, stimulation of skeletal muscle fatty acid oxidation and muscle glucose uptake, and modulation of insulin secretion by pancreatic beta-cells. In skeletal muscle, AMPK is activated by contraction. Type 2 diabetes mellitus is likely to be a disease of numerous etiologies. However, defects or disuse (due to a sedentary lifestyle) of the AMPK signaling system would be predicted to result in many of the metabolic perturbations observed in Type 2 diabetes mellitus. Increased recruitment of the AMPK signaling system, either by exercise or pharmaceutical activators, may be effective in correcting insulin resistance in patients with forms of impaired glucose tolerance and Type 2 diabetes resulting from defects in the insulin signaling cascade.

Fatty acid oxidation affects food intake by altering hepatic energy status.

Inhibition of fatty acid oxidation stimulates feeding behavior in rats. To determine whether a decrease in hepatic fatty acid oxidation triggers this behavioral response, we compared the effects of different doses of methyl palmoxirate (MP), an inhibitor of fatty acid oxidation, on food intake with those on in vivo and in vitro liver and muscle metabolism. Administration of 1 mg/kg MP selectively decreased hepatic fatty acid oxidation but did not stimulate food intake. In contrast, feeding behavior increased in rats given 5 or 10 mg/kg MP, which inhibited hepatic fatty acid oxidation to the same extent as did the low dose but in addition suppressed fatty acid oxidation in muscle and produced a marked depletion of liver glycogen. Dose-related increases in food intake tracked dose-related reductions in liver ATP content, ATP-to-ADP ratio, and phosphorylation potential. The findings suggest that a decrease in hepatic fatty acid oxidation can stimulate feeding behavior by reducing hepatic energy production.

Higher mitochondrial fatty acid oxidation following intermittent versus continuous endurance exercise training

It has been well documented that skeletal muscle fatty acid oxidation can be elevated by continuous endurance exercise training. However, it remains questionable whether similar adaptations can be induced with intermittent interval exercise training. This study was undertaken to directly compare the rates of fatty acid oxidation in isolated subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria following these different exercise training regimes. Mitochondria were isolated from the gastrocnemius-plantaris muscles of male Sprague-Dawley rats following exercise training 6 days per week for 12 weeks. Exercise training consisted of either continuous, submaximal, endurance treadmill running (n = 10) or intermittent, high intensity, interval running (n = 10). Both modes of training enhanced the oxidation of palmityl-carnitine-malate in both mitochondrial populations (p < 0.05). However, the increase associated with the intermittent, high intensity exercise training was significantly greater than that achieved with the continuous exercise training (p < 0.05). Also, the increases associated with the IMF mitochondria were greater than the SS mitochondria (p < 0.05). These data suggest that high intensity, intermittent interval exercise training is more effective for stimulation of fatty acid oxidation than continuous submaximal exercise training and that this adaptation occurs preferentially within IMF mitochondria.Key words: muscle, subsarcolemmal mitochondria, intermyofibrillar mitochondria.

Fatty acid binding protein, a major protein in the flight muscle of migrating Western Sandpipers

Migratory flight in birds is fueled primarily by fatty acid oxidation imposing a requirement for high rates of fatty acid: (a) transport; (b) uptake; and (c) delivery to intracellular sites of β-oxidation. Muscle fatty acid binding protein (M-FABP) is a cytosolic protein involved in the intracellular transport of fatty acids. Its expression appears to be correlated with muscle fatty acid oxidation capacity. The M-FABP was isolated for the first time from a long distance migrant bird using: (i) size exclusion; (ii) anion exchange; and (iii) hydroxyapatite chromatography. M-FABP has a molecular weight of approximately 14 000 Da and an isoelectric point of pH 4.8. A partial amino acid sequence of the protein demonstrated homology to M-FABPs from other species (80% identical to human heart FABP). It was estimated that M-FABP comprises approximately 14 and 21% of total cytosolic protein of the pectoralis and heart, respectively; the highest values yet reported from any vertebrate muscle. The abundance of M-FABP in these tissues suggests that the protein may play a key role in fatty acid supply during endurance flight. Thus, it is proposed that a seasonal increase in M-FABP expression could be a component of physiological preparation for migration.

5 Malonyl-CoA???Regulator of Fatty Acid Oxidation in Muscle During Exercise

As muscle goes from a resting state to exercise, the following sequence of events occurs (Figure 5.5): (1) The rise in AMP accompanying contraction allosterically activates AMPK and an AMPK kinase; (2) The activated AMPK kinase phosphorylates and further activates AMPK; (3) The activated AMPK phosphorylates and inactivates ACC; and (4) The consequent decline in malonyl-CoA (product of ACC reaction) relieves inhibition of CPT-1 and allows an increased rate of fatty acid oxidation when fatty acids become available.

Higher mitochondrial fatty acid oxidation following intermittent versus continuous endurance exercise training.

It has been well documented that skeletal muscle fatty acid oxidation can be elevated by continuous endurance exercise training. However, it remains questionable whether similar adaptations can be induced with intermittent interval exercise training. This study was undertaken to directly compare the rates of fatty acid oxidation in isolated subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria following these different exercise training regimes. Mitochondria were isolated from the gastrocnemius-plantaris muscles of male Sprague-Dawley rats following exercise training 6 days per week for 12 weeks. Exercise training consisted of either continuous, submaximal, endurance treadmill running (n = 10) or intermittent, high intensity, interval running (n = 10). Both modes of training enhanced the oxidation of palmityl-carnitine-malate in both mitochondrial populations (p < 0.05). However, the increase associated with the intermittent, high intensity exercise training was significantly greater than that achieved with the continuous exercise training (p < 0.05). Also, the increases associated with the IMF mitochondria were greater than the SS mitochondria (p < 0.05). These data suggest that high intensity, intermittent interval exercise training is more effective for stimulation of fatty acid oxidation than continuous submaximal exercise training and that this adaptation occurs preferentially within IMF mitochondria.

Some thoughts on the importance of insulin in the regulation of the blood glucose level.

Insulin can influence rates of glucose utilization by muscle and possibly other tissues via both direct and indirect effects. It can control the rate of fatty acid mobilization from adipose tissue and the rate of fatty acid oxidation in muscle, and the latter inhibits glucose utilization and oxidation. Insulin may influence the levels of insulin-like growth factors I and II, both of which have effects on rates of glucose utilization by muscle. The inter-tissue cycle between glucose and lactate-the Cori cycle, which is influenced by insulin-may provide another novel mechanism for control of blood glucose. How far other anti-insulin hormones affect these processes is not clear.

Peroxisomal and mitochondrial palmitate oxidation in rat cardiac and skeletal muscles: Effects of diabetes, fasting and fat consumption

Peroxisomal lipid metabolism in several tissues is altered under conditions in which there is a relative increase in the use of fatty acids for energy (eg. diabetes, fasting and high fat-feeding). The present study was undertaken to determine if these same conditions affect peroxisomal lipid metabolism in rat cardiac and skeletal muscle; catalase activity and mitochondrial palmitate oxidation were also measured. Rats were made diabetic by Streptozotocin injection; 4 weeks later peroxisomal palmitate oxidation capacity in vitro was elevated in heart and soleus, but not in two other skeletal muscles. Mitochondrial palmitate oxidation was significantly increased in heart by the diabetic condition. In fasted rats total peroxisomal palmitate oxidation capacity was decreased in cardiac and three skeletal muscles, whereas in rats fed a high fat (70% kcals lard) diet for 3 weeks, peroxisomal palmitate oxidation capacity was increased in psoas and soleus, but not in the heart or extensor digitorus longus. Muscle catalase activity was elevated in rats made diabetic, fasted or fed a high fat diet. The results support the concept that peroxisomal fatty acid oxidation in muscle participates in the overall regulation of lipid metabolism, but may not respond solely to the amount of fatty acid potentially available as substrate.

Is intravenous glucose the only treatment for neonatal hypoglycaemia? : J.M. Hawdon and M.P. Ward Platt, Department of Child Health, University of Newcastle upon Tyne, U.K.

The unusual energy metabolism of elasmobranchs is characterized by limited or absent fatty acid oxidation in cardiac and skeletal muscle and a great reliance on ketone bodies and amino acids as oxidative fuels in these tissues. Other extrahepatic tissues in elasmobranchs rely on ketone bodies and amino acids for aerobic energy production but, unlike muscle, also appear to possess a significant capacity to oxidize fatty acids. This organization of energy metabolism is reflected by relatively low plasma levels of non-esterified fatty acids (NEFA) and by plasma levels of the ketone body ß-hydroxybutyrate that are as high as those seen in fasted mammals. The preference for ketone body oxidation rather than fatty acid oxidation in muscle of elasmobranchs under routine conditions is opposite to the situation in teleosts and mammals. Carbohydrates appear to be utilized as a fuel source in elasmobranchs, similar to other vertebrates. Amino acid- and lipid-fueled ketogenesis in the liver, the lipid storage site in elasmobranchs, sustains the demand for ketone bodies as oxidative fuels. The liver also appears to export NEFA and serves a buoyancy role. The regulation of energy metabolism in elasmobranchs and the effects of environmental factors remain poorly understood. The metabolic organization of elasmobranchs was likely present in the common ancestor of the Chondrichthyes ca. 400 million years ago and, speculatively, it may reflect the ancestral metabolism of jawed vertebrates. We assess hypotheses for the evolution of the unusual energy metabolism of elasmobranchs and propose that the need to synthesize urea has influenced the utilization of ketone bodies and amino acids as oxidative fuels.

7 Defects of fatty-acid oxidation in muscle

Long-chain fatty acids (LCFA) are oxidized by muscle mitochondria after transport in the cytosol by fatty-acid-binding protein(s) and their activation by a thiokinase. Carnitine, two forms of carnitine palmitoyltransferase(s) and carnitine acylcarnitine translocase are involved in LCFA gating. A primary genetic carnitine deficiency occurs in children with dilated cardiomyopathy, hypoglycaemia and low carnitine content in plasma, liver and muscle, owing to a defect in a common high-affinity transport system. This high-affinity transport in muscle differs from a low-affinity transport that has modifications during muscle maturation. The genetic enzyme defects of beta-oxidation (long-chain acyl-CoA dehydrogenase, medium- and short-chain acyl-CoA-dehydrogenase) present with Reye-like attacks that may lead to non-ketotic hypoglycaemia, coma and sudden infant death syndrome. There is elevated urinary excretion of dicarboxylic acids, acylcarnitines and acylglycines. Secondary carnitine deficiency may occur. ETF and ETF dehydrogenase deficiencies may present in a neonatal form with congenital anomalies, or in a later-onset form with ethylmalonic adipic aciduria. A still-unidentified defect leads to LCFA accumulation in fibroblasts, bone marrow, liver and muscle cells in a multisystem triglyceride disorder.

Muscle malonyl-CoA decreases during exercise

Malonyl-CoA, the inhibitor of carnitine acyltransferase I, is an important regulator of fatty acid oxidation and ketogenesis in the liver. Muscle carnitine acyltransferase I has previously been reported to be more sensitive to malonyl-CoA inhibition than is liver carnitine acyltransferase I. Fluctuations in malonyl-CoA concentration may therefore be important in regulating the rate of fatty acid oxidation in muscle during exercise. Male rats were anesthetized (pentobarbital via venous catheters) at rest or after 30 min of treadmill exercise (21 m/min, 15% grade). The gastrocnemius/plantaris muscles were frozen at liquid N2 temperature. Muscle malonyl-CoA decreased from 1.66 +/- 0.17 to 0.60 +/- 0.05 nmol/g during the exercise. This change was accompanied by a 31% increase in cAMP in the muscle. The decline in malonyl-CoA occurred before muscle glycogen depletion and before onset of hypoglycemia. Plasma catecholamines, corticosterone, and free fatty acids were all significantly increased during the exercise. This exercise-induced decrease in malonyl-CoA may be important for allowing the increase in muscle fatty acid oxidation during exercise.

Verapamil reverses PTH- or CRF-induced abnormal fatty acids oxidation in muscle

Chronic renal failure (CRF) is associated with impaired long chain fatty acids (LCFA) oxidation by skeletal muscle mitochondria. This is due to reduced activity of carnitine palmitoyl transferase (CPT). These derangements were attributed to the secondary hyperparathyroidism of CRF, since prior parathyroidectomy in CRF rats reversed these abnormalities and PTH administration to normal rats reproduced them. It was proposed that these effects of PTH are mediated by its ionophoric property leading to increased entry of calcium into skeletal muscle. A calcium channel blocker may, therefore, correct these derangements. The present study examined the effects of verapamil on LCFA oxidation, CPT activity by skeletal muscle mitochondria, and 45Ca uptake by skeletal muscle obtained from CRF rats and normal animals treated with PTH with and without verapamil. Both four days of PTH administration and 21 days of CRF produced significant (P less than 0.01) reduction in LCFA oxidation and CPT activity of skeletal muscle mitochondria, and significant (P less than 0.01) increment in 45Ca uptake by skeletal muscle. Simultaneous treatment with verapamil corrected all these derangements. Administration of verapamil alone to normal rats did not cause a significant change in any of these parameters. The data are consistent with the proposition that the alterations in LCFA in CRF or after PTH treatment are related to the ionophoric action of the hormone and could be reversed by a calcium channel blocker.

The metabolic response to the eugylcemic insulin clamp in type I diabetes and normal humans

The euglycemic insulin clamp has been utilized extensively to measure in vivo tissue sensitivity to insulin under various circumstances. Insulin sensitivity is determined from the amount of glucose metabolized under steady state conditions. To assess the effect of abnormalities in other insulin responsive metabolic pathways on glucose metabolism and thus insulin sensitivity as measured by the glucose clamp, the concentration of lactate, pyruvate, 3-hydroxybutyrate, glycerol, alanine, and free fatty acids were measured at baseline and during a two-hour euglycemic clamp in 13 nonobese subjects with type I diabetes. The observed responses were compared to 11 normal controls. Insulin sensitivity as measured by M (glucose metabolized), MCRg (metabolic clearance of glucose), and M I ratio (glucose metabolized per unit insulin) were all significantly decreased in the diabetic subjects ( P < 0.005). Free fatty acids (FFA) and 3-hydroxybutyrate were significantly elevated at baseline in the diabetic subjects ( P < 0.05) and decreased significantly at 60 and 120 minutes in both groups. Baseline blood pyruvate and lactate concentrations were similar in the control and diabetic subjects. Pyruvate increased significantly at 60 minutes in both groups ( P < 0.05) and returned to baseline in the control subjects but remained elevated at 120 minutes in the diabetic subjects ( P < 0.001). Lactate increased similarly in both groups and remained elevated at 60 and 120 minutes. In summary, insulin sensitivity as assessed by the euglycemic insulin clamp is decreased in type I diabetes. However, specific differences in the concentration of several other metabolites both at baseline and in response to hyperinsulinemia were also identified in the diabetic subjects. The chronically increased circulating concentrations and increased muscle oxidation of FFA in type I diabetes as reflected by the elevated baseline levels may be responsible for inhibiting muscle glucose uptake and oxidation during the glucose clamp via the glucose fatty acid cycle. In addition, inhibition of pyruvate dehydrogenase secondary to increased muscle fatty acid oxidation would also result in decreased pyruvate metabolism and may explain the sustained elevations in this metabolite during the glucose clamp. These differences may account for the decreased metabolism of glucose in response to insulin infusion during the glucose clamp and help explain the insulin insensitivity of type I diabetes.

THE GLUCOSE-FATTY ACID CYCLE: Relationship between glucose utilization in muscle, fatty acid oxidation in muscle and lipolysis in adipose tissue

THE GLUCOSE-FATTY ACID CYCLE: Relationship between glucose utilization in muscle, fatty acid oxidation in muscle and lipolysis in adipose tissue

Studies of fatty acid oxidation in homogenates of the cardiomyopathic hamster

The rate of fatty acid oxidation and the activities of some enzymes of β-oxidation and the TCA cycle were measured in homogenates prepared from heart, muscle and liver of 185 day old cardiomyopathic hamsters and age-matched normal controls. Fatty acid oxidation was severely depressed in heart and muscle homogenates but was not affected in liver homogenates of the diseased animals. The activities of enoyl-CoA hydratase, β-hydroxyacyl-CoA dehydrogenase and thiolase were significantly depressed in heart homogenates of the diseased animals. Neither fatty acyl-CoA synthetase, acylcarnitine transferase or any of the enzymes of the TCA cycle were affected. In muscle homogenates, the activities of some of the enzymes of the TCA cycle and β-oxidation were significantly elevated in the diseased animals. Only acylcarnitine transferase activity was significantly depressed in the cardiomyopathic hamsters. It is concluded that the depression in fatty acid oxidation in heart may be related to the depression in the activities of the enzymes of β-oxidation and that the depression in fatty acid oxidation in muscle may be related to a defective transport of fatty acids across the mitochondrial membrane.

Availability of substrates and capacity for prolonged heavy exercise in man.

Availability of substrates and capacity for prolonged heavy exercise in man.

Diisopropylammonium dichloroacetate (DIPA) and sodium dichloroacetate (DCA): Effect on glucose and fat metabolism in normal and diabetic tissue

Diisopropylammonium dichloroacetate (DIPA) was found to exert a significant and prolonged hypoglycemic effect in alloxan diabetic rats, but did not alter blood sugar levels of normal rats. It did not affect blood glycerol levels in either the diabetic or nondiabetic group. In order to establish the site of action of DIPA, we carried out a series of in vitro studies. Both DIPA and sodium dichloroacetate (DCA) significantly stimulated glucose-U- 14C oxidation to 14CO 2 in isolated hemidiaphragms from diabetic but not from nondiabetic rats. Diisopropylammonium hydrochloride (DIA) was not effective in promoting glucose oxidation in tissues from diabetic or nondiabetic rats. Thus, the effect of the compound in vivo may be due entirely to its acid moiety. DCA (and presumably DIPA) produced no significant changes in glycerol output or on glucose-U- 14C conversion to 14C-triglyceride in hemidiaphragms or epididymal fat pads from normal or diabetic rats. In addition, DCA did not alter oleate-1- 14C conversion to 14C-triglyceride in muscle or adipose tissue from diabetic animals. However, DCA markedly inhibited oleate-1- 14C oxidation to 14CO 2 in muscle from diabetic rats. It is possible that the hypoglycemic activity of DIPA, and presumably DCA, may be due, at least partly, to a suppression of fatty acid oxidation in muscles of diabetic rats. The high levels of circulating free fatty acids and ketone bodies which commonly occur in diabetes increase intracellular concentrations of citrate, a known inhibitor of phosphofructokinase (PFK). By suppressing fatty acid oxidation in muscle, DIPA and DCA may thus reduce citrate levels and reactivate PFK. Such an effect may explain the selective action of both DIPA and DCA on diabetic but not on normal tissue.

Uptake and release of free fatty acids and other metabolites in the legs of exercising men.

Uptake and release of free fatty acids and other metabolites in the legs of exercising men.

UTILIZATION OF LIPIDS BY FISH. II. FATTY ACID OXIDATION BY A PARTICULATE FRACTION FROM LATERAL LINE MUSCLE.

The fatty acid oxidizing system present in lateral line muscle of rainbow trout (Salmo gairdnerii) and sockeye salmon (Oncorhynchus nerka) was studied by using subcellular particles, having the sedimentation characteristics of mitochondria. The rate of oxidation of K-myristate-1-C14, K-octanoate-1-C14, and Na-hexanoate-1-C14was determined at 25 °C by measuring the formation of C14O2. Oxidation was stimulated by adenosine triphosphate Mg++, coenzyme A and tricarboxylic acid cycle intermediates, but not by cytochrome c. It was optimum at pH 7.5–8.5.The data are consistent with the assumption that in the lateral line muscle fatty acid oxidation takes place through the known mechanism involving CoA derivatives.