Postnatal development of palmitate oxidation and mitochondrial enzyme activities in rat cardiac and skeletal muscle

Reorganization of mitochondria-organelle interactions during postnatal development in skeletal muscle

Skeletal muscle cellular development requires the integrated assembly of mitochondria and other organelles adjacent to the sarcomere in support of muscle contractile performance. However, it remains unclear how interactions among organelles and with the sarcomere relates to the development of muscle cell function. Here, we combine 3D volume electron microscopy, proteomic analyses, and live cell functional imaging to investigate the postnatal reorganization of mitochondria-organelle interactions in skeletal muscle. We show that while mitochondrial networks are disorganized and loosely associated with the contractile apparatus at birth, contact sites among mitochondria, lipid droplets and the sarcoplasmic reticulum are highly abundant in neonatal muscles. The maturation process is characterized by a transition to highly organized mitochondrial networks wrapped tightly around the muscle sarcomere but also to less frequent interactions with both lipid droplets and the sarcoplasmic reticulum. Concomitantly, expression of proteins involved in mitochondria-organelle membrane contact sites decreases during postnatal development in tandem with a decrease in abundance of proteins associated with sarcomere assembly despite an overall increase in contractile protein abundance. Functionally, parallel measures of mitochondrial membrane potential, NADH redox status, and NADH flux within intact cells revealed that mitochondria in adult skeletal muscle fibres maintain a more activated electron transport chain compared with neonatal muscle mitochondria. These data demonstrate a developmental redesign reflecting a shift from muscle cell assembly and frequent inter-organelle communication toward a muscle fibre with mitochondrial structure, interactions, composition and function specialized to support contractile function. KEY POINTS: Mitochondrial network organization is remodelled during skeletal muscle postnatal development. The mitochondrial outer membrane is in frequent contact with other organelles at birth and transitions to more close associations with the contractile apparatus in mature muscles. Mitochondrial energy metabolism becomes more activated during postnatal development. Understanding the developmental redesign process within skeletal muscle cells may help pinpoint specific areas of deficit in muscles with developmental disorders.

Sex-specific differences in the metabolic enzyme activity and transporter levels in mouse skeletal muscle during postnatal development

Although sex-associated differences in energy metabolism in adults are well-characterized, developmental sex-specific changes in skeletal muscle metabolism are largely unknown. This study investigated sex differences in high-energy phosphate, glycolytic, and mitochondrial enzyme activities and metabolite transporter protein levels in mouse skeletal muscles during the early postnatal period (day 10), post-weaning (day 28), sexual maturity (day 56), and adult life (day 140). No significant sex-specific differences were observed on days 10 and 28, except for glucose transporter (GLUT) 4 level. The hexokinase, phosphofructokinase, and lactate dehydrogenase activities of skeletal muscle were higher and the citrate synthase, cytochrome c oxidase, and β-hydroxyacyl-CoA dehydrogenase activities were lower in female mice than those in male mice on days 56 and 140. The GLUT4 and FAT/CD36 protein levels were higher and the monocarboxylate transporter 4 level was lower in the skeletal muscles of female mice than those of male mice, particularly on days 56 and 140. At 140 days of age, the respiratory exchange ratio during treadmill running (15 m/min, 60 min) was lower in females than that in males, despite no sex differences at rest. In summary, sex differences were not evident in the early postnatal and post-weaning periods but became apparent after the mice reached sexual maturity. These findings indicate that sexually mature animals are a better model for investigating sex differences, particularly in the context of studying energy metabolism in mice.

Nutritional modulation of heart failure in mitochondrial pyruvate carrier-deficient mice

The myocardium is metabolically flexible; however, impaired flexibility is associated with cardiac dysfunction in conditions including diabetes and heart failure. The mitochondrial pyruvate carrier (MPC) complex, composed of MPC1 and MPC2, is required for pyruvate import into the mitochondria. Here we show that MPC1 and MPC2 expression is downregulated in failing human and mouse hearts. Mice with cardiac-specific deletion of Mpc2 (CS-MPC2-/-) exhibited normal cardiac size and function at 6 weeks old, but progressively developed cardiac dilation and contractile dysfunction, which was completely reversed by a high-fat, low-carbohydrate ketogenic diet. Diets with higher fat content, but enough carbohydrate to limit ketosis, also improved heart failure, while direct ketone body provisioning provided only minor improvements in cardiac remodelling in CS-MPC2-/- mice. An acute fast also improved cardiac remodelling. Together, our results reveal a critical role for mitochondrial pyruvate use in cardiac function, and highlight the potential of dietary interventions to enhance cardiac fat metabolism to prevent or reverse cardiac dysfunction and remodelling in the setting of MPC deficiency.

Adverse Effects of Fenofibrate in Mice Deficient in the Protein Quality Control Regulator, CHIP

We previously reported how the loss of CHIP expression (Carboxyl terminus of Hsc70-Interacting Protein) during pressure overload resulted in robust cardiac dysfunction, which was accompanied by a failure to maintain ATP levels in the face of increased energy demand. In this study, we analyzed the cardiac metabolome after seven days of pressure overload and found an increase in long-chain and medium-chain fatty acid metabolites in wild-type hearts. This response was attenuated in mice that lack expression of CHIP (CHIP^−/−). These findings suggest that CHIP may play an essential role in regulating oxidative metabolism pathways that are regulated, in part, by the nuclear receptor PPARα (Peroxisome Proliferator-Activated Receptor alpha). Next, we challenged CHIP^−/− mice with the PPARα agonist called fenofibrate. We found that treating CHIP^−/− mice with fenofibrate for five weeks under non-pressure overload conditions resulted in decreased skeletal muscle mass, compared to wild-type mice, and a marked increase in cardiac fibrosis accompanied by a decrease in cardiac function. Fenofibrate resulted in decreased mitochondrial cristae density in CHIP^−/− hearts as well as decreased expression of genes involved in the initiation of autophagy and mitophagy, which suggests that a metabolic challenge, in the absence of CHIP expression, impacts pathways that contribute to mitochondrial quality control. In conclusion, in the absence of functional CHIP expression, fenofibrate results in unexpected skeletal muscle and cardiac pathologies. These findings are particularly relevant to patients harboring loss-of-function mutations in CHIP and are consistent with a prominent role for CHIP in regulating cardiac metabolism.

Maturation of Cardiac Energy Metabolism During Perinatal Development

As one of the highest energy consumer organ in mammals, the heart has to be provided with a high amount of energy as soon as its first beats in utero. During the development of this organ, energy is produced within the cardiac muscle cell depending on substrate availability, oxygen pressure and cardiac workload that drastically change at birth. Thus, energy metabolism relying essentially on carbohydrates in fetal heart is very different from the adult one and birth is the trigger of a profound maturation which ensures the transition to a highly oxidative metabolism depending on lipid utilization. To face the substantial increase in cardiac workload resulting from the growth of the organism during the postnatal period, the heart not only develops its capacity for energy production but also undergoes a hypertrophic growth to adapt its contractile capacity to its new function. This leads to a profound cytoarchitectural remodeling of the cardiomyocyte which becomes a highly compartmentalized structure. As a consequence, within the mature cardiac muscle, energy transfer between energy producing and consuming compartments requires organized energy transfer systems that are established in the early postnatal life. This review aims at describing the major rearrangements of energy metabolism during the perinatal development.

Stage of perinatal development regulates skeletal muscle mitochondrial biogenesis and myogenic regulatory factor genes with little impact of growth restriction or cross-fostering

Foetal growth restriction impairs skeletal muscle development and adult muscle mitochondrial biogenesis. We hypothesized that key genes involved in muscle development and mitochondrial biogenesis would be altered following uteroplacental insufficiency in rat pups, and improving postnatal nutrition by cross-fostering would ameliorate these deficits. Bilateral uterine vessel ligation (Restricted) or sham (Control) surgery was performed on day 18 of gestation. Males and females were investigated at day 20 of gestation (E20), 1 (PN1), 7 (PN7) and 35 (PN35) days postnatally. A separate cohort of Control and Restricted pups were cross-fostered onto a different Control or Restricted mother and examined at PN7. In both sexes, peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), cytochrome c oxidase subunits 3 and 4 (COX III and IV) and myogenic regulatory factor 4 expression increased from late gestation to postnatal life, whereas mitochondrial transcription factor A, myogenic differentiation 1 (MyoD), myogenin and insulin-like growth factor I (IGF-I) decreased. Foetal growth restriction increased MyoD mRNA in females at PN7, whereas in males IGF-I mRNA was higher at E20 and PN1. Cross-fostering Restricted pups onto a Control mother significantly increased COX III mRNA in males and COX IV mRNA in both sexes above controls with little effect on other genes. Developmental age appears to be a major factor regulating skeletal muscle mitochondrial and developmental genes, with growth restriction and cross-fostering having only subtle effects. It therefore appears that reductions in adult mitochondrial biogenesis markers likely develop after weaning.

Octanoate and nonaoate oxidation increases 50-80% over the first two days of life in piglet triceps brachii and gracilis muscle strips

An in vitro muscle strip incubation system was developed to measure the rate of catabolism of 1 mmol/L [1-(14)C]octanoate, 1 mmol/L [1-(14)C]nonanoate, 1 mmol/L [9-(14)C]nonanoate, and 10 mmol/L [U-(14)C]glucose by measuring the recovery of (14)CO(2). Muscle strips (13 mm × 1.5 mm, ~50 mg) were isolated from triceps brachii and gracilis muscles of newborn and 2-d-old, small (<950 g) and large (>1450 g) piglets. The position of the (14)C label in the substrate affected the rate and amount of recovery in (14)CO(2). Therefore, comparisons were made between age groups (0 vs. 2 d old) within substrates but limited across substrates to comparisons of [1-(14)C]-labeled fatty acids. The medium-chain fatty acid (MCFA) oxidation rates [pmol/(h · mg)] in muscle strips isolated from piglets from the 2 weight groups (<950 and >1450 g) did not differ (P > 0.99), there was a trend towards a difference between triceps brachii and gracilis muscle (P = 0.09; data not shown), and there were no significant interactions involving pig weight or muscle type; therefore, results were pooled across these factors. During the first 2 d of life, MCFA oxidation [pmol/(h • · mg muscle strip)] increased (P < 0.05) 50-80%, but the glucose oxidation rate did not change (P > 0.82). By d 2, the oxidation rate of nonanoate as represented by the one carbon was 25% greater than for octanoate (P < 0.05). The conversion of [9-(14)C]nonanoate to (14)CO(2) indicated that muscle had the capacity to oxidize the propionyl-CoA produced by β-oxidation of nonanoate and that odd-chain C-9 MCFA provided anabolic carbon to the citric acid cycle.

Mouse cardiac acyl coenzyme a synthetase 1 deficiency impairs Fatty Acid oxidation and induces cardiac hypertrophy

Long-chain acyl coenzyme A (acyl-CoA) synthetase isoform 1 (ACSL1) catalyzes the synthesis of acyl-CoA from long-chain fatty acids and contributes the majority of cardiac long-chain acyl-CoA synthetase activity. To understand its functional role in the heart, we studied mice lacking ACSL1 globally (Acsl1(T-/-)) and mice lacking ACSL1 in heart ventricles (Acsl1(H-/-)) at different times. Compared to littermate controls, heart ventricular ACSL activity in Acsl1(T-/-) mice was reduced more than 90%, acyl-CoA content was 65% lower, and long-chain acyl-carnitine content was 80 to 90% lower. The rate of [(14)C]palmitate oxidation in both heart homogenate and mitochondria was 90% lower than in the controls, and the maximal rates of [(14)C]pyruvate and [(14)C]glucose oxidation were each 20% higher. The mitochondrial area was 54% greater than in the controls with twice as much mitochondrial DNA, and the mRNA abundance of Pgc1α and Errα increased by 100% and 41%, respectively. Compared to the controls, Acsl1(T-/-) and Acsl1(H-/-) hearts were hypertrophied, and the phosphorylation of S6 kinase, a target of mammalian target of rapamycin (mTOR) kinase, increased 5-fold. Our data suggest that ACSL1 is required to synthesize the acyl-CoAs that are oxidized by the heart, and that without ACSL1, diminished fatty acid (FA) oxidation and compensatory catabolism of glucose and amino acids lead to mTOR activation and cardiac hypertrophy without lipid accumulation or immediate cardiac dysfunction.

Changes in creatine transporter function during cardiac maturation in the rat

BACKGROUND

It is well established that the immature myocardium preferentially utilises non-oxidative energy-generating pathways. It exhibits low energy-transfer capacity via the creatine kinase (CK) shuttle, reflected in phosphocreatine (PCr), total creatine and CK levels that are much lower than those of adult myocardium. The mechanisms leading to gradually increasing energy transfer capacity during maturation are poorly understood. Creatine is not synthesised in the heart, but taken up exclusively by the action of the creatine transporter protein (CrT). To determine whether this transporter is ontogenically regulated, the present study serially examined CrT gene expression pattern, together with creatine uptake kinetics and resulting myocardial creatine levels, in rats over the first 80 days of age.

RESULTS

Rats were studied during the late prenatal period (-2 days before birth) and 7, 13, 21, 33, 50 and 80 days after birth. Activity of cardiac citrate synthase, creatine kinase and its isoenzymes as well as lactate dehydrogenase (LDH) and its isoenzymes demonstrated the well-described shift from anaerobic towards aerobic metabolism. mRNA levels of CrT in the foetal rat hearts, as determined by real-time PCR, were about 30% of the mRNA levels in the adult rat heart and gradually increased during development. Creatine uptake in isolated perfused rat hearts increased significantly from 3.0 nmol/min/gww at 13 days old to 4.9 nmol/min/gww in 80 day old rats. Accordingly, total creatine content in hearts, measured by HPLC, increased steadily during maturation (30 nmol/mg protein (-2 days) vs 87 nmol/mg protein (80 days)), and correlated closely with CrT gene expression.

CONCLUSIONS

The maturation-dependant alterations of CK and LDH isoenzyme activities and of mitochondrial oxidative capacity were paralleled by a progressive increase of CrT expression, creatine uptake kinetics and creatine content in the heart.

Development of aerobic and anaerobic metabolism in cardiac and skeletal muscles from harp and hooded seals

In diving animals, skeletal muscle adaptations to extend underwater time despite selective vasoconstriction include elevated myoglobin (Mb) concentrations, high acid buffering ability (beta) and high aerobic and anaerobic enzyme activities. However, because cardiac muscle is perfused during dives, it may rely less heavily on Mb, beta and anaerobic pathways to support contractile activity. In addition, because cardiac tissue must sustain contractile activity even before birth, it may be more physiologically mature at birth and/or develop faster than skeletal muscles. To test these hypotheses, we measured Mb levels, beta and the activities of citrate synthase (CS), beta-hydroxyacyl-CoA dehydrogenase (HOAD) and lactate dehydrogenase (LDH) in cardiac and skeletal muscle samples from 72 harp and hooded seals, ranging in age from fetuses to adults. Results indicate that in adults cardiac muscle had lower Mb levels (14.7%), beta (55.5%) and LDH activity (36.2%) but higher CS (459.6%) and HOAD (371.3%) activities (all P<0.05) than skeletal muscle. In addition, while the cardiac muscle of young seals had significantly lower [Mb] (44.7%) beta (80.7%) and LDH activity (89.5%) than adults (all P<0.05), it was relatively more mature at birth and weaning than skeletal muscle. These patterns are similar to those in terrestrial species, suggesting that seal hearts do not exhibit unique adaptations to the challenges of an aquatic existence.

Postnatal development of muscle biochemistry in nursing harbor seal (Phoca vitulina) pups: limitations to diving behavior?

Adult marine mammal muscles rely upon a suite of adaptations for sustained aerobic metabolism in the absence of freely available oxygen (O(2)). Although the importance of these adaptations for supporting aerobic diving patterns of adults is well understood, little is known about postnatal muscle development in young marine mammals. However, the typical pattern of vertebrate muscle development, and reduced tissue O(2) stores and diving ability of young marine mammals suggest that the physiological properties of harbor seal (Phoca vitulina) pup muscle will differ from those of adults. We examined myoglobin (Mb) concentration, and the activities of citrate synthase (CS), beta-hydroxyacyl coA dehydrogenase (HOAD), and lactate dehydrogenase (LDH) in muscle biopsies from harbor seal pups throughout the nursing period, and compared these biochemical parameters to those of adults. Pups had reduced O(2) carrying capacity ([Mb] 28-41% lower than adults) and reduced metabolically scaled catabolic enzyme activities (LDH/RMR 20-58% and CS/RMR 29-89% lower than adults), indicating that harbor seal pup muscles are biochemically immature at birth and weaning. This suggests that pup muscles do not have the ability to support either the aerobic or anaerobic performance of adult seals. This immaturity may contribute to the lower diving capacity and behavior in younger pups. In addition, the trends in myoglobin concentration and enzyme activity seen in this study appear to be developmental and/or exercise-driven responses that together work to produce the hypoxic endurance phenotype seen in adults, rather than allometric effects due to body size.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

Alterations in energy metabolism in cardiomyopathies

Despite the fact that the heart requires huge amounts of energy to sustain contractile function, it has limited energy reserves and must therefore continually produce large amounts of adenosine triphosphate (ATP) to sustain function. Fatty acids are the primary energy substrate of the adult heart, with more than 60% of the energy normally obtained from the oxidation of fatty acids, the remainder coming from the metabolism of carbohydrates. Alterations in both the rates of ATP production and the type of energy substrate used by the heart can have consequences on contractile function, as well as on its ability to respond to energetic stresses. Switches in myocardial substrate utilization and energy production rates have been shown to occur in various cardiomyopathies, as well as in any subsequent heart failure. Heart failure is characterized by an inefficient pumping of the heart, which fails to meet the energy requirements of the body. A number of cardiomyopathies can lead to heart failure. This paper will review the alterations in energy metabolism that occur in a number cardiomyopathies, including ischemic and diabetic cardiomyopathies, as well as hypertrophic cardiomyopathies resulting from mutations in enzymes involved in energy metabolism, such as 5' adenosine monophosphate-activated protein kinase (AMPK).

Tissue-Specific and Fatty Acid Transporter-Specific Changes in Heart and Soleus Muscle Over a 1-yr Period

Rates of fatty acid oxidation increase rapidly in both rat heart and skeletal muscle in the early postnatal period. Therefore, we examined in heart and soleus muscle, (a) whether there were rapid changes in fatty acid transporter (FAT/CD36, FABPpm) mRNA and protein expression early in life (days 10 -36) and thereafter (days 84, 160, 365), and (b) whether the rates of fatty acid transport and the plasmalemmal content of FAT/CD36 and FABPpm were altered. Protein expression was altered rapidly from day 10-36 in both heart (FAT/CD36 only, +21%, P < 0.05)) and soleus muscle (FAT/CD36 + 100%, P < 0.05; FABPpm -20%, P < 0.05), with no further changes thereafter (P < 0.05). Rates of fatty acid transport (day 10 vs day 160) were increased in heart (+33%, P < 0.05) and muscle (+85%, P < 0.05), and were associated with concomitant increases in plasmalemmal FABPpm (+44%, P < 0.05) and FAT/CD36 (+16%, P < 0.05) in the heart, and only plasmalemmal FAT/CD36 in muscle (+90%, P < 0.05). Therefore, known changes in the rates of fatty acid oxidation in heart and muscle early in life appear to be accompanied by a concurrent upregulation in the rates of fatty acid transport and the expression of FAT/CD36 in heart and muscle, as well as an increase in plasmalemmal FAT/CD36 and FABPpm in the heart, and only plasmalemmal FAT/CD36 in soleus muscle. We speculate that the rapid upregulation of fatty acid transport rates in heart and muscle are needed to support the increased rates of fatty oxidation that have been previously observed in these tissues.

Myocardial substrate metabolism in the normal and failing heart

The alterations in myocardial energy substrate metabolism that occur in heart failure, and the causes and consequences of these abnormalities, are poorly understood. There is evidence to suggest that impaired substrate metabolism contributes to contractile dysfunction and to the progressive left ventricular remodeling that are characteristic of the heart failure state. The general concept that has recently emerged is that myocardial substrate selection is relatively normal during the early stages of heart failure; however, in the advanced stages there is a downregulation in fatty acid oxidation, increased glycolysis and glucose oxidation, reduced respiratory chain activity, and an impaired reserve for mitochondrial oxidative flux. This review discusses 1) the metabolic changes that occur in chronic heart failure, with emphasis on the mechanisms that regulate the changes in the expression of metabolic genes and the function of metabolic pathways; 2) the consequences of these metabolic changes on cardiac function; 3) the role of changes in myocardial substrate metabolism on ventricular remodeling and disease progression; and 4) the therapeutic potential of acute and long-term manipulation of cardiac substrate metabolism in heart failure.

Postnatal increase in insulin-sensitive glucose transporter expression is associated with improved recovery of postischemic myocardial function

OBJECTIVE

Glucose is an important substrate for energy production in the developing heart. Increased glucose uptake rate and metabolism during ischemia and reperfusion are closely linked to postischemic myocardial recovery. The initial rate-limiting step for glycolysis is the transport of glucose across the plasma membrane by glucose transporters (GLUT-1 and GLUT-4). We hypothesized that changes in GLUT-1 and GLUT-4 expression in developing hearts lead to age-dependent adaptive changes in glucose uptake capacity and influence tolerance to ischemia.

METHODS

Western-immunoblotting was performed to determine GLUT-1 and GLUT-4 expression in myocardial tissue from 1, 2, and 3-week-old and adult rabbits. Glucose uptake rate was measured with (31)P-nuclear magnetic resonance spectroscopy using 2-deoxyglucose as substrate in isolated perfused hearts. Hearts from same age rabbits were perfused in the Langendorff mode with crystalloid buffer or buffer plus a GLUT-4 specific antibody in order to determine GLUT-4 mediated effects on myocardial protection. The hearts were subjected to 30 minutes of normothermic ischemia followed by reperfusion. Cardiac contractile function measurements were obtained pre- and postischemia. Tissue lactate accumulation was measured in all groups at end-ischemia

CONCLUSIONS

Insulin-regulated glucose transporter (GLUT-4) expression in the heart increased gradually after birth reaching nearly adult levels by 3 weeks of age. Corresponding with the higher amount of GLUT-4 protein, improved recovery of postischemic contractile function was seen in older hearts in association with increased anaerobic glycolytic capacity. Interventions to accelerate postnatal GLUT-4 expression may improve ischemic tolerance in the neonatal heart.

Functional and metabolic adaptation of the heart to prolonged thyroid hormone treatment

In heart failure, thyroid hormone (TH) treatment improves cardiac performance. The long-term effects of TH on cardiac function and metabolism, however, are incompletely known. To investigate the effects of up to 28 days of TH treatment, male Wistar rats received 3,3',5-triiodo-l-thyronine (200 microg/kg sc per day) leading to a 2.5-fold rise in plasma fatty acid (FA) level and progressive cardiac hypertrophy (+47% after 28 days) (P < 0.001). Ejection fraction (echocardiography) was increased (+12%; P < 0.05) between 7 and 14 days and declined thereafter. Neither cardiac FA oxidation, glycolytic capacity (homogenates) per unit muscle mass, nor mRNA levels of proteins involved in FA and glucose uptake and metabolism (Northern blots and microarray) were altered. After 28 days of treatment, mRNA levels of uncoupling proteins (UCP) 2 and 3 and atrial natriuretic factor were increased (P < 0.05). This indicates that TH-induced hypertrophy is associated with an initial increase in cardiac performance, followed by a decline in cardiac function and increased expression of UCPs and atrial natriuretic factor, suggesting that detrimental effects eventually prevail.

Critical steps in cellular fatty acid uptake and utilization

Despite decades of extensive research, the transport routes, mechanisms of uptake and points of flux control of long-chain fatty acids (FA) in mammalian organs are still incompletely understood. In non-fenestratred organs such as heart and skeletal muscle, membrane barriers for blood-borne FA are the luminal and abluminal membranes of endothelial cells, the sarcolemma and the mitochondrial membranes. Transport of FA through the phospholipid bilayer of the cellular membrane is most likely accomplished by diffusion of protonated FA. Evidence is accumulating that membrane-associated proteins, such as plasmalemmal fatty acid-binding protein (FABPpm) and fatty acid translocase (FAT/CD36), either alone or in conjunction with albumin binding protein (ABP), are instrumental in enhancing the delivery of FA to the cellular membrane. Inside the cell, cytoplasmic fatty acid-binding proteins (FABPc) are involved in diffusion of FA from the plasmalemma to the intracellular sites of conversion, such as the mitochondrial outer membrane. After conversion of FA to FACoA, the fatty acyl chain is transported across the mitochondrial inner membrane in a carnitine-mediated fashion. Uptake and utilization of FA by muscle cells are finely tuned, most likely to avoid the intracellular accumulation of FA, as these are cytotoxic at high concentrations. On a short-term basis, net uptake is, among others, regulated by intracellular translocation of FAT from intracellular stores to the sarcolemma and by the concentration gradient of FA across the sarcolemma. The latter implies that, among others, the rate of FA utilization determines the rate of uptake. The rate of utilization is governed by a variety of factors, including malonylCoA, the ratio acetylCoA/CoA and the availability of competing substrates such as glucose, lactate, and ketone bodies. Long-term regulation of uptake and utilization is accomplished by alterations in the rate of expression of genes, encoding for FA-handling proteins. Circumstantial evidence indicates that FA themselves are able to modulate the expression of FA-handling genes via nuclear transcription factors such as peroxisome proliferator-activated receptors (PPARs).

Regulation of mitochondrial contact sites in neonatal, juvenile and diabetic hearts

Mitochondrial contact sites (MiCS) are dynamic structures involved in high capacity transport of energy from mitochondria into the cytosole. Previous studies revealed that in normal conditions the actual number of MiCS is in correlation with the energy requirements of the heart, particularly with those for its contractile work. Although the detailed mechanisms of signalling between the processes of energy utilisation and MiCS formation in the heart are not yet elucidated, it is known that intracellular Ca2+ transients are intimately involved in this crosstalk. The present study is devoted to investigation of Ca2+-linked MiCS formation in healthy adult hearts and in hearts with modified Ca2+-handling such as in developing, in juvenile and diabetic myocardium. Experiments were performed on hearts of healthy rats on the 22nd embryonal day, 1st, 4th, 7th and 14th postnatal days as well as on adult hearts. Diabetic hearts were investigated on the 8th day after streptozotocin injection (45 mg x kg(-1) iv.) to adult rats. Intracellular Ca2+ movements were affected by modulation of Ca2+ concentration in perfusion solution (1.6 or 2.2 mmol l(-1) in isolated, Langendorff-perfused hearts, by calcium paradox (CaP) or by replacing of Ca2+ by Cd2+ ions. Elevation of extracellular Ca2+ was reflected by 30.1, 10.4 and 24.1% increase in intracellular free Ca2+ concentration in healthy adult, diabetic and 14-day old hearts respectively. In developing hearts the amount of MiCS was culminating on the 4th postnatal day. In adult hearts, elevated calcium in the perfusion solution, CaP as well as diabetes led to a significant increase in the amounts of MiCS formed (58.1, 77.2 and 86.5% respectively; p < 0.05). Diabetic and 14-day old hearts naturally exhibited amounts of MiCS comparable to those obtained by Ca2+-stimulation of MiCS formation in adult healthy hearts. In contrast to healthy controls, perfusion of diabetic and 14-day old hearts with elevated Ca2+ as well as induction of CaP exerted little influence on MiCS formation (4.4 and 8.2% for elevated Ca2+; 2.9 and 10.7% for CaP; p > 0.05). A replacement of Ca2+ by Cd2+ ions lowered the amount of MiCS in healthy adult and diabetic hearts (61 and 52.2%; p < 0.05). In conclusion, during development, the formation of MiCS may be influenced by both, permanent stimulation by Ca2+-signalling and the availability of mCPK. In healthy adult hearts the amount of MiCS is modulated by intracellular Ca2+ transients in response to changes in extracellular Ca2+ concentration. In diabetic hearts the modulation of MiCS formation is naturally attenuated, apparently as a consequence of persisting alterations in Ca2+-handling.

Tissue-specific and isoform-specific changes in MCT1 and MCT4 in heart and soleus muscle during a 1-yr period

We examined the postnatal changes (days 10, 36, 84, 160, 365) of monocarboxylate transporters (MCT)1 and MCT4 in rat heart and soleus muscle. In the heart, MCT1 protein and mRNA remained unaltered from day 10 until 1 yr of age. Both MCT4 protein and mRNA in the heart were detected at 10 days of age, but the MCT4 protein and transcript were not detected thereafter. In the soleus muscle, MCT1 protein (+38%) and mRNA (+136%) increased during the first 84 days and remained stable until 1 yr of age. In contrast, soleus MCT4 protein decreased by 90% over the course of 1 yr, with the most rapid decrease (-60%) occurring by day 84 (P < 0.05). At the same time, MCT4 mRNA was increased by 74% from days 10 to 84 (P < 0.05), remaining stable thereafter. In conclusion, developmental changes in MCT transport proteins are tissue specific and isoform specific. Furthermore, it appears that MCT1 expression in the heart and MCT1 and MCT4 expression in the soleus are regulated by pretranslational processes, whereas posttranscriptional processes regulate MCT4 expression in the soleus muscle.

Demonstration of direct effects of growth hormone on neonatal cardiomyocytes

The cellular and molecular basis of growth hormone (GH) actions on the heart remain poorly defined, and it is unclear whether GH effects on the myocardium are direct or mediated at least in part via insulin-like growth factor (IGF-1). Here, we demonstrate that the cultured neonatal cardiomyocyte is not an appropriate model to study the effects of GH because of artifactual loss of GH receptors (GHRs). To circumvent this problem, rat neonatal cardiomyocytes were infected with a recombinant adenovirus expressing the murine GHR. Functional integrity of GHR was suggested by GH-induced activation of the cognate JAK2/STAT5, MAPK, and Akt intracellular pathways in the cells expressing GHR. Although exposure to GH resulted in a significant increase in the size of the cardiomyocyte and increased expression of c-fos, myosin light chain 2, and skeletal alpha-actin mRNAs, there were no significant changes in IGF-1 or atrial natriuretic factor mRNA levels in response to GH stimulation. In this model, GH increased incorporation of leucine, uptake of palmitic acid, and abundance of fatty acid transport protein mRNA. In contrast, GH decreased uptake of 2-deoxy-d-glucose and levels of Glut1 protein. Thus, in isolated rat neonatal cardiomyocytes expressing GHR, GH induces hypertrophy and causes alterations in cellular metabolic profile in the absence of demonstrable changes in IGF-1 mRNA, suggesting that these effects may be independent of IGF-1.

Developmental changes in mitochondrial activity and energy metabolism in fetal and neonatal rat brain

Experiments were undertaken to investigate mitochondrial activity and energy metabolism in the developing rat brain from the late fetal stage to the neonatal stage. Samples of cerebral cortical tissue were obtained from fetuses at 14, 16, 18, and 20 days of gestation, and from pups at 1 h, 1 day and 7 days after birth. Mitochondrial respiration was measured polarographically using homogenates. Fetal and neonatal brains were frozen in situ and fluorometric enzymatic techniques were used for the analysis of ATP, ADP, AMP, and lactate. In the fetal brain, there was a gradual increase in stimulated (+ADP) and uncoupled respiratory rates using glutamate and malate as substrates, from 14 days to 20 days of gestation, together with a moderate increase in ATP concentration and in the sum total of adenine nucleotides, and a significant decrease in lactate. Since non-stimulated (-ADP) respiratory rates did not change with increasing gestational age, the respiratory control ratio appeared to increase over the same period. An increase in mitochondrial activity was more pronounced immediately after birth, together with a marked increase in ATP concentration and in the sum total of adenine nucleotides. The highest rate of mitochondrial respiration was observed in 1-hour-old pups. These results indicate that, in the rat brain, there is maturation of oxidative metabolism in mitochondria that is initiated in late gestation. Acceleration in mitochondrial respiration occurs immediately after birth in order to maintain high-energy phosphate levels, and this may be crucial for the successful outcome of the newborn.

Developmental changes in regulation of mitochondrial respiration by ADP and creatine in rat heart in vivo

In saponin-skinned muscle fibers from adult rat heart and m. soleus the apparent affinity of the mitochondrial oxidative phosphorylation system for ADP (Km = 200-400 microM) is much lower than in isolated mitochondria (Km = 10-20 microM). This suggests a limited permeability of the outer mitochondrial membrane (OMM) to adenine nucleotides in slow-twitch muscle cells. We have studied the postnatal changes in the affinity of mitochondrial respiration for ADP, in relation to morphological alterations and expression of mitochondrial creatine kinase (mi-CK) in rat heart in vivo. Analysis of respiration of skinned fibers revealed a gradual decrease in the apparent affinity of mitochondria to ADP throughout 6 weeks post partum that indicates the development of mechanism which increasingly limits the access of ADP to mitochondria. The expression of mi-CK started between the 1st and 2nd weeks and reached the adult levels after 6 weeks. This process was associated with increases in creatine-activated respiration and affinity of oxidative phosphorylation to ADP thus reflecting the progressive coupling of mi-CK to adenine nucleotide translocase. Laser confocal microscopy revealed significant changes in rearrangement of mitochondria in cardiac cells: while the mitochondria of variable shape and size appeared to be random-clustered in the cardiomyocytes of 1 day old rat, they formed a fine network between the myofibrils by the age of 3 weeks. These results allow to conclude that in early period of development, i.e. within 2-3 weeks, the diffusion of ADP to mitochondria becomes progressively restricted, that appears to be related to significant structural rearrangements such as formation of the mitochondrial network. Later (after 3 weeks) the control shifts to mi-CK, which by coupling to adenine nucleotide translocase, allows to maximally activate the processes of oxidative phosphorylation despite limited access of ADP through the OMM.

Dietary coconut oil affects more lipoprotein lipase activity than the mitochondria oxidative capacities in muscles of preruminant calves

The presence of coconut oil in a milk replacer stimulates the growth rate of calves, suggesting a better oxidation of fatty acid in muscles. Because dietary fatty acid composition influences carnitine palmitoyltransferase I (CPT I) activity in rat muscles, this study was designed to examine the effects of a milk replacer containing either tallow (TA) or coconut oil (CO) on fatty acid utilization and oxidation and on the characteristics of intermyofibrillar (IM) and subsarcolemmal (SS) mitochondria in the heart and skeletal muscles of preruminant calves. Feeding CO did not affect palmitate oxidation rate by whole homogenates, but induced higher palmitate oxidation by IM mitochondria (+37%, P < 0.05). CPT I activity did not significantly differ between the two groups of calves. Heart and longissimus thoracis muscle of calves fed CO had higher lipoprotein lipase activity (+27% and 58%, respectively; P < 0.05) but showed no differences in fatty acid binding protein content or activity of oxidative enzymes. Whatever the muscle and the diet, IM mitochondria had higher respiration rates and enzyme activities than those of SS mitochondria (P < 0.05). Furthermore, CPT I activity of the heart was 28-fold less sensitive to malonyl-coenzyme A inhibition in IM mitochondria than in SS mitochondria. In conclusion, dietary CO marginally affected the activity of the two mitochondrial populations and the oxidative activity of muscles in the preruminant calf. In addition, this study showed that differences between IM and SS mitochondria in the heart and muscles were higher in calves than in other species studied so far.

Co-expression in rat heart and skeletal muscle of four genes coding for proteins implicated in long-chain fatty acid uptake

It has been suggested that specific membrane-associated and cytoplasmic proteins cooperate in the uptake of long-chain fatty acids by cardiac and skeletal muscle cells. A prerequisite for this hypothesis would be the co-occurrence of these proteins in muscle. Thus, we studied the possible co-expression in rat muscles of the genes coding for the integral membrane proteins fatty acid transport protein (FATP) and fatty acid translocase (FAT), the membrane-associated plasmalemmal fatty acid-binding protein (FABPpm) and the cytoplasmic heart-type fatty acid-binding protein (H-FABPc). The transcripts of the four proteins were assessed in heart and skeletal muscles of adult Wistar rats, in isolated cells and cell lines from rat heart and also in rat heart during development and upon streptozotocin-induced diabetes. All four genes showed high expression levels in heart, somewhat lower in red skeletal muscle (soleus) and appreciably lower in white skeletal muscle (extensor digitorum longus). FATP, FAT and H-FABPc showed a 3- to 5-fold increase in mRNA expression during maturational growth of the heart, while the FABPpm expression remained virtually constant. In the heart, streptozotocin-diabetes induced a slight, but statistically not significant, increase in the expression of all four genes. In conclusion, this study shows the co-expression of FATP, FAT, FABPpm and H-FABPc in rat muscles. This finding supports the possible cooperation of these proteins in the uptake of long-chain fatty acids by muscle cells.

Contribution of mitochondria and peroxisomes to palmitate oxidation in rat and bovine tissues

Total and peroxisomal palmitate oxidation capacities and mitochondrial enzyme activities were compared in tissues from growing rats, preruminant calves and 15-month-old bulls. Total palmitate oxidation rates were 1.9-5.2-fold higher in rat than in bovine tissues and 1.7-fold higher in the heart and muscles from calves than from growing bulls. The peroxisomal contribution to palmitate oxidation was similar between rats and bovines (i.e. calves and bulls) in liver (35-51%), heart (26%) but not in muscles (14 +/- 3% in rats vs 33 +/- 4.5% in bovines, P < 0.05). Mitochondrial enzyme activities were 1.8-4.8-fold higher in rat than in bovine tissues but the citrate synthase to cytochrome-c oxidase ratio was the highest in the liver (17-38), intermediate in the heart and muscles from calves and rats (6-10) and the lowest in heart and muscles from bulls (2-3, P < 0.05). In all tissues and animal groups, palmitate oxidation rates were similar per unit cytochrome-c oxidase activity, but not always per unit citrate synthase activity. Therefore, differences in mitochondrial contents (as between rats and bovines) or in mitochondrial characteristics (as between liver and muscles) relate to the differences in palmitate oxidation capacity.

Perinatal changes in myocardial supply and flux of fatty acids, carbohydrates, and ketone bodies in lambs

No information is available on perinatal changes in myocardial metabolism in vivo. We measured myocardial supply and flux of fatty acids, carbohydrates, and ketone bodies in chronically instrumented fetal, newborn (1-4 days), and juvenile (7 wk) lambs, by measuring aorta-coronary sinus concentration differences and blood flow. In the fetal lambs, myocardial supply and flux of fatty acids were zero. In the newborn lambs, the supply of fatty acids increased tenfold, but there was no flux of fatty acids. Carbohydrates were the major energy source in fetal and newborn lambs, accounting for 89 and 69% of myocardial oxygen consumption, respectively. In the juvenile lambs, the flux of fatty acids was increased threefold. The supply and flux of carbohydrates were decreased (by 31 and 82%, respectively). The supply and flux of ketone bodies gradually increased with age. We show that the myocardium of the lamb in vivo does not switch immediately after birth from carbohydrates to fatty acids. The mechanisms involved in the development of myocardial fatty acid oxidation remain to be elucidated.

Effect of age on in vivo rates of mitochondrial protein synthesis in human skeletal muscle

A progressive decline in muscle performance in the rapidly expanding aging population is causing a dramatic increase in disability and health care costs. A decrease in muscle endurance capacity due to mitochondrial decay likely contributes to this decline in muscle performance. We developed a novel stable isotope technique to measure in vivo rates of mitochondrial protein synthesis in human skeletal muscle using needle biopsy samples and applied this technique to elucidate a potential mechanism for the age-related decline in the mitochondrial content and function of skeletal muscle. The fractional rate of muscle mitochondrial protein synthesis in young humans (24 +/- 1 year) was 0.081 +/- 0.004%.h-1, and this rate declined to 0.047 +/- 0.005%.h-1 by middle age (54 +/- 1 year; P < 0.01). No further decline in the rate of mitochondrial protein synthesis (0.051 +/- 0.004%.h-1) occurred with advancing age (73 +/- 2 years). The mitochondrial synthesis rate was about 95% higher than that of mixed protein in the young, whereas it was approximately 35% higher in the middle-aged and elderly subjects. In addition, decreasing activities of mitochondrial enzymes were observed in muscle homogenates (cytochrome c oxidase and citrate synthase) and in isolated mitochondria (citrate synthase) with increasing age, indicating declines in muscle oxidative capacity and mitochondrial function, respectively. The decrease in the rates of mitochondrial protein synthesis is likely to be responsible for this decline in muscle oxidative capacity and mitochondrial function. These changes in muscle mitochondrial protein metabolism may contribute to the age-related decline in aerobic capacity and muscle performance.

Substrate metabolism in the developing heart

The developing heart undergoes a remarkable metabolic transformation as it adjusts to the higher-oxygen, extrauterine environment. During gestation, glycolysis and lactate oxidation constitute the major sources of adenosine triphosphate (ATP) for the fetal heart. After birth, however, there is a rapid shift from carbohydrate to fatty acid utilization. Despite the transition to primarily aerobic metabolism, the neonatal heart retains an enhanced capacity for anaerobic energy production. This unique metabolic adaptation is important when assessing the immature heart's responses to states of oxygen insufficiency, such as ischemia, hypoxia, and tachycardia. This article reviews the dramatic changes in enzyme activities, mitochondrial morphology and function, and substrate availability that underlie this change in metabolism in the maturing heart.

Membrane-associated and cytoplasmic fatty acid-binding proteins

A number of cellular fatty acid-binding proteins are being implicated in the uptake and intracellular transport of long-chain fatty acids by parenchymal cells. Having been a topic of research for more than 20 years, cytoplasmic fatty acid-binding proteins now are assigned various pivotal functions in intracellular fatty acid transport and metabolism. More recently several membrane-associated fatty acid-binding proteins have been identified and these proteins are thought to function in the transmembrane transport of fatty acids. In this review, a short summary is provided of the latest developments in this research area.

Role of fatty acid-binding protein in lipid metabolism of insect flight muscle

Since insect flight muscles are among the most active muscles in nature, their extremely high rates of fuel supply and oxidation pose interesting physiological problems. Long-distance flights of species like locusts and hawkmoths are fueled through fatty acid oxidation. The lipid substrate is transported as diacylglycerol in the blood, employing a unique and efficient lipoprotein shuttle system. Following diacyglycerol hydrolysis by a flight muscle lipoprotein lipase, the liberated fatty acids are ultimately oxidized in the mitochondria. Locusta flight muscle cytoplasm contains an abundant fatty acid-binding protein (FABP). The flight muscle FABP of Locusta migratoria is a 15 kDa protein with an isoelectric point of 5.8, binding fatty acids in a 1:1 molar stoichiometric ratio. Binding affinity of the FABP for long-chain fatty acids (apparent dissociation constant Kd = 5.21 +/- 0.16 microM) is however markedly lower than that of mammalian FABPs. The NH2-terminal amino acid sequence shares structural homologies with two insect FABPs recently purified from hawkmoth midgut, as well as with mammalian FABPs. In contrast to all other isolated FABPs, the NH2 terminus of locust flight muscle FABP appeared not to be acetylated. During development of the insect, a marked increase in fatty acid binding capacity of flight muscle homogenate was measured, along with similar increases in both fatty acid oxidation capacity and citrate synthase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle phosphorus energy state in very-low-birth-weight infants: effect of exercise

Skeletal muscle hypotonia is a hallmark clinical finding in very-low-birth-weight (VLBW) human infants. Although the biochemical basis for this phenomenon is not completely understood, one hypothesis is that the phosphorylation potential is abnormally low in the skeletal muscle of these infants. Therefore, we used 31P-nuclear magnetic resonance (NMR) spectroscopy to measure phosphorus metabolites in the skeletal muscle of VLBW infants during rest and during reflex-induced muscle contractions. Compared with healthy larger infants or to adults, the total phosphorus NMR signal is lower in VLBW infants. In VLBW infants during rest, [PCr]/([PCr]+[Pi]), where PCr is phosphocreatine and brackets denote concentration, was 89% and [ATP]/[ADP][Pi] was 59% of that found in larger infants (P less than 0.05). During reflex-induced isometric contractions in VLBW infants, [PCr]/([PCr]+[Pi]) declined by 24% and [ATP]/[ADP][Pi] declined by 35% (P less than 0.05 vs. rest). In all conditions, muscle pH remained 7.1. Overall, the differences in skeletal muscle energy state during rest and the corresponding changes in concentration of high-energy phosphates during mild exercise suggest a very limited energy reserve in the hypotonic muscle of VLBW infants.

Developmental regulation of lipoprotein lipase in rats

To evaluate changes in lipoprotein lipase (LPL) expression during development, levels of LPL mRNA, protein, and enzyme activity were measured in heart, epididymal fat, kidney, and brain of rats, from late gestation through 24 mo. LPL mRNA, protein, and enzyme activity were low in fetal and neonatal hearts. LPL mRNA increased 11-fold by 60 days and remained at this level thereafter; LPL protein and enzyme activity increased 10-fold by weaning, before declining to low values by 3 mo. LPL mRNA levels, protein, and enzyme activity did not change in epididymal fat from 3 wk to 21 mo. In the kidney, LPL mRNA levels were high at the end of gestation but fluctuated during the first month. LPL protein and activity were low at day 1 and rose eightfold to peak values by day 7 before decreasing to low levels by weaning. LPL mRNA levels were relatively high in fetal brains and then fell 60% during the neonatal period. LPL protein peaked at day 7 before falling 95% by weaning. Thus LPL is under complex tissue-specific regulation involving transcriptional and posttranscriptional mechanisms.

Adaptation of energy metabolism of canine latissimus dorsi muscle in response to chronic electrical stimulation

Transformation of the latissimus dorsi (LD) muscle from a fast-twitch, fatigue-prone to a fatigue-resistant ("heart-like") muscle, necessary to allow its application in cardiac assist devices, can be induced by chronic electrical stimulation. In adult dogs we studied the nature and time course of myofibrillar and metabolic adaptations in the LD muscle when exposed in situ to 24 weeks of continuous electrical stimulation. In addition, the metabolic properties of the stimulated muscle were compared with those of canine cardiac muscle. The proportion of immunohistochemically identified type I fibres increased on stimulation from 28% to 80%, while that of type II fibres decreased from 69% to 16%. Fibres of intermediate type (IIC and IC) appeared transiently; the highest levels were found between 4 and 8 weeks of stimulation. The activities of fructose-6-phosphate kinase and lactate dehydrogenase (LDH), which before stimulation were similar to those in heart, decreased to 18% and 34% of their initial values respectively. However, the LDH isozyme pattern changed towards that typical for cardiac muscle. These changes indicate a markedly decreased flux capacity through the glycolytic pathway which, however, is directed more towards the oxidative conversion of substrates. The mitochondrial capacity (maximal palmitate oxidation and pyruvate dehydrogenase complex activities) of the muscle did not change and remained at a level less than half of that of cardiac ventricular muscle. Contents of adenine nucleotides and endogenous substrates were maintained during stimulation. No further changes in the observed adaptations occurred after week 12 of stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal fatty acid utilization by cultured cardiac cells from 7-day-old obese Zucker rats

Fatty acid utilization by muscle and nonmuscle heart cells in culture has been investigated in the 7-day-old Zucker rat to determine if this tissue could contribute to the lower energy expenditure reported in obese rats at the onset of obesity. The partitioning of oleate to oxidation and esterification products and the effect of genotype on this partitioning according to cell types were studied. Results showed that the fatty acid beta-oxidation and its esterification in neutral lipid was decreased by 30% in beating muscle cells from obese animals when compared with those from lean animals. In contrast, nonmuscle cells exhibited a decreased beta-oxidation alone. A similar fatty acid composition of the phospholipids was found in non-muscle cells of obese animals and their lean litter mates. In muscle cultures, palmitic and oleic acids are lower in cells of obese rats than in those of lean rats. The present study indicates that a defect in energy metabolism could be found in heart cells at the onset of obesity, suggesting that this defect is determined by intrinisic factor(s).

Human cytochrome c oxidase during cardiac growth and development

Human heart mitochondrial cytochrome c oxidase specific content and specific activity was measured in five fetuses 15-21 weeks gestational age and in five patients whose age ranged from 6 days to 22 years. None had evidence of cardiac pathology. An increase in cytochrome c oxidase specific content and specific activity was observed in the fetal heart with increasing gestational age (0.13-0.38 nmol heme a/mg protein and 67-295 nmol O2 utilized/min/mg protein) and from the neonatal period (0.35 nmol heme a/mg protein and 140 nmol O2/min/mg protein) to adulthood (1.2 nmol heme a/mg protein and 1104 nmol O2/min/mg protein). A marked increase was observed postnatally between 4 and 19 months.

Postnatal development of the actual and total activity of the branched-chain 2-oxo acid dehydrogenase complex in rat tissues

Actual and total activities of the branched-chain 2-oxo acid dehydrogenase complex were determined in homogenates of quadriceps muscle, heart, liver, kidney and brain from rats of 0-70 days age. All rat tissues except quadriceps muscle showed a marked increase of total activity between 0 and 21 days, heart and kidney also after weaning. The actual activity rose after birth in liver, kidney and brain and after weaning in liver, kidney and heart. The activity state was always about 100% in liver and varied between 40-60% in kidney and brain, 10-23% in heart and 6-12% in quadriceps muscle. The actual activities measured indicate, that the degradation of branched-chain 2-oxo acids mainly takes place in the liver of the newborn, suckling and young-adult rat.

Effect of dietary fat on total and peroxisomal fatty acid oxidation in rat tissues

In this study the effect of dietary trans fatty acids on the peroxisomal and mitochondrial beta-oxidation is compared with that of saturated or cis-monounsaturated fatty acids. Oxidation of [1-14C]- and [16-14C]palmitate was assayed in the absence as well as in the presence of antimycin plus rotenone in homogenates of liver, heart and skeletal muscle of four groups of rats fed diets containing 40 energy% fat of different fatty acid composition. Three groups were given fat blends rich in C16, C18 saturated (cocoa butter), cis-monounsaturated (low-linoleic-acid olive oil) or trans fatty acids (partially hydrogenated soybean oil), respectively. The fourth group received a mixture of these fats with half the amount of trans fatty acids of the third group. Total oxidation rates of [1-14C]- and [16-14C]palmitate in the absence of antimycin were not significantly influenced by the type of dietary fat in the investigated tissues. The antimycin-insensitive [1-14C]palmitate oxidation rate and the proportion of peroxisomal oxidation of the total oxidation were lower in all tissues of those animals fed the mixed dietary fat than in those fed the other diets; both parameters were higher in the liver of cocoa butter-fed rats than in those of the other groups. Comparison of the results with literature data and with previous results obtained with a low-fat diet (Veerkamp and Van Moerkerk (1986) Biochim. Biophys. Acta 875, 301-310) indicates that high-fat diets only induce peroxisomal beta-oxidation activity if they also contain C20, C22 fatty acids. High dietary concentrations of trans C18 fatty acids do not result in a higher peroxisomal activity than that observed for other fatty acids with the same chain length.

Peroxisomal fatty acid oxidation in rat and human tissues. Effect of nutritional state, clofibrate treatment and postnatal development in the rat

Oxidation of palmitate 14C-labeled in different positions was assayed in the absence and presence of antimycin and rotenone in homogenates of various rat and human tissues to determine total and peroxisomal oxidation and acetyl group production. Total and antimycin-insensitive palmitate oxidation rates were higher in rat heart, liver and quadriceps muscle than in the corresponding human tissues. The proportion of antimycin-insensitive oxidation of [1-14C]palmitate was 17-35% in tissues of starved rats and in human muscles and fibroblasts, but peroxisomal production of acetyl groups amounted only to 5-11% of that by mitochondria. The mean number of peroxisomal beta-oxidation cycles was 1.5-2.5 per palmitate molecule. The nutritional state markedly influenced the total oxidation rate and the antimycin-insensitive proportion in rat liver. Clofibrate feeding increased total and antimycin-insensitive oxidation rates in liver, heart and kidney, but not in quadriceps muscle. Total oxidation capacity was maximal in rat liver at weaning, and in rat heart at an age of 70 days. Antimycin-insensitive oxidation rates increased in rat liver and heart at postnatal development up to weaning. A marked proportion of lignocerate oxidation was antimycin-sensitive in rat tissues.

Fatty acid binding proteins from heart

Heart contains a fatty acid binding protein (FABP) concentration comparable to liver, when it is determined with a fatty acid-binding assay. The low concentration detected with anti-liver FABP antibodies is related to the different chemical forms and physiochemical properties of liver and heart FABP. The ratio of fatty acid bound per purified protein molecule is one or lower. Rat heart mitochondria oxidize FABP-bound fatty acids. The FABP content of rat heart is dependent on sex and diurnal cycle, but is not influenced by starvation or clofibrate feeding. It is also not different in the newborn rat. FABP was obtained from human heart in a yield of 11%. It shows similar binding characteristics to palmitic, oleic and arachidonic acid. The functional significance of the specific heart FABP is discussed in relation to myocardial fatty acid metabolism in normal and pathological conditions.

Postnatal growth and differentiation in three hindlimb muscles of the rat. Characterization with biochemical and enzyme-histochemical methods

The postnatal development, between 0 and 90 days, of three hindlimb muscles and diaphragm of the rat was investigated with respect to fiber types and diameter (histochemistry) and substrate oxidation rates and enzyme activities (biochemistry). The process of muscle fiber differentiation into mature patterns was evaluated by visual classification into 3 or 4 groups having different staining intensities for 3 enzyme-histochemical reactions, enabling 26 fiber types to be distinguished. These exhibited specific sizes and growth rates that varied among the muscles. One of the hindleg muscles (flexor digitorum brevis) remained much more immature than soleus and extensor digitorum longus. The histochemical and biochemical findings correlated well. The capacity for pyruvate and palmitate oxidation, and the activities of cytochrome c oxidase and citrate synthase, increased markedly between 9 and 37 days in soleus and extensor digitorum longus (except citrate synthase in the latter) but not in flexor digitorum brevis. Creatine kinase activity increased in all hindlimb muscles. Both the capacity and the activity of pyruvate oxidation (determined in homogenates and intact isolated muscles, respectively), were in accordance with the fiber type composition. In contrast to oxidation capacity, the activity of pyruvate oxidation decreased after birth until the mature stage, when a value of 18-42% of that of early postnatal muscles was recorded.

Changes of the NADP-linked malic enzyme in the developing rat skeletal muscle

The activities of NADP-linked malic enzyme, hexose monophosphate shunt dehydrogenases and NADP-linked isocitrate dehydrogenase were studied during development of skeletal muscle and compared with those in the liver. The variation patterns of malic enzyme activity in the liver and in the skeletal muscle were very similar, however the amplitude of the changes was different. The enzyme activity increased approx 16-fold in the liver and about 2-fold in skeletal muscle at the same stage of development. In skeletal muscle the increase of the malic enzyme activity was only slightly higher than of lactic dehydrogenase and citrate synthase. Studies on the intracellular distribution of malic enzyme in skeletal muscle showed that both mitochondrial and extramitochondrial enzymes increased between 20th and 37th day of life, the increase of the extramitochondrial enzyme being more pronounced. The hexose monophosphate shunt dehydrogenases activity showed an increase in the liver but no change was observed in the skeletal muscle at the weaning time. Changes in the activity of the liver and skeletal muscle isocitrate dehydrogenase were not significant between 10th and 80th day of life. The results suggest that the malic enzyme in the liver is playing a different physiological role than in the skeletal muscle.

Palmitate oxidation by intact preparations of skeletal muscle

1. The palmitate oxidation by intact preparations of rat hemidaphragm, m.soleus and m.flexor digitorum brevis and by teased fibers of human m.pectoralis was studied. 2. The structural and metabolic viability of the in vitro preparations was shown by a low leakage of soluble creatine kinase, a constant rate of palmitate oxidation and only a small stimulatory effect of L-carnitine. 3. With hemidaphragm the palmitate oxidation rate increases with both the palmitate concentration (0-3 mM) and the palmitate/albumin molar ratio (0.5-5.0). 4. The apparent Km for palmitate oxidation was about 1.5 mM at 0.1 and 0.2 mM albumin and about 2.7 mM at 0.4 and 0.6 mM albumin, which correlates with the higher affinity of albumin for palmitate at lower palmitate/albumin molar ratios. 5. After prolonged starvation the apparent Km at 0.4 mM albumin is markedly lower. In whole homogenates of diaphragm the apparent Km at 0.4 mM albumin is only about 370 microM. 6. The calculated maximal oxidation rate was not significantly different for all albumin concentrations examined (23-32 nmol/min per g), did not change after starvation and appears to be of the same order of magnitude as the rate of endogenous fatty acid consumption (30-40 nmol/min per g). 7. Results suggest that substrate availability is a main factor for the oxidation rate of exogenous palmitate by hemidiaphragm in vitro and that the magnitude of the apparent Km is largely dependent upon the degree of label dilution with fatty acids of endogenous origin.