Competition between fatty acids and carbohydrate or ketone bodies as metabolic fuels for the isolated perfused heart

The ability of carbohydrate fuels (lactate, pyruvate, glucose) and the ketone bodies (acetoacetate, beta-hydroxybutyrate) to compete with fatty acids as fuels of respiration in the isolated Langendorf-perfused heart was studied. Oleate and octanoate were used as fatty acid fuels since oleate requires carnitine for entry into mitochondria, whereas octanoate does not. The two ketone bodies inhibited the oxidation of both oleate and octanoate implying an intramitochondrial site of action. Pyruvate, lactate, and lactate plus glucose inhibited oleate oxidation but not octanoate oxidation, indicating a mechanism of inhibition that involves the carnitine system. Pyruvate was a more potent inhibitor than lactate at equal concentrations, but the effect of lactate could be greatly increased by dichloroacetate, an inhibitor of pyruvate dehydrogenase kinase. The physiological and mechanistic implications of these observations are discussed.     

gAd-globular head domain of adiponectin increases fatty acid oxidation in newborn rabbit hearts

Adiponectin is an adipocyte-derived hormone that has a number of metabolic effects in the body, including the control of both glucose and fatty acid metabolism. The globular head domain of adiponectin, gAd, has also been shown to increase fatty acid oxidation in skeletal muscle. Within days after birth, a rapid increase in fatty acid oxidation occurs in the heart. We examined whether adiponectin or gAd plays a role in this maturation of cardiac fatty acid oxidation. Plasma adiponectin increased in newborn rabbits following birth: 1.2 +/- 0.3 microg/ml in 1-day-old, 6.8 +/- 1.8 microg/ml in 7-day-old, and 45 +/- 5 microg/ml in 6-week-old rabbits. Because plasma insulin levels decrease and remain low throughout the suckling period, and because this decrease may contribute to the maturation of fatty acid oxidation, we examined the effects of adiponectin and gAd on fatty acid oxidation in isolated perfused 1-day-old rabbit hearts in the presence or absence of 100 microunits/ml insulin. Adiponectin (10 microg/ml) did not alter fatty acid oxidation in the presence of insulin. In the absence of insulin, the addition of recombinant gAd (1.5 microg/ml) increased fatty acid oxidation compared with control (129 +/- 18 versus 66 +/- 11 nmol.g dry weight(-1).min(-1), respectively (p < 0.05). In 7-day-old hearts, where fatty acid oxidation rates were 5-fold higher than 1-day-old hearts, gAd did not alter fatty acid oxidation rates. The increase in fatty acid oxidation in 1-day-old hearts occurred independently of changes in 5'-AMP-activated protein kinase, acetyl-CoA carboxylase, or malonyl-CoA. The effect of gAd on fatty acid oxidation was reversed in the presence of 100 microunits/ml insulin. These results suggest that a decrease in plasma insulin and increase in gAd are involved in the increase of cardiac fatty acid oxidation in the immediate newborn period.     

Differential modulation of citrate synthesis and release by fatty acids in perfused working rat hearts

The objective of this study was to test the effect of increasing fatty acid concentrations on substrate fluxes through pathways leading to citrate synthesis and release in the heart. This was accomplished using semirecirculating work-performing rat hearts perfused with substrate mixtures mimicking the in situ milieu (5.5 mM glucose, 8 nM insulin, 1 mM lactate, 0.2 mM pyruvate, and 0.4 mM oleate-albumin) and 13C methods. Raising the fatty acid concentration from 0.4 to 1 mM with long-chain oleate or medium-chain octanoate resulted in a lowering ( approximately 20%) of cardiac output and efficiency with unaltered O2 consumption. At the metabolic level, beyond the expected effects of high fatty acid levels on the contribution of pyruvate decarboxylation (reduced >3-fold) and beta-oxidation (enhanced approximately 3-fold) to citrate synthesis, there was also a 2.4-fold lowering of anaplerotic pyruvate carboxylation. Despite the dual inhibitory effect of high fatty acids on pyruvate decarboxylation and carboxylation, tissue citrate levels were twofold higher, but citrate release rates remained unchanged at 11-14 nmol/min, representing <0.5% of citric acid cycle flux. A similar trend was observed for most metabolic parameters after oleate or octanoate addition. Together, these results emphasize a differential modulation of anaplerotic pyruvate carboxylation and citrate release in the heart by fatty acids. We interpret the lack of effects of high fatty acid concentrations on citrate release rates as suggesting that, under physiological conditions, this process is maximal, probably limited by the activity of its mitochondrial or plasma membrane transporter. Limited citrate release at high fatty acid concentrations may have important consequences for the heart's fuel metabolism and function.     

Metabolic fate of non-esterified fatty acids in isolated hepatocytes from newborn and young pigs. Evidence for a limited capacity for oxidation and increased capacity for esterification.

In hepatocytes isolated from 48 h-old starved of suckling newborn pigs or from 15-day-old starved piglets, the rate of ketogenesis from oleate or from octanoate is very low. This is not due to an inappropriate fatty acid uptake by the isolated liver cells, but results from a limited capacity for fatty acid oxidation. Some 80-95% of oleate taken up is converted into esterified fats, whatever the age or the nutritional conditions. Three lines of indirect evidences suggest that fatty acid oxidation is not controlled primarily by malonyl-CoA concentration in newborn pig liver. Firstly, the addition of glucagon does not increase fatty acid oxidation or ketogenesis. Secondly, the rate of lipogenesis is very low in isolated hepatocytes from newborn pigs. Thirdly, the rates of oxidation and ketogenesis from octanoate are also decreased in isolated hepatocytes from newborn and young piglets. The huge rate of esterification of fatty acids in the liver of the newborn pigs probably represents a species-specific difference in intrahepatic fatty acid metabolism.     

Modulation of the sympathetic nerve action on carbohydrate and ketone body metabolism by fatty acids, glucagon und insulin in perfused rat liver

Rat liver was perfused in situ via the portal vein without recirculation: 1) Nerve stimulation (20 Hz, 2 ms, 20 V) increased glucose output and shifted lactate uptake to output; the alterations were diminished by oleate but not octanoate. 2) Glucagon (1nM) stimulated glucose output maximally also in the presence of the fatty acids, so that nerve stimulation could not increase it further. The hormone also enhanced lactate uptake and nerve stimulation counteracted this effect. The counteraction was diminished by oleate but not octanoate. 3) Insulin (100nM) slightly lowered glucose output and had no effect on lactate balance. It antagonized the increase of glucose output by nerve stimulation, but left the shift of lactate uptake to release unaffected. These events were not influenced by the fatty acids. 4) Nerve stimulation decreased ketone body production from oleate and octanoate. 5) Glucagon increased ketogenesis from oleate, but not octanoate. In the presence of glucagon nerve stimulation also lowered ketogenesis. This decrease was diminished in the presence of oleate. 6) Insulin lowered ketogenesis from oleate but not octanoate. In the presence of insulin nerve stimulation decreased ketogenesis; the relative change was independent of the fatty acids. The complex interactions between fatty acids, glucagon and insulin in the modulation of sympathetic nerve actions can be summarized as follows: Oleate, which enters the mitochondria via the carnitine system, but not octanoate, which enters independently from this system, as well as insulin but not glucagon effectively modulated the nerve actions on carbohydrate metabolism. Glucagon but not insulin modulated the nerve effects on ketogenesis from oleate but not octanoate. The regulatory interactions between substrates, hormones and nerves can best be explained on the basis of the model of metabolic zonation.     

Octanoate oxidation measured by 13C-NMR spectroscopy in rat skeletal muscle, heart, and liver.

Contribution of octanoate to the oxidative metabolism of the major sites of fatty acid oxidation (heart, liver, and resting and contracting skeletal muscle) was assessed in the intact rat with 13C-NMR spectroscopy. Under inhalation anesthesia, [2,4,6,8-13C4]octanoate was infused into the jugular vein and the sciatic nerve of one limb was stimulated for 1 h. Octanoate was a principal contributor to the acetyl-CoA pool in all tissues examined, with highest oxidation occurring in heart and soleus muscle followed by predominantly red portion of gastrocnemius muscle (RG), liver, and then white portion of gastrocnemius muscle (WG). Fractional contribution of 13C-labeled octanoate to the acetyl-CoA pool (Fc2) was 0.563 +/- 0.066 for heart and 0.367 +/- 0.054 for liver. Significant differences were observed between each of the muscle types during both rest and contraction. In muscle, Fc2 was highest in soleus (0.565 +/- 0.089 rested, 0.564 +/- 0.096 contracted), followed by RG (0.470 +/- 0.092 rested, 0.438 +/- 0.072 contracted), and lowest in WG (0.340 +/- 0.081 rested, 0.272 +/- 0.065 contracted). Our findings demonstrate that the fractional contribution of octanoate to oxidative metabolism correlates with oxidative capacity of the tissue and that octanoate metabolism increases in contracted muscle in proportion to the overall increase in oxidative rate.     

Profiling substrate fluxes in the isolated working mouse heart using 13C-labeled substrates: focusing on the origin and fate of pyruvate and citrate carbons

The availability of genetically modified mice requires the development of methods to assess heart function and metabolism in the intact beating organ. With the use of radioactive substrates and ex vivo perfusion of the mouse heart in the working mode, previous studies have documented glucose and fatty acid oxidation pathways. This study was aimed at characterizing the metabolism of other potentially important exogenous carbohydrate sources, namely, lactate and pyruvate. This was achieved by using (13)C-labeling methods. The mouse heart perfusion setup and buffer composition were optimized to reproduce conditions close to the in vivo milieu in terms of workload, cardiac functions, and substrate-hormone supply to the heart (11 mM glucose, 0.8 nM insulin, 50 microM carnitine, 1.5 mM lactate, 0.2 mM pyruvate, 5 nM epinephrine, 0.7 mM oleate, and 3% albumin). The use of three differentially (13)C-labeled carbohydrates and a (13)C-labeled long-chain fatty acid allowed the quantitative assessment of the metabolic origin and fate of tissue pyruvate as well as the relative contribution of substrates feeding acetyl-CoA (pyruvate and fatty acids) and oxaloacetate (pyruvate) for mitochondrial citrate synthesis. Beyond concurring with the notion that the mouse heart preferentially uses fatty acids for energy production (63.5 +/- 3.9%) and regulates its fuel selection according to the Randle cycle, our study reports for the first time in the mouse heart the following findings. First, exogenous lactate is the major carbohydrate contributing to pyruvate formation (42.0 +/- 2.3%). Second, lactate and pyruvate are constantly being taken up and released by the heart, supporting the concept of compartmentation of lactate and glucose metabolism. Finally, mitochondrial anaplerotic pyruvate carboxylation and citrate efflux represent 4.9 +/- 1.8 and 0.8 +/- 0.1%, respectively, of the citric acid cycle flux and are modulated by substrate supply. The described (13)C-labeling strategy combined with an experimental setup that enables continuous monitoring of physiological parameters offers a unique model to clarify the link between metabolic alterations, cardiac dysfunction, and disease development.     

Carbohydrate metabolism during prolonged exercise and recovery: interactions between pyruvate dehydrogenase, fatty acids, and amino acids.

During prolonged exercise, carbohydrate oxidation may result from decreased pyruvate production and increased fatty acid supply and ultimately lead to reduced pyruvate dehydrogenase (PDH) activity. Pyruvate also interacts with the amino acids alanine, glutamine, and glutamate, whereby the decline in pyruvate production could affect tricarboxycylic acid cycle flux as well as gluconeogenesis. To enhance our understanding of these interactions, we studied the time course of changes in substrate utilization in six men who cycled at 44+/-1% peak oxygen consumption (mean+/-SE) until exhaustion (exhaustion at 3 h 23 min+/-11 min). Femoral arterial and venous blood, blood flow measurements, and muscle samples were obtained hourly during exercise and recovery (3 h). Carbohydrate oxidation peaked at 30 min of exercise and subsequently decreased for the remainder of the exercise bout (P<0.05). PDH activity peaked at 2 h of exercise, whereas pyruvate production peaked at 1 h of exercise and was reduced (approximately 30%) thereafter, suggesting that pyruvate availability primarily accounted for reduced carbohydrate oxidation. Increased free fatty acid uptake (P<0.05) was also associated with decreasing PDH activity (P<0.05) and increased PDH kinase 4 mRNA (P<0.05) during exercise and recovery. At 1 h of exercise, pyruvate production was greatest and was closely linked to glutamate, which was the predominant amino acid taken up during exercise and recovery. Alanine and glutamine were also associated with pyruvate metabolism, and they comprised approximately 68% of total amino-acid release during exercise and recovery. Thus reduced pyruvate production was primarily associated with reduced carbohydrate oxidation, whereas the greatest production of pyruvate was related to glutamate, glutamine, and alanine metabolism in early exercise.     

Development and regulation of ketogenesis in hepatocytes isolated from newborn rats.

The development of fatty acid metabolism was studied in isolated hepatocytes from newborn rats. Ketone-body production from oleate is increased 6-fold between 0 and 16 h after birth. This increase is related to an enhanced beta-oxidation rather than to a channeling of acetyl-CoA from the tricarboxylic acid cycle to ketone-body synthesis. The increase in oleate oxidation is not related to a decreased esterification rate, as the latter is already low at birth and does not decrease further. At birth, lipogenic rate is 2-3-fold lower than in fed adult rats and it decreases to undetectable values in 16 h-old rats. A 90% inhibition of lipogenesis in hepatocytes of newborn rats (0 h) by glucagon and 5-(tetradecyloxy)-2-furoic acid does not lead to an increased oxidation of non-esterified fatty acids. This suggests that the inverse relationship between lipogenesis and ketogenesis in the starved newborn rat is not responsible for the switch-on of fatty acid oxidation at birth. Moreover, ketogenesis from octanoate, a medium-chain fatty acid the oxidation of which is independent of carnitine acyltransferase, follows the same developmental pattern at birth as that from oleate.     

Fatty acid oxidation and its impact on response of spontaneously hypertensive rat hearts to an adrenergic stress: benefits of a medium-chain fatty acid

The spontaneously hypertensive rat (SHR) is a model of cardiomyopathy characterized by a restricted use of exogenous long-chain fatty acid (LCFA) for energy production. The aims of the present study were to document the functional and metabolic response of the SHR heart under conditions of increased energy demand and the effects of a medium-chain fatty acid (MCFA; octanoate) supplementation in this situation. Hearts were perfused ex vivo in a working mode with physiological concentrations of substrates and hormones and subjected to an adrenergic stimulation (epinephrine, 10 microM). (13)C-labeled substrates were used to assess substrate selection for energy production. Compared with control Wistar rat hearts, SHR hearts showed an impaired response to the adrenergic stimulation as reflected by 1) a smaller increase in contractility and developed pressure, 2) a faster decline in the aortic flow, and 3) greater cardiac tissue damage (lactate dehydrogenase release: 1,577 +/- 118 vs. 825 +/- 44 mU/min, P < 0.01). At the metabolic level, SHR hearts presented 1) a reduced exogenous LCFA contribution to the citric acid cycle flux (16 +/- 1 vs. 44 +/- 4%, P < 0.001) and an enhanced contribution of endogenous substrates (20 +/- 4 vs. 1 +/- 4%, P < 0.01); and 2) an increased lactate production from glycolysis, with a greater lactate-to-pyruvate production ratio. Addition of 0.2 mM octanoate reduced lactate dehydrogenase release (1,145 +/- 155 vs. 1,890 +/- 89 mU/min, P < 0.001) and increased exogenous fatty acid contribution to energy metabolism (23.7 +/- 1.3 vs. 15.8 +/- 0.8%, P < 0.01), which was accompanied by an equivalent decrease in unlabeled endogenous substrate contribution, possibly triglycerides (11.6 +/- 1.5 vs. 19.0 +/- 1.2%, P < 0.01). Taken altogether, these results demonstrate that the SHR heart shows an impaired capacity to withstand an acute adrenergic stress, which can be improved by increasing the contribution of exogenous fatty acid oxidation to energy production by MCFA supplementation.     

Coronary nitric oxide production controls cardiac substrate metabolism during pregnancy in the dog

The aim of this study was to examine the role of nitric oxide (NO) in the control of cardiac metabolism at 60 days of pregnancy (P60) in the dog. There was a basal increase in diastolic coronary blood flow during pregnancy and a statistically significant increase in cardiac output (55 +/- 4%) and in cardiac NOx production (44 +/- 4 to 59 +/- 3 nmol/min, P < 0.05). Immunohistochemistry of the left ventricle showed an increase in endothelial nitric oxide synthase staining in the endothelial cells at P60. NO-dependent coronary vasodilation (Bezold-Jarisch reflex) was increased by 20% and blocked by N(G)-nitro-l-arginine methyl ester (l-NAME). Isotopically labeled substrates were infused to measure oleate, glucose uptake, and oxidation. Glucose oxidation was not significantly different in P60 hearts (5.4 +/- 0.5 vs. 6.2 +/- 0.4 micromol/min) but greatly increased in response to l-NAME injection (to 19.9 +/- 0.9 micromol/min, P < 0.05). Free fatty acid (FFA) oxidation was increased in P60 (from 5.3 +/- 0.6 to 10.4 +/- 0.5 micromol/min, P < 0.05) and decreased in response to l-NAME (to 4.5 +/- 0.5 micromol/min, P < 0.05). There was an increased oxidation of FFA for ATP production but no change in the respiratory quotient during pregnancy. Genes associated with glucose and glycogen metabolism were downregulated, whereas genes involved in FFA oxidation were elevated. The acute inhibition of NO shifts the heart away from FFA and toward glucose metabolism despite the downregulation of the carbohydrate oxidative pathway. The increase in endothelium-derived NO during pregnancy results in a tonic inhibition of glucose oxidation and reliance on FFA uptake and oxidation to support ATP synthesis in conjunction with upregulation of FFA metabolic enzymes.     

Peroxisomal and mitochondrial oxidation of fatty acids in the heart, assessed from the 13C labeling of malonyl-CoA and the acetyl moiety of citrate

We previously showed that a fraction of the acetyls used to synthesize malonyl-CoA in rat heart derives from partial peroxisomal oxidation of very long and long-chain fatty acids. The 13C labeling ratio (malonyl-CoA)/(acetyl moiety of citrate) was >1.0 with 13C-fatty acids, which yields [13C]acetyl-CoA in both mitochondria and peroxisomes and < 1.0 with substrates, which yields [13C]acetyl-CoA only in mitochondria. In this study, we tested the influence of 13C-fatty acid concentration and chain length on the labeling of acetyl-CoA formed in mitochondria and/or peroxisomes. Hearts were perfused with increasing concentrations of labeled docosanoate, oleate, octanoate, hexanoate, butyrate, acetate, or dodecanedioate. In contrast to the liver, peroxisomal oxidation of 1-13C-fatty acids in heart does not form [1-13C]acetate. With [1-13C]docosanoate and [1,12-13C2]dodecanedioate, malonyl-CoA enrichment plateaued at 11 and 9%, respectively, with no detectable labeling of the acetyl moiety of citrate. Thus, in the intact rat heart, docosanoate and dodecanedioate appear to be oxidized only in peroxisomes. With [1-13C]oleate or [1-13C]octanoate, the labeling ratio >1 indicates the partial peroxisomal oxidation of oleate and octanoate. In contrast, with [3-13C]octanoate, [1-13C]hexanoate, [1-13C]butyrate, or [1,2-13C2]acetate, the labeling ratio was <0.7 at all concentrations. Therefore, in rat heart, (i) n-fatty acids shorter than 8 carbons do not undergo peroxisomal oxidation, (ii) octanoate undergoes only one cycle of peroxisomal beta-oxidation, (iii) there is no detectable transfer to the mitochondria of acetyl-CoA from the cytosol or the peroxisomes, and (iv) the capacity of C2-C18 fatty acids to generate mitochondrial acetyl-CoA decreases with chain length.     

Metabolism of saturated fatty acids by Paramecium tetraurelia

Paramecium requires oleate for growth. The phospholipids of the ciliate contain high concentrations of palmitate and 18- and 20-carbon unsaturated fatty acids. We previously showed that radiolabeled oleate is desaturated and elongated to provide these 18- and 20-carbon unsaturated acids. We now report on saturated fatty acid (SFA) metabolism in Paramecium. Radiolabeled palmitate and stearate were incorporated directly into cellular phospholipids with little or no desaturation and/or elongation. Radiolabeled acetate, malonate, pyruvate, citrate, or glucose added to cultures were not incorporated into cellular phospholipid fatty acids indicating that these exogenously supplied putative precursors were not utilized for fatty acid synthesis by Paramecium. Radiolabel from octanoate or hexanoate appeared in fatty acyl groups of phospholipids, possibly by partial beta-oxidation and reincorporation of the label. Under oleate-free conditions in which cultures do not grow, radiolabel from these shorter chain SFA were beta-oxidized and preferentially used for the formation of arachidonate, the major end-product of fatty acid synthesis in Paramecium. Cerulenin inhibited culture growth apparently by inhibiting de novo fatty acid synthesis. Cerulenin-treated cells did not incorporate radioactivity from [1-14C]octanoate into esterified palmitate. However, total saponifiable phospholipid fatty acids, including SFA, per cell increased under these conditions.     

Nitric oxide generation by endothelial cells exposed to shear stress in glass tubes perfused with red blood cell suspensions: role of aggregation

When recovering from heart failure (HF), the myocardium displays a marked plasticity and can regain normal gene expression and function; however, recovery of substrate oxidation capacity has not been explored. We tested whether cardiac functional recovery is matched by normalization of energy substrate utilization during post-HF recovery. HF was induced in dogs by pacing the left ventricle (LV) at 210-240 beats/min for 4 wk. Tachycardia was discontinued, and the heart was allowed to recover. An additional group was studied in HF, and healthy dogs served as controls (n = 8/group). Cardiac free fatty acids (FFAs) and glucose oxidation were measured with [3H]oleate and [14C]glucose. At 10 days of recovery, hemodynamic parameters returned to control values; however, the contractile response to dobutamine remained depressed, LV end-diastolic volume was 28% higher than control, and the heart mass-to-body mass ratio was increased (9.8 +/- 0.4 vs. 7.5 +/- 0.2 g/kg, P < 0.05). HF increased glucose oxidation (76.8 +/- 19.7 nmol.min(-1).g(-1)) and decreased FFA oxidation (20.7 +/- 6.4 nmol.min(-1).g(-1)), compared with normal dogs (24.5 +/- 6.3 and 51.7 +/- 9.6 nmol.min(-1).g(-1), respectively), and reversed to normal values at 10 days of recovery (25.4 +/- 6.0 and 46.6 +/- 6.7 nmol.min(-1).g(-1), respectively). However, similar to HF, the recovered dogs failed to increase glucose and fatty acid uptake in response to pacing stress. The activity of myocardial citrate synthase and aconitase was significantly decreased during recovery compared with that in control dogs (58 and 27% lower, respectively, P < 0.05), indicating a persistent reduction in mitochondrial oxidative capacity. In conclusion, cardiac energy substrate utilization is normalized in the early stage of post-HF recovery at baseline, but not under stress conditions.     

Altered cardiac metabolic phenotype after prolonged inhibition of NO synthesis in chronically instrumented dogs

Acute inhibition of nitric oxide (NO) synthase causes a reversible alteration in myocardial substrate metabolism. We tested the hypothesis that prolonged NO synthase inhibition alters cardiac metabolic phenotype. Seven chronically instrumented dogs were treated with N(omega)-nitro-L-arginine methyl ester (L-NAME, 35 mg.kg(-1).day(-1) po) for 10 days to inhibit NO synthesis, and seven were used as controls. Cardiac free fatty acid, glucose, and lactate oxidation were measured by infusion of [(3)H]oleate, [(14)C]glucose, and [(13)C]lactate, respectively. After 10 days of L-NAME administration, despite no differences in left ventricular afterload, cardiac O(2) consumption was significantly increased by 30%, consistent with a marked enhancement in baseline oxidation of glucose (6.9 +/- 2.0 vs. 1.7 +/- 0.5 micromol.min(-1).100 g(-1), P < 0.05 vs. control) and lactate (21.6 +/- 5.6 vs. 11.8 +/- 2.6 micromol.min(-1).100 g(-1), P < 0.05 vs. control). When left ventricular afterload was increased by ANG II infusion to stimulate myocardial metabolism, glucose oxidation was augmented further in the L-NAME than in the control group, whereas free fatty acid oxidation decreased. Exogenous NO (diethylamine nonoate, 0.01 micromol.kg(-1).min(-1) iv) could not reverse this metabolic alteration. Consistent with the accelerated rate of carbohydrate oxidation, total myocardial pyruvate dehydrogenase activity and protein expression were higher (38 and 34%, respectively) in the L-NAME than in the control group. Also, protein expression of the constitutively active glucose transporter GLUT-1 was significantly elevated (46%) vs. control. We conclude that prolonged NO deficiency causes a profound alteration in cardiac metabolic phenotype, characterized by selective potentiation of carbohydrate oxidation, that cannot be reversed by a short-term infusion of exogenous NO. This phenomenon may constitute an adaptive mechanism to counterbalance cardiac mechanical inefficiency.     

Transmembrane transport of fatty acids in the heart

Summary Although fatty acid uptake by the myocardium is rapid and efficient, the mechanism of their transmembrane transport has been unclear. Fatty acids are presented to the plasma membrane of cardiomyocytes as albumin complexes within the plasma. Since albumin is not taken up by the cells, it was postulated that specific high affinity binding sites at the sarcolemma may mediate the dissociation of fatty acids from the albumin molecules, before they are transported into the cells. In studies with a representative long-chain fatty acid, oleate, it was in fact shown that fatty acids bind with high affinity to isolated plasma membranes of rat heart myocytes revealing a K_D of 42 nM. Moreover, a specific membrane fatty acid-binding protein (MFABP) was isolated from these membranes. It had a molecular weight of 40 kD, an isoelectric point of 9.0, and lacked carbohydrate or lipid components. Binding to a specific membrane protein might represent the first step of a carrier mediated uptake process. Therefore, the uptake kinetics of oleate by isolated rat heart myocytes was determined under conditions where only cellular influx and not metabolism occurred. Uptake revealed saturation kinetics and was temperature dependent which were considered as specific criteria for a facilitated transport mechanism. For evaluation whether uptake is mediated by MFABP, the effect of a monospecific antibody to this protein on cellular influx of oleate was examined. Inhibition of uptake of fatty acids but not of glucose by the antibody to MFABP indicated the physiologic significance of this protein as transmembrane carrier in the cellular uptake process of fatty acids. Such a transporter might represent an important site for the metabolic regulation of fatty acid influx into the myocardium.     

A more detailed fatty acid composition of human lipoprotein(a)--a comparison with low density lipoprotein

Lipoprotein(a)'s (Lp(a)'s) fatty acid composition is partially known for the cholesteryl ester (CE), triglyceride (TG) and total phospholipid (PL) fractions. Individual PLs' fatty acids are unknown. This study sought to confirm and extend existing data and elucidate the individual PLs of Lp(a). For Lp(a) versus LDL, the mole percentage saturated fatty acids comprised 11.3+/-1.3 versus 16.8+/-1.2 (CE) (P<0.05), 43.4+/-5.2 versus 39.2+/-4.0 (TG) (P<0.05), 55.7+/-6.3 versus 54.7+/-5.9 (PL) (P>0.05), 51.9+/-3.5 versus 50.2+/-4.2 (choline-containing phospholipids (PC)) (P>0.05), 40.2+/-4.6 versus 43.1+/-3.9 (ethanolamine-containing phospholipids (PE)) (P>0.05), 73.2+/-7.6 versus 81.2+/-8.2 (sphingomyelin (SPH)) (P<0.05). Linoleic acid was CE's major fatty acid and while palmitic acid was the major fatty acid in all other fractions except PE.     

Effects of phosphodiesterase inhibitors on glucose utilization in isolated cardiac myocytes

The phosphodiesterase (PDE) inhibitor, enoximone, enhances the oxidation of fatty acids in cardiac myocytes. Since carbohydrate oxidation is tightly coupled and inversely related in cardiac tissue to fatty acid oxidation, this study was designed to investigate enoximone's effects on glucose metabolism in the heart. To determine if enoximone alters this reciprocal relationship, the effects of enoximone on [U-¹⁴C]glucose and [2-¹⁴C]pyruvate oxidation were determined in isolated cardiac myocytes. The effect of PDE inhibitors was also examined on pyruvate dehydrogenase complex (PDH) activity, a key component of oxidative glucose metabolism. Two PDE inhibitors, enoximone and milrinone, decreased PDH activity by 69 and 64%, respectively at 0.5 mM. This inhibition of PDH activity by enoximone was completely reversed after removing enoximone from the myocyte medium. PDH activity was unaffected by agents which alter cyclic nucleotide signaling: cGMP, dibutyryl cyclic AMP, and AMP. The effect of enoximone on [2-¹⁴C]pyruvate oxidation was similar to that on PDH. Interestingly, the oxidation of glucose was decreased 35% by 0.5 mM enoximone. In isolated rat heart mitochondria (RHM), enoximone decreased PDH activity by 37%. These studies suggest that PDE inhibitors decrease carbohydrate utilization by inhibiting the PDH complex in the heart. The inhibition of PDH by PDE inhibitors appears unrelated to their effects on cAMP or cGMP. This inhibition of PDH by PDE inhibitors may occur, at least in part, secondary to stimulating fatty acid oxidation.