SUBSTRATE UTILIZATION BY THE HEART
One of the particularities of cardiac muscle is its ability to produce energy from a wide range of substrates, including dietary fatty acids (FAs) and carbohydrates, but also from various molecules produced by metabolism, such as lactate, pyruvate, and ketone bodies. The cardiac muscle shifts continuously from one source of energy to another, according to the supply availability in the blood elicited by food, exercise, or pathophysiologic state. In the human heart, FAs are considered to account for 60-70% of oxygen consumption for energy production (Fig. 1A). The FAs are first acylated in acyl CoA by the acyl CoA synthase. The acyl CoA enters the mitochondria using the carnitine shuttle and undergoes β-oxidation to produce hydrogen and acetyl CoA. Acetyl CoA is further converted to hydrogen in the Krebs cycle. The hydrogen atoms produced by these reactions allow the production of NADH2 and FADH2, which, in turn, induce the production of ATP through the cytochrome chain and the mitochondrial ATPase system. Carbohydrates are used by the heart in the form of glucose-6-phosphate (G6P) produced from either glucose or glycogen. G6P is converted to pyruvate in a multistep reaction, and pyruvate is then further metabolized into acetyl CoA by pyruvate dehydrogenase to enter the Krebs cycle. Pyruvate can also be produced from lactate by lactate dehydrogenase. Acetyl CoA therefore appears to be a key step in the metabolic pathways from the three main energetic substrates of the heart (FAs, glucose, and lactate). However, FA utilization may vary up to extreme values according to situations (nutritional or pathophysiologic) that occur within the same day, and the adaptation of heart must be very rapid.
The consumption of a carbohydrate-rich meal increases blood levels of glucose and insulin. Insulin favors glucose uptake and decreases FA uptake by the heart (Fig. 1B). Such conditions lower the cytoplasmic acyl CoA availability, reduce the β-oxidation pathway, and increase the contribution of the carbohydrate pathway to energy production. The high glucose and low FA availability contribute to rapid conversion of the pyruvate dehydrogenase from the inactive to the active form, which increases the glycolytic flux. Under such conditions, carbohydrates (including lactate) become the major substrate and may contribute 60 to 100% of oxygen consumption for energy production. Lactate can be an important substrate for the heart when its blood concentration increases noticeably, as in exercise (1). This pathway involves inhibition of glucose oxidation by citrate and a reduction in fatty acid oxidation by inhibition of the thiokinase, an enzyme of the β-oxidation pathway.
During fasting, the FAs are oxidized in the heart preferentially to other substrates. The free FA concentration in blood is high, which increases the FA uptake by the heart, and allows the FAs to become the major source of energy and to account for up to 80% of oxygen consumption (Fig. 1C). This situation is associated with increased β-oxidation favored by the intracellular FA availability, and also with a decrease in the utilization of carbohydrates (2). Two major mechanisms account for this reduction in glycolytic flux: an accumulation of citrate, which inhibits phosphofructokinase, the second kinase of the glycolytic chain, and a reduced activation of pyruvate dehydrogenase. Similar conditions can be observed in pathophysiologic situations such as diabetes mellitus (3) and possibly in excess catecholamine syndromes that increase lipolysis in adipose tissue. The FA contribution can be even more increased during postprandial lipemia induced by fat-rich food intake (3). The triglycerides transported by the lipoproteins are used by the heart after FA release by lipoprotein lipases, and the FAs enter the oxidation pathways.
When the oxygen supply decreases below the cardiac cell requirement, such as in mild ischemic conditions, the glycolytic flux is increased and the FA uptake is reduced to meet the reduction in mitochondrial oxidation activity (Fig. 1D). The release of lactate dehydrogenase, the modification of the substrate-to-product ratio, and the decreased activity in the Krebs cycle contribute to lactate production, and the glycolysis becomes partly anaerobic. Such a situation is associated with a more or less pronounced energy deficiency.
THE COST IN OXYGEN
As stated above, the heart can shift very rapidly from the use of one substrate to another so as to produce the energy required by its activity and any increase of activity required by physiologic or pathologic events. It has been known since the 1960s that all of these substrates compete for oxygen (4), the preferential route being partly controlled by the intracellular citrate level. However, these substrates are not equivalent in terms of oxygen requirement for ATP production (Fig. 2). One molecule of glucose requires 12 oxygen atoms (two per carbon) to produce 38 ATP molecules 6.3 per carbon). The oxidation of one molecule of palmitate produces 130 ATP molecules (8.2 per carbon) and requires 46 oxygen atoms (2.8 per carbon). In this context, FAs appear to be the most efficient substrate for energy production in terms of fuel supply, but the worst in terms of oxygen consumption. Under physiologic conditions, the oxygen supply is not a limiting factor, and substrate availability is the main determinant of the fuel source (e.g., postprandial vs. fasting, lactate level elicited by exercise). In pathologic conditions, when the oxygen supply becomes insufficient, the shift from an FA pathway to a carbohydrate pathway can thus increase by 15-20% the amount of ATP that can be produced from the available oxygen. This shift might enable the heart to resist the mild hypoxic of conditions induced by moderate impairment of coronary flow and in more severe ischemic conditions as well, although more briefly.
Such an increase in the oxygen cost of cardiac contraction was observed in the stunned myocardium (5,6). Under these conditions, neither the β-oxidation rate nor the [1-14C]-palmitate incorporation into triglycerides and phospholipids was modified compared with the physiologic myocardium (Fig. 3). However, the ratio of β-oxidation rate to oxygen consumption was increased in the stunned myocardium (6). The resulting increased participation of FA oxidation in the oxygen consumption might be partly responsible for the reduced myocardial metabolic efficiency. These data underscore the deleterious effect of an excess FA oxidation rate and the interest in avoiding the β-oxidation pathway under ischemic and/or reperfusion conditions.
IS IT POSSIBLE TO INFLUENCE SUBSTRATE SELECTION?
A dietary approach
The possibility of influencing cardiac metabolism by switching the fuel used by the myocardium could become strategically important. It is possible to interfere with the metabolic choice at different levels. One of these possible interventions is related to a dietary approach. It is well known that modification in the composition of dietary polyunsaturated FA intake can induce alterations in the tissue membranes, which in the heart are particularly pronounced (7). Most of the cardiac muscle membrane systems are affected by these modifications in composition, including the mitochondrial membranes. An example is given in Fig. 4 (8), which shows that adding fish oil to the diet of rats induces membrane alterations that strongly affect both arachidonic acid (20:4 n-6) and docosahexaenoic (22:6 n-3), two FAs often considered to play a functional role. In these n-3 PUFA-rich rats, the mitochondrial activity is characterized (Fig. 5A) by a slight decrease in oxygen consumption and creatine phosphate production when palmitoylcarnitine is used as substrate (9). As shown in Fig. 5B, the mitochondrial activity recovers completely during postischemic reperfusion when the hearts are rich in n-3 polyunsaturated FAs (FO group). Conversely, the recovery is significantly lower in hearts of the n-6-rich group (SSO group) and the stunning is more pronounced. Although it is not possible from the published data to conclude that the postischemic recovery is correlated with the low mitochondrial oxygen consumption under basal conditions, the data reviewed here strongly support this hypothesis, and point out that diet-induced membrane alterations may contribute to enhanced cardiac resistance by influencing energy production.
It has often been reported that when the cardiac FA intake is elevated (e.g., excess blood free FAs), the increase in oxygen uptake is higher than expected from the ATP production requirements. Opie (3) suggested that this could be due to the activation by FAs of futile metabolic cycles that may induce wastage of ATP, and thus of oxygen, when ATP production must be maintained. Similar futile cycles were also suggested to describe the calcium effects on mitochondria. At low calcium concentrations, respiration in isolated rat cardiac mitochondria induces a complete disappearance of ADP, with a correlated increase of ATP (and creatine phosphate when creatine is present), but AMP cannot be detected. As shown in Fig. 6, when respiration is studied in a medium with increased calcium concentrations (at concentrations close to those reported in ischemic cardiomyocytes), ATP production is not complete and part of the oxygen is used for production of a noticeable amount of AMP (9a). This futile cycle of oxygen, which reduces the ATP produced per oxygen atom, occurs only with palmitoylcarnitine as substrate and was not observed when pyruvate was used to model the glycolysis pathway.
Interestingly, the calcium-induced decrease in mitochondrial ADP/O ratio can be influenced by the polyunsaturated FA profile of cardiac mitochondria. Increasing the n-3 polyunsaturated fatty acids in mitochondria phospholipids may lower the sensitivity to calcium (9). This phenomenon could be related to the lower β-oxidation capacity and might contribute to the lower stunning observed in the heart of fish oil-fed rats. Increasing the β-oxidation rate in normal as well as stunned hearts would allow excess acetate production and reactivation of this accumulated acetate in acetyl CoA by acetyl CoA synthase. This reaction would be responsible for the energy wastage (constant AMP production), which decreases the metabolic efficiency observed in the whole heart under conditions of excess FA oxidation (increased exogenous FA or stunning) and the resulting contractile abnormalities.
A pharmacologic approach
Another possible approach is the use of molecules designed to interfere with cardiac metabolism to limit the use of FAs (and/or to enhance the glycolytic flux). Partial inhibition of mitochondrial β-oxidation can hardly be considered as a possible strategy. Several cases have been reported in the literature of inborn deficiencies affecting one or more of the enzymes involved in β-oxidation (10). These defects in the mitochondrial oxidation of long-chain FAs are known to induce severe cardiomyopathy and alterations in blood and urine composition involving a wide range of compounds, including glucose, FAs, ketone bodies, carnitine, lactate, ammonia, organic acids, and glycine conjugates. Similarly, a number of studies have described the lipidosis and cardiomyopathy induced in several species by feeding rapeseed oil. These hearts were characterized by lipid droplet deposition and increases in triglyceride stores. The major component (50%) of rapeseed oil is the long-chain erucic acid (22:1 n-9), which was shown to be a very poor substrate and to inhibit β-oxidation in cardiac myocytes (11), a phenomenon that could in turn result in triglyceride overstorage when the FA uptake is not altered.
Other attempts were made to lower FA utilization for energy by decreasing the transfer of long-chain FAs into the mitochondria by inhibition of palmitoylcarnitine transferase. Etomoxir, a selective inhibitor of palmitoylcarnitine transferase 1, is a good example of these attempts. The molecule, developed as a hypoglycemic drug, was shown to increase glucose oxidation in normal and diabetic hearts (12). The mechanism of the shift in cardiac fuel utilization was associated with an increase in pyruvate dehydrogenase and phosphofructokinase activity (13), which should be correlated with a reduction in citrate level due to a decrease in β-oxidation. However, treatment of rats with etomoxir induces changes in the cardiac phenotype, as evidenced by alterations in the expression of myosin isoenzymes and sarcoplasmic calcium pumps, which result in cardiac growth (14). These changes might be attributed to the triglyceride excess and lipidosis described with etomoxir and with erucic acid. The pathways that lead from reduction of FA utilization to phenotypic changes are not fully understood, but a large decrease in cardiac FA consumption has been reported to induce cardiac growth. Panos and Finerty (15) observed the induction of cardiac growth by fat-free diets.
Trimetazidine, another interesting molecule, was introduced as an antianginal drug several years ago (16) and was reported to exert beneficial effects on ischemic injury (17) without displaying any hemodynamic activity. In vitro experiments have outlined the cardioprotective effect of trimetazidine, suggesting that the molecule may interfere with calcium accumulation and/or intracellular acidosis (18). Trimetazidine was reported to decrease the cardiac work in isolated rat heart under normoxic conditions and was therefore suggested to favor energy preservation by limiting the decrease in intracellular ATP content caused by ischemia (19). The molecule exerts a direct effect on the cultured cardiac myocyte, as evidenced by a strong limitation of hypoxia-induced action potential decrease, and prevention of arrhythmias (20). In this protection at the cellular level, some data argue for a mechanism related to a modification of the fuel requirements in the ventricular myocytes. As shown in Fig. 7B, release of lactate dehydrogenase during hypoxia is strongly reduced in the presence of trimetazidine (21). This may favor the production of pyruvate and may limit the amount of lactate (which could be related to the altered development of acidosis). Treatment of the cells with the drug induced a slight decrease in cellular ATP, which was observed in normoxia as well as in hypoxia provided that the medium was glucose-free (Fig. 7A). This difference disappeared when glucose was added to the medium. The hypothesis was raised that the ratio of carbohydrates to FAs in fuel contribution to energy production could be enhanced in the trimetazidine-treated cells. In vitro experiments in isolated heart mitochondria (Fig. 8) confirmed that mitochondrial respiration was reduced when the substrate was palmitoylcarnitine, but was not affected when any other substrate was used (20). This difference concerned both oxygen consumption and ATP production. Although trimetazidine probably affects FA oxidation, the mechanism is still unknown and appears more complex than those described above. Although the drug has been used for years in humans, lipidosis, cardiac growth, excess of triglyceride stores, or alterations of circulating FAs have never been reported in clinical trials or experimental studies. This raises the question of whether the primary site of action of trimetazidine could be related to another pathway that may utilize the FAs for a purpose other than β-oxidation. This would decrease the use of FAs for energy production without increasing storage lipids.
The decrease in fatty acid oxidation for energy production (and the related increase in carbohydrate oxidation) is the mechanism by which the heart naturally faces a (mild) deficiency in oxygen supply. Undoubtedly, any action that would mimic and/or favor this switch would be of high efficacy in maintaining cardiac function. Moreover, the close relationship between the oxidation of FAs and their incorporation into the membrane structure is of primary importance in the cardiac cell, as in all depolarizing cells. In addition to the decrease in circulating lipids, alteration of intracellular FA utilization could become a strategic route in cardiology. There are clear experimental indications that both nutrition and pharmacology may constitute promising approaches, but this domain remains largely underexplored.
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Satellite symposium held during the XVII Congress of the European Society of Cardiology; Amsterdam, The Netherlands, August 22, 1995
Published with an educational grant provided by Groupe de Recherche Servier, France
Keywords:© Lippincott-Raven Publishers.
Fatty acids; β-Oxidation; Glucose; Pyruvate; Lactate; ATP; Myocardial contraction