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Applied physiology of the heart
Basic science
Applied physiology of the heart
supply and demand of oxygen (see Ward, CROSS REFERENCES). They are stored in adipose tissue as part of triacylglycerol and transported to the myocyte either: • within chylomicrons or very low density lipoproteins • as NEFA complexed to albumin. NEFA enter the myocyte passively down a steep diffusion gradient, maintained by interactions with fatty acid-binding pro teins in the cytoplasm. They are activated to an acyl form and ‘ferried’ through the outer and inner mitochondrial membranes as an acyl-carnitine by carnitine palmitoyl transferase trans porters. Once regenerated to the acyl form, they undergo serial β-oxidation (which involves oxidation by acetyl CoA dehydroge nase followed by the repeated release of acyl units). A molecule of acetyl CoA is produced at the end of each series. The ratelimiting steps for β-oxidation are transport of NEFA into the cell and mitochondria.
D W Quinn D Pagano
Abstract This contribution focuses on the applied physiology of the heart.
Keywords myocardial metabolism; fatty acids; glucose; tricarboxylic acid cycle; electron transport chain; NEFA; ischaemia; reperfusion; insulin
Metabolism of glucose (Figure 3) The primary source of glucose for myocyte metabolism is from the plasma, transported passively down a steep diffusion gradi ent into the cell by glucose transporters (GLUT). These proteins (which are normally stored in cytoplasmic membranes) can be recruited to the plasma membrane as needed. Five subtypes have been identified; GLUT 4 predominates in the adult sarcolemma during normal oxygenated blood flow. Glucose is initially metabolized to glycogen (glycogenesis) or by glycolysis to pyruvate or lactate (depending on oxygen avail ability; see Maughan, CROSS REFERENCES). Glycolysis occurs in the cytoplasm and the ATP generated is directed preferentially toward ion homeostasis and glycogenesis (rather than contrac tion). Pyruvate may have a number of fates, including: • decarboxylation to acetyl CoA • carboxylation to oxaloacetate and malonate • dehydrogenation to lactate. Under aerobic conditions, pyruvate is transported with a pro ton into the mitochondria for decarboxylation to acetyl CoA and carbon dioxide. This reaction is facilitated by the pyruvate de hydrogenase complex of enzymes, and involves the reduction of NAD+ to NADH. It commits acetyl CoA to the tricarboxylic acid cycle and subsequent complete oxidation of glucose, and can proceed only if there is sufficient oxygen for the electron trans port chain. If glucose transport and glycolysis exceeds glucose oxidation, glycolysis is maintained by the metabolism of pyruvate to lactic acid by lactate dehydrogenase. This process: • occurs in the cytoplasm • does not require oxygen (i.e. it is anaerobic) • regenerates the essential glycolytic co-enzymes NAD+ from NADH. Lactic acid is metabolized back to pyruvate after oxygen is reintroduced or released into the circulation and transported to the liver for gluconeogenesis. Glycogenolysis can also contribute glucose derivatives for glycolysis, generating more net produc tion of ATP than glucose. These derivatives are preferentially metabolized by oxidative (rather than anaerobic) pathways. Irrespective of the source, NADH generated within the cytoplasm must be transported to the mitochondrial matrix to donate electrons to the electron transport chain. A malate– aspartate ‘metabolic shuttle’ achieves this by using cytoplasmic
Normal myocardial metabolism Myocardial function requires energy expenditure. The pre dominant energy source within the cell is adenosine triphosphate (ATP), which is dephosphorylated to adenosine diphosphate (ADP) and inorganic phosphate (Pi). This reaction provides energy for cellular ion homeostasis, biochemical synthesis and contraction. Other energy sources include the release of Pi from creatine phosphate, ADP and adenosine monophosphate (AMP). The myocyte requires continous production of ATP to survive because it cannot be stored. This is achieved by metabolism of non-esterified fatty acids (NEFA), glucose and amino acids, which generate ATP directly or through the production of reduced AMPlinked nicotinamide and flavin dinucleotide substrates (NADH, FADH2). These compounds are capable of donating electrons to a series of important prosthetic groups (electron transport chain) on the inner membrane of the mitochondrion. The electrons pass along this chain by sequential oxidation and reduction reactions, generating ATP by oxidative phosphorylation at three potential sites before finally reducing oxygen to water. The highest yields of ATP occur with complete NEFA or glucose oxidative metabolism with the production of a key intermediary compound: acetyl coenzyme A (acetyl CoA). This generates NADH and FADH2 from reactions along the tricarboxylic acid cycle (Figure 1) that enter the electron transport chain to drive oxidative phosphorylation.
Metabolism of fatty acids (Figure 2) Fatty acids (see Gleeson, CROSS REFERENCES) are the principal source of ATP (70–80%) for the myocardium during matched
D W Quinn FRCS is a Specialist Registrar in Cardiothoracic Surgery on the West Midlands rotation, UK. Conflicts of interest: none declared. D Pagano FRCS is a Consultant Cardiothoracic Surgeon at University Hospitals Birmingham NHS Trust, Birmingham, UK. Conflicts of interest: none declared. This article was first published in Surgery 2004; 22(6): 144a–e.
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Tricarboxylic acid cycle
Acetyl CoA
Citrate synthase
Oxaloacetate
NADH
NAD: Nicotinamide adenine dinucleotide NADH: Nicotinamide adenine dinucleotide (reduced form) CoASH: Coenzyme A with sulphdryl functional group GDP: Guanosine diphosphate GTP: Guanosine triphosphate FAD: Flavin adenine dinucleotide FADH: Flavin adenine dinucleotide (reduced form)
Citrate Aconitate hydratase
Malate dehydrogenase
Isocitrate
NAD Malate
NAD
Fumarate dehydrogenase
Isocitrate dehydrogenase
Fumarate FADH
NADH Succinate dehydrogenase FAD
CoASH
2-oxoglutarate
Succinate NAD
CO2 GTP + CoASH
Oxoglutarate dehydrogenase
Succinyl CoA
Succinyl CoA synthetase
NADH
GDP + Pi Figure 1
xaloacetate to accept a proton from NADH and synthesize o malate. This is transported across the mitochondrial membranes, where the process is reversed, regenerating NADH. The oxalo acetate is returned to the cytoplasm by a reaction with glutamate, which generates transportable aspartate and 2-oxoglutarate, thereby maintaining the shuttle service. The metabolism of pyru vate to oxalate and malate allows some of the intermediates of the malate–aspartate shuttles and the tricarboxylic acid cycle to be replenished.
High concentrations of NADH and acetyl CoA generated from β-oxidation inhibit the oxidation of glucose. High concentrations of citrate (an intermediary in the tricarboxylic acid cycle) inhibit phosphofructokinase. These inhibitors suppress complete glucose oxidation via pyruvate relative to glycolysis to lactate, so reducing the competition between β- and glucose oxidation for the electron transport chain and increasing production of lactic acid. Conversely, acetyl CoA derived from pyruvate metabolism can be converted to malonyl CoA, a process stimulated by insulin. Glucose, lactate and malonyl CoA in turn diminish the availability of NEFA for subsequent β-oxidation by inhibiting the transport of NEFA to the mitochondrion.
The tricarboxylic acid cycle and the electron transport chain Acetyl CoA (from β- and glucose oxidation) participates in a series of reactions within the mitochondrial matrix near the inner mito chondrial membrane. The metabolism of acetyl CoA in turn gen erates NADH and FADH2 for the electron transport chain. NADH and FADH2 (from the tricarboxylic acid cycle and that generated and transported into the mitochondria from the cytoplasm) are made available for the electron transport chain.
Efficiency of metabolism The higher carbon content of NEFA generates more ATP per mole during β-oxidation (e.g. palmitate produces 129 molecules of ATP) than is generated per mole during aerobic (38 molecules of ATP) and anaerobic (2 molecules of ATP) glucose metabolism. The hydrophobic nature of NEFA reduces the water content of adipose stores, making them, on a weight basis, a more efficient energy store compared to tissues rich in hydrophilic glycogen. β-oxidation requires more oxygen/ATP to be generated than complete glucose oxidation, making glucose metabolism about 10–15% more oxygen efficient. The accelerated generation of lactic acid during high rates of β-oxidation and the subsequent
The relationship between the metabolism of glucose and NEFA Metabolism of glucose and NEFA compete for the delivery of acetyl CoA to the tricarboxylic acid cycle. The products of β-oxidation inhibit the transport and metabolism of glucose at a number of sites in glycolysis/glucose oxidation.
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sympathetic tone, cardiac output and release of local vasodilators (nitric oxide, adenosine, hydrogen ions). This attempts to: • redress mismatching of the supply and demand of oxygen • provide additional glucose to support anaerobic and aerobic metabolism • remove accumulating lactic acid. After exercise, lactate is remetabolized by the recovering myocytes to pyruvate and acetyl CoA, a process facilitated by high concentrations of citrate and NADH, which inhibit glyco lysis. Metabolism of glucose is slowly superseded by metabolism of NEFA during sustained, less severe exercise.
-oxidation CH3(CH2)nCH2-CH2CO.ScoA FAD Acyl-CoA dehydrogenase FADH2 CH3(CH2)nCH=CHCO.SCoA H2O Enoyl-CoA hydratase
Metabolism during ischaemia During the initial phases of low-flow ischaemia (<75% reduc tion in coronary flow), up to two-thirds of glucose derivatives are provided by glycogenolysis. Hypoxia and release of adrenaline provide the stimulus for translocation of GLUT 1 and GLUT 4 to the sarcolemma. This increases glucose extraction, which main tains or elevates glucose uptake despite low delivery. Glucose quickly replaces glycogen as the source of substrate for glyco lysis, preventing depletion of glycogen stores. Preservation of glycogen is associated with improved myocyte viability and/or function during ischaemia/reperfusion. This prevents ischaemic contracture, possibly through altered handling of calcium. Falling concentrations of intracellular ATP and rising concentrations of intracellular AMP stimulate phosphofructokinase, which accele rates glycolysis and generates ATP in the cytoplasm for ion homeostasis. Remaining oxygen provision is diverted from βoxidation to glucose oxidation by the competitive mechanisms described above, improving the efficiency of oxygen metabolism. Release of catecholamines stimulates the mobilization of NEFA from adipose tissue, but delivery and transport of NEFA to the myocyte is hampered in ischaemia. High intracytoplasmic and intramitochondrial concentrations of NEFA are associated with membrane damage by lipid peroxidation, which reduces contractility and is pro-arrhythmogenic. β-oxidation diverts glycolysis toward production of lactic acid. With prolonged lowflow ischaemia, glycolysis gradually slows, being reduced by the accumulation of lactic acid and by membrane damage. In no-flow ischaemia, endogenous delivery of glucose is eliminated and the myocytes rely on glycogenolysis. The failure to remove accumulating lactic acid uncouples glycolysis, reduces intra cellular pH and further disrupts the metabolic pathways and ion homeostasis of myocytes, and reduces their contractile function. Irrespective of coronary blood flow, a reduction in production of ATP is associated with reduced contractility. The biochemistry and mechanics of the myocardium adapts to preserve viability in some cases of chronic or repetitive acute myocardial ischaemia. These areas of myocardium show relatively higher glycogen con centrations, plasma glucose extraction and a greater reliance on glucose-based metabolism. They also show reduced contractility (hibernating myocardium). Metabolic and contractile function is reversible on reinstitution of normal blood flow.
CH3(CH2)nOHCH.CH2CO.SCoA NAD+ 3-hydroxyacyl-CoA dehydrogenase NADH CH3(CH2)nCOH=CH2CO.SCoA CoASH Acetyl-CoA acyl transferase
CH3(CH2)nCO.S CoA + CH3CO.SCoA (acetyl CoA)
FAD: Flavin adenine dinucleotide; FADH2: Flavin adenine dinucleotide (reduced form); NAD+: Nicotinamide adenine dinucleotide (oxidized form); NADH: Nicotinamide adenine dinucleotide (reduced form); CoASH: Coenzyme A with sulphdryl functional group.
The sequential removal of acyl groups during β-oxidation generates FADH and NADH, which can feed into the electron transfer chain and acetyl CoA, which is the main substrate for the tricarboxylic acid cycle. Figure 2
clearance of intracellular protons leads to other intracellular ion imbalances (e.g. sodium, calcium), which require ATP for homeo static restoration. NEFA metabolism at a cellular level is less efficient at releasing ATP for cell use than glucose metabolism.
Eating and exercise Metabolism of NEFA predominates between meals, supplied by mobilization of triacyglycerol from adipose tissue. Intracellular glycogen provides for glycolysis until a meal rich in carbohydrate is eaten. Postprandial increases in the concentration of glucose in plasma stimulate insulin release and consequent increased transport and metabolism of glucose and glycogenesis, while also inhibiting β-oxidation. Glucose is the predominant source of ATP in these circumstances. As high-intensity exercise begins, additional ATP is provided from creatine phosphate and glycogenolysis, before anaerobic metabolism of glucose increases due to relative oxygen deficiency. Increases in coronary blood flow occur in response to increased
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Metabolism during reperfusion Re-establishing blood flow, glucose delivery and removal of lactic acid accelerates glycolysis and increases ATP in the cytoplasm. 200
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Glycolysis Glucose +ve
Glycogen
ATP
Glycogen phosphorylase
Hexokinase
Glucose
Pi
ADP Glucose-6phosphate
Glucose-1phosphate Glucose phosphate isomerase
Fructose-6phosphate
ATP Citrate H+
–ve
ATP Phosphofructokinase (PFK) ADP
+ve Fructose-1,6bisphosphate
F-6-P F-2,6 bisP AMP
Dihydroxyacetone phosphate
Glycerol-3phosphate GAPDH
Pi
NAD+
NADH 3-phosphoglycerol phosphate ADP: Adenosine diphosphate ATP: Adenosine triphosphate Pi: Inorganic phosphate AMP: Adenosine monophosphate GAPDH: D-glyceraldehyde phosphate dehydrogenase NAD+: Nicotinamide adenine dinucleotide (oxidized form) NADH: Nicotinamide adenine dinucleotide (reduced form) PC: Pyruvate carboxylase
ADP Phosphoglycerate kinase ATP 3-Phosphoglycerate Phosphoglycerolmutase
2-Phosphoglycerate
Phosphoenolpyruvate Pyruvate kinase
ADP
ATP PC
–ve +ve
ATP
ADP
Pyruvate NADH Lactate
NADH NAD+
Acetyl-CoA
The common pathway for the metabolism of glucose to pyruvate (glycolysis) generates less net ATP (2 ATP molecules consumed, 4 generated) than does the metabolism of glycogen to pyruvate (1 ATP consumed, 4 generated). The conversion to pyruvate or lactate is dependent on oxygen availability. The conversion to lactate generates NAD+, which is capable of maintaining glycolysis by continuing to provide NAD+ for the conversion of glycerol-3-phosphate to 3-phosphoglycerol phosphate. Lactate can be metabolized back to pyruvate with the reintroduction of oxygen. Figure 3
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The reintroduction of an adequate delivery of oxygen permits a return to glucose oxidation. Improved ion homeostasis and reac tivation of pyruvate dehydrogenase complex consumes protons that would otherwise have increased concentrations of intra cellular calcium. Increased formation of pyruvate replenishes the tricarboxylic acid cycle. While oxidative phosphorylation pre cedes restoration of function, the rapid recovery of β-oxidation reduces the return of contractile efficiency until normal, balanced metabolism resumes.
studies; it has increased cardiac contractility in coronary artery disease in clinical trials.
Drugs that reduce the metabolism of NEFA Etomoxir: experimentally, etomoxir indirectly promotes oxidation of glucose by reducing the transport of NEFA into the mitochon drial matrix (thereby reducing the substrate for β-oxidation). Trimetazidine inhibits β-oxidation by reducing the production of acetyl CoA from NEFA, permitting more pyruvate to proceed to glucose oxidation. Trimetazidine has been shown to have cardioprotective effects during clinical trials of coronary artery bypass grafting and percutaneous coronary interventions, as well as improving functional recovery during myocardial reperfusion in experimental studies.
Metabolic interventions during ischaemia and reperfusion Myocardial protection from ischaemia and reperfusion centres on maintaining sufficient production of ATP to match the metabolic demands of the myocardium. Historically, techniques of myo cardial protection have been applied to cardiac surgery, where cardiopulmonary bypass (see page 217) permits elective arrest of myocardial contractile activity, improving the ratio of the demand and supply of ATP. This can be achieved by temporary fibrillation of the heart at low temperatures or by administering solutions into the coronary circulation that induce warm or cold diastolic arrest (cardioplegia). Another approach is to improve the tolerance of myocardial cells to ischaemia and their ability to reconstitute during reper fusion (so improving their survivability and returning sufficient mechanical function rapidly). This may be achieved by inducing a sustained shift in metabolism away from NEFA and towards glucose in anticipation of ischaemia. A number of drugs (some with many actions) have been proposed as metabolic modu lators. These modulators have been used in cardiac surgery where coronary flow is deliberately globally reduced to low- or zeroflow conditions. They have also been used in other conditions of anticipated ischaemia (percutaneous coronary intervention) and unanticipated ischaemia (acute coronary syndromes/myocardial infarction), where flow is acutely low.
Glucose, insulin and potassium Insulin, alone or more commonly in combination with glucose and potassium (GIK), has been widely studied. Insulin and glu cose act directly and indirectly at a number of key points in the metabolism of glucose and NEFA. Glycolysis is encouraged by promoting glucose transport through an increase in the number of GLUT 1 and GLUT 4 transporters in the sarcolemma. Increased availability of glucose is supplemented by accelerated reactions involving: • hexokinase (glycolysis) which increases the availability of pyruvate • pyruvate dehydrogenase (glucose oxidation) which increases the production of lactic acid • glycogen synthase, which increases the production of glycogen. Increased systemic concentrations of insulin and glucose reduce circulating transport of NEFA by myocytes, so reducing intra cellular accumulation of acyl-carnitine. This further promotes glucose oxidation, reduces oxygen wasting and may reduce NEFA-mediated damage to the membrane in ischaemia or reper fusion. In the experimental setting at various temperatures, the provision of additional glucose and/or insulin stimulates gly colysis, glucose oxidation and preserves glycogen stores during low or limited duration no-flow ischaemia (albeit at the expense of increased production of lactic acid in tissues). This improves post-ischaemic preservation of ATP/phospho-creatinine/glyco gen and contractile functional recovery. Infarct size and dys rhythmic potential can also be reduced. In the clinical setting, GIK has been studied in acute myo cardial infarction (± reperfusion) and before, during and after surgery of the coronary or mitral valve. Many of the earlier tri als have suggested clinical benefits. GIK reduces mortality in patients receiving thrombolysis, and in patients without heart failure receiving primary angioplasty for myocardial infarction. Dextrose and insulin given in sufficient quantities to achieve strict normoglycaemic glucose control reduces one-year mortality in all patients. Preoperative use of GIK to pre-empt ischaemic and reperfu sion insults with metabolic adaptations results in peri-ischaemic glycogen preservation and reduced dysrhythmias postopera tively. Using GIK before, during and after ischaemia results in reduced postoperative inotropic requirements and dysrhythmias,
Drugs that promote the oxidation of glucose The promotion of glucose oxidation reduces: • wastage of oxygen • myocyte damage related to β-oxidation • production of lactic acid in low-flow ischaemia; it may allow accelerated glycolysis to better preserve intracellular concen trations of glycogen. Carnitine and palmitate participate in activated transport of NEFA into mitochondria for β-oxidation. High cytoplasmic concentrations lower the concentration ratios of acetyl CoA:CoA by the transport of acyl CoA into the mitochondrial membranes. This serves to stimulate pyruvate dehydrogenase complex, allowing glucose (rather than β-) oxidation to proceed relative to oxygen delivery. Carnitine can reduce ischaemic ECG changes during exercise stress testing in patients with coronary artery disease. Dichloroacetate promotes glucose oxidation by directly stimulat ing the pyruvate dehydrogenase complex. It converts most of the pyruvate dehydrogenase complex into its active form. This has improved post-ischaemic contractile recovery in experimental
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and reduces the proportion of patients suffering significant myocardial injury. While improved ATP handling, reduced NEFA-related injury or other anti-apoptotic effects may account for some of these outcomes, insulin also has haemodynamic actions, and the rela tionship between the two is unclear. Insulin is a vasodilator, low ering the systemic vascular resistance. The ability of insulin to reduce the requirement of postoperative use of inotropes may be related to this or it may act as a direct positive inotrope. There are potential problems with the use of GIK in prolonged low-flow ischaemia, where it may promote accumulation of lactic acid and subsequent myocardial damage and contractile dysfunction. Continuous or multidose intermittent cardioplegia may reduce this by assisting removal of lactic acid. Animal experiments suggest that GIK present only at reper fusion provides equivalent protection from infarction as that present throughout ischaemia and reperfusion. This may repre sent an anti-apoptotic effect of insulin, mediated via potassium
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ion-ATP channel signalling and inhibited by pre-ischaemic hyper glycaemia. The relationship between this effect and the other metabolic effects of GIK is unclear. The haemodynamic effects of GIK during reperfusion are maintained because its use after cardiac surgery for the treatment of low cardiac output syndrome results in higher cardiac indices and a reduced need for inotropic and mechanical circulatory sup port. Experimental studies of GIK as an integral part of cardio plegia indicate that it provides protection from ATP depletion, but large clinical trials have failed to show benefit. ◆
Cross references Gleeson M. Basic metabolism I: fat. Surgery 2005; 23(3): 83–8. Maughan R. Basic metabolism II: carbohydrate. Surgery 2005; 23(5): 154–8. Ward J. Oxygen delivery and demand. Surgery 2006; 24(10): 354–60.
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supply and demand of oxygen (see Ward, CROSS REFERENCES). They are stored in adipose tissue as part of triacylglycerol and transported to the myocyte either: • within chylomicrons or very low density lipoproteins • as NEFA complexed to albumin. NEFA enter the myocyte passively down a steep diffusion gradient, maintained by interactions with fatty acid-binding pro teins in the cytoplasm. They are activated to an acyl form and ‘ferried’ through the outer and inner mitochondrial membranes as an acyl-carnitine by carnitine palmitoyl transferase trans porters. Once regenerated to the acyl form, they undergo serial β-oxidation (which involves oxidation by acetyl CoA dehydroge nase followed by the repeated release of acyl units). A molecule of acetyl CoA is produced at the end of each series. The ratelimiting steps for β-oxidation are transport of NEFA into the cell and mitochondria.
D W Quinn D Pagano
Abstract This contribution focuses on the applied physiology of the heart.
Keywords myocardial metabolism; fatty acids; glucose; tricarboxylic acid cycle; electron transport chain; NEFA; ischaemia; reperfusion; insulin
Metabolism of glucose (Figure 3) The primary source of glucose for myocyte metabolism is from the plasma, transported passively down a steep diffusion gradi ent into the cell by glucose transporters (GLUT). These proteins (which are normally stored in cytoplasmic membranes) can be recruited to the plasma membrane as needed. Five subtypes have been identified; GLUT 4 predominates in the adult sarcolemma during normal oxygenated blood flow. Glucose is initially metabolized to glycogen (glycogenesis) or by glycolysis to pyruvate or lactate (depending on oxygen avail ability; see Maughan, CROSS REFERENCES). Glycolysis occurs in the cytoplasm and the ATP generated is directed preferentially toward ion homeostasis and glycogenesis (rather than contrac tion). Pyruvate may have a number of fates, including: • decarboxylation to acetyl CoA • carboxylation to oxaloacetate and malonate • dehydrogenation to lactate. Under aerobic conditions, pyruvate is transported with a pro ton into the mitochondria for decarboxylation to acetyl CoA and carbon dioxide. This reaction is facilitated by the pyruvate de hydrogenase complex of enzymes, and involves the reduction of NAD+ to NADH. It commits acetyl CoA to the tricarboxylic acid cycle and subsequent complete oxidation of glucose, and can proceed only if there is sufficient oxygen for the electron trans port chain. If glucose transport and glycolysis exceeds glucose oxidation, glycolysis is maintained by the metabolism of pyruvate to lactic acid by lactate dehydrogenase. This process: • occurs in the cytoplasm • does not require oxygen (i.e. it is anaerobic) • regenerates the essential glycolytic co-enzymes NAD+ from NADH. Lactic acid is metabolized back to pyruvate after oxygen is reintroduced or released into the circulation and transported to the liver for gluconeogenesis. Glycogenolysis can also contribute glucose derivatives for glycolysis, generating more net produc tion of ATP than glucose. These derivatives are preferentially metabolized by oxidative (rather than anaerobic) pathways. Irrespective of the source, NADH generated within the cytoplasm must be transported to the mitochondrial matrix to donate electrons to the electron transport chain. A malate– aspartate ‘metabolic shuttle’ achieves this by using cytoplasmic
Normal myocardial metabolism Myocardial function requires energy expenditure. The pre dominant energy source within the cell is adenosine triphosphate (ATP), which is dephosphorylated to adenosine diphosphate (ADP) and inorganic phosphate (Pi). This reaction provides energy for cellular ion homeostasis, biochemical synthesis and contraction. Other energy sources include the release of Pi from creatine phosphate, ADP and adenosine monophosphate (AMP). The myocyte requires continous production of ATP to survive because it cannot be stored. This is achieved by metabolism of non-esterified fatty acids (NEFA), glucose and amino acids, which generate ATP directly or through the production of reduced AMPlinked nicotinamide and flavin dinucleotide substrates (NADH, FADH2). These compounds are capable of donating electrons to a series of important prosthetic groups (electron transport chain) on the inner membrane of the mitochondrion. The electrons pass along this chain by sequential oxidation and reduction reactions, generating ATP by oxidative phosphorylation at three potential sites before finally reducing oxygen to water. The highest yields of ATP occur with complete NEFA or glucose oxidative metabolism with the production of a key intermediary compound: acetyl coenzyme A (acetyl CoA). This generates NADH and FADH2 from reactions along the tricarboxylic acid cycle (Figure 1) that enter the electron transport chain to drive oxidative phosphorylation.
Metabolism of fatty acids (Figure 2) Fatty acids (see Gleeson, CROSS REFERENCES) are the principal source of ATP (70–80%) for the myocardium during matched
D W Quinn FRCS is a Specialist Registrar in Cardiothoracic Surgery on the West Midlands rotation, UK. Conflicts of interest: none declared. D Pagano FRCS is a Consultant Cardiothoracic Surgeon at University Hospitals Birmingham NHS Trust, Birmingham, UK. Conflicts of interest: none declared. This article was first published in Surgery 2004; 22(6): 144a–e.
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Tricarboxylic acid cycle
Acetyl CoA
Citrate synthase
Oxaloacetate
NADH
NAD: Nicotinamide adenine dinucleotide NADH: Nicotinamide adenine dinucleotide (reduced form) CoASH: Coenzyme A with sulphdryl functional group GDP: Guanosine diphosphate GTP: Guanosine triphosphate FAD: Flavin adenine dinucleotide FADH: Flavin adenine dinucleotide (reduced form)
Citrate Aconitate hydratase
Malate dehydrogenase
Isocitrate
NAD Malate
NAD
Fumarate dehydrogenase
Isocitrate dehydrogenase
Fumarate FADH
NADH Succinate dehydrogenase FAD
CoASH
2-oxoglutarate
Succinate NAD
CO2 GTP + CoASH
Oxoglutarate dehydrogenase
Succinyl CoA
Succinyl CoA synthetase
NADH
GDP + Pi Figure 1
xaloacetate to accept a proton from NADH and synthesize o malate. This is transported across the mitochondrial membranes, where the process is reversed, regenerating NADH. The oxalo acetate is returned to the cytoplasm by a reaction with glutamate, which generates transportable aspartate and 2-oxoglutarate, thereby maintaining the shuttle service. The metabolism of pyru vate to oxalate and malate allows some of the intermediates of the malate–aspartate shuttles and the tricarboxylic acid cycle to be replenished.
High concentrations of NADH and acetyl CoA generated from β-oxidation inhibit the oxidation of glucose. High concentrations of citrate (an intermediary in the tricarboxylic acid cycle) inhibit phosphofructokinase. These inhibitors suppress complete glucose oxidation via pyruvate relative to glycolysis to lactate, so reducing the competition between β- and glucose oxidation for the electron transport chain and increasing production of lactic acid. Conversely, acetyl CoA derived from pyruvate metabolism can be converted to malonyl CoA, a process stimulated by insulin. Glucose, lactate and malonyl CoA in turn diminish the availability of NEFA for subsequent β-oxidation by inhibiting the transport of NEFA to the mitochondrion.
The tricarboxylic acid cycle and the electron transport chain Acetyl CoA (from β- and glucose oxidation) participates in a series of reactions within the mitochondrial matrix near the inner mito chondrial membrane. The metabolism of acetyl CoA in turn gen erates NADH and FADH2 for the electron transport chain. NADH and FADH2 (from the tricarboxylic acid cycle and that generated and transported into the mitochondria from the cytoplasm) are made available for the electron transport chain.
Efficiency of metabolism The higher carbon content of NEFA generates more ATP per mole during β-oxidation (e.g. palmitate produces 129 molecules of ATP) than is generated per mole during aerobic (38 molecules of ATP) and anaerobic (2 molecules of ATP) glucose metabolism. The hydrophobic nature of NEFA reduces the water content of adipose stores, making them, on a weight basis, a more efficient energy store compared to tissues rich in hydrophilic glycogen. β-oxidation requires more oxygen/ATP to be generated than complete glucose oxidation, making glucose metabolism about 10–15% more oxygen efficient. The accelerated generation of lactic acid during high rates of β-oxidation and the subsequent
The relationship between the metabolism of glucose and NEFA Metabolism of glucose and NEFA compete for the delivery of acetyl CoA to the tricarboxylic acid cycle. The products of β-oxidation inhibit the transport and metabolism of glucose at a number of sites in glycolysis/glucose oxidation.
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sympathetic tone, cardiac output and release of local vasodilators (nitric oxide, adenosine, hydrogen ions). This attempts to: • redress mismatching of the supply and demand of oxygen • provide additional glucose to support anaerobic and aerobic metabolism • remove accumulating lactic acid. After exercise, lactate is remetabolized by the recovering myocytes to pyruvate and acetyl CoA, a process facilitated by high concentrations of citrate and NADH, which inhibit glyco lysis. Metabolism of glucose is slowly superseded by metabolism of NEFA during sustained, less severe exercise.
-oxidation CH3(CH2)nCH2-CH2CO.ScoA FAD Acyl-CoA dehydrogenase FADH2 CH3(CH2)nCH=CHCO.SCoA H2O Enoyl-CoA hydratase
Metabolism during ischaemia During the initial phases of low-flow ischaemia (<75% reduc tion in coronary flow), up to two-thirds of glucose derivatives are provided by glycogenolysis. Hypoxia and release of adrenaline provide the stimulus for translocation of GLUT 1 and GLUT 4 to the sarcolemma. This increases glucose extraction, which main tains or elevates glucose uptake despite low delivery. Glucose quickly replaces glycogen as the source of substrate for glyco lysis, preventing depletion of glycogen stores. Preservation of glycogen is associated with improved myocyte viability and/or function during ischaemia/reperfusion. This prevents ischaemic contracture, possibly through altered handling of calcium. Falling concentrations of intracellular ATP and rising concentrations of intracellular AMP stimulate phosphofructokinase, which accele rates glycolysis and generates ATP in the cytoplasm for ion homeostasis. Remaining oxygen provision is diverted from βoxidation to glucose oxidation by the competitive mechanisms described above, improving the efficiency of oxygen metabolism. Release of catecholamines stimulates the mobilization of NEFA from adipose tissue, but delivery and transport of NEFA to the myocyte is hampered in ischaemia. High intracytoplasmic and intramitochondrial concentrations of NEFA are associated with membrane damage by lipid peroxidation, which reduces contractility and is pro-arrhythmogenic. β-oxidation diverts glycolysis toward production of lactic acid. With prolonged lowflow ischaemia, glycolysis gradually slows, being reduced by the accumulation of lactic acid and by membrane damage. In no-flow ischaemia, endogenous delivery of glucose is eliminated and the myocytes rely on glycogenolysis. The failure to remove accumulating lactic acid uncouples glycolysis, reduces intra cellular pH and further disrupts the metabolic pathways and ion homeostasis of myocytes, and reduces their contractile function. Irrespective of coronary blood flow, a reduction in production of ATP is associated with reduced contractility. The biochemistry and mechanics of the myocardium adapts to preserve viability in some cases of chronic or repetitive acute myocardial ischaemia. These areas of myocardium show relatively higher glycogen con centrations, plasma glucose extraction and a greater reliance on glucose-based metabolism. They also show reduced contractility (hibernating myocardium). Metabolic and contractile function is reversible on reinstitution of normal blood flow.
CH3(CH2)nOHCH.CH2CO.SCoA NAD+ 3-hydroxyacyl-CoA dehydrogenase NADH CH3(CH2)nCOH=CH2CO.SCoA CoASH Acetyl-CoA acyl transferase
CH3(CH2)nCO.S CoA + CH3CO.SCoA (acetyl CoA)
FAD: Flavin adenine dinucleotide; FADH2: Flavin adenine dinucleotide (reduced form); NAD+: Nicotinamide adenine dinucleotide (oxidized form); NADH: Nicotinamide adenine dinucleotide (reduced form); CoASH: Coenzyme A with sulphdryl functional group.
The sequential removal of acyl groups during β-oxidation generates FADH and NADH, which can feed into the electron transfer chain and acetyl CoA, which is the main substrate for the tricarboxylic acid cycle. Figure 2
clearance of intracellular protons leads to other intracellular ion imbalances (e.g. sodium, calcium), which require ATP for homeo static restoration. NEFA metabolism at a cellular level is less efficient at releasing ATP for cell use than glucose metabolism.
Eating and exercise Metabolism of NEFA predominates between meals, supplied by mobilization of triacyglycerol from adipose tissue. Intracellular glycogen provides for glycolysis until a meal rich in carbohydrate is eaten. Postprandial increases in the concentration of glucose in plasma stimulate insulin release and consequent increased transport and metabolism of glucose and glycogenesis, while also inhibiting β-oxidation. Glucose is the predominant source of ATP in these circumstances. As high-intensity exercise begins, additional ATP is provided from creatine phosphate and glycogenolysis, before anaerobic metabolism of glucose increases due to relative oxygen deficiency. Increases in coronary blood flow occur in response to increased
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Metabolism during reperfusion Re-establishing blood flow, glucose delivery and removal of lactic acid accelerates glycolysis and increases ATP in the cytoplasm. 200
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Glycolysis Glucose +ve
Glycogen
ATP
Glycogen phosphorylase
Hexokinase
Glucose
Pi
ADP Glucose-6phosphate
Glucose-1phosphate Glucose phosphate isomerase
Fructose-6phosphate
ATP Citrate H+
–ve
ATP Phosphofructokinase (PFK) ADP
+ve Fructose-1,6bisphosphate
F-6-P F-2,6 bisP AMP
Dihydroxyacetone phosphate
Glycerol-3phosphate GAPDH
Pi
NAD+
NADH 3-phosphoglycerol phosphate ADP: Adenosine diphosphate ATP: Adenosine triphosphate Pi: Inorganic phosphate AMP: Adenosine monophosphate GAPDH: D-glyceraldehyde phosphate dehydrogenase NAD+: Nicotinamide adenine dinucleotide (oxidized form) NADH: Nicotinamide adenine dinucleotide (reduced form) PC: Pyruvate carboxylase
ADP Phosphoglycerate kinase ATP 3-Phosphoglycerate Phosphoglycerolmutase
2-Phosphoglycerate
Phosphoenolpyruvate Pyruvate kinase
ADP
ATP PC
–ve +ve
ATP
ADP
Pyruvate NADH Lactate
NADH NAD+
Acetyl-CoA
The common pathway for the metabolism of glucose to pyruvate (glycolysis) generates less net ATP (2 ATP molecules consumed, 4 generated) than does the metabolism of glycogen to pyruvate (1 ATP consumed, 4 generated). The conversion to pyruvate or lactate is dependent on oxygen availability. The conversion to lactate generates NAD+, which is capable of maintaining glycolysis by continuing to provide NAD+ for the conversion of glycerol-3-phosphate to 3-phosphoglycerol phosphate. Lactate can be metabolized back to pyruvate with the reintroduction of oxygen. Figure 3
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The reintroduction of an adequate delivery of oxygen permits a return to glucose oxidation. Improved ion homeostasis and reac tivation of pyruvate dehydrogenase complex consumes protons that would otherwise have increased concentrations of intra cellular calcium. Increased formation of pyruvate replenishes the tricarboxylic acid cycle. While oxidative phosphorylation pre cedes restoration of function, the rapid recovery of β-oxidation reduces the return of contractile efficiency until normal, balanced metabolism resumes.
studies; it has increased cardiac contractility in coronary artery disease in clinical trials.
Drugs that reduce the metabolism of NEFA Etomoxir: experimentally, etomoxir indirectly promotes oxidation of glucose by reducing the transport of NEFA into the mitochon drial matrix (thereby reducing the substrate for β-oxidation). Trimetazidine inhibits β-oxidation by reducing the production of acetyl CoA from NEFA, permitting more pyruvate to proceed to glucose oxidation. Trimetazidine has been shown to have cardioprotective effects during clinical trials of coronary artery bypass grafting and percutaneous coronary interventions, as well as improving functional recovery during myocardial reperfusion in experimental studies.
Metabolic interventions during ischaemia and reperfusion Myocardial protection from ischaemia and reperfusion centres on maintaining sufficient production of ATP to match the metabolic demands of the myocardium. Historically, techniques of myo cardial protection have been applied to cardiac surgery, where cardiopulmonary bypass (see page 217) permits elective arrest of myocardial contractile activity, improving the ratio of the demand and supply of ATP. This can be achieved by temporary fibrillation of the heart at low temperatures or by administering solutions into the coronary circulation that induce warm or cold diastolic arrest (cardioplegia). Another approach is to improve the tolerance of myocardial cells to ischaemia and their ability to reconstitute during reper fusion (so improving their survivability and returning sufficient mechanical function rapidly). This may be achieved by inducing a sustained shift in metabolism away from NEFA and towards glucose in anticipation of ischaemia. A number of drugs (some with many actions) have been proposed as metabolic modu lators. These modulators have been used in cardiac surgery where coronary flow is deliberately globally reduced to low- or zeroflow conditions. They have also been used in other conditions of anticipated ischaemia (percutaneous coronary intervention) and unanticipated ischaemia (acute coronary syndromes/myocardial infarction), where flow is acutely low.
Glucose, insulin and potassium Insulin, alone or more commonly in combination with glucose and potassium (GIK), has been widely studied. Insulin and glu cose act directly and indirectly at a number of key points in the metabolism of glucose and NEFA. Glycolysis is encouraged by promoting glucose transport through an increase in the number of GLUT 1 and GLUT 4 transporters in the sarcolemma. Increased availability of glucose is supplemented by accelerated reactions involving: • hexokinase (glycolysis) which increases the availability of pyruvate • pyruvate dehydrogenase (glucose oxidation) which increases the production of lactic acid • glycogen synthase, which increases the production of glycogen. Increased systemic concentrations of insulin and glucose reduce circulating transport of NEFA by myocytes, so reducing intra cellular accumulation of acyl-carnitine. This further promotes glucose oxidation, reduces oxygen wasting and may reduce NEFA-mediated damage to the membrane in ischaemia or reper fusion. In the experimental setting at various temperatures, the provision of additional glucose and/or insulin stimulates gly colysis, glucose oxidation and preserves glycogen stores during low or limited duration no-flow ischaemia (albeit at the expense of increased production of lactic acid in tissues). This improves post-ischaemic preservation of ATP/phospho-creatinine/glyco gen and contractile functional recovery. Infarct size and dys rhythmic potential can also be reduced. In the clinical setting, GIK has been studied in acute myo cardial infarction (± reperfusion) and before, during and after surgery of the coronary or mitral valve. Many of the earlier tri als have suggested clinical benefits. GIK reduces mortality in patients receiving thrombolysis, and in patients without heart failure receiving primary angioplasty for myocardial infarction. Dextrose and insulin given in sufficient quantities to achieve strict normoglycaemic glucose control reduces one-year mortality in all patients. Preoperative use of GIK to pre-empt ischaemic and reperfu sion insults with metabolic adaptations results in peri-ischaemic glycogen preservation and reduced dysrhythmias postopera tively. Using GIK before, during and after ischaemia results in reduced postoperative inotropic requirements and dysrhythmias,
Drugs that promote the oxidation of glucose The promotion of glucose oxidation reduces: • wastage of oxygen • myocyte damage related to β-oxidation • production of lactic acid in low-flow ischaemia; it may allow accelerated glycolysis to better preserve intracellular concen trations of glycogen. Carnitine and palmitate participate in activated transport of NEFA into mitochondria for β-oxidation. High cytoplasmic concentrations lower the concentration ratios of acetyl CoA:CoA by the transport of acyl CoA into the mitochondrial membranes. This serves to stimulate pyruvate dehydrogenase complex, allowing glucose (rather than β-) oxidation to proceed relative to oxygen delivery. Carnitine can reduce ischaemic ECG changes during exercise stress testing in patients with coronary artery disease. Dichloroacetate promotes glucose oxidation by directly stimulat ing the pyruvate dehydrogenase complex. It converts most of the pyruvate dehydrogenase complex into its active form. This has improved post-ischaemic contractile recovery in experimental
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and reduces the proportion of patients suffering significant myocardial injury. While improved ATP handling, reduced NEFA-related injury or other anti-apoptotic effects may account for some of these outcomes, insulin also has haemodynamic actions, and the rela tionship between the two is unclear. Insulin is a vasodilator, low ering the systemic vascular resistance. The ability of insulin to reduce the requirement of postoperative use of inotropes may be related to this or it may act as a direct positive inotrope. There are potential problems with the use of GIK in prolonged low-flow ischaemia, where it may promote accumulation of lactic acid and subsequent myocardial damage and contractile dysfunction. Continuous or multidose intermittent cardioplegia may reduce this by assisting removal of lactic acid. Animal experiments suggest that GIK present only at reper fusion provides equivalent protection from infarction as that present throughout ischaemia and reperfusion. This may repre sent an anti-apoptotic effect of insulin, mediated via potassium
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ion-ATP channel signalling and inhibited by pre-ischaemic hyper glycaemia. The relationship between this effect and the other metabolic effects of GIK is unclear. The haemodynamic effects of GIK during reperfusion are maintained because its use after cardiac surgery for the treatment of low cardiac output syndrome results in higher cardiac indices and a reduced need for inotropic and mechanical circulatory sup port. Experimental studies of GIK as an integral part of cardio plegia indicate that it provides protection from ATP depletion, but large clinical trials have failed to show benefit. ◆
Cross references Gleeson M. Basic metabolism I: fat. Surgery 2005; 23(3): 83–8. Maughan R. Basic metabolism II: carbohydrate. Surgery 2005; 23(5): 154–8. Ward J. Oxygen delivery and demand. Surgery 2006; 24(10): 354–60.
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