Metabolism, the Stress Response to Surgery and Perioperative Thermoregulation
Metabolism may be defined as the chemical processes which enable cells to function. Basal metabolic rate (BMR) is the minimum amount of energy required to maintain basic autonomic function and normal homeostasis. For example, energy is required by the myocardium to maintain heart rate and stroke volume and by nerve and muscle membranes to maintain membrane potentials. In a healthy resting adult, BMR is in the region of 2000 kcal day–1 (equivalent to 40 kcal m–2 h–1). One calorie is the energy required in joules to raise the temperature of 1 g of water from 15 °C to 16 °C. Because this is a very small unit, a more practical measure in human physiology is the kcal or Calorie (C).
Adenosine triphosphate (ATP) is the ‘energy currency’ of the body. It contains two high-energy phosphate bonds and is present in all cells. Most physiological processes acquire energy from it. Oxidation of nutrients in cells releases energy, which is used to regenerate ATP. Conversion of one mole of ATP to adenosine diphosphate (ADP) releases 8 kcal of energy. Additional hydrolysis of the phosphate bond from ADP to AMP also releases 8 kcal (Fig. 11.1). Other high-energy compounds include creatine phosphate and acetyl CoA. The generation of energy through the oxidation of carbohydrate, protein and fat is termed catabolism, whereas the generation of stored energy as energy-rich phosphate bonds, carbohydrates, proteins or fats is termed anabolism (Fig. 11.2). The amount of energy released by carbohydrate, protein and fat metabolism is: carbohydrate 4.1 kcal g–1, protein 4.1 kcal g–1 and fat 9.3 kcal g–1.
The final product of carbohydrate digestion is glucose, which is used to form ATP in cells. Because the cellular membrane is impermeable to glucose, it is transported by a carrier protein (GLUT4) across the membrane in a process termed facilitated diffusion. Activation of insulin receptors speeds translocation of GLUT4-containing endosomes into the cell membrane which then mediate glucose transport into the cell. Facilitated diffusion of glucose into cells is increased 10-fold in the presence of insulin, without which the rate of uptake would be inadequate. This is a passive process (i.e. it does not require energy expenditure by the cell). In contrast, glucose absorption in the gastrointestinal tract and reabsorption in the renal tubule are both active processes (i.e. are energy-consuming processes). They involve co-transport with sodium ions via sodium-dependent glucose transporters (SGLT).
After absorption into cells, glucose may be used immediately or stored in the form of glycogen, particularly in liver and muscle. The process of releasing glucose molecules from the glycogen molecule in times of high metabolic demand is termed glycogenolysis. This process is initiated by the enzyme phosphorylase, which is activated in the presence of adrenaline (epinephrine) and glucagon. Adrenaline is released by the sympathetic nervous system, while glucagon is released from the α cells of the pancreas in response to hypoglycaemia.
The mechanism of glucose catabolism involves an extensive series of enzyme-controlled steps, rather than a single reaction. This is because the oxidation of one mol of glucose (180 g) releases almost 686 kcal of energy, whereas only 8 kcal is required to form one molecule of ATP. Therefore, an elaborate series of reactions, termed the glycolytic pathway, releases small quantities of energy at a time, resulting in the synthesis of 38 mol of ATP from each mol of glucose (Fig. 11.3). As each molecule of ATP releases 8 kcal, a total of 304 kcal of energy in the form of ATP is synthesized. Hence, the efficiency of the glycolytic pathway is 44%, the remainder of the energy being released as heat.
FIGURE 11.3 Summary of the glycolytic pathway. Krebs citric acid cycle. FFA, free fatty acid. Note that two molecules of pyruvic acid are produced for each molecule of glucose metabolized. Each pyruvic acid molecule enters the Krebs citric acid cycle.
1. Glycolysis, i.e. splitting the glucose (6 carbon atoms) molecule into two molecules of pyruvic acid (3 carbon atoms each). This results in the net formation of two molecules of ATP anaerobically but also generates two pairs of H+ for entry into the respiratory chain (see below) (Fig. 11.4).
2. Oxidation of each of the pyruvic acid (3 carbon atom) molecules in the Krebs citric acid cycle results in the generation of five pairs of H+ per 3-carbon moiety, i.e. 10 pairs of H+ per 6 carbon glucose molecule (Fig. 11.5).
FIGURE 11.5 The Krebs citric acid cycle. Note that five pairs of H+ are generated by the oxidation of each pyruvate molecule. Each pair of H+ generates three molecules of ATP in the respiratory chain in the mitochondria.
3. Oxidative phosphorylation, i.e. the formation of ATP by the oxidation of hydrogen to water. This process is also known as the respiratory chain. For each molecule of glucose, a total of 12 pairs of H+ are fed into the respiratory chain, each pair generating three molecules of ATP. Thus, oxidative phosphorylation results in 36 molecules of ATP per molecule of glucose. A further two molecules of ATP are produced anaerobically. Therefore, one molecule of glucose generates 38 molecules of ATP. Uncoupling of oxidative phosphorylation allows ATP production to be sacrificed for heat production as part of thermoregulatory homeostasis.
This is the process of ATP formation in the absence of oxygen and is possible because the first two steps of glycolysis do not require oxygen. In the absence of oxygen, pyruvic acid molecules and hydrogen ions accumulate, which would normally stop the reaction. However, pyruvic acid and hydrogen ions combine in the presence of the enzyme lactic dehydrogenase to form lactic acid, which diffuses easily out of cells, allowing anaerobic glycolysis to continue. Lactic acidosis is a feature of shock caused by, for example, severe sepsis. This is a highly inefficient use of the energy within glucose. When oxygen is again available to the cells, lactic acid is reconverted to glucose or used directly for energy.
The glycolytic pathway metabolizes 70% of glucose. A second mechanism, the phosphogluconate pathway (also known as the hexose monophosphate shunt) is responsible for metabolism of the remaining 30%. The importance of this pathway is that ATP is formed independently of the enzymes needed in the glycolytic pathway, and hence an enzymatic abnormality in the glycolytic pathway does not completely inhibit energy metabolism. It also provides for the production of pentoses, which are needed for nucleic acid production.
Proteins are composed of amino acids, of which there are more than 20 different types in humans. All amino acids have a weak acid group (-COOH) and an amine group (-NH2). They are joined by peptide linkages to form peptide chains (primary structure), a reaction which releases a molecule of water in the process. The blood concentration of amino acids is approximately 1–2 mmol L–1. Entry into cells requires facilitated or active transport using carrier mechanisms. They are then conjugated into proteins by the formation of peptide linkages. Formation of the peptide link requires 0.5–4.0 kcal derived from ATP. Large proteins may be composed of several peptide chains wrapped around each other (secondary structure) and bound by weaker links, e.g. hydrogen bonds, electrostatic forces and sulfhydryl bonds (tertiary structure).
Some amino acids present in the body are not present in proteins to any appreciable extent including, for example, ornithine, 5-hydroxytryptophan, L-dopa and thyroxine. Catecholamines, histamine and serotonin are formed from specific amino acids. Sulphur-containing amino acids are the source of urinary sulphate and provide sulphur for incorporation into various proteins, e.g. Coenzyme A.
There is equilibrium between the amino acids in plasma, plasma proteins and tissue proteins. Proteins may be synthesized from amino acids in all cells of the body, the type of protein depending on the genetic material in the DNA, which determines the sequence of amino acids formed and hence controls the nature of the synthesized proteins. Essential amino acids must be ingested as they cannot be synthesized in the body. Table 11.1 lists the eight essential amino acids. If there is dietary deficiency of any of these, the subject develops a negative nitrogen balance. Others are non-essential (i.e. may be synthesized in the cells). Synthesis is by the process of transamination, whereby an amine radical (-NH2) is transferred to the corresponding α-keto acid. Breakdown of excess amino acids into glucose (gluconeogenesis) generates energy or storage as fat, both of which occur in the liver. The breakdown of amino acids occurs by the process of deamination, which takes place in the liver. It involves the removal of the amine group with the formation of the corresponding ketoacid. The amine radical may be recycled to other molecules or released as ammonia. In the liver, two molecules of ammonia are combined to form urea (Fig. 11.6). Amino acids may also take up ammonia to form the corresponding amide.
During starvation or when no protein is ingested (e.g. after major surgery), 20–30 g day–1 of protein is catabolized for energy purposes. This occurs despite the continuing availability of some stored carbohydrates and fats. When carbohydrate and fat stores are exhausted, the rate of protein catabolism is increased to > 100 g day–1, resulting in a rapid decline in tissue function. During the systemic inflammatory response syndrome (SIRS) or after major surgery, there is functional catabolism also. Several hormones influence protein metabolism. Growth hormone, insulin and testosterone are anabolic, i.e. they increase the rate of cellular protein synthesis. Other hormones, e.g. glucocorticoids, are catabolic, i.e. they decrease the amount of protein in most tissues, except the liver. Glucagon promotes gluconeogenesis and protein breakdown. Thyroxine indirectly affects protein metabolism by affecting metabolic rate. If insufficient energy sources are available to cells, thyroxine may contribute to excess protein breakdown. Conversely, if adequate amino acid and energy sources are available, thyroxine may increase the rate of protein synthesis.
Lipids are a diverse group of compounds characterized by their insolubility in water and solubility in non- polar solvents such as ether or benzene. They include fats, oils, steroids, waxes, etc. They serve as an immediate energy source but also provide storage energy. They include cholesterol, which is a precursor of steroids. They provide electrical insulation for nerve conduction and when combined with protein they are known as lipoproteins, an important component of cell membranes. Lipoproteins are also the predominant means for the transport of bloodstream lipids.
Lipids include triglycerides (TGs), phospholipids (PLs) and cholesterol. The basic structure of TGs and PLs is the fatty acid. Fatty acids are long-chain hydrocarbon organic acids. TGs are composed of three long-chain fatty acids bound with one molecule of glycerol (Fig. 11.7). Phospholipids have two long-chain fatty acids bound to glycerol with the third fatty acid replaced by attached compounds such as inositol, choline or ethanolamine. Although cholesterol does not contain fatty acid, its sterol nucleus is formed from fatty acid molecules.
Some polyunsaturated fatty acids are considered essential because they cannot be synthesized in humans and because they are precursors for eicosanoids. They must be acquired from plant sources. These essential fatty acids are linolenic acid and linoleic acid which together with their derivative arachidonic acid form prostaglandins, lipoxins and leukotrienes (collectively termed eicosanoids).
After absorption in the gastrointestinal tract, lipids are aggregated into droplets (diameter 90–1000 nm), termed chylomicrons, composed mainly of TGs. These molecules are too large to pass the endothelial cells of the portal system and so enter the circulation via the thoracic duct. Chylomicrons are metabolized by lipoprotein lipase adherent to the endothelium of many tissues throughout the body including adipose tissue but not adult liver. Chylomicrons carry cholesterol to the liver. The fatty acids and lipoproteins released from the liver into the circulation are derived from secondary products of chylomicron metabolism.
Transport of lipids from the liver or adipose cells to other tissues that need it as an energy source occurs by means of binding to plasma albumin. The fatty acids are then referred to as free fatty acids (FFAs), to distinguish them from other fatty acids in the plasma. After 12 h of fasting, all chylomicrons have been removed from the blood, and circulating lipids then occur in the form of lipoproteins. Lipoproteins are smaller particles than chylomicrons but are also composed of TGs, PLs and cholesterol. They may be classified as:
Cholesterol is a lipid with a sterol nucleus and is formed from acetyl CoA. It may be absorbed from food (animal sources only) but is also synthesized in the liver and to a lesser extent other tissue. Its function is predominantly the formation of bile salts in the liver, which promote the digestion and absorption of lipids. The remainder is used in the formation of adrenocortical and sex hormones and it is deposited also in the skin, where it resists the absorption of water-soluble chemicals.
The serum cholesterol concentration is correlated with the incidences of atherosclerosis and coronary artery disease. Prolonged elevations of VLDL, LDL and chylomicron remnants are associated with atherosclerosis. Conversely, HDL is protective. Factors affecting blood cholesterol concentration are outlined in Figure 11.8.
There is a feedback mechanism whereby increased cholesterol absorption from the diet results in inhibition of the enzyme HMG-CoA reductase, which regulates synthesis of cholesterol. There are many hormonal influences in cholesterol metabolism also, including increased plasma concentrations in response to abnormally low concentrations of thyroid hormone, insulin and androgens. Oestrogen reduces cholesterol concentration by an unknown mechanism. The family of cholesterol-lowering drugs termed statins are inhibitors of the enzyme HMG-CoA reductase.
Lipids are ingested in similar proportions to carbohydrates and may be used as an energy source immediately or stored in the liver or adipose cells for later use as an energy source. The stages in the use of TGs as an energy source are as follows. TG is hydrolysed to its constituent glycerol and three fatty acids; glycerol is then conjugated to glycerol 3-phosphate and enters the glycolytic pathway, which generates ATP as described above. Fatty acids need carnitine as a carrier agent to enter mitochondria, where they undergo beta oxidation. The precise number of ATP molecules formed from a molecule of TG depends on the length of the fatty acid chain, longer chains providing more acetyl CoA and hence more molecules of ATP. Newborns have a special type of fat, termed brown fat, which on exposure to a cold stressor is stimulated to break down into free fatty acids and glycerol. In brown adipose tissue, oxidation and phosphorylation are not coupled and therefore the metabolism of brown fat is especially thermogenic.
Initial degradation of fatty acids occurs in the liver, but the acetyl-CoA may not be used either immediately or completely. Ketones, or keto acids, are either acetoacetic acid, formed from two molecules of acetyl CoA, β-hydroxybutyric acid, formed from the reduction of acetoacetic acid, or acetone, formed when a smaller quantity of acetoacetic acid is decarboxylated (Fig. 11.9