Glucose Production

By convention, the basal or postabsorptive state is defined as the metabolic condition that prevails in the morning after an overnight (10 to 14 hours) fast. For most individuals, this time represents the longest period of fasting in everyday life. For the rest of the day, most people are more or less in the fed state. Maintenance of the fasting plasma glucose concentration is primarily the responsibility of the liver.9,10 The liver provides glucose for all tissues of the body, either by breaking down its own stores of glycogen or by synthesizing glucose from gluconeogenic precursors, of which the most important are lactate, pyruvate, glycerol, alanine, and other gluconeogenic amino acids. The central role of the liver in providing a constant supply of glucose to the body is related to the presence of G6Pase, which catalyzes the conversion of glucose 6-phosphate (G6P) to glucose within the hepatocyte. Although a number of tissues, including muscle and adipocytes, possess the enzymatic machinery necessary to degrade glycogen and synthesize G6P from lactate and amino acids, they either completely lack or possess too little of the key enzyme, G6Pase, to release significant amounts of free glucose into the circulation. The kidney, like the liver, also possesses the necessary enzymatic apparatus to produce glucose via the gluconeogenic pathway and to release it into the circulation. In normal subjects after an overnight fast, the kidney contributes ∼10% of total body glucose production.¹¹ During prolonged starvation and metabolic acidosis, renal gluconeogenesis is enhanced and may contribute as much as 25% of basal glucose production. Unlike the liver, the major gluconeogenic precursor for the kidney is glutamine.

Under postabsorptive conditions, glucose output in healthy adults averages 140 mg/min (778 µmol/min) or 2.0 mg/min per kg of body weight (11 µmol/min per kilogram) in a 70-kg individual (Fig. 10-2). The variation around this mean is significant (20% to 30%), with an unknown contribution of genetic and environmental factors. Little information is available concerning how much the fasting glucose output varies as a consequence of changes in dietary habits, caloric intake, or physical fitness. Intrafamilial covariance of this physiologic variable also is undetermined. Under standard nutritional conditions, the normal liver contains approximately 80 g of glycogen (see Table 10-1), and during fasting, liver glycogen stores decline at a rate of approximately 110 mg/min (611 µmol/min) or 8% per hour. From this it follows that hepatic glycogen depots would become empty after approximately 12 hours. Because fasting can be prolonged well beyond 12 hours, it is obvious that gluconeogenesis must progressively replace glycogenolysis as the fast continues.¹² In animals, the basal rate of glucose turnover is considerably higher than in humans—for example, dogs (3.6 mg/min per kilogram) and rats (7.2 mg/min per kilogram)—and the limited capacity of the liver to store glycogen confers an increasing role to gluconeogenesis for the maintenance of basal glycemia. This limitation on glycogen accumulation has an anatomic basis, because stuffing the cytoplasm with glycogen granules impairs cellular functions and results in liver damage, as seen in patients with glycogen storage diseases. In healthy subjects, after a 10- to 12-hour overnight fast, gluconeogenesis accounts for approximately 50% of total hepatic glucose release (see Fig. 10-2).¹² The substrates for this de novo glucose synthesis remain somewhat elusive. Circulating lactate, pyruvate, glycerol, alanine, and other gluconeogenic amino acids are natural candidate precursors and have been shown to transfer their carbons to newly synthesized glucose molecules, as documented by the incorporation of labeled lactate into glucose, that is, the Cori cycle. However, transsplanchnic catheterization in humans has shown that the net uptake of known circulating gluconeogenic precursors (lactate, pyruvate, glycerol, amino acids) can account for only 15% to 20% of total endogenous glucose production.¹³ The discrepancy between radioisotopic estimates of basal gluconeogenic rate and accountable circulating precursors suggests that the bloodborne substrates may not be the only source of gluconeogenic precursors. Within the splanchnic area, the intestine returns 10% to 20% of its glucose uptake to the liver as lactate, but this fills only part of the gap. It has been suggested that intrahepatic proteolysis and/or lipolysis could provide ample amounts of gluconeogenic precursors in addition to those entering from the systemic circulation.

The regulation of basal hepatic glucose production is controlled by the sum of multiple neural, hormonal, and metabolic stimuli, some stimulatory and others inhibitory.9,10 Fig. 10-3 portrays the control system as a simple balance between inhibition and stimulation. Insulin and glucagon provide the primary hormonal signals that regulate the production of glucose by the liver under postabsorptive conditions. Of the two, the action of insulin normally predominates. Hepatic glucose production is exquisitely sensitive to very small fluctuations in the circulating plasma insulin concentration. Increments in the plasma insulin concentration of as little as 5 to 10 µU/mL cause a marked, rapid suppression of glycogenolysis and decline in hepatic glucose output, whereas inhibition of gluconeogenesis is less sensitive.¹⁰

By restraining lipolysis and proteolysis, insulin also reduces the delivery of potential glucose precursors (glycerol and amino acids) from peripheral tissues (adipocytes and muscle) to the liver, and this further reduces hepatic glucose output. In its capacity as the inhibitory signal for glucose release, insulin is greatly favored by the anatomic connection between the pancreas and the liver. Because the pancreatic vein is a tributary of the portal vein, insulin, which is secreted by the β cells, reaches the liver in fasting humans at a concentration that is three to four times higher than the peripheral (arterial) concentration. This steep portosystemic gradient is maintained by the high rate of insulin degradation by hepatic tissues (fractional insulin extraction = 50%). Consequently, a small secretory stimulus to the β cells will disproportionately raise the portal insulin concentration, thereby selectively acting on glucose production rather than enhancing peripheral glucose utilization. In addition to short-circuiting the general circulation, pancreatic insulin release is potentiated by a number of gastrointestinal hormones (e.g., glucose-dependent insulinotropic polypeptide, glucagon-like peptide-1, secretin, cholecystokinin, pancreozymin, and others, which are released in response to meal ingestion).¹⁴ Therefore, anatomic and physiologic connections that comprise the gut-liver-pancreas axis ensure that the primary station for the handling of foodstuff, the liver, is under close control by a nearby, well-informed unit, the β cell.

Conversely, small decrements in the peripheral plasma insulin concentration, as little as 1 to 2 µU/mL, lead to an increase in hepatic glucose production.¹⁵ This highly sensitive interaction between the liver and insulin plays a critical role in the maintenance of basal glucose levels when fasting is prolonged. As glycogen stores become depleted, there is a small decrease in the arterial glucose concentration, which in turn leads to a decline in pancreatic insulin secretion and stimulation of glucagon release. The resultant hypoinsulinemia removes the constraint on lipolysis, and plasma FFA levels rise. By mass action, FFAs enhance their own uptake by all cells in the body, including liver and muscle. Enhanced FFA oxidation by the hepatocytes provides an energy source to drive gluconeogenesis, and the end product of beta oxidation, acetyl-CoA, stimulates the first committed enzyme, pyruvate carboxylase, in the gluconeogenic pathway (Fig. 10-4).^1,2 The combination of hypoinsulinemia, hypoglycemia, and increased FFA and amino acid supply also stimulates hepatic gluconeogenesis. In peripheral tissues, enhanced FFA and ketone oxidation spare glucose utilization (Randle cycle; see subsequent discussion), thereby minimizing the need for carbohydrate as an energy source.^1,2

Another important consequence of the fasting-related decline in plasma insulin concentration is the stimulation of proteolysis.¹⁵ This augments the outflow of amino acids, especially alanine, which accounts for approximately 50% of total α-amino nitrogen release. The predominance of alanine in the amino acid efflux from muscle cannot be explained by its presence in cellular proteins, of which alanine accounts for only 7% to 10%. The major source of alanine outflow from muscle during starvation is derived from the transamination of glucose-derived (from muscle glycogen and circulating glucose) pyruvate. The branched-chain amino acids (valine, leucine, isoleucine) provide the amino groups for muscle alanine synthesis. The alanine that is released from muscle is transported via the bloodstream to the liver, where it is converted to glucose, thus completing the glucose-alanine cycle (see Fig. 10-4).¹³ The glucose-lactate cycle (Cori cycle) also provides an important source of three-carbon skeletons for gluconeogenesis during fasting.¹⁵ Insulinopenia enhances the breakdown of glycogen and leads to accumulation of pyruvate. Because the Krebs cycle has been inhibited by the accelerated rate of FFA oxidation (Randle cycle), the pyruvate can either be transaminated to alanine (glucose-alanine cycle) or converted to lactate and released into the circulation, where it is carried to the liver and synthesized into glucose (glucose-lactate cycle). From the quantitative standpoint, approximately twice as many carbon skeletons are recycled to glucose via the Cori cycle compared with the alanine cycle.

The counterregulatory hormones (glucagon, epinephrine, growth hormone, cortisol, thyroid hormones) all are capable of offsetting the action of insulin on the liver and work by stimulating both glycogenolysis and gluconeogenesis.¹⁶ Glucagon plays a major role in the tonic support of basal hepatic glucose release and, in humans and animals, experimental suppression of endogenous glucagon secretion with preservation of basal insulin levels causes a 30% to 40% decline in hepatic glucose production.¹⁷ This suppression of glucose production involves both the glycogenolytic and gluconeogenic pathways. The precise quantitative contribution of the other counterregulatory hormones to the maintenance of basal glucose output under normal conditions of fasting has not been assessed. However, it is likely that they (i.e., epinephrine, cortisol, and growth hormone) also exert a tonic effect on hepatic glucose production in the postabsorptive state, and the withdrawal of insulin (i.e., hypoinsulinemia) that occurs with prolonged fasting allows their stimulatory effect on glycogenolysis and gluconeogenesis to occur unopposed. The net result of their unopposed action is an increase in hepatic glucose and renal output.

During more pronounced hypoglycemia, as may occur with insulin administration, all the counterregulatory hormones are released and act synergistically to restore normoglycemia.¹⁶ However, they do so with different dose-response kinetics and time courses. Glucagon and catecholamines act rapidly, whereas cortisol, growth hormone, and thyroid hormones (in that order) are involved in the long-range control of hepatic glucose release. Small, acute increases in plasma glucagon and epinephrine concentrations markedly stimulate both glycogenolysis and gluconeogenesis. Acute elevations in plasma cortisol, growth hormone, and thyroid hormones have no stimulatory effect on total hepatic glucose release. However, both cortisol and growth hormone have been shown to markedly potentiate the effects of epinephrine and glucagon on hepatic glucose release.

In addition to hormonal regulation, the central nervous system has an important role in the maintenance of hepatic glucose production.¹⁸ Both parasympathetic and sympathetic fibers reach the liver via the splanchnic nerves, thereby supplying autonomic neural modulation of both glucose production and uptake. In animals, parasympathetic stimulation restrains glycogenolysis and enhances glycogen synthesis, whereas activation of the sympathetic nerves innervating the liver stimulates glucose output via potentiation of both glycogenolysis and gluconeogenesis. In humans, the influence of the sympathetic nervous system on hepatic glucose metabolism can be demonstrated under conditions of acute stimulation, but the contribution of the autonomic nervous system to the maintenance of basal hepatic glucose production remains undetermined.

A number of metabolic signals play an important role in the control of hepatic glucose production in the postabsorptive state.9,10,19 Hyperglycemia per se inhibits liver glucose output. In normal adults, hyperglycemia and hyperinsulinemia occur concurrently, and in combination they provide a potent stimulus to suppress hepatic glucose release. As shown in Fig. 10-5, physiologic hyperglycemia (maintained using the hyperglycemic clamp technique), while maintaining basal insulinemia, is as effective as insulin in suppressing hepatic glucose production. More impressive is the observation that hyperglycemia in the presence of hypoinsulinemia (hyperglycemic clamp with somatostatin) causes a greater than 50% suppression of hepatic glucose release (see Fig. 10-5). Conversely, hypoglycemia by itself provides a trigger to increase hepatic glucose release. During insulin-induced hypoglycemia in humans and animals, an increase in plasma glucose occurs even when the counterregulatory hormonal response is inhibited (glucose autoregulation). Recent studies suggest that this effect of hypoglycemia is mediated via glucose sensors in the hypothalamic region of the brain, which activate hepatic glycogenolysis via sympathetic connections to the liver.²⁰

Altered substrate delivery to the liver also influences glucose release by the liver. In nondiabetic humans, it is difficult to demonstrate a detectable increase in hepatic glucose production by infusing large quantities of glycerol, lactate, or a mixture of amino acids, as long as there occurs a physiologic increase in plasma insulin concentration to balance out such gluconeogenic push. However, even though total hepatic glucose output does not increase, there is a marked stimulation of gluconeogenesis that is precisely counterbalanced by an inhibition of glycogenolysis. The increased provision of gluconeogenic precursors leads to an increase in the intrahepatic formation of G6P, but the eventual fate of this intermediate is glycogen rather than free glucose, because the rate-limiting enzyme for glucose production, G6Pase, is not simultaneously activated. FFAs play an important role in setting the level of hepatic glucose production. Only the odd-chain FFAs (i.e., propionate) can donate their carbon atoms to oxaloacetate in the tricarboxylic acid cycle and thus directly contribute to net gluconeogenesis. Most physiologic FFAs are of even chain, and although they can exchange their carbon moieties with tricarboxylic acid cycle intermediates, they do not contribute to de novo glucose synthesis. Nonetheless, when the perfusion medium of isolated rat liver is enriched with oleate or palmitate, new glucose formation from lactate or pyruvate is enhanced. The biochemical mechanisms involved in this stimulation of gluconeogenesis have been well worked out.²¹ The products of FFA oxidation, citrate and acetyl-CoA, activate the key enzymes that control gluconeogenesis, pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and G6Pase (see Fig. 10-4). In addition, elevated plasma FFA concentrations in vivo are usually accompanied by raised glycerol levels because both result from the hydrolysis of triglycerides (see Fig. 10-4). Therefore, accelerated lipolysis supplies both the stimulus (FFA), the substrate (glycerol), and the energy source (ATP) to drive gluconeogenesis. In isolated hepatocytes, FFAs in micromolar amounts also have been shown to inhibit glycogen synthase. This suggests that an additional interaction of FFA metabolism with hepatic glucose production may be at the level of glycogen metabolism. In healthy volunteers, short-term infusion of triglycerides (with heparin to activate lipoprotein lipase) increases the plasma FFA concentration, leading to the stimulation of hepatic glucose output under conditions (i.e., hyperglycemia and insulinopenia induced by somatostatin plus glucose infusion) that mimic the diabetic state.²² A large part of this effect can be reproduced by infusing, under the same experimental circumstances, glycerol alone. On the other hand, when endogenous insulin is allowed to increase or when exogenous insulin is administered, the stimulatory effect of triglyceride infusion to increase the plasma FFA concentration on hepatic glucose release is easily overcome. In summary, the long-chain FFAs can regulate hepatic glucose production both by acting on the key enzymes of gluconeogenesis (i.e., through buildup of the products of FFA oxidation) and by virtue of the substrate push of glycerol. This regulatory loop is operative particularly when insulin secretion is not stimulated (i.e., in the basal state). Conversely, studies in both animals and humans have documented that a significant part of the suppressive action of insulin on hepatic glucose production is mediated via the hormone’s antilipolytic effect on adipocytes.^10,23 If the plasma FFA concentration is maintained during insulin infusion, the inhibitory effect of physiologic hyperinsulinemia on hepatic glucose production is impaired.

Glucose Disposal

In the basal (postabsorptive) state, the rate of whole-body glucose disposal equals the rate of hepatic glucose production, and the plasma glucose concentration remains constant.1,2 Information about the contribution of individual organs and tissues to total glucose uptake has been obtained in regional catheterization studies performed in combination with glucose tracers and indirect calorimetry.²⁴ By collating the available information, the organ circulation model depicted in Fig. 10-6 can be constructed.⁵ In this synthesis, steady-state interorgan exchanges of glucose, tissue blood flow, and regional glucose gradients are calculated based on a rate of hepatic glucose production of 140 mg/min (778 µmol/min). For a 70-kg man, this equals 2.0 mg/min per kilogram or 11 µmol/min per kilogram. In the postabsorptive state, approximately 70% of basal glucose disposal takes place in insulin-independent tissues (brain, liver, kidney, intestine, RBC). Of these, the brain predominates and accounts for almost half of the total hepatic glucose production. The liver plus gastrointestinal (splanchnic) tissues account for an additional 20%. It also can be appreciated that the fractional extraction of glucose (as defined earlier) is quite low everywhere in the body (ranging from 1.0% to 3.5%) except in the brain (9%) (Table 10-2). Because skeletal muscle represents 40% of total body weight and receives 16% of the cardiac output, one can calculate that it accounts for one third of the overall glucose disposal in the basal state (i.e., ∼245 µmol/min or 44 mg/min) (see Fig. 10-6).^1,2,5 As shown in Table 10-2, the muscle glucose clearance averages 1.3 mL/min per kilogram of tissue. Glucose clearance is a useful metabolic concept and is defined as the amount of plasma that is completely cleared of glucose in a given period; as such, it provides an index of the efficiency of tissue glucose removal. In the rank of efficiency of tissue glucose clearance in the basal state, resting muscle is last, being 10 times less active than the liver and 50 times less avid than the brain. It is noteworthy that tissues (brain, liver, kidneys) that have a high glucose clearance in the basal state are insulin independent. Increasing the plasma insulin concentration above fasting values has no effect on glucose clearance by these tissues (i.e., brain, liver, kidneys), whereas in muscle, glucose clearance increases by a factor of 10 or greater over the physiologic range of insulin concentrations. The intermediate position of heart muscle in the list is accounted for by its constant working state.

These organ-specific glucose clearance characteristics (see Table 10-2) represent the physiologic equivalent of the type and abundance of specific glucose transporters (GLUTs) the various tissues express (Table 10-3).^2,6 They also help to define the concept of an insulin-independent tissue. Thus in tissues in which an increase in the plasma insulin concentration does not accelerate the glucose clearance, the GLUT is not responsive to acute changes in the plasma insulin concentration. At present, several GLUT isoforms have been isolated.^2,6 A non-insulin-regulatable GLUT (GLUT-1) effects facilitated glucose transport in RBCs. The abundance of this transporter in RBCs ensures rapid diffusion of glucose across the RBC membrane, and this characteristic confers on RBCs an important role in the interorgan exchange of glucose. The same GLUT-1 transporter is present in the brain. Because of its low K_m (∼1 mmol/L), it saturates at plasma glucose concentrations well below the normal fasting plasma glucose concentration (∼5 mmol/L), thereby ensuring a constant flux of glucose into the brain cells. This is an important adaptive mechanism that provides the cerebral tissues an adequate supply of fuel even in the face of hypoglycemia. Another unique feature of GLUT-1 is its low V_max (∼3 mmol/L). This protects the brain against acute fluid shifts and cerebral edema that otherwise would accompany hyperglycemia. Thus GLUT-1 is well suited for its physiologic function, especially in the individual with insulin-dependent diabetes in whom extreme shifts in plasma glucose concentration (from hypoglycemia to hyperglycemia) are common. Another important corollary of GLUT-1 is that an increase in plasma glucose concentration above fasting levels (i.e., >5 mmol/L) will necessarily lead to a decline in brain glucose clearance because the transporter saturates at approximately 3 mmol/L. Moreover, because under postabsorptive conditions the brain is responsible for approximately half of the total body glucose disposal, it also follows that an increase in fasting plasma glucose concentration (with or without an increase in plasma insulin) will be associated with a decline in whole-body glucose clearance.

A totally distinct (from the physiologic standpoint) GLUT, GLUT-2, is present in liver and pancreatic β cells.2,6 It has a high K_m (∼15 to 20 mmol/L), and as a consequence, the free glucose concentration in cells expressing this transporter increases in direct proportion to the increase in plasma glucose concentration. This characteristic allows these cells to respond as “glucose sensors.”^1,2 As the ambient glucose concentration increases, more glucose enters the β cell, which responds by appropriately augmenting its secretion of insulin, whereas the liver reads the rising plasma glucose level and decreases its output of glucose. As a corollary of this, an increase in the plasma glucose concentration is associated with a proportional increase in glucose uptake by these tissues with an unchanging glucose clearance. Because GLUT-2 does not respond to insulin, hyperinsulinemia is not associated with an increase in hepatic or β-cell glucose clearance.

It is noteworthy that each GLUT is associated with a specific hexokinase, which has a K_m that parallels that of its associated GLUT.²⁵ For liver and β cells, the phosphorylating enzyme is hexokinase IV or glucokinase. Its high K_m constant has led investigators to propose glucokinase as the β-cell sensor. Consistent with this, recent studies have demonstrated that some forms of maturity-onset diabetes of the young (MODY) are associated with mutations in the glucokinase gene, and the physiologic counterpart of this is a defect in insulin secretion. Insulin-sensitive tissues, muscle, and adipocytes contain GLUT-4 and its physiologic coupler, hexokinase II. GLUT-4 has a K_m constant of approximately 5 mmol/L, which is close to that of the plasma glucose concentration. In the basal state, the majority of GLUT-4 are not located in the plasma membrane but reside in vesicles within the cell. After exposure to insulin, the concentration of GLUT-4 in the plasma membrane of adipocytes and muscle increases markedly, and there is a reciprocal decline in the intracellular GLUT-4 pool.²⁶ Insulin not only enhances their translocation and insertion into the plasma membrane but also augments their intrinsic activity. Thus, muscle glucose clearance increases markedly, 10-fold or greater, in response to increments in plasma insulin concentration within the physiologic range.^1,2,27 GLUT-3, the other major GLUT, is present in the gut and kidney.^2,6 It is sodium-dependent and does not respond to insulin. In the gut, it mediates unidirectional gastrointestinal absorption of glucose in the small intestine, whereas in the kidney, it regulates unidirectional glucose absorption in the proximal tubule. Although this transporter is insulin insensitive, it plays a crucial role in glucose homeostasis by regulating its entry into the body and preventing its loss via the kidney.

The intracellular disposition of transported glucose can be studied in vivo by using glucose tracers and then measuring the appearance of the label in specific metabolic products such as lactate (i.e., anaerobic glycolysis) and carbon dioxide (i.e, complete oxidation).⁸ These techniques, even when correctly applied, provide only estimates of the metabolic fate of plasma glucose. For example, should glycogen in muscle be oxidized directly, the plasma glucose–specific activity would miss it completely because plasma glucose does not equilibrate with the intracellular free-glucose pool. To circumvent this problem, investigators have employed indirect calorimetry, which measures total carbon dioxide production from all carbohydrate sources, both intracellular and extracellular.²⁸ Although indirect calorimetry depends on a number of assumptions, these are reasonable and have been largely validated. Moreover, this technique is easy to apply and noninvasive. Indirect calorimetry also provides a good estimate of the rate of energy expenditure and complements information obtained by tracer methods. In the basal state and under ordinary nutritionally circumstances, oxygen consumption averages 250 mL/min, whereas carbon dioxide production is 200 mL/min—that is, the whole-body respiratory quotient equals 0.8 (respiratory quotient = carbon dioxide production/oxygen consumption). From the equations depicted in Table 10-4, whole-body carbohydrate oxidation can be estimated to account for approximately 60% of total glucose uptake in the postabsorptive state.²⁴ Because the brain uses 46% of the total glucose turnover (see Table 10-2), and because essentially all brain glucose uptake is accounted for by oxidation, it follows that three fourths (i.e., 46/0.6 or 77%) of basal glucose oxidation occurs in the brain. Little is left for other tissues, which preferentially derive their metabolic energy from the oxidation of FFAs and other lipids under postabsorptive conditions. Skeletal muscle, for example, has a respiratory quotient of 0.75 and relies on fat oxidation for the production of 80% of the energy that it needs in the resting state. Thus the basal state is characterized by parsimonious use of glucose as a metabolic fuel.¹⁵ Moreover, the glucose is selectively channeled to organs that cannot rely on alternative energy sources. In the postabsorptive state, more than half of the total energy production is generated via oxidation of fat, of which there are plentiful stores (see Table 10-1). Insulin is the principal regulator that determines the metabolic mix of fuels in the basal state. A small decrement in the circulating hormone level releases the brake on lipolysis, and the plasma FFA concentration increases, thereby allowing fat to override glucose in the competition between the two substrates. Although these very small changes in plasma insulin concentration are sufficient to promote a shift in fuel metabolism from carbohydrate to fat, the plasma insulin level is still sufficiently elevated to maintain glucose transport and metabolism in target tissues at minimal rates and to restrain protein breakdown, which contributes only approximately 15% to basal energy metabolism.¹⁵ The role that counterregulatory (glucagon, epinephrine, cortisol, growth hormone, thyroid hormones) hormones play in basal glucose uptake is less well defined but probably centers on the maintenance of lipolysis, because all the insulin antagonistic hormones are more or less potent lipolytic stimuli.

Glucose Cycles

After entry of glucose into the cell through a specific GLUT, the sugar does not necessarily follow a direct path to its eventual fate, be it glycogen, lactate, carbon dioxide, or pentoses. Rather, it may in part reach its destination via a number of circuitous routes that have become known as futile cycles. A metabolic futile cycle is one in which a precursor is converted into a product by a forward reaction and then resynthesized to the precursor. In such a reaction, there is no net product accumulation, but energy (ATP) is used. There are multiple examples of such futile cycles in the glucose metabolic pathway.29,30 The first involves the conversion of glucose to G6P by glucokinase and its subsequent reconversion to intracellular free glucose by G6Pase in the liver. Each turn of this cycle uses one molecule of ATP. Another example of a futile cycle is represented by the conversion of G6P to fructose-6-phosphate and back through the phosphoglucoisomerase reaction. Perhaps the best-studied futile cycle that is under the control of insulin is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate in the liver. The reverse reaction is regulated by fructose-1,6-bisphosphatase, whereas the forward reaction is catalyzed by phosphofructokinase (PFK). The latter enzyme is controlled by the energy status of the cell and key intracellular metabolites. High levels of ATP, acidosis, and citrate inhibit PFK, whereas ADP and alkalosis stimulate PFK. The most potent activator of PFK is fructose-2,6-bisphosphate, whose synthesis is stimulated by the enzyme fructose-2,6-bisphosphate kinase. This latter enzyme is under the control of insulin, and the PFK step, therefore, represents an important regulatory control point for insulin action.³¹

In general terms, whenever bidirectional flux through a metabolic pathway is simultaneously operative, there exists a cycle, regardless of the number of intermediate reactions and regardless of whether one or more tissues are involved. In the examples cited previously, the cycles occurred within individual cells. However, cycles also can exist between organs. In this regard, lipolysis in adipose tissue followed by partial reesterification of FFAs in the liver is a complete cycle. Another important cycle is the breakdown of proteins in the liver or other tissues. The glucose-alanine and glucose-lactate (Cori) loops¹³ also represent important cycles (see previous discussion) that provide conservation of carbon skeletons and transfer of α-amino groups between muscle and liver.

The derogatory connotation of futility has traditionally been reserved for those cycles that go on in the same cell. These cycles are, however, anything but futile. As elegantly discussed by Newsholme and Leech,²⁹ a metabolic cycle with a reverberating internal loop provides the best kinetic stratagem to maintain the enzymes of a dormant pathway at a minimum of activity, while at the same time ensuring a high sensitivity gain for rapid amplification of incoming signals. The ATP cost of these cycles is itself a means of increasing the efficiency of energy dissipation. The fact that the activity of these cycles is under hormonal control (e.g., catecholamines, glucagon, and thyroid hormones enhance the cycling rate) establishes a mechanism for rapid modulation. In this way, these cycles become components of facultative thermogenesis. Equally important, the operation of these futile or substrate cycles allows the generation of metabolic intermediates that can modulate the activities of key enzymes and allow allosteric regulation.