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Home / Resources / Clinical Gems / International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #77: Insulin Actions In Vivo: Glucose Metabolism Part 3 of 9

International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #77: Insulin Actions In Vivo: Glucose Metabolism Part 3 of 9

The basal state 

Glucose production   

By convention, the basal state is the metabolic condition prevailing in the morning after an overnight (10–14h) fast. This time marks the end of the longest period of fasting in ordinary life and also is the most common point of clinical observation and physiologic measurements. The true value of basal endogenous (liver plus kidney) glucose production is the one that would be reproducibly measured with the use of an irreversible glucose tracer, which loses its label at the earliest possible intracellular step without ever getting it reincorporated into a circulating tracer molecule [14,15]. Carbon isotopes systematically underestimate glucose turnover because of the efficient reincorporation of carbons (via three-carbon fragments such as lactate) into new glucose in the liver (via gluconeogenesis) (Figure 14.2).

Chapter14Fig14.2Gluconeogenesis can occur in other tissues (e.g. skeletal muscle [17]), but the absence of significant G6Pase activity in tissues other than the liver and kidney prevents labeled break- down products from re-entering the circulation as glucose, where they would alter the estimation of glucose turnover. As for tritiated or deuterated isotopes, labels in positions 3, 4, 5, and 6 are lost at the triosephosphate step or further downstream in anaerobic glycolysis, while a label in position 2 is largely lost at the phosphoglucoisomerase step (a near-equilibrium reaction) soon after phosphorylation [14,15,18] (Figure 14.2). In neither case does the detached label (essentially in equilibrium with the hydrogen of the body water pool) recycle back into a new glucose molecule to any detectable extent.

In the basal state, glucose output in healthy adults averages ∼840 μmol min−1 (or ∼12 μmol min−1 per kg of body weight) [19]. The dispersion around this mean estimate is significant (20–30%), with an unknown contribution of genetic and environmental factors. In nondiabetic subjects total body endogenous glucose output variability is wide and is largely explained by the amount of lean mass [20] and this, in turn, explains differences in total endogenous glucose output due to sex, obesity, and age. Under standard nutritional conditions, the fasting liver depletes its glycogen stores at a rate of about 5% per hour, such that glycogen depots are empty after 24 h. Since fasting can be prolonged well beyond 24h, obviously gluconeogenesis must progressively replace glycogenolysis as fast continues [21,22].

In animal species in which the basal rate of glucose turnover is higher than in humans (e.g. dogs, 20 μmol min−1 kg−1 ; rats, 40 μmol min−1 kg−1 ), 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 anatomical basis: overcrowding of cytoplasm with glycogen granules impairs cellular functions, leading to infiltration of nuclei and, eventually, to cell death, as observed in several glycogen storage diseases. In normal healthy adults in the overnight fasted state, gluconeogenesis and glycogenolysis approximately equally contribute to hepatic glucose release [21–23]. In obese nondiabetic and in type 2 diabetic subjects the contribution of gluconeogenesis to hepatic glucose production is increased compared to lean normal glucose tolerant subjects [22,24,25]. In subjects with variable degrees of overweight and hyperglycemia, it has been established that the percent contribution of gluconeogenesis to fasting glucose release rises with increasing body mass index (by ∼1% per body mass index unit) and increasing fasting hyperglycemia (by ∼3% per mmol L−1 ) [22]. In healthy subjects, physiologic hyperinsulinemia suppresses percent gluconeogenesis by ∼20% while completely blocking glycogenolysis [24,26]. As long as hyperinsulinemia restrains glycogenolysis — as is the case of obese nondiabetic subjects—basal endogenous glucose release will be normal in absolute terms. As glycogenolysis also becomes resistant to the inhibitory effect of insulin—as in the case of type 2 diabetic patients [26] — fasting glucose output will increase [12,27]. Circulating lactate, pyruvate, glycerol, alanine, and other gluconeogenic amino acids are important gluconeogenic precursors [28,29]. However, transsplanchnic catheterization in humans has shown that net uptake of circulating precursors accounts for less than 50% of endogenous glucose production in the basal state [28,29]. The discrepancy (amounting to 1–2μmolmin−1 kg−1) between radioisotopic estimates of basal gluconeogenic rate [22] and accountable circulating precursors [28,29] indicates that the blood is not the only route of gluconeogenic supply. Within the splanchnic area, the intestine returns 10–20% of its glucose uptake to the liver as alanine (0.5 μmol min−1 kg−1 ) [30], thereby filling part of the gap. Intrahepatic lipolysis (i.e., glycerol), proteolysis, and glycolysis theoretically could guarantee ample provision of precursors as an alternative or in addition to those in the circulation. However, the liver can supply gluconeogenic amino acids only at the cost of breaking down liver tissue. It has been calculated that to produce glucose at a rate of 5.6 μmol min−1 kg−1 (i.e., ∼50% of fasting hepatic output) would require the consumption of all the protein in 40 g of liver tissue every hour [22]. Thus, both the amount and source of the gluconeogenic flux in fasting humans remain elusive.

Chapter14Fig14.3The main control of basal hepatic and renal glucose production is exerted by the sum of several neurohormonal and metabolic influences, some stimulatory, others inhibitory. Figure 14.3 depicts the control system in the liver as a simple balance between inhibition and stimulation. Both parasympathetic and sympathetic fibers reach the liver via the splanchnic nerves, thereby supplying autonomic nervous modulation of both glucose production and uptake. An increase in parasympathetic impulses restrains glycogenesis and enhances glycogen synthesis, while an increase in sympathetic outflow to the liver stimulates glucose output via potentiation of both glycogenolysis and gluconeogenesis [31]. 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 glucose production remains undetermined.

Chapter14Fig14.4As for metabolic signals, hyperglycemia per se can effectively inhibit liver glucose output in normal humans [29]. As shown in Figure 14.4, during constant hyperglycemia (maintained by the hyperglycemic clamp technique) hepatic glucose production is significantly reduced in comparison with euglycemia even when the endogenous insulin response to hyperglycemia is blocked by somatostatin. It is evident from Figure 14.4 that normally the two inhibitory signals, hyperglycemia and endogenous hyperinsulinemia, concur to shut off glucose production [32–34]. Hypoglycemia by itself may trigger an increase in hepatic glucose release even when one or more counterregulatory influences are paralyzed [35]. Studies in humans [33,36] and in dog [37] have established a role for a portosystemic glucose concentration gradient in controlling hepatic glucose handling. Thus, intraportal glucose infusion specifically alters the partitioning of a glucose load among tissues, by stimulating net hepatic glucose uptake and reducing peripheral glucose uptake. Of other metabolic signals, it is difficult to demonstrate in humans an increase in hepatic glucose production by infusing large quantities of glycerol, lactate, or a mixture of amino acids, as long as there is physiologic hyperinsulinemia to balance out such a gluconeogenic push.

 

Note, that an increased provision of precursors may well lead to increased intrahepatic formation of glucose-6-phosophate (G6P), but the eventual fate of this intermediate may be glycogen rather than free glucose if control of the rate-limiting step for free glucose production, namely G6Pase activity, is not simultaneously loosened. Free fatty acids (FFA) stand out as one substrate that may play an important role in setting the level of hepatic glucose production. Since only odd-chain FFA (i.e., propionate) can donate their carbon to oxaloacetate in the tricarboxylic acid cycle, FFA do not represent an important precursor for gluconeogenesis. Most physiologic FFA are even-chain, and they exchange their carbon moieties with tricarboxylic acid cycle intermediates but do not contribute to de novo glucose synthesis. Nevertheless, enriching the perfusion medium of isolated rat liver with oleate or palmitate induces an increase in new glucose formation from lactate or pyruvate [38]. Moreover, FFA and/or products of their oxidation (e.g. citrate and acetyl-CoA) activate key gluconeogenic enzymes such as pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and G6Pase [38,39]. In addition, raised FFA concentrations in vivo are accompanied by raised glycerol levels, resulting from hydrolysis of triglycerides. Therefore, accelerated lipolysis normally supplies both the stimulus (FFA) and the substrate (glycerol) for gluconeogenesis. Finally, the liver takes up FFA avidly (with an extraction ratio of ∼30%), and oxidizes them efficiently (as indicated by the low respiratory quotient of the organ) [40,41]. Thus, there are all the requisites to consider FFA oxidation in the liver as the energy-providing process that is coupled to energy-requiring gluconeogenesis. In isolated hepatocytes, FFA in micromolar amounts inhibit glycogen synthase [42], which suggests that an additional interaction of this substrate with hepatic glucose metabolism may be at the level of glycogen metabolism. In healthy volunteers, short-term infusion of triglycerides with heparin (to activate lipoprotein lipase and elevate the plasma FFA concentration) results in an increase in hepatic glucose output under conditions (hyperglycemia and somatostatin block of endogenous insulin response) mimicking the key features of diabetes [43]. When endogenous insulin is allowed to rise, or when exogenous insulin is administered, the stimulatory effect of triglyceride infusion on hepatic glucose release is easily overcome. Consistent with this, FFA infusion in healthy subjects augments hepatic gluconeogenesis but basal hepatic glucose production does not rise because of a reciprocal decrease in hepatic glycogenolysis [40,44].

In summary, long-chain FFA may regulate glucose production both by acting upon key enzymes of gluconeogenesis (through products of FFA oxidation) and by virtue of the substrate push of glycerol. In glucose-tolerant normal subjects, this regulatory mechanism is primarily operative when insulin secretion is not stimulated, that is, in the basal state. Insulin and the classic counterregulatory hormones—glucagon, cortisol, growth hormone (GH), catecholamines, and triiodothyronine (T3 ) — form one of the best-described agonist – antagonist regulatory systems (Figure 14.3). Insulin is a potent, specific, and rapid-acting inhibitor of hepatic glucose production [12,45]. It restricts both glycogenolysis and gluconeogenesis, although with different dose – response characteristics, gluconeogenesis being less sensitive [24,46]. Moreover, by restraining lipolysis and proteolysis, insulin also reduces the delivery of potential glucose precursors (glycerol, amino acids) to the liver. In its capacity as the inhibitory signal for glucose release, insulin is greatly favored by the anatomical connection between the pancreas and the liver, and secreted insulin reaches the liver at a concentration that in fasting humans is three- to fourfold higher than the peripheral (arterial) concentration [47]. Such portosystemic gradient is maintained by a high rate of insulin degradation by hepatic tissues (with a fractional extraction of about 50%). Thus, a small secretory stimulus to the β cell primarily serves to increase portal insulin levels, thereby selectively acting upon glucose production rather than also enhancing peripheral glucose utilization. In addition to short-circuiting the systemic circulation, pancreatic insulin release is potentiated by several gastrointestinal hormones (e.g. glucose-dependent insulinotropic polypeptide [gastric inhibitory polypeptide] and glucagon-like peptide 1). Therefore, anatomical and physiologic connections in the gut – liver – pancreas circle 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.

The anti-insulin hormones all counter insulin action on the liver by facilitating both glycogenolysis and gluconeogenesis. They do so, however, with different dose – response kinetics and time courses. Thus, glucagon and catecholamines act rapidly, while cortisol, growth hormone, and thyroid hormones (in that order) are involved in the long-term control of glucose release [48]. Glucagon plays a major part in the tonic support of hepatic glucose release: in humans suppression of glucagon release with preservation of basal insulin secretion causes a fall of glucose production of over one third [48]. The precise quantitative contribution of the other counterregulatory hormones system to the maintenance of basal glucose output has not been assessed. What has become established, however, is the strongly synergistic pattern of interaction between the anti-insulin hormones, such that their cumulative effect is larger than the sum of the individual effects [49,50]. This interaction also is expressed at the level of the components of glucose production, glycogenolysis and gluconeogenesis, as well as at the level of peripheral tissues (as discussed later in this chapter). An added feature of this homeostatic complex is the dual negative feedback between agonists and antagonists (Figure 14.3). Alpha cells and β cells “talk” to each other in the islets of Langerhans via paracrine influences (some of which may possibly be mediated by somatostatin). For example, insulin infusion in vivo reduces circulating glucagon levels by 20 – 30% even when changes in glycemia are prevented [51].

Another example is the direct modulation of β-cell secretion by catecholamines, with α-adrenergic stimulation (mostly, α-2) inhibiting and β-adrenergic stimulation increasing insulin secretion. It is noteworthy that in this control system a host of hormones is required to balance the action of only one agonist, insulin. This fact arises from the inhibitory nature of insulin’s effect on the production of a fuel upon which brain cell viability depends in an obligatory manner.

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