Hypoglycemia (a low concentration of blood glucose ) is defined as a blood glucose concentration below 2.5 mmol/L (45 mg/dL) (see Fig. 20.8). A decrease in plasma glucose concentration stimulates the sympathetic nervous system. Epinephrine and glucagon are released, resulting in a stress response, the manifestations of which may include sweating, trembling, increased heart rate, and a feeling of hunger. If blood glucose continues to fall, brain function is compromised owing to lack of glucose (neuroglycopenia). The patient becomes confused and may lose consciousness. Profound hypoglycemia can be fatal.
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| Figure 20.6 Regulation of gluconeogenesis and glycolysis by phosphofructokinase (see Fig. 20.7). Glucagon regulates gluconeogenesis by controling the bifunctional enzyme complex that contains both phosphofructokinase-2 (PFK-2) and fructose 2,6-biphosphatase-2 (Fru-2,6-BPase-2) activity. This changes the concentration of fructose-2,6-biphosphate (Fru-2,6-BP), which in turn regulates the activity of 'mainline' enzymes: PFK-1 and Fru-2,6-BPase-1. See also Chapters 11 and 12. |
| GLYCOGEN STORAGE DISEASES |
| Glycogen is degraded in response to a falling blood glucose concentration. A defect in the glycogenolytic pathway can lead to insufficient glucose supply and may cause hypoglycemia. This happens in patients with inherited deficiencies of enzymes controling glycogen metabolism. Seven types of glycogen storage disease are known. They are very rare, but provide an excellent insight into human energy metabolism (see Chapter 12). The symptoms of glycogen storage diseases vary and depend on the site of the enzyme defect. For instance, type 1 glycogen storage disease (von Gierke's disease) is a deficiency of Glc-6-Pase, which leads to a fasting hypoglycemia unresponsive to epinephrine and glucagon. On the other hand, patients with type V disease (McArdle's disease), which is caused by muscle phosphorylase deficiency, do not experience hypoglycemia, but have a limited ability to perform strenuous exercise (see clinical box on p. 163). |
| Hypoglycemia in healthy individuals is usually mild and may occur during exercise after a period of fasting, or as a result of drinking alcohol. Alcohol increases the intracellular NADH + H+/NAD+ ratio, which favors conversion of pyruvate to lactate and reduces the pool of pyruvate available for gluconeogenesis. Hypoglycemia may also be a feature of endocrine syndromes characterized by a low cortisol concentration caused by an insufficient amount of adrenocorticotrophic hormone (ACTH; see Chapter 37). Another endocrine cause of hypoglycemia is a rare insulin-secreting
tumor of β-cells - insulinoma. Other causes are listed in Figure 20.9.
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