In the fasting state, glucose turnover in a 70 kg (154 lb) individual is approximately 2 mg/kg/min (200 g/24 h). The plasma glucose concentration reflects the balance between intake (glucose absorption from the gut), tissue utilization (glycolysis, pentose phosphate pathway, tricarboxylic acid (TCA) cycle, glycogen synthesis) and endogenous production (glycogenolysis and gluconeogenesis). Glucose homeostasis is controlled primarily by the anabolic hormone insulin and also by several insulin-like growth factors. Several catabolic hormones (glucagon, catecholamines, cortisol, and growth hormone) oppose the action of insulin; they are known as anti-insulin or counter-regulatory hormones (Fig. 20.1).
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| Figure 20.1 Hormonal control of glucose homeostasis. Plasma glucose concentration reflects the balance between the hypoglycemic action of insulin and the hyperglycemic action of anti-insulin hormones. The lower part of the figure illustrates the daily patterns of insulin, glucagon and plasma glucose concentrations. Glucose concentration throughout the day remains in a relatively narrow range (see also Fig. 12.1). To obtain glucose concentrations in mg/dL, multiply by 18. |
| Insulin and glucagon are the main hormones responsible for controlling plasma glucose levels
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| Insulin is secreted from the pancreas in response to the increase in plasma glucose following a meal. Insulin decreases the plasma glucose concentration by promoting the uptake of glucose into tissues, intracellular glucose metabolism, and glycogen synthesis. Anti-insulin hormones stimulate both the release of glucose from glycogen stores and its de novo synthesis, thus causing an increase in glucose concentration in plasma (hyperglycemia). The balance between the effects of insulin and glucagon is a key factor in the control of fuel metabolism. Insulin and glucagon are both secreted from the same anatomic location - the pancreatic islets of Langerhans. Insulin is secreted by β-cells (which constitute approximately 70% of all islet cells) and glucagon is secreted by the α-cells.
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