REGULATION OF BLOOD GLUCOSE
Adequate concentrations of glucose in the blood are necessary for brain cells as they cannot metabolize substances other than glucose and ketone bodies as energy sources nor can they store or synthesize glucose. After a meal, any released glucose is absorbed by the gastrointestinal tract enters the bloodstream and is delivered to the peripheral tissues where it may be metabolized to allow ATP production. Surplus glucose is converted to glycogen and stored in the liver and skeletal muscles or converted to triacylglycerols and stored in adipose tissue. During fasting, the liver produces glucose by glycogenolysis or gluconeogenesis and this is used to maintain blood glucose concentration (Figure 7.22).
The concentration of glucose in plasma is regulated by the hormones insulin and glucagon. Insulin is synthesized as preproinsulin in the β cells of the islets of Langerhans in the pancreas but during its secretion is enzymatically converted to active insulin (Figures 7.3 (B) and 7.23). Insulin has a number of functions. It inhibits glycogenolysis, gluconeogenesis, lipolysis, ketogenesis
and proteolysis and stimulates glucose uptake by muscle and adipose tissues, glycolysis, glycogenesis, protein synthesis and uptake of K+ and Pi. Glucagon is released by the α cells of the pancreas. Its effects are antagonistic to those of insulin. An increase in blood glucose stimulates the pancreas to produce insulin which, in turn, promotes the uptake and utilization of glucose by cells lowering its concentration. A reduction in blood glucose stimulates release of glucagon that promotes glycogenolysis in the liver thereby increasing blood glucose levels (Figure 7.24). Disorders of insulin release or activity can cause an increase in blood glucose, hyperglycemia, or its reduction, hypoglycemia.