Glucose levels in the blood rise after a meal. This increase stimulates the β-cells in the islets of Langerhans to secrete insulin. Insulin has effects on several tissues: liver, adipose tissue, muscle tissue and other body cells. In general insulin stimulates glucose uptake. In the liver and in muscle tissue insulin also induces glycogen synthesis. These events result in a decrease of the blood glucose levels to normal values. In adipose tissue the synthesis of triglyceride is stimulated.
The physiological regulation of glucose uptake is depicted above. After food intake (1) the complex sugars are broken down in the small intestine into glucose molecules. L-cells in the distal intestinal wall secrete the incretin GLP-1 upon stimulation by food/glucose (2).
This glucagon-like peptide-1 is released proportional to the amount of food post-prandial in the distal intestine. GLP-1 is key player in many processes after a meal. It stimulates the release of insulin by the β-cells in the pancreas (3). Similarly, GLP-1 inhibits the release of glucagon by the α-cells (4). The rise in insulin will stimulate the glucose uptake from the blood into the cells (8). Further, GLP-1 inhibits gastric emptying (5) and induces the feeling of satiety (6) in order to reduce further carbohydrate intake. GLP-1 is metabolised by the enzyme dipeptidyl peptidase 4 (DDP-IV) which is present in the cell wall (7).
I. Insulin has a plasma half-life of 10-20 minutes.
II. Fasting induces secretion of insulin.
Extra info: Insulin has a short half-life of 10-20 minutes. Fasting inhibits the secretion of insulin, because plasma glucose levels decrease.