Blood Glucose Regulation
- - Alpha cells are glucagon- synthesizing
- - Beta cells are insulin-producing
- They act as fuel sensors secreting glucagon and insulin during fasting and fed states.
- Glucagon and insulin are intimately but independently involved in the regulation of blood glucose levels.
- Effects are antagonistic; insulin is hypoglycemic hormone glucagon is a hyperglycemic hormone.
Falling blood glucose levels (humoral stimuli) prompt the alpha cells to secrete glucagon. Other stimuli include sympathetic nervous system stimulation and rising amino acid levels. Glucagon targets the liver where it breaks down glycogen to glucose; glucose is then released into the blood by liver cells, causing blood glucose levels to rise. Glucagon release is suppressed by rising blood glucose levels, insulin and somatostain (GHIH).
Pancreatic beta cells are stimulated to secrete insulin chiefly by elevated blood glucose levels (also by rising plasma levels of amino acids and fatty acids). Insulin stimulates glycogen production and inhibits the breakdown of glycogen to glucose in the liver. Insulin also stimulates amino acid uptake and protein synthesis in muscle tissue. Insulin sweeps glucose out of the blood causing it to be used for energy or converted to other forms. As body cells take up glucose and other nutrients, and plasma levels of these substances drop, insulin secretion is suppressed.