- Anabolic hormone synthesized in β cells of islets (70% of cells, lie centrally)
- Promotes glucose transport into all cells
- Except β cells, hepatocytes, and CNS
- Increased glucose, fatty acid, and amino acid storage
- 56–amino acid polypeptide
- T1/2: 7-10 mins
- Synthesized as precursor proinsulin, in response to glucose
- Cleaved into insulin and C peptide (equal amounts of each)
- Insulin moved into secretory granules
- Released via exocytosis directly into blood stream
- β cell highly sensitive to increases in glucose concentration
- Immediately reacts with a short burst of stored insulin (4 to 6 minutes)
- Followed by sustained secretion of insulin
- Requires active synthesis within the islet cell
- Primarily metabolized by the liver
- Brain cells and RBCs do not take up insulin
- Insulin binds to a specific insulin receptor
- Actively transported across cell membranes throughout the body
- Several classes of glucose transporters, with varying affinities for glucose
- Insulin resistance (in T2DM) can be the result of
- Decreased numbers of receptors
- or a decreased affinity of receptors for insulin
- Sulfonylureas stimulate insulin secretion, are used in T2DM insulin resistance