INSULIN HORMONE ACTION
Insulin hormone is synthesized in and secreted from the islets of Langerhans in the endocrine tissue of the pancreas gland. The islets develop from endodermal outgrowths from fetal gut. The normal pancreas has about a million scattered islets of variable size. They comprise only 2% of pancreas volume. The islets contain four main cell types that produce different hormones:
❏ β-cells produce insulin hormone.
❏ α-cells produce glucagons.
❏ δ-cells produce somatostatin.
❏ PP cells produce pancreatic polypeptide.
β-cells are located mainly in the centre of the islet whereas the α- and δ-cells are located towards the periphery. These islet cells interact through direct contact and secretions. The pancreatic islets are innervated with autonomic nerves. Parasympathetic nerves stimulate insulin release while adrenergic sympathetic nerves inhibit insulin and stimulate glucagon. There are additional nerves that stimulate hormone release-producing neuropeptides such as vasointestinal peptide (VIP) and others that inhibit insulin secretion by producing neuropeptide Y (1a).
The main stimulator of insulin production is glucose. The stimulation occurs in a biphasic pattern. There is an acute first phase that lasts several minutes followed by a sustained second phase. The rate of insulin release is controlled by the activity of the enzyme glucokinase. Maximal insulin release occurs at glucose concentrations of 20 mmol/l. Glucose levels below 4 mmol/l, however, do not stimulate insulin release.
Glucose enters the β-cell by the GLUT-2 transporter before being phosphorylated by glucokinase and coupled to insulin release. Glycolysis produces adenosine triphosphate (ATP). This closes ATP sensitive potassium channels, depolarizes the β-cell membrane and leads to an influx of calcium. This leads to granule exocytosis and release of the insulin hormone (1b).
E234 – Insulin Hormone Action – www.diabetic.today