Somatostatin Inhibits Glucagon and Insulin Secretion
The delta cells of the islets of Langerhans secrete the hormone somatostatin, a polypeptide containing only 14 amino acids that has an extremely short half-life of only 3 minutes in the circulating blood. Almost all factors related to the ingestion of food stimulate somatostatin secretion. They include (1) increased blood glucose, (2) increased amino acids, (3) increased fatty acids, and (4) increased concentrations of several of the gastrointesti-nal hormones released from the upper gastrointestinal tract in response to food intake.
In turn, somatostatin has multiple inhibitory effects as follows:
1. Somatostatin acts locally within the islets of Langerhans themselves to depress the secretion of both insulin and glucagon.
2. Somatostatin decreases the motility of the stomach, duodenum, and gallbladder.
3. Somatostatin decreases both secretion and absorption in the gastrointestinal tract.
Putting all this information together, it has been sug-gested that the principal role of somatostatin is to extend the period of time over which the food nutrients are assimilated into the blood. At the same time, the effect of somatostatin to depress insulin and glucagon secretion decreases the utilization of the absorbed nutrients by the tissues, thus preventing rapid exhaus-tion of the food and therefore making it available over a longer period of time.
It should also be recalled that somatostatin is the same chemical substance as growth hormone inhibitoryhormone, which is secreted in the hypothalamus andsuppresses anterior pituitary gland growth hormone secretion.
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