Somatostatin inhibits the secretion of growth hormone, insulin, and other hormones
Somatostatin is made in the δ cells of the pancreatic islets (see Fig. 51-1), as well as in the D cells of the gastrointestinal tract (see pp. 868–870), in the hypothalamus, and in several other sites in the CNS (see pp. 993–994). Somatostatin was first described as a hypothalamic peptide that suppresses the release of growth hormone; growth hormone had also been called somatotropin, which accounts for the name somatostatin. In both pancreatic δ cells and the hypothalamus, somatostatin exists as both 14– and 28–amino-acid peptides. In the hypothalamus, the 14–amino-acid form is predominant, whereas in the gastrointestinal tract (including the δ cells), the 28–amino-acid form predominates. The 14–amino-acid form is the C-terminal portion of the 28–amino-acid form. The biological activity of somatostatin resides in these 14 amino acids.
Somatostatin inhibits the secretion of multiple hormones, including growth hormone, insulin, glucagon, gastrin, vasoactive intestinal peptide (VIP), and thyroid-stimulating hormone. This property has led to therapeutic use of a long-acting somatostatin analog (octreotide) in some difficult-to-treat endocrine tumors, including those that produce growth hormone (acromegaly), insulin (insulinoma), or serotonin (carcinoid), among others. The concentration of somatostatin found in pancreatic venous drainage is sufficiently high to inhibit basal insulin secretion. Recall that blood flows from the center of each islet—which is where the bulk of the β cells are—to the periphery of the islet—which is where the δ cells tend to be located (see Fig. 51-1). This spatial arrangement minimizes the effect of somatostatin on the islet from which it is secreted. Whether somatostatin has important paracrine actions on some β cells or on α cells remains controversial.
The islet cells also make other peptides; for example, pancreatic polypeptide is made in the F cells of the pancreas. As with insulin and glucagon, secretion of pancreatic polypeptide is altered by dietary intake of nutrients. However, whether pancreatic polypeptide has any actions in mammalian fuel metabolism is not clearly understood.
Occasionally, islet cell tumors may develop and secrete gastrin, VIP, growth hormone–releasing factor, or other hormones. Although these individual instances prove that these peptides can be made by islet tissue, they have no known normal function in the islet.