Definition
An inhibitory peptide hormone that suppresses growth hormone, insulin, glucagon and many other secretions.
Somatostatin is a cyclic peptide hormone existing in two active forms, of 14 and 28 residues, each containing a disulfide bridge. It is produced by the hypothalamus, pancreatic delta cells, the gut and other tissues. It acts through five G protein-coupled receptor subtypes, SSTR1 to SSTR5, mainly via Gi to lower cAMP, broadly inhibiting hormone secretion, including growth hormone, TSH, insulin and glucagon. In the growth hormone axis it opposes GHRH. Native somatostatin has a half-life of a few minutes, so stabilised cyclic analogues containing D-amino acids were developed; octreotide, lanreotide and pasireotide are approved prescription medicines. Labelled analogues are also used in receptor imaging research.