Which hormone inhibits both insulin and glucagon secretion?

• A. Amylin
• B. Somatostatin
• C. Triiodothyronine (T3)
• D. Pancreatic polypeptide

Answer: (B)

Somatostatin serves as a paracrine brake within the pancreatic islets, produced by delta cells to dampen the secretion of both insulin from beta cells and glucagon from alpha cells. By binding to its receptors on these cells, it reduces signaling that leads to hormone release, helping stabilize blood glucose after meals. Mechanistically, it lowers cAMP signaling and calcium influx, which lowers the exocytosis of hormone-containing granules. This balancing action is triggered by high glucose and amino acids, making somatostatin the hormone that can simultaneously inhibit both insulin and glucagon. Amylin is mainly co-secreted with insulin and primarily helps suppress postprandial glucagon while slowing gastric emptying; it does not inhibit insulin. Triiodothyronine increases metabolic rate and does not directly suppress these pancreatic hormones. Pancreatic polypeptide modulates exocrine secretions and gut motility rather than specifically inhibiting both islet hormones.