secretin receptor-like group of hormone receptors, member of the class B family of seven-transmembrane G protein-coupled receptors
This group represents G protein-coupled receptors for structurally similar peptide hormones that include secretin, growth-hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating polypeptide (PACAP), and vasoactive intestinal peptide (VIP). These receptors are classified into the subfamily B1 of class B GRCRs that consists of the classical hormone receptors and have been identified in all the vertebrates, from fishes to mammals, but are not present in plants, fungi, or prokaryotes. For all class B receptors, the large N-terminal extracellular domain plays a critical role in peptide hormone recognition. Secretin, a polypeptide secreted by entero-endocrine S cells in the small intestine, is involved in maintaining body fluid balance. This polypeptide regulates the secretion of bile and bicarbonate into the duodenum from the pancreatic and biliary ducts, as well as regulates the duodenal pH by the control of gastric acid secretion. Studies with secretin receptor-null mice indicate that secretin plays a role in regulating renal water reabsorption. Secretin mediates its biological actions by elevating intracellular cAMP via G protein-coupled secretin receptors, which are expressed in the brain, pancreas, stomach, kidney, and liver. GHRHR is a specific receptor for the growth hormone-releasing hormone (GHRH) that controls the synthesis and release of growth hormone (GH) from the anterior pituitary somatotrophs. Mutations in the gene encoding GHRHR have been connected to isolated growth hormone deficiency (IGHD), a short-stature condition caused by deficient production of GH or lack of GH action. VIP and PACAP exert their effects through three G protein-coupled receptors, PACAP-R1, VIP-R1 (vasoactive intestinal receptor type 1, also known as VPAC1) and VIP-R2 (or VPAC2). PACAP-R1 binds only PACAP with high affinity, whereas VIP-R1 and -R2 specifically bind and respond to both VIP and PACAP. VIP and PACAP and their receptors are widely expressed in the brain and periphery. They are upregulated in neurons and immune cells in responses to CNS injury and/or inflammation and exert potent anti-inflammatory effects, as well as play important roles in the control of circadian rhythms and stress responses, among many others. All B1 subfamily GPCRs are able to increase intracellular cAMP levels by coupling to adenylate cyclase via a stimulatory Gs protein. However, depending on its cellular location, some members of subfamily B1 are also capable of coupling to additional G proteins such as G(i/o) and/or G(q) proteins, thereby leading to activation of phospholipase C and intracellular calcium influx.