There are several different types which secrete the hormones secretin, cholecystokinin and several others. Prosecretin is a precursor to secretin, which is present in digestion. Starling's subject was hormones, with secretin being prominent. They named this intestinal secretion "secretin". "Secretin, its discovery, and the introduction of the hormone concept.". Functional imaging of the pancreas is performed following administration of secretin. Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin. Glucagon belongs to the secretin family of hormones. They called the responsible chemical secretin and named such messenger chemicals hormones. A recombinant human secretin has been available since 2004 for these diagnostic purposes. These include secretin, acetylcholine, ATP, and bombesin. It is a member of the secretin family of G protein-coupled receptors. The outer membrane complex is made up largely by the secretin GspD. Pituitary adenylate cyclase-activating peptide has been shown to interact with secretin receptor. Vasoactive intestinal peptide, cholecystokinin, and secretin all inhibit production.