discovered in 1902 by E. H. Starling. It was later linked to chemical regulation and was the first substance to be deemed a hormone. Secretin Glucagon Glicentin (GLI) Vasoactive intestinal peptide (VIP) Gastric inhibitory polypeptide (GIP) Others Motilin Neurotensin Substance P Somatostatin Bombesin Serotonin Angiotensin Nitric Oxide Kinins Histamine References Mark H. Whitnall, William G. Haynes, David J. Webb (1997). Principles of Medical Biology. SF Gilbert. (2000). Developmental Biology 6th Edition. Jan M. Keppel Hesselink.(2016). "Autacoids: A New Fundament for Pain Medicine of the 21th Century". McKinley, Michael P., et al. Anatomy & Physiology: an Integrative Approach. McGraw-Hill Higher Education, 2012 Rehfeld JF1, Friis-Hansen L, Goetze JP, Hansen TV. (2007). "The biology of cholecystokinin and gastrin peptides". Curr Top Med Chem. 2007;7(12):1154-65. Henriksen JH, de Muckadell OB. (2000). "Secretin, its discovery, and the introduction of the hormone concept.". Scand J Clin Lab Invest. 2000 Oct;60(6):463-71. Signal transduction Hormones Endocrine system Cell signaling