The Gut Peptide Hormone Family, Motilin and Ghrelin

Abstract

Endocrine hormones are a system of glands, each of which secretes a type of hormone into the bloodstream to regulate multiple physiology of the body. In the past several decades, many hormones from the gastrointestinal tract has been identified and cloned, and their physiological functions have been studied. Although the pituitary gland was considered to be the main endocrine organ of the body in early studies, there are other organs that produce endocrine hormones such as adipose tissue, reproductive organ, adrenal gland, and gastrointestinal tract. Among those, the gastrointestinal tract is the largest endocrine organ of the body in volume, and hormones produced in the gastrointestinal tract are physiologically important for their roles in development, growth, cardiovascular, gastric motility, behavior and maintenance of energy homeostasis. Many hormones have been identified in each different parts of the gastrointestinal tract. For instance, in the stomach, gastrin, histamine (Dornonville de la Cour, et al. 2001), somatostatin (Bolkent, et al. 2001), neuropeptide Y (Wang, et al. 1987), ghrelin (Sakata, et al. 2002) and leptin (Bado, et al. 1998) are produced in the mucosal layer and/or myentric plexus, and cholecystokinin (CCK) (Miyamoto and Miyamoto 2004), glucagon-like peptide-1 (Theodorakis, et al. 2006), motilin (Sakai, et al. 1994a) (Satoh, et al. 1995), serotonin (Ku, et al. 2004) and PYY3-36 (Rozengurt, et al. 2006) are produced in the upper and lower intestine. Motilin and ghrelin are considered to comprise a peptide family based on similarity of their structures and also their similarity in each specific G protein coupled receptor, growth hormone secretagogue receptor (GHS-R) and motilin receptor (MTL-R, also known as GPR38). In this chapter, we review recent research and knowledge of the peptides, motilin and ghrelin regarding their structures, distribution of motilinand ghrelin-producing cells, distribution of their receptors, plasma profies and secretion of motilin and ghrelin, and the role of motilin and ghrelin in gastric motility. However, there is a lack of basic information for motilin study such as information on the detailed distribution of motilin and motilin receptor in the body and changes in motilin release under some physiological states. One of the reasons for the difficulty in motilin study is that rodents such as rats and mice cannot be used for motilin study because the motilin gene is inactivated in the common ancestor of mice and rats (He, et al. 2010). For this reason, motilin has been studied using relatively largesized animals, such as dogs and rabbits, which has made it difficult to investigate in detail the mechanisms underlying the actions of motilin. Recently, we characterized the house musk shrew (Suncus murinus, order: Insectivora, suncus named as laboratory strain) as a motilinand ghrelin-producing small