Ghrelin – Physiological Functions and Regulation

Abstract

Ghrelin is an orexigenic peptide predominantly secreted from the stomach and stimulates appetite and growth hormone (GH) release.Studies have provided evidence that ghrelin exercises a wide range of functions, including regulation of food intake and energy metabolism, modulation of cardiovascular function, stimulation of osteoblast proliferation and bone formation and stimulation of neurogenesis and myogenesis. In the gastrointestinal system, ghrelin affects multiple functions, including secretion of gastric acid, gastric motility and pancreatic protein output. Most of these functions have been attributed to the actions of acylated ghrelin. The balance among its secretion rate, degradation rate and clearance rate determines the circulating level of ghrelin. This review explains what ghrelin is, its physiological functions and the factors that influence its level. KeywordsGhrelin, food intake, obesity, lipolysis, ghrelin receptors, regulation Disclosure: Mona Mohamed Ibrahim Abdalla has no conflicts of interest to declare. Acknowledgement:A note of appreciation to Research Management Centre of MAHSA University, Malaysia for funding publication of this article.

Plasma levels of acylated ghrelin in patients with insulinoma and expression of ghrelin and its receptor in insulinomas

“…On the other hand, the effect of ghrelin on insulin secretion is controversial, either stimulating the secretion of insulin [37] or inhibiting the secretion of insulin [18,38,39]. Therefore, we try to understand the potential interaction between the insulin and ghrelin, both of which are associated with obesity [9,10,40]. Finally, the acylated ghrelin is the primary form to exert its biological activity [6,7,41].…”

“…In our present study, the plasma levels of acylated ghrelin are much lower than plasma levels of total ghrelin shown in other studies. One reason is that acylated ghrelin constitutes only a small part of total ghrelin; normally acylated ghrelin accounts for <10% of the total ghrelin in the circulation [8,10]. Another reason might be that a small part of ghrelin is lost when we extracted and purified plasma ghrelin by using Sep-Pak C18 cartridge according to the manufacturer's protocol.…”

“…Interestingly, it has been reported that ghrelin is an orexigenic peptide and closely associated with obesity by increasing body weight in humans [9]. Endogenous ghrelin stimulates hunger and food intake more effectively than any other known peptides except neuropeptide Y, with which it is approximately equipotent; and exogenous injection of ghrelin also stimulates food intake rapidly by increasing appetite and the number of meals [10][11][12][13]. Moreover, ghrelin has been reported to stimulate appetite and food intake even more in obese than lean humans [14].…”

“…Moreover, ghrelin has been reported to stimulate appetite and food intake even more in obese than lean humans [14]. Most biological actions of ghrelin, especially those involving endocrine and anabolic effects, require acylation [7,10,15,16]. Acylation of ghrelin is also required for activating its functional receptor in vivo [6].…”

“…Growth hormone secretagogue receptor type1A (GHS-R1A) is the functional receptor of ghrelin [10,20,21], which is expressed widely in human body (including a number of endocrine organs such as the pituitary, pancreas, and thyroid). It has been shown that ghrelin and GHS-R1A are expressed in neuroendocrine tumors [22][23][24][25][26][27][28][29].…”