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BIOL 2P93 (93)

GI Hormones

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Brock University
Jenny Janke

Role of Hormones in Digestion I. Introduction a. Four types of control systems i. Neuronal—CNS & enteric NS ii. Hormonal—acts via hormones in circulation iii. Paracrine—acts on adjacent cells iv. Lumicrine—hormones released from 1 cell into gut lumen to affect another cell (like paracrine but further acting) b. Endocrine system—diffuse in GI; due to many different organs c. Hormones—released from cells in small amounts; affect target cells thru circulation i. six GI hormones: gastrin, CCK, secretin, GIP, GLP, motilin d. Peptides—regulate GI fxn via non-hormonal mechanisms; not delivered thru circulation i. Somatostatin—paracrine regulating peptide ii. GRP (gastrin-releasing peptide)—neurocrine regulation of enteric nervous system; released from enteric nerves in response to a meal iii. Also lumicrine regulation via peptides e. Feedback Loops—key inregulating hormonal function; mainly negative i. Most hormones have >1 negative (or positive) feedback loop ii. Stimulus (food/ pH / distension) => hormone secretion => changes in upstream & downstream GI system II. Response to a Meal a. Cephalic Phase i. Initiated in anticipation of a meal, smell/taste of food & controlled by neuronal pathways (parasympathetic & vagal) ii. Limited amounts of digestive secretions enter gut iii. What happens: ↑ salivary flow; small ↑ [Gastrin] in lumen & serum; small ↑ CCK & secretin secretion; stomach relaxes b. Gastric Phase i. Initiated by food in the stomach ii. Major hormone is Gastrin iii. What happens: ↓ saliva secretion; ↑ gastrin secretion; ↑ pancreatic enzyme secretion c. Early Intestinal Phase (Digestive Phase) i. Initiated by chyme entering the duodenum ii. Major hormones are CCK & secretin as well as incretins (GIP & GLP) iii. What happens: GB contracts => bile release; ↑ CCK, ↑ secretin, ↓ gastrin, ↓ gastric acid output, ↑ pancreatic enzyme secretion d. Late Intestinal Phase i. Initiated by chyme entering ileum => gradual return of activity to interdigestive state ii. Mobility pattern shifts => MMC (migrating myoelectric complex) 1 iii. What happens: ↓ luminal levels of all digestive secretions; ↑ volume of GB (as liver ↑ production); e. Interdigestive Period (Fasting) i. Secretory cells replenish stores; GB concentrates bile; salivary glands secrete at basal rate; MMC characterizes motor activity of gut III. Gastrin a. Structure i. Made as preprohormone (preprogastrin) which is cleaved => progastrin which is cleaved => gastrin ii. 2 major human forms: 1. G-34—predominant in duodenum; ½ life = 10 min; involved in growth of GI epithelium & preparation of duodenum 2. G-17—predominant in antral stomach; ½ life = 3 min; involved in regulation of acid secretion during a meal b. Storage—in G cells in antrum of stomach (& duodenum) c. Release i. Stimulated by peptides, amino acids, & Ca in stomach ii. Inhibited by low pH (<2.5) & somatostatin iii. GRP (released from enteric nerves) => potent effect (↑ gastrin) d. Cellular Targets i. CCK-B receptor—serpentine 7-transmembrane domain, GPCR ii. Binds to. . . 1. Gastric parietal cells— ↑ HCl secretion 2. Enterocromaffin-like cel
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