How Do Hormones and Other Secretions interact in Human Digestion?

3. Pancreatic exocrine secretions: Within the 1.5 L of fluid secreted per day two major substances can be found: an alkaline fluid rich in HCO3- , and hydrolytic enzymes. (Bray et al, 1999) Bicarbonate ions, secreted by the ductular epithelial cells, are produced from CA catalysing of CO2 and H2O. (Thomas, 2006) They neutralise the chyme, entering the duodenum so that a suitable PH of 6.7 to 9.0 for the pancreatic enzymes is established. The pancreatic secretions can be categorised into four groups:

            1. Protease zymogen forms; such as trypsinogen which is converted to trypsin by enterokinase- secreted by duodenal mucosa. Other precursors are activated by trypsin. (Berne et al, 2004)

            2. Lipases; which hydrolyse triglycerides and phospholipids. (Bray et al, 1999)

            3. An α amylase; which acts on starch and break it down to maltose and glucose by splitting glycosidic bonds. (After Table 17.4 from Bray et al, 1999)

            4. Other enzymes such as ribonuclease and cholesterol ester hydrolase which act on RNA, and cholesterol and fatty acids, and their products are nucleotides, and cholesterol and free fatty acids respectively. (After Table 17.4 from Bray et al, 1999)

4. Biliary secretions: 0.5 L of bile enters the duodenum everyday from the common bile duct, formed of the cystic duct from the gallbladder and the common hepatic duct from the liver. The closure of sphincter of Oddi causes the bile to be accumulated in the gallbladder. Bile salts and HCO3- are the two components of the bile. Bile salts contribute to the fat digestion, and absorption of fats and fat-soluble vitamins. The HCO3- neutralises the acid chyme from the stomach. (Bray et al, 1999)

5. Intestinal secretions: 0.15 L daily supply of mucus, electrolytes, water (Berne et al, 2004), and alkaline fluid are produced by the Brunner’s glands of the duodenum, and the mucosa of the small intestine and the colon. The mucus in duodenum is very viscid and entraps alkaline fluid to protect the cells from stomach acid. A variety of enzymes such as aminopeptidases and amylases are released from desquamated intestinal cells, which are important in digestion. (Bray et al, 1999)

Gastrointestinal Hormones

The gastrointestinal mucosa produces peptides that can only be identified as hormones, under the following conditions: 1.The peptide must be released from one region of the gut and affect on the other; 2.This effect has to be continuous till there is no more nervous connection between regions; 3.The peptide entered the bloodstream must imitate the physiological stimulus effect, which was applied to the gut; 4. The peptide must be chemically known. (Bray et al, 1999)

The major hormones are:

1. Gastrin: Released by stomach and leads to the secretion of HCl and gastric motility; (After Table 17.5 from Bray et al, 1999)

2. Cholecystokinin (CCK): Released by upper small intestine, leads to the secretion of enzyme-rich fluid from pancreas; (After Table 17.5 from Bray et al, 1999, and Table 8.3 from Brook & Marshall, 2001)

3. Secretin: Released by upper small intestine, leads to the secretion of bile and alkaline fluid from pancreas and liver; (After Table 17.5 from Bray et al, 1999, and Table 21.2 from Germann & Stanfield, 2005)

 

4. Glucose-dependent insulinotrophic peptide (GIP): Released by upper small intestine, leads to the secretion of insulin from pancreas; (After Table 17.5 from Bray et al, 1999 and Table 8.3 from Brook & Marshall, 2001)

 

5. Enteroglucagon: Released by ileum and colon, causes the inhibition of gastric secretion and motility; (After Table 17.5 from Bray et al, 1999)

6. Somatostatin: Released by intestine and pancreas, inhibits gastric acid and pancreatic enzymes release. (After Table 17.5 from Bray et al, 1999)

Two Examples of Gastrointestinal Secretions

1) GASTRIC ACID

Gastric acid (HCL) is produced by the parietal cells. The rate of secretion varies from 1 mEq/hr (basal) to 40 mEq/hr (maximal stimulation). Inside the parietal cells there are tubules and vesicles, which build the tubulovesicular system. (Berne et al, 2004) H+ and Cl- are generated inside the cells from the reaction of CO2 with H2O, using catalyser CA. (Germann & Stanfield, 2005) When stimulated, the tubulovesicular membranes combine to the membrane of the secretory canaliculi, which are connected to the cell’s surface. Then, H+ is pumped out of the cell, against its concentration gradient, by H+,K+-ATPase, situated on the apical membrane. Cl- is transported inside the cell, against its concentration, by the Cl-, HCO3- countertransporter and concentrated in the cytoplasm of the parietal cell. At the end, Cl- leaves the cell through an electrogenic channel. (Berne et al, 2004)  

The gastric acid provides the stomach lumen with an acidic environment (PH 0.8). This environment is necessary for the proteolytic enzymes and pepsins activity, also for denaturizing ingested proteins and destroying bacteria. (Bray et al, 1999)

The gastric secretions are stimulated by action of acetylcholine (ACh), gastrin, and histamine. However, the physiological acid stimulator is food, whose stimulation occurs in three phases: Cephalic phase, in which acid secretion is initiated by the cause of sight or smell of food, and chewing; Gastric phase of secretion, which starts by entering the food into the stomach and as a result, stomach distension. Also chemical nature of the food (e.g. coffee and calcium) is important; and Intestinal phase, in which the gastric secretion is lowered when the chyme enters duodenum. The hormones secretin, GIP and CCK are involved. (Bray et al, 1999)

2) GASTRIN

The longest molecule of gastrin found in humans has 34 amino acids, which is produced between meals. However, following a meal, two related gastrins, I and II, both having 17 amino acids have been isolated. (Bray et al, 1999) Also, 14 amino acid gastrins have been identified, but in fact, what are required for biological activity are the four amino acids at the carboxy terminus. (Brook & Marshall, 2001) The sequence (Try-Met-Asp-Phe-NH2 ) determines the hormone characteristics. (Bray et al, 1999)

Gastrin is produced by specialised G cells, which are located in the mucosa of the stomach distal region and the upper small intestine. The secretion of the gastrin is stimulated by: 1.Digestion products in the stomach, such as coffee; 2.Nervous activity during the gastric cephalic phase (vagus nerve involved); 3.Stomach distal region distension. Gastrin release is inhibited by the gastric contents’ PH being dropped to 2.5 (Brook & Marshall, 2001), also by secretin, GIP and somatostatin hormones. (Bray et al, 1999)

Gastrin stimulates the secretion of HCl, pepsins and intrinsic factor. Also, ‘’it enhances gastric motility and increases the tone of the lower oesophageal sphincter’’. (Bray et al, 1999) Furthermore, it causes the pancreatic secretions to increase and contracts the gallbladder, while increasing the mucosa of the stomach, duodenum and colon. (Bray et al, 1999)

Conclusion

The digestive secretions, throughout the GI tract, are released for the purpose of the food digestion and subsequently its absorption into the bloodstream. In order for the gastrointestinal system to be efficient, its secretions have to be controlled and regulated step by step. The factors involved in digestive regulation are: 1.for salivary glands: medullary salivary centre producing autonomic input, 2.for stomach secretions: neurons and hormones controlling cephalic-phase, gastric-phase, and intestinal-phase stimuli, 3.for pancreatic secretions: nervous reflexes and hormones such as secretin and CCK (Potentiation happens when both are present), 4.for liver (bile) secretion: hormones (CCK and secretin). These hormones also cause the contraction of the gallbladder. (Germann & Stanfield, 2005)

Bibliography

Germann, W.J. & Stanfield, C.L. (2005): Principles of Human Physiology; 2nd edition; Chapter 21: p.647; pp.650-653; pp.668-673; p.682; Benjamin Cummings

Bray, J.J. & Cragg, P.A. & Macknight, A.D.C. & Mills, R.G. (1999): Lecture Notes on Human Physiology; 4th edition; Chapter 17: pp.482-499; Blackwell Science Ltd

Berne, R.M. & Levy, M.N. & Koeppen, B.M. & Stanton, B.A. (2004): Physiology; 5th edition; Chapter 32: p.566; pp.568-574; p.583; pp.592-593; Elsevier, Inc.

Brook, C.G.D. & Marshall, N.J. (2001): Essential Endocrinology; 4th edition; Chapter 8: pp.154-155; Blackwell Sciences Ltd

Thomas, A. (2006): ‘’Gastrointestinal secretions and vomiting’’;  

 Anaesthesia & Intensive Care Medicine; Volume 7, Issue 2; 1 February 2006; p.56; Elsevier Ltd

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