GASTROINTESTINAL PHYSIOLOGY JOHNSON PDF
Gastrointestinal Physiology, Johnson - Ebook download as PDF File .pdf), Text File .txt) or read book online. Gastrointestinal Physiology, Johnson. Gastrointestinal Physiology. 8th Edition. Mosby Physiology Monograph Series ( With STUDENT CONSULT Online Access). Authors: Leonard Johnson. Gastrointestinal Physiology. 7th Edition. Mosby Physiology Monograph Series. Authors: Leonard Johnson. eBook ISBN: eBook ISBN.
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Get this from a library! Gastrointestinal physiology. [Leonard R Johnson;] -- This volume in the Mosby Physiology Monograph Series explains the fundamentals. Gastrointestinal Physiology Leonard R. Johnson PhD, Leonard R. Johnson This concise, readable text provides You Can Download the PDF. Gastrointestinal Physiology. Leonard R. Johnson, Eighth edition, , p. Elsevier Mosby, Philadelphia, ISBN Gastrointestinal.
Fourth, the substance must be identied chemically, and its structure must be conrmed by synthesis.
Five GI peptides have achieved full status as hormones. There is also an extensive list of candidate hormones whose signicance has not been established. This list includes several chemically dened peptides that have signicant actions in physiology or pathology but whose hormonal status has not been proved.
These are pancreatic polypeptide, neurotensin, and substance P. In addition, two known hormones, glucagon and somatostatin, have been identied in GI tract mucosa; their possible function as GI hormones is currently being investigated. Some of these peptides function physiologically as paracrines or neurocrines.
Another GI peptide, Ghrelin, is released from the body of the stomach and functions as a hormone to regulate food intake. This topic is covered in Chapter Secretin, the rst hormone, was discovered in by Bayliss and Starling and was described as a substance, released from the duodenal mucosa by hydrochloric acid, that stimulated pancreatic bicarbonate and uid secretion.
Jorpes and Mu isolated it and identied its amino acid sequence in It was synthesized by Bodanszky and coworkers later the same year. Edkins discovered gastrin in , stating to the Royal Society that in the process of the absorption of digested food in the stomach a substance may be separated from the cells of the mucous membrane which, passing into the blood or lymph, later stimulates the secretory cells of the stomach to functional activity.
For 43 years investigators were preoccupied by the controversy over the existence of gastrin. The debate intensied when Popielski demonstrated that histamine, a ubiquitous substance present in large quantities throughout the body including the gastric mucosa , was a powerful gastric secretagogue. In Komarov demonstrated that gastrin was a polypeptide and was dierent from histamine. By Gregory and his colleagues had extracted and isolated hog gastrin; Kenner and his group synthesized it the same year.
A er 60 years all of the criteria for establishing the existence of a GI hormone had been satisfied. In Ivy and Oldberg described a humoral mechanism for the stimulation of gallbladder contraction initiated by the presence of fat in the intestine. The hormone was named cholecystokinin a er its primary action. The only controversy involving CCK was a mild one over nomenclature. In Harper and Raper described a hormone released from the small intestine that stimulated pancreatic enzyme secretion and accordingly named it pancreozymin.
As Jorpes and Mu carried out the purication of these two substances in , it became obvious that both properties resided in the same peptide. For the sake of convenience and because it was the rst action described, this hormone is called CCK. In Brown and his coworkers described the purication of a powerful enterogastrone from intestinal mucosa.
Enterogastrone literally means a substance from the intestine entero- that inhibits -one the stomach gastr-. By this peptide had been puried, isolated, sequenced, and named gastric inhibitory peptide for its ability to inhibit gastric secretion. Released from the intestinal mucosa by fat and glucose, GIP also stimulates insulin release. Following proof that the release of insulin was a physiologic action of the peptide, GIP became the fourth GI hormone. The insulinotropic eect of GIP requires elevated amounts of serum glucose.
For this reason, and because it is doubtful whether the inhibitory eects of the peptide on the stomach are physiologic, it has been suggested that its name be changed to glucose-dependent insulinotropic peptide. In either case it is still referred to as GIP. Brown and his coworkers also described the purication of motilin in the early s.
Motilin is a linear 22amino acid peptide puried from the upper small intestine. During fasting it is released cyclically and stimulates upper GI motility. Its release is under neural control and accounts for the interdigestive migrating myoelectric complex. Chemistry The GI hormones and some related peptides can be divided into two structurally homologous families.
The rst consists of gastrin Fig. The ve carboxyl-terminal C-terminal amino acids are identical in these two hormones.
All the biologic activity of gastrin can be reproduced by the four C-terminal amino acids. This tetrapeptide, then, is the minimum fragment of gastrin needed for strong activity and is about one sixth as active as the whole 17amino acid molecule. The sixth amino acid from the C-terminus of gastrin is tyrosine, which may or may not be sulfated. When sulfated, the hormone is called gastrin II. Both forms occur with equal frequency in nature.
The amino-terminus N-terminus of gastrin is pyroglutamyl, and the C-terminus is phenylalamide see Fig. The NH2 group following Phe does not signify that this is the N-terminus; rather, it indicates that this C-terminal amino acid is amidated. These alterations in structure protect the molecules from aminopeptidases and carboxypeptidases and allow most of them to pass through the liver without being inactivated.
Gastrin is rst synthesized as a large, biologically inactive precursor called progastrin. A glycine-extended G-Gly form of gastrin is then formed by endoproteolytic processing within the G cells. G-Gly is the substrate for an amidation reaction that results in the formation of the mature, amidated gastrin. CCK, which has 33 amino acids, contains a sulfated tyrosyl residue in position 7 from the C-terminus see Fig.
CCK can activate gastrin receptors e. Each hormone, however, is more potent at its own receptor than at those of its homologue. CCK is always sulfated in nature, and desulfation produces a peptide with the gastrin pa ern of activity. The minimally active fragment for the CCK pa ern of activity is therefore the C-terminal heptapeptide.
Peptides with a sulfated tyrosyl residue in position 7 act on CCK-1 receptors, have cholecystokinetic potency, and are weak stimulators of gastric acid secretion.
Obviously the tetrapeptide itself and all fragments less than seven amino acids long possess gastrin-like activity. The second group of peptides is homologous to secretin and includes vasoactive intestinal peptide VIP , GIP, and glucagon, in addition to secretin Fig.
Secretin has 27 amino acids, all of which are required for substantial activity. Pancreatic glucagon has 29 amino acids, 14 of which are identical to those of secretin. Glucagon-like immunoreactivity has been isolated from the small intestine, but the physiologic signicance of this enteroglucagon has not been established.
Glucagon has no active fragment, and, like secretin, the whole molecule is required before any activity is observed. Evidence indicates that secretin exists as a helix; thus the entire amino acid sequence may be necessary to form a tertiary structure with biologic activity. Each has many of the same actions as those of secretin and glucagon.
This group of peptides is discussed in greater detail later in the chapter. Most peptide hormones are heterogeneous and occur in two or more molecular forms. Gastrin, secretin, and CCK all have been shown to exist in more than one form. Gastrin was originally isolated from hog antral mucosa as a heptadecapeptide see Fig.
The Gastrointestinal System
Yalow and Berson demonstrated heterogeneity by showing that the major component of gastrin immunoactivity in the serum was a larger molecule that they called big gastrin.
On isolation, big gastrin was found to contain 34 amino acids; hence it is called G Trypsin splits G 34 to yield G 17 plus a heptadecapeptide dierent from G Therefore, G 34 is not simply a dimer of G An additional gastrin molecule G 14 has been isolated from tissue and contains the C-terminal tetradecapeptide of gastrin. Current evidence indicates that most G 17 is produced from pro G 17 and most G 34 is derived from pro G Thus G 34 is not a necessary intermediate in the production of G During the interdigestive basal state, most human serum gastrin is G Unlike those of other species, the duodenal mucosa of humans contains signicant amounts of gastrin.
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Nerves and Smooth Muscle 3. Swallowing 4.
Gastric Emptying 5. Motility of the Small Intestine 6. Motility of the Large Intestine 7. Salivary Secretion 8.This disorder has been a ributed to abnormalities in the enteric nerves. Coordinating multiple organs and physiologic processes is a significant challenge that must be solved to achieve the overall functions of the gastrointestinal system.
The small intestine is also capable of eliciting a highly coordinated contractile. Swallowing -- 4. Skip to content. English Copyright: Hardy and used by Starling in to describe secretin and gastrin and to convey the concept of bloodborne chemical messengers.