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Inhibits gastric motor motility and acid secretion

Somatostatin

Gastrin

Nerve cells in intestine

Exerts inhibitory actions on release of CCK, VIP/ GIP

" Table 8-2 lists neurotransmitters produced by the gastrointestinal tract.

b CCK is the abbreviation for the peptide hormone cholecystokinin-pancreozymin; it is usually designated simply as cholecystokinin. c GIP is known as both glucose-dependent insulinotropic polypeptide and also gastrin-inhibiting polypeptide.

d The pancreatic polypeptide family consists of three peptides, pancreatic polypeptide (PP), peptide YY (PYY), and neuropeptide Y (NPY; see Table 8-2). All three are amino acid peptides. e See Table 8-2.

" Table 8-2 lists neurotransmitters produced by the gastrointestinal tract.

b CCK is the abbreviation for the peptide hormone cholecystokinin-pancreozymin; it is usually designated simply as cholecystokinin. c GIP is known as both glucose-dependent insulinotropic polypeptide and also gastrin-inhibiting polypeptide.

d The pancreatic polypeptide family consists of three peptides, pancreatic polypeptide (PP), peptide YY (PYY), and neuropeptide Y (NPY; see Table 8-2). All three are amino acid peptides. e See Table 8-2.

TABLE 8-2 Neurotransmitters Produced by the Gastrointestinal Tract"

Neurotransmitter

Bombesin1 (gastrin releasing peptide GRP) Calcitonin gene-related peptide (CGRP)

Endothelin

Galanin

Neuropeptide Y (NPY) Neuromedin B

Neuromedin U

Substance P and the tachykinins Vasoactive intestinal peptide (VIP)

Actions

Stimulates gastrin release

Stimulates somatostatin release and smooth muscle contraction Potent vasoconstrictor; acts locally in the GI tract; function not known (see Chapter 15) Not definitely known; may inhibit postprandial release of somatostatin, neurotensin, and PP

Potent vasoconstrictor; inhibits acetylcholine release Stimulates gastrin release; inhibits hypothalamus TSH release

Regulates anterior pituitary function Smooth muscle contractions

Smooth muscle relaxation; stimulates pancreatic HC03~ secretion

" Table 8-1 lists hormones produced by the gastrointestinal tract.

b Bombesin is a member of the bombesin-like family of peptides found in the skin, gut, and brain of frogs; it shares biological activity with the mammalian gastrin-releasing peptide (GRP), the tachykinins, and neuromedin B.

components—amino acids, mono- and disaccharides, fatty acids, and glycerol, respectively—before they can be efficiently absorbed.

The overall physical and chemical properties of the various foodstuffs (e.g., water solubility, net charge), as well as the nature of their structural units and bonding (e.g., amide, glycoside, ester), pose diverse biochemical problems to be resolved by the digestive system. The system of digestion, then, is the processing of the ingested bulk food into molecular forms capable of entering the intestinal absorptive process.

Table 8-3 summarizes the steps and processes associated with the digestive system. The digestive process involves the complex integration of voluntary and involuntary muscle contractions, parasympathetic and sympathetic neural actions, release of gastrointestinal and other hormones, and biosynthesis and release of a host of digestive enzymes (e.g., pepsin, trypsin, chy-motrypsin, amylase) and digestive reagents (e.g., HC1 and HCO3), along with digestive detergents (e.g., bile acids). These substances all collectively work together to process the food and present it to the intestinal absorptive machinery in an optimal form for absorption. The intestinal absorptive machinery includes the integrated functioning of the mouth, stomach, small and large intestines, pancreas, liver, and gallbladder.

As a consequence of evolutionary pressures, a number of hormonal systems have emerged to participate in the regulation of digestion and absorption of key dietary nutrients. These include the gastrointestinal hormones (this chapter), insulin and glucagon (see Chapter 7), and vitamin D metabolites (one of the calcium-regulating hormones—see Chapter 9). In addition, many other hormones are known to exert effects on either the digestion process or the gastrointestinal tract to modulate the absorption of various nutrients.

II. ANATOMICAL, MORPHOLOGICAL, AND PHYSIOLOGICAL RELATIONSHIPS

A. Gastroenteropancreatic System

The gross anatomical organization of the digestive tract, the stomach, the small intestine, and the large intestine of humans is shown in Figure 8-1. The anatomical relationships between the liver, pancreas, and gallbladder are given in Figure 7-2.

The digestive tube or alimentary canal in humans is a 9-m-long muscular tube extending from the lips to the anus. Indicated in Figure 8-1 are the individual components of the alimentary canal; each part makes an essential contribution to the overall process of making food available to the cells of the body. The operations within the digestive tube are supplemented by several accessory organs, including the teeth, tongue, salivary glands, pancreas, and liver.

B. Stomach

Figure 8-2 presents a cross-sectional view of the stomach. The gastric secretions of hydrochloric acid and pepsin are dependent upon the functional capacity and mass of the various secretory elements in the gastric mucosa, as well as being a consequence of the action of the several hormonal and neural stimulants and inhibitors.

The gastric mucosa is divided into three anatomical regions: cardiac, oxyntic, and pyloric. The total surface area of the gastric mucosa is —800 cm2. Anatomically, the cardiac portion encompasses the first few millimeters below the gastroesophageal junction and includes the cardiac mucus glands; it secretes alkaline mucus and a small amount of electrolytes. The oxyntic or fundic portion comprises 75-80% of the entire stomach. The oxyntic glands, which number —35 million

Hormones, Second Edition TABLE 8-3 Steps and Components of the Digestive Process

Process

Anatomical location

Purpose

Food entry Consumption of food / mastication Salivary secretion (stimulated by the nervous system): 1200 ml is secreted per day. Voluntary swallowing of food Involuntary peristaltic muscular contraction Digestion in the stomach

Secretion of gastric juice (HC1 + pepsin) as a consequence of neural, mechanical, and hormonal (gastrin) stimuli (2000 ml secreted/day) Involuntary mechanical contractions

Digestion in the small intestine Neural and hormonal (secretion of pancreozymes); mediated secretion of pancreatic juice (1200 ml secreted/day) Bicarbonate secretion

Amylase, trypsinogen, and chymotrypsinogen secretion and conversion to "active" enzymes Neural stimulation and duodenal muscular distension and stimulation of intestinal juice (4000-5000 ml secreted/ day)

Enterokinase (activates trypsinogen conversion into trypsin) Proteases (chymotrypsinogens, trypsinogen, lipases, amylases) Sucrase, maltase, lactase; specific transport systems (all bound to brush border membranes) Synthesis and secretion of bile acids (by the liver) followed by storage in the gallbladder; hormonally stimulated secretion (by cholecystokinin and secretin) of the gallbladder to release bile acids (600-800 ml secreted/day) Involuntary mechanical activity Segmentation

Peristalsis Chemical activity

Digestion in the large intestine Involuntary mechanical activity

Bacterial action

Mouth

Salivary glands

Tongue / pharnyx / esophagus Esophagus

Stomach

Stomach

Exocine or acinar pancreas secretions are ducted into the duodenum

Duodenum

Large intestine

Large intestine

Support bodily nutrient requirements Conversion of starches into dextrins and maltose

Entry of food into digestive tract To move food along the digestive tract

Conversion of proteins to polypeptides

Mixing of intestinal contents and when pylorus opens transfer of stomach contents (chyme) to the small intestine

HC03" neutralizes HC1 from stomach and creates a favorable duodenal pH (6.0-7.0) so that pancreatic enzymes may operate efficiently

Hydrolysis of amide, glycoside, and ester bonds of peptides, carbohydrates, and lipids

Liver (site of production), gallbladder (storage), transport (through bile duct) into duodenum

Duodenum, jejunum, and ileum

Duodenum, jejunum, and ileum Duodenum, jejunum, and ileum

Further processing of proteins —> peptides

Further processing of proteins —> peptides

Further processing of dextrins and other partially hydrolyzed substances

Emulsify fats to facilitate absorption of fatty acids

Squeezing and mixing of intestinal contents

(8-10 contractions/min in duodenum) Onward movement of intestinal contents Continued saponification of fat and hydrolysis of proteins —» peptides to amino acids and carbohydrates-dextrins to mono- and disaccharides followed by cellular absorption and transport to the blood or lymph compartments Mixing movements that promote reabsorption of H20 and compaction of intestinal contents (7000-8000 ml of H20 reabsorbed/day) Acts on undigested residues to effect fermentation of carbohydrate and further degradation of proteins as well as biosynthesis of vitamin K

Pharynx

Esophagus

Parotid gland and duct

Stomach

Sigmoid colon Rectum

Anal sphincters Anus

Pancreas with duel Transverse colon Descending colon

FIGURE 8-1 Diagram of the human digestive system.

Esophagus

Parotid gland and duct

Vestibule

Sublingual gland Submaxillary gland

Pharynx

Gallbladder Liver

Hepatic duct

Cystic duct Common bile duct Duodenum Jejunum

Ascending colon Ileum Cecum Appendix

Stomach

Sigmoid colon Rectum

Anal sphincters Anus

Pancreas with duel Transverse colon Descending colon

FIGURE 8-1 Diagram of the human digestive system.

in the stomach of humans, are straight or slightly coiled tubules and are largely composed of parietal cells and mucus-secreting cells. The parietal cells, constituting 75% of the mucosal volume, have large numbers of mitochondria, which are required for the metabolic energy demands involved in the production of concentrated hydrochloric acid. The parietal or oxyntic cells undergo a dramatic morphological transformation during stimulation, so that the internal tubulovesicles coalesce and become transformed into microvillar membranes. The chief cells are the primary site of the secretion of pepsin. The pyloric gland area constitutes the lower 20% of the gastric mucosa and contains pyloric glands, which are largely composed of mucus-secreting cells.

C. Small and Large Intestines

The small intestine of humans is divided anatomically into the duodenum (—25 cm), which is connected to the stomach via the pylorus, the jejunum (—275 cm), and the ileum (—425 cm), which connects to the large

Esophagus

Fundus

Cardiac sphincter

Lesser

Smooth muscle layers

Greater curvature

FIGURE 8-2 Longitudinal section of the human stomach and upper duodenum.

Smooth muscle layers

Greater curvature

Esophagus

Fundus

Cardiac sphincter

Lesser

FIGURE 8-2 Longitudinal section of the human stomach and upper duodenum.

intestine. The large intestine (~180 cm) is sequentially divided into the colon, appendix, rectum, and anal canal.

The morphology and cellular organization of the intestine are superbly adapted to efficiently effect the absorption of dietary constituents. The lumen of the small intestine is composed of a multitude of fingerlike projections, which are termed villi (see Figure 8-3A). Typically these villi range from 0.5 to 1.5 mm in length. They are composed principally of two types of cells: the columnar epithelial cell and the goblet or mucus-secreting cell. The cells on the villus are rapidly renewed. The mucosal cells originate in a progenitor population existent at the base of the villus in the crypt of Lieberkuhn, and ultimately, after migration up the villus, the cells are sloughed off into the lumen of the intestine. Only after epithelial cells enter the villus do they differentiate into fully functional absorptive cells. This differentiation process is characterized by a change in the RNA/protein ratio and a change in the activity of various enzymes as a function of the position of the epithelial cell on the villus. Intestinal epithelial cell turnover times of 70-100 hr have been reported in mice, rats, and humans. The rate of epithelial cell renewal increases when there is an increased demand for new cells, as in the recovery from radiation injury, response to parasite infestation, or adaptation to partial intestinal resection.

The apical surface of these cells, which is exposed to the lumen, is referred to as the brush border or the microvillar membrane (see Figure 8-3B). These microvilli are ~1 jum long and ~0.1 /um wide. As a consequence, the surface area of the mucosa is increased by 600-fold compared to the total area of a hypothetical cell without microvilli. A variety of carbohydrate hydrolases and an alkaline phosphatase are specifically associated with this brush border region of the intestinal mucosal cell. This is believed to be the site of localization of permeases that are postulated to be involved in the absorption of food materials into the cell. Beneath the columnar epithelial cells lie the lymphatic and blood vascular systems, which project into each villus and thus provide an efficient mechanism for the translocation of the various substrates once they exit from the epithelial cells.

The outermost surface of the columnar epithelial cell, that is, the microvilli or brush border region, is covered with a filamentous, mucopolysaccharide coat termed the glycocalyx or "fuzzy coat." The function of the glycocalyx is not specifically known.

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