Fundamentals of Pharmacology
for Veterinary Technicians
Chapter 11
Gastrointestinal Drugs
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Basic Anatomy and Physiology
• The term gastrointestinal (GI) tract
describes a long, muscular tube that
begins at the mouth and ends at the anus.
• Structures:
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Oral cavity
Esophagus
Stomach
Small intestine
Large intestine
• Structures vary from monogastric animals
with simple stomachs to ruminant animals
with multichambered forestomachs
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Control Mechanisms of the GI
Tract
• One control mechanism of the GI tract is the
autonomic nervous system (parasympathetic and
sympathetic branches)
• Parasympathetic stimulation increases intestinal
motility, increases GI secretions, and relaxes
sphincters
– Cholinergic drugs simulate these actions
– Anticholinergic drugs inhibit these actions
• Sympathetic stimulation decreases intestinal
motility, decreases GI secretions, and inhibits the
action of sphincters
– Sympathetic nerves simulate these actions
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Drugs Affecting the GI Tract
• Antisialogues
– Drugs that decrease salivary flow
– Used to limit the flow of excess saliva,
which often occurs secondary to
anesthetic drug use
• Examples include anticholinergics such as
glycopyrrolate and atropine
• These drugs can also affect peristalsis
because they are also used to treat
vomiting, diarrhea, and excess gastric
secretion
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antidiarrheals
– Drugs that decrease peristalsis, thereby
allowing fluid absorption from the
intestinal contents
– Examples:
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Anticholinergics
Protectants/adsorbents
Opiate-related agents
Probiotics
Metronidazole
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antidiarrheals (cont.)
– Anticholinergics are used to treat tenemus and
vomiting
– Examples:
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Atropine
Aminopentamide
Isopropamide
Propantheline
Methscopolamine
– Side effects include dry mucous membranes,
urine retention, tachycardia, and constipation
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antidiarrheals (cont.)
– Protectants/adsorbents coat inflamed
intestinal mucosa with a protective layer
(protectants) or bind bacteria and/or digestive
enzymes and/or toxins to protect intestinal
mucosa from damaging effects (adsorbents)
– Examples:
• Bismuth subsalicylate (bismuth + aspirin-like
product)
• Kaolin/pectin
• Activated charcoal
– Side effects include constipation
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antidiarrheals (cont.)
– Opiate-related agents control diarrhea by
decreasing both intestinal secretions and the
flow of feces and increasing segmental
contractions
– Examples:
• Diphenoxylate
• Loperamide
• Paregoric
– Side effects include CNS depression, ileus,
urine retention, bloat, and constipation
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antidiarrheals (cont.)
– Probiotics seed the GI tract with beneficial
bacteria; use is based on the theory that some
forms of diarrhea are caused by disruption of
the normal bacterial flora of the GI tract
– Must be refrigerated to maintain the viability of
the bacteria
– Examples:
• Plain yogurt with active cultures
• Variety of trade-name products
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antidiarrheals (cont.)
– A theory regarding the development of
diarrhea is that anaerobic bacteria may
increase due to disruption of normal GI
flora
– One way to treat this is to use an
antibiotic effective against anaerobic
bacteria
– Metronidazole is an example of an
antibiotic used to treat diarrhea
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Laxatives
– A laxative loosens the bowel contents and
encourages evacuation of stool
– Laxatives help animals evacuate without
excessive straining; treat chronic constipation
from nondietary causes and movable intestinal
blockages; and evacuate the GI tract before
surgery, radiography, or diagnostic procedures
– Cathartics are harsher laxatives; purgatives
are harsh cathartics
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Types of Laxatives
– Osmotic
• Pull water into the colon and increase water
content in the feces, thereby increasing
bulk and stimulating peristalsis
• Are salts or saline product that may cause
electrolyte imbalances if absorbed
systemically
• Examples include: lactulose, sodium
phosphate with sodium biphosphate,
magnesium sulfate, magnesium hydroxide
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Types of Laxatives (cont.)
– Stimulant
• Increase peristalsis by chemically irritating
sensory nerve endings in the intestinal
mucosa
• Many are absorbed systemically and cause
a variety of side effects
• Examples include bisacodyl,
phenolphthalein, and castor oil
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Types of Laxatives (cont.)
– Bulk-forming
• Substances that absorb water into the
intestine, increase fecal bulk, and stimulate
peristalsis, resulting in large, soft stool
production (which tends to look normal)
• Are not systemically absorbed, so side
effects are rare
• Examples include psyllium hydrophilic
mucilloid, polycarbophil,and bran
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Types of Laxatives (cont.)
– Emollients
• Can be stool softeners (reduce stool surface tension
and reduce water absorption through the colon),
lubricants (facilitate the passage of fecal material,
increasing water retention in stool), or fecal wetting
agents (detergent-like drugs that permit easier
penetration and mixing of fats and fluid with the fecal
mass)
• Examples include docusate sodium, docusate
calcium, docusate potassium, and petroleum
products
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Drugs Affecting the GI Tract
• Antiemetics
– Drugs that control vomiting that help alleviate
discomfort and help control electrolyte balance
– Most are given parenterally, as the patient
may vomit the medication before it can be
absorbed through the GI tract
– Examples:
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Phenothiazine derivatives
Antihistamines
Anticholinergics
Procainamide derivatives
Serotonin receptor antagonists
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiemetics (cont.)
– Phenothiazine derivatives
• Inhibit dopamine in the chemoreceptor trigger zone,
thus decreasing the stimulation to vomit
• Side effects include hypotension and sedation
• Examples:
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Acepromazine
Chlorpromazine
Prochlorperzine
Perphenazine
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Drugs Affecting the GI Tract
• Antiemetics (cont.)
– Antihistamines
• Controls vomiting when the vomiting is due to
motion sickness, vaccine reactions, or inner ear
problems
• Work by blocking input from the vestibular system to
the CRTZ
• A side effect is sedation
• Examples:
– Trimethobenzamide
– Dimenhydrinate
– Diphenhydramine
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiemetics (cont.)
– Anticholinergics
• Block acetylcholine peripherally, which decreases
intestinal motility and secretions
• May decrease gastric emptying (which may increase
the tendency to vomit)
• Side effects include dry mouth, constipation, urinary
retention, and tachycardia
• Examples:
– Aminopentamide
– Atropine
– Propantheline
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiemetics (cont.)
– Procainamide derivatives
• Work centrally by blocking the CRTZ and
peripherally by speeding gastric emptying,
strengthening cardiac sphincter tone, and
increasing the force of gastric contractions
• Should not be used in animals with GI
obstructions, GI perforation, or GI
hemorrhage
• An example used in veterinary medicine is
metoclopramide
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiemetics (cont.)
– Serotonin receptor antagonists
• Work selectively on 5-HT3 receptors, which
are located peripherally and centrally
• Work on the theory that some chemicals
cause vomiting because they increase
serotonin release from small intestinal cells
• Examples:
– Ondansetron
– Dolasetron
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Emetics
– Drugs that induce vomiting
– Used in the treatment of poisoning and drug
overdose
– Vomiting should not be induced if caustic
substances have been ingested
• Always check with poison control prior to inducing
vomiting
– Activated charcoal is given if emesis is
contraindicated (it absorbs many chemicals
and drugs in the upper GI tract)
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Emetics (cont.)
– Can be centrally acting (working on the
CRTZ) or peripherally acting (working
on receptors locally)
– Centrally acting
• Apomorphine
• Xylazine
– Peripherally acting
• Ipecac syrup
• Home remedies
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiulcer drugs
– Help prevent the formation of ulcers
– Categories include
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Antacids
Histamine-2 receptor antagonists
Mucosal protective drugs
Prostaglandin analogs
Proton pump inhibitors
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiulcer drugs (cont.)
– Antacids
• Promote ulcer healing by neutralizing HCl and
reducing pepsin activity
• Interact with other drugs
– By adsorption or binding the other drugs
– By increasing stomach pH
– By increasing urinary pH
• May be systemic or nonsystemic
• Examples:
– Systemic: sodium bicarbonate, calcium carbonate
– Nonsystemic: magnesium hydroxide,
aluminum/magnesium hydroxide, aluminum
hydroxide
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiulcer drugs (cont.)
– Histamine-2 receptor antagonists
• Prevent acid reflux by competitively blocking the H2
receptors of the parietal cells in the stomach, thus
reducing gastric acid secretion
• Examples:
– Cimetidine
– Ranitidine
– Famotidine
– Mucosal protective drugs
• Combine with protein to form an adherent substance
that covers the ulcer and protects it from stomach
acid and pepsin
• An example is sucralfate
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antiulcer drugs (cont.)
– Prostaglandin analogs
• Suppress gastric secretions and increase mucus
production in the GI tract
• An example is misoprostol, which is usually given to
animals taking NSAIDs
– Proton pump inhibitors
• Bind irreversibly to the H+-K+-ATPase enzyme on
the surface of parietal cells of the stomach; this
inhibits hydrogen ion transport into the stomach so
that it cannot secrete HCl
• Examples:
– Omeprazole
– Lansoprazole
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Antifoaming drugs
– Reduce or prevent the formation of foam
– Used in ruminants, whose rumens are subject
to acute frothy bloat
– Make this foam less stable, breaking it up to
promote gas release through belching
– Administered as solutions by stomach tube
directly into the forestomach
– Examples include poloxalene and polymerized
methyl silicone
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Prokinetic agents
– Increase the motility of parts of the GI tract to
enhance movement of material through it
– Types of prokinetic agents are
parasympathomimetics, dopaminergic
antagonists, and serotonergic agents
– Parasympathomimetic agents include
• Acetylcholinesterase inhibitors, which compete with
ACh for acetylcholinesterase, resulting in increased
intestinal tone and salivation
– An example is neostigmine
• Cholinergics, which make a precursor to
acetylcholine
– An example is dexpanthenol
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.
Drugs Affecting the GI Tract
• Prokinetic agents (cont.)
– Dopaminergic agents stimulate
gastroesophageal sphincter, stomach, and
intestinal motility by sensitizing tissues to the
action of the neurotransmitter ACh
• Examples include metoclopramide and domeridone
– Serotonergic agents stimulate motility of the
gastroesophageal sphincter, stomach, small
intestine, and colon
• An example is cisapride
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Drugs Affecting the GI Tract
• Enzyme supplements
– Pancreatic enzymes must be
supplemented in the diet when the
pancreas is not functioning properly (as
in pancreatic exocrine insufficiency)
– Pancrealipase contains primarily lipase,
but also contains amylase and protease
– Can be irritating to the skin on contact
and to nasal passages upon inhalation
• See Table 11-1
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Dental Prophylaxis and
Treatment
• See Table 11-1
© 2004 by Thomson Delmar Learning, a part of the Thomson Corporation.