Gastrointestinal System Drugs

Gastrointestinal System Drugs Part 1

Gastrointestinal (GI) Tract

  • Definition: The gastrointestinal tract is a long, muscular tube that begins at the mouth and ends at the anus.

  • Components:

    • Oral Cavity and Pharynx

    • Esophagus

    • Stomach

    • Intestinal Structure:

    • Small Intestine: Consists of duodenum, jejunum, and ileum.

    • Large Intestine: Comprises cecum, colon, and rectum.

  • Animal Classification:

    • Carnivores and Omnivores: Classified as monogastric.

    • Ruminants: Possess a 4-chambered foregut.

Cecum Comparisons

  • Cecum Size Order:

    • Herbivores > Omnivores > Carnivores

Colon Modifications

  • Spiral Colon: Present in cattle and swine.

  • Large and Small Colons: Seen in horses and rabbits.

GI Control Mechanisms

  • Autonomic Nervous System Regulation:

    • Parasympathetic Stimulation:

    • Increases intestinal motility.

    • Increases GI secretions.

    • Relaxes sphincters.

    • Cholinergic Drugs: Mimic these actions.

    • Anticholinergic Drugs: Inhibit these actions.

    • Sympathetic Stimulation:

    • Decreases intestinal motility.

    • Decreases GI secretions.

    • Inhibits the action of sphincters.

    • Sympathetic Drugs: Mimic these actions.

Common GI Disorders

  • Overview: Among the most prevalent complaints in veterinary medicine.

  • Clinical Signs Include:

    • Diarrhea

    • Vomiting

    • Regurgitation

    • Constipation

  • Underlying Causes May Include:

    • Infectious agents

    • Dietary change or indiscretion

    • Adverse effects of drugs

    • Systemic disease

    • Others (as noted humorously by the speaker)

GI Drugs Overview

Antisialogues

  • Function: Decrease salivary flow.

  • Purpose: Used to limit excess saliva, especially following anesthetic use.

  • Examples:

    • Anticholinergics:

    • Glycopyrrolate

    • Atropine - not used in rabbits, also will cross the placental line, comes in small and large animal concentrations

  • Additional Effects:

    • Can slow peristalsis, effective in treating excess gastric secretion and diarrhea.

  • Adverse Effects:

    • Dry mucous membranes

    • Urine retention

    • Tachycardia

    • Constipation

    • Caution: Use with care in horses!

Antidiarrheals

Categories of Antidiarrheals:

  • Anticholinergics:

    • Purpose: Treat tenesmus - straining to defecate and vomiting.

    • Examples Include:

    • Atropine

    • Aminopentamide

    • Propantheline

    • N-butylscopolammonium bromide (Buscopan®) - fda approved for use in horses for colic, one time use IV

    • Adverse Effects: Note similar side effects as antisialogues. Caution in horses!

Protectants/Adsorbents

  • Function:

    • Coat inflamed intestinal mucosa.

    • Bind bacteria, digestive enzymes, or toxins to protect intestinal mucosa from damaging effects.

  • Examples:

    • Bismuth subsalicylate (combination of bismuth and aspirin-like product) - don’t use with cats, drug interactions

    • Kaolin/pectin

    • Activated charcoal

  • Adverse Effects: Constipation

Motility Reducers

  • Function: Opiate-related agents that decrease peristaltic contractions and increase segmental contractions.

  • Examples:

    • Diphenoxylate CV

    • Loperamide

  • Adverse Effects:

    • Constipation

    • Ileus - complete stop of GI - can lead to bloat

    • Bloat

    • Urine retention

    • CNS depression

Probiotics

  • Purpose: Seed the GI tract with beneficial bacteria.

  • Common Strains:

    • Lactobacillus spp.

    • Enterococcus faecium

    • Bifidobacterium spp.

  • Examples:

    • Plain yogurt with active cultures

    • Various trade-name products (examples provided in parallel resources)

Antibacterial Treatment

  • Theory: Disruption of normal GI flora allows anaerobic bacteria to increase.

  • Effective Drug: Metronidazole is effective against anaerobic bacteria, has activity against Giardia, and provides direct anti-inflammatory properties in the gut.

Laxatives

  • Function: Loosen bowel contents.

  • Indications for Use:

    • Help animals evacuate without excessive straining

    • Treat chronic constipation

    • Evacuate GI tract before procedures

  • Types of Laxatives:

    • Osmotic - pull water into the colon

      • lactulose

      • sodium or magnesium salts - never use in cats or animals with cardiac or renal disease

      • polyethylene glycol

    • Stimulant - increase peristalsis by chemically irritationg sensory nerve endings in the intestinal mucosa

      • absorbed systemically and cause a variety of adverse effects

      • no FDA approved veterinary products

      • bisacodly

      • castor oil

      • senna

    • Emollient - fecal wetting agents

    • stool softeners

    • lubricants

      • docusate sodium, docusate calcium

      • petroleum/mineral oil

        • mineral oil should always be given via nasogastric tube - can be fatal if aspirated into the lungs

    • Bulk-forming - poorly digestible fibers

    • absorb water into the intestine, increase fecal bulk, and stimulate peristalsis

      • may also bind endotoxin

      • not systemically absorbed, adverse effects are rare but hydration should be maintained

    • psyllium hydrophilic mucilloid

    • calcium polycarbophil

    • bran

Osmotic Laxatives

  • Mechanism: Pull water into the colon to increase fecal water content.

  • Examples:

    • Lactulose

    • Sodium or magnesium salts

    • Polyethylene glycol (PEG)-electrolyte solution

  • Adverse Effects:

    • Electrolyte imbalances from salts, caution in cardiovascular or renal disease.

    • Prolonged use can lead to dehydration.

Stimulant Laxatives

  • Mechanism: Increase peristalsis by chemically irritating sensory nerve endings in the intestinal mucosa.

  • FDA Status: No current FDA-approved veterinary products.

  • Examples:

    • Bisacodyl

    • Castor oil

    • Senna

  • Adverse Effects: Limited knowledge on animal effects, but many are absorbed systemically.

Bulk-Forming Laxatives

  • Mechanism: Include poorly digestible fibers that absorb water, increase fecal bulk, and stimulate peristalsis.

  • Additional Effects: May bind endotoxins.

  • Examples:

    • Psyllium hydrophilic mucilloid

    • Calcium polycarbophil

    • Bran

  • Adverse Effects: Rare due to non-systemic absorption; hydration should be maintained when using.

Emollient Laxatives

  • Also Known As: Fecal wetting agents and stool softeners.

  • Mechanism: They act as lubricants for stool easing.

  • Examples:

    • Docusate sodium

    • Docusate calcium

    • Petroleum (petrolatum, mineral oil)

Antiemetics

  • Overview: Vomiting can arise from various causes including viral and bacterial infections, dietary indiscretion, food intolerance, surgical situations, pain, or other drug interactions.

  • Mechanism of Vomiting: The vomiting center of the brain has several inputs that activate it, including:

    • Equilibrium changes in the ear

    • Pain or fear responses due to pain

    • Intracranial pressure changes

    • Stimulation of the vagus nerve in the GI tract

    • Activity in the chemoreceptor trigger zone (CRTZ)

Types of Antiemetics

  • Purpose: Drugs control vomiting and alleviate discomfort while managing electrolyte balance.

  • Route of Administration: Most are given parenterally.

  • Examples Include:

    • Phenothiazine derivatives

    • Antihistamines

    • Prokinetic agents

    • Serotonin receptor antagonists

    • Maropitant

Phenothiazine Derivatives

  • Mechanism: Inhibit dopamine in the CRTZ reducing the stimulation to initiate vomiting.

  • Adverse Effects:

    • Hypotension

    • Protrusion of the nictitating membrane

  • Examples:

    • Acepromazine

    • Chlorpromazine

    • Prochlorperazine

Antihistamines

  • Mechanism: Block input from the vestibular system to the CRTZ; significant to note that cats lack histamine receptors in CRTZ.

  • Adverse Effects:

    • Drowsiness

    • Paradoxical excitement

  • Examples:

    • Diphenhydramine (Benadryl® and variations)

    • Dimenhydrinate (Dramamine®)

    • Meclizine

Prokinetic Agents

  • Function: Blocks the CRTZ, increases gastric emptying, and enhances gastro-esophageal sphincter tone.

  • Adverse Effects:

    • Changes in mentation and behavior

    • Contraindications: Not for use with GI hemorrhage, obstruction, or perforation!

  • Example: Metoclopramide

Serotonin Receptor (5-HT3) Antagonists

  • Mechanism: Chemicals causing vomiting may increase serotonin release from small intestinal cells; these antagonists act on both central and peripheral 5-HT3 receptors.

  • Examples:

    • Ondansetron

    • Dolasetron

    • Granisetron

  • Adverse Effects: Uncommon but may include constipation, head shaking, sedation, arrhythmias, and hypotension.

Neurokinin (NK1) Antagonist

  • Example: Maropitant (Cerenia®)

  • Mechanism: Substance P involved in vomiting; used to prevent acute vomiting and motion sickness.

  • Adverse Effects Include: Injection site pain, pre-travel vomiting, hypersalivation, and diarrhea.

Conclusion: Each class of drug discussed plays a critical role in the management of various gastrointestinal disorders, highlighting the importance of proper pharmacological intervention in veterinary care.

Note: The contents herein are a detailed delineation of gastrointestinal system drugs as outlined in Romich, Chapter 11, contributing to a comprehensive understanding necessary for veterinary practice by healthcare professionals.