VPT 311: General and Systemic Veterinary Pharmacology (2+1)

GENERAL MECHANISM OF ACTION

• All purgatives increase the water content of feces by:
  • A hydrophilic or osmotic action, retaining water and electrolytes in the intestinal lumen- increase volume of colonic content and make it easily propelled.
  • Acting on intestinal mucosa to decrease net absorption of water and electrolyte intestinal transit is enhanced indirectly by fluid bulk
  • Increasing propulsive motility as primary action- allowing less time for absorption of salt and water as a secondary effect
• Certain purgatives increase motility through an action on the myenteric plexus.
• Laxatives modify the fluid dynamics of the mucosal cell and may cause fluid accumulation in gut lumen by one or more of the following mechanisms:
  • inhibiting Na⁺K⁺ ATPase of villus cells – impairing electrolyte and water adsorption
  • Stimulating adnylyl cyclase in crypt cells – increasing water and electrolyte secretion
  • Enhancing prostaglandin synthesis in mucosa which increases secretion
  • Structural injury to absorbing intestinal mucosal cells.