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

OSMOTIC DIURETICS

• These are inert substances filtered into the glomerulus.
  
• Their main effect is exerted in the proximal tubule, descending limb of Henle and the collecting duct which are freely permeable to water.
• By remaining in the tubular lumen, these agents cause an increase in colloid osmotic pressure in the tubular lumen causing suction of water into the tubular lumen.
• There is also reduced water reabsorption due to renal medullary hyperemia.
• There is impairment of Na⁺ reabsorption because the diluted solution in the lumen reduces the Na⁺ concentration gradient, making it harder for Na⁺ to be reabsorbed.
• They are not useful in treating conditions with sodium retention.
• These drugs are useful in acutely raised intracranial pressure or intraocular pressure.
• They are also useful in prevention of acute renal failure.
  
• Unwanted effects include transient expansion of extracellular fluid volume and hyponatraemia, headache and vomiting.
• Mannitol is commonly used as an osmotic diuretic and has to be administered intravenously.
• Isosorbide has the advantage of being absorbed after oral administration in monogastric species.