CARDIOVASCULAR PHARMACOLOGY
MNA Program
Fall 1999

2. Describe the mechanism(s) of action of the osmotic diuretic, mannitol. What would be an indication (rationale) for administering it during surgery? How could administration of sodium nitroprusside interfere with its diuretic efficacy?

Mannitol mechanism of action
    After administration, mannitol is completely filtered at the glomeruli, and none of the filtered drug is reabsorbed from the renal tubules.  As a result, mannitol 's the osmotic pressure of renal tubular fluid and prevents reabsorption of H₂O.  Na+ is diluted in this retained H₂O in the tubules, leading to less reabsorption of this ion.  As a result of this osmotic effect in the tubular fluid, there is an osmotic diuretic effect with urinary excretion of H₂O, Na⁺, Cl^- and HCO₃.  

Mannitol clinical uses

1. Prophylaxis against acute renal failure

2. Differential diagnosis of acute oliguria (in intravascular volume vs. renal function)

3. Treatment of intracranial pressure

4. To intraocular pressure (short term opthalmic. surgery)

Sodium Nitroprusside (SNP) and Mannitol
Act against each other.  

• Mannitol 

  • 's ICP

  • urine output which 's intravascular volume and 's Na⁺ (slightly).  

• SNP

  • 's Na⁺

  • 's intravascular volume

  • activity of RAA system

  • ICP by vasodilation of cerebral vasculature

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