Last updated 04/10/00 12:26 PM
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
Prophylaxis against acute renal failure
Differential diagnosis of acute oliguria (in intravascular volume vs. renal function)
Treatment of intracranial
pressure
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