Mannitol moa
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mannitol mechanism of action pharmacology ICP osmotic diuretic
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Here is a detailed breakdown of mannitol's mechanism of action, drawn from multiple pharmacology and physiology textbooks:
Mannitol - Mechanism of Action
What is Mannitol?
Mannitol is a six-carbon sugar alcohol (reduced fructose) that is the prototypic osmotic diuretic. It is given intravenously because oral administration causes osmotic diarrhea rather than diuresis.
Core Mechanism: Osmotic Action
Mannitol works by being an osmotically active, non-reabsorbable solute. Because it is:
- Freely filtered at the glomerulus
- Not metabolized
- Poorly/not reabsorbed in the renal tubules
...it exerts its effects purely through osmotic forces.
1. Osmotic Diuresis (Renal Mechanism)
Site of action: Proximal tubule + descending limb of loop of Henle + collecting duct
- Filtered mannitol accumulates in the proximal tubule and descending limb, which are normally freely permeable to water.
- The presence of this non-reabsorbable solute creates a countervailing osmotic force that opposes normal passive water reabsorption.
- Ongoing Na+ reabsorption lowers tubular fluid [Na+], creating a gradient for back-flux of reabsorbed Na+ into the tubule - impairing net sodium reabsorption.
- The same mechanism impairs water and solute reabsorption in the loop of Henle.
- Mannitol also opposes ADH action in the collecting tubule via osmotic effects.
- Net result: large water diuresis > natriuresis (leads to hypernatremia if free water not replaced).
(Morgan & Mikhail's Clinical Anesthesiology; Katzung's Basic & Clinical Pharmacology; Brenner & Rector's The Kidney)
2. Reduction of Intracranial Pressure (ICP)
Mechanism - osmotic gradient across the BBB:
- IV mannitol raises plasma osmolality, creating an osmotic gradient between the blood and the brain.
- Water moves out of cerebral tissue (across the relatively intact blood-brain barrier) into the intravascular compartment.
- This reduces brain water content and cell volume, lowering ICP.
- Intracranial pressure falls within 60-90 minutes of administration.
- Dose: 0.25-1 g/kg IV (Morgan & Mikhail); 1-2 g/kg (Katzung) as 15-20% solution.
(Katzung; Morgan & Mikhail)
3. Reduction of Intraocular Pressure
- Same osmotic principle applies to the aqueous humor of the eye.
- Mannitol extracts water from the vitreous/aqueous compartment, reducing intraocular pressure.
- Used perioperatively in ophthalmologic procedures.
4. Additional Mechanisms
| Mechanism | Effect |
|---|---|
| Increases renal blood flow (RBF) | Via hemodilution + decreased hematocrit/viscosity |
| Activates intrarenal prostaglandin synthesis | Vasodilation, contributing to increased RBF |
| Decreases medullary solute gradient | Prevents urinary concentration |
| Free radical scavenging | Proposed role in renal protection |
| BBB disruption (high-dose intra-arterial) | Transiently opens tight junctions in brain endothelium - used experimentally for drug delivery |
(Brenner & Rector; Morgan & Mikhail; Goodman & Gilman)
Pharmacokinetics Summary
| Parameter | Detail |
|---|---|
| Route | IV only (oral = osmotic diarrhea) |
| Distribution | Extracellular fluid only |
| Metabolism | Not metabolized |
| Elimination | Glomerular filtration; t½ ~1 hour (normal), up to 36 hours in severe CKD |
Key Clinical Uses
- Raised ICP (traumatic brain injury, cerebral edema)
- Raised intraocular pressure (pre-operative ophthalmology)
- Acute kidney injury - to promote diuresis, flush tubular toxins
- Dialysis disequilibrium syndrome - can reverse it
- Ciguatera fish poisoning - reduces Schwann cell edema
Important Adverse Effects
- Before diuresis: Expands ECF - can precipitate pulmonary edema in heart failure
- After diuresis: Dehydration, hypernatremia, hyperkalemia, metabolic acidosis
- High doses: Nephrotoxicity (especially in pre-existing renal impairment)
- Headache, nausea, vomiting
Contraindicated in anuria, severe heart failure, and active intracranial bleeding (paradoxical ICP rise if BBB disrupted).
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