Ostmotic diuretics
Mechanism of action
- Are membrane impermeant polyhydric alcohols; pharmacologically inert
- Enter nephron by glomerular filtration, but are not reabsorbed
- Increase osmolarity of the filtrate, opposing the absorption of water in parts of the nephron that are freely permeable to water
- Major site of action in the kidney is the proximal tubule where most iso-osmotic reabsorption of water occurs
- Secondarily decrease in sodium reabsorption in the proximal tubule (larger fluid volume decreases sodium concentration and electrochemical gradient for reabsorption)
Clinical indications
- Used in the prevention of acute hypovolaemic renal failure to maintain urine flow
- In urgent treatment of acutely raised intracranial and intraocular pressure
- The solute does not enter the eye or brain, but increased plasma osmolality extracts water from these compartments
Adverse effects
- Transient expansion of blood volume
- Hyponatraemia
Other causes of osmotic diuresis
Osmotic diuresis may also occur:
- In hyperglycaemia - reabsorptive capacity of the proximal tubule for glucose (by SGLT1 and SGLT2)is exceeded
- Glucose remaining in the filtrate retains fluid
- As a consequence of use of iodine-based radiocontrast dyes in imaging
- Dye is filtered at glomerulus but it is not reabsorbed constituting an osmotic load
- Patients with borderline cardiovascular status may experience hypotension due to reduction in intravascular volume
Carbonic anhydrase inhibitors