Pharmacodynamics and Mechanism of Action
The primary target of this drug class is the enzyme carbonic anhydrase. In the kidneys, it is located predominantly on the apical membrane of proximal tubular epithelial cells. Normally, this enzyme catalyzes the critical hydration and dehydration reactions of carbonic acid.
When carbonic anhydrase is pharmacologically blocked by acetazolamide, the following occurs:
- The reabsorption of bicarbonate ions ($HCO_3^-$) and sodium ions ($Na^+$) from the renal tubular lumen into the bloodstream is impaired.
- The excretion of bicarbonates in the urine is dramatically increased.
- As a result, urinary pH shifts toward alkalinity (alkalinization of urine).
- Simultaneously, bicarbonate stores in the blood and tissues are depleted, inevitably leading to blood acidification—thus developing metabolic acidosis.
Use as a Diuretic
Historically, acetazolamide was prescribed for edema associated with heart failure, liver cirrhosis, or renal failure. However, today it is rarely used as a classical diuretic due to its low efficacy and inconvenient dosing schedule.
The main limitation of the drug is tolerance (tachyphylaxis), which is directly linked to its mechanism of action. The diuretic effect of acetazolamide is short-lived and strictly dependent on the availability of bicarbonate ions in the body. Once these stores are depleted and metabolic acidosis sets in, the drug simply stops working.
To restore its efficacy, the alkaline reserve of the blood must be replenished. Therefore, acetazolamide is prescribed exclusively in intermittent regimens: for example, a patient takes the tablets for 5 days, followed by a mandatory 2-day break.
Extranephral Effects and Ophthalmologic Applications
Because carbonic anhydrase is present in tissues outside the kidneys, inhibiting this enzyme produces several significant extranephral effects. These effects define the modern clinical utility of the drug class.
Effects on the Eye (Primary Use): In the ciliary body of the eye, the drug inhibits carbonic anhydrase, leading to reduced production of aqueous humor. This results in a drop in intraocular pressure.
- Glaucoma: Acetazolamide is administered orally (per os) for the rapid management of acute glaucoma attacks, as well as for preoperative preparation.
- Topical Alternative: For chronic glaucoma management, another inhibitor is used—dorzolamide, which is formulated as eye drops.
Effects on the Central Nervous System: In the CNS, enzyme blockade leads to a decrease in cerebrospinal fluid (CSF) production. This property allows the drug to be successfully used for elevated intracranial pressure (pseudotumor cerebri / intracranial hypertension).
Pharmacokinetics and Other Indications
Acetazolamide is well absorbed when taken orally. Its onset of action begins within 40–60 minutes, peaking after 2–4 hours. The total duration of the clinical effect ranges from 10 to 12 hours.
In addition to glaucoma, edema, and intracranial hypertension, the drug is used in the following settings:
- Altitude Sickness (Acute Mountain Sickness) (accelerates acclimatization to high altitudes by inducing a non-anion gap metabolic acidosis, which stimulates respiration).
- Epilepsy (specifically absence seizures in pediatric practice, as adjunctive therapy).
- Correction of Metabolic Alkalosis (because the drug deliberately induces acidosis, it is logical to use it to compensate for excess blood alkalinization).