Acetazolamide

Acetazolamide, sold under the trade name Diamox among others, is a medication used to treat glaucoma, epilepsy, acute mountain sickness, periodic paralysis, idiopathic intracranial hypertension (raised brain pressure of unclear cause), heart failure and to alkalinize urine.^[1][2] See also Acetazolamide.

Acetazolamide is a carbonic anhydrase inhibitor, hence causing the accumulation of carbonic acid.^[3] Carbonic anhydrase is an enzyme found in red blood cells and many other tissues that catalyses the following reaction:^[4]

H2CO3 ⇌ H2O + CO2

hence lowering blood pH, by means of the following reaction that carbonic acid undergoes:^[5]

H2CO3 ⇌ HCO3− + H+

which has a pKa of 6.3.^[5]

The mechanism of diuresis involves the proximal tubule of the kidney. The enzyme carbonic anhydrase is found here, allowing the reabsorption of bicarbonate, sodium, and chloride. By inhibiting this enzyme, these ions are excreted, along with excess water, lowering blood pressure, intracranial pressure, and intraocular pressure. A general side effect of carbonic anhydrase inhibitors is loss of potassium due to this function. By excreting bicarbonate, the blood becomes acidic, causing compensatory hyperventilation with deep respiration (Kussmaul breathing), increasing levels of oxygen and decreasing levels of carbon dioxide in the blood.^[6]

In the eye this results in a reduction in aqueous humour.

Bicarbonate (HCO3−) has a pKa of 10.3 with carbonate (CO32−), far further from physiologic pH (7.35–7.45), and so it is more likely to accept a proton than to donate one, but it is also far less likely for it to do either, thus bicarbonate will be the major species at physiological pH.

Under normal conditions in the proximal convoluted tubule of the kidney, most of the carbonic acid (H2CO3) produced intracellularly by the action of carbonic anhydrase quickly dissociates in the cell to bicarbonate (HCO3−) and an H+ ion (a proton), as previously mentioned. The bicarbonate (HCO3−) exits at the basal portion of the cell via sodium (Na+) symport and chloride (Cl−) antiport and re-enters circulation, where it may accept a proton if blood pH decreases, thus acting as a weak, basic buffer. The remaining H+ left over from the intracellular production of carbonic acid (H2CO3) exits the apical (urinary lumen) portion of the cell by Na+ antiport, acidifying the urine. There, it may join with another bicarbonate (HCO3−) that dissociated from its H+ in the lumen of the urinary space only after exiting the proximal convoluted kidney cells/glomerulus as carbonic acid (H2CO3) because bicarbonate (HCO3−) itself can not diffuse across the cell membrane in its polar state. This will replenish carbonic acid (H2CO3) so that it then may be reabsorbed into the cell as itself or CO2 and H2O (produced via a luminal carbonic anhydrase). As a result of this whole process, there is a greater net balance of H+ in the urinary lumen than bicarbonate (HCO3−), and so this space is more acidic than physiologic pH. Thus, there is an increased likelihood that any bicarbonate (HCO3−) that was left over in the lumen diffuses back into the cell as carbonic acid, CO2, or H2O.

In short, under normal conditions, the net effect of carbonic anhydrase in the urinary lumen and cells of the proximal convoluted tubule is to acidify the urine and transport bicarbonate (HCO3−) into the body. Another effect is excretion of Cl− as it is needed to maintain electroneutrality in the lumen, as well as the reabsorption of Na+ into the body.

Thus, by disrupting this process with acetazolamide, urinary Na+ and bicarbonate (HCO3−) are increased, and urinary H+ and Cl− are decreased. Inversely, serum Na+ and bicarbonate (HCO3−) are decreased, and serum H+ and Cl− are increased. H2O generally follows sodium, and so this is how the clinical diuretic effect is achieved, which reduces blood volume and thus preload on the heart to improve contractility and reduce blood pressure, or achieve other desired clinical effects of reduced blood volume such as reducing edema or intracranial pressure.^[7]