Figure 1211

Carbonic anhydrase II deficiency. Carbonic anhydrase II deficiency is an autosomal recessive disorder characterized by renal tubular acidosis (RTA), with both proximal and distal components, osteopetrosis, and cerebral calcification. Carbonic anhydrase catalyzes the reversible hydration of carbon dioxide (CO2), and thereby accelerates the conversion of carbon dioxide and water to hydrogen ions (H+) and bicarbonate (HCO-3) [21]. A least two isoenzymes of carbonic anhydrase are expressed in the kidney and play critical roles in urinary acidification. In the proximal tubule, bicarbonate reabsorption is accomplished by the combined action of both luminal carbonic anhydrase type IV (CA4) and cytosolic carbonic anhydrase type II (CA2), the luminal sodium-hydrogen exchanger, and the baso-lateral sodium-bicarbonate exchanger. Impaired bicarbonate reabsorption in the proximal tubule is the underlying defect in type II or proximal RTA. In the distal nephron, carbonic anhydrase type II is expressed in the intercalated cells of the cortical collecting duct. There carbonic anhydrase type II plays a critical role in catalyzing the condensation of hydroxy ions, generated by the proton-translocating H+-adenosine triphosphatase (H+ ATPase), with carbon dioxide to form bicarbonate. In carbonic anhydrase type II deficiency, the increase in intracellular pH impairs the activity of the proton-translocating H-ATPase. Carbonic anhydrase inhibitors (eg, acetazo-lamide) act as weak diuretics by blocking bicarbonate reabsorption. Cl-—chloride ion; H2CO3—carbonic acid; K+—potassium ion; Na+—sodium ion.

Cortical collecting

Principal cell

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