Figure 1212

Distal renal tubular acidosis (RTA). The collecting duct is the principal site of distal tubule acidification, where the final 5% to 10% of the filtered bicarbonate load is reabsorbed and the hydrogen ions (H+) generated from dietary protein catabolism are secreted. The distal nephron is composed of several distinct segments, eg, the connecting tubule, cortical collecting duct, and medullary collecting duct. The tubular epithelia within these segments are composed of two cell types: principal cells that transport sodium, potassium, and water; and intercalated cells that secrete hydrogen ions and bicarbonate (HCO-3) [22].

Urinary acidification in the distal nephron depends on several factors: an impermeant luminal membrane capable of sustaining large pH gradients; a lumen-negative potential difference in the cortical collecting duct that supports both hydrogen and potassium ion (K+) secretion; and secretion of hydrogen ions by the intercalated cells of the cortical and medullary collecting ducts at a rate sufficient to regenerate the bicarbonate consumed by metabolic protons [22]. Abnormalities in any of these processes could result in a distal acidification defect.

Recent studies in families with isolated autosomal dominant distal RTA have identified defects in the basolateral chloride-bicarbonate exchanger, AE1 [23,24]. Defects in various components of the H+-adenosine triphosphatase (H+ ATPase) and subunits of the H+-K+ ATPase (H+\K+ ATPase) also have been proposed as the basis for other hereditary forms of distal RTA. CA2—cytosolic carbonic anhydrase type II; Cl-—chloride ion; CO2—carbon dioxide; Na+—sodium ion; OH-—hydroxy ions.

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