Distal renal tubular acidosis and high urine carbon dioxide tension in a patient with Southeast Asian ovalocytosis

Abstract

Southeast Asian ovalocytosis (SAO) is the best-documented disease in which mutation in the anion exchanger-1 (AE1) causes decreased anion (chloride [Cl-]/bicarbonate [HCO3-]) transport. Because AE1 is also found in the basolateral membrane of type A intercalated cells of the kidney, distal renal tubular acidosis (dRTA) might develop if the function of AE1 is critical for the net excretion of acid. Studies were performed in a 33-year- old woman with SAO who presented with proximal muscle weakness, hypokalemia (potassium, 2.7 mmol/L), a normal anion gap type of metabolic acidosis (venous plasma pH, 7.32; bicarbonate, 17 mmol/L; anion gap, 11 mEq/L), and a low rate of ammonium (NH4+) excretion in the face of metabolic acidosis (26 μmol/min). However, the capacity to produce NH4+did not appear to be low because during a furosemide-induced diuresis, NH4+excretion increased almost threefold to a near-normal value (75 μmol/L/min). Nevertheless, her minimum urine pH (6.3) did not decrease appreciably with this diuresis. The basis of the renal acidification defect was most likely a low distal H+secretion rate, the result of an alkalinized type A intercalated cell in the distal nephron. Unexpectedly, when her urine pH increased to 7.7 after sodium bicarbonate administration, her urine minus blood carbon dioxide tension difference (U-B PCO2) was 27 mm Hg. We speculate that the increase in U-B PCO2might arise from a misdirection of AE1 to the apical membrane of type A intercalated cells.