Case Report
Distal renal tubular acidosis and high urine carbon dioxide tension in a patient with southeast asian ovalocytosis

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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 Pco2 might arise from a misdirection of AE1 to the apical membrane of type A intercalated cells.

Section snippets

Case report

The chief complaint in a 33-year-old woman was generalized muscle weakness for the past 4 months. Her past medical history and review of systems were unremarkable. She was an elder sister in a family with two sibs. Only she and her father had SAO.

On admission, she was afebrile, blood pressure was 110/70 mm Hg while supine, respiratory rate was 20 breaths/min, and pulse rate was 86 beats/min. Postural changes in blood pressure and pulse rate were not detected. The patient was alert and oriented.

Discussion

The findings of hyperchloremic metabolic acidosis, hypokalemia with a relatively high TTKG and potassium excretion rate, a relatively low rate of excretion of NH4+ and citrate, and high urine pH (>5.5) are all typical for a patient with a decreased rate of distal H+ secretion.20 These results describe our patient with dRTA, whose disease is associated with the mutation of AE1 gene (deletion of 27-bp in exon 11). Nevertheless, no specific renal tissue was obtained because we could not do so on

Acknowledgements

Acknowledgment: The authors thank Nunghathai Sawasdee for her laboratory assistance.

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    Received January 23, 1998; accepted in revised form October 16, 1998.

    Address reprint requests to Somkiat Vasuvattakul, MD, Renal Unit, Department of Medicine, Siriraj Hospital, Mahidol University, Bangkok, Thailand. E-mail: [email protected]

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