A 44-year-old woman with a known history of Sjögren's syndrome develops distal renal tubular acidosis (dRTA). Arterial blood gas: pH 7.29, HCO3 14 mEq/L, pCO2 30 mmHg. Serum K+ 2.9 mEq/L. Urine pH is 6.8 despite systemic acidosis. Which mechanism best explains dRTA pathogenesis in Sjögren's syndrome?
- A Antibodies against carbonic anhydrase II causing reduced H+ generation in the proximal tubule
- B Reduced aldosterone secretion causing decreased electrogenic H+ excretion — type 4 RTA
- C Autoantibodies targeting the H+-ATPase (vacuolar ATPase) on α-intercalated cells of the collecting duct, impairing proton secretion ✓
- D Bicarbonate wasting from the proximal tubule due to defective NBC1 cotransporter — type 2 RTA
Explanation
In Sjögren's syndrome-associated dRTA (type 1 RTA), autoantibodies target the H+-ATPase (V-ATPase) on α-intercalated cells of the collecting duct, impairing the ability to secrete H+ into the tubular lumen. This is confirmed by inability to acidify urine below pH 5.5 despite systemic acidosis. The mechanism is impaired proton secretion (not bicarbonate wasting), hence urine anion gap is positive and urine pH remains > 5.5. Sjögren's can also produce anti-CA-II antibodies but the primary RTA mechanism involves H+-ATPase. Type 4 RTA involves aldosterone deficiency/resistance and causes hyperkalaemia, not hypokalaemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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