Distal renal tubular acidosis with severe hypokalaemia, probably caused bycolonic H+-K+-ATPase deficiency

We describe a 21 month old male infant who presented with failure to thrive associated with severe hypokalaemia and metabolic acidosis, together with hypomagnesaemia. Evaluation revealed marked renal and probable faecal potas sium wasting, distal renal tubular acidosis, mild urinary magnesium wasting , and a normal gastric pH (gastric H+-K+-ATPase). Hypokalaemic forms of met abolic acidosis, such as diabetic ketoacidosis and proximal renal tubular a cidosis were ruled out from the clinical picture. The hypokalaemia of dista l renal tubular acidosis usually improves with alkali therapy, but this was not observed: despite correction of acidosis with 5 mmol/kg potassium citr ate per day, an additional 5 mmol/kg potassium chloride was required to bri ng serum potassium to 3.5 mmol/l. At 3 years of age potassium was provided in the absence of potential alkali and acidosis ensued; serum bicarbonate f ell to 10 mmol/l. Although a specific genetic analysis is not yet possible, the abnormalities are consistent with a novel form of distal renal tubular acidosis. The pathophysiology probably does not stem from defects in the v acuolar H+-ATPase but more likely from deficient activity of the colonic is oform of H+-K+-ATPase that is resident in the medullary collecting duct and mediates potassium absorption and proton secretion.