title: Studies on the pathophysiology of the low urine pH in patients with uric acid stones.
abstract: BACKGROUND
  A very low urine pH is the major risk factor for uric acid stone formation.
  
  
  METHODS
  A subgroup of patients with a history of uric acid stones and a persistently low urine pH (<5.5 for at least 12 h/day) were selected for detailed study. Based on their relative ammonium (NH(+)(4)) and sulfate (SO(2-)(4)) excretions, patients were divided into two groups.
  
  
  RESULTS
  The first group (N = 2) excreted 173 and 139% more NH(+)(4) than SO(2-)(4). Their daily urinary unmeasured anion excretion was higher than their calculated net diet alkali input (38 and 61 vs. 24 and 49 mEq, respectively). In the second group (N = 12), NH(+)(4) excretion was 69 +/- 5% that of SO(2-)(4). In 2 of 12, decreased renal ammoniagenesis was suspected due to a plasma potassium of 5.3 mmol/L and/or a lower GFR (65 and 59 L/day); these patients had an extremely low citrate excretion (3 and 1 mEq/day). In contrast, citrate excretion was not low in the remaining 10 patients (10.4 +/- 1.3 mEq/day).
  
  
  CONCLUSIONS
  Patients in group 1 needed a higher NH(+)(4) excretion possibly because of a H+ load from excessive renal excretion of organic anions. We speculate that an alkaline proximal tubular cell pH could be the basis for the low NH(+)(4) and high citrate excretions in 10 of 12 patients in group 2. Dietary factors and/or a molecular lesion may contribute to their pathophysiology.
authors.affiliation: Renal Division, St. Michael's Hospital, University of Toronto, 30 Bond Street, Toronto, Ontario, Canada M5B 1A6. kamel.kamel@utoronto.ca
authors.forename: Kamel S
authors.initials: KS
authors.lastname: Kamel
authors.forename: Surinder
authors.initials: S
authors.lastname: Cheema-Dhadli
authors.forename: Mitchell L
authors.initials: ML
authors.lastname: Halperin
mesh_terms: D000328:Adult; D000641:Ammonia / Q000652:urine; D018360:Crystallography, X-Ray; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007669:Kidney Calculi / Q000378:metabolism* / Q000652:urine; D008297:Male; D008875:Middle Aged; D012016:Reference Values; D013431:Sulfates / Q000652:urine; D014527:Uric Acid / Q000378:metabolism*
chemical_list: D013431:Sulfates; D014527:Uric Acid; D000641:Ammonia