title: Titratable acidity: a Pitts concept revisited.
abstract: Titratable Acidity (TA) in urine can be measured directly or calculated from actual and reference pH, by using the pKa₂ 6,8 for phosphate. In urine, H₂PO₄(-) represents the excretion of filtered H₂PO₄(-), filtrated HPO₄(2-) being completely reabsorbed by the proximal tubule (the Van Slyke approach). Since excretion of H₂PO₄(-) frequently exceeds its glomerular filtration, this approach is considered inadequate by Pitts. He claimed that it is the tubular H(+) secretion which converts filtered HPO₄(2-) to H₂PO₄(-), thereafter excreted in urine. This is only true under conditions of inorganic acid or neutral phosphate loading, when the maximum tubular phosphate reabsorption (TmPi) is overcharged. In controls, H₂PO₄(-) excretion is lower than its glomerular filtration, provided that acid-base status is normal and tubular phosphate reabsorption is below the TmPi. The TmPi is lower than its glomerular filtration, provided that acid-base status is normal and tubular phosphate reabsorption is below the TmPi. When the TmPi is exceeded, a portion of HPO₄(2-) escapes proximal reabsorption, reaching the distal tubule where its absorption is precluded, while tubular H(+) secretion converts HPO₄(2-) to H₂PO₄(-). In man and dog, the attainment of TmPi is evidenced by a FE% of 20%, and only beyond this limit H₂PO₄(-) excretion exceeds glomerular filtration. When FE% is lower than 20%, H₂PO₄(-) filtration exceeds excretion, HPO4(2-) being completely reabsorbed at the proximal tubule by NaPi-2a and 2c cotransporters. While Van Slyke's approach is always valid, Pitts' approach is only valid under loading conditions, when the two processes of H₂PO₄(-) excretion overlap each other. NH (+4) increases inversely to TA excretion in conditions of acidosis and tP restriction, but is independent of TA in Pi-replete dogs, independently of acidosis.
authors.affiliation: Division of Nephrology, Civil and University Hospital, Santa Maria della Misericordia , Udine , Italy.
authors.forename: Roberto
authors.initials: R
authors.lastname: Mioni
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Mioni
mesh_terms: D000138:Acidosis / Q000652:urine*; D000818:Animals; D004285:Dogs; D006801:Humans; D006863:Hydrogen-Ion Concentration; D051379:Mice; D010756:Phosphoric Acids / Q000652:urine*; D017186:Titrimetry; D016482:Urinalysis
chemical_list: D010756:Phosphoric Acids
keywords: Acid phosphate; dibasic phosphate; fractional phosphate excretion; inorganic phosphate; sodium hydrogen antiporter; sodium phosphate T2 cotransporter; sodium phosphate-2a cotransporter; sodium phosphate-2c cotransporter; tubular maximum Pi transport