IMPAIRED URINARY AMMONIUM EXCRETION IN PATIENTS WITH ISOLATED PROXIMAL RENAL TUBULAR-ACIDOSIS

During previous studies in patients with isolated proximal renal tubular acidosis (pRTA), the rates of urinary ammonium excretion were considered inappropriately low for their state of chronic metabolic acidosis. These observations were made while the patients were on a normal diet as well as when they were undergoing a short ammonium chloride test. Because these findings suggested an impaired ability to excrete maximal amounts of ammonium, the response to the 3-day acid loading test was evaluated in eight patients with isolated pRTA and in 10 normal control subjects. Plasma creatinine, acid-base, and electrolyte values were analyzed before and after 3 days of ingesting 2 mmol/kg.24 h of ammonium chloride. Twenty-four-hour urine specimens were collected the day before and on the third day of acid loading to determine urine pH, as well as the rate of excretion of NH4+ and titratable acid in milliequivalents per 24 h per 1.73 m2. During the basal state, all patients with pRTA had hyperchloremic metabolic acidosis and they excreted urine of lower pH (5.51 +/- 0.18 versus 6.00 +/- 0.13; P < 0.05) and greater titratable acid (29.1 +/- 4.3 versus 21.8 +/- 1.4; P < 0.05); however, they had rates of NH4+ excretion similar to those of controls. On the third day of acid loading, they excreted urine of lower pH (4.66 +/- 0.03 versus 5.00 +/- 0.03; P < 0.05) and equivalent amounts of titratable acid, whereas their NH4+ excretion was significantly less than that of controls (47.7 +/- 4.4 versus 76.3 +/- 5.7; P< 0.05). These results show that patients with isolated pRTA have an impaired excretion of urinary ammonium and suggest that a decreased supply of NH3 to the distal tubular fluid may contribute to their urine of lower pH.