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).(ABSTRACT TRUNCATED AT 250 WORDS)