Effect of an acute oral protein load on renal acidification in healthy humans and in patients with chronic renal failure.

The effect of a meat load on the renal handling of acid-base balance was studied in ten healthy subjects (GFR by inulin clearance = 98.5 +/- 8.14 ml.min-1.1.73 m-2) and in ten patients affected by chronic renal failure (CRF) (GFR = 39.9 +/- 5.3 ml.min-1.1.73 m-2). After the meat load (2 g.kg-1 body weight of cooked unsalted red meat), GFR increased by 26.9% (peak value) over baseline in healthy subjects and by 32% in CRF patients. The acid-base status of the healthy subjects was in the normal range, whereas the CRF patients disclosed a slight metabolic acidosis. After a meat load, there was, in the healthy subjects, an increase in the filtered load of bicarbonate coupled to an enhanced tubular reabsorption and urinary excretion. The time course between bicarbonate load and urinary excretion was coincident. In CRF patients, the increase of bicarbonate tubular load after the meal was associated with an increase in tubular reabsorption but not in urinary excretion of this anion. The relationship between bicarbonate load and reabsorption was linear in both groups up to the highest filtered loads. Baseline titratable acidity (TA) and ammonium (NH4+) excretion (expressed per ml GFR) were increased in CRF patients as compared with control subjects, but no changes were found after the meat load in both groups in these experimental conditions. The data indicate that the renal tubules contribute to the maintenance of acid-base balance both in healthy subjects and in CRF patients by reabsorbing most of the additional bicarbonate load. The transient, but significant, increase in bicarbonate excretion observed in healthy subjects could be related to the increased tubular load of bicarbonate. In CRF patients, tubular bicarbonate reabsorption was more complete, possibly because of the stimulation of H+ secretion by the mild metabolic acidosis. TA and NH4+ did not participate in tubular compensation of the increased buffer load.