Impaired distal nephron acidification in chronically phosphate depleted rats.

Renal tubular bicarbonate reabsorption and acidification were evaluated in phosphate depleted rats (PD) and controls. After 33 days of phosphate depletion, urine pH of PD rats (N = 5, 5.36 +/- 0.15) was significantly higher than control (N = 5, 5.64 +/- 0.09, P less than 0.005) following an NH4Cl load. Urinary titratable acid of PD rats (9.6 +/- 1.8) was significantly reduced compared to control (117.2 +/- 19.7 muEq/3 h, P less than 0.001), whereas NH+4 excretion was not different. The plasma HCO-3 thresholds at which bicarbonaturia occurred (approximately 25 mEq/l) were identical in controls and phosphate depleted rats during isotonic bicarbonate infusion. The higher urine pH of phosphate depleted rats following NH4Cl administration was not due to low urinary phosphate as 3-day phosphate depleted rats could normally acidify urine after NH4Cl (pH = 5.86 +/- 0.09, N = 6 vs. control 5.87 +/- 0.08, N = 6, P = N.S.) despite urinary phosphate excretion as low as in 33-day PD rats. These data indicate the presence of impaired distal tubular acidification in chronically phosphate depleted rats.