Dissociation between antidiuretic response and renal medullary cyclic AMP levels in the rat

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, 6.36±0.15) was significantly higher than control (N=5, 5.64±0.09,P<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 Eq/3 h,P<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.Former trainee of the Cardiovascular Research Program and currently a third year medical student at The University of Michigan Medical School.