Tubular effects of acute metabolic acidosis in the rat.

Micropuncture and clearance studies were performed to evaluate the effects of acute NH4Cl-induced metabolic acidosis. Studies were performed in four groups of rats: controls, urea-saline, saline, and NH4Cl. Acidosis (pH: 7.25 +/- 0.02[S.E.M.], HCO3- = 12.4 +/- 0.7 mEq/L) was associated with increased fractional sodium excretion, 9.13% +/- 0.52 and calcium excretion, 1.32% +/- 0.43, which was significantly greater than controls, 0.30% +/- 0.07 and 0.30% +/- 0.07; urea-saline, 0.54% +/- 0.1 and 0.54% +/- 0.13; saline, 0.71% +/- 0.17 and 0.47% +/- 0.09. In the proximal tubule, however, fractional reabsorption was less with saline, 41.5 +/- 1.7, than either urea-saline, 51.1 +/- 1.3, or the NH4Cl-infused group, 46.9 +/- 0.4. The data indicate that natriuresis of acute NH4Cl loading is associated with but cannot be accounted for by inhibition of proximal tubular reabsorption. The mechanism of inhibition of transport at sites beyond the proximal tubule is not clear but cannot be either volume expansion or increased solute load due to urea excretion.