In vivo analysis of amantadine renal clearance in the uninephrectomized rat: functional significance of in vitro bicarbonate-dependent amantadine renal tubule transport.

Amantadine transport into renal proximal and distal tubules is bicarbonate dependent. In the present study, we addressed the effects of bicarbonate on renal clearance and urinary excretion of amantadine. Renal clearance of kynurenic acid was also studied to determine whether bicarbonate effects are specific for organic base transport by the kidney. After a moderate diuresis was established, animals received i.v. [(3)H]amantadine or [(3)H]kynurenic acid followed by an acute dose of sodium bicarbonate or physiological saline. Urine and blood samples were analyzed for [(3)H]amantadine or [(3)H]kynurenic acid, blood gases, and pH. Amantadine and kynurenic acid were excreted by the kidneys, and both compounds underwent renal tubular secretion. Amantadine metabolism occurred, and one metabolite was detected in the urine. In the bicarbonate-treated rats, the total amount of amantadine excreted in the urine was decreased, whereas the amount of metabolite recovered was similar in both groups. Bicarbonate treatment caused a sustained increase in blood bicarbonate levels, a mild increase in blood pH, and a decrease in amantadine renal clearance and in the amantadine/creatinine clearance ratio. Only a transient decrease in the renal clearance of kynurenic acid and the kynurenic acid/creatinine clearance ratio was observed. This study demonstrates that short-term changes in bicarbonate concentration may have significant effects on renal organic cation elimination. Coupled with our previous in vitro demonstration of bicarbonate-dependent organic cation transport, the present study suggests that bicarbonate inhibition of renal tubule organic cation secretion may explain the previous observation that bicarbonate dosing decreases amantadine excretion by the kidney.