SITE-SELECTIVE EFFECT OF BICARBONATE ON AMANTADINE RENAL TRANSPORT - QUININE-SENSITIVE IN PROXIMAL VS QUINIDINE-SENSITIVE SITES IN DISTAL TUBULES

The mechanism for bicarbonate enhancement of amantadine (A(+)) uptake was studied further. A selective modulatory effect of bicarbonate on t he stereoselectivity of inhibition of A(+) uptake by the stereoisomers , quinine (Q) and quinidine (QD), as reflected by the reversal of pote ncy (Q > QD in KHS vs. QD > Q in phosphate), is reported. Studies were performed in bicarbonate (KHS) and phosphate buffers using purified c ortical proximal (PT) and distal tubules (DT) from rat kidneys. Variat ions of extracellular K+ and Ca++ were used to assess the effect of me mbrane depolarization and calcium on A(+) uptake by the tubules. K+ co ncentration manipulation (1.5-100 mM) did not change A(+) uptake by PT or DT in KHS buffer. High Ca++ (5.0 mM) decreased A(+) uptake by PT i n KHS (P < .001), whereas in phosphate buffer, both 2.5 and 5.0 mM Ca decreased uptake (P < .001). In DT, 0.25 mM Ca++ enhanced A(+) uptake (P < .05) in KHS, but 2.5 and 5.0 mM Ca++ decreased it in both buffers . Inhibition studies with Q and QD were performed to characterize the bicarbonate-dependent and bicarbonate-independent A(+) transport sites further. Stereoselectivity of inhibition was observed in PT in all bu ffers used. Potency of Q was higher than QD in KHS but lower than QD i n phosphate-buffered PT. QD potency remained unchanged. In phosphate-p lus-bicarbonate-buffered PT, the inhibitory potencies for Q and QD inc reased, with the potency of QD being greater than in KHS (P < .001). F or DT, Q and QD were equipotent in all buffers used. The potency of QD in phosphate buffer decreased compared with that in KHS (P < .001). T he increase in potency of QD on addition of bicarbonate to phosphate b uffer was of the same magnitude observed in PT. These studies indicate that intracellular negativity and extracellular calcium are not rate limiting for entry of the organic cation into the tubule cell. Moreove r, bicarbonate appears to modulate the Q-sensitive A(+) transporters i n PT and the QD-sensitive A(+) transporters in the DT. Our studies ind icate that the QD-sensitive site may be similar between PT and DT.