NH4+ modulates renal tubule amantadine transport independently of intracellular pH changes

A bicarbonate-dependent organic cation transporter, unique from rOCT1 and rOCT2, primarily mediates amantadine uptake into renal proximal tubules. We examined whether intracellular pH regulates bicarbonate-dependent amantadine transporter function in these tubules. NH4Cl treatment resulted in immediate intracellular alkalinization of tubules for up to 30 s followed by gradual acidification that was maximal at 5 min. Proximal tubule amantadine uptake was similarly inhibited (60%) by NH4Cl during both the early intracellular alkalinization and later acidification phases. Sodium propionate treatment resulted in immediate intracellular acidification of proximal tubules without inhibiting amantadine uptake. NH4Cl inhibition of bicarbonate-dependent amantadine uptake was dose-dependent, competitive and sex-dependent. NH4Cl, NH4NO3, (NH4)2SO4 and (NH4)2HPO4 inhibited amantadine uptake into proximal tubules similarly. NH4Cl also stimulated efflux of amantadine and tetraethylammonium from preloaded proximal tubules, suggesting mediation of a facilitated process. These data suggest the potential for direct modulation of organic cation transporters by NH4+ in rat kidney proximal tubules.