KINETICS OF INTERACTIONS OF PARA-AMINOHIPPURATE, PROBENECID, CYSTEINECONJUGATES AND N-ACETYL CYSTEINE CONJUGATES WITH BASOLATERAL ORGANIC ANION TRANSPORTER IN ISOLATED RABBIT PROXIMAL RENAL TUBULES

Kinetics of the first 15 s of para-aminohippurate (PAH) uptake across the basolateral membrane of single isolated S2 segments of rabbit prox imal renal tubules and the effects of probenecid and cysteine conjugat es on them were determined. For PAH uptake in control tubules, K-t (th e concentration of PAH at 1/2 J(max)) was about 110 mu M and J(max) (m aximal rate of PAH transport) was about 6.5 pmol min(-1) nl(-1). In tu bules preloaded with alpha-ketoglutarate (alpha-KG), thereby stimulati ng PAH/alpha-KG countertransport, J(max) doubled with little change in K-t. Probenecid cis-inhibited PAH uptake with an apparent K-i of abou t 13 to 15 mu M whether or not the tubules were preloaded with alpha-K G. High probenecid concentrations cis-inhibited PAH uptake by >98%, in dicating that essentially ail movement of PAH across the basolateral m embrane is carrier mediated. Zwitterionic nephrotoxic cysteine conjuga te, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and nontoxic cysteine con jugate, S-(2-benzothiazole)-L-cysteine (BTC) cis-inhibited PAH uptake (apparent K-i: similar to 86 mu M for DCVC; similar to 37 mu M for BTC ) at least as effectively as their negatively charged N-acetyl derivat ives (NAC-DCVC and NAC-BTC) (apparent K-i: similar to 310 mu M for NAC -DCVC; similar to 35 mu M for NAG-ETC). The inhibition by both DCVC an d NAC-DCVC was competitive in nature. NAC-DCVC also cia-inhibited net transepithelial secretion of PAH by isolated, perfused S2 segments. Th e presence of DCVC and NAC-DCVC, as well as PAH itself, in the bathing medium trans-stimulated the 15 s efflux of PAH across the basolateral membrane of single S2 segments with oil-filled lumens. These data ind icate that these cysteine conjugates and their N-acetyl derivatives, n ot only interact competitively with the PAH transporter, but are trans ported by it.