Kinetics of interactions of para-aminohippurate, probenecid, cysteine conjugates 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 proximal renal tubules and the effects of probenecid and cysteine conjugates on them were determined. For PAH uptake in control tubules, Kt (the concentration of PAH at 1/2 Jmax) was about 110 microM and Jmax (maximal rate of PAH transport) was about 6.5 pmol min-1 nl-1. In tubules preloaded with alpha-ketoglutarate (alpha-KG), thereby stimulating PAH/alpha-KG countertransport, Jmax doubled with little change in Kt. Probenecid cis-inhibited PAH uptake with an apparent Ki of about 13 to 15 microM whether or not the tubules were preloaded with alpha-KG. High probenecid concentrations cis-inhibited PAH uptake by > 98%, indicating that essentially all movement of PAH across the basolateral membrane is carrier mediated. Zwitterionic nephrotoxic cysteine conjugate, S-(1,2-dichlorovinyl)-L-cysteine (DCVC), and nontoxic cysteine conjugate, S-(2-benzothiazole)-L-cysteine (BTC) cis-inhibited PAH uptake (apparent Ki: approximately 86 microM for DCVC; approximately 37 microM for BTC) at least as effectively as their negatively charged N-acetyl derivatives (NAC-DCVC and NAC-BTC) (apparent Ki: approximately 310 microM for NAC-DCVC; approximately 35 microM for NAC-BTC). The inhibition by both DCVC and NAC-DCVC was competitive in nature. NAC-DCVC also cis-inhibited net transepithelial secretion of PAH by isolated, perfused S2 segments. The 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 indicate that these cysteine conjugates and their N-acetyl derivatives, not only interact competitively with the PAH transporter, but are transported by it.