Transport of cefadroxil in rat kidney brush-border membranes is mediated by two electrogenic H⁺-coupled systems

The transport characteristics of the aminocephalosporin [³H]cefadroxil have been studied in brush-border membrane vesicles (BBMV) of rat kidney cortex by a rapid filtration technique and by use of a potential sensitive fluorescent dye. Influx of [³H]cefadroxil (0.25 μM) into BBMV as a function of time displayed a pronounced overshoot phenomenon in the presence of a transmembrane pH gradient (pH(in) > pH(out)). Evidence for an electrogenic cefadroxil/H⁺-symport in the presence of an inwardly directed proton gradient is provided by the demonstration of: 1) reduced uptake in the presence of a protonophore; 2) reduced uptake under voltage clamp conditions; and 3) increased uptake in the presence of a valinomycin-induced inside negative K⁺-diffusion potential. pH-gradient dependent uptake of [³H]cefadroxil as a function of substrate concentration revealed the presence of multiple carrier systems. By kinetic analysis, a high-affinity carrier system (K(m), 8.8 ± 1.3 μM) and a low-affinity system (K(m), 2.62 ± 0.80 mM) could be resolved. The high-affinity transport system was found to be very specific for substrates (cephalosporins and di- and tripeptides) carrying an α-amino group. By use of a potential sensitive fluorescent dye 3,3'-dipropylthiadicarbocyanine iodide, the low-affinity transport system was characterized with respect to its driving force and its kinetic features. This transporter was found also to be electrogenic in nature, representing a second cefadroxil/H⁺-symport system. In summary, our studies demonstrate for the first time uphill transport of cefadroxil in kidney BBMV mediated by multiple carrier systems. Transport is rheogenic, energized by the proton motive force and shared by other aminocephalosporins as well as di- and tripeptides. These potential reabsorption pathways for aminocephalosporins in the tubular cells may counteract the proposed secretory fluxes for aminocephalosporins via the organic anion and cation exchangers present in the apical membrane.