Sodium and pH dependent carrier-mediated transport of antibiotic, fosfomycin, in the rat intestinal brush-border membrane

Abstract

The mechanism of intestinal absorption of an antimicrobial agent, fosfomycin (FOM), was investigated in rats using small intestinal brush-border membrane vesicles (BBMV). The uptake of [3H]FOM by BBMV was osmolarity- and temperature-sensitive and showed apparently saturable uptake kinetics consistent with the Michaelis-Menten equation, having Kt = 15.3 mM and Jmax = 7.78 nmol/30 s/mg protein at 37 degrees C. An overshoot uptake of FOM was observed in the presence of an inwardly direct Na+ gradient. The replacement of extravesicular Na+ with choline or mannitol significantly reduced the uptake. An addition of a protonophore, FCCP, significantly decreased the initial uptake of FOM in the absence of Na+ gradient but in the presence of a H+ gradient (pHin = 7.5, pHout = 6.0), whereas in the absence of a H+ gradient no significant difference was observed between the uptakes at an acidic pH (pHin = pHout = 6.0) and a neutral pH (pHin = pHout = 7.5). An inside negative potassium diffusion potential induced by valinomycin enhanced significantly the uptake of FOM. The uptake of FOM in the presence of both Na(+)- and H(+)-gradients was significantly inhibited by phosphate, arsenate and phosphonoformic acid (PFA), which are specific inhibitors of phosphate transport, but not by D-glucose. Based on these results, it is concluded that FOM transport in the small intestine is partially shared with the Na(+)-phosphate cotransport system and in part occurs via a H(+)-gradient dependent carrier-mediated system.