Sodium and pH Dependent Carrier-Mediated Transport of Antibiotic, Fosfomycin, in the Rat Intestinal Brush-Border Membrane

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 K_t = 15.3 mM and J_max = 7.78 nmol/30 s/mg protein at 37 °C. An overshoot uptake of FOM was observed in the presence of an inwardly directed Na⁺ gradient. The replacement of extra vesicular 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 (pH_in = 7.5, pH_out = 6.0), whereas in the absence of a H⁺ gradient no significant difference was observed between the uptakes at an acidic pH (pH_in = pH_out = 6.0) and a neutral pH (pH_in = pH_out = 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.