Sodium-dependence of p-aminohippurate transport by rat kidney cortex slices

The effect of medium Na+ concentration on p-amino-hippuric acid (PAH) uptake and efflux were investigated in rat kidney cortex slices. The uptake during 10 min incubation was taken as a measure of the initial influx. At a fixed Na+ concentration, the uptake curve (amount of uptake against substrate concentration) consisted of two distinct components; one linear with medium PAH concentration and the other obeying Michaelis-Menten kinetics. Only the latter was dependent on the Na+ concentration. A reduction of medium Na+ concentration resulted in an increase in Kt for PAH, while Vmax for PAH remained unchanged. In the absence of Na+, the component obeying Michaelis-Menten kinetics still existed, and Vmax for PAH was identical with that in the presence of Na+. The reciprocals of Kt were linearly proportional to medium Na+ concentration. Such kinetic properties could be interpreted on the basis of a kinetic model in which PAH was assumed to be transported across the cell membrane by forming both binary (carrier-PAH, SX) and ternary (NaXS) complexes. An elevation or reduction of Na+ concentration in efflux media caused an immediate decrease or increase in the rate constant for PAH efflux, respectively. Addition of a competitive inhibitor (hippurate) to efflux medium also caused an immediate increase in the rate constant. From these observations, it was concluded that PAH transport in rat kidney was mediated through both Na+-dependent and Na+-independent carrier processes at the peritubular membrane.