Rat renal cortical slices demonstrate p-aminohippurate/glutarate exchange and sodium/glutarate coupled p-aminohippurate transport.

In isolated basolateral membrane vesicles p-aminohippurate (PAH) transport may be coupled indirectly to the sodium gradient through PAH/glutarate [or alpha-ketoglutarate (alpha-KG)] exchange and Na/glutarate cotransport. In this study, rat renal cortical slices were used to examine indirect coupling of PAH transport to sodium in intact renal tissue. Like basolateral membrane vesicles, slices demonstrated avid uptake of 50 microM [14C]glutarate. Steady-state tissue/medium ratios of 30 were achieved by 90 to 120 min. Uptake was inhibited markedly by lithium and fumarate. PAH also inhibited glutarate accumulation, but through acceleration of glutarate efflux i.e., PAH/glutarate exchange, rather than direct inhibition of uptake. PAH-driven efflux of glutarate from slices was blocked by probenecid, which inhibits PAH/glutarate exchange in vesicles. Inasmuch as slices showed both Na/glutarate uptake and PAH/glutarate exchange, externally added glutarate should stimulate PAH uptake in slices. Indeed, in the presence of sodium, 50 microM external glutarate approximately doubled PAH accumulation by the slices. Stimulation by glutarate was abolished by either lithium or fumarate, or by elimination of sodium from the external buffer. The stimulatory effect was specific for glutarate or alpha-KG. Acetate, fumarate and succinate stimulated PAH uptake poorly, if at all. Neither fumarate nor lithium was able to eliminate concentrative PAH uptake completely, suggesting that a portion of PAH transport may occur without Na-dependent glutarate or alpha-KG recycling, i.e., that it may run directly off metabolic alpha-KG production or via a completely glutarate/alpha-KG independent mechanism.