INTERACTION OF ALPHA-KG WITH BASOLATERAL ORGANIC ANION TRANSPORTER IN ISOLATED RABBIT RENAL S3 PROXIMAL TUBULES

To understand the basolateral p-aminohippurate (PAH) transporter in the S3 segment of rabbit proximal tubules and its relationship to the transporter in the S2 segment, we measured the 30-s uptake and efffux of PAH across the basolateral membrane of single isolated S3 segments at 37 degrees C in bicarbonate-buffered media. Kinetic analysis of uptake data revealed a concentration of PAH at one-half J(max) of similar to 107 mu M (same as in the S2 segment) but a J(max) of 600 fmol . min(-1). nl(-1) (one-tenth that of S2 segment). The coefficient for efflux across the basolateral membrane was also only one-sixth to one-tenth of that in the S2 segment. These data suggest that the basolateral PAH transporter is the same in both segments but that there are fewer transporters in the S3 than in the S2 segment. However, the apparent inhibitor constant values for cis-inhibition by probenecid (similar to 29 mu M in S3, similar to 15 mu M in S2) and by alpha-ketoglutarate (alpha-KG) in the presence of LiCl (similar to 40 mu M in S3, similar to 160 mu M in S2) suggest that the transporters may not be identical in the two segments. In bicarbonate-buffered medium, preloading the tubules with 100 mu M alpha-KG did not trans-stimulate PAH uptake across the basolateral membrane, whereas preloading with 1.0 mu M alpha-KG caused a significant stimulation of 43%. However, in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered medium, preloading the tubules with 100 mu M alpha-KG caused a twofold increase in PAH uptake. These data suggest that the initial intracellular concentration of alpha-KG (or metabolic state of the tubule) may influence PAH uptake and the trans-stimulatory effect of alpha-KG.