Renal palmitate transport: possible sites for interaction with a plasma membrane fatty acid-binding protein

In previous studies from this laboratory, a mediated transport system for long chain fatty acids was observed in rat renal basolateral membrane vesicles. Transport was measured in the absence of albumin and indicated the presence of a Na+ independent anion exchange mechanism. The present experiments were done to characterize renal transport of fatty acids derived from fatty acid-albumin complexes. 3H-palmitate uptake by brush border (BBMV) and basolateral membrane vesicles (BLMV) isolated from rat renal cortex was determined using a rapid filtration technique. All incubation media contained 100 microM 3H-palmitate complexed to 100 microM bovine serum albumin. Up to 65% of initially bound fatty acid-albumin complexes were displaceable by washing with solution containing 0.1% albumin. Total palmitate uptake was measured as the remaining non-displaceable radioactivity. In BBMV in low ionic strength (300 mM mannitol) or ionic buffers (100 mM mannitol + 100 mM NaCl or KCl), total palmitate uptake at 15 sec did not differ from equilibrium (60 min) values of 10-11 nmoles/mg protein. Uptake was primarily due to binding. A similar pattern was seen with BLMV in 300 mM mannitol buffer. In BLMV in 100 mM NaCl or KCl buffers, equilibrium uptake was 10-fold lower than at 15 sec. This suggests binding followed by cation-dependent translocation. If a putative FABPPM is involved in transport only, its presence should be confined to BLMV.