Transport and metabolism of octanoate by the perfused rat kidney

These studies were done to determine the capacity of the perfused rat kidney to metabolize and transport the medium-chain fatty acid, octanoate (C8). Use of C8, which is water soluble, facilitated the study of transport, since protein, normally needed to transport long-chain fatty acids in solution, could be omitted from the perfusate. Using a filtering kidney it was found that total metabolism and incorporation of [1-14C]octanoate occurred at a rate of 3.9 +/- 0.3 micromol-g wet wt-1-20 min-1 when the perfusate octanoate was 2.8 mM, and reabsorptive uptake occurred at the rate of 7.5 +/- 1.3 micromol-g wet wt-1-20 min-1 at the same octanoate concentration. Use of a nonfiltering kidney (10% dextran perfusate) allowed quantitation of peritubular octanoate uptake. This peritubular uptake showed saturation above 0.7 mM perfusate octanoate with an apparent transport maximum (Tmax) at 2.1 micromol-g wet wt-1-20 min-1. Many previous experiments have linked renal fatty acid transport with that of the organic anion transport system. The apparent peritubular Tmax observed for octanoate suggests carrier-mediated transport. However, this transport did not appear to be inhibited by other organic acids such as probenecid (1--2 mM) and p-aminohippurate (1--2 mM).