Kinetic analysis of L-carnitine uptake by the choroid plexus

Choroid plexuses from the lateral (LVCP) and fourth ventricles (FVCP) of rats or rabbits were incubated in artificial cerebrospinal fluid (CSF) containing 1 microM [14C]L-carnitine with various concentrations of L-carnitine ranging from 0.01 mM to 1.0 mM. The time course of 1 microM [14C]L-carnitine uptake by the choroid plexus indicated that it increased linearly for the first 15 min. Steady-state levels were reached by 30 min with tissue concentrations more than 20 (FVCP)- to 30 (LVCP)-fold greater than the concentration in the medium. The uptake of [14C]L-carnitine was increased with increasing concentrations of the substrate in the medium and this uptake followed Michaelis-Menten kinetics. The uptake by the rat choroid plexus took place against a concentration gradient via a saturable process and kinetic analysis revealed Km of 32 microM (LVCP) and 34 microM (FVCP) and Vmax of 21 (LVCP) and 17 nmol/ml/min (FVCP), respectively. Ouabain inhibited the uptake by 51% (FVCP) and 48% (LVCP) and hypothermia (0 degrees C) produced inhibition by 97% (FVCP) and 96% (LVCP), respectively. However, the uptake of L-carnitine was not sensitive to probenecid or tyrosine, which indicates the presence of an independent carrier for L-carnitine in the choroid plexus. Similar results were obtained with the rabbit choroid plexus.