Uptake of acetyl-L-carnitine in the brain

Analysis in mouse brain slices of the uptake of acetyl-L-[N-methyl-14C]carnitine with time showed it to be concentrative, and kinetic analysis gave a Km of 1.92 mM and a Vmax of 1.96 mumol/min per ml, indicating the presence of a low-affinity carrier system. The uptake was energy-requiring and sodium-dependent, being inhibited in the presence of nitrogen (absence of O2), sodium cyanide, low temperature (4 degrees C), and ouabain, and in the absence of Na+. The uptake of acetyl-L-carnitine was not strictly substrate-specific; gamma-butyrobetaine, L-carnitine, L-DABA, and GABA were potent inhibitors, hypotaurine and L-glutamate were moderate inhibitors, and glycine and beta-alanine were only weakly inhibitory. In vivo, acetyl-L-carnitine transport across the blood-brain barrier had a brain uptake index of 2.4 +/- 0.2, which was similar to that of GABA. These results indicate an affinity of acetyl-L-carnitine to the GABA transport system.