Uptake of leucine and alanine by cultured cerebral capillary endothelial cells.

Pure cultures of rat cerebral capillary endothelium have been used to study the A- and L-systems of amino acid transport. Leucine is taken up by a non-concentrative mechanism that can be saturated, and competitively inhibited by phenylalanine. Uptake is rapid, with equilibration apparent after 3-5 min (all experiments performed at 37 degrees C). The Km for transport was 83 microM +/- 26 (mean +/- S.E.M., n = 3) which is in good agreement with recent in vivo reports using unanaesthetised rats. Alanine was transported by a saturable, concentrative mechanism. Dependence on Na+-ions was demonstrated by lack of specific uptake in Na+-free buffer and reduced uptake after preincubation in ouabain--a Na+,K+-ATPase inhibitor. The Km for transport was 325 microM +/- 88 (mean +/- S.E.M., n = 3). The finding of an active A-system transporter in vitro suggests that the cells may have lost the polarity they demonstrate in vivo. The relevance of these findings to transport of nutrients and drugs across the blood-brain barrier is discussed.