An active transport system in the blood-brain barrier may reduce levodopa availability.

Levodopa, the primary drug used to treat patients with Parkinson's disease, is transported into the brain by the facilitative amino acid transporter (L1). We present here an unanticipated discovery: levodopa may be pumped out of the brain by a Na(+)-dependent transport system that couples the naturally occurring Na(+) gradient existing between the brain's extracellular fluid and the cytoplasm of capillary endothelial cells. The activity of this system reduces the net availability of levodopa.