Transport of 3-methoxy-4-hydroxymandelic acid by isolated rat choroid plexus.

3-Methoxy-4-hydroxymandelic acid (VMA) is transported into the isolated choroid plexus against a concentration gradient by a saturable, energy-dependent system. The apparentK m for transport is 35 nM and theV max is 1 pmol/mg tissue/hr. Concentrations of probenecid (0.1 mM) that block the transport of other acidic biogenic amine metabolites did not block the transport of VMA. The transport system of the choroid plexus probably plays a role in clearing VMA from the CSF.