ACTIVE-TRANSPORT OF ORGANIC-ACIDS (FLUORESCEIN) IN THE CHOROID-PLEXUS OF THE RABBIT

The kinetics of active transport of an organic acid (fluorescein) through the membranes of the choroid plexus from the lateral ventricules of the brain of rabbit was studied morphologically and functionally. Fluorescein is actively translocated through the apical and basal membrane of the epithelium and is accumulated in blood capillaries at a concentration exceeding 1 order of magnitude that in the incubation medium. The kinetic curves displaying saturation and the demonstration of inhibition by other acids shows that a specific carrier is involved in the transfer across the membrane. The active transport of fluorescein at 20.degree. C was Na independent. Total exclusion of Na from the incubation medium does not change Km and Vmax. The active transport depends on the operation of (Na+ + K+)-ATPase as energy source but obviously no specific complexes with the participation of Na are involved.