Kinetics and Na independence of amino acid uptake by blood side of perfused sheep choroid plexus.

The kinetic constants and sodium independence of amino acid uptake by the basolateral face (blood side) of the isolated perfused choroid plexus of the sheep were investigated. Uptake of 3H-labeled L-alanine, glycine, L-glutamine, L-leucine, L-glutamate, and L-lysine was not significantly inhibited when the sodium level of the perfusate was lowered to less than 6 mM. The cerebrospinal fluid (CSF) secretion rate was, however, reduced by 65.6%. The Michaelis constants for L-serine, glycine, L-phenylalanine, L-glutamate, and L-arginine were measured and varied between 25.4 microM for L-arginine and 2.6 microM for L-glutamate. The Vmax for the amino acids varied by about the same range as Km, with L-serine having the highest Vmax of 28.8 nmol.min-1.g-1, and L-phenylalanine having the lowest at 4.6 nmol.min-1.g-1. At normal plasma concentrations of 30-155 microM for individual amino acids, the carriers for glycine, L-phenylalanine, and L-glutamate would be almost fully saturated (greater than 90%), while those for L-serine and L-arginine would be greater than 60% saturated. This restricted uptake of amino acids by the basolateral face of the choroid plexus, coupled to clearance of these substances out of the CSF (5), could account for the low levels in the CSF compared with that in the plasma.