CEREBRAL PASSAGE OF FREE AMINO ACIDS 557 
TRANSPORT OF AMINO ACIDS FROM THE BRAIN AGAINST 
A CONCENTRATION GRADIENT 
A more direct test for the existence of active transport processes in cerebral amino 
acid passage can be performed by studying transport against a concentration gradient. 
Plasma levels of an amino acid in rats can be kept elevated by constant infusion or 
repeated intraperitoneal injections, and subsequently cerebral levels can be increased 
500 e e 
a a as eed 
e e 
300 
fe) 
— 
oD 
= 
100 
Brain 
(eo) 
50 
| El 
100 200 3000 FR 
minutes 
Fig. 2. Lysine transport from brain against a concentration gradient; 2 wmoles of lysine were 
administered subarachnoidally. 
by intracerebral administration. The results of such experiments with leucine are 
shown in Fig. 11’. In the beginning of the experiment plasma and brain levels were 
elevated, plasma levels being considerably above that of brain. In time brain levels 
decreased against the concentration gradient of elevated plasma levels till a plasma 
300 
200 
@ = §=Brain 
g/g 
100 

60 120 120 240 
minutes minutes 
Fig. 3. Phenylalanine transport from brain against a concentration gradient; Fig. 3a against 
lower, Fig. 3b against higher plasma levels. In both experiments, 2 ~moles of phenylalanine were 
administered subarachnoidally. 
to brain ratio of 11-15 was reached. Similar plasma to brain ratios were reached at 
higher (experiment A) or lower (experiment B) plasma levels, showing that the 
value of this ratio is to some degree independent of the absolute leucine levels in 
the plasma. Such experiments were performed with lysine (Fig. 2); here the decrease 
References p. 563 
