552 G. GUROFF AND S. UDENFRIEND 
The metabolic component responsible for the concentrative uptake of tyrosine by 
the slice is not evident since hexoses stimulate uptake, anaerobic conditions eliminate 
it, and inhibitors of both anaerobic and aerobic metabolism depress it. The only 
conclusion that can be reached at this time is that the aerobic metabolism of hexose 
best supports the uptake mechanism. 
Comparison between 7m vivo and in vitro data reveals that although the concen- 
tration by the brain is not radically different under the two conditions, the speed of 
uptake by the slice is much greater. The marked stereospecificity observed 7 vivo is 
not seen 77 vitro. Also, p-hydroxyphenylacetic acid, which does not enter the intact 
TABLE VIII 
EFFECT OF VARIOUS AGENTS ON UPTAKE OF L-TYROSINE 
OR p-HY DROXY PHENYLACETIC ACID BY BRAIN SLICES 
Slices incubated for 60 min in to ml of Krebs—Ringer—bicarbonate 
buffer containing L-tyrosine (10-3 /) or p-hydroxyphenylacetic 
acid (10-3 M). 

Intracellular concn. (g/ml) 




Compounds Medium concn. (ug/ml) 
L-T vrosine p-Hydroxyphenylacetic 
acid 
Control 257, 0.86 
+ Glucose (10-2 W) 4.6 0.88 
+ Citrate (10-2 M) Pit 0.97 
+ Iodoacetate (10-2 M) iy 0.84 
+ Ca? (12.7 x 10-3 M) 3.8 0.95 

brain, diffuses into the slice. The list of amino acid inhibitors is the same 7m vivo and 
im vitro, but in the intact animal some competitors eliminate all the tyrosine uptake, 
while in the slice they eliminate only a part, the concentrative part. In general, it 
seems that the concentrative component of uptake by the slice represents the uptake 
observed in vivo. 
On the basis of these findings it may be suggested that two mechanisms are in- 
volved in tyrosine uptake by the brain in vivo. The cellular mechanism is concen- 
trative, metabolically linked, and subjective to inhibition by other amino acids. The 
other mechanism, missing in the slice and possibly identified with the “blood-brain 
barrier”, limits the speed of entry of tyrosine, possesses great stereospecificity, and 
prevents the entry of such molecules as p-hydroxyphenylacetic acid. The properties 
of this second mechanism cannot yet be investigated directly but it must be con- 
sidered active in at least one sense: if a “blood-brain barrier” exists to the uptake 
of such molecules as p-hydroxyphenylacetic acid and p-tyrosine, some mechanism 
for facilitating the entry of L-tyrosine and other metabolites must exist at the same 
site. 
Preliminary studies indicate that the uptake systems for tryptophane and phenyl- 
alanine are similar to that for tyrosine. It is of interest to determine how general 
these relationships are and which amino acids share common uptake mechanisms. 
References p. 553 
