CEREBRAL PASSAGE OF FREE AMINO ACIDS 555 
acid levels there was a net efflux from the brain with time. The rate of exchange was 
to some extent proportional to the level of the amino acid in brain, in that increased 
cerebral levels resulted in a higher rate of exchange as measured by the appearance 
in the brain of label from the intravenously administered tracer doses (Table I)’*. 
TABLET 
THE EFFECT OF INCREASED CEREBRAL LEVELS 
ON AMINO ACID EXCHANGE 



Administered substance Counts/g 
= + ; = fresh brain|min 
Intravenously Subarachnoidally 
Lysine — 3,300 
53 Lysine 4,800 
An Leucine 4,300 
Re Saline 3,800 
Leucine — 5,500 
a Lysine 5,500 
* Leucine 7,100 
A Saline 4,100 

Intravenous injection: 0.2 umole “C-lysine or 'C-leucine 
(1.2 < ro® cts/min) in 0.02 ml. 
Subarachnoid injection: 0.02 ml of 0.9% saline or 0.9% saline 
containing 1.9 mole, 'C-lysine or 2.1 wmole 1C-leucine. 
In the case of the two amino acids studied up to the present time this effect seems 
to be fairly specific, in that each amino acid increased its own exchange rate more 
than that of the other amino acid. The dependence of exchange rate on cerebral levels 
could be shown without disturbing physiological equilibria. Lysine levels varied 
from one brain part to another under normal conditions, without the administration 
of lysine; when exchange was estimated by measuring the appearance of the label 
following intravenous tracer doses, the rate of lysine exchange was higher in the 
parts of brain in which lysine level was higher. On the other hand, leucine distribution 
and consequently leucine exchange were more homogeneous (Table IT)}®. 
TABLE II 
THE RELATIONSHIP BETWEEN DISTRIBUTION AND EXCHANGE OF CEREBRAL AMINO ACIDS 



L-Lysine p-Lysine L-Leucine 
Distribution* Exchange** Distribution* Distribution* Exchange** 
Cerebellum-pons-medulla 150 140 140 100 100 
Posterior half of cerebrum 98 99 120 98 95 
Anterior half of cerebrum 73 79 60 100 IIo 

* Distribution = swmoles/g fresh brain, whole brain = roo. 
** Exchange = counts/min/g fresh brain, whole brain = foo. 
Exchange was measured 5 min after the intravenous administration of tracer doses 
of the #C amino acid. 
References p. 563 
