550 A. LAJTHA 
Exchange of an amino acid between plasma and brain may occur through any 
number of possible mechanisms and not necessarily through a single reversible 
process. Experiments such as those shown in Tables I and II may measure the final 
result or the average of a number of processes. Amino acid transport occurs not only 
at the cell membrane but also at other membranes, such as the nuclear membranes2°, 
In the brain the complexity of the problem is further increased by the anatomical 
and histological complexity of the organ itself. Thus an investigation of cerebral 
transport processes may have to consider a bewildering number of membranes—organ 
membranes, membranes of the various cell types, and intracellular membranes—in 
addition to other factors peculiar to the brain, such as cell processes, myelin sheaths, 
Plasma A 
800 -----5---------7=----- 
fe) 
600) ° 
400) 
2 ie} .. ie} 
J 5 PlasmaB o 
fea) - (eo) 
=| 

Brain B 


60 120 
minutes 
Fig. 1. Leucine transport from brain against a concentration gradient; experiment A against 
higher, experiment B against lower plasma levels. In both experiments, 2 wmoles of leucine were 
administered subarachnoidally to young rats in 0.02 ml 0.9% saline. Plasma levels were kept 
high by intraperitoneal injections. 
and fluid spaces. It is likely, however, that the transport of any one amino acid in the 
brain will be governed or limited by one or only a few transport mechanisms and 
that valid comparisons and conclusions can be drawn from investigations concerning 
even such a complex organ. 
Findings have been made in other systems comparable to the above findings of 
increased exchange caused by increased cerebral levels of an amino acid. Apparent 
uphill transport induced by counterflow has been shown in several systems (see?!) ; 
in particular, amino acid transport in Ehrlich ascites tumor-cell has been studied 
in detail. Preloading the ascites cells with glycine22, 23 increased glycine influx from 
the medium, the influx coefficient rising parallel with the internal concentration and 
the efflux coefficient remaining unchanged. It is possible that the model of exchange 
diffusion of amino acids (and it was suggested that exchange diffusion and active 
transport refer to the same mechanism) proposed for ascites cells?’ is applicable for 
amino acid exchange in the brain as well. I hope Dr. HEtnz will discuss his findings 
on exchange diffusion at greater detail in this volume. Similar to the finding of rapid 
exchange in spite of a barrier to net uptake, the effect of increased brain levels on the 
rate of exchange cannot be satisfactorily explained by passive diffusion. 
References p. 563 
