534 H. N. CHRISTENSEN et al. 
In Fig. 4 we have arranged results obtained by FINCH AND Hirp!4 on the uptake 
(not the absorption) of amino acids by isolated intestinal wall. You will note that 
the rates of uptake at 10 mM levels tend to be reversed from those seen at 1 mM. 
At high levels a high affinity solute may leave so few sites available that only rarely 
14 
EM eT VAL 
ISOL 
12 
' ALA 
ae GLUNH> 
10F-- 
= 
is ASPNH» 
io} 8! PRO SER 
g HIS 
o 
pe 
26 
He} 
Qa 
=) 
4 
GLY 
2 ————E SS eres eee 
20 40 60 
Uptake rate at 10 M 
Fig. 4. Rates of uptake of amino acid by isolated small intestine of the rat at 1 mM plotted 
against the rates at 1o-m/V/ levels. Data of FINCH AND HirpD". 
can a new molecule of solute combine with the transport site!®. This plot also brings 
out the distinct position occupied by glycine. a-Aminoisobutyrate turns out to be a 
somewhat specific model for glycine; on a similar plot showing transport rates, it 
occupies a position even nearer the origin than glycine. Here, as for the ascites cell, 
glycine and a-aminoisobutyrate show distinctive behavior, including low affinity 
and access to a transport site that can scarcely be saturated. This behavior is not 
a consequence of the optical symmetry of these two amino acids, since cycloleucine 
4 

g 
Be 
19) 
3 
E 
oO 
SS 
els 
fe) 
i 
o 
1) 
oO 
O 
oO 20 40 60 80 
Minutes 
Fig. 5. Concentrative transfer of cycloleucine across the everted, isolated jejunum of the rat. 
The technique of CRANE AND WILSON was used!2. The initial cycloleucine level on the mucosal 
side was 2 mV; on the serosal side it was 2, 4, or 8 mM as indicated above by the initial distri- 
bution ratio. Reproduced with permission from AKEDO AND CHRISTENSEN”. 
References p. 538 
