Ion and Metabolite Transport Through the Intestinal Membranes of T. bernacchii
241
overshoot phenomenon, always correlated to the activity of a Na + -coupledD-glucose transport mechanism, was completely inhibited by phloridzin, a
Na+-D-glucose cotransport inhibitor, or by substitution of NaCI with
choline Cl. This result confirms the presence of a Na+-D-glucose
cotransport mechanism on the apical membranes of the Antarctic fish
enterocyte.
250
~ 200
e c..
0)150
E
•
~ 100
(5
E
c.. 50
2
4
- - NaO 99rrM + D-Glucose 0.1 rrM
_____ Choline ChlOride 99mM + D-Glucose 0 1 rrM
- - NaO 99mM + D-Glucose O.1mM+ Phloridzin O.01rrM
6
time (min)
8
t------,100
Fig. 3. Time-course of D-e 4 C]-glucose uptake in T bernacchii intestinal BBMV. Vesicles
were resuspended in the same solution as Fig. 1. A valinomycin concentration equal to
0.005 mM was present in all the external media
Table 1. Main characteristics of Na+-D-glucose cotransport in fish and other vertebrates
T bernacchii
A. anguilla
Other
vertebrates
N a-dependence
+
+
+
Electrogenicity
+
+
+
Substrate
p-Me-glucose ~ a-Mesame
specificity
glucose ~ D-glucose ~
D-galactose >3-0-Meglucose ~ D-fructose >
2-deoxy-D-glucose
Km (mM)
0.16±0.Q3
0.10 ± 0.02
0.03 - 4.0
Jmax (nmol/mg
0.23 ± 0.009
6.87 ± 0.89
0.65 - 11.0
protein/min)
Na+-D-glucose
?
2: I
2:1; 1:1
stoichiometric ratio
Temperature
-2/0°C
> 12°C
optimum