Ion and Metabolite Transport Through the Intestinal Membranes of T. bernacchii
243
sugar transport and of the ouabain-sensitive basolaterally located Na + fK+ATPase. Figure 4 shows that while Na + -dependent DC 4 C]-glucose uptake
of eel intestine increases exponentially with temperature in the full range
tested (3 - 28 0q, the sugar transport activity of the Antarctic fish reached
its maximum at -2/0 °e and slowly decreased at higher temperatures. On
the other hand, Fig. 5 shows that the activity of Na+fK+-ATPase of both
species' enterocytes exponentially increased with temperature over the
range of experimental temperature used.
Discussion and Conclusions
The success of notothenioids in surviving in the Antarctic Sea seems to
involve, among other things, adaptive modifications of intrinsic proteins
involved with the transport of water and electrolytes through the plasma
membranes. Although many researchers have concentrated their efforts on
the analysis of various enzymes involved in different cellular functions,
reaching many interesting conclusions regarding the molecular adaptive
features of these enzymes at low temperatures, very few data exist on
membrane transport proteins, which have the peculiarity of needing to
function in a lipid microenvironment with a specific physical state of
fluidity. Since 1983 our group has studied the characteristics of various
transport systems in different epithelia of teleosts. One of the first epithelia
used for these studies was the intestine, because a variety of different
transport systems for ions, nutrients and other substrates are localized in
this tissue. By using brush border and basolateral membranes isolated as
vesicles, in association with radioactive and fluorescent tracers, we have
demonstrated in the intestine of carnivorous and herbivorous teleost
species the existence of transport systems for ions and metabolites with
characteristics similar to those previously described in higher vertebrates
[6-10]. Recently we have applied similar experimental procedures in the
study of intestinal transport systems of Trematomus bernacchii. The main
conclusions deriving from these studies are summarized in Fig. 6. This
Figure suggests that on the apical membranes of T bernacchii enterocytes
are present: 1) the N a + -dependent, electrogenic transport mechanisms for
sugars and amino acids [3]; 2) a H+/dipeptide cotransport (Rollo et al.
unpublished data) with characteristics similar to those previously reported
in A. anguilla [10] and 3) an Na+/H+ exchanger (Acierno et al. unpublished
data) that does not exist on eel intestinal BBMV [11]. This exchanger,
however, has been found in other fish species, confirming that an
exchanger commonly present on apical membranes of mammalian intestine
[12] is also expressed in teleosts. The Na+fK+-ATPase enzymatic activity
(mU/mg protein) of the Antarctic fish intestine is comparable, at -1 °e, to
that measured in eel, at 18 °e, suggesting that one of the main homeostatic
243
sugar transport and of the ouabain-sensitive basolaterally located Na + fK+ATPase. Figure 4 shows that while Na + -dependent DC 4 C]-glucose uptake
of eel intestine increases exponentially with temperature in the full range
tested (3 - 28 0q, the sugar transport activity of the Antarctic fish reached
its maximum at -2/0 °e and slowly decreased at higher temperatures. On
the other hand, Fig. 5 shows that the activity of Na+fK+-ATPase of both
species' enterocytes exponentially increased with temperature over the
range of experimental temperature used.
Discussion and Conclusions
The success of notothenioids in surviving in the Antarctic Sea seems to
involve, among other things, adaptive modifications of intrinsic proteins
involved with the transport of water and electrolytes through the plasma
membranes. Although many researchers have concentrated their efforts on
the analysis of various enzymes involved in different cellular functions,
reaching many interesting conclusions regarding the molecular adaptive
features of these enzymes at low temperatures, very few data exist on
membrane transport proteins, which have the peculiarity of needing to
function in a lipid microenvironment with a specific physical state of
fluidity. Since 1983 our group has studied the characteristics of various
transport systems in different epithelia of teleosts. One of the first epithelia
used for these studies was the intestine, because a variety of different
transport systems for ions, nutrients and other substrates are localized in
this tissue. By using brush border and basolateral membranes isolated as
vesicles, in association with radioactive and fluorescent tracers, we have
demonstrated in the intestine of carnivorous and herbivorous teleost
species the existence of transport systems for ions and metabolites with
characteristics similar to those previously described in higher vertebrates
[6-10]. Recently we have applied similar experimental procedures in the
study of intestinal transport systems of Trematomus bernacchii. The main
conclusions deriving from these studies are summarized in Fig. 6. This
Figure suggests that on the apical membranes of T bernacchii enterocytes
are present: 1) the N a + -dependent, electrogenic transport mechanisms for
sugars and amino acids [3]; 2) a H+/dipeptide cotransport (Rollo et al.
unpublished data) with characteristics similar to those previously reported
in A. anguilla [10] and 3) an Na+/H+ exchanger (Acierno et al. unpublished
data) that does not exist on eel intestinal BBMV [11]. This exchanger,
however, has been found in other fish species, confirming that an
exchanger commonly present on apical membranes of mammalian intestine
[12] is also expressed in teleosts. The Na+fK+-ATPase enzymatic activity
(mU/mg protein) of the Antarctic fish intestine is comparable, at -1 °e, to
that measured in eel, at 18 °e, suggesting that one of the main homeostatic