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Abstract
l-Selenomethionine (SeMet) is a dominant form of selenium (Se) found in organisms
at all levels of aquatic food chains and a key source of Se bioaccumulation and ecotoxicity.
In mammals, intestinal absorption of SeMet is at least partly via the Na(+)-dependent
neutral amino acid transporter. The mechanism of SeMet absorption and competitive
effects of other dietary components on SeMet absorption in fish are unknown. Thus
the in vitro uptake rates of l-methionine (Met) and the competitive effect of SeMet
on Met absorption, an indicator that SeMet uses the same nutrient transporter(s) for
absorption, in the various regions of the green sturgeon (Acipenser medirostris) intestine
were investigated using intact tissues (a modified everted sleeve method). Intestinal
tissue was incubated in Ringer's solution containing 0-10mmolL(-1) Met or SeMet (n=5
for each substrate's concentration and intestinal region), respectively, as well as
constant tracer levels of isotope-labeled Met. The data indicate that SeMet uptake
was mediated by the same transporter(s) as Met and that the absorption kinetics were
similar for both substrates. When there were differences in absorption they appeared
to be mostly due to higher permeability (passive uptake) of the tissue for Met than
for SeMet, particularly in the pyloric caeca (PC) and distal intestine (DI). Maximum
rates of absorption, on the other hand, tended to be higher for SeMet than Met in
the mid intestine and DI, whereas differences in affinity for the transporters varied
between these tissues but were very similar in the PC. These differences may be due
to differences in regional intestinal characteristics such as amount of mucus secreted
and degree of tissue contraction, and/or substrate differences regarding solubility
in and movement through the mucus, influence on tissue contraction, permeability through
membranes or between cells, intracellular metabolism, as well as basolateral transport.
Interestingly, an increasing proximal-to-distal gradient for rates of methionine and
putative SeMet absorption was observed in green sturgeon which differs from the declining
gradient usually observed for substrate absorption in other fish species and mammals.
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