for these transport proteins are identified,
the study of how the feeding of elevated
amounts of lysine in the diet ultimately
caused an increase in transport system
activity can be determined.
In terms of other transport systems in
the chicken gastrointestinal tract, a high-
affinity, Na+-dependent anionic amino acid
transport has been described, presumably
corresponding to system XAG-like activity
(Wingrove and Kimmach, 1998). Also, the
expression of H+/peptide transporter
activity has been reported in the BBM of
the small intestine, caecum and rectum in
chicks (Calonge et al., 1990). Accordingly,
with the noted exceptions for systems y+
and L (above), it appears that the chicken
intestinal tract expresses a complement of
amino acid and peptide transporter
activities that is similar to that of mam-
mals. However, the molecular characteriza-
tion of intestinal transporters in the
chicken may increase our ability to
manipulate the capacity for methionine
and lysine transport, and, thereby, may
optimize our ability to balance the supple-
mentation of these limiting amino acids.
PigsIn the pig, as with other monogastrics, the
jejunum has the greatest capacity to absorb
amino acids (Leibholz, 1998). As reviewed
by Munck (1997), the BBM of the pig small
intestine is known to express systems
IMINO, B and probably xc, but, unlike
other monogastrics, not Bo,+. Consistent
with this conclusion, measurements of
amino acid fluxes across the small intestine
suggest that transport systems capable of
cationic amino acid transport in pig entero-
cytes are Na+-independent in nature. As
discussed previously (Matthews et al.,
1996c), the use of distal duodenal and
proximal ileal cannulas in growing pigs
resulted in the quantification of mutually
inhibitable absorption of lysine and
arginine. Additionally, the flux of lysine
across isolated neonatal pig jejunal tissue
sheets has been described as having
saturable (Km= 200 μM) and non-saturable
components. Because uptake was not
measured in the absence of Na+, mediated
lysine uptake could have occurred by
either systems Bo,+, y+and/or bo,+. Lysine
absorption in the presence of Na+by villi-
tip enterocytes in the jejunal tissue of pigs
(at approximately <1–28 days of age) has
been shown to be non-concentrative,
whereas alanine uptake in the presence of
Na+was. Accordingly, lysine uptake could
have occurred by systems y+and/or bo,+, but
not by Bo,+. Likewise, the non-concentrative
component of alanine uptake was probably
by system L and/or bo,+. Consistent with
the hypothesis that system bo,+is expressed
by jejunal tissue, mRNA isolated from
the jejunum of pigs encoded system
bo,+ activity when expressed in oocytes
(Matthews et al., 1996c). As with other
monogastrics, the extracellular affinity for
lysine was greater than that for leucine.
Because system Bo,+ apparently is not
expressed in the intestinal BBM of pigs,
the observed Na+-dependent concentrative
alanine uptake may have been by systems
A or ASC. Interestingly, however, neither
of these transport systems have been
shown to be expressed in intestinal BBM of
other mammals (Table 1.1).
Clearly the complement of amino acid
transporter activities expressed by the pig
gastrointestinal tract has not been well
defined. Additionally, reports of peptide
transport systems are not known to this
reviewer. Because the presence of leucine
has been shown to stimulate both the
influx and efflux of lysine across the
basolateral membranes of enterocytes,
ostensibly by the allosteric binding to
system y+transporters (Deves and Boyd,
1998), and because system bo,+transport
uptake of lysine is inhibitable by leucine
(Matthews et al., 1996c), the flux of lysine
across pig jejunal enterocytes may be
dependent on the luminal and serosal
concentrations of leucine. If future research
supports this hypothesis, then the deter-
mination of an optimal leucine:lysine
dietary ratio may promote the more
efficient feeding of lysine to pigs.Amino Acid and Peptide Transport Systems 17