drag forces that were present across ruminal
epithelia, depending on the relative water
and substrate absorption rates.
In a parabiotic study designed to
evaluate the potential for forestomach
epithelia to absorb dipeptides, the omasum
demonstrated a markedly greater (approxi-
mately sixfold) ability to absorb carnosine
and Met–Gly than did the rumen
(Matthews and Webb, 1995). The expres-
sion of sheep omasal mRNA in oocytes
(Matthews et al., 1996b) resulted in H+-
dependent Gly-Sar uptake (Km= 0.4 mM)
that is consistent with PepT1 transport
characteristics (Table 1.2). Additional work
confirmed these findings and demonstrated
that sheep omasal peptide transport pro-
tein(s) display differential affinities for di-,
tri- and tetra-peptides (Pan et al., 1997). As
observed for PepT1 and PepT2 monogastric
transporters (Daniel et al., 1992), no rela-
tionship existed between the ability of
these peptides to be transported and their
molecular weight, hydrophobicity or net
electrical charge.
The above data indicate that the
ruminant gastrointestinal tract tissue of
sheep and cattle possesses the capacity to
absorb peptide-bound amino acids by
peptide transport systems similar to
those expressed by monogastric species.
Unresolved questions regarding peptide
absorption in ruminants include: (i) what
the relative contribution of free versus
peptide-bound amino acids is compared
with the absorption of total amino
acids across the gastrointestinal tract of
ruminants; (ii) whether the forestomach
epithelium is capable of absorbing nutri-
tionally significant quantities of peptides;
and (iii) whether specific dietary proteins
and/or peptides can be supplied in a
manner that increases total amino acid
absorption.
Conclusions
The absorption of either free and peptide
amino acids traditionally has been
studied as separate events. Most of what
is known regarding the structure, function
and distribution of amino acid and
peptide transport systems and proteins
has been derived from the study of labora-
tory animal species and man. Information
regarding the individual proteins respons-
ible for the absorption of amino acids and
peptides in farm animal species is
limited. Amino acids typically are recog-
nized by several transport systems,
whereas most peptides apparently are
transported by a single low-affinity,
high-capacity and/or a high-affinity, low-
capacity transporter. A variety of trans-
porters in cells and cellular membranes
provide a range of free and peptide-bound
amino acid transport affinities and uptake
capacities, thus promoting efficient
absorption of these substrates, regardless
of their extracellular concentration. In
cells and membranes exposed to high sub-
strate concentrations, the expression of
low-affinity, high-capacity transporters is
typical, whereas those normally exposed
to low substrate concentrations often
express high-affinity, low-capacity trans-
porters. Some cells and membranes
express both high- and low-affinity trans-
porters.
The relative contribution of each trans-
porter system/protein(s) to total amino acid
nourishment of animals varies throughout
the gastrointestinal and peripheral tissues,
depending on substrate supply, cell type,
tissue, species and/or the metabolic status
of the animal. Accordingly, the ratio of free
versus peptide-bound amino acid absorp-
tion will probably vary with production
state. In the final analysis, however, it is the
aggregate contribution of all these absorp-
tion phenomena, with their overlapping
substrate affinities, that supplies the animal
with its requisite amino acids. Therefore,
the more we understand about how cells
absorb amino acids, the greater our
potential is to develop diets and feeding
strategies that optimize protein synthesis
and retention.20 J.C. Matthews