affinity, high-capacity peptide transport
(PepT1) in monogastrics (Dyer et al.,
1996). Further research will be required to
determine whether the dual expression of
high- and low-affinity peptide transport
systems is a unique property of ruminant
intestinal epithelia, as compared with
monogastrics.
Forestomach absorption of amino acids and
peptides
The study of transport systems in fore-
stomach tissues (rumen, reticulum and
omasum) is complicated by the structure of
their keratinized squamous epithelia.
Starting proximal to the forestomach
liquor, and ending adjacent to the
basement membrane, the four strata of
epithelial cells that make up these tissues
have been classified as the basale,
spinosum, granulosum and corneum. The
number of cell layers that comprise the
whole epithelium can range from eight to
- Given the technical difficulties in
performing kinetic characterization in such
a complex tissue, and the classic hypo-
thesis that the forestomach is responsible
for little, if any, net absorption of amino
acids, little research has been conducted to
identify the capacity for forestomach
absorption of amino acids or peptides.
Consequently, mechanisms capable of -
amino nitrogen absorption in forestomach
tissues are poorly characterized.
Given that pre-feeding levels of free
amino acid concentrations in strained
ruminal fluid of 0.12–1.5 mg dl^1 and
post-feeding concentrations of 0.72–6 mg
dl^1 have been reported in the rumen
liquor of sheep fed common diets (as sum-
marized by Matthews et al., 1996a), it is
likely that amino acid transporters would
be of the high-affinity type. As discussed
previously (Matthews and Webb, 1995),
studies on measurement of the trans-
epithelial passage of histidine (from 0.66
to 20 mM) across sheep ruminal epithelial
sheets concluded that absorption was
interpreted to mean that absorption was
not saturatable. In retrospect, however, the
fact that methionine, arginine and glycine
inhibited histidine transfer indicates that
histidine transfer may have been at least
partially mediated by transport proteins.
Additional evidence that cationic amino
acid absorption from forestomach liquor is
mediated comes from the observation that
the flux of lysine and arginine across
ruminal tissue sheets is saturable from 0.3
to 30 mM. In omasal tissue, preliminary
data from substrate competition trials in
parabiotic chamber and oocyte expression
models indicate that system bo,+is prob-
ably responsible for the absorption of
cationic amino acids from across the
omasal epithelium (Matthews et al.,
1996a). Because a lumen-to-blood Na+
gradient does not typically exist, these
competition profiles are consistent with
the overlapping substrate recognition
patterns of transporter systems y+or bo,+.
From a teleological perspective, it
seems reasonable to suggest that fore-
stomach epithelial tissues may have evolved
to express H+/peptide co-transporters to
absorb peptides from forestomach liquor.
H+ gradients of the magnitude used to
demonstrate the presence of carrier-
mediated dipeptide transport by intestinal
and renal epithelial BBM vesicles and in
cultured colon cells can develop between
forestomach liquor and cells that comprise
omasal and ruminal epithelia. Importantly,
the Na+/H+exchanger and Na+/K+ATPase
proteins, considered to be essential in re-
establishing H+ gradients in epithelial
cells, are reported to exist and function in
both ruminal and omasal epithelia. Finally,
it is reasonable to expect pre-feeding
concentrations of 1.5–5 mg dl^1 of peptide
N and post-feeding concentrations of
10–27 mg dl^1 of peptide N in ruminal
fluid of sheep and cattle (Matthews et al.,
1996a). Because omasal liquor amino acid
and peptide N concentration values are
unknown, it is not possible to predict
the potential relative driving forces for
amino acid absorption across the omasal
epithelium. However, the absorption of
water by omasal tissues would presumably
result in the concentration of rumen liquor
solutes, thereby potentially re-establishing
(or generating greater) omasal liquor-to-
blood concentration gradients and solventAmino Acid and Peptide Transport Systems 19