across tissue beds (Seal and Parker, 1996).
There is increasing evidence that dipep-
tides may be metabolized (hydrolysed) at
different rates in different tissues (Druml et
al., 1997), which raises the intriguing
possibility of targeting substrates to
specific sites in the body. Recent research
on the expression of peptide transporters
suggests that these may be inducible under
different dietary regimes (Walker and Hirst,
1997), again suggesting that these sub-
strates may be important under different
physiological situations. However, the
contribution of these processes to overall
-amino N economy remains unresolved
and will be a key area for future research.
Absorption of Amino Acids Across
the Gastrointestinal Tract
The development of catheterization proce-
dures for determining the composition of
blood supplying and draining the gastro-
intestinal tissues, together with blood flow
measurements and the use of isotopically
labelled substrates, has substantially
increased our understanding of the flux of
amino acids across gut tissues (Seal and
Reynolds, 1993). Where experiments have
coupled these procedures with measure-
ments of amino acid disappearance from
the gut lumen, it is clear that fewer amino
acids arrive in the portal vein than
apparently disappear from digesta (Tagari
and Bergman, 1978; MacRae and Reeds,
1980; Neutze et al., 1990; MacRae et al.,
1996). Experiments with ruminant and
non-ruminant animals have shown that
amino acid flux across the portal-drained
viscera is responsive to changes in diet and
the supply of energy-yielding substrates
within the gastrointestinal tissues (Seal et
al., 1992; Balcells et al., 1995; Seal and
Parker, 1996; van der Meulen et al., 1997).
These studies confirm that the net flux of
amino acids is sensitive to many factors
including metabolism of sequestered
amino acids from the arterial supply. This
latter process has been determined in
sheep using a combination or arterio-
venous difference procedures and isotopic
kinetic measurements (MacRae et al.,
1996). In this study, intravenous and intra-
luminal infusions of^13 C-labelled amino
acids were used to apportion unidirec-
tional fluxes from the arterial blood pool
and from the gut lumen. The results of this
experiment suggest that, whilst some
essential amino acids are sequestered from
the lumen, a much greater proportion
comes from circulating blood supply to the
tissue. Extraction of amino acids in this
way reflects the high rates of protein
turnover (relative to tissue mass) observed
for gastrointestinal tissues (Lobley et al.,
1980). Stoll et al. (1998) also used^13 C-
labelled amino acids from algal protein and
showed in young pigs that first-pass
splanchnic metabolism of enteral amino
acids reflects intestinal rather than hepatic
metabolism. In addition, they demon-
strated that although this accounted for
35% of dietary protein intake, the portal-
drained viscera continued to utilize arterial
amino acids, and amino acid catabolism
accounted for 80% of CO 2 production by
the portal-drained viscera, predominantly
through oxidation of glutamate and
aspartate.
Net extraction of amino acids (i.e. a
lower concentration of amino acids in
portal blood compared with arterial blood
supplying the tissues) across the gastro-
intestinal tissues is often observed, indicat-
ing that gastrointestinal tissue requirements
for some amino acids cannot be met from
luminal sources and must be derived from
the arterial supply, especially in the fasted
state. Glutamine, for example, is utilized
readily by gut tissues for energy production
(Windmeuller and Spaeth, 1980) and in the
supply of amide nitrogen for purine and
pyrimidine synthesis (Duée et al., 1995).
This intestinal utilization of glutamine is
extensive. For example, in lambs, incor-
poration of [5–^15 N]glutamine into intestinal
proteins accounted for 73% of the gross
flux of the amino acid across the small
intestine (Gate et al., 1997). In the pig,
portal-drained visceral flux of glutamine is
negative after feeding a wide range of diets
(see van der Meulen and Jansman, 1997).
The use of glutamine in enterocytes falls inInter-organ Amino Acid Flux 51