Farm Animal Metabolism and Nutrition

with the strong relationship between
ammonia N flux and N intake (Seal and
Reynolds, 1993). Metabolizable energy
intake appears to be a better predictor of
portal amino acid flux than protein intake
(Reynolds et al., 1994) since this is most
closely linked to ruminally digestible
organic matter, the principal determinant
of microbial protein synthesis (Hoover and
Stokes, 1991). Amino acid flux is also
affected by the supply of glucose to the
small intestine. In studies with pigs fed
diets containing starches with differing
rates of digestion, van der Meulen et al.
(1997) showed that portal flux of amino
acids was reduced when pea starch was fed
compared with more rapidly digested
maize starch. We have also shown that
increasing glucose availability to the
gastrointestinal tissues of sheep and steers,
either by increasing ruminal propionate
availability (Seal and Parker, 1996) or by
infusion of glucose or starch into the small
intestine (Seal et al., 1994; Piccioli-
Cappelli et al., 1997), results in increased
portal amino acid flux. The mechanism by
which these responses are mediated is
unclear but may be due to reduced cata-
bolism of amino acids within gastro-
intestinal tissues, changes in transit of
digesta in the gut lumen or, in the case of
poorly digested starches in the small
intestine, sequestration of protein within
the digesta matrix.

The absorption of low-molecular

weight peptides

There is increasing evidence that amino
acids may be absorbed across intestinal
tissues both as ‘free’ amino acids and
‘bound’ as low-molecular weight peptides
(see Chapter 1), and studies in chronically
catheterized steers have shown that the
pattern of amino acids in the peptide
fraction changes across both mesenteric-
and portal-drained tissues (Seal and Parker,
1996). Experiments with sheep, in which
labelled dipeptides have been infused into
the rumen at rates calculated to maintain
peptide concentrations seen post-feeding,

show that these dipeptides were absorbed

intact into both the mesenteric and portal

vein (Mesgaran, 1996). However, the con-

centrations recovered in blood were very

small and were achieved using high

infusion rates in order to elevate intra-

ruminal peptide concentrations to levels

which would not normally be maintained

under physiological conditions. In contrast

to these studies, Neutze et al. (1996) in

lambs, and Dawson and Holdsworth (1962)

in rats, were unable to show absorption of

labelled peptides across the gut wall,

suggesting that peptide material must be

hydrolysed either at the cell wall, or within

the cytosol before being transported across

the basolateral membrane as free amino

acids. Peptide transporters have been

identified in tissues from different sites

along the gastrointestinal tract in many

non-ruminant and ruminant species

(Walker and Hirst, 1997). These trans-

porters show a wide substrate specificity,

and the contribution of peptide transport to

total amino acid flux across the gut wall

remains an area of debate; at the time of

writing, it is still generally considered that

the great majority of amino acids are

absorbed from the small intestine as free

amino acids.

Trans-hepatic Flux of Amino Acids

Only a small proportion of the total amino

acids absorbed from the small intestine

into the hepatic portal vein reach the

peripheral circulation in free form because

substantial and variable amounts are

removed by the liver (Lobley et al., 1995,

1996). The proportion of individual amino

acids which is removed is also species

dependent (Wray-Cahen et al., 1997). The

balance of amino acids leaving the liver is

important in influencing the subsequent

availability of amino acids for extrahepatic

use. For example, hepatic extraction of

histidine (Wolff et al., 1972) may leave

insufficient amounts of this amino acid for

milk synthesis. Data for hepatic extractions

of individual amino acids in large animals

are shown in Table 3.3. Such data are

54 C.J. Seal and D.S. Parker