growing pigs compared with neonatal pigs
in which the amino acid is the preferred
energy substrate compared with ATP and
glucose. In ruminant animals, portal flux of
glutamine is also negative, but the high
concentration of the amino acid in herbage
means that duodenal glutamine supply is
much higher than in non-ruminant species.
There is, however, considerable variability
in portal glutamine flux across species and
under different dietary regimes (Table 3.2).
These data emphasize the sensitivity of the
gut tissues not only to dietary amino acid
supply but also to other luminal and arter-
ial nutrients. In addition, different
responses between ruminant species fed
the same diets (see, for example, Prior et
al., 1981) question the application of mean
data in the development of mechanistic
models of nutrient metabolism.
Net absorption of amino acids is
strongly correlated with increasing protein
intake in non-ruminants (Rérat et al., 1988)
although the absorption coefficients of
individual amino acids fall with increasing
protein supply. These responses are
different for non-essential and essential
amino acids; increasing duodenal supply52 C.J. Seal and D.S. Parker
Table 3.2. Effect of diet on net glutamine flux across portal-drained viscera of various species.
Species Net fluxa Diet Reference
Sheep +ve Cereal/straw Balcells et al.(1995)
ve Cereal/straw + i.v. glucose
Sheep ve Forage Piccioli Capelli et al.(1997)
+ve Forage + i.v. glucose
++ve Forage + intraduodenal glucose
Sheep ve High protein forage Tagari and Bergman (1978)
ve Medium protein cereal by-products
Sheep +ve Forage (hay based) Prior et al.(1981)
++ve Concentrate
Steers +ve Forage (hay based)
ve Concentrate
Steers +ve Forage (lucerne) Taniguchi et al.(1995)
ve Forage + abomasal starch, intraruminal casein
ve Forage + intraruminal starch and casein
Steers ve Concentrate (maize based) Guerino et al.(1991)
+ve Concentrate + postruminal casein (300 g day^1 )
Steers +ve Forage Seal et al.(1992)
ve Concentrate
Steers ve Forage Seal and Parker (1996)
+ve Forage + intraruminal propionate
Steers ve Concentrate fed and starved Koeln et al.(1993)
Dairy cows +ve Silage/concentrate 4 weeks post-partum Reynolds et al.(1988)
ve 8 weeks post-partum
Heifers ve High concentrate Harmon et al.(1988)
ve High concentrate + ionophores
Pigs ve Starch-based diets van der Meulen et al.(1997)
Pigs ve Pre-weaning Wu et al.(1994)
ve Maize-based diet
aNegative values indicate net extraction and positive values net release of glutamine by portal-drained
visceral tissues.