Agriculture Reference
In-Depth Information
growth factors is supported by the observations that a higher apoptotic rate was seen
in formula-fed piglets which received no milk-borne growth factors (Godlewski
et al.
,
2005), or which were deprived of colostral growth factors such as epidermal growth
factor (Clark
et al.
, 2005), insulin-like growth factor (Xu
et al.
, 1994a) or leptin (Wolinski
et al.
, 2001). Mucosal growth is also related to transient hypertrophy of enterocytes due
to endocytosis of colostral Ig. In pigs, two mechanisms which are either dependent or
independent of an intestinal receptor (neonatal Fc receptor, FcRn) that binds to the Fc
segment of IgG (Cervenak
et al.
, 2009, Stirling
et al.
, 2005), explain the early transfer of
macromolecules in their intact form from the gut lumen into the circulation. The passage
of IgG is gradually stopped by a mechanism commonly called 'gut closure' (epithelial
closure) which occurs after 48 h of postnatal life in pigs. Recent studies indicate that
FcRn may also be involved in protection mechanisms against luminal Ig degradation
and could therefore play a role of immunological sensor in the intestine throughout life
(Baker
et al.
, 2009).
15.2.3
Digestion and absorption of nutrients
The digestion and absorption of nutrients are of particular importance in neonatal
piglets whose nutrient requirements are enormous in order to support their rapid growth
and high metabolic rate during the first postnatal week. At birth, the piglet has all the
digestive equipment and the complete array of transporters needed to start extra-uterine
life, but considerable changes occur during the suckling period (Boudry
et al.
, 2010, Le
Huërou-Luron, 2002, Le Huërou-Luron
et al.
, 2010). Indeed, colostrum intake accelerates
enzymatic maturation (Jensen
et al.
, 2001).
Neonates absorb glucose and galactose after lactase digestion of the mother's milk.
Intestinal lactase activity peaks at birth and decreases thereafter, whereas maltase and
sucrase activities increase with postnatal age. The sodium-glucose linked transporter
(SGLT1) transports glucose and galactose across the apical membrane while glucose
transporter 2 (GLUT2) acts as the basolateral membrane. Intestinal SGLT1 and GLUT2
activities are high at birth and during suckling, thus contributing substantially to neonatal
glucose homeostasis (Gabler
et al.
, 2009, Yang
et al.
, 2011). Protein digestion is also
effective at birth. Peptidase activities, such as aminopeptidase N and dipeptidyl peptidase
IV, are already well developed at birth. In addition, high expression of intestinal peptide
and amino acid transport systems ensures maximum protein absorption at birth while
it declines thereafter until weaning. Nevertheless, the total capacity of the intestine to
absorb amino acids increases with age due to increases in length and mass of the intestine.
Di- and tri-peptides are absorbed at the apical membrane through the peptide transporter
1 (PepT 1), and are finally hydrolysed by peptidases in the cytoplasm of enterocytes.
Transport systems in the basolateral membrane mediate the exit of free amino acids
from the cytoplasm to the portal circulation. Quantitatively, entry of peptides by PepT
1 is the predominant mode of absorption of protein digestion products. Small peptides
resistant to hydrolysis by cytosolic peptidases do enter the blood but these constitute a
relatively minor component. During the perinatal period, PepT 1 level peaks at birth
in the small intestine and then declines until weaning (Boudry and Le Huërou-Luron,
personal communication). Presence of PepT 1 in the colon of neonatal pigs suggests a
