Agriculture Reference
In-Depth Information
An
in vivo
study showed that when sows were fed different levels of crude protein, arterial
insulin and prolactin concentrations were strongly correlated with mammary amino acid
AVD, thereby providing evidence of a link between insulin and prolactin concentrations
and amino acid uptake by the mammary gland. It may therefore be that amino acid
utilization by the sow mammary gland could be regulated via circulating concentrations
of insulin and prolactin binding to porcine mammary cells (Farmer
et al.
, 2008). he
mRNA abundance, however, for the prolactin receptor encoding gene in sow mammary
tissue was greater on d 17 than on d 5 of lactation, but was not correlated to the expression
of genes encoding the milk proteins α-lactalbumin and β-casein (Manjarín
et al.
, 2012a).
A recent study also showed that increasing circulating concentrations of prolactin in late
gestation led to greater piglet growth rate, and also augmented the postpartum increase in
mRNA expression of genes encoding for β-casein, α-lactalbumin and glucose transporter
1 (VanKlompenberg
et al.
, 2013). In the study by Farmer
et al.
(2008) however glucose
AVD was poorly correlated with circulating concentrations of prolactin. In that same
study, glucose AVD was also poorly correlated with circulating concentrations of insulin.
The extent of insulin dependence for glucose transport across the porcine mammary gland
is unknown. The bulk of glucose transport is likely mediated via an insulin independent
glucose transporter. As mentioned earlier, transcript for the genes encoding GLUT1, an
insulin independent glucose transporter was twice as abundant as that of GLUT4, the
insulin dependent glucose transporter. Furthermore, transport per se of neither glucose
nor amino acids appeared to be under IGF-I regulation (Farmer
et al.
, 2008). Transgenic
sows over-expressing mammary IGF-I also had similar milk composition than control
sows (Monaco
et al.
, 2005), further suggesting little role of IGF-I on nutrient uptake by
mammary tissue.
Finally, there is little information on the role of glucocorticoids in sows. In one study, the
expression of the gene encoding for the glucocorticoid receptor during the prepartum
until the post-weaning period was positively correlated with expression of α-lactalbumin
and β-casein (Manjarín
et al.
, 2012a), indicating a likely role in amino acid uptake.
14.4
Conclusions
The amount of milk produced by lactating sows is ultimately dependent on the availability
of nutrients to the mammary gland (Boyd
et al.
, 1995). This availability is in turn affected
by mammary blood flow and nutrient uptake. Estimation of mammary blood flow is quite
sophisticated and requires catheterization of blood vessels. Most reported plasma flow
values range between 1.9 and 4.9 l/min, and the one major factor involved in modulating
mammary blood flow is milk removal. Studies based on mammary AVD showed that
there is major uptake of glucose and amino acids by the sow mammary gland and, to
a lesser extent, there is also uptake of other energetic precursors such as triglycerides,
phospholipids, acetate, propionate and lactate. Nutrient uptake by the sow udder is
largely regulation by the diet and is also affected by factors such as stage of lactation and
nursing intensity. In the case of glucose and amino acids, there are transporters located on
the mammary glands that are needed for their uptake, yet the nutritional and molecular
regulations of these are not yet elucidated. It is evident that the mechanisms involved
