Agriculture Reference
In-Depth Information
found in transcript abundance of these same genes in response to dietary crude protein
levels and amino acid availability. Manjarín
et al.
(2012b) therefore suggested that the
changes in AVD of amino acids in response to dietary amino acid availability are likely
a result of competitive inhibitory processes at the blood mammary cell interface rather
than changes in gene expression. Continued characterization of amino acid transporter
systems and their respective proteins, particularly in response to dietary and physiological
challenges, will allow to better understand how milk production in sows is governed. For
instance, in the study by Manjarín
et al.
(2011), transcript abundance of CAT-1, ATB
0,+
,
and y
+
LAT2 increased between day 112 of gestation and day 17 of lactation. Expression
of these three genes was positively correlated with expression for genes encoding for
β-casein and α-lactalbumin, which are two mammary synthesized proteins. These
authors (Manjarín
et al.,
2011) proposed that CAT-1, ATB
0,+
, and y
+
LAT2 may become
molecular targets for improving sow milk production since these proteins are responsible
for transport of the typically limiting amino acid lysine in the lactating sow's diet.
14.3.6
Hormones and mammary uptake of nutrients
Numerous hormones play a major role in the regulation of milk synthetic processes
and the direct involvement of specific hormones is evidenced by the presence of their
receptors in mammary tissue. Receptors for growth hormone (Manjarín
et al.
, 2012a),
IGF-I (Lee
et al.
, 1993; Manjarín
et al.
, 2012a; Theil
et al.
, 2006), leptin (Palin
et al.
,
2004), prolactin (Manjarín
et al.
, 2012a; Palin
et al.
, 2004; Plaut
et al.
, 1989; Theil
et al.
,
2005; Trott
et al.
, 2009), insulin (Manjarín
et al.
, 2012a), glucocorticoids (Manjarín
et
al.
, 2012a), and oxytocin (Lundin-Schiller
et al.
, 1996) are present in porcine mammary
tissue. These receptors are under complex regulation, which will not be discussed because
it is beyond the scope of the present chapter.
There is a scarcity of information on the role of lactogenic hormones in the regulation
of mammary nutrient uptake during lactation in swine. Pettigrew
et al.
(1993) showed
that circulating insulin concentrations were related to protein (r=0.42), fat (r=0.39) and
lactose (r=0.36) contents in milk, thereby indicating that insulin may be involved in
uptake of plasma substrates by the mammary gland during lactation. The transcript of the
gene encoding for the insulin receptor protein is relatively abundant in porcine mammary
tissue and remains invariant between the end of gestation, throughout lactation and post-
weaning (Manjarín
et al.
, 2012a).
An
in vitro
study showed that exogenous insulin, in combination with prolactin and
cortisol, can lead to significant increases in rates of lipid synthesis and glucose oxidation by
porcine mammary explants (Jerry
et al.
, 1989). Furthermore, a close relationship between
concentrations of prolactin and rate of glucose metabolism in porcine mammary tissue
was reported (Jerry
et al.
, 1989) and prolactin binding to its receptor on sow mammary
tissue was postulated as being a major effector of mammary metabolic rate (Plaut
et
al.
, 1989). Such an involvement of prolactin in nutrient metabolism within mammary
tissue would not be surprising in light of its essential role for both the initiation and the
maintenance of lactation in sows (Farmer
et al.
, 1998).
