Agriculture Reference
In-Depth Information
34% was oxidized to CO and the remaining 13% was used for fatty acid or amino acids
synthesis. For instance, Dourmad
et al.
(2000) showed that free fatty acids AVD decreased
to negative values post-feeding during the peak of glucose AVD, indicating that glucose
utilization for fatty acid synthesis increases under absorptive state. The same authors also
reported lactate AVD to increase post-feeding compared with pre-feeding, indicative
of increased glucose oxidation under absorptive state. The mammary gland appears to
respond to dietary glucose availability by altering the AVD; in the study by Dourmad
et al.
(2000), AVD for glucose markedly decreased 16 h post-prandially from 24.2. to
12.2 mg/dl, a decrease that was directly related to that of arterial glucose concentrations
from 126.8 to 62.4 mg/dl. Albeit based on only one study, given that the extraction rate
of glucose by the sow mammary gland remained constant over a relatively wide range of
arterial glucose concentrations (i.e. 126.8 to 62.4 mg/dl), glucose uptake by the mammary
gland is likely mediated by high capacity, low affinity glucose transporter proteins.
Transcript abundance for the genes
SLC2A1
,
SLC2A4
,
and
INSR
, respectively encoding for
the non-insulin dependent glucose transporter GLUT1, the insulin-dependent glucose
transporter GLUT4, and the insulin receptor proteins, was measured in mammary tissue
at the end of pregnancy, throughout lactation, and post-weaning (Manjarín
et al.
, 2012a).
Relative abundance of all 3 genes was relatively high across all stages, with GLUT1
being two-fold greater than GLUT4 and insulin receptor, with no change in expression
occurring between early and peak lactation. Both GLUT1 and GLUT4 are high capacity,
low affinity and high Km transporter proteins, and as such, monosaccharide uptake by
the mammary gland would be expected to increase with an increase in arterial glucose
concentrations. This is not surprising given the importance of glucose as a substrate for
lactose synthesis in sow milk. Renaudeau
et al.
(2003) estimated that 1,300 g of glucose
was needed to support a milk yield of 11 kg. From the work of Guan
et al.
(2004b), it
was estimated that the porcine udder removes approximately 2,000 g of glucose per day
to support a daily milk yield of 11.4 kg (Farmer
et al.
, 2008a). The ratio between lactose
output and glucose uptake ranges from 0.35 to 0.68 across studies (Dourmad
et al.
, 2000;
Linzell
et al.
, 1969b; Spincer
et al.
, 1969).
14.3.3
Uptake of other energetic precursors
Three classic studies, namely those of Spincer
et al.
(1969), Linzell
et al.
(1969b) and
Spincer and Rook (1971), have reported mammary uptake of triglycerides, phospholipids,
acetate, propionate and lactate in sows (Table 14.2). Other than glucose, the major sources
of non-amino acid carbon uptake are triglycerides and lactate (Table 14.2). Linzell
et al.
(1969b) used five sows, ranging from 12 to 61 d of lactation to estimate mammary AVD.
Lactate and triglycerides AVD were considerable whereas AVD for phospholipids, free
fatty acids, volatile fatty acids and β-hydroxybutyrate were small and/or variable. Spincer
et al.
(1969) reported that of the total recorded mammary uptake of plasma constituents,
11% was accounted for by plasma triglycerides and only 2% by acetate. Of the plasma
triglycerides, oleate (23%), linoleate (21%), palmitate (19%) and stearate (16%) were the
most prominent and when radioactive triglycerides were infused intravenously, more
than 60% of palmitic acid and 70% of stearic acid in milk fat were accounted for by uptake
from plasma triglycerides (Spincer and Rook, 1971). Recent findings by Dourmad
et al.
