Mammary blood flow and nutrient uptake - The Gestating and Lactating Sow - page 312

Responses of the milk vein and the cranial mammary artery to potential other vasoactive
substances were studied in isolated blood vessels of lactating sows. Noradrenaline,
serotonin, prostaglandin F, prostacyclin, histamine and potassium had vasoconstrictive
effects on both these vessels whereas acetylcholine had a relaxing effect on both arterial
and vein and β-adenosine had a relaxing effect on the milk vein only (Busk et al. , 1999).
In rodents and humans, adrenergic nerve fibers cause constriction of blood vessels in the
mammary gland thereby reducing blood flow through the glands (Erikson et al. , 1996).
Franke-Radowiecka and Wasowicz (2002) showed that, in swine, the majority of adrenergic
and acetylcholinesterase-positive nerve fibers are localized in the subcutaneous tissue of
the nipple and mammary glands. Franke-Radowiecka and Wasowicz (2002) reported
that cholinergic innervation is considerably less abundant than adrenergic innervation
in porcine mammary glands. Nerve supply to the cranial mammary glands of sows differs
from that to the inguinal glands. Cranial mammary glands receive their innervation from
thoracic nerves whereas inguinal mammary glands receive their innervation mainly from
the pudendal nerve (Klopfenstein et al. , 2006).

14.3 Uptakes of nutrients

14.3.1 Measure of mammary arteriovenous difference

Measurements of mammary nutrient arteriovenous difference (AVD) and uptake
necessitate cannulation of an artery and of the major venous system draining the
mammary parenchyma. In these in vivo studies, arterial blood was collected following
cannulation of the carotid and venous blood following cannulation of the abdominal
vein (Trottier et al., 1995a; Figure 14.1). The cranial end of the anterior portion of the
mammary system is the preferred cannulation site for representative measurement of
nutrient concentrations contributed by glands located in both the thoracic and abdominal
regions. In addition, mammary involution appears to be initiated in the inguinal region
in smaller litters, whereas the abdominal and thoracic glands usually remain functional
until the end of the lactation period (Kim et al. , 2001).

14.3.2 Uptake of glucose

Blood glucose is the major precursor for the synthesis of lactose. Glucose represents
40 to 60% of the total carbon mass taken up by sow mammary tissue (Dourmad et al. ,
2000; Linzell et al. , 1969b; Renaudeau et al. , 2003; Spincer et al. , 1969). Glucose uptake
by the mammary gland makes up a considerable proportion of the total body glucose
requirement. Once in the mammary gland, glucose is used as the main substrate for
lactose, glycerol and fatty acid synthesis (Linzell et al. , 1969b) and may also provide
energy for metabolic processes associated with maintenance of the mammary gland.
Linzell et al. (1969a) reported an extraction rate of glucose by the mammary gland of
about 26%. This is consistent with the estimates of 31% obtained by Spincer et al. (1969)
and 26% by Renaudeau et al. (2003), whereas Dourmad et al. (2000) and Trottier et
al. (1995b) reported lower values of 20 and 20.7%, respectively. Using labeled-carbon
glucose, Linzell et al. (1969b) estimated that 53% of glucose was used for lactose synthesis,