Agriculture Reference
In-Depth Information
it offers the advantage of falling under the definition of 'inert' marker. On the other hand,
given the low arteriovenous difference and extraction rate of Ca (Nielsen
et al.
, 2002a) its
validity as an internal marker is questionable.
Renaudeau
et al.
(2002) obtained direct measurements of blood flow through the
mammary glands of lactating sows by implanting an ultrasonic blood flow probe around
the right external pudic artery. The average pudic mammary blood flow was 910±283
ml/min, corresponding to 3.6 l/min or 4,984 l/day. These values compare to those obtained
from previously-cited authors using the Fick principle (Table 14.1). This approach offers
the advantage of evaluating short-term changes (i.e. minutes) in mammary blood flow
14.2.3
Regulation of mammary blood flow
Regulation of blood flow to the sow udder and each of the respective mammary glands is
poorly understood. One major factor that appears to play a significant role in modulating
blood flow is milk removal. Renaudeau
et al.
(2002) reported that mammary blood flow
decreased markedly shortly after intravenous administration of oxytocin in sows and
returned to basal values as quickly as 12 min following initiation of nursing. Thereafter,
blood flow increased and reached a maximum value 22 min after the onset of milking. It
was suggested that the post-suckling increase in mammary blood flow may be in response
to the nursing event or to the decrease in milk volume. Olsson
et al.
(2003) showed that
a jugular vein infusion of oxytocin caused a short-term increase in mammary blood flow
in lactating goats. It was also demonstrated that there is an increase in intra-mammary
pressure during natural milk ejection or following an intravenous injection of oxytocin in
sows and that the timing of milk ejection between teats may be influenced by blood flow
(Kent
et al.
, 2003). The link between nursing and mammary blood flow is also apparent
at weaning, whereby blood flow decreases by 40 and 60%, respectively, 8 and 16 h after
weaning in pigs (Renaudeau
et al.
, 2002). A positive relationship between litter size and
mammary plasma flow was reported by Nielsen
et al.
(2002b); as litter size increased
from 3 to 13, mammary plasma flow increased linearly from approximately 2,000 l/d to
over 5,000 l/d, reflecting an increase in the number of functional glands and total milk
demand. Minor changes in mammary blood flow in sows are also seen with postural
changes and time relative to feeding. Renaudeau
et al.
(2002) reported a 6% decrease in
mammary blood flow when sows were standing compared with lying, and a 7.7% increase
after a meal reaching a peak 65 min post-prandially. The effect of ambient temperature
on mammary blood flow (Renaudeau
et al.
, 2003) was also examined. While exposure
of sows to an ambient temperature of 28 °C did not affect milk yield, mammary blood
flow increased by approximately 8%. The authors suggested that this small increase was
consequent to an increase in blood flow to subcutaneous capillaries for dissipating heat.
Nitric oxide, also known as the endothelium-derived factor, stimulates vascular smooth
muscle relaxation resulting in vasodilation and increase in blood flow. Lacasse
et al.
(1996)
demonstrated that intra-mammary infusion of a nitric oxide donor in lactating goats
rapidly increased mammary blood flow. Yet, a causal relationship between mammary
blood flow and milk yield remains to be demonstrated. Mammary blood flow of goats
was affected unilaterally and milk yield was measured.
