Biomedical Engineering Reference
In-Depth Information
other heat-stressed-induced changes (Rhoads
et al.
2007
) .
The negative energy balance associated with
the early postpartum period is coupled with
increased risk of metabolic disorders and health
problems (Goff and Horst
1997
; Drackley
1999
) ,
decreased milk yield and reduced reproductive
performance (Lucy et al.
1992
; Beam and Butler
1999
; Baumgard et al.
2002,
2006
) . It is likely
that many of the negative effects of heat stress on
production, animal health and reproduction indi-
ces are mediated by the reduction in energy
balance. However, it is not clear how much of the
reduction in performance (yield and reproduc-
tion) can be attributed or accounted for by the
biological parameters effected by heat stress (i.e.
reduced feed intake vs. increased maintenance
costs). Seasonal differences in milk production
are caused by periodic changes of environment
over the year, which has (1) a direct effect on ani-
mal's milk production through decreased DMI
and (2) an indirect effect through fluctuation in
quantity and quality of feed. Process of lactation
is a physiological process that presents a substan-
tial challenge to the homeostasis of the cardio-
vascular and fluid secretory system (Silanikove
1994,
2000
; Maltz and Silanikove
1996
). The acute
and large decrease in milk secretion may, therefore,
be considered as having vital importance that
makes it necessary to enable the cows to survive
under heat stress. The initial reactions to acute heat
stress may be emotional responses (Silanikove
2000
), but prolonged challenges impact milk
yields and composition; milk protein fat contents
were found to be reduced. These reductions may
be a part of their adaptive response (Kadzere et al.
2002
; Collier et al.
2006
; Igono et al.
1992
) .
1990
). Sustained changes in the frequency of
milking and milk secretion are associated with
metabolic adaptations (Shennan and McNeillie
1994
) and with long-term adaptations in the
degree of differentiation and the number of
mammary epithelial cells (Liu et al.
1997
; Quarrie
et al.
1998
). The fast modulation of milk secre-
tion in response to external factors, such as emo-
tional stress, heat stress and water deprivation
also depends on a negative feedback regulatory
system, required for survival of a species increases
(Silanikove et al.
2006
) . The negative feedback
system has been shown to comprise an endoge-
nous milk enzymatic system, the plasminogen
activator (PA)-plasminogen (PG)-plasmin (PL),
that specifically forms a b-casein (CN) fragment
(f) (1-28) from b-CN, which acts as the negative
control signal by closing potassium channels on
the apical membrane of the epithelial cells of
the mammary gland (Silanikove et al.
2000,
2006
). Inhibition of milk secretion occurs due to
downregulation of these channels by inducing
undefined inwardly directed cellular signals.
A further activation of the PA-PG-PL system,
coupled with more extensive degradation of
casein-induced involution of the mammary gland
in lactating goats and cows, has been observed
that forcefully activate the innate immune system
(Silanikove et al.
2005,
2006
) . The concept that
PA-PG-PL-b-CN f (1-28) is involved in milk-
borne negative feedback regulation of milk
secretion was supported experimentally under
conditions that simulated stress (intramammary
treatment with dexamethasone) (Silanikove et al.
2000
; Shamay et al.
2000
) and by exposing the
cows to dehydration (Silanikove et al.
2000
) .
The involvement of PA-PG-PL system in reg-
ulation of milk secretion (Shamay et al.
2003
)
and the induction of mammary gland involution
(Silanikove et al.
2005
) are well documented, and
the effects of the PA-PG-PL system are related to
enhanced degradation of the extracellular matrix
(Lund et al.
2000
) . The PA-PG-PL system works
in mammary secretion by increasing casein deg-
radation and liberation of active components and
that b-CN f (1-28) is a principal casein degrada-
tion product that is involved in negative control
of milk secretion in cows under heat stress.
9
Mechanism of Regulation of
Milk Secretion and Mammary
Function
Milk secretion and mammary function are regu-
lated acutely by local autocrine feedback mecha-
nisms that involve milk-borne factors which are
sensitive to the frequency and efficiency of milking
of cows (Daly et al.
1993
; Wilde and Peaker
