Biomedical Engineering Reference
In-Depth Information
increase milk yield by 2-3 kg/day, regardless of the
stage of lactation. This LD effect is attributed to
increased circulating insulin-like growth factor-I
that is independent of any effect on growth hor-
mone concentrations (Dahl et al.
2011
) . Cows
housed under SD during the dry period have
higher mammary growth and produce 3-4 kg/day
more milk in the subsequent lactation compared
with cows on LD when dry. While on SD, circu-
lating prolactin (PRL) diminishes, but expression
of PRL receptor increases in mammary, liver and
immune cells. The PRL-signalling pathways
within these tissues are affected by photoperiod.
The replacement of PRL to cows on SD is able to
partially reverse the effects of SD on production
in the next lactation. Therefore, effects on cows
are likely mediated through a PRL-dependent
pathway. In prepubertal stage, LD improves
mammary parenchymal accumulation and lean
body growth which lead to higher yields and pro-
duction in the first lactation (Dahl et al.
2011
) .
Therefore, photoperiod manipulation can be one of
the important tools to improve the efficiency of
production across the life cycle of the dairy cow
particularly under temperate climate.
Intracellular circadian rhythm generation
occurs through an auto-regulatory transcription-
translation feedback loop (Reppert and Wever
2001
). The positive loop consists of ARNTL (aka
BMAL1) and CLOCK gene products (and NPAS2
outside the SCN), and the negative loop consists
of the PER and CRY gene products (Gekakis
1998
). ARNTL expression is also regulated
by Rev-erba (NR1D1) and Rora (RORA) that
respectively repress or activate ARNTL tran-
scription (Preitner et al.
2002
; Guillaumond et al.
2005
). The genes that RORA and NR1D1 regu-
late are often coordinately regulated by these two
molecules, and crosstalk between RORA and
NR1D1 likely acts to fine-tune their target
physiologic networks, such as circadian rhythms,
metabolic homeostasis and in fl ammation (Forman
et al.
1994
) .
Bovine mammary epithelial cells have been
postulated to possess a functional clock that can be
synchronized by external stimuli and the expres-
sion of ARNTL (ARNTL and CLOCK gene
products make up core clock elements to generate
circadian rhythms) (Casey et al.
2009
) . A positive
limb of the core clock is responsive to prolactin
in bovine mammary explants. Others showed that
7% of genes expressed in breasts of lactating
women had circadian patterns of expression, and
the diurnal variation of composition of cow's
milk is associated with changes in expression
of mammary core clock genes. These studies
indicate that the circadian system coordinates the
metabolic and hormonal changes required for
lactation initiation and sustaining it. Thus, cow's
capacity to produce milk and cope with metabolic
stresses in early lactation depends on its ability to
set circadian rhythms during the transition period
(Casey et al.
2009
) .
12
Oxidative Stress
Living organisms and their systems attempt to
maintain homeostasis by adjusting to the
continuous state of flux in the external environ-
ment (Crawford and Davies
1994
) . Circadian
synchronisation of physiological events and cel-
lular processes is essential for survival and wel-
fare of the organism. The synchronisation of
gene expression to external stimulus can help
minimise the damage caused by external stres-
sors (Langmesser and Albrecht
2006
) and adap-
tive process. The redox balance, the balance
between oxidants and antioxidants, is one exam-
ple of such a system that helps maintaining reac-
tive oxygen species (ROS) homeostasis
(Krishnan et al.
2008
) . The rhythmic expression
of genes involved in various metabolic pathways
and stress resistance has been reported in flies
(Ceriani et al.
2002
) and in mammals (Miller
et al.
2007
). Daily rhythms are likely to occur in
the expression of some antioxidants (catalase,
SOD or GST) and may be involved in protect-
ing the organism from excessive levels of ROS
that may damage biological macromolecules
(Hardeland et al.
2003
; Kondratov
2007
) . ROS
are generated by normal physiological process
(Sugino
2006
), but when produced in excessive
quantities or at rates faster than they are removed
