Biomedical Engineering Reference
In-Depth Information
Hansen
1997
; Edwards et al.
1997
; Rivera and
Hansen
2001
; Rivera et al.
2003
) . Thermal stress
triggers a dramatic and complex programme of
altered gene expression in bovine mammary epi-
thelial cells (BMEC) similar to patterns reported
in other cell types exposed to thermal stress. As
reported by Sonna et al. (
2002
) , these changes
include (1) inhibition of DNA synthesis, tran-
scription, RNA processing and translation; (2)
inhibition of progression through the cell cycle;
(3) denaturation and misaggregation of proteins;
(4) increased degradation of proteins through
proteasomal and lysosomal pathways; (5) disrup-
tion of cytoskeletal components; (6) alterations
to metabolism that lead to a net reduction in
cellular ATP; and (7) changes in membrane per-
meability that lead to an increase in intracellular
Na
+
, H
+
and Ca
2+
concentrations. Thermal stress
induced the changes in gene expression along
with rapid regression of BMEC ductal structures.
Transcriptional activity indicated a downregula-
tion of a number of genes associated with branch-
ing morphogenesis and microtubule activity,
thereby suggesting a repression of the genomic
signals responsible for promoting ductal growth
and networking (Collier et al.
2006
) . Overall, the
transcriptome profile indicated downregulation
of genes involved in cell structure, metabolism,
biosynthesis and intracellular transport. The
upregulated genes during heat shock in BMEC
mainly were involved in cellular repair, protein
repair and degradation and apoptosis after loss of
thermotolerance when HSP-70 gene expression
fell to basal levels (Edwards et al.
1997
) . These
data indicate that morphogenic activity in the
mammary epithelium might depend upon the
expression profile of a core set of genes and that
structural assembly is under a positive mode of
regulation (i.e. morphogenesis is 'on' by default).
In turn, the transition from structural assembly to
disassembly might be controlled at the genomic
level by simply shutting down cellular biosynthe-
sis and core morphogenic genes. In contrast,
transcription of genes encoding repair enzymes
and apoptotic proteins is kept off until the cell
requires them and removes the inhibition, which
suggests a negative mode of regulation.
An additional group of genes dominated by
patterns of downregulation were those involved
in BMEC differentiation and milk synthesis. This
suggests that (1) even during growth and mor-
phogenesis, BMEC expresses detectable mRNA
levels of some lactogenic genes; and (2) heat-
induced BMEC regression includes transcrip-
tional repression of genes involved in milk
synthesis. This strongly implies that milk yield
losses in lactating dairy cows exposed to thermal
stress are due in part to direct repression of genes
associated with milk synthesis.
Thermotolerance in BMEC was found to be
lost after 8 h of exposure to thermal stress when
HSP-70 gene expression returned to basal levels,
which was associated with increased expression
of genes in the apoptotic pathways, indicating
these cells were in the process of undergoing
apoptosis. These studies were carried out using
BMEC from non-adapted and non-acclimated
cattle (Collier et al.
2006
). Thus, a portion of the
loss in milk yield during acute thermal stress is
associated with direct effects of thermal stress on
BMEC. Acute thermal stress of growing bovine
mammary epithelial cells directly reduces cellu-
lar growth and ductal branching and downregu-
lates genes associated with protein synthesis and
cellular metabolism. Chronic thermal stress
would likely reduce mammary growth during
pregnancy. Furthermore, negative effects of ther-
mal stress on expression of milk protein genes
indicate that thermal stress likely has direct nega-
tive effects on milk yield.
12
Reducing Heat Stress for
Improving Milk Production
The body temperatures of the cow should be
maintained below 102.5°F (39.2°C) and respira-
tion rates below 80 per minute. Heat stress should
be evaluated at the cows' nose level both lying
down and standing at the bunk and in the holding
pen. Often, considerable heat stress occurs in the
holding area while cows are waiting to be milked.
Igono et al. (
1992
) found that despite high ambi-
ent temperatures during the day, a cool period of
less than 21°C for 3-6 h minimises the decline in
milk yield.
These findings suggest that it is critical to
minimise increase in cow's body temperature
