Biomedical Engineering Reference
In-Depth Information
levels also differ with season and it may be
involved in acclimation (Leining et al.
1979
) .
Reduced leptin and/or Ob-Rb expression in
heat-shocked lymphocytes may represent an
adaptive mechanism to high environmental tem-
peratures, which may limitedly explain the immu-
nosuppression mechanisms observed in cows
during summer. Therefore, a greater understanding
is required to understand mechanism associated
with immune suppression and hormonal changes
during high milk production in heat stressed cows.
12
Conclusions
The biological mechanism by which heat stress
impacts animal production and reproduction is
both direct and indirect. The decrease in feed
intake and also because of an altered endocrine
status, reduction in nutrient absorption and
increased maintenance requirements result in a
net decrease in nutrient/energy availability. Heat
stress acclimation is accomplished by changes in
homeostatic responses and may include homeor-
hetic processes involving an altered endocrine
status that ultimately affects target tissue respon-
siveness to environmental stimuli. Stress hor-
mones are also implicated in the acclimatory
response to heat stress and they primarily include
thyroid hormones, prolactin, growth hormone,
glucocorticoids and mineralocorticoids. The
thyroid hormones, T
4
and T
3
, provide a major
mechanism important for acclimation and have
received considerable research attention in ani-
mal production system. It is established that heat
acclimation decreases endogenous levels of
thyroid hormones to reduce endogenous heat
production. The decrease in thyroid hormones
during heat stress is to facilitate decrease in basal
metabolic rate and muscle activity by decreasing
heat production. Under heat stress, cortisol hor-
mone is another protective mechanism prevent-
ing metabolic heat production. In other words, it
is thermogenic and consequently reduces adreno-
cortical activity, under thermal stress. Therefore,
cortisol is involved in adaptation to short- and
long-term heat stress. Plasma levels of LH and
oestradiol are reduced during summer in cows
affecting fertility. Heat stress may affect the
secretion of the gonadotrophins through mecha-
nisms that modify the synthesis or the secretion
of GnRH, the responsiveness of the gonadotro-
phs to the actions of GnRH or the feedback
actions of gonadal hormones. Prolonged or
chronic heat stress results in suppressed gonad-
otrophin secretion and inhibition of reproduction
but, when the duration of the stress response is
transient or acute, the effects are less clear.
Appendices
Stressor
: A threatening or unpredictable stimulus
that causes a stress response.
Stress response
: The physiological, hormonal
and behavioural changes that result from expo-
sure to a stressor.
Chronic stress
: A state that an organism enters
when repetitive or long-term exposure to a stressor
has exceeded an organism's regulatory capacities.
Context of a stressor
: The physical and psycholo-
gical conditions present when a stressor appears.
Acclimation
: After repeated or chronic exposure
to a single stressor, an animal no longer perceives
the stressor to be threatening and reduces its phys-
iological stress response. The decrease in stress
response is specific to that stressor and does not
generalise to other stressors as long as the animal
is capable of distinguishing between them.
Sensitisation
: When acclimation to one stressor
increases subsequent stress responses to novel
stressors.
'Stress hormones'
: A generic and non-scientific
term for hormones whose concentrations change
in response to stressors and are indicative of a
stress response. They are divided in two main
types: catecholamines (e.g. epinephrine/adrena-
line, norepinephrine/noradrenaline) and gluco-
corticoid-steroid hormones (e.g. cortisol,
corticosterone). Some hormones (e.g. cortisol)
have been traditionally used as indicative of
stress. However, they may exhaust under repeti-
tive stimuli and may not reflect chronic stress.
Steroid hormones
: A class of hormones (includ-
ing testosterone, oestradiol and cortisol) typified
by a four-ring structure.
(Romero
2004
)
