Biomedical Engineering Reference
In-Depth Information
will lead to improved genetic selection of heat
stress resistant genotypes. Hormones known to
be homeorhetic regulators are also implicated in
acclimatory responses to thermal stress and
altered photoperiod. These include thyroid
hormones, prolactin (PRL), somatotropin (ST),
glucocorticoids and mineralocorticoids. One
example of acclimatory change in an endocrine
regulator is the seasonal rhythm in prolactin con-
centration when animals acclimate to seasonal
changes in temperature and day length. The
hypothalamic-pituitary-adrenal axes including
corticotropin-releasing hormone, adrenocorti-
cotropic hormone (corticotrophin), cortisol
and aldosterone are also altered by thermal stress
and are involved in acclimatory responses to
thermal stress (Maloyan and Horowitz
2002
) .
Corticotropin-releasing hormone stimulates
somatostatin release from the hypothalamus,
which can inhibit secretion of ST and thyroid-
stimulating hormone from the pituitary and
downregulate the thermogenic effects of both ST
and thyroid hormones. In dairy cattle, the gluco-
corticoids decrease during acclimation at 35°C
and are lower in thermally acclimated animals
compared with controls.
Most of these examples deal with decreased
heat production within the body and the cow's
increased ability to dissipate heat obtained from
the environment. In lactating cows, when long-
term heat acclimation occurs, the low-producing
cow's milk output will reach a level comparable
to what the cow should produce while not under
heat stress, but in the higher producing cow, it
could still be below the milk production possible
in a thermoneutral environment (Johnson and
Vanjonack
1976
) . Some environmental stresses,
such as dehydration (Silanikove
1994
; Silanikove
and Tadmor
1989
) and acute heat stress (Maltz
et al.
1994
; Silanikove
2000
) , may take cows
beyond their current acclimatised-adaptive range
quickly (within 24-48 h) which necessitates the
induction of emergency physiological responses
to avoid lethal effects or death. Such immediate
measures include an acute reduction in milk
yield, because milk production, particularly in
high-yielding dairy cows, intensifies the effects
of these external stresses (Silanikove
1994,
2000
) .
6
Role of Acclimation During
Heat Stress
Acclimation is a phenotypic response developed
by the subject or an animal to an individual
stressor within the environment (Fregley
1996
) .
Acclimatization is the process by which an animal
adapts to several stressors within its natural envi-
ronment (Bligh
1976
) . Acclimation and acclimati-
zation are therefore not evolutionary adaptation or
natural selection, which are defined as changes
allowing for preferential selection of an animal's
phenotype and are based on a genetic component
passed to the next generation. The altered pheno-
type of acclimated animals returns to normal if
environmental stressors are removed. This is not
the case in animals which are genetically adapted
to their environment. Acclimatization is a process
that takes several days to weeks to occur via home-
orhetic and not homeostatic mechanism. There are
three functional differences between acclimatory
responses and homeostatic or 'reflex responses':
1. The acclimatory response takes much longer to
occur (days or weeks vs. seconds or minutes).
2. The acclimatory responses generally have a
hormonal link in the pathway from the central
nervous system to the effector cell.
3. The acclimatory effect usually alters the ability
of an effector cell or organ to respond to envi-
ronmental change (Bligh
1976
) .
These acclimatory responses are characteristic of
homeorhetic mechanisms in a species, and the
net effect is to coordinate metabolism to achieve
a new physiological state. Thus, the seasonally
adapted animal is different metabolically during
winter than during summer.
Acclimation involves an altered expression of
pre-existing features and is a process driven by
the endocrine system with the goal of maintaining
animal well-being regardless of environmental
challenges. The long-term acclimation to heat
was classically referred to as acclamatory homeo-
stasis (Horowitz
2001
), but has been proposed to
be a homeorhetic mechanism (Collier et al.
2005
) ,
because it alters the set points of homeostatic-
related systems (i.e. basal and stimulated carbo-
hydrate metabolism). Understanding this process
