Biomedical Engineering Reference
In-Depth Information
Hamilton et al.
1999
). Since plasma levels of
IGF-1 are directly related to energy status and
IGF-1 is critical to ovarian follicular develop-
ment (Beam and Butler
1999
), it is likely that
IGF-1 is associated with poor reproductive per-
formance of cows during hot summer. These
effects may be mediated through reduced feed
intake as cows in negative energy balance have
lower IGF-1 levels (Webb et al.
1999
; Bousquet
et al.
2004
) and an increase in dietary intake
improves LH pulse frequency (Spicer et al.
1990
)
and the diameter of the dominant follicle in heifers
(Diskin et al.
1999
) .
leads to tissue destruction, thereby eliminating
minerals from the body in the urine and faeces.
The relationship between plasma aldosterone
concentration and urine electrolyte excretion
during heat stress in bovines has been docu-
mented by El-Nouty et al. (
1980
) . Plasma aldos-
terone levels during the first few hours of
prolonged heat exposure decline (40% lower),
and thereafter, the decline occurs rapidly. This
decline in aldosterone level has been observed
due to a fall in serum K levels mainly because of
its increased excretion in sweat (El-Nouty et al.
1980
). The differences in the ruminants and
nonruminants are likely to occur with respect to
Na and K loss during thermal stress, as nonru-
minants produce sweat high in Na and low in K
concentrations (Lippsett et al.
1961
) contrary to
ruminants.
6
Growth Hormone
Growth hormone is a calorigenic hormone which
exerts its effects on almost all tissues of the body.
The plasma GH levels decline at thermoneutrality
in heat-stressed Jersey cows (Mitra et al.
1972
) .
Igono et al. (
1988
) observed that GH content in
milk of low, medium and high production groups
declined when THI exceeded 70. Plasma growth
hormone reductions that occurred with heat-
stressed cows did not occur in thermoneutral con-
ditions for cows fed restricted intakes that were
similar to those consumed during heat stress. The
decreased GH leads to less calorigenesis aimed in
the maintenance of heat in the body (Bauman and
Currie
1980
). In addition to calorigenesis, GH also
enhances heat production by stimulating thyroid
activity (Yousef and Johnson
1966a
) . Therefore, a
reduced secretion of GH may help survival of the
homeotherm under high ambient temperatures.
8
Adiponectin and Leptin
The transition dairy cows undergo large meta-
bolic adaptations in glucose, fatty acid and mineral
metabolism to support lactation and avoid meta-
bolic dysfunction (Overton and Waldron
2004
) .
But the level of feeding of cows during the last
month before calving has been postulated to have
only small effects on the cows' metabolic and
hormonal status (Roche et al
.
2005
) . Since leptin
is thought to play a critical role in regulating
energy metabolism in mammals (Block et al.
2003
; Aggarwal et al.
2010
) , nutritional changes
induced by heat stress may influence leptin.
Adipose tissue performs its regulatory function
by secreting biologically active molecules called
adipocytokines (Trayhurn
2005
) . Among these,
adiponectin and leptin are expressed almost
exclusively in differentiated adipocytes and are
crucial for the regulation of energy balance and
carbohydrate/lipid metabolism (Havel
2002,
2004
). Adiponectin is one of the most important
and abundant adipocytokines and have been
observed to exert anti-diabetic, anti-atherogenic
and anti-inflammatory roles (Goldstein and Scalia
2004
; Pittas et al.
2004
). However, leptin is a
modulator of the appetite and the energetic
balance and may function as an endocrine, a
7
Aldosterone
Plasma aldosterone concentration in
B. taurus
decreases significantly due to high environ-
mental temperature (El-Nouty et al.
1980
; Niles
et al.
1980
; Aboul-Naga
1987
) and prolonged
heat exposure reduce mineralocorticoids. The
decrease in aldosterone and parathyroid hormone
secretion during heat stress in cattle is associated
with a rise in body fluids and water turnover rate.
In addition, the increase of glucocorticoid hormone
