Biomedical Engineering Reference
In-Depth Information
neuroendocrine changes, thereby promoting
homeostasis within the neuroendocrine-immune
system against stress (Nakamura et al.
1997
) . The
increase in plasma cortisol during summer in
calving cows provides at least a partial explana-
tion for the changes in cell-mediated and humoral
immunity. Studies in humans and laboratory
animals under a large variety of experimental
conditions have also demonstrated that intense
adrenocortical activity depress T-cell activity and
elevate B-cell activity (MacMurray et al.
1983
;
Kok et al.
1995
; Elenkov and Chrousos
1999
;
Moynihan
2003
) . Therefore, under stressful con-
ditions, glucocorticoids may suppress cellular
immunity and boost humoral immunity via an
effect on T helper 1 (Th1)/Th2 cells and type 1/
type 2 cytokine production (Elenkov and Chrousos
1999
) . Webster et al. (
2002
) have indicated that
glucocorticoids induce a shift from a Th1 (cellular)
to a Th2 (humoral) pattern of immunity and that
this is mainly to downregulate the Th1 cytokines
for allowing dominant expression of the Th2
cytokines (Franchimont et al.
1998
; Agarwal and
Marshall
2001
) .
mechanisms through which nutrition mediates its
effects are not well understood. Clearly, changes
in the availability of nutrients are perceived by
the hypothalamus and influence gonadotrophin
secretion via effects on hypothalamic GnRH
release (Foster et al.
1989
; Ebling et al.
1990
) .
Insulin and leptin are hypothesised to be the adi-
posity signal for the brain to regulate body weight.
Jonsson et al. (
1997
) suggested that the reduction
in dry matter intake of heat-stressed animals
results into a negative energy balance that may
prolong the postpartum period and decrease fer-
tility in dairy cows. Negative energy balance
leads to decreased plasma concentrations of insu-
lin and glucose that affect follicle development.
Lower plasma insulin and glucose lead to
impaired follicular development and delayed
ovulation. Insulin also appears to play a role in
regulating synthesis and secretion of leptin.
Adipose tissue fragments cultured in the pres-
ence of insulin have been observed to increase
both synthesis and secretion of leptin (Barr et al.
1997
). A positive correlation between insulin and
leptin levels has been observed in cows (Block
et al.
2001,
2003
; Aggarwal et al.
2010
) . The
depression of insulin (Habeeb
1987
; Aggarwal
2004
), thyroxine (El-Mastry and Habeeb
1989
;
Aggarwal
2004
; Aggarwal and Singh
2009
) and
cortisol (Kamal et al.
1989
; Aggarwal
2004
) may
contribute to a decrease in milk synthesis and
milk yield, and changes in milk composition may
be observed.
4
Insulin
Plasma insulin decreases due to exposure to ele-
vated temperature in cattle. The decreased values
were 30, 54 and 33% in Friesian calves (Habeeb
1987
), Friesian heifers (Sejrsen et al.
1980
) and
Holstein cows, respectively (Abdel-Samee et al.
1989
). A decrease in insulin level is likely to
facilitate decrease in heat production. However,
other researchers either did not detect significant
difference between the stressed and unstressed
Friesian bulls in plasma insulin concentration
(McVeign and Tarrent
1982
) or reported an
increase in insulin levels during exposure to
elevated temperature (Chaiyabuter et al.
1987
) ,
and the increase in insulin levels during expo-
sure to elevated temperature was correlated
with a marked increase in plasma glucose level.
Such increase may be related to the action of
glucocorticoids.
Nutrition plays a major role in controlling
reproductive processes. However, the physiological
5
Insulin-Like Growth
Hormone 1 (IGF-1)
IGF-1 is synthesised by most of body tissues and
may act in an autocrine or paracrine manner
(Cohick and Clemmons
1993
) . The liver is the
primary source of circulating IGF-1, which may
directly affect target tissues, such as the mam-
mary gland of pregnant and lactating animal to
stimulate the synthesis of milk components. Most
of the effects of growth hormone are mediated by
IGF-1. The concentration of IGF-1 has been
found to decrease during summer months
(Ingraham et al.
1982
; Butler and Smith
1989
;
