Biomedical Engineering Reference
In-Depth Information
Heat Stress and Hormones
Contents
1
Introduction
......................................................
Abstract
Activation of the hypothalamic-pituitary-adrenal
axis and the consequent increase in plasma
glucocorticoid concentrations are two of the
most important responses of the animals to
heat stress. The short- and long-term environ-
mental heat affects endocrine glands and in
turn release of hormones, namely, thyroxine,
cortisol, growth hormone and catecholamines.
Some of them result in initial increase due to
acute stressors and a decline in plasma levels
after prolonged exposure to stressors has been
observed. The relationship of amounts in
plasma of these hormones to milk production
appears to be related directly for cortisol,
growth hormone and prolactin with an inverse
relationship with thyroxine. Epinephrine and
norepinephrine are elevated with prolonged
environmental heat stress. Hormones in
plasma are important as potential indicators of
the physiological status of a cow and reflect
the physiological compensations a cow under-
goes at various stages of lactation and expo-
sure to heat stress. The plasma thyroxine (T
4
)
and triiodothyronine (T
3
) levels have been
observed to decline under heat stress as com-
pared to thermoneutral conditions. The decline
in thyroid hormones along with decreased
plasma growth hormone (GH) level has a
synergistic effect to reduce heat production.
A reduced secretion of GH is required for
survival of the homeotherm during heat stress.
The concentration of insulin-like growth factor-1
(IGF-1) has been observed to decrease during
28
2
Thyroxine (T
4
)
and Triiodothyronine (T
3
)
................................ 29
3
Cortisol
.............................................................. 31
4
Insulin
................................................................ 33
5
Insulin-Like Growth Hormone 1
(IGF-1)
.............................................................. 33
6
Growth Hormone
............................................. 34
7
Aldosterone
....................................................... 34
8
Adiponectin and Leptin
................................... 34
9
Reproductive Hormones
.................................. 35
9.1 FSH and LH .............................................. 35
9.2 Progesterone.............................................. 36
9.3 Oestrogen Concentrations ......................... 38
9.4 Gonadotrophins and Corticosteroids......... 40
9.5 Prostaglandin............................................. 40
9.6 Androstenedione ....................................... 41
10
Catecholamines
................................................
41
11
Prolactin
............................................................
41
12
Conclusions
.......................................................
42
Appendices
................................................................
42
References
.................................................................
44
