Biomedical Engineering Reference
In-Depth Information
Plasma oestradiol concentrations are affected
by heat stress in dairy cows (Wolfenson et al.
1995
;
Wilson et al.
1998
), and effects may be consistent
with decreased concentrations of LH and reduced
dominance of the selected follicle. Similarly, the
effect of heat stress on plasma progesterone con-
centration is unequivocal and controversial.
Wilson et al. (
1998
) found that heat stress had no
effect on the plasma progesterone concentrations
but that luteolysis was delayed. However, other
studies have reported increased progesterone
concentrations (Trout et al.
1998
; Abilay et al.
1975
; Vaught et al.
1977
) , decreased progester-
one concentrations (Ronchi et al.
2001
; Rosenberg
et al.
1977
; Jonsson et al.
1997
) or concentration
remain unchanged (Roth et al.
2000
; Guzeloglu
et al.
2001
) during heat stress in dairy cows.
These differences probably arise because of
uncontrolled changes in other factors that affect
blood progesterone concentrations. Several fac-
tors like the type and magnitude of heat stress
(i.e. acute or chronic) and differences in dry matter
intake independently affect blood progesterone
concentrations. Plasma progesterone concentra-
tions are determined by the differences between
the rate of luteal production and the rate of hepatic
metabolism, and both of these are also affected
by changes in dry matter intake. Low plasma pro-
gesterone concentrations during the luteal phase
of the preconception oestrous cycle can compro-
mise follicular development leading to abnormal
oocyte maturation and early embryonic death
(Ahmad et al.
1995
) . During the conception
cycle, low progesterone concentrations lead to
the failure of implantation (Mann et al.
1999
;
Lamming and Royal
2001
) . In the conception
cycle, the effect of progesterone has been proba-
bly related to the need for synchronous develop-
ment of the embryo and delayed or advanced
development of the corpus luteum leading to
higher rates of implantation failure (Lamming
and Royal
2001
). The pattern of the postovula-
tory rise in progesterone has been reported to be
associated with fertility (Darwash et al.
1999
) .
Cows producing high milk production often
have high dry matter intake (Staples et al.
1990
;
Hommeida et al.
2004
) and low circulating
progesterone concentrations in lactating
(Hommeida et al.
2004
) . Acute feeding reduced
circulating progesterone by 25% in pregnant
cows (Vasconcelos et al.
2003
) . Lucy et al. (
1998
)
found that circulating progesterone was lower in
cattle genetically selected for high milk produc-
tion. Sangsritavong et al. (
2002
) demonstrated
that lactating cows have a much greater steroid
metabolism than non-lactating cows. As a result,
lactating cows may have larger luteal tissue vol-
ume on the ovary (Sartori et al.
2002,
2004
) yet
have lower circulating progesterone and oestra-
diol concentrations than heifers and dry cows
(Wolfenson et al.
2004
) . Low progesterone secre-
tion compromises fertility in dairy cattle (Mann
and Lamming
1999
), and an increase in proges-
terone secretion may facilitate embryonic devel-
opment. Progesterone provides nourishment for
the conceptus via induction of secretion of pro-
teins and other molecules from the endometrium
(Garrett et al.
1988
) . Low peripheral concentra-
tions of progesterone are also associated with
increased luteinising hormone (LH) pulses
(Ireland and Roche
1982
) that can stimulate lute-
olytic signals in favour of pregnancy failure.
Skarzynski and Okuda (
1999
) reported that block-
ing the progesterone receptor with a progesterone
antagonist (onapristone) increased prostaglandin
F2a (PGF2 a) production by bovine luteal cells
harvested from mid-cycle corpora lutea (CL)
(Days 8-12). Also, it was revealed that the bovine
corpus luteum (CL) does not undergo apoptosis
until progesterone production has declined
(Juengel et al.
1993
; Rueda et al.
1995
) .
Increased corticosteroid secretion inhibits
GnRH and thus LH secretion (Gilad et al.
1993
) .
The high concentrations of oestradiol can counter-
act the effect of heat stress, or alternatively, the
neuroendocrine mechanism controlling gonado-
trophin secretion is more sensitive to heat stress
particularly in animals having low concentrations
of plasma oestradiol. Heat stress also impairs ovar-
ian functions directly to decrease its sensitivity to
gonadotrophin stimulation (Wolfenson et al.
1997
). The alteration in the secretory activity of
the follicle and the corpus luteum to heat stress
may influence fertility in cows and buffaloes.
