Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 21 Clinical Reproductive Endocrinology
through hysterectomy ( Stabenfeldt et al. , 1974b ). Removal
of the uterus from these species during the luteal phase
prolongs luteal activity. It is well established that the uterus
in these species synthesizes and releases PGF 2 α , which
causes the CL to regress ( McCracken et al. , 1972 ).
The temporal release patterns of PGF 2 α ( Fig. 21-4 ),
usually in a pulsatile mode lasting a few hours, have been
described in the ewe ( Barcikowski et al. , 1974 ; Harrison
et al. , 1972 ), sow ( Gleeson et al. , 1974 ), doe ( Fredriksson
et al. , 1984 ), and cow ( Nancarrow et al. , 1973 ). Some of
the problems involved in determining PGF 2 α (e.g., a short
half-life and formation by platelets at collection) can be
avoided if the main blood plasma metabolite, 15-keto-
13,14-dihydro-PGF 2 α , is determined. Data are available on
the patterns of the metabolite during luteolysis in the cow
( Kindahl et al. , 1976 ), ewe ( Peterson et al. , 1976 ), mare
( Neely et al. , 1979 ), doe ( Fredriksson et al. , 1984 ), and
sow ( Shille et al. , 1979a ).
Regression of CL is usually accomplished within 48h
following the onset of the prostaglandin release. It is likely
that estrogens, presumably of ovarian follicle origin, initiate
PGF 2 α release. Estrogen also initiates the formation of
endometrial oxytocin receptors ( McCracken et al. , 1984 ),
which in sheep are important for pulsatile synthesis and
release of PGF 2 α in that oxytocin can initiate the release of
PGF 2 α ( Sharma and Fitzpatrick, 1974 ). In ruminants, the CL
is able to synthesize oxytocin and oxytocin release from the
CL in response to PGF 2 α , which, in turn, initiates the synthesis
and release of PGF 2 α by the uterus; this is the basis
for the pulsatile secretion of PGF 2 α (for a review, see Flint
et al. [1992] , and Whates and Denning-Kendall [1992] ). In
the dog and cat, PGF 2 α does not appear to be involved in
luteolysis, although its precise role is still uncertain.
C . Early Pregnancy
Modification of PGF 2 α release is essential for the establishment
of pregnancy in the species (cow, ewe, mare, and sow)
in which this compound serves as the luteolysin ( Kindahl
et al. , 1976 ; Nett et al. , 1976 ; Shille et al. , 1979a ). The
rapid elongation of fetal membranes, which precedes the
critical time of the initiation of luteal regression by about
3 days in the nonpregnant animal, appears to be important
for modifying prostaglandin release. Fredriksson et al.
(1984) in goats and Zarco et al. (1988) in sheep have shown
that the main change in PGF 2 α synthesis and release, nonpregnant
versus pregnant animal, involves a continuous
mode of secretion, not pulsatile. In fact, PGF 2 α concentrations
increase at the onset of pregnancy, but the release is
continuous, which prolongs luteal activity.
Maternal recognition of pregnancy in the cow, ewe, doe,
and sow involves mechanisms that alter the prostaglandin
release to protect the CL from luteolysis. In the cow,
ewe, and doe, embryonic interferon tau (IFN- τ ), which
is secreted between days 10 and 21 to 24 of pregnancy
Estrone sulfate
Mare
Sow
0 20 40 60 80 100
Days of pregnancy
Estrone sulfate
Doe
Ewe
Cow
0 40 80 120 160 200
Days of pregnancy
FIGURE 21-6 Blood levels of conjugated estrogens (mainly estrone
sulfate) during early pregnancy in the mare, sow, doe, ewe, and cow. Data
derived from Kindahl et al. (1982) (mare); Robertson and King (1974)
(sow); Chaplin and Holdsworth (1982) (doe); Tsang (1978) (ewe); and
Gaiani et al. (1982) (cow).
by trophectoderm cells of the blastocysts ( Roberts et al. ,
1990 ), inhibits uterine PGF 2 α pulses ( Spencer et al. , 1999 ;
Thatcher et al. , 2001 ). IFN-τ is secreted into the uterine
lumen and inhibits the expression of estrogen and oxytocin
receptors, thus blocking the episodic uterine PGF 2 α secretion
and luteolysis (for a review, see Spencer et al. [2004] ).
In the sow, the maternal recognition of pregnancy is controlled
by conceptus-derived estrogens ( Geisert et al. , 1982 ;
Perry et al. , 1976 ) (Fig. 21-6 ).
Modification of the release pattern of PGF 2 α (pulsatile
to continuous) by the luteotropic products from the
conceptus and uterus is probably the most important factor
that allows luteal activity to continue. The net result
is that luteal activity is extended in the cow, ewe, mare,
and sow beginning at about 14 days following ovulation.
Modification of PGF 2 α release appears not to be important
for the establishment of pregnancy in the dog and cat.
D . Pregnancy and Parturition
The presence of a CL is necessary for the maintenance of
pregnancy in a vast majority of cows ( Estergreen et al. ,
1967 ). The pig also requires luteal support throughout
gestation (see Bazer et al. , 1979 ). In the ewe, the presence
of CL is required for the first 50 to 60 days of gestation