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

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Chapter | 21 Clinical Reproductive Endocrinology
Progesterone (nmol/L)
100
80
60
40
20
0
Estradiol Progesterone LH
Proestrus
3–21 days
Recommended time
for insemination
Ovulation
Estrus
3–21 days
Metestrus
FIGURE 21-9 Schematic representation of the temporal relationship
among estradiol, progesterone, peak of luteinizing hormone (LH), and
ovulation in the bitch. The recommended time of mating or artificial
insemination is based on the progesterone concentration.
progesterone in daily samples of blood obtained during the
periovulatory period gives a more precise timing of ovulation
than can be obtained by vaginal cytology.
In situations wherein a particular pairing of animals
does not result in a mutual sexual attraction and where artificial
insemination must be used, progesterone analysis is a
useful tool to verify ovulation. Artificial insemination with
frozen-thawed semen usually results in lower pregnancy
rates than artificial inseminations using fresh semen. Using
progesterone determinations to pinpoint ovulation it was
found that the pregnancy rate following insemination with
frozen-thawed semen could be improved when the insemination
was performed at progesterone concentrations
30 nmol/liter ( Fig. 21-9 ) ( Linde-Forsberg and Forsberg,
1989, 1993 ).
Progesterone analysis can also be useful in cases of
short estrus cycle intervals to determine if ovulatory failure
has occurred, a situation in which progesterone concentrations
are low following the termination of estrus.
Analysis of progesterone (alone or in combination with
estrogen) is also useful in cases when there is some question
as to the completeness of removal of ovarian tissue
during an ovariohysterectomy (ovarian remnant syndrome).
Stimulation by hCG/eCG or GnRH can improve the possibility
of finding any remnant ovarian tissue. An important
distinction as concerns the dog is that, unlike other domestic
species, the bitch is keyed into sexual receptivity at
the end of the follicular phase by progesterone. Dogs may
be sexually attractive because of odors arising from the
vagina because of factors such as infection, but bitches
will only accept males in the presence of increased progesterone
concentrations, a situation that is almost always
associated with luteal tissue as part of a remaining ovarian
remnant.
2 . Testosterone
The most common endocrine test in male dogs is testosterone
for the purpose of checking the secretory status
of the Leydig cells. Testosterone values in normal dogs
range from about 1 ng/ml (3.5 nmol/liter) to about 10 ng/ml
(35 nmol/liter) because of the pulsatile release pattern of
testosterone. Testosterone concentrations in castrated dogs
are less than 0.5 nmol/liter.
Assays of basal testosterone concentration may allow the
diagnosis of the absence of testicular tissue in the same manner
as described previously for the male horse. Sometimes
the positive confirmation of presence of testicular tissue
may require the use of hCG or GnRH to stimulate testosterone
production. In this situation, a resting blood plasma
sample is collected immediately before the administration
of the stimulating hormone, and a second blood sample is
collected an hour later. A significant increase in plasma testosterone
concentration is diagnostic of testicular tissue.
Testosterone analysis is done in conjunction with fertility
examinations, often in stud animals that are presented
as infertility cases following a prolonged show tour. Most
of these animals have testosterone concentrations that are
compatible with normal spermatogenesis, although sperm
counts are often very low. The management of this syndrome
is still uncertain.
Feminizing syndromes in intact male dogs have been
observed, in which concentrations of testosterone are greatly
decreased below normal (to 100 pg/ml or less; 350 pmol/
liter) and estradiol values are two to three times normal (30
to 45 pg/ml versus the normal 15 pg/ml; 110 to 165 pmol/liter
and 55 pmol/liter). It is thought that a majority of these cases
involve Sertoli cell tumors. Dogs with confirmed Sertoli cell
tumors have high peripheral blood plasma levels of inhibin
and suppressed levels of LH and testosterone ( Grootenhuis
et al. , 1990 ). These authors were, however, unable to detect
differences in blood concentrations of estradiol between
dogs with Sertoli cell tumors and control dogs.
3 . Relaxin
Relaxin is in the pregnant dog is first detected at day 18 to 25
of gestation but is undetectable during anestrus, throughout
nonpregnant ovarian cycles, and in male dogs ( Steinetz et al. ,
1987, 1996 ). Maximal concentrations are attained by days
40 to 50 of pregnancy and are followed by slight declines
before parturition. Relaxin is produced predominantly
by syncytiotrophoblasts in the placenta ( Klonisch et al. ,
1999 ) but also by the ovary, and it is the closest thing to a
pregnancy-specific canine hormone ( Tsutsui and Stewart,
1991 ). Commercial available assays for canine relaxin can
be used to diagnose pregnancy as early as 21 days after
breeding, although negative results should be rechecked
after a week. However, relaxin cannot be used to estimate
litter size and remains elevated after pregnancy loss.