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

V. General Comments
655
F . Cat
1 . Progesterone
Progesterone analysis can be used to verify the occurrence
of ovulation in the cat following coitus. Ovulation usually
occurs 24 to 36 h after coital contact with a male at the
appropriate time of the follicular phase of the estrus cycle
( Shille et al. , 1983 ). If the breeding schedule is very limited
in time, it is possible for a cat to be bred too early in the
follicular phase with coitus failing to elicit LH release. Or
more rarely, females may allow copulation at times other
than the follicular phase. In both these situations, ovulatory
failure would be documented by the finding of low
progesterone values 10 days postbreeding. In one study,
progesterone concentration 1.87 ng/ml; 6 nmol/liter was
consistently associated with luteal-phase ovaries, and values
0.15 ng/ml; 0.5 nmol/liter were associated with follicular
phase ovaries ( Lawler et al. , 1991 ).
2 . Estrogen
As the cat is an induced ovulator (requires coitus), estradiol
analysis can be used to assess the presence of ovarian follicle
activity with values ranging from 10 pg/ml (37 pmol/liter) in
the interfollicular phase to 60 pg/ml (220 pmol/liter) during
folliculogenesis. As indicated previously, queens have ovarian
follicle growth patterns that last 5 to 7 days followed by a
slightly longer interval before the next growth phase ( Shille
et al. , 1979b ). Estrogen analysis could document the presence
or absence of ovarian follicular activity in animals that
fail to manifest sexual activity. It also could be used to assess
the completeness of an ovariohysterectomy in cats that are
spayed but who present signs suggestive of sexual receptivity.
The usual finding is that of low estrogen concentrations,
which indicates the behavior is not sexual in orientation.
In the domestic cat, estrogen metabolites are primarily
excreted in the feces ( Möstl et al. , 1993 ; Shille et al. ,
1990 ). Analysis of fecal steroids is a useful noninvasive
approach of monitoring ovarian function in exotic Felidae
( Graham et al. , 1993 ).
3 . Testosterone
Testosterone analyses can be used to evaluate Leydig cell
function in the testis of the male cat. The range of values is
usually between 1 and 10 ng/ml (3.5 to 35 nmol/liter).
4 . Relaxin
Relaxin is produced by the fetoplacental unit beginning
about day 20 of gestation; maximal concentrations are
achieved by day 30 to 35 ( Addiego et al. , 1987 ). Relaxin
concentrations thus can be used to assess pregnancy status
in the cat and even to assess its normalcy based on the fact
that fetoplacental units in jeopardy produce less relaxin.
V . GENERAL COMMENTS
A . Hormone Concentrations
The foregoing presentation did not emphasize hormone
concentrations. This is because there is still some variability
as to the values reported by various laboratories. It is
important that clinical endocrinology laboratories understand
and have experience with their assay systems in
relation to particular clinical syndromes. For example, the
actual concentration of progesterone during the follicular
phase of the estrus cycle of domestic animals is approximately
100 pg/ml (318 pmol/liter), certainly no greater than
200 pg/ml (636 pmol/liter) plasma. Some laboratories, however,
report basal values of 1 to 2 ng/ml (3.2 to 6.4 nmol/
liter) for progesterone. The reference values developed by
laboratories depend on the type of assay used.
It is possible to have a wide range of values for a particular
hormone and still have normal physiological conditions.
Luteal phase progesterone concentrations in the cow,
for example, range between 2 and 12 ng/ml (6 to 38 nmol/
liter) because of pulsatile gonadotropin secretion ( Walters
et al. , 1984 ) with no adverse effect on the preparation of
the animal for the nurture of a potential embryo. Hormone
values can also depend on the type of material used in the
assay. Luteal phase progesterone values are approximately
4 ng/ml (13 nmol/liter) in fat-free milk, between 5 and 35 ng/
ml (16 to 111 nmol/liter) in whole milk, and approximately
250 ng/ml (795 nmol/liter) in milk fat.
B . Analysis and Storage Effects
1 . Blood
The treatment and storage of samples before analysis
can influence the hormone value obtained for a blood
sample. It has been known that red blood cells from cow
have the capacity to metabolize progesterone to other steroids
( Short, 1958 ; Fig. 21-10 ). The rapid decrease, about
10% to 20% reduction/hour, in the progesterone content
in heparinized bovine blood samples appears to be due to
the conversion of progesterone to 20 β-hydroxylated gestagens.
Also other 20-keto-gestagens like pregnenolone and
17 α -hydroxyprogesterone undergo a similar conversion to
20 β -hydroxylated gestagens and reduce their blood concentration
during storage ( Choi et al. , 1989 ). The drop in
progesterone concentration in heparinized whole blood follows
glycolysis, and when glucose concentrations are low
the enzymatic degradation of progesterone in heparinized
cow blood is to some degree reversed ( Oltner and Edqvist,
1982 ; Fig. 21-10 ). The use of anticoagulants blocking or
preventing glucolysis like sodium fluoride much reduces
the rate of decline in whole blood progesterone concentrations
( Pulido et al. , 1991 ; Vahdat et al. , 1979 ). Temperature
also plays a significant role in regulating the rate of decline