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

II. Assay Methods
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to another form. For example, many of the tissues that are
particularly responsive to androgens have the enzyme
5 α -reductase that converts testosterone to 5 α-dihydrotestosterone
(5 α-DHT); 5α -DHT has a much higher affinity for
the androgen receptor within the target cell, which makes
5 α -DHT more biologically active than testosterone as to its
androgenic effects. The androgenic potency of 5 α-DHT is
twice that of testosterone.
Another important interconversion of steroid hormones
is the one resulting from the increase in circulating cortisol
concentrations that occur in the fetal lamb before parturition.
The elevated fetal cortisol concentrations stimulate
17 α -hydroxylase, C17-C20 lyase activity, and probably aromatase
activity in the plaxcenta. This makes it possible for
progesterone to be converted to estrogen synthesis in the
placenta. Estrogens then affect the synthesis of prostaglandin
F 2 α , which precipitates delivery. The interconversion of
progesterone to estrogen is well documented in the sheep and
probably also occurs in a similar way in the goat and the cow.
E . Synthesis and Clearance of Hormones
The determination of concentrations of hormones in biological
fluids including plasma, urine, saliva, milk, and
feces has been useful in determining the reproductive status
of animals. Although a number of factors can influence
hormone concentrations, the overriding factors are synthesis
and clearance. We are usually concerned about the
rate of synthesis of a hormone from a particular endocrine
gland because factors that govern clearance are usually stable,
and the concentration of the hormone usually reflects
the rate of synthesis or secretion.
The synthesis of steroid hormones of the reproductive
system is under the control of gonadotropins, which are
released in pulsatile fashion. This has a profound influence
on the secretion of testosterone in the male in that changes
in pulsatile rate can occur a number of times a day with
increases in pulse rate resulting in greatly increased concentrations
of testosterone. For example, in males of many
domestic species, testosterone values can range from 3.5 to
20 nmol/l within a period of a few hours with the extremes
still representing normal production of testosterone by the
testes. The usual judgment as to normalcy is based on an
animal having at least the minimal or basal concentration.
In the female, estrogen and progesterone synthesis
by the ovary is also under the control of a pulsatile mode
of gonadotropin secretion. The pulse rate usually remains
relatively stable over certain periods of time so that fluctuations
in concentration of these hormones are not as acute
as for androgens.
In the female, synthesis rates for ovarian steroid
hormones are obviously related to ovarian function.
Progesterone concentrations, relatively stable during the
luteal phase of the estrus cycle, decline rapidly over a 24- to
36-h period during luteolysis. Estrogen values continually
increase during the follicular phase of the cycle, declining
with the onset of the gonadotropin preovulatory surge as
the granulosa is converted from estrogen to progesterone
production. Even though secretion rates can change for
both progesterone and estrogen, analysis of these hormones
usually brings useful information as to luteal or follicular
activity, respectively. One other factor must be considered
if one wishes to use hormone values (in blood, for example)
as an indication for secretory activity of an endocrine organ
(i.e., the conversion of steroid hormones by peripheral tissues).
For example, in primates, estrone concentrations are
derived mainly from the conversion of ovarian estradiol-17 β
and adrenal androstenedione by tissues such as the liver.
Steroids are eliminated via conjugation with glucuronic
acid or sulfates to form inactive mono- or diglucosiduronates
or sulfates. These conjugates are all water soluble with
excretion occurring via urine or bile (feces). The conjugation
occurs mainly in the liver, and the conjugates formed
lack steroidal activity. Steroids are also rendered inactive by
their metabolism to compounds that have greatly reduced
biological activity. In this way, steroids are rapidly cleared
from the bloodstream. Clearance is defined as the volume
of blood that would be totally cleared of a particular steroid
per unit time. Clearance can thus be expressed as liters/minute,
and the clearance for most steroid hormones is around
1 liter/minute. In most situations, the clearance rate of steroids
is relatively constant, so that blood concentrations are
a fairly good measure of fluctuations in production rates.
Placental gonadotropic hormones like hCG and eCG are
produced in high concentrations and have much longer halflives
than the pituitary gonadotropins and prolactin. The
latter have half-lives that are around 10 to 30 min, whereas
the corresponding figures for the placental hormones are
from 1.5 days for hCG to 6 days for eCG. One exception is
equine LH, which shows structural similarities to eCG and
also has a much longer half-life (days) than LH from other
species. The increased half-lives are due to the fact that the
molecules are composed of a larger portion of carbohydrate
moiety as compared to FSH and LH of most species.
In the blood circulation, prostaglandins are rapidly
metabolized to their respective 15-keto-13,14-dihydro
compounds ( Fig. 21-2 ). Primary prostaglandins like PGF 2 α
have a half-life in the peripheral circulation that is less than
20sec, whereas the 15-keto-13,14-dihydro-PGF 2 α have a
somewhat longer half-life of about 8 min; 90% or more of
PGF 2 α is metabolized by one passage through the lungs.
The 15-keto metabolites are biologically inactive, and
before being excreted into urine they are degraded into
short dicarboxylic acids ( Neff et al. , 1981 ).
II . ASSAY METHODS
The radioimmunoassay (RIA) technique was originally
introduced for the measurement of plasma insulin ( Berson
and Yalow, 1959 ) and the enzyme immunoassay (EIA)