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

IV. Assessment of Pituitary Function
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4 . Action
OT causes contraction of the myoepithelial elements of
the excretory ducts of the mammary glands, resulting in
milk ejection. In addition, it exerts a constricting effect
on the uterine muscle. Estrogens increase the response of
the uterus to OT, whereas progesterone inhibits it. During
pregnancy, the sensitivity of the uterus to OT is greatly
reduced. However, during delivery and immediately thereafter,
the uterus is particularly sensitive to OT. In birds,
AVT is far more active in stimulating the contraction of the
oviduct than is OT ( Bentley, 1971 ).
5 . Disease/Tests
To our knowledge no pituitary dysfunction concerning
OT release in domestic animals has been reported. Basal
plasma levels of OT are in approximately the same range
as plasma VP concentrations (Table 18-8) . In the human
and the goat ( Thornton et al. , 1986 ), OT release cannot be
stimulated by hypertonic saline infusion but does increase
in response to insulin-induced hypoglycemia ( Nussey et
al. , 1986 ). In the rat, nausea-producing chemical agents
and cholecystokinin cause a dose-dependent increase in
OT ( Verbalis et al. , 1986 ).
IV . ASSESSMENT OF PITUITARY
FUNCTION
Endocrine studies in animals with suspected hypothalamus-pituitary
disease may be carried out for three reasons:
(1) documentation of specific endocrine deficits; (2)
insight into the type, size, and progression of the lesion;
and (3) characterization of hyperfunction. Specific diagnostic
tests for several of the pituitary hormones have been
described in the previous sections. Here the discussion will
concentrate on a more general approach in an attempt to
determine the extent of involvement of AL, IL, or NL.
A . Adenohypophysis
1 . Anterior Lobe
The species specificity of some of the AL hormones precludes
their measurement in several animal species with
the assays available. For many other hormones, however,
homologous or heterologous assays are available, allowing
assessment of AL function.
Although baseline hormone levels can provide much
information, the pulsatile nature of AL hormone secretion
often makes interpretation difficult. It is particularly difficult
to distinguish between normal and low levels of these hormones.
For this reason, provocative tests should be used to
examine hypothalamus-pituitary adequacy. Now that hypothalamic-releasing
hormones have become available and are
applicable in most species, appropriate testing of AL function
is possible.
In all instances blood samples are collected at 15, 0,
10, 20, 30, and 45 min (for the short test), and 60, 90, and
120 min (for the long test) for measurement of the pituitary
hormones. At time zero the test substance is injected intravenously.
It may be one of the following:
1. 1 μ g GHRH/kg, 2 μ g ghrelin/kg, or 10 μ g clonidine/kg
to test GH and IGF-1 secretion.
2. 1 μ g CRH/kg or 0.6 μ g lysine vasopressin/kg to test
ACTH ( cortisol) secretion.
3. 10 μ g TRH/kg to test PRL or TSH secretion (see
Section II.C.2).
4. 10 μ g GnRH/kg to test FSH or LH secretion.
Because of the rapid proteolytic degradation of some
pituitary hormones, blood samples should always be
chilled on ice as soon as possible and stored at 20 ° C (see
also Section V.E of Chapter 19 on adrenocortical function) .
Combined function tests are coming into use in
humans. The anterior pituitary is stimulated with four
hypothalamic-releasing hormones in a single procedure.
Despite the fact that there is some interaction of the different
releasing hormones, combined administration of
releasing hormones seems to be a useful test for the assessment
of pituitary function ( Schopohl et al. , 1986 ). Such a
combined function test has been introduced for the dog.
In this test, the anterior pituitary is stimulated with four
hypothalamic-releasing hormones (CRH, GHRH, TRH,
and GnRH) with measurements of ACTH ( cortisol), GH,
PRL, TSH, and LH ( Meij et al. , 1996a, 1996b ). Compared
with the single administration of these secretagogues, there
is little interference in this test, except for the LH response,
which is lower in the combined test than following single
GnRH administration. The test has been used successfully
to document combined pituitary hormone deficiency
in German shepherd dogs with dwarfism ( Kooistra et al. ,
2000c ) and to document AL function in bitches treated
with medroxyprogesterone acetate ( Beijerink et al. , 2007 ).
2 . Intermediate Lobe
α -MSH is secreted specifically by the IL. Its release can
be stimulated with dopamine agonists such as haloperidol
in a dose of 0.2 mg/kg (Table) . The times of sampling for
α -MSH can be as for the tests of adenohypophyseal function
(see Section IV.A), and the test has been used to document
IL function before and after hypophysectomy in dogs
( Meij et al. , 1997c ).
B . Neurohypophysis
As explained in Section III.A, VP secretion can be examined
indirectly by means of a modified water-deprivation