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

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Chapter | 19 Adrenocortical Function
TABLE 19-4 Plasma Corticoids (nmol/l) Following ACTH Stimulation
Species
ACTH Dose and
Route
Sampling
Time
Hormone Range (Mean SEM) Reference
Dog 250 μ g I.V. 90min Cortisol 120–620 ( Meijer et al ., 1978c )
Dog 5 μ g/kg I.V. 60min Cortisol 220–560 ( Behrend et al ., 2005 )
Corticosterone 18–102 ( Behrend et al ., 2005 )
Cat 125 μ g I.V. 60min Cortisol (274 19) ( Peterson and Kemppainen, 1993 )
Horse 250 μ g I.V. 120min Cortisol 308–602 ( van der Kolk et al ., 2001 )
Cattle 2IU/kg BW 0.75 60min Cortisol (145 27) ( Beerda et al ., 2004 )
1989 ). It does not seem to be advisable to increase saliva
flow by pilocarpine ( Blackshaw and Blackshaw, 1989 ). It
has also been reported that in calves ( Fell and Shutt, 1986 )
and goats ( Greenwood and Shutt, 1992 ), saliva can be aspirated
from the side of the mouth with minimum restraint.
5 . Cortisol in Milk
Monitoring adrenocortical activity by measurements in
milk may be of value in searching for stressors in the livestock
industry ( Gygax et al ., 2006 ; Stephens, 1980 ). In a
comparison of different milking systems, no differences
in milk cortisol concentrations were found ( Gygax et al .,
2006 ). The low concentrations indicate that in milk also
the biologically active free fraction instead of the total cortisol
fraction is measured.
6 . Plasma ACTH
Determinations of resting plasma ACTH concentrations are
not useful in the diagnosis of hyperadrenocorticism because
of the overlapping values for normal dogs and dogs with
hyperadrenocorticism. Its value lies mainly in the differential
diagnosis of hyperadrenocorticism (discussed later). In
animals with primary adrenocortical failure, plasma ACTH
concentrations are usually extremely high. However, in most
cases such a determination is primarily of academic interest
as the diagnosis can be made with the more practical ACTH
stimulation test (see Section IV.C.1). Plasma ACTH measurements
are useful when in doubt about a primary or secondary
adrenocortical insufficiency ( Kooistra et al ., 1995 ).
C . Dynamic Tests
Apart from the above-described measurements of reflections
of basal adrenocortical secretions, various maneuvers
have been introduced to test the physiology of the hypothalamo-pituitary-adrenocortical
axis. As in other areas of
endocrinology, stimulation tests are used when hypofunction
is suspected, and suppression tests are used when
hyperfunction is suspected.
1 . Tests of Adrenocortical Reserve
The critical feature of the test is that a substantial increase in
plasma (or urinary) corticoids must occur in response to ACTH
if one is to prove the existence of adrenal reserve ( Table 19-4 ).
One method is described in detail in Section IV.F.
In veterinary medicine, ACTH stimulation has also been
used to confirm adrenocortical hyperfunction. This is based
on the assumption that hyperplastic adrenal cortices have
abnormally large cortisol reserves. Indeed, hyperresponsiveness
to ACTH or lysine-vasopressin ( Meijer et al ., 1978a )
stimulation is found in pituitary-dependent hyperadrenocorticism,
but there is considerable overlap with normal dogs.
Over the years, several modifications/combinations of the
ACTH stimulation test ( Eiler and Oliver, 1980 ; Eiler et al .,
1984 ; Feldman, 1986 ) for the diagnosis of hyperadrenocorticism
have been used, but it still appears to be a dynamic
test with less diagnostic accuracy than the low-dose dexamethasone
suppression test (LDDST) ( Behrend et al ., 2005 ;
Feldman, 1986 ; Peterson, 1986 ). This seems to hold true for
the horse as well ( Dybdal et al ., 1994 ), although the ACTH
stimulation test is recommended for the diagnosis of equine
hyperadrenocorticism ( Van der Kolk et al ., 1993 ). The value
of the ACTH stimulation test lies in the diagnosis of hypoadrenocorticism
(see Section IV.F). When used for the diagnosis
of hyperadrenocorticism, the modification of a combination
with a dexamethasone suppression test seems the least
desirable, as doses of 0.1 mg of dexamethasone/kg can alter
the results of the ACTH stimulation test for at least 3 days
(Kemppainen et al ., 1989 ). Measurement of plasma corticosterone
or 17 α -hydroxyprogesterone after ACTH stimulation
is of little additive value as the changes measured are proportionate
with changes in cortisol and may be high in dogs with
nonadrenal neoplasia as well ( Behrend et al ., 2005 ).
2 . Tests of Pituitary (ACTH) Reserve
In animals with adrenals responsive to ACTH, administration
of the 11 β -hydroxylase inhibitor metyrapone decreases cortisol
secretion and consequently increases ACTH secretion.
As a result, cortisol precursor synthesis increases, which can