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

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Chapter | 18 Pituitary Function
test and directly by measuring plasma VP during hypertonic
saline infusion. Both procedures are described here
in detail, as they have been developed for use in the dog
(Biewenga et al. , 1987 ; Mulnix et al. , 1976 ). It should be
noted that because of interference of plasma proteins, the
radioimmunoassay for VP requires prior extraction of the
peptide with ethanol and validation of the assay for the dog
(Biewenga et al. , 1991 ; Hellebrekers et al. , 1987 ). Antisera
against AVP may possess 100% cross-reactivity with AVT
( Choy and Watkins, 1986 ; Hellebrekers et al. , 1987 ).
1 . Modifi ed Water-Deprivation Test
a . Indication
Differentiation between neurogenic and nephrogenic diabetes
insipidus, when the administration of DDAVP (Minirin)
has not resolved the diagnosis.
b . Procedure
Following 12 h of fasting, water is withheld and plasma
and urine are collected every hour or 2 h, depending on the
severity of the polyuria. Osmolality is measured in both
samples. At each collection the animal is weighed. When
the weight loss approaches 5% of initial body weight, the
test should be stopped.
When, in the presence of an adequate osmotic stimulus
(Posm 305 mOsm/kg), urine concentration is maximal
(less than 5% increase in Uosm between consecutive collections),
2IU of lysine vasopressin are administered subcutaneously.
Uosm is measured again 1h later.
c . Interpretation
In both nephrogenic diabetes insipidus and neurogenic
diabetes insipidus, Uosm will remain low during water
deprivation. In neurogenic diabetes insipidus, Uosm will
rise by 50% or more following the injection of vasopressin,
whereas in nephrogenic diabetes insipidus, there will
be little or no rise in Uosm.
However, as mentioned earlier, in some cases the outcome
may not be conclusive and erroneous results may
also occur.
2 . VP Measurements during Hypertonic Saline Infusion
a . Indications
Differential diagnosis of polyuric conditions and suspicion
of inappropriate VP secretion.
b . Procedure
The euhydrated animal is infused for 2 h via the jugular
vein with 20% NaCl solution at a rate of 0.03 ml/kg body
weight per minute. Samples for plasma VP and Posm
are obtained at 20-minute intervals. Especially in the
severely polyuric animal, there is a risk of inducing critical
hypertonicity. In these cases the samples collected at 0, 40,
and 80 min should be checked for Posm immediately.
c . Interpretation
The slope of the regression line for Posm and plasma VP
concentration is used as a measure of the sensitivity of the
osmoregulatory system. In the nomogram developed by
Biewenga et al. , (1987), the 90% range for sensitivity was
0.24 to 2.47 (pg/ml)/(mOsm/kg). The 90% range for the
threshold of the system was 276 to 309 mOsm/kg. After
hypophysectomy in the dog, there is no VP response to
hypertonic saline infusion ( Meij et al. , 1997c ).
However, as mentioned earlier, the occurrence of spontaneous
VP pulses makes it difficult to differentiate these
from VP pulses (to the magnitude of “ erratic bursts ”) during
hypertonic saline infusion ( van Vonderen et al. , 2004b ).
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