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

II. Physiology of Adrenocortical Hormones
609
Unbound steroids readily diffuse into the salivary
glands. Because of the close relationship between free cortisol
in blood and saliva ( Riad-Fahmy et al ., 1982 ), techniques
have been developed for the collection of saliva
from cattle ( Murphy and Connell, 1970 ), sheep ( Fell et al .,
1985 ), and dogs ( Phillips et al ., 1983 ). Saliva cortisol concentrations
correlated significantly with plasma cortisol
concentrations after an insulin-induced hypoglycemia and
varied from 7% to 12% of total plasma concentrations, in
line with other reports of free plasma cortisol concentrations
in dogs ( Beerda et al ., 1996 ). In cow’s milk, about
60% of the cortisol is present in the ultrafiltrate. Following
parturition, the percentage of unbound cortisol (in colostrum)
decreases to 40% ( Shutt and Fell, 1985 ) owing to the
higher concentrations of CBG-like protein.
The physiological significance of protein binding probably
lies in a buffering effect, which prevents rapid variations
of the plasma cortisol level. Transcortin restrains the
active cortisol from reaching the target organ and also protects
it from rapid inactivation by the liver and excretion
through the kidneys.
Plasma aldosterone is predominantly bound to albumin,
which has a low affinity. The relatively low degree
of protein binding of plasma aldosterone partially explains
the very low plasma concentration and the short biological
half-life of this hormone.
E . Metabolic Breakdown and Excretion
Only unbound cortisol and its metabolites are filterable at
the glomerulus. Most of this filtered cortisol is reabsorbed,
whereby a tubular maximum is only achieved at very high
filtered loads of free cortisol ( Boonayathap and Marotta,
1974 ). Less than 20% of the filtered cortisol is excreted
unchanged in the urine. Nevertheless, in most mammals
the kidneys account for 50% to 80% of the excretion of the
metabolized steroids. The remainder is lost via the gut. To
render them suitable for renal elimination, the steroids are
inactivated and made more water soluble through enzymatic
modifications. The liver is the major organ responsible
for steroid inactivation and conjugation to form
water-soluble compounds, although in the dog—contrary to
humans—the kidney and the gastrointestinal tract also contribute
to the metabolic clearance of cortisol (McCormick
et al ., 1974) . In the canine, kidney cortisol glucuronide is
both secreted and reabsorbed, without a tubular maximum
or a plasma threshold ( Boonayathap and Marotta, 1974 ).
Cortisol is cleared from the plasma with a half-life of
60 min or less. For pigs, the metabolic clearance rate of cortisol
was calculated to be about 1 l.h 1 .kg 1 ( Hennesy et al .,
1986 ). In dogs, about 60% of infused cortisol is eliminated
within 24 h in the urine (Rijnberk et al ., 1968a ). The 11β -
hydroxyl group of cortisol can be oxidized to the ketone,
forming cortisone ( Fig. 19-4 ). The reaction is reversible, and
in general the equilibrium is shifted to favor the 11 β-hydroxyl
FIGURE 19-4 Metabolism of cortisol in the dog (simplified).
group. However, because the adrenal cortex produces much
more cortisol than cortisone (if any), there is substantial
cortisol-to-cortisone conversion. These two steroids have
similar subsequent metabolic fates.
Apart from this 11β -hydroxylation, cortisol metabolism
in the dog involves the following: (1) reduction of ring A
to tetrahydro derivatives, (2) reduction of the 20-keto group
to a hydroxyl, and (3) conjugation with glucuronic acid to
form glucuronides ( Gold, 1961 ). In addition, unconjugated
20-hydroxycortisol/cortisone has been found in canine urine.
In total glucuronide fraction of urinary corticoids in dogs, at
least steroids reduced at C-20 represent 60%. This is of prime
importance when it comes to assessing adrenocortical function
by measuring urinary cortisol metabolites. Measurements
directed at steroids containing a 17 α ,21-dihydroxy, 20-keto
arrangement (i.e., the Porter-Silber reaction) will detect only
a small part of the cortisol metabolites and therefore have
limited value ( Siegel, 1965 ). Instead, preferably, measurements
are preformed involving reduction of the urine metabolites
at C-20, followed by oxidation to 17-ketosteroids, which
are then quantitated. Further details on this measurement
of total 17-hydroxycorticosteroids are given in Section IV.B.3 .
Aldosterone is converted not only to tetrahydroaldosterone-3-glucuronide
but also to aldosterone-18-glucuronide.
Most of aldosterone metabolism takes place in liver and