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

IV. Laboratory Assessment of Hepatic Function
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is diminished primarily in chronic liver disease in which
there is significant loss of hepatocellular mass.
Measurement of protein C has been validated for use as
a biomarker for the assessment of experimental liver injury
(Saha et al., 2007 ), and when combined with other conventional
laboratory tests, it was shown to be of value in the
recognition of portosystemic shunts and other severe clinical
forms of hepatobiliary disease in dogs ( Toulza et al., 2006 ) .
Plasmin, a serine protease synthesized by the liver, is
necessary for the degradation of fibrin. Antithrombin has
potent protease activity against thrombin, plasmin, and
other coagulation factors, and its primary physiological
function is to modulate the activity of thrombin. Heparin
enhances the inhibition of thrombin by antithrombin.
The complement system plays a critical role in the
inflammatory response and in host defense mechanisms
against infection, and most of the plasma proteins of the
complement system are synthesized in the liver. De novo
synthesis of C2, C3, C4, factor B, and other complement
proteins has been demonstrated in cultured mammalian
hepatocytes ( Anthony et al., 1985 ; Ramadori et al.,
1986 ). The liver also is the site of synthesis of the protease
inhibitor α -1-antitrypsin, a major plasma protein that
inhibits serine proteases including granulocyte elastase,
and α -2-macroglobulin, an inhibitor of a variety of other
proteases.
E . Dye Excretion
The rate of removal of organic anions can be measured
and used to assess the functional capacity of the liver and
hepatic blood flow. Sulfobromophthalein (BSP, bromsulphalein;
Fig. 13-11 ) and indocyanine green (ICG; Fig. 13-12 )
have been used most frequently to assess hepatic function.
Following bolus intravenous administration, these dyes are
removed rapidly from the plasma primarily by the liver
and excreted in the bile. Delayed plasma clearance is taken
to be indicative of abnormal hepatocellular or biliary tract
function or hepatic circulation (Center et al., 1983c).
The overall process of hepatic excretion of BSP is similar
but not identical to that of bilirubin. Hepatic uptake of
BSP across the sinusoidal plasma membrane is facilitated
primarily by sodium-independent OATP, but the
sodium-dependent NTCP also may contribute significantly
(Hagenbuch et al., 1996 ; Hata et al., 2003 ). Within the cell,
BSP competes with other organic anions for binding to cytosolic
ligandin, which by serving as an ion “ sink ” is a driving
force for hepatic uptake ( Levi et al., 1969 ). The conjugation
mechanisms for bilirubin and BSP are completely separate.
BSP is conjugated with glutathione by action of the cytosolic
enzyme glutathione-S-aryl transferase-B. Although
conjugation appears to facilitate BSP excretion, it is not a
required step because approximately 50% of the BSP in bile
is unconjugated. The canalicular transport of BSP is similar
to that of conjugated bilirubin and involves Mrp2.
Hepatic uptake of ICG is similar to that of BSP, but
canalicular transport into bile is remarkably different and
involves Mdr2, the canalicular transport protein responsible
for transport of cholesterol and phospholipids into bile
( Huang and Vore, 2001 ).
In the icteric patient, the question arises whether competition
between BSP and bilirubin alters the results of the
BSP excretion test. In general, the BSP test is seldom justified
in hyperbilirubinemic patients because hepatic disease
is evident and no important additional information is likely
to be provided. In general, dye excretion tests are most useful
for situations in which a suggestion of occult liver disease
exists but in which the results of other liver function
tests are equivocal. The net competitive effect of bilirubin
on BSP excretion is not great. For example, horses starved
for 72 h developed a three-fold elevation in total serum bilirubin
Fig. 13-9, but BSP excretion was delayed less than
25% (Tennant, unpublished).
In the dog and cat, a standard dose of 5mg/kg of BSP
is administered as an intravenous bolus. A sample of blood
is removed at 30 min, and the BSP concentration is determined
spectrophotometrically. It is assumed that the original
dose of BSP (5mg/kg) is distributed in a plasma volume
of 5 ml/kg so that the concentration of BSP in plasma at
time zero is (by definition) 1 mg/ml. The percentage retention
at 30min is calculated from the ratio of the concentration
at time zero and at 30 min. Retention of 5% or less is
considered to be within normal limits ( Cornelius, 1970 ).
In the large domestic species, because it is inconvenient
or impossible to obtain body weight, plasma clearance of
BSP is measured. The initial slope of the disappearance
curve is independent of BSP dose and a standard 1.0 g
dosage is administered to normal-sized horses (450 kg)
or a dose of 0.5 g to smaller horses. Blood samples are
obtained at 6, 9, and 12 min following injection and the
fractional clearance rate or plasma half-life (T 1/2 ) is calculated
( Fig. 13-13 ). In the horse, normal plasma T 1/2 values
range from 2.5 and 3.5 min. In cattle, the rate of BSP excretion
is similar to the horse. Sheep have a more rapid excretion
rate requiring samples to be taken at 3, 5, and 7min
following injection. Normal T 1/2 values for sheep range
from 1.6 to 2.7 min ( Cornelius, 1970 ).
The BSP test is safe although the dye is irritating if
infiltrated perivascularly. The test should be used only
when cardiovascular function is normal. If hypovolemia or
hypotension is present, hepatic perfusion will be reduced
and erroneously prolong the rate of BSP clearance.
The clearance rate of ICG provides a useful estimate of
hepatic excretory function. Since the hepatic extraction ratio
approaches 1, and hepatic clearance corresponds to hepatic
blood flow. In contrast to BSP, the plasma disappearance
of ICG generally follows a single exponential after mixing.
Unlike BSP, ICG is available commercially and is the dye
excretion test most readily available for clinical use.
ICG clearance has been used to estimate circulation
time and hepatic blood flow. The ICG clearance rate from