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

III. Clinical Manifestations of Hepatic Insufficiency
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approximately 30% of normal results in a modest increase
in the serum bilirubin concentration. Greater deficiency of
bilirubin-UGT leads to ineffective esterification of bilirubin
and to more severe unconjugated hyperbilirubinemia
and icterus. Bilirubin conjugation in the liver of neonatal
animals is relatively low compared to the adult. Coupled
with the high rate of bilirubin production related to accelerated
neonatal erythrocyte turnover, the risk of hyperbilirubinemia
and icterus may be increased in the neonate.
Defective bilirubin conjugation as a result of inherited
UDP-glu curonosyltransferase deficiency causes the severe
unconjugated hyperbilirubinemia of the Crigler-Najjar
syndrome of humans and the Gunn rat. Gilbert’s disease is
a more benign, inherited form of UDP-glucuronosyltransferase
deficiency in humans that is characterized by intermittent
unconjugated hyperbilirubinemia and icterus. A
similar enzyme deficiency may explain the benign unconjugated
hyperbilirubinemia observed in some horses.
The final step in hepatic excretion is the transport of
conjugated bilirubin across the bile canaliculus and into
the biliary system. The experimental intravenous infusion
of unconjugated bilirubin at a rate that exceeds the maximal
hepatic excretory capacity results in accumulation of
conjugated bilirubin in plasma. This indicates that under
normal conditions, the rate-limiting step in the transfer of
bilirubin from plasma to bile is canalicular transport rather
than either hepatic uptake or conjugation ( Arias et al.,
1961 ). The concentration of bilirubin glucuronide in bile is
150-fold greater than in hepatocytes.
The unidirectional transport of bilirubin conjugates
from the cytoplasm across the canalicular plasma
membrane into bile is mediated by the multidrug resistance-associated
protein 2 (Mrp2; ABCC2; canalicular
multispecific organic anion transporter, cMOAT ), a member
of the ATP-binding cassette (ABC) superfamily, and
an integral part of the bile canaliculus ( Chowdhury et al.,
1994 ; Nies and Keppler, 2007 ). The Mrp2 transport mechanism
is functionally distinct from the ATP-dependent, bile
salt export pump of the canaliculus described later in the
section on bile acids (Bsep; Alpert et al., 1969 ; Arias et al.,
1993 ; Muller et al., 1991 ; Stieger et al., 2007). Although
mechanistically separate from the Mrp2 system, bile salt
excretion enhances bile flow and increases the maximum
transport capacity for bilirubin and other organic anions
( Goresky et al., 1974 ).
Mutations of the Mrp2 gene are responsible for the Dubin-
Johnson syndrome of humans, a benign, inherited hepatic
disease associated with conjugated hyperbilirubinemia
and accumulation of melanin like pigment in hepatocytes.
A similar disease has been observed in two separate strains
of rats. In the GY/TR(-) rat ( Jansen et al., 1985 ; Paulusma
et al., 1996 ) and in the similar Eisai hyperbilirubinemic
rat (EHBR; Ito et al., 1997, 2001 ; Takikawa et al., 1991 ),
genetic sequence variants lead to premature stop codons
and to the absence of the Mrp2 gene product in the canaliculus.
A similar genetic abnormality is likely in Corriedale
sheep, which develop an inherited disease that is clinically
and biochemically identical to the Dubin-Johnson syndrome
of humans ( Cornelius et al., 1965a, 1968b ) and rats
(Kitamura et al., 1990 ).
3 . Extrahepatic Metabolism of Bilirubin
From the bile, conjugated bilirubin enters the intestine.
Conjugated bilirubin, a polar compound, is poorly absorbed
in the small intestine and passes to the large intestine where
it is reduced to a series of colorless derivatives collectively
called urobilinogens (stercobilinogens). Reduction is catalyzed
by dehydrogenases of anaerobic colonic bacteria.
In germ-free animals that lack intestinal microorganisms,
bilirubin passes unaltered into the feces and urobilinogen
is not produced ( Gustafsson and Lanke, 1960 ). Most of the
urobilinogen formed in the colon is passed in the feces, but
some is absorbed into the portal circulation, transported to
the liver, and most of that is excreted in the bile. A small
fraction (1% to 5%) of absorbed urobilinogen, however,
passes into the general circulation and is excreted by the
kidney. In the dog, urobilinogen is excreted by both glomerular
filtration and tubular secretion, the latter being
enhanced in acid urine ( Levy et al., 1968 ).
Although the liver is the principal site of bilirubin conjugation
and excretion, alternate pathways have been demonstrated.
In normal animals, these alternate mechanisms are
of minor significance but may become quantitatively more
important in liver disease. After total hepatectomy, dogs
have been shown to develop moderate hyperbilirubinemia
and bilirubinuria. In addition to unconjugated bilirubin, the
plasma of hepatectomized dogs contains the monoglucuronide
conjugate ( Hoffman et al., 1960 ) and, in some studies,
the diglucuronide of bilirubin ( Royer et al., 1965 ). The
kidney and intestine both have been shown experimentally
to be sites with the capacity to conjugate bilirubin ( Royer
et al., 1974 ). Differences in extrahepatic metabolism of
bilirubin may explain some of the remarkable differences
between species in the bilirubin levels reached after bile
duct obstruction (see the following section).
4 . Icterus
The clinical sign of icterus or jaundice develops when the
yellow pigment bilirubin accumulates in plasma and other
tissues. Yellow discoloration of tissues can first be noted
by careful observation when the plasma bilirubin value
exceeds 2 to 3 mg/dl and can be appreciated even by an
untrained observer when the concentration exceeds 3 to
4 mg/dl. The correlation between the plasma bilirubin concentration
and the degree of clinical icterus is not, however,