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

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Chapter | 12 Diagnostic Enzymology of Domestic Animals
canine corticosteroid induced isoenzyme of ALP. A study
of 270 dogs with suspected hepatic disease had the results
of hepatic biopsies compared to serum hepatic enzyme
results ( Center et al. , 1992 ). Hepatic disease was confirmed
by histology in 207 of the 270 cases. The sensitivity
of serum ALP and GGT activity to detect histologically
confirmed hepatic disease was 85% and 46%, respectively.
However, the specificity of serum ALP was only 51%,
whereas the specificity of serum GGT was 87%. In a study
of 69 cats with suspected hepatic disease, of which 54 had
histological evidence of hepatic disease, the overall sensitivity
of serum ALP and GGT activity was 48% and 83%,
respectively (Center et al. , 1886). Serum GGT activity was
more sensitive than serum ALP activity for detection of
extrahepatic cholestasis, cholangiohepatitis, and cirrhosis.
In contrast, the percentage increase of serum ALP activity
was greater than serum GGT activity in 11 of 15 cats with
hepatic lipidosis. This is likely because hepatic lipidosis is
a form of intracellular cholestasis, ALP is primarily associated
with hepatocytes, and GGT is primarily associated
with the biliary epithelial cells.
Several laboratory animal models have been used to
study the mechanism of increase of serum GGT activity.
These include bile duct ligation, treatment of the animals
with alpha naphthyl isothiocyanate (ANIT) to cause necrosis
of biliary epithelial cells, and a choledochocaval shunt (CCS)
model that shunts bile from the common bile duct directly
into the anterior vena cava ( Bulle et al. , 1990 ; Hardison
et al. , 1983 ; Kryszewski et al. , 1973 ; Leonard et al. , 1984 ;
Putzki et al. , 1989 ). In the bile duct ligation model, there is
an initial increase of serum GGT activity and a decrease in
liver tissue GGT activity. The rise in serum GGT activity is
associated with a parallel increase in serum bile acids, and it
is almost certain that the bile acids alone or in conjunction
with a hydrolytic enzyme facilitate solubilization or release
of GGT from the membrane. With persistence of cholestasis,
there is a proliferation of biliary epithelial cells and an
increase in bile duct volume in the liver, which is paralleled
by an increase in GGT activity in the liver and a second
phase of increase in serum GGT activity. In the ANIT
model, chronic treatment with ANIT results in repeated
necrosis of the biliary epithelial cells and bile duct proliferation.
As the bile duct mass increases, there is a persistent
increase in serum GGT activity. These two models both support
the concept that there is an initial release of GGT activity
from injury to biliary epithelial cells and retention of bile,
the magnitude of which is determined in part by whether the
species has low or high tissue GGT activity. Persistently
increased serum GGT activity may indicate biliary hyperplasia
that provides an increased source of GGT for release.
Although not confirmed experimentally, clinical observations
suggest that domestic animals with markedly increased
serum GGT activity often have biliary hyperplasia.
The CCS model in rats is unique in that it provides
persistently increased liver and blood bile acids and
increased bile flow, but without increased biliary pressure
( Hardison et al. , 1983 ). In this model, within 24 h, serum
GGT activity nearly equals that seen with experimental
bile duct ligation, suggesting that bile acids or other
bile constituents mediate the release of GGT into serum.
Moreover, increased biliary pressure and regurgitation
of GGT through tight junctions are not necessary for the
observation of increased serum GGT activity ( Putzki et al. ,
1989 ). The actual pathway traveled by GGT from the biliary
epithelial cells to blood is unclear. It is also unclear if
the amount of GGT on the sinusoidal surfaces of hepatocytes
is adequate to account for the magnitude of increase
of serum GGT activity observed in any of the three models
described previously. A mechanism of release from hepatocytes
analogous to that described for ALP, but with a different
hydrolytic enzyme, might be considered.
Increased serum GGT activity in calves led to the recognition
that species that produce large amounts of GGT
activity in mammary glands may excrete or release GGT
into colostrum. Colostral GGT is taken up by passive transfer
in the newborn and serves as an easy, inexpensive, and
automated test for successful passive transfer ( Braun et al. ,
1982 ; Perino et al. , 1993 ; Wilson et al. , 1999 ; Zanker
et al. , 2001 ). The difference between presuckling and postsuckling
serum GGT activity can be more than a 100 fold
in calves ( Braun et al. , 1982 ). This increase correlates well
enough with increased serum immunoglobulin to allow
serum GGT activity to substitute as a test of adequate
immunoglobulin transfer ( Perino et al. , 1993 ). However,
GGT activity decreases steadily for the first 18 to 20 days,
so the ability of serum GGT activity to accurately conclude
failure of passive transfer is reduced after approximately
8 days ( Wilson et al. , 1999 ). In a review of failure of passive
transfer in calves, the authors concluded that the loss of correlation
between serum GGT activity and immunoglobulin
concentrations after the first few days of suckling negates
the value of the test, and its use should be discouraged in
cattle ( Weaver et al. , 2000 ). Serum GGT activity also indicates
passive transfer of immunoglobulins in goats, but in
foals there is no difference in pre- and postsuckling serum
GGT activity ( Braun et al. , 1984 ; Patterson and Brown,
1986 ). In canine pups postsuckling serum GGT activity can
reach up to 100 times the upper interval because of high
colostral GGT activity ( Center et al. , 1991 ).
Although renal tissue has the highest concentration of
GGT activity per gram of tissue in all species studied, there
is no evidence supporting the presence of renal tubular GGT
in blood. This is likely the result of the location of GGT on
the luminal surface of the tubular epithelial cells and possibly
rapid clearance from blood by the galactose receptor on
hepatocytes. However, the location of GGT on the tubular
epithelial cells means that the enzyme is readily shed into
urine and can indicate renal tubular cell injury. Because of
the variability of urine volume, urine GGT activity must be
normalized to urine creatinine concentration using a GGT
activity:creatinine ratio. Numerous reports show that the
urine GGT activity:creatinine ratio in dogs is a sensitive