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

III. Methodology
65
level, 6.6 to 7.7 mmol/l (120 to 140 mg/dl) was reached
at 1 h and returned to the fasting level, 3.6 to 5.3 mmol/l
(65 to 95 mg/dl), in 2 to 3 h. The OGTT may be simplified
by taking a single sample at 2 h after giving the glucose
(i.e., 2 h postprandial glucose). A normal blood glucose
level at 2 h postprandially indicates that diabetes is unlikely.
A hyperglycemia at 2 h is indicative of a diabetic curve and
should be confirmed with the complete GTT. The insulin
response curve during the OGTT can also be evaluated.
In the OGTT, for a given level of blood glucose, the total
insulin secretion (TIS) is greater than in the IVGTT. This
is most likely due to the insulin releasing effect of the gut
hormones (Section VII.D).
PLASMA GLUCOSE (mg/dl)
700
600
400
200
IVGTT IN DIABETES MELLITUS OF DOGS
TYPE I
TYPE II
TYPE III
NORMAL
T 1/2 240 min; K 0.39% /min
T 1/2 232 min; K 0.34% /min
T 1/2 106 min; K 0.76% /min
b . Intravenous Glucose Tolerance Test and
the Insulin Response
The intravenous glucose tolerance test (IVGTT) in animals
must also be standardized for best results ( Kaneko et al .,
1978a ). This is necessary because glucose clearance halftimes
(T 1/2 ) and urinary glucose losses are directly proportional
to the glucose dose. The recommended method gives
optimal results because (1) it does not overload the animal
with glucose, (2) the infusion can be given easily within
the time limits, (3) the blood glucose level is high enough
to give a maximal insulin response, and (4) urinary loss of
glucose is minimal. After a standard overnight (12 to 16 h)
fast (except for an adult ruminant), a zero-time heparinized
blood sample is taken. Next, 0.5 gm glucose/kg b.w.
is infused I.V. as a sterile 50% solution in 30 s. Timing of
the test is begun at the midpoint or at 15 s after start of the
injection. In large animals, the glucose is given within 2 to
3 min or more quickly if possible. Subsequent blood samples
are taken at 5, 15, 25, 35, 45, and 60 min. The results
are plotted on semilogarithmic coordinates from which the
time required for the glucose concentration to fall by half,
the T 1/2 , is graphically estimated between 15 and 45 min
postinfusion. From the T 1/2 , the fractional turnover rate, k,
can also be calculated:
0.
693
k 100
%/m
T
12 /
The fractional turnover rate, k, can also be calculated
without graphing the data and using the relationships:
Ln1
Ln 2
k
100
%/m
T T
2 1
from the k value, the T ½ may be calculated:
T
1/
2
0.
693
100m
k
100
90
80
0 15 30 45 60
TIME (min)
T 1/2 26 9 min (X S.D.)
K 2.76 0.9 (X S.D.) %/min
FIGURE 3-13 The intravenous glucose tolerance test (IVGTT) in
normal dogs and in dogs with various types of diabetes mellitus. From
Kaneko et al. (1977) .
The fractional turnover rate has been variously expressed
as the glucose turnover rate, the glucose disappearance
rate, the glucose disappearance coefficient, or simply as the
k-value. The normal T½ and k in dogs are 25 8 min and
2.76 0.91%/min, respectively (Kaneko et al ., 1977 ). The
diabetic animal with a decreased glucose tolerance has a
longer T 1/2 and lower k.
The method is equally applicable to and the only practical
method in large animals. The k-value in a spontaneously
diabetic cow was 0.38%/m (T 1/2 182 m) as compared to
a reference value of 1.98%/m (T 1/2 35 m) (Kaneko and
Rhode, 1964 ) and was comparable to the k-values obtained
using 14C-glucose ( Kaneko et al ., 1966 ).
Standardization of the IVGTT as described also has
the advantages that an adequate insulin response is provoked,
the influence of urinary glucose loss is minimized,
and reproducible clearance values are obtained ( Fig. 3-13 ).
Other areas of the IVGTT with diagnostic significance
for diabetes are the 5-m peak, which is inordinately high,
and the 60-m glucose level, which has not returned to the
preinfusion level.
The insulin response curve to the glucose load is
obtained from the same samples as for glucose ( Fig. 3-14 ).
In the normal response curve to a glucose load, the peak
insulin response occurs at 5 min followed by a return to
normal at 60 min ( Kaneko et al ., 1977 ). The early 5-min
peak is due to the stimulation of release of stored insulin
by the beta cells by glucose. In humans, a second peak is
seen at 20 to 30 min, which is attributed to the de novo synthesis
of insulin by the beta cells. This peak has not been
experimentally discernible in dogs ( Kaneko et al ., 1978b ).