daily 3MH produced in the urine, swine
excrete 1.5% day^1. Therefore, accurate
determination of 3MH production in sheep
and swine requires an exit out of the
system from compartment 3. This exit
accounts for appreciable loss of 3MH into a
balenine ‘sink’ which turns over slowly or
not at all during the time frame of the
study.
Representative plasma decay curves
following a single dosing of 3-[methyl-
(^2) H
3 ]methylhistidine tracer are illustrated in
Fig. 2.9. In general, each species exhibited
a similar exponential decay, characterized
by rapid decay over the first 2–3 h,
followed by a slower decay up to 12 h and
a steady-state decay over the remainder of
the study. The decays of the tracer are
representative of the models used.
Humans, cattle and dogs exhibit very
similar decays, while sheep and swine are
very different.
Measurement and Significance of Protein Turnover 39
Cattle
P > 0.25
14
12
10
8
6
4
2
0
3-methylhistidrie production
(μ
mol kg
^1
day
^1
)
Dogs Humans
Model Urine
P > 0.25
P < 0.25
Fig. 2.9.Daily 3-methylhistidine production expressed as μmol kg^1 day^1 for dogs, cattle and humans as
calculated from urinary excretion and by a three-compartment model of 3-methylhistidine production.
*There was no mean difference between urinary and model 3-methylhistidine production for cattle and
humans, P> 0.25.
Table 2.1.Comparison of 3-methylhistidine kinetic parameters.
Species
Parameter Cattle Humans Dogs Swine Sheep
Animals, n 39 4 5 20 40
Urinary 3MH loss, % of total 100 100 100 1 17
Fractional transfer rate, min1a
L2,1 0.18 0.08 0.11 0.23 0.21
L1,2 0.06 0.06 0.06 0.09 0.08
L3,2 0.003 0.009 0.006 0.014 0.007
L2,3 0.002 0.002 0.008 0.006 0.005
L0,3 NA NA NA 0.0009 0.0004
L0,1 0.006 0.004 0.02 NA 0.0003
aFractional transfer rate (Li,j) from compartment j to i.
NA, not applicable.