tissues maintain homeostasis more readily
than others. Furthermore, there is a large
variation among species. Responses to
changes in dietary intake usually vary with
level of intake, and several elements have
specific storage tissues while others do not.
One objective of this chapter is to identify
the variations in homeostatic responses of
trace elements among tissues and species.
Many studies of trace element
dynamics, especially in larger animals and
humans, have given little consideration to
the dietary intake prior to the experimental
period and to the effect of previous diet on
body stores at the time of initiating the
study. The effect of previous history on Zn
metabolism was investigated by Johnson et
al. (1988). By studying rats for two sequen-
tial dietary periods, it was shown that Zn
absorption was affected only by the current
dietary Zn intake, but that endogenous
excretion was influenced by both current
and past intake. Rats previously fed a
deficient Zn diet, 1.5 μg Zn g^1 , had lower
rates of endogenous excretion over a range
of Zn intakes than rats previously fed 12.5
or 50.3 μg Zn g^1 diet (Johnson et al.,
1988). The differences in endogenous
excretion allowed the previously deficient
rats to recover body Zn more rapidly
during the second dietary period.
Larger animals and humans may take
months or even years to establish a new
steady-state for some trace elements
following a change in intake (Buckley,
1996). Although adaptive changes in rates
of absorption may occur relatively rapidly,
endogenous excretion will be in a state of
flux until a new steady-state has been
established. Furthermore, steady-state
cannot be achieved in growing animals
with respect to major trace element pools,
which are constantly increasing in size.
Although measurements of endogenous
excretion can be made under non-steady-
state conditions, investigators should be
aware that the results apply only at the
time of making the measurements. A goal
of this chapter is to develop an apprecia-
tion of the rate of change of trace element
status and its impact on interpretation of
tracer and balance studies.
An isotope dilution technique
(Weigand and Kirchgessner, 1976) has been
used frequently for measuring true
absorption and endogenous faecal excre-
tion in studies of trace element dynamics.
The technique depends on sampling a
reference tissue or fluid which contains the
labelled element assumed to be at the same
specific activity as endogenous secretions.
Endogenous faecal excretion is calculated
from the quantity of tracer in faeces and
the specific activity in the reference tissue
or fluid. The validity of selecting certain
tissues or fluids for the isotope dilution
technique has been tested for several
elements and species. Although support for
acceptable reference tissues and fluids has
been obtained, not all sources of error have
been investigated, and some caution is
required. For example, liver Mn specific
activity has been used to represent specific
activity of endogenous secretions; however,
liver specific activity was shown recently
to be indistinguishable from biliary specific
activity in Mn-replete rats, but it was only
about one-third of biliary-specific activity
in Mn-deficient rats (Malecki et al., 1996).
Since bile is a significant source of endo-
genous Mn secretions, estimates of endo-
genous faecal excretion based on liver
specific activity may be in error in Mn-
deficient rats. On the other hand, endo-
genous secretions of Mn are very low in
deficient rats, and correction for the poten-
tial error would only make the estimates
lower and may have little effect on inter-
pretation of the results. The effect of the
choice of reference tissue and time of
sampling on the accuracy of determina-
tions of endogenous excretion has been
discussed by Weigand et al. (1988a). Some
aspects of trace element dynamics related
to measurement technique are mentioned
in this chapter.
The homeostatic control of Zn in rats
has been studied much more intensively
than other element and species combina-
tions; consequently, Zn dynamics is
emphasized in this chapter. Knowledge of
Zn metabolism in the rat is sufficient to
construct a kinetic simulation of whole-
body and tissue responses to changes in162 W.T. Buckley