and other gastrointestinal secretions, with
only a small percentage of endogenous loss
in the urine. Surface losses from sweat,
skin and hair or feathers are small, but
exceed urinary losses, at least in humans.
Biliary Cu is reabsorbed poorly in
mammals and chickens, which increases
the efficiency of endogenous excretion.
The liver plays an important role in
the quantitative whole-body metabolism of
Cu, especially in ruminants. Up to 80% of
whole-body Cu is found in the liver of
healthy sheep and cattle, as opposed to 8
or 9% in the liver of monogastrics. Bone
also is a major Cu pool in all species,
varying in Cu content from about 18% of
whole-body Cu in cattle to 40% in humans.
Muscle comprises about 13% of whole-
body Cu in cattle, 23% in humans and
31% in rats. In small animals, the skin also
becomes a major component of whole-body
Cu, about 23% in rats.
There is considerable variability
among species in their ability to maintain
liver Cu homeostasis (Fig. 8.14). Sheep
appear to have little or no ability to
regulate liver Cu concentration. Even small
increments in dietary Cu to unsupple-
mented diets yield substantial increases in
liver Cu. Sheep are very susceptible to
chronic Cu poisoning, which is probably a
result of their inability to regulate liver Cu
accumulation. Pre-ruminant calves, which
also are susceptible to chronic Cu poison-
ing, have a limited ability to control liver
Cu accumulation, although that ability
improves after weaning. Although the liver
Cu of adult cattle increases with increasing
Cu supplementation above the nutritional
requirement, the concentration tends to
level off at subtoxic accumulations,
indicating some homeostatic regulation.
Rats, on the other hand, demonstrate the
same characteristic plateau in response to
increasing dietary intake as has been
observed with Zn. Liver Cu increases up to
the nutritional requirement of about
5–10 μg Cu g^1 diet and thereafter remains
constant up to about 100 μg Cu g^1 diet,
beyond which a substantial increase in174 W.T. Buckley
Fig. 8.13.Tissue Zn accumulation of young growing male rats fed deficient dietary Zn (3 μg g^1 ) in a
semi-synthetic diet. The figure is a simulation as described in Fig. 8.12.