United States
Environmental Protection
Agency
Health Effects Research
Laboratory
Cincinnati OH 45268
V-/EPA
Research and Development
EPA-600/S1-81-063 Feb. 1982
Project Summary
Mechanisms of Cadmium
Absorption in Rats
E. C. Foulkes, D. R. Johnson, N. Sugawara, R. F. Bonewitz, and C. Voner
This study was undertaken in order
to help clarify the factors which
determine the fractional absorption of
an oral load of cadmium (Cd) from the
intestine of the rat. The experiments
utilized intact segments of intestine,
perfused or incubated in situ with their
blood supply intact. Absorption of Cd
from the jejuna! lumen can be ascribed
to a saturable membrane system; that
is, after short periods of exposure
essentially all the metal removed from
the lumen was recovered in mucosal
tissue (Step I). The second step in Cd
absorption, i.e., transfer of the metal
from mucosa into blood, proceeded at
only 1 -2% of the rate of uptake from
the lumen (Step I). No evidence was
obtained for a role of metallothionein
in the mucosal retention of Cd. Step I
of Cd absorption was inhibited by a
variety of exogenous and endogenous
factors. Thus, zinc was found to
depress Cd transport in an apparently
competitive manner. Addition of milk
to the lumen also inhibited Cd uptake,
an effect entirely due to the Ca
content. Bile salts act as endogenous
modulators of Cd absorption; their
effect may be related to micelle
formation.
The research also included studies
of duodenal and ileal Cd transport.
Heal Cd absorption differed from that
in the jejunum by a relatively much
faster Step II. Unlike the low ratio of
Steps ll/l for the toxic metal in the
jejunum, the ratio for the essential
metals Cu and Zn was much higher
(about 50%). Absorption of Cd by the
gut in neonatal rats proceeds much
faster than in adults; reasons for this
difference have not yet been clarified.
Another question remaining under
study is the extent to which different
metals such as Cd and Zn share
common absorptive mechanisms.
This Project Summary was devel-
oped by EPA's Health Effects Research
Laboratory, Cincinnati, OH, to an-
nounce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The human environment contains a
variety of heavy metals, originating from
both natural and anthropogenic sources.
Some of these metals are clearly
essential for life, while for some others
a biological function has been sug-
gested. However, many of these ele-
ments, in the form of organic or
inorganic compounds, are highly toxic.
The level of such compounds in the
environment may therefore bear directly
on human health. Analysis of their
effects assumes additional urgency
because of the likelihood that ambient
concentrations of these metals may be
increasing.
A special problem in this regard is
posed by cadmium. Significant amounts
of this element are being added to the
environment through use of sewage
sludges and other fertilizers on agri-
cultural land, from combustion of fossil
fuels, and by other processes The
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problem is exacerbated by the fact that
the half life of Cd in the body, and
especially the kidneys, is very long; in
effect, Cd acts as a cumulative poison.
Its prime target organ is the kidney, and
nephropathy has been reported in
exposed human populations.
Main sources of the human body
burden of Cd outside of occupational
environments are Cd in food and
tobacco smoke. The non-smoker derives
most of his Cd through gastrointestinal
absorption. In spite of this fact, relatively
little is known about mechanisms of
intestinal Cd absorption, and about the
factors which on the whole reduce net
fractional absorption to only a few
percent of the oral load. This considera-
tion defined the objective of the research
descirbed in this report. A scientific
interest in the basic mechanism of
metal transport, and the possibility of
applying knowledge gained to the
control of metal absorption, formed the
starting points.
Throughout the work attempts were
made to follow absorption under as
physiological conditions as possible,
using isolated segments of intestine in
situ in the living animal. No attempt was
made to repeat in detail material already
published. Copies of papers and abstracts
based on these results are appended to
the report. These papers should be
consulted for details.
Results
The results reported here extend
earlier work by other investigators.
However, unlike several of these
investigators, the present findings were
made with intact intestinal segments in
situ at Cd concentrations one might
conceivably encounter in heavily polluted
areas. Previous work in many cases
used excessively high Cd concentra-
tions. In addition, the preferred tech-
niques often were the analysis of
absorption in the intact animal, or the
measurement of transport by everted
sacs of intestine in vitro. In both cases,
the avid retention of Cd in the intestinal
wall was a source of difficulty.
Thus, in sacs, cadmium is not likely to
diffuse across the submucosal tissues
into serosal fluid as readily as, for
instance, sugars or amino acids. There-
fore, the release of Cd from mucosa in
sacs may differ quantitatively from that
occurring under physiological condi-
tions. Also, in the intact animal entero-
hepatic recirculation makes it impossible
to obtain absolute values for the
unidirectional movement of Cd from
lumen into mucosa and blood. The
method employed in this study proved a
more appropriate procedure for the
detailed analysis of Cd absorption under
reasonably physiological conditions.
From results obtained the following
conclusions may be drawn:
(1) Cd is removed from the lumen of
the rat jejunum by a membrane-
related process which exhibits
saturation kinetics (Step I). After
short periods, essentially all Cd
thus removed can be recovered
from the mucosa. An activity
gradient exists along the jejunum.
(2) Step I or Cd transport is modulated
by bile salts as well as by a variety
of food constituents.
(3) Zinc interacts in an apparently
competitive manner with Cd for
transport by Step I (Figure 1). The
inhibitory effect of Zn on Cd
uptake is illustrated in Figure 2,
using more conventional units.
Unlike Cd, however, Zn is not
appreciably retained in the muco-
sa. In spite of the competition for
Step I, the transmural movement
of Zn and Cd is not mediated by
identical mechanisms.
1.8-
• 7.2H
So. 6-
00
-2?
o
0.4-
Zn 5.0 mM,
10
20
30 40 50
Cd
Figure 1. Effect of Zn and Cd up-
take. Cd was transported
out of the lumen at a con-
stand exponential rate for
30 mm. from which the
mean initial rate (M±SE)
of transport was calculated
in nmole Cd/g tissue/min
for 4 to 6 rats.
20-
18-
16-
14-
5 8-
! 6H
2-
0.02 mM Cd
072345
mM Zn
Figure 2. Effect of Zn on Cd uptake.
(4) Step II in the absorption of Cd, i.e.,
its movement from mucosa into
blood, proceeds at only 1 -2% of
the rate of Step I. Step II is in
series with Step I, and under4
present conditions determines^
the rate of Cd absorption into the
body.
(5) Metallothionein, the low molecu-
lar weight protein able to bind 7
moles Cd/mole, could not be
shown to play any role in the
aborption of Cd.
(6) Cd absorption in the duodenum
resembles qualitatively that de-
scribed for the jejunum. In con-
trast, Step II of Cd transport is
relatively much faster in the
ileum.
This investigation has contributed to a
better def i nition of factors which may be
responsible for the control of Cd
absorption in vivo. In addition, it has
confirmed the possibility of altering
fractional absorption of an oral load of
Cd by dietary manipulations. Work
along these lines is continuing.
The results of this research have been
published intwojournalarticles: "Some
determinants of intestinal cadmium
transport in the rat", by E.G. Foulkes, J.
Environ Path and Toxicol. 3.471-81
(1980), and "On the role of metallo-
thionem in cadmium absorption by rat
jejunum in situ", by D. Kello et al..
Toxicology 14:199-208 (1979).
I
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£. C Foulkes, D R. Johnson, N Sugawara, R. F. Bonewitz, andC, Voner are with
the University of Cincinnati, Cincinnati, OH 45267.
Norman E. Kowal is the EPA Project Officer (see below).
The complete report, entitled "Mechanisms of Cadmium Absorption in Rats,"
(Order No. PB 82-108 184: Cost. $8.00, subject to change) will be available
only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone 703-487-4650
The EPA Project Officer can be contacted at
Health Effects Research Laboratory
U S Environmental Protection Agency
Cincinnati, OH 45268
•ff U S GOVERNMENT PRINTING OFFICE 1982 — 559-017/7449
-------
E. C. Foulkes, D. Ft. Johnson, N. Sugawara, R. F. Bonewitz, andC. Voner are with
the University of Cincinnati, Cincinnati, OH 45267
Norman E. Kowal is the EPA Project Officer (see below).
The complete report, entitled "Mechanisms of Cadmium Absorption in Rats,"
(Order No PB 82-108 184. Cosf $8 00, subject to change) will be available
only from:
National Technical Information Service
5285 Port Royal Road
Spring field, VA 22161
Telephone 703-487-4650
The EPA Project Officer can be contacted at
Health Effects Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
•fa U S GOVERNMENT PRINTING OFFICE, 1982 — 559-017/7449
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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