Introduction
Chromium (Cr) is ubiquitous in the
environment occurring naturally in soils, rocks and living
organisms. Chromium exists primarily in two forms, trivalent Cr
(III) and hexavalent Cr (VI), with the latter primarily
produced by anthropogenic source (1). Chromium (III) is an
essential ultratrace element and plays an important role in the
biological system production of insulin (2). This element is
also produced by many different industries including welding
chrome plating, chrome pigmenting, leather tanning, wood
preserving, and in ferrochrome industry (3). Occupational
exposure to Cr (III) and Cr (VI) by inhalation depends upon the
job function and industry (4). Chromium enters the air and soil
mostly in the chromium (III) and chromium (IV) forms (5). In
the air, chromium compounds are presented mostly as fine dust
particles which eventually settle over land and water (6). The
chemistry of chromium is very interesting and complicated. The
inter-conversion of chromium (III) and chromium (IV) is
controlled by several factors including the presence and
concentrations of chromium species, oxidizing and reducing
agents, the electrochemical potentials of the oxidation and
reduction reactions, acid-base reactions, complex forming
agents, and so on. The reduction of Cr under physiological
conditions is illustrated by the following equation.
It is important to note that trivalent
chromium is the most stable from of chromium which can be
produce by Cr (VI) by a number of reductants (8-10) including
vitamin C, reduced glutathione (GSH) and cysteine. The initial
step involves a two electron reduction to Cr (IV) followed by
one electron reduction to Cr (III), in the presence of
intracellular reductants. This can produce a number of diseases
including bone and renal diseases (10, 11).
Patients with chronic renal failure have a
slightly higher mean serum chromium concentration than normal
subjects. Whereas, patients on hemodialysis have mean serum
chromium concentration over 20 times higher than normal
individuals (12). It has been reported that high serum chromium
concentration is a result of dialysis treatment and not of
renal failure (12) since transplanted patients have serum
chromium levels similar to those of chronic renal failure.
Thus, the restoration of normal kidney function by
transplantation leads to a drop in serum chromium concentration
to almost normal levels (14). The source of chromium is from
the hemodialysis concentrate and also dialysis apparatus but
not from the water supply as previously reported for aluminum
toxicity in these patients (15-16).
The increased body burden of chromium in
hemodialysis patients appears, therefore, to be confined to
plasma compartment where it binds to serum transferrin (17).
Serum human transferrin is a -glycoprotein with a
molecular weight of approximately 80 KD and it is the major
iron carrier protein in the plasma (18). Due to physiochemical
similarities with iron a number of other elements including Mn
(19), Zn (20), Cd (21) and Indium (22) bind to this protein in
the plasma.
Chromium transfers across dialysis
membrane and therefore binds to serum transferrin (17). It has
been reported that this binding activity may lead to the
disturbances of iron metabolism (23). Morris et al found that
the concentration of transferrin in patients with Alzheimer's
disease is much higher than normal subjects (24). Therefore,
the interaction of chromium with transferrin may disturb brain
function.
Therefore, the major aim of the present
study was to investigate the short and long term effects of
chromium on the level of rat brain catecholamines and
acetylcholinesterase activity.
Experimental
Animals
Male wistar rats weighing (100-150) gram
were purchased from Pasteur Institute (Tehran, Iran) and
maintained in animal house until the desired weight (200-220)
gram was attained. All rats were fed with Food and water under
standard condition. Four rats each served as experimental and
controls for each individual studied. The indicated dose of
chromium as chromium chloride was dissolved in saline and
injected intraperitoneally to experimental groups. Controls
were injected only with saline. The onset and duration of each
injection series are given in the tables. Animals were killed
by decapitation. Brains were carefully removed and dissected in
to cerebellum, mid-brain and brain cortex. Catecholamines
levels of each section were determined according to the method
described by Messripour and Haddady. Brain was homogenized in
acidic pH, centrifuged and the catecholamines fraction was
separated using Al2O3 and measured by using spectrofluorimetry
technique. Lowry's method was used for protein determination
(26). Blood samples were collected and sera were stored in
pre-acid washed tubes for chromium determination.
Acetylcholinesterase activity of brain
fractions was measured using the method of metanitrophenol
(27).
Chromium determinations were carried out
using a Perkin-Elmer (HG-Ao600) flameless atomic absorption
spectrophotometry as reported for aluminum determinations (28).
Chemicals
All chemicals were reagent grade and were
obtained from Sigma Chemical Company (Germany). Deionized water
was used throughout this project. Statistical analysis was done
using student's t-test.
Results and Discussion
Prior to study, the baseline serum
chromium concentrations of experimental and untreated control
animals were determined (Table 1). Daily administration of
chromium as CrCl3 led to the significant elevation of serum Cr
after 15 to 60 days of injection P<0.05. Administration of a
single dose of 8 mmol/kg of chromium in two hours reduced
catecholamines levels of cerebellum (22.8%), Mid-brain (19.4%)
and Brain-cortex (21.2%) in comparison to untreated chromium
controls (Table 2).Daily dose (38 µmol/kg) of chromium
for 15, 30 and 60 days reduced catecholamines levels of
cerebellum by 8.3, 14.3 and 32.8%, midbrain by 4.5, 8.6 and
20.3% and brain-cortex by 6.1, 10.4 and 21.3% respectively
(Table 3).
The short and long term effects of
chromium on different regions of rat brain acetylcholinesterase
activity were studied next.
A single dose of 8 mmole/kg of chromium
after two hours reduced acetylcholinesterase activity of
cerebellum (36.1%). Mid-brain (29.0%) and brain cortex by 26.7%
rrespectively table 2.
Administration of 38 µmol/kg of
chromium daily for 15, 30 and 60 days reduced cerebellum
acetylcholinesterase activity by 9.4, 17.2 and 27.1%, mid-brain
by 6.8, 13.4 and 22.6% and brain cortex by 7.2, 12.8 and 24.9%
respectively (Table 4).
Published studies from various countries
have documented significantly raised serum and whole blood
chromium concentrations in dialysis patients (29-30). Chromium
is widely used in many metal alloys and contaminations of the
dialysis fluid during manufacturing process leads to the
transfer of chromium into systemic circulation during the
dialysis process (29-31).
The probable mechanism by which chromium
causes neurological disease is still a matter of discussion.
Previously, it has been reported that transferrin which is an
iron carrier protein is also responsible for the transportation
of chromium into the circulation and it has high affinity for
chromium and transferrin receptors on the lumen of brain
capillaries (24) which may be able to mediate the uptake of
chromium in the brain.
Data which have been presented in this
manuscript show that a single dose of chromium caused an
approximate parallel reduction in the levels of catecholamines
and acetylcholinesterase activity of various part of brain.
When lower doses of chromium (2 mg/kg) were administered for 15
to 60 days, significant reductions in the levels of
catecholamines content and acetylcholinesterase activity were
seen particularly in 60 days of chromium administration since
both catecholamines and acetylcholinesterase are necessary for
biochemical function of brain and their reduction may be due to
the interference of high level of chromium with the synthesis
of any specific enzymes which may be responsible for the
production of catecholamines and acetylcholinesterase.
Alternatively chromium (VI)-containing compounds after
reduction to chromium (III), interfere with DNA synthesis in
the treated cells (32). Chromium treatment rapidly inhibits DNA
replication and secondarily blocks RNA and protein synthesis
(32), which seems to be possibly related to the depletion of
intracellular nucleotide triphosphates (adenylate and
guanylate) pools resulting from the formation of Cr (III)
dependent coordinate complexes with desoxynucleotid three
phosphate (dNTP). This might be considered for the reduction in
the production of acetylcholinesterase and also those enzymes
which are involved in the biochemical pathways of
catecholamine.
It has been already reported by this
laboratory and others that aluminum administration to rats
significantly reduces catecholamines content of cerebellum,
mid-brain and brain cortex (34). Due to the chemical
similarities between aluminum and chromium, both metals may
follow the same processes in the brain for the disturbances of
brain function. It has been also reported that when aluminum
salts are administered to experimental animals, a slow
progressive encephalopathy characterized by neurofibrillar
degeneration occurs (35). However, the exact mechanism by which
chromium interferes with brain function and causes neurological
disorders is not fully clear and similar to aluminum more
investigation should be done to elucidate this speculation.
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