Mechanisms of aluminium neurotoxicity in oxidative stress-induced ...

Excretion
INTRODUCTION
As insoluble aluminium hydroxyl coumpounds are formed at neutral pH most of
the dietary aluminium is excreted in the faeces without ever being absorbed. If renal
functions are not compromised approximately 95% of the absorbed aluminium is
quickly excreted in the urine by kidneys, presumably as aluminium citrate. Biliary
system accounts for less than 2% of total aluminium elimination (Kovalchik et al. 1978,
Priest et al. 1995, Yokel et al. 1996). Decreased renal functionality increases the risk of
aluminium accumulation and toxicity (Greger and Sutherland 2007). Indeed, dialysis
encephalopathy syndrome (DES) was developed by renal-impaired people who received
aluminium in dialysis fluids or parenterally (Alfrey et al. 1976).
Elimination rate
Aluminium accumulation is not only due to impaired renal functions or exposure
to high quantities of aluminium but also occurs physiologically with aging (McDermott
et al. 1979). Aluminium contents of brain, serum, lungs, blood, liver, kidneys, and bone
have been demonstrated to increase with age (Markesbery et al. 1984, Zapatero et al.
1995, Greger and Sutherland 1997, Shimizu et al. 1994, U. S. Public Health Service
1992, Stitch 1957, Tipton and Shafer 1964, Roider and Drasch 1999, Markesbery et al.
1981) and younger individuals absorbe less aluminium than older people. Actually, it
has been estimated that aluminium deposits in the brain at a rate of 6 μg per year of life
(Edwardson 1991). This increasing body burden with age in the brain may be produced
by a slow, or no, elimination of aluminium coupled to a continued exposure to the metal
and a decreased ability to remove the metal from the brain with age. Different
aluminium half-lives (t½) have been reported suggesting that there is more than one
compartment of aluminium storage from which the metal is slowly eliminated (Wilhelm
et al. 1990, Ljunggren et al. 1991, Priest et al. 1995). The t½ of aluminium elimination
positively correlates with the duration of the exposure as longer t½ were observed when
the duration of sampling after exposure was increased. Bone stores about 58% of the
human aluminium body burden and its aluminium clearance is more rapid than from
brain, which is logical regarding to bone turnover and lack of neuron turnover.
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