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

INTRODUCTION
et al. 1993), probably through formation of insoluble complexes with aluminium ion in
the gut (Schaefer et al. 1988). Three mechanisms, reviewed by Greger and Sutherland
(1997), were proposed to explain the increase in aluminium absorption provoked by
citrate: enhanced aluminium solubility in the gastrointestinal tract, transport of
aluminium citrate into mucosal cells, and opening of epithelial tight junctions that are
present between mucosal cells (Taylor et al. 1998).
58
Aluminium absorption does not seem to happen in the stomach where most
aluminium is converted to soluble monomolecular species at low pH (Froment et al.
1989). At near-neutral pH aluminium precipitation occurs in the intestine.
Consequently, the small portion of aluminium accessible for transport is the part that
has been complexed with organic molecules in the stomach, allowing it to remain
soluble at higher pH of the small intestine (Reiber et al. 1995). The primary site of
aluminium absorption is the proximal intestine (Greger and Sutherland 1997). The
precise mechanism of gastrointestinal absorption is not yet fully known. It has been
suggested that intestinal absorption of aluminium occurs paracellularly along
enterocytes and through tight junctions by passives processes (Exley et al. 1996) and
transcellularly through enterocytes involving passive facilitated and active transport
processes, such as calcium uptake and sodium transport processes, and a role for
transferrin (Greger 1993, Greger and Sutherland 1997). Interestingly, it was suggested
that each aluminium species likely has its own absorption mechanism (Van der Voet
1992).
Intranasal absorption
Inhalation exposure to aluminium occurs from cosmetic (aerosols),
environmental and occupational sources (fumes, dusts, flakes). Inhaled aluminium was
suggested to accumulate in the brain through absorption via the olfactory system
(Roberts 1986, Exley et al. 1996) or through systemic absorption via the lung epithelia
(Gitelman et al. 1995) and through the gastrointestinal tract as particulates are
swallowed (Rollin et al. 1993). Pulmonary absorption seems to be more efficient than
gastrointestinal absorption. Actually, Jones and Benett (1986) estimated that