Renal filtration, reabsorption and excretion of aluminium in the rat

1. Plasma and urinary aluminium levels, and renal function, were investigated in a control group of rats (n = 5) and in two groups that received an intravenous bolus dose of aluminium chloride (either 25 μg or 800 μg of aluminium, n = 7 and 5, respectively).

2. In the control group (plasma aluminium concentration 76.8 ± 14.2 ng/ml), 59.4 ± 3.5% of the plasma aluminium was ultrafilterable. The percentage ultrafilterable after the administration of 25 μg of aluminium was 41.9 ± 7.8 (plasma concentration 154.3 ± 18.6 ng/ml). However, after administration of 800 μg of aluminium, to give a plasma concentration of 19800 ± 2956 ng/ml, only 1.06 ± 0.13% was ultrafilterable.

3. Such results have generally been interpreted as indicating an increase in protein-binding of aluminium with increasing aluminium concentration. In buffered aqueous solutions of aluminium chloride at pH 7.4, with an aluminium concentration of 189 ± 6 ng/ml, 96.12 ± 0.02% was ultrafilterable (n = 6). This concentration is comparable with that attained in the low-dose (25 μg) aluminium group of animals and suggests that the difference between the ultrafilterable percentage of aluminium in plasma compared with that in aqueous solution is indeed due to the binding of aluminium to high M_r material (proteins). In contrast, however, in an aqueous buffered (pH 7.4) solution containing 28200 ng of aluminium/ml, only 1.05 ± 0.09% was ultrafilterable. This indicates insolubility (i.e. colloid formation) of the aluminium at this high concentration. The same percentage (1.06 ± 0.13) was ultrafilterable from plasma from the high-dose (800 μg) aluminium group with a plasma aluminium concentration of 19 800 ± 2956 ng/ ml. This low ultrafilterable fraction is thus attributable to the insolubility of aluminium at this concentration, rather than to protein binding.

4. The fractional excretion of aluminium in the control group was approximately 10%. It was somewhat higher in the low-dose aluminium group, and reached 53% in the high-dose aluminium group. Nevertheless, it is clear that the kidneys are inefficient at excreting aluminium, so that after aluminium administration there is a prolonged period (several hours) during which the plasma aluminium concentration remains elevated and thus tissue deposition may occur.