Some observations on the magnesium metabolism of the rabbit lens.

The (Mg) concentration in the rabbit lens is about 3.25 mmol/kg wet wt or 4.7 mM. This level is much greater than that in the bathing aqueous and vitreous humors. The Mg concentration in the peripheral part of this tissue is about four times greater than that in the inner 'nuclear' regions. Exchanges of Mg between the lens and its bathing fluids are very slow; under control conditions the lens appears to be virtually impermeable to Mg. Exposure to metabolic inhibitors, depolarizing concentrations of potassium, ouabain or replacement of Na with Li had little effect on the Mg content of the lens. Incubation of lenses in solutions containing EDTA or following freezing and thawing suggested that much of the Mg is present in a bound form. The ionophore A-23187 increased the permeability of the lens to Mg. Adjustment of the external Mg level in the presence of A-23187 indicated that the internal 'free' Mg concentration was about 3 mM. This result suggests that about 35% of the lenticular magnesium is bound.