The role of divalent cations in controlling amphibian lens membrane permeability; the mechanisms of toxic cataracts.

The effects of a range of divalent ions on lens sodium and potassium permeability characteristics were studied in calcium competition and replacement experiments. Resting voltage and conductance were measured and also voltage-independent conductance. Strontium and manganese were the only divalent ions able to maintain, in the absence of calcium, both sodium and potassium permeability at or near the control level. Neither cobalt nor magnesium had any effect on lens voltage of conductance in the presence of calcium, but neither of these ions could maintain lens permeability properties in the absence of calcium. Cadmium and barium had little effect on sodium permeability, but the former increased potassium permeability while the latter reduced it. Barium was the only divalent studied that appeared to inactivate voltage-sensitive potassium channels in the presence of calcium. Nickel, zinc and copper increased both sodium and potassium permeability in the presence of calcium and so they are likely to be particularly damaging to the lens. Copper was extremely toxic since it was able to overturn the regulatory influence of calcium when it was present in concentrations as low as 10(-6)M.