Glucose-induced membrane permeability changes in the lens

Abstract

The effect of glucose and other hexose sugars on the electrical conductance of the amphibian lens was investigated using voltage-clamp techniques. Perifusing the lens with a Ringers solution made hyperosmotic by the addition of 40 m m-glucose to the control solution caused a steady increase in the conductance with the voltage clamped at the resting value ( ∼ −80 mV). Return to the control solution did not establish the original control conductance, but rather a further increase was observed. Perifusion with an isosmotic glucose-containing solution (40 m m-glucose added and 20 m m-NaCl removed) also caused the voltage-clamped conductance to increase and on return to the control solution a steady, raised baseline conductance was obtained. Similar conductance changes were observed in the presence of galactose, fructose and 3- o-methyl glucose and as the latter is not metabolized by the lens it is unlikely that the conductance changes are due to an accumulation of sugar alcohol. They are in fact interpreted in terms of a swelling of the lens in the presence of a permeant solute. Small fluctuations in external glucose levels, such as occur in diabetes, thus cause marked changes in membrane permeability and these changes are likely to impose an additional strain on lens volume regulation. Phloretin, which is an inhibitor of glucose transport, also significantly increased the lens conductance in concentrations as low as 10 −5 m. As there was a concomitant hyperpolarization of the lens membrane potential, the conductance change was probably due to an increase in potassium permeability. Similar changes have also been reported in nervous tissue in the presence of low concentrations of phloretin.