Early receptor potential measurements in human ocular siderosis.

There is electrophysiologic evidence that photoreceptors have normal visual pigment density (outer segment length) at a stage of ocular siderosis when the photoreceptor membrane function is defective. The early receptor potentials (ERPs) from two patients showed normal amplitudes, although the electroretinograms (ERGs) had reduced a- and b-waves for both cone and rod ERG responses. Since the ERP is generated by photolysis of visual pigment and requires orientation of the pigment molecules by outer segment disc membranes, the normal ERP amplitudes suggest that the photoreceptor outer segments have normal photopigment density, are of normal length, and are properly oriented. However, the transduction mechanism that converts visual pigment photolysis into membrane electrical potentials (the ERG a wave) is defective at this stage of ocular siderosis.