Regional distribution of cGMP-activated ion channels in the plasma membrane of the rod photoreceptor.

In patch-clamp recordings from excised membrane patches, the distribution of cGMP-activated channels in the plasma membrane of the rod photoreceptor was examined. These channels have been shown to be the light-sensitive channels that carry the inward dark current in the rod outer segment, where phototransduction occurs (Matthews, 1987); thus, they are centrally involved in photoreceptor transduction. In the outer segment, cGMP-activated channels were present at high density. In the inner segment, cGMP-activated channels were also present, but their density was much lower than in the outer segment. Calcium-activated potassium channels had the opposite distribution, with a higher density in the inner segment. The results suggest that there is a barrier preventing the unrestricted spread of membrane proteins between the inner and outer segments.