Ionic flux and membrane characteristics of isolated rod outer segments

Abstract

The osmotic behavior of freshly isolated rod outer segments reveals that Na ions can flow rapidly into dark-adapted outer segments through the plasma membrane, but K ions cannot. The Na flux is rectified since the rate of Na influx is found to be at least a hundred times larger than its rate of efflux. The influx of Na is light-sensitive: illumination of the outer segments produces a specific increase in the resistance of the plasma membrane to Na influx. This Na resistance increases linearly with the number of rhodopsin molecules bleached. In frog outer segments the rate of the Na entry is permanently reduced by half when about 10 6 rhodopsin molecules are bleached per rod. At lower light levels the rate of Na entry is transiently (∼ 1 sec) reduced by half when about 75 rhodopsin molecules are bleached per rod per sec. For a Na dark flux of 2 × 10 9 Na/rod sec this means that excitation of a single rhodopsin molecule can prevent about 10 7 Na ions from entering the outer segment. This is the same photosensitivity which has been described for the intact receptor cell in the living retina.