Efflux of potassium from isolated rod outer segments: a photic effect.

1. Illumination of the isolated outer segments of rod photoreceptors loaded with (42)K or (86)Rb reduces the efflux of these ions.2. During the perfusion of the isolated rod outer segments with a solution containing only 2.36 mM-Na the effect of light is absent, and the amplitude of the photic effect is linearly related to the logarithm of the extracellular Na concentration.3. In darkness, raising the concentration of K in the fluid of perfusion gives an increase of the efflux of (86)Rb and increasing the extracellular concentration of Ca yields a retention. The efflux of (86)Rb and (42)K is greater in darkness when sucrose or choline substitute for Na.4. It is suggested that in darkness the isolated outer segments are permeable both to Na and to K. Light appears to decrease the permeability for Na ions. There is no evidence that the permeability for K ions is modified by light.