The effect of ions on the photoresponses of pigeon cones.

1. Pigeon cone receptor potentials have been identified in a preparation of isolated, incubated retina.2. There is an approximately linear relationship between response amplitude and sodium concentration of the bathing medium.3. A decrease of chloride ions produces results equivalent to a decrease of sodium ions. The response amplitude is a power function of the product of the sodium and chloride concentrations. For most experiments, the exponent is ca. 1.4. After treatment of the retina with ouabain, responses vanish, but can be temporarily restored, in normal, or inverted polarity, by appropriate changes in sodium or chloride concentration.5. Ammonium and potassium ions can substitute quantitatively for sodium ions. Lithium ions initially carry response currents, but later block all responses. Bromide, nitrate and thiocyanate can substitute for chloride ions.6. The response wave form, and the amplitude/light intensity relationship are independent of ions.7. Models of the photoreceptor are discussed. It is concluded that the membrane potential of the cone in light is chiefly determined by the distribution of anions across the membrane, and in darkness there is an increase in outer limb cation permeability.