A comparison of the behaviour to ions of the P III component of the pigeon cone and rat rod electroretinogram

1. Isolated pigeon and rat retinas were incubated in various media and the effect of changing ions on the P III component of the electroretinogram was studied, with particular regard to the influence of calcium on other ions. 2. In pigeon, without calcium, reduction of sodium and chloride both reduce P III. This behaviour continues in the presence of calcium but only if the retina has been treated with ouabain . Under these conditions, the effect of calcium is to prevent the appearance of inverted responses. 3. Under other conditions, calcium renders the pigeon P III insensitive to change in chloride. Sensitivity is rapidly restored by the use of ouabain, 2,4‐DNP, or by cooling. 4. Calcium does not alter the linear relationship between sodium and response amplitude. 5. In pigeon, increasing potassium increases P III amplitude, though the exact relationship is altered by calcium. 6. In the rat retina treated with ouabain , increasing sodium increases P III, and increasing potassium decreases P III. This result is similar to that of Sillman, Ito & Tomita (1969 b ). If calcium is absent, inverted P IIIs can be obtained. 7. If calcium is present in the medium, reducing sodium also reduces rat P III. 8. If calcium is reduced below 10 −7 M , then reducing sodium increases P III, and large responses are obtainable from solutions which contain almost no ions. These responses vanish very rapidly if the retina is cooled or treated with ouabain.