1. Photosensitivities of visual pigments were determined by measuring early receptor currents (ERCs) in voltage‐clamped photoreceptors from larval salamanders. 2. As expected from previous work of others, the ERC elicited by a brief flash consisted of a rapid inward component followed by a larger and slower outward component. The magnitude of the outward component corresponded to the movement of about 0.18 electronic charge across the membrane per photoisomerization. 3. The time course of the ERC was independent of the flash intensity, the flash wavelength and the magnitude of the response. The outward component of the cone ERC declined about twice as rapidly as the outward component of the rod ERC.. 4. The amplitude of the ERC decreased as successive flashes bleached the cell's pigment. Using the proportional relation between the size of the ERC and the number of pigment molecules photoisomerized, photosensitivities of the native A2 pigments in rods, red‐sensitive cones, blue‐sensitive cones and UV‐sensitive cones were determined. Calculated solution photosensitivities for rhodopsin, red‐sensitive and blue‐sensitive cone pigments were not significantly different and the average value for all three pigments at their respective absorption maxima was (7.3 +/‐ 1.6) x 10(‐9) micron 2 molecule‐1. A value of 44.0 x 10(‐9) micron 2 molecule‐1 was obtained in a single UV‐sensitive cone. 5. Substitution of the native dehydroretinal chromophore in the red‐sensitive cone pigment with 11‐cis‐retinal increased the solution photosensitivity to (9.6 +/‐ 0.62) x 10(‐9) micron 2 molecule‐1. 6. We conclude that cone pigments have large molecular absorption cross‐sections and high quantum efficiencies of photoisomerization. These properties seem well suited for the receptive molecules of a highly sensitive, miniaturized transducer.
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