Molecular and thermal origins of fast photoelectric effects in the squid retina.

When a short, intense flash of light is absorbed by the outer segments of squid photoreceptors fixed in glutaraldehyde, a voltage appears briefly across the retina. The waveform depends on the relative amounts of rhodopsin and its various stable photoproducts present at the beginning of the flash. Each light-absorbing species present contributes a characteristic voltage component which is summed in the gross waveform. The heating effect of the absorbed light produces a small, long-lasting thermoelectric voltage as well. When this thermal effect is corrected for, interconversion of equal numbers of rhodopsin and acid metarhodopsin molecules by a flash results in a fast voltage waveform whose time integral is zero. Thus the charge flowing in one direction in the retina when rhodopsin is converted to acid metarhodopsin by one photon is apparently exactly reversed when acid metarhodopsin is reconverted to rhodopsin by another.