Rhodopsin kinetics in the cat retina.

Abstract

The bleaching and regeneration of rhodopsin in the living cat retina was studied by means of fundus reflectometry. Bleaching was effected by continuous light exposures of 1 or 20 min, and the changes in retinal absorbance were measured at 29 wavelengths. For all the conditions studied (fractional bleaches of from 65-100%), regeneration of rhodopsin to its prebleach levels required > 60 min in darkness. After 1 min exposure, the difference spectra recorded during the first 10 min of dark adaptation were dominated by photoproduct absorption, and rhodopsin regeneration kinetics were obscured by these intermediate processes. Extending the bleaching duration to 20 min gave the products of photolysis an opportunity to dissipate, and the regenerative process could be followed over its full time-course. It was not possible to fit these data with the simple exponential function predicted by 1st-order reaction kinetics. Other possible mechanisms were considered and are presented in the text. The kinetics of regeneration compared favorably with the temporal changes in log sensitivity determined electrophysiologically by other investigators. Based on the bleaching curve for cat rhodopsin, the photosensitivity was determined and approximated closely the value obtained for human rhodopsin; i.e., the energy Ec required to bleach 1 - e-1 of the available rhodopsin was 7.09 log scotopic troland-seconds (corrected for the optics of the cat eye), as compared with .apprx. 7.0 in man.