PHOSPHORYLATION OF NON-BLEACHED RHODOPSIN IN INTACT RETINAS AND LIVING FROGS

The photoresponse in retinal photoreceptors begins when a molecule of rhodopsin is excited by a photon of light. Photoexcited rhodopsin acti vates an enzymatic cascade including the G-protein transducin and cycl ic GMP phosphodiesterase. As a result, cytoplasmic cyclic GMP concentr ation is decreased and the photoresponse is initiated. This process is terminated when rhodopsin is phosphorylated by rhodopsin kinase and s ubsequently blocked by a protein called arrestin. It has been noted by several investigators that light can cause phosphorylation of not onl y photoexcited but also non-excited rhodopsin in rod photoreceptors. A goal of this study was to determine how much non-bleached rhodopsin i s phosphorylated. To determine how the structural integrity of the pho toreceptor influences the extent of non-breached rhodopsin phosphoryla tion, we studied the reaction in electropermeabilized rod outer segmen ts, in rod outer segments still attached to isolated retinas and in li ving frogs. In the first two preparations, we found that the maximum e xtent of non-bleached rhodopsin phosphorylation was approximately 1% o f the total rhodopsin pool. In living frogs, the maximal amount of non -bleached rhodopsin phosphorylation was similar to 2% of the total rho dopsin pool and occurred after prolonged illumination by the relativel y dim light intensity of 20 lux. These data appear to exclude models f or light adaptation that postulate high levels of phosphorylation of n on-bleached rhodopsin in rod photoreceptors.