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 activates an enzymatic cascade including the G-protein transducin and cyclic GMP phosphodiesterase. As a result, cytoplasmic cyclic GMP concentration is decreased and the photoresponse is initiated. This process is terminated when rhodopsin is phosphorylated by rhodopsin kinase and subsequently blocked by a protein called arrestin. It has been noted by several investigators that light can cause phosphorylation of not only photoexcited but also non-excited rhodopsin in rod photoreceptors. A goal of this study was to determine how much non-bleached rhodopsin is phosphorylated. To determine how the structural integrity of the photoreceptor influences the extent of non-bleached rhodopsin phosphorylation, we studied the reaction in electropermeabilized rod outer segments, in rod outer segments still attached to isolated retinas and in living frogs. In the first two preparations, we found that the maximum extent of non-bleached rhodopsin phosphorylation was approximately 1% of the total rhodopsin pool. In living frogs, the maximal amount of non-bleached rhodopsin phosphorylation was approximately 2% of the total rhodopsin pool and occurred after prolonged illumination by the relatively dim light intensity of 20 lux. These data appear to exclude models for light adaptation that postulate high levels of phosphorylation of non-bleached rhodopsin in rod photoreceptors.