Dim background light and Cerenkov radiation from32P block reversal of rhodopsin phosphorylation in intact frog retinal rods

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Abstract
The phosphorylation of photoexcited rhodopsin (Rho*) is thought to inactivate this receptor by inhibiting its interaction with the GTP-binding protein transducin (Gt). Here we report that the time course of phosphorylation-dephosphorylation after bright illumination of intact rod outer and inner segments (ROS-RIS) incubated in33Pican be altered if the ROS-RIS are first exposed to levels of dim illumination that cause light adaptation in these ROS-RIS. The dephosphorylation of >107phosphorylated rhodopsin molecules/ROS following a bright flash can be blocked by prior dim continuous illumination (generating 103Rho*/ROS/s) that cumulatively bleaches ≍ 105rhodopsin molecules/ROS. The phenomenon has not been previously noted because these low levels of light are emitted as a result of Cerenkov radiation from the32P isotope that is usually employed to monitor rhodopsin phosphorylation. The inhibition of rhodopsin dephosphorylation by dim conditioning illumination is observed in intact ROS-RIS but is lost when ROS-RIS are electropermeabilized or fragmented.