Involvement of cGMP and calcium in the photoresponse in vertebrate photoreceptor cells.

The biochemical and physiological events responsible for reaction of the rod photoreceptor cells in the vertebrate eye have provided insight into the mechanisms involved in signal transduction in other cells. The light-mediated changes in the photopigment, rhodopsin, in the photoreceptive region of these photoreceptors initiates a cascade of events that result in changes in conductance through a specific ligand-gated receptor. This process involves activation of a specific heterotrimeric GTP-binding protein, transducin, and is mediated by changes in the intracellular concentration of two intracellular second messengers, cGMP and calcium. The current hypotheses are discussed of the mechanisms involved in various aspects of the photoresponse, particularly photoexcitation, termination, and recovery.