CYCLIC NUCLEOTIDES have been proposed as mediators of the decreased Na + permeability caused by illumination of vertebrate photoreceptors1,2. We show here that cyclic GMP depolarises the rod outer segment (ROS) approximately to the Na+ equilibrium potential within milliseconds after being injected intracellularly, that previous light adaptation antagonises this depolarisation, and that the injection of cyclic GMP without illumination initiates a repolarisation after a time lag which is longer than that following a light flash and proportional to the injection time. The results are interpreted in terms of a model in which cyclic GMP levels are controlled by the resultant of cyclase and phosphodiesterase (PDE) velocities. The PDE velocity of hydrolysis in the dark is increased by prior light adaptation in the presence of increased substrate (cyclic GMP). The data suggest that cyclic GMP is an important factor in the regulation of the membrane potential of the ROS.
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