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Department of Physiology, Boston University School of Medicine, MA 02118, USA.

1. Both theory and analysis of photocurrents in retinal rods show that phosphodiesterase activity after a flash rises initially as a delayed ramp. 2. The effect of light adaptation on the flash-induced rise in phosphodiesterase activity deduced from photocurrent responses was investigated. 3. Background adaptation reduces the deduced rate of rise of phosphodiesterase activity. The effect is most prominent for bright backgrounds and moderate flashes. There is little reduction for bright flashes, even in bright backgrounds. There is no effect for weak backgrounds. 4. Light adaptation after bleaching visual pigment produces a reduction in the deduced rise of phosphodiesterase activity for all flashes. For bright flashes, the reduction is explained by the reduction in quantum catch. For moderate flashes, there is an extra reduction, similar to the reduction produced by the equivalent background. 5. The results provide support for the idea that a reduction in the amplification step of phototransduction functions as part of the mechanism of light adaptation in rods. The dependence on flash intensity of the background-induced reduction in phosphodiesterase activation could imply a feedback mechanism on the activation steps of phototransduction.

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