LIGHT ADAPTATION AND THE RISING PHASE OF THE FLASH PHOTOCURRENT OF SALAMANDER RETINAL RODS

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 in vestigated. 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 brigh t flashes, even in bright backgrounds. There is no effect for weak bac kgrounds. 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 reducti on in quantum catch. For moderate flashes, there is an extra reduction , similar to the reduction produced by the equivalent background. 5. T he results provide support for the idea that a reduction in the amplif ication step of phototransduction functions as part of the mechanism o f light adaptation in rods. The dependence on flash intensity of the b ackground-induced reduction in phosphodiesterase activation could impl y a feedback mechanism on the activation steps of phototransduction.