The AT (amplified transient) signal is a flash-induced increase of the near-infrared light scattering from isolated bovine rod outer segments and is interpreted as a monitor of cGMP-phosphodiesterase activation [(1985) FEBS Lett. 188 15-20]. We have investigated the effects of ATP and cyclic GMP on this signal. It has been found that ATP enhances the AT signal the relative effect being the largest for low photoexcitation (~ 1 rhodopsin per disc membrane). At a high rhodopsin turnover which saturates the AT amplitude the effect of ATP is to accelerate the rise of the signal. ATP can also accelerate the falling phase of the signal. This deactivating effect depends on the simultaneous presence of cyclic GMP. The results indicate that ATP acts on the phosphodiesterase activation cycle promoting activation as well as deactivation dependent on cGMP as a cofactor.
