Light adaptation in vertebrate photoreceptors is thought to be mediated through a number of biochemical feedback reactions that reduce the sensitivity of the photoreceptor and accelerate the kinetics of the photoresponse. Ca 2� plays a major role in this process by regulating several components of the phototransduction cascade. Guanylate cyclase and rhodopsin kinase are suggested to be the major sites regulated by Ca 2� . Recently, it was proposed that cGMP may be another messenger of light adaptation since it is able to regulate the rate of transducin GTPase and thus the lifetime of activated cGMP phosphodiesterase. Here we report measurements of the rates at which the changes in Ca 2� and cGMP are followed by the changes in the rates of corresponding enzymatic reactions in frog rod outer segments. Our data indicate that there is a temporal hierarchy among reactions that underlie light adaptation. Guanylate cyclase activity and rhodopsin phosphorylation respond to changes in Ca 2� very rapidly, on a subsecond time scale. This enables them to accelerate the falling phase of the flash response and to modulate flash sensitivity during continuous illumination. To the contrary, the acceleration of transducin GTPase, even after significant reduction in cGMP, occurs over several tens of seconds. It is substantially delayed by the slow dissociation of cGMP from the noncatalytic sites for cGMP binding located on cGMP phosphodiesterase. Therefore, cGMP-dependent regulation of transducin GTPase is likely to occur only during prolonged bright illumination. tor outer segment (reviewed in Chabre and Deterre, regulation is guanylate cyclase, the enzyme responsible for cGMP synthesis. This regulation is conferred through 1989; Pugh and Lamb, 1990). Photoreceptors adapt to the Ca 2� binding proteins known as guanylate cyclase ambient light through feedback reactions that regulate activating proteins (GCAPs) (Palczewski et al., 1994; either the catalytic activity or the catalytic lifetime of individual components of the phototransduction cas- Dizhoon et al., 1995). Cyclase activity is low in darkness cade. It is well established that Ca 2� plays a significant and increases when Ca 2� levels drop in response to light. The third site is the regulation of the sensitivity of role in this process (reviewed by Lagnado and Baylor, the cationic channel to cGMP. Lowering Ca 2� causes 1992; Koutalos and Yau, 1993;Bownds and Arshavsky, 1995). Ca 2� declines in response to light because it can the channel to become more sensitive to cGMP due to no longer enter through the cationic channels while it the dissociation of calmodulin (CaM) or a closely related Ca 2� binding protein. This might result in the accontinues to be extruded through the Na/Ca,K excelerated recovery of the photoresponse by facilitating changer. There are at least three sites of Ca 2� regulathe reopening of channels. In addition, the gain of the tion in the cascade. The first is the regulation of the lifetime of light-activated rhodopsin. Rhodopsin is turned casc...