ONSET OF FEEDBACK REACTIONS UNDERLYING VERTEBRATE ROD PHOTORECEPTOR LIGHT ADAPTATION

Light adaptation in vertebrate photoreceptors is thought to be mediate d through a number of biochemical feedback reactions that reduce the s ensitivity of the photoreceptor and accelerate the kinetics of the pho toresponse. Ca2+ plays a major role in this process by regulating seve ral components of the phototransduction cascade. Guanylate cyclase and rhodopsin kinase are suggested to be the major sites regulated by Ca2 +. Recently, it was proposed that cGMP may be another messenger of lig ht adaptation since it is able to regulate the rate of transducin GTPa se and thus the lifetime of activated cGMP phosphodiesterase. Here we report measurements of the rates at which the changes in Ca2+ and cGMP are followed by the changes in the rates of corresponding enzymatic r eactions in frog rod outer segments. Our data indicate that there is a temporal hierarcht among reactions that underlie light adaptation. Gu anylate cyclase activity and rhodopsin phosphorylation respond to chan ges in Ca2+ very rapidly, on a subsecond time scale. This enables them to accelerate the ailing 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 i n cGMP, occurs over several tens of seconds. It is substantially delay ed by the slow dissociation of cGMP from the noncatalytic sites for cG MP binding located an cGMP phosphodiesterase. Therefore, cGMP-dependen t regulation of transducin GTPase is likely to occur only during prolo nged bright illumination.