Does rod phototransduction involve the delayed transition of activated rhodopsin to a second, more active catalytic state?

Abstract

Recovery kinetics of the saturating photocurrent response in amphibian rods suggest regulation of the visual signal by a first-order deactivation reaction with an exponential time constant (τ_c) of about 2 s. The original hypothesis that τ_c represents the lifetime of activated rhodopsin (R^*) in a single-step deactivation appears at odds with several recent findings, for example, that Ca²⁺, a known regulator of the enzymatic phosphorylation of R^*, does not regulate the value of τ_c. A recently proposed alternative hypothesis, that τ_c is the lifetime of activated transducin and that the R^* lifetime is relatively short (∼0.4 s), appears consistent with the Ca²⁺ data but is difficult to reconcile with a high specific catalytic activity of R^*. The present theoretical study proposes a rate-equation model of R^* activation and deactivation in amphibian rods that is generally consistent with observed properties of the τ_c-associated reaction and the action of Ca²⁺ as well as with the stereotyped nature of the single-photon response. The model is developed by considering the effect of background light on a time-dependent variable, R_eff^*, defined as the effective total level of R^* activity. Central starting assumptions are that Ca²⁺ reduction mediates the effect of background light on R_eff^*(t) and that background desensitization of the photocurrent flash response derives from this action of Ca²⁺. Construction of the model is guided by criteria based on previous experimental findings. Among these are the approximate constancy of background desensitization expressed at near-peak and later times in the flash response, and the large (∼10-fold) dynamic range of this desensitization. The proposed model hypothesizes that an event regulated by Ca²⁺ feedback causes activated rhodopsin to become susceptible to a two-phase, stochastic deactivation process, the second phase of which is characterized by τ_c. A central prediction of the model is the regulated transition of flash-activated R^* to “R^**”, a state exhibiting greatly increased catalytic activity.