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 (tau(c)) of about 2 s. The original hypothesis that tau(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 tau(c). A recently proposed alternative hypothesis, that tau(c) is the lifetime of activated transducin and that the R* lifetime is relatively short (approximately 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 tau(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 (approximately 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 tau(c). A central prediction of the model is the regulated transition of flash-activated R* to "R**", a state exhibiting greatly increased catalytic activity.