Recovery Kinetics of Human ROD Photoresponses: A Model Suggested by The Two-Branched a-WAVE "Saturation Function"

Abstract

Recovery of the human ERG a-wave can be tracked using a paired-flash protocol, in which the test flash is followed at varying times by a bright probe flash that saturates the rods. Birch et al.1 used this approach to measure the period (T) of apparent saturation of the rod-mediated a-wave2 in relation to the intensity (If) of the test flash. Fig. 1 shows the "saturation function" (T vs. ln If function) obtained in a group of experiments. Over the investigated range of If (40 to 106 scot-td-s), the saturation function exhibits two essentially linear branches, the slopes of which [ΔT/Δ(ln If)] are ≃0.3 s and ≃2.3 s. Results obtained in studies of salamander rods3-5 led Birch et al. to hypothesize that the upper-branch slope of 2.3 s represents "τR*", the exponential lifetime of photoactivated rhodopsin (R*). The occurrence of two branches in the saturation function of Fig. 1 could in principle reflect a variation of τR* with flash intensity, i.e., an increase of τR* from -0.3 s to =2.3 s with increasing If. However, the evidence that τR* in salamander rods is unaffected by changing illumination conditions3-6 suggests that τR* in human rods may be constant. Here we present a model of rod transduction based on the assumption that τR* is constant. We show that this model can account for the two-branched property of the a-wave saturation function.