A Comparisons between predicted and experimental responses to steps of light. The two temporal phases of light adaptation operate at distinct levels of current suppression. Furthermore, in the intact rod, the close proximity of the exchanger to the channel37,38 causes the Ca2+ concentration localized around the channel to fall even more quickly than in the bulk cytoplasm see also Ref. Assuming that the Ca2+ diffusion coefficient is the same in the rod outer segments of other species, we can estimate the rate of propagation of the Ca2+ decline signal on the basis of the radii of the outer segments. A Schematic diagram of the rod photoreceptor showing the overall response of the cell to light. Abnormal photoresponses and light-induced apoptosis in rods lacking rhodopsin kinase. The solid lines in the panels of Figure 9 are fits to the data points based on a diffusion model that allows the estimation of the apparent Ca2+ diffusion coefficient that can account for the observed fluorescence profiles.
Trends Biochem Sci 1991; 16:387-391. In order to relate these effects of Ca2+ to the response of the cell, we also need the relation between Ca2+ concentration and light intensity. Light threshold is the intensity required to produce a criterion response. Calcium Channels at the Photoreceptor Synapse; S. The line through the points is the best fitting regression line.
The difference in saturation time after the two sets of steps over the linear regions gray vertical line was 11. In contrast, no significant effects were observed in rats treated with steroid or cyclosporin A. Rhodopsin phosphorylation in bovine rod outer segments is mor sensitive to the inhibitory action of recoverin at the low rhodopsin bleaching than it is at the high bleaching. Biochem Biophys Res Commun 1989; 162:544-549. Koutalos Y, Nakatani K, Yau K-W.
Therefore the underlying fast phase process es must have a time constant of approximately 1-2 s. Copyright 1999 Association for Research in Vision and Ophthalmology 114 H. An additional 47-fold sensitivity loss evolved with a time constant of 9. What might be the biological sense of high-gain rhodopsin phosphorylation? The curve is derived from j N x,0 τ N x as in the text eq. Purification and characterization of rhodopsin kinase.
Role of the acylated amio terminus of recoverin in Ca2+-dependent membrane interaction. A photoreceptor calcium binding protein is recognized by autoantibodies obtained from patients with cancer-associated retinopathy. This conclusion was based on the following experiments. For the cell in Fig. Only the final 2 s of the light exposure and the initial response recoveries are shown. Biochim Biophys Acta 2000; 1498:112-121. Recoverin is known to function as an inhibitor of rhodopsin kinase in a calcium dependent manner as shown in Figure 6.
What Ca2+-binding proteins are involved? B Contribution of the different Ca2+ modulations to the overall response of the cell. Eur J Biochem 1993; 21:881-890. The effects of Ca2+ on the different components of rod phototransduction have been characterized and can quantitatively account for the steady state responses of the rod cell to background illumination. This strongly suggests that a molecular mechanism s underlying light adaptation engaged between 2. While their overall conclusion appears to be similar to ours, several questions regarding their experimental designs and explanations of their data have arisen as described below.
Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients. Static and dynamic actions of cytoplasmic Ca2+ in the adaptation of responses to saturating flashes in salamander rods. . It was found that at high Ca2+, myristoylated recoverin prolonged the recovery phase of the bright flash response but had less effect on the dim flash response. Science 1993; 259 5096 :829-832. The exchanger has a Km for Ca2+ of 1-2 µM,9,42 and so, for the physiological Ca2+ concentrations, operates in the linear range.
Calcium Channels at the Photoreceptor Synapse -- 29. Data were obtained from truncated salamander rods in the presence of 0. The nature and identity of the internal excitational transmitter of vertebrate phototransduction. We substituted saturating steps for flashes to probe the adaptation that occurs during responses to continuous illumination. In the presence of the feedback, the light-sensitive current declines immediately after the switching on of the light, but after a while it slowly recovers to a new steady state. Given enough time, it will reach a steady level. Key among these are: what are the functional consequences of 475Ser phosphorylation? Calcium-bound recoverin targets rhodopsin kinase to membranes to inhibit rhodopsin phosphorylation.
Kawamura S, Hisatomi O, Kayada S et al. J Biol Chem 2000; 275:3313-3319. For the dimmest intensity, the response to a single trial is shown in gray. J Cell Biol 1967; 33:61-72. If intracellular Ca2+ is prevented from changing in a rod, neither the fast nor the slow phase of light adaptation appears to develop Fig. Therefore the negative results ought to be interpreted cautiously.
Invest Ophtalmol Vis Sci 1993; 34:81-90. J Neurosci 2000; 20 6 :2209-2217. The normalized time constant τN in sec. Localization of the sites for Ca2+-binding proteins on G protein-coupled recepor kinases. By 10 s, the loss in sensitivity reached 125-fold, while an additional 21-fold loss occurred over the next 50 s.