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Kinetics of turn-offs of frog rod phototransduction cascade.

Astakhova LA, Firsov ML, Govardovskii VI - J. Gen. Physiol. (2008)

Bottom Line: The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs.The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding.This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

View Article: PubMed Central - PubMed

Affiliation: Sechenov Institute for Evolutionary Physiology & Biochemistry, Russian Academy of Sciences, 194223 St. Petersburg, Russia.

ABSTRACT
The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs. The two processes are among the key factors that set the speed and sensitivity of the photoresponse and whose regulation contributes to light adaptation. The aim of this study was to determine time courses of flash-induced PDE activity in frog rods that were dark adapted or subjected to nonsaturating steady background illumination. PDE activity was computed from the responses recorded from solitary rods with the suction pipette technique in Ca(2+)-clamping solution. A flash applied in the dark-adapted state elicits a wave of PDE activity whose rising and decaying phases have characteristic times near 0.5 and 2 seconds, respectively. Nonsaturating steady background shortens both phases roughly to the same extent. The acceleration may exceed fivefold at the backgrounds that suppress approximately 70% of the dark current. The time constant of the process that controls the recovery from super-saturating flashes (so-called dominant time constant) is adaptation independent and, hence, cannot be attributed to either of the processes that shape the main part of the PDE wave. We hypothesize that the dominant time constant in frog rods characterizes arrestin binding to rhodopsin partially inactivated by phosphorylation. A mathematical model of the cascade that considers two-stage rhodopsin quenching and transducin inactivation can mimic experimental PDE activity quite well. The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding. This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

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Modeling the sensitivity reduction by light adaptation with accelerated cascade shut-offs. Experimental data from the same cell as in Figs. 8 and 9. Fold sensitivity reduction is defined as the shift of the Pepperberg plot along the intensity axis. Smooth lines are predictions of the three-stage quenching model described in the Appendix, with parameters given in the main text. Concurrent acceleration of rhodopsin phosphorylation and transducin shut-off is sufficient to reproduce experimental data fairy well. Acceleration factors are given near the curves.
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fig11: Modeling the sensitivity reduction by light adaptation with accelerated cascade shut-offs. Experimental data from the same cell as in Figs. 8 and 9. Fold sensitivity reduction is defined as the shift of the Pepperberg plot along the intensity axis. Smooth lines are predictions of the three-stage quenching model described in the Appendix, with parameters given in the main text. Concurrent acceleration of rhodopsin phosphorylation and transducin shut-off is sufficient to reproduce experimental data fairy well. Acceleration factors are given near the curves.

Mentions: One may tentatively attribute τD to the time constant of arrestin binding to rhodopsin partially quenched by phosphorylation. This means that the main phase of the cascade inactivation is controlled by rhodopsin phosphorylation and transducin/PDE shut-off. The two main processes shape the waveform of the photoresponse to nonsaturating stimuli. Arrestin binding is then responsible for a low-amplitude slow tail of the response that hardly can be revealed by routine curve fitting of noisy experimental recordings, as clearly seen from detailed mathematical modeling (Hamer et al., 2003, 2005). However, at bright flashes the main phase of quenching is complete while the cell still stays in saturation, so it is arrestin binding that now controls the recovery time. The process is believed to be Ca2+ independent, which would explain the constant slope of the Tsat versus ln(Intensity) plot regardless of the adaptation state (Fig. 8). A mathematical model based on this assumption provides a good description of the Pepperberg plot in dark adaptation and on light backgrounds (Fig. 11; see Appendix and the next section).


Kinetics of turn-offs of frog rod phototransduction cascade.

Astakhova LA, Firsov ML, Govardovskii VI - J. Gen. Physiol. (2008)

Modeling the sensitivity reduction by light adaptation with accelerated cascade shut-offs. Experimental data from the same cell as in Figs. 8 and 9. Fold sensitivity reduction is defined as the shift of the Pepperberg plot along the intensity axis. Smooth lines are predictions of the three-stage quenching model described in the Appendix, with parameters given in the main text. Concurrent acceleration of rhodopsin phosphorylation and transducin shut-off is sufficient to reproduce experimental data fairy well. Acceleration factors are given near the curves.
© Copyright Policy
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2571975&req=5

fig11: Modeling the sensitivity reduction by light adaptation with accelerated cascade shut-offs. Experimental data from the same cell as in Figs. 8 and 9. Fold sensitivity reduction is defined as the shift of the Pepperberg plot along the intensity axis. Smooth lines are predictions of the three-stage quenching model described in the Appendix, with parameters given in the main text. Concurrent acceleration of rhodopsin phosphorylation and transducin shut-off is sufficient to reproduce experimental data fairy well. Acceleration factors are given near the curves.
Mentions: One may tentatively attribute τD to the time constant of arrestin binding to rhodopsin partially quenched by phosphorylation. This means that the main phase of the cascade inactivation is controlled by rhodopsin phosphorylation and transducin/PDE shut-off. The two main processes shape the waveform of the photoresponse to nonsaturating stimuli. Arrestin binding is then responsible for a low-amplitude slow tail of the response that hardly can be revealed by routine curve fitting of noisy experimental recordings, as clearly seen from detailed mathematical modeling (Hamer et al., 2003, 2005). However, at bright flashes the main phase of quenching is complete while the cell still stays in saturation, so it is arrestin binding that now controls the recovery time. The process is believed to be Ca2+ independent, which would explain the constant slope of the Tsat versus ln(Intensity) plot regardless of the adaptation state (Fig. 8). A mathematical model based on this assumption provides a good description of the Pepperberg plot in dark adaptation and on light backgrounds (Fig. 11; see Appendix and the next section).

Bottom Line: The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs.The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding.This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

View Article: PubMed Central - PubMed

Affiliation: Sechenov Institute for Evolutionary Physiology & Biochemistry, Russian Academy of Sciences, 194223 St. Petersburg, Russia.

ABSTRACT
The time course of the light-induced activity of phototrandsuction effector enzyme cGMP-phosphodiesterase (PDE) is shaped by kinetics of rhodopsin and transducin shut-offs. The two processes are among the key factors that set the speed and sensitivity of the photoresponse and whose regulation contributes to light adaptation. The aim of this study was to determine time courses of flash-induced PDE activity in frog rods that were dark adapted or subjected to nonsaturating steady background illumination. PDE activity was computed from the responses recorded from solitary rods with the suction pipette technique in Ca(2+)-clamping solution. A flash applied in the dark-adapted state elicits a wave of PDE activity whose rising and decaying phases have characteristic times near 0.5 and 2 seconds, respectively. Nonsaturating steady background shortens both phases roughly to the same extent. The acceleration may exceed fivefold at the backgrounds that suppress approximately 70% of the dark current. The time constant of the process that controls the recovery from super-saturating flashes (so-called dominant time constant) is adaptation independent and, hence, cannot be attributed to either of the processes that shape the main part of the PDE wave. We hypothesize that the dominant time constant in frog rods characterizes arrestin binding to rhodopsin partially inactivated by phosphorylation. A mathematical model of the cascade that considers two-stage rhodopsin quenching and transducin inactivation can mimic experimental PDE activity quite well. The effect of light adaptation on the PDE kinetics can be reproduced in the model by concomitant acceleration on both rhodopsin phosphorylation and transducin turn-off, but not by accelerated arrestin binding. This suggests that not only rhodopsin but also transducin shut-off is under adaptation control.

Show MeSH
Related in: MedlinePlus