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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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Light adaptation shortens both the rising and falling phases of flash PDE response. Same cell as in Figs. 3 and 5. (A) In this and many other cells, responses on backgrounds of varying intensity can be made to coincide with the dark-adapted response by proper expansion along the time axis. Legend near the curves shows the fraction of dark current blocked by the background, and corresponding expansion factor. (B) Curves from A plotted on log scale against common time axis. Solid straight lines show exponential approximation of the recovery phase. (C) Fronts of the curves from A, with circles marking half-rising time t0.5.
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fig6: Light adaptation shortens both the rising and falling phases of flash PDE response. Same cell as in Figs. 3 and 5. (A) In this and many other cells, responses on backgrounds of varying intensity can be made to coincide with the dark-adapted response by proper expansion along the time axis. Legend near the curves shows the fraction of dark current blocked by the background, and corresponding expansion factor. (B) Curves from A plotted on log scale against common time axis. Solid straight lines show exponential approximation of the recovery phase. (C) Fronts of the curves from A, with circles marking half-rising time t0.5.

Mentions: It appeared that in many rods normalized light-adapted PDE responses could be made to virtually coincide with the dark response by a simple expansion along the time axis (Fig. 6 A). This suggests that both shut-off processes shaping the response are under light-adaptation control and are accelerated by background light to approximately the same extent. In the rod shown in Fig. 6, the background that reduced circulating current by 27% resulted in a 2.5-fold acceleration of the entire βfl(t) curve. The background light that closed 42% of the channels produced an ≈4.6-fold acceleration.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Light adaptation shortens both the rising and falling phases of flash PDE response. Same cell as in Figs. 3 and 5. (A) In this and many other cells, responses on backgrounds of varying intensity can be made to coincide with the dark-adapted response by proper expansion along the time axis. Legend near the curves shows the fraction of dark current blocked by the background, and corresponding expansion factor. (B) Curves from A plotted on log scale against common time axis. Solid straight lines show exponential approximation of the recovery phase. (C) Fronts of the curves from A, with circles marking half-rising time t0.5.
© Copyright Policy
Related In: Results  -  Collection

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

fig6: Light adaptation shortens both the rising and falling phases of flash PDE response. Same cell as in Figs. 3 and 5. (A) In this and many other cells, responses on backgrounds of varying intensity can be made to coincide with the dark-adapted response by proper expansion along the time axis. Legend near the curves shows the fraction of dark current blocked by the background, and corresponding expansion factor. (B) Curves from A plotted on log scale against common time axis. Solid straight lines show exponential approximation of the recovery phase. (C) Fronts of the curves from A, with circles marking half-rising time t0.5.
Mentions: It appeared that in many rods normalized light-adapted PDE responses could be made to virtually coincide with the dark response by a simple expansion along the time axis (Fig. 6 A). This suggests that both shut-off processes shaping the response are under light-adaptation control and are accelerated by background light to approximately the same extent. In the rod shown in Fig. 6, the background that reduced circulating current by 27% resulted in a 2.5-fold acceleration of the entire βfl(t) curve. The background light that closed 42% of the channels produced an ≈4.6-fold acceleration.

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