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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 does not affect the cascade amplification. Inset shows fronts of two current responses to saturating flashes of nearly identical intensities (filled circles, dark adaptation; open circles, background blocking 45% of the dark current). Smooth lines are fits to the data with Eq. 12. No reduction of amplification by light adaptation is seen. Main panel shows the apparent amplification constant (A in Eq. 12) as a function of the flash strength and adaptation state. Smooth exponential line is drawn to visually connect the points and bears no mechanistic meaning. Same cell as in Fig. 8.
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fig9: Light adaptation does not affect the cascade amplification. Inset shows fronts of two current responses to saturating flashes of nearly identical intensities (filled circles, dark adaptation; open circles, background blocking 45% of the dark current). Smooth lines are fits to the data with Eq. 12. No reduction of amplification by light adaptation is seen. Main panel shows the apparent amplification constant (A in Eq. 12) as a function of the flash strength and adaptation state. Smooth exponential line is drawn to visually connect the points and bears no mechanistic meaning. Same cell as in Fig. 8.

Mentions: The horizontal shift of the Pepperberg line induced by steady backgrounds characterizes desensitization due to light adaptation. On average, the background light that resulted in closure of 50% of the light-sensitive channels produced 41 ± 17-fold desensitization (four cells, mean ± SEM). Maximum desensitization observed was 85-fold at the background that closed 55% dark current channels. To test whether the decrease of amplification within the cascade contributes to desensitization, we determined the amplification by fitting the rising phases of normalized saturated responses by(12)\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}r(t)/r_{{\mathrm{max}}}=1-{\mathrm{exp}}(-{\frac{1}{2}}A{\cdot}R^{{^\ast}}{\cdot}(t-t_{0})^{2}),\end{equation*}\end{document}where t is time, t0 is the transduction delay, R* is the flash intensity expressed in the number of activated rhodopsins, and A is the amplification (Pugh and Lamb, 2000). An example of the fit is given in the inset in Fig. 9. Symbols show the front of the response to ≈3,000 R* in dark adaptation (solid dots) and on 50% background (circles), of the cell whose Tsat versus I functions are plotted in Fig. 8. Smooth lines are fits to each set of data with Eq. 12. No change in the cascade amplification is seen.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Light adaptation does not affect the cascade amplification. Inset shows fronts of two current responses to saturating flashes of nearly identical intensities (filled circles, dark adaptation; open circles, background blocking 45% of the dark current). Smooth lines are fits to the data with Eq. 12. No reduction of amplification by light adaptation is seen. Main panel shows the apparent amplification constant (A in Eq. 12) as a function of the flash strength and adaptation state. Smooth exponential line is drawn to visually connect the points and bears no mechanistic meaning. Same cell as in Fig. 8.
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getmorefigures.php?uid=PMC2571975&req=5

fig9: Light adaptation does not affect the cascade amplification. Inset shows fronts of two current responses to saturating flashes of nearly identical intensities (filled circles, dark adaptation; open circles, background blocking 45% of the dark current). Smooth lines are fits to the data with Eq. 12. No reduction of amplification by light adaptation is seen. Main panel shows the apparent amplification constant (A in Eq. 12) as a function of the flash strength and adaptation state. Smooth exponential line is drawn to visually connect the points and bears no mechanistic meaning. Same cell as in Fig. 8.
Mentions: The horizontal shift of the Pepperberg line induced by steady backgrounds characterizes desensitization due to light adaptation. On average, the background light that resulted in closure of 50% of the light-sensitive channels produced 41 ± 17-fold desensitization (four cells, mean ± SEM). Maximum desensitization observed was 85-fold at the background that closed 55% dark current channels. To test whether the decrease of amplification within the cascade contributes to desensitization, we determined the amplification by fitting the rising phases of normalized saturated responses by(12)\documentclass[10pt]{article}\usepackage{amsmath}\usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{pmc}\usepackage[Euler]{upgreek}\pagestyle{empty}\oddsidemargin -1.0in\begin{document}\begin{equation*}r(t)/r_{{\mathrm{max}}}=1-{\mathrm{exp}}(-{\frac{1}{2}}A{\cdot}R^{{^\ast}}{\cdot}(t-t_{0})^{2}),\end{equation*}\end{document}where t is time, t0 is the transduction delay, R* is the flash intensity expressed in the number of activated rhodopsins, and A is the amplification (Pugh and Lamb, 2000). An example of the fit is given in the inset in Fig. 9. Symbols show the front of the response to ≈3,000 R* in dark adaptation (solid dots) and on 50% background (circles), of the cell whose Tsat versus I functions are plotted in Fig. 8. Smooth lines are fits to each set of data with Eq. 12. No change in the cascade amplification is seen.

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