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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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Dependence of the acceleration of the cascade shut-off on the dark current suppression by background. Data for individual cells are shown by different symbols/line styles. (A) Time constant of the recovery phase τt. (B) Half-rising time t0.5. (C) Average ± SEM obtained by interpolation between experimental points. Filled circles, τt; empty circles, t0.5. Smooth curves show Hill-like approximations (Eq. 11) with the parameters given in the text.
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fig7: Dependence of the acceleration of the cascade shut-off on the dark current suppression by background. Data for individual cells are shown by different symbols/line styles. (A) Time constant of the recovery phase τt. (B) Half-rising time t0.5. (C) Average ± SEM obtained by interpolation between experimental points. Filled circles, τt; empty circles, t0.5. Smooth curves show Hill-like approximations (Eq. 11) with the parameters given in the text.

Mentions: There was a big scatter among cells with respect to steepness and magnitude of background-induced acceleration of turn-offs (Fig. 7, A and B). Acceleration is plotted versus fold dark current suppression Id/Ib, which is supposed to reflect the decline in the intracellular free Ca2+ level (Gray-Keller and Detwiler, 1994, 1996; Younger et al., 1996). Maximum acceleration of the tail was 5.5-fold at 58% (2.4-fold) dark current suppression (Fig. 7 A, the cell labeled by open diamonds). Maximum shortening of the front was 7.6-fold at 69% (3.2-fold) dark current suppression (Fig. 7 B, the cell labeled by filled triangles). By linear interpolation between data points for individual cells, we calculated an average acceleration versus dark current suppression functions (Fig. 7 C, filled and empty circles). To approximate front and tail acceleration data, we used a Hill-type equation:(11)\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*}a(f)=\frac{a_{{\mathrm{max}}}{\cdot}(kf)^{h}}{1+(kf)^{h}},\end{equation*}\end{document}where a(f) is the acceleration factor, f = Id/Ib, k is a scaling constant, h is the Hill coefficient, and amax = 1+1/kh. The function is forced to pass through (1, 1) point. The least-square fit yields amax = 3.6, h = 5.2, k = 0.83 for τt, and amax = 5, h = 5.8, k = 0.79 for t0.5. However, the steepest initial part of the curves can be fitted by a(f) = f h, with h = 2.5–3.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Dependence of the acceleration of the cascade shut-off on the dark current suppression by background. Data for individual cells are shown by different symbols/line styles. (A) Time constant of the recovery phase τt. (B) Half-rising time t0.5. (C) Average ± SEM obtained by interpolation between experimental points. Filled circles, τt; empty circles, t0.5. Smooth curves show Hill-like approximations (Eq. 11) with the parameters given in the text.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2571975&req=5

fig7: Dependence of the acceleration of the cascade shut-off on the dark current suppression by background. Data for individual cells are shown by different symbols/line styles. (A) Time constant of the recovery phase τt. (B) Half-rising time t0.5. (C) Average ± SEM obtained by interpolation between experimental points. Filled circles, τt; empty circles, t0.5. Smooth curves show Hill-like approximations (Eq. 11) with the parameters given in the text.
Mentions: There was a big scatter among cells with respect to steepness and magnitude of background-induced acceleration of turn-offs (Fig. 7, A and B). Acceleration is plotted versus fold dark current suppression Id/Ib, which is supposed to reflect the decline in the intracellular free Ca2+ level (Gray-Keller and Detwiler, 1994, 1996; Younger et al., 1996). Maximum acceleration of the tail was 5.5-fold at 58% (2.4-fold) dark current suppression (Fig. 7 A, the cell labeled by open diamonds). Maximum shortening of the front was 7.6-fold at 69% (3.2-fold) dark current suppression (Fig. 7 B, the cell labeled by filled triangles). By linear interpolation between data points for individual cells, we calculated an average acceleration versus dark current suppression functions (Fig. 7 C, filled and empty circles). To approximate front and tail acceleration data, we used a Hill-type equation:(11)\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*}a(f)=\frac{a_{{\mathrm{max}}}{\cdot}(kf)^{h}}{1+(kf)^{h}},\end{equation*}\end{document}where a(f) is the acceleration factor, f = Id/Ib, k is a scaling constant, h is the Hill coefficient, and amax = 1+1/kh. The function is forced to pass through (1, 1) point. The least-square fit yields amax = 3.6, h = 5.2, k = 0.83 for τt, and amax = 5, h = 5.8, k = 0.79 for t0.5. However, the steepest initial part of the curves can be fitted by a(f) = f h, with h = 2.5–3.

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