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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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Intensity dependence of the PDE activity. (A) Series of PDE responses of the same cell to flashes of various intensities computed assuming ncG = 3 (solid lines). Numbers near the curves show relative intensities of the flashes. Relative intensity 1 = 10 R*. Dashed lines are responses to weaker flashes scaled up to the brightest flash proportional to the intensity ratio. Trace I = 2.5 is average of three recordings; other traces, single recordings. (B, inset) Response versus intensity curves of the cell shown in A, plotted under assumption ncG = 3 (solid line through dots) or ncG = 2 (interrupted line through empty circles). ncG = 3 yields linear R versus I function, whereas that for ncG = 2 is nonlinear. Thus, the value ncG = 3 is accepted in all calculations.
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fig4: Intensity dependence of the PDE activity. (A) Series of PDE responses of the same cell to flashes of various intensities computed assuming ncG = 3 (solid lines). Numbers near the curves show relative intensities of the flashes. Relative intensity 1 = 10 R*. Dashed lines are responses to weaker flashes scaled up to the brightest flash proportional to the intensity ratio. Trace I = 2.5 is average of three recordings; other traces, single recordings. (B, inset) Response versus intensity curves of the cell shown in A, plotted under assumption ncG = 3 (solid line through dots) or ncG = 2 (interrupted line through empty circles). ncG = 3 yields linear R versus I function, whereas that for ncG = 2 is nonlinear. Thus, the value ncG = 3 is accepted in all calculations.

Mentions: The ground for choosing this particular value for ncG is explained in Fig. 4. Here, solid lines show a series of PDE responses to flashes of different intensities. The curves are calculated using ncG = 3.0 for the cooperativity of channel gating (see Eqs. 2, 3, and 5). Dashed lines show responses to two weaker flashes scaled up proportionally to the difference in intensity between them and the brightest flash. It is seen that the derived PDE responses scale linearly with the light intensity, as should be expected for the cascade that is far from saturation (see also Fig. 4 B, solid line). On the other hand, assuming ncG = 2 does not allow linear scaling (Fig. 4 B, dot-dashed line). In most cells, the test was not sensitive and only detected ncG changes >0.5. Computations on five cells yielded the average ncG = 3.2 ± 0.3. Therefore, in Fig. 3 and for further analysis we accept the whole number ncG = 3.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Intensity dependence of the PDE activity. (A) Series of PDE responses of the same cell to flashes of various intensities computed assuming ncG = 3 (solid lines). Numbers near the curves show relative intensities of the flashes. Relative intensity 1 = 10 R*. Dashed lines are responses to weaker flashes scaled up to the brightest flash proportional to the intensity ratio. Trace I = 2.5 is average of three recordings; other traces, single recordings. (B, inset) Response versus intensity curves of the cell shown in A, plotted under assumption ncG = 3 (solid line through dots) or ncG = 2 (interrupted line through empty circles). ncG = 3 yields linear R versus I function, whereas that for ncG = 2 is nonlinear. Thus, the value ncG = 3 is accepted in all calculations.
© Copyright Policy
Related In: Results  -  Collection

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

fig4: Intensity dependence of the PDE activity. (A) Series of PDE responses of the same cell to flashes of various intensities computed assuming ncG = 3 (solid lines). Numbers near the curves show relative intensities of the flashes. Relative intensity 1 = 10 R*. Dashed lines are responses to weaker flashes scaled up to the brightest flash proportional to the intensity ratio. Trace I = 2.5 is average of three recordings; other traces, single recordings. (B, inset) Response versus intensity curves of the cell shown in A, plotted under assumption ncG = 3 (solid line through dots) or ncG = 2 (interrupted line through empty circles). ncG = 3 yields linear R versus I function, whereas that for ncG = 2 is nonlinear. Thus, the value ncG = 3 is accepted in all calculations.
Mentions: The ground for choosing this particular value for ncG is explained in Fig. 4. Here, solid lines show a series of PDE responses to flashes of different intensities. The curves are calculated using ncG = 3.0 for the cooperativity of channel gating (see Eqs. 2, 3, and 5). Dashed lines show responses to two weaker flashes scaled up proportionally to the difference in intensity between them and the brightest flash. It is seen that the derived PDE responses scale linearly with the light intensity, as should be expected for the cascade that is far from saturation (see also Fig. 4 B, solid line). On the other hand, assuming ncG = 2 does not allow linear scaling (Fig. 4 B, dot-dashed line). In most cells, the test was not sensitive and only detected ncG changes >0.5. Computations on five cells yielded the average ncG = 3.2 ± 0.3. Therefore, in Fig. 3 and for further analysis we accept the whole number ncG = 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