Limits...
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.

Show MeSH

Related in: MedlinePlus

Independence of the dominant time constant of the cascade quenching from light adaptation. Saturation time versus flash intensity functions are shown for a dark-adapted rod and at two backgrounds of different intensities. Each point in dark-adapted state and at 45% background is the average of two recordings. Four recordings are averaged at brighter (72%) background. Recovery from saturation is taken as the moment of regaining 1.5 pA ROS current, which is equal to 10% dark current. Straight lines are least-square fits to data with Eq. 7; three initial points on the shallow part of the background curves are excluded from fitting. Numbers near the lines show τD values derived from the fit (mean ± SEM).
© Copyright Policy
Related In: Results  -  Collection

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

fig8: Independence of the dominant time constant of the cascade quenching from light adaptation. Saturation time versus flash intensity functions are shown for a dark-adapted rod and at two backgrounds of different intensities. Each point in dark-adapted state and at 45% background is the average of two recordings. Four recordings are averaged at brighter (72%) background. Recovery from saturation is taken as the moment of regaining 1.5 pA ROS current, which is equal to 10% dark current. Straight lines are least-square fits to data with Eq. 7; three initial points on the shallow part of the background curves are excluded from fitting. Numbers near the lines show τD values derived from the fit (mean ± SEM).

Mentions: The dominant shut-off time constant τD has previously been tentatively identified with transducin/PDE shut-off time constant τE in salamander (Lyubarsky et al., 1996; Nikonov et al., 2000). The conclusion was based on independence of τD from background light that would accelerate rhodopsin phosphorylation via Ca2+ feedback and thus reduce τR. We confirm background invariance of τD in R. ridibunda rods (Fig. 8). For example, in the cell shown in Fig. 8, the background illumination that closed 72% light-sensitive channels and accelerated the recovery from saturation by ≈7 s had no significant impact on the slope of the Pepperberg line. The same result was obtained in all rods tested (six cells) at background intensities blocking from 30 to 75% of the dark current.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Independence of the dominant time constant of the cascade quenching from light adaptation. Saturation time versus flash intensity functions are shown for a dark-adapted rod and at two backgrounds of different intensities. Each point in dark-adapted state and at 45% background is the average of two recordings. Four recordings are averaged at brighter (72%) background. Recovery from saturation is taken as the moment of regaining 1.5 pA ROS current, which is equal to 10% dark current. Straight lines are least-square fits to data with Eq. 7; three initial points on the shallow part of the background curves are excluded from fitting. Numbers near the lines show τD values derived from the fit (mean ± SEM).
© Copyright Policy
Related In: Results  -  Collection

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

fig8: Independence of the dominant time constant of the cascade quenching from light adaptation. Saturation time versus flash intensity functions are shown for a dark-adapted rod and at two backgrounds of different intensities. Each point in dark-adapted state and at 45% background is the average of two recordings. Four recordings are averaged at brighter (72%) background. Recovery from saturation is taken as the moment of regaining 1.5 pA ROS current, which is equal to 10% dark current. Straight lines are least-square fits to data with Eq. 7; three initial points on the shallow part of the background curves are excluded from fitting. Numbers near the lines show τD values derived from the fit (mean ± SEM).
Mentions: The dominant shut-off time constant τD has previously been tentatively identified with transducin/PDE shut-off time constant τE in salamander (Lyubarsky et al., 1996; Nikonov et al., 2000). The conclusion was based on independence of τD from background light that would accelerate rhodopsin phosphorylation via Ca2+ feedback and thus reduce τR. We confirm background invariance of τD in R. ridibunda rods (Fig. 8). For example, in the cell shown in Fig. 8, the background illumination that closed 72% light-sensitive channels and accelerated the recovery from saturation by ≈7 s had no significant impact on the slope of the Pepperberg line. The same result was obtained in all rods tested (six cells) at background intensities blocking from 30 to 75% of the dark current.

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