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

Effect of light adaptation on the time course and amplification of PDE activation. Same cell as in Fig. 3. (A) Flash-induced PDE response in dark adaptation (curve 1) and on the steady backgrounds that was applied before the jump into 0 Na+, 0 Ca2+ solution and resulted in 27% (curve 2) and 42% (curve 3) suppression of the dark current. Responses are scaled to unity with respect to their peaks to more clearly show kinetic changes. Flash intensity 13 R* (dark), 159 R* (on background 170 R*s−1), and 82 R* (on background 490 R*s−1). Background-induced steady PDE activity (βs) is 2.7 s−1 on weaker and 3.6 s−1 on brighter background. (B) Same responses as in A, but the responses on background, instead of normalization to unity, are scaled down inversely proportional to the ratio of flash intensities. Fronts of the three curves virtually coincide, showing that the rate of activation of the cascade (amplification) is not affected by light adaptation.
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Effect of light adaptation on the time course and amplification of PDE activation. Same cell as in Fig. 3. (A) Flash-induced PDE response in dark adaptation (curve 1) and on the steady backgrounds that was applied before the jump into 0 Na+, 0 Ca2+ solution and resulted in 27% (curve 2) and 42% (curve 3) suppression of the dark current. Responses are scaled to unity with respect to their peaks to more clearly show kinetic changes. Flash intensity 13 R* (dark), 159 R* (on background 170 R*s−1), and 82 R* (on background 490 R*s−1). Background-induced steady PDE activity (βs) is 2.7 s−1 on weaker and 3.6 s−1 on brighter background. (B) Same responses as in A, but the responses on background, instead of normalization to unity, are scaled down inversely proportional to the ratio of flash intensities. Fronts of the three curves virtually coincide, showing that the rate of activation of the cascade (amplification) is not affected by light adaptation.

Mentions: Background light significantly accelerated the time course of the flash-induced PDE activity (Fig. 5 A). Remarkably, when flash responses in dark adaptation and on a steady background were normalized to flash intensity, light-adapted response returned to the pre-flash level earlier, yet rising phases of the two responses coincided (Fig. 5 B). This means that light adaptation accelerated the turn-off process(es) but had no effect on cascade amplification. The conclusion is robust for the background intensities that blocked not more than ≈50% of the dark current. At stronger backgrounds, irregular variations of the circulating current in Ca clamp conditions resulted in large scatter of deduced βfl(t) amplitudes among trials. However, the shape of the βfl(t) curve at a given background was well-reproducible, so further conclusions regarding the effect of light adaptation on the turn-off kinetics are reliable.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Effect of light adaptation on the time course and amplification of PDE activation. Same cell as in Fig. 3. (A) Flash-induced PDE response in dark adaptation (curve 1) and on the steady backgrounds that was applied before the jump into 0 Na+, 0 Ca2+ solution and resulted in 27% (curve 2) and 42% (curve 3) suppression of the dark current. Responses are scaled to unity with respect to their peaks to more clearly show kinetic changes. Flash intensity 13 R* (dark), 159 R* (on background 170 R*s−1), and 82 R* (on background 490 R*s−1). Background-induced steady PDE activity (βs) is 2.7 s−1 on weaker and 3.6 s−1 on brighter background. (B) Same responses as in A, but the responses on background, instead of normalization to unity, are scaled down inversely proportional to the ratio of flash intensities. Fronts of the three curves virtually coincide, showing that the rate of activation of the cascade (amplification) is not affected by light adaptation.
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Effect of light adaptation on the time course and amplification of PDE activation. Same cell as in Fig. 3. (A) Flash-induced PDE response in dark adaptation (curve 1) and on the steady backgrounds that was applied before the jump into 0 Na+, 0 Ca2+ solution and resulted in 27% (curve 2) and 42% (curve 3) suppression of the dark current. Responses are scaled to unity with respect to their peaks to more clearly show kinetic changes. Flash intensity 13 R* (dark), 159 R* (on background 170 R*s−1), and 82 R* (on background 490 R*s−1). Background-induced steady PDE activity (βs) is 2.7 s−1 on weaker and 3.6 s−1 on brighter background. (B) Same responses as in A, but the responses on background, instead of normalization to unity, are scaled down inversely proportional to the ratio of flash intensities. Fronts of the three curves virtually coincide, showing that the rate of activation of the cascade (amplification) is not affected by light adaptation.
Mentions: Background light significantly accelerated the time course of the flash-induced PDE activity (Fig. 5 A). Remarkably, when flash responses in dark adaptation and on a steady background were normalized to flash intensity, light-adapted response returned to the pre-flash level earlier, yet rising phases of the two responses coincided (Fig. 5 B). This means that light adaptation accelerated the turn-off process(es) but had no effect on cascade amplification. The conclusion is robust for the background intensities that blocked not more than ≈50% of the dark current. At stronger backgrounds, irregular variations of the circulating current in Ca clamp conditions resulted in large scatter of deduced βfl(t) amplitudes among trials. However, the shape of the βfl(t) curve at a given background was well-reproducible, so further conclusions regarding the effect of light adaptation on the turn-off kinetics are reliable.

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