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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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Finding flash-induced PDE activity from photoresponse recorded in Ca2+-clamping solution. (A) Current responses of a rod to the same 10-ms, 13 R* flash applied in normal Ringer (curve 1, average of four responses) and in 0 Na+, 0 Ca2+ solution (curve 2, single response). Responses are normalized to the dark current level present before the flash. (B) The wave of flash-induced PDE activity (noisy curve) calculated from the curve 2 in A using Eqs. 3–5 in the text. Assumed: βdark = 3.4 s−1, ncG = 3. Smooth curve shows an exponential approximation of the recovery phase, starting from 60% maximum downward. Circle marks the half-height of the rising phase.
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fig3: Finding flash-induced PDE activity from photoresponse recorded in Ca2+-clamping solution. (A) Current responses of a rod to the same 10-ms, 13 R* flash applied in normal Ringer (curve 1, average of four responses) and in 0 Na+, 0 Ca2+ solution (curve 2, single response). Responses are normalized to the dark current level present before the flash. (B) The wave of flash-induced PDE activity (noisy curve) calculated from the curve 2 in A using Eqs. 3–5 in the text. Assumed: βdark = 3.4 s−1, ncG = 3. Smooth curve shows an exponential approximation of the recovery phase, starting from 60% maximum downward. Circle marks the half-height of the rising phase.

Mentions: Fig. 3 A shows normalized nonsaturated current responses of a rod in a normal bath solution (curve 1) and after the jump into the jet of 0 Na+, 0 Ca2+ solution (curve 2). As expected, the trace in the Ca-clamping solution initially follows a normal response, but then reaches higher amplitude and lasts longer. After correcting for saturation, gL = 4.8 in this cell. Noisy line in Fig. 3 B shows the time course of the light-induced PDE activity, βfl(t), as derived from the curve 2 Fig. 3 A using Eqs. 3 and 5, and assuming ncG = 3.


Kinetics of turn-offs of frog rod phototransduction cascade.

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

Finding flash-induced PDE activity from photoresponse recorded in Ca2+-clamping solution. (A) Current responses of a rod to the same 10-ms, 13 R* flash applied in normal Ringer (curve 1, average of four responses) and in 0 Na+, 0 Ca2+ solution (curve 2, single response). Responses are normalized to the dark current level present before the flash. (B) The wave of flash-induced PDE activity (noisy curve) calculated from the curve 2 in A using Eqs. 3–5 in the text. Assumed: βdark = 3.4 s−1, ncG = 3. Smooth curve shows an exponential approximation of the recovery phase, starting from 60% maximum downward. Circle marks the half-height of the rising phase.
© Copyright Policy
Related In: Results  -  Collection

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

fig3: Finding flash-induced PDE activity from photoresponse recorded in Ca2+-clamping solution. (A) Current responses of a rod to the same 10-ms, 13 R* flash applied in normal Ringer (curve 1, average of four responses) and in 0 Na+, 0 Ca2+ solution (curve 2, single response). Responses are normalized to the dark current level present before the flash. (B) The wave of flash-induced PDE activity (noisy curve) calculated from the curve 2 in A using Eqs. 3–5 in the text. Assumed: βdark = 3.4 s−1, ncG = 3. Smooth curve shows an exponential approximation of the recovery phase, starting from 60% maximum downward. Circle marks the half-height of the rising phase.
Mentions: Fig. 3 A shows normalized nonsaturated current responses of a rod in a normal bath solution (curve 1) and after the jump into the jet of 0 Na+, 0 Ca2+ solution (curve 2). As expected, the trace in the Ca-clamping solution initially follows a normal response, but then reaches higher amplitude and lasts longer. After correcting for saturation, gL = 4.8 in this cell. Noisy line in Fig. 3 B shows the time course of the light-induced PDE activity, βfl(t), as derived from the curve 2 Fig. 3 A using Eqs. 3 and 5, and assuming 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