Actions of Ca2+ on an Early Stage in Phototransduction Revealed by the Dynamic Fall in Ca2+ Concentration during the Bright Flash Response

To study the actions of Ca2+ on “early” stages of the transduction cascade, changes in cytoplasmic calcium concentration (Ca2+i) were opposed by manipulating Ca2+ fluxes across the rod outer segment membrane immediately following a bright flash. If the outer segment was exposed to 0 Ca2+/0 Na+ solution for a brief period immediately after the flash, then the period of response saturation was prolonged in comparison with that in Ringer solution. But if the exposure to 0 Ca2+/0 Na+ solution instead came before or was delayed until 1 s after the flash then it had little effect. The degree of response prolongation increased with the duration of the exposure to 0 Ca2+/0 Na+ solution, revealing a time constant of 0.49 ± 0.03 s. By the time the response begins to recover from saturation, Ca2+i seems likely to have fallen to a similar level in each case. Therefore the prolongation of the response when Ca2+i was prevented from changing immediately after the flash seems likely to reflect the abolition of actions of the usual dynamic fall in Ca2+i on an early stage in the transduction cascade at a site which is available for only a brief period after the flash. One possibility is that the observed time constant corresponds to the phosphorylation of photoisomerized rhodopsin.

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Prolongation of Actions of Ca2+ Early in Phototransduction by 9-Demethylretinal

The Journal of General Physiology ◽

10.1085/jgp.118.4.377 ◽

2001 ◽

Vol 118 (4) ◽

pp. 377-390 ◽

Cited By ~ 6

Author(s):

Hugh R. Matthews ◽

M.C. Cornwall ◽

R.K. Crouch

Keyword(s):

Outer Segment ◽

Time Course ◽

Calcium Concentration ◽

Surface Membrane ◽

Background Intensity ◽

Cytoplasmic Calcium ◽

Response Recovery ◽

Bright Flash ◽

Flash Response

During adaptation Ca2+ acts on a step early in phototransduction, which is normally available for only a brief period after excitation. To investigate the identity of this step, we studied the effect of the light-induced decline in intracellular Ca2+ concentration on the response to a bright flash in normal rods, and in rods bleached and regenerated with 11-cis 9-demethylretinal, which forms a photopigment with a prolonged photoactivated lifetime. Changes in cytoplasmic Ca2+ were opposed by rapid superfusion of the outer segment with a 0Na+/0Ca2+ solution designed to minimize Ca2+ fluxes across the surface membrane. After regeneration of a bleached rod with 9-demethlyretinal, the response in Ringer's to a 440-nm bright flash was prolonged in comparison with the unbleached control, and the response remained in saturation for 10–15s. If the dynamic fall in Ca2+i induced by the flash was delayed by stepping the outer segment to 0Na+/0Ca2+ solution just before the flash and returning it to Ringer's shortly before recovery, then the response saturation was prolonged further, increasing linearly by 0.41 ± 0.01 of the time spent in this solution. In contrast, even long exposures to 0Na+/0Ca2+ solution of rods containing native photopigment evoked only a modest response prolongation on the return to Ringer's. Furthermore, if the rod was preexposed to steady subsaturating light, thereby reducing the cytoplasmic calcium concentration, then the prolongation of the bright flash response evoked by 0Na+/0Ca2+ solution was reduced in a graded manner with increasing background intensity. These results indicate that altering the chromophore of rhodopsin prolongs the time course of the Ca2+-dependent step early in the transduction cascade so that it dominates response recovery, and suggest that it is associated with photopigment quenching by phosphorylation.

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Origin and control of the dominant time constant of salamander cone photoreceptors

The Journal of General Physiology ◽

10.1085/jgp.201110762 ◽

2012 ◽

Vol 140 (2) ◽

pp. 219-233 ◽

Cited By ~ 5

Author(s):

Jingjing Zang ◽

Hugh R. Matthews

Keyword(s):

Time Constant ◽

External Solution ◽

Light Response ◽

Response Recovery ◽

Bright Flash ◽

Active Intermediates ◽

And Control ◽

Flash Response ◽

Phototransduction Cascade ◽

External Ph

Recovery of the light response in vertebrate photoreceptors requires the shutoff of both active intermediates in the phototransduction cascade: the visual pigment and the transducin–phosphodiesterase complex. Whichever intermediate quenches more slowly will dominate photoresponse recovery. In suction pipette recordings from isolated salamander ultraviolet- and blue-sensitive cones, response recovery was delayed, and the dominant time constant slowed when internal [Ca2+] was prevented from changing after a bright flash by exposure to 0Ca2+/0Na+ solution. Taken together with a similar prior observation in salamander red-sensitive cones, these observations indicate that the dominance of response recovery by a Ca2+-sensitive process is a general feature of amphibian cone phototransduction. Moreover, changes in the external pH also influenced the dominant time constant of red-sensitive cones even when changes in internal [Ca2+] were prevented. Because the cone photopigment is, uniquely, exposed to the external solution, this may represent a direct effect of protons on the equilibrium between its inactive Meta I and active Meta II forms, consistent with the notion that the process dominating recovery of the bright flash response represents quenching of the active Meta II form of the cone photopigment.

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High time resolution enzyme cytochemistry of rod outer segment

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100161953 ◽

1990 ◽

Vol 48 (3) ◽

pp. 878-879

Author(s):

Takuma Saito ◽

Toshihiro Takizawa

Keyword(s):

Enzyme Activities ◽

Outer Segment ◽

Rapid Change ◽

High Time Resolution ◽

Visual Cell ◽

Enzyme Cytochemistry ◽

Activation Effect ◽

Rod Outer Segment ◽

Enzymatic Cascades ◽

Rapid Drop

Cells and tissues live on a number of dynamic metabolic pathways, which are made up of sequential enzymatic cascades.Recent biochemical and physiological studies of vision research showed the importance of cGMP metabolism in the rod outer segment of visual cell, indicat ing that the photon activated rhodopsin exerts activation effect on the GTP binding protein, transducin, and this act ivated transducin further activates phosphodiesterase (PDEase) to result in a rapid drop in cGMP concentration in the cytoplasm of rod outer segment. This rapid drop of cGMP concentration exerts to close the ion channel on the plasma membrane and to stop of inward current brings hyperpolarization and evokes an action potential.These sequential change of enzyme activities, known as cGMP cascade, proceeds quite rapidly within msec order. Such a rapid change of enzyme activities, such as PDEase in rod outer segment, was not a matter of conventional histochemical invest igations.

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