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Light-dependent changes in outer segment free-Ca2+ concentration in salamander cone photoreceptors
A P Sampath1, H R Matthews, M C Cornwall
1Department of Physiological Science, University of California, Los Angeles, Los Angeles, California 90095, USA. apsampat@ucla.edu
The Journal of General Physiology
|February 2, 1999
Summary
Researchers measured photocurrent and calcium levels in salamander cone photoreceptors. Light exposure rapidly reduced outer segment calcium, which recovered upon photopigment regeneration, showing a graded decline during light adaptation.
Area of Science:
- Vision science
- Photoreceptor physiology
- Calcium signaling
Background:
- Cone photoreceptors are crucial for color vision and light adaptation.
- Calcium ions play a vital role in regulating photoreceptor function.
- Understanding calcium dynamics is key to elucidating visual processing.
Purpose of the Study:
- To simultaneously measure photocurrent and outer segment calcium concentration in isolated salamander cone photoreceptors.
- To investigate the relationship between light exposure, calcium levels, and photopigment status.
- To characterize the kinetics of calcium changes in different cone types during light adaptation.
Main Methods:
- Isolated salamander cone photoreceptors were used for simultaneous photocurrent and calcium measurements.
- A laser spot confocal technique with the calcium indicator fluo-3 was employed.
- Photocurrent was recorded from the inner segment, while fluorescence changes in the outer segment were monitored.
- Photopigment regeneration was induced using exogenous 11-cis-retinal.
Main Results:
- Intense illumination suppressed circulating current and rapidly decreased outer segment calcium concentration.
- Red-sensitive cones showed biexponential calcium decay (time constants ~43 and 640 ms), while blue-sensitive cones exhibited slower decay (~140 and 1400 ms).
- Dark-adapted outer segment calcium was ~410 nM, dropping to ~5.5 nM after intense light; regeneration restored these levels.
- Outer segment calcium concentration declined linearly with circulating current during adaptation to background light.
Conclusions:
- Light-induced suppression of photocurrent is tightly linked to a rapid decrease in outer segment calcium.
- Calcium dynamics differ between red- and blue-sensitive cones, suggesting distinct adaptation mechanisms.
- Photopigment status critically influences light sensitivity and calcium homeostasis in cones.
- Outer segment calcium concentration provides a graded signal reflecting light intensity during adaptation.