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Light-induced calcium release and re-uptake in toad rods
1Department of Ophthalmology, Jules Stein Eye Institute, UCLA School of Medicine 90024.
Summary
Light exposure causes calcium (Ca) release from toad retinal rods, primarily via Na/Ca exchange. Following illumination, Ca is actively reabsorbed into disks, a process enhanced by the pigment epithelium.
Area of Science:
- Biophysics
- Neuroscience
- Cell Biology
Background:
- Rod outer segments (ROS) are critical for vision, and their function is modulated by calcium (Ca) levels.
- Understanding Ca dynamics in ROS is essential for elucidating phototransduction mechanisms.
Purpose of the Study:
- To investigate the effect of light on Ca content in toad retinal rod outer segments.
- To differentiate between Ca release and uptake mechanisms under light and dark conditions.
Main Methods:
- Laser micromass analysis was employed on isolated toad retinas and eyecups.
- Isotopically labeled Ca (44Ca) was used to distinguish external Ca uptake from internal Ca release (40Ca).
- Sodium (Na) substitution experiments were performed to assess the role of Na/Ca exchange.
Main Results:
- Continuous illumination induced a sigmoidal decrease in outer segment 40Ca content, indicating light-dependent Ca release.
- Light-induced Ca release was significantly reduced by replacing external Na with choline or Li.
- Ca uptake into rods was minimally affected by illumination but markedly increased in darkness post-illumination.
- Ca re-uptake occurred at a higher rate in eyecups compared to isolated retinas.
Conclusions:
- Light increases Ca permeability or transport across the disk membrane in ROS.
- Ca is extruded from rods via Na/Ca (or Na/Ca-K) exchange across the plasma membrane.
- Cessation of illumination stimulates Ca resequestration into disks, enhanced by the pigment epithelium.